TECHNICAL TRAININGPRODUCT TECHNOLOGY - WHEELED PRODUCTS

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Product Technology

Engine techniques...........................................................................3

Crankcase..........................................................................................9

Crankshaft......................................................................................10

Cylinder..........................................................................................11

Piston..............................................................................................12

Fuel systems...................................................................................13

Lubrication systems......................................................................19

Ignition systems.............................................................................21

Electrical systems...........................................................................25

Drive systems.................................................................................33

Hydraulics.......................................................................................37

Chassis............................................................................................39

Protective equipment....................................................................41

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Induction stroke

Compression stroke

Four-stroke engine cycle

The two-stroke engine has a power stroke each time the piston passes top dead centre. The four-stroke engine on the other hand has a power stroke every second revolution. To permit the four different strokes to take place, the four-stroke engine is equipped with a valve arrangement that usually consists of an inlet valve and an exhaust valve. The advantage of a four-stroke engine is that the combustion process is very efficient at varying rpm. A four-stroke engine also develops significant torque at low rpm. Four-stroke engines are more complex and heavier as a result of additional parts but this additional complexity has a proven track record in reliability and dependability.

Induction stroke

The working cycle of the four-stroke engine starts with an induction stroke. When the piston moves down in the cylinder the inlet valve opens at the same time. The fuel-air mixture from the carburettor is drawn into the cylinder for the entire period the valve is open. The exhaust valve remains closed.

Compression stroke

The second stroke of the four-cycle engine is the compression stroke. The compression stroke starts as the piston passes bottom dead centre and starts to move upward in the cylinder. The inlet valve closes at this time and the exhaust valve remains closed. This allows the piston to compress the fuel-air mixture into a small volume. Compressing the fuel-air mix is important for developing maximum power. The higher the compression, the greater pressure exerted on the piston when the fuel-air mix ignites. Compression also pre-heats the mixture, which helps it burn efficiently.

Engine technique

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Combustion stroke (Power stroke)

Exhaust stroke

Engine technique

Combustion stroke (Power stroke)

The third stroke is the combustion stroke (power stroke). The combustion stroke starts as the compressed fuel-air mix is ignited in the combustion chamber. Both inlet and exhaust valves remain closed during this stroke. A spark plug, located in the cylinder head, generates an electrical spark in the combustion chamber, which ignites the compressed fuel-air mix. The burning fuel rapidly expands, creating high pressure against the top of the piston. This pressure drives the piston downward. This downward motion provides the power to turn the crankshaft that, in turn, drives the engine’s outlet shaft. The crankshaft reciprocates the linear movement of the piston into a rotary movement.

Exhaust stroke

The final stroke of the cycle is the exhaust stroke. Just after the piston has passed bottom dead centre and starts to move back up the cylinder the exhaust valve opens. The inlet valve remains closed. On its way, upwards the piston forces the burnt gases through the exhaust valve and out into the exhaust pipe. When the piston reaches the top of its travel, the exhaust valve closes, and the intake valve opens. The exhaust stroke completes the combustion process. The opening of the intake valve signals the beginning of a new cycle.

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Side-valve engine

Overhead-valve engine (OHV)

Overhead camshafts (OHC)

Engine technique

Valves

Valves are opened and closed in two ways. One is by cams directly actuating the valve stems. The cams are located on a camshaft. Another is by way of rocker arms actuating the valve stems through push rods. The push rods are actuating by valve lifters or cams on the crankshaft. The cams or valve lifters determine when the valves open and close. This timing is critical.

The valves that control the flow of gas in the engine the inlet and exhaust valve may be located either at the side of the cylinder, which is known as a side-valve arrangement, or in the cylinder head, which is known as an overhead-valve engine.

Side-valve engine

The side-valve layout engine is used in older types of engine design. The valves are located in a special housing known as the valve housing on the side of the cylinder. Each is linked to its’ own ports, inlet port and exhaust port. A valve lifter driven by a rotating cam on a camshaft in the crankcase actuates the valve stem.

Overhead-valve engine (OHV)

This arrangement is suited to engines that have high power requirements and where higher engine speeds are needed. On this type of engine the valves are located in the cylinder head and are each connected to their own port. The opening and closing of the valves may be directly controlled by cams on an overhead camshaft that act directly on the valve stems, or by rocker arms acting on the valve stems.

Overhead camshafts (OHC)

Overhead valve engines can have overhead camshafts (OHC) which actuate the valves. The overhead camshaft (OHC) is then located in the cylinder head instead of in the engine block or crankcase.

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Engine technique

Valve clearance

Valve clearance

Regardless of whether an engine has side-valves or overhead-valves there has to be a clearance between the valve stem and the valve lifter, the cam or the rocker arm. This is necessary because the length of the valve stem increases considerably when the engine is hot. If there were no valve clearance the valve would soon press on the lifter, preventing a tight seal, and this would lead to damage to the valve head and seat. Note that the clearance should always be set when the engine is cold as the measurement result will be inaccurate if the engine is measured hot as metal expands when heated. The valve clearance is usually larger for the exhaust valve than for the inlet valve (0.25 – 0.45 mm (0.010 - 0.020 inch)) resp. 0.20 – 0.25 mm (0.008 - 0.010 inch)). The valves are made of different materials and must not be mixed up during servicing. Some manufacturers label the inlet valve “IN” and exhaust valve “EX”. If there are no labels, you easily notice the difference when the valves are to be cleaned. The exhaust valve is a lot more difficult to clean.

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A = RotorB = Carbon brush

C = Permanent magnetD = Interference suppression device

Engine techniques

Battery driven lawn mower

Electric motors

The electric engine motor comes in handy in the context of lawn mowers. To avoid a long unwieldy electric wire the engine is powered by two 12-volt lead batteries which are recharged the usual way with a battery charger. The motor is a 4-pole permanent magnet motor and has a constant rotation speed (2.900 r/min). Permanent magnet motors are characterised by the fact that they can only drive on direct current 6, 12 or 24 volt. The rotation speed with this kind of motor is high and very even.

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The crankcase on a four-stroke engine has the same main functions as on a two-stroke engine, with the exception that it does not act as a scavenging pump. Instead it acts as an oil container for the lubricant that is needed to lubricate the moving parts of the engine. In addition to the functions described it has one more important function: to provide bearing mountings for the camshafts of the valve mechanism, if the engine has pushrods or valve actuators.

Single casting crankcase

In applications where the engine is used as a power source for tractors, riders and lawn mowers, the crankcase and cylinder are produced as a single casting, whether they are made from cast iron or aluminium. This manufacturing method offers several advantages from the production point of view, and when it comes to servicing and maintenance.

Oil pump

The bottom of the crankcase also acts as an oil sump to collect, store and help cool the oil that is passing through the engine. A gear-type oil pump, driven by the camshaft, sucks the oil through the oil pickup tube from the sump via a strainer in the bottom of the crankcase and then forces it through an oil cooler and into the engine’s oil galleries. The crankcase contains a number of oil galleries. These are internal passages that feed oil to various locations, the most important of which are the main bearings for the crankshaft and camshaft bearings. The engine uses plain shell type bearings that rely on a plentiful supply of lubricating oil. The conrod bearings are then supplied via drilled journals within the crankshaft.

The crankcase provides bearing mountings for the camshafts

A single casting crankcase

Crankcase

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The main purpose of the crankshaft is to convert the up-and-down movement of the piston into rotation, and hence transfer the force that is generated in the cylinder’s combustion chamber into rotary torque at the engine’s output shaft. The crankshaft must be very precisely balanced or the engine will vibrate itself to pieces.

The crankshaft can be constructed in a variety of ways depending on the intended application of the engine, production factors, etc. A crankshaft for a small four-stroke engine is manufactured in a single piece and is made of cast iron. The most common method is to make it from several components, and this is known as a built-up crankshaft. The starting materials for such a crankshaft are forged blanks that are machined in various ways to give the correct shape, dimensions, and tolerances.

One-piece crankshaft

The one-piece crankshaft is made from a single casting. This type of crankshaft is used mainly in low-speed engines with lower demands for precision and vibration levels. A one-piece crankshaft generally requires a connecting rod with a split big end and a bearing that is made up of separate bearing rollers or bearing shells fitted to the connecting rod and bearing cap. The exception is where the connecting rod has a one-piece big end, despite the fact that the crankshaft is a single casting. In this case, one of the counterweights is made so small that the connecting rod can be slid over it. This is the most common construction method and it offers certain advantages for production.

Built-up crankshaft

One-piece crankshaft

Crankshaft rotation

Crankshaft

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Cylinder

The cylinder and piston, in conjunction with the crankcase and crankshaft are the main components of a combustion engine. The cylinder can be compared with a container in which fuel-air mixture is ignited by a spark generated from the spark plug. The ignited fuel-air mixture forces the piston down the cylinder, a force that in turn also drives the engine’s output shaft.

The cylinder is a basic part of the engine. It is a casting generally made out of iron or aluminium. A cylinder head is bolted to the top of each bank in order to seal each individual cylinders containing or isolating the combustion process within the cylinder. On overhead-valve engines (OHV) the cylinder head contains at least one intake valve and one exhaust valve on each respective cylinder. This allows the fuel-air mixture to enter and the burned exhaust gas to exit the cylinder.

Cooling fins

The cylinder cooling fins vary in size in order to produce a uniform cooling effect. The cooling fins are larger on the lee side (furthest from the source of cooling, providing a larger heat dissipation area and compensating for the difference in temperature of the warmer air passing over the lee side and the cooler air hitting the cylinder directly from the source of cooling. As a result, the temperature is kept more uniform around the entire cylinder.

Overhead valve engine (OHV)

Overhead valve engine (OHV)

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The design of a piston in a four-stroke engine varies depending on whether the engine has a cast cylinder liner or a cylinder head that is made of aluminium. In the first instance the head of the piston is marked with the letter ”L” and is easily recognisable since the piston has a dull surface coating and has an expansion ring in the groove for the oil scraper ring. The piston is chromium plated when used in an aluminium cylinder (polished finish) and does not include an expansion ring in the groove for the oil scraper ring.

The piston in a four-stroke engine generally has two compression rings and one oil scraper ring. The two top rings seal the compression gasses in the combustion chamber. The bottom ring scrapes the oil off the cylinder walls, expelling it through the oil return slot in the piston, to the sump.

Compression rings are usually made of cast iron and the oil scraper ring of steel. The rings can be combined in different ways, but it is important that they are always positioned correctly. The scraper groove should be down. Some piston manufacturers mark the piston ring with a “T” or “TOP” on the surface that should face upwards.

Piston

There is a cast cylinder liner to the left and to the right a cylinder head of aluminium

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The typical fuel system and related components include the fuel tank, in-line fuel filter, fuel pump, carburettor, and fuel lines. Some applications use gravity feed without a fuel pump. The fuel from the tank is moved through the in-line filter and fuel lines by the fuel pump. On engines not equipped with a fuel pump, the fuel tank outlet is located above the carburettor inlet and gravity moves the fuel. Fuel then enters the carburettor float bowl and is moved into the carburettor body. There the fuel is mixed with air and is burned in the engine combustion chamber.

Fuel-air mixture

A fuel-powered engine needs both fuel and air (oxygen) in order to operate. Fuel in its pure liquid form doesn’t burn. If the engine doesn’t get both fuel and oxygen at the right time and in the right quantities, the spark plug will have nothing to ignite and the engine will not run. One of the major problems is getting the correct amount of fuel and air into the engine. Since air is so much lighter than fuel, it takes enormous quantities of air to mere drops of fuel to create a burnable ratio of fuel and air. This ratio offers a rather wide variation depending on the outside temperature. If there is too much fuel mixed with the air, the engine runs rich and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. Use only clean, fresh and unleaded fuel with octane rating 87 or higher as it leaves less combustion chamber deposits.

Fuel filter

The fuel tank has an inlet and outlet pipe. The outlet pipe has a fitting for the fuel line connection and may be positioned in the top or in the side of the tank. The lower end is above the bottom of the tank so that any collected sediment will not be flushed out into the carburettor. In order to ensure clean fuel, fuel filters are installed in the fuel line. Fuel filters can be located at any position between the fuel tank and the carburettor.

Fuel systems

Walbro carburettor

Husqvarna recommends Aspen fuel

Fuel filter

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Dirt and rust can build up inside the fuel tank, which can be caused by normal condensation and moisture. Every time the fuel tank is refilled the potential is there for dirt to enter, which can clog the fuel filters rapidly, causing loss of power, acceleration hesitation and hard starting. As the fuel filter gets older more dirt collects inside it, gradually restricting the flow of fuel to the engine. A partially restricted filter will usually allow the passage of enough fuel to keep the engine running at idle or low speed, but may starve the engine for fuel at higher speeds or loads.

Fuel pump

All modern fuel systems are fed using a fuel pump. The fuel pump has three functions:

•To deliver enough fuel to supply the requirements of an engine under all operating conditions, to maintain enough pressure in the line between the carburettor and the pump.•To keep the fuel from boiling. •To prevent a vapour lock situation.

Operation of the fuel pump

The fuel pump has two internal chambers separated with a diaphragm. The air chamber is connected to the engine crankcase by a rubber hose. The fuel chamber has an inlet from the fuel tank, and an outlet to the carburettor. The inlet and outlet each have an internal, one-way check valve. Alternating negative and positive pressures in the crankcase activate the pump. When the piston moves up in the cylinder, negative pressure (vacuum) is created in the crankcase and in the air chamber of the pump. The diaphragm flexes toward the negative pressure, and the suction draws fuel past the inlet check valve, into the fuel chamber. Downward movement of the piston causes a positive pressure in the crankcase and air chamber, pushing the diaphragm in the opposite direction, putting pressure on the fuel. The inlet check valve has now closed, so the fuel is forced past the outlet check valve, to the carburettor.

Fuel pump

Fuel systems

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Excessive pressure can hold the carburettor float needle off its seat, causing a higher fuel level in the float chamber. This will result in high fuel consumption. The highest pressure occurs at idling speed and the lowest at top speed. Although fuel pumps all work to produce the same effect, there are various types of fuel pumps that may operate somewhat differently.

Troubleshooting the carburettor

If engine troubles are experienced that appear to be fuel system related, check the following areas before adjusting or disassembling the carburettor:

•Make sure the fuel tank is filled with clean, fresh petrol and that the fuel cap vent is not blocked and is operating properly.

•Check that the fuel is reaching the carburettor, fuel shut-off valve, fuel tank filter screen, in-line fuel filter, fuel lines and fuel pump and that they are working properly.

•Check that all air cleaner components and carburettor are clean and fastened securely, make also sure that the ignition system, governor system, exhaust system, throttle and choke controls are working properly before adjusting or disassembling the carburettor.

Float carburettor

In principle the float carburettor is the simplest type of carburettor. A float carburettor can only work in an upright position, since it operates with a given volume of fuel in a reservoir, the float chamber. This makes the float carburettor ideal for engines on products such as lawn mowers, riders, and tractors. The fuel must be kept at a preset level, which is determined by the float, in order for the carburettor to work properly. The float carburettor consists of two main parts, the mixing chamber (A) and the float chambers (B), (C) is the venturi tube.

Float carburettor

Fuel systems

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The mixing chamber is simply a tube, which has a constriction at a certain point. This constriction is called a venturi tube. The venturi tube has the capacity to increase the speed at which the air passes through it. In addition, it causes a reduction of pressure (vacuum) just where the diameter is at its narrowest. This can be exploited when constructing the carburettor by allowing one duct from the float chamber to discharge into the venturi tube. When air rushes through the venturi tube, a vacuum builds up. Fuel is sucked up from the float chamber, mixed with the air rushing by and is then conveyed into the engine. The fuel level in the float-chamber is slightly lower than the outlet opening in the venturi so that the fuel does not flood into the carburettor.

The main components of the float carburettor are:

A = Throttle valveB = Choke valveC = VenturiD = Main jetE = Main nozzleF = Needle valve for fuelG = Float for fuel level control in the float chamberH = Air nozzle, full throttleJ = Air nozzle, low speedK = Idle jetL = Fuel inletM = Fuel levelN = Part throttle jets

The fuel flows into the carburettor past the needle valve (F) and fills the float chamber to a specific level (M) determined by the float. When the engine starts and consumes fuel, the level drops and the float opens the needle valve to fill more fuel into the float chamber. The air space above the fuel level must be in connection with the surrounding air in order for this to happen. The fuel level in the float-chamber is slightly lower than the outlet opening in the venturi so that the fuel does not flood into the carburettor. Consequently, it is extremely important during servicing to check that this connection is not blocked.

Main compontents of a float carburettor

Float chamber

The mixing chamber is a tube called venturi tube

Fuel systems

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Starting with the choke valve closed

When the piston moves downwards in the cylinder and the inlet valve is open, a negative pressure is created behind the choke valve (B). The fuel is pressed upwards through both the main jet (D) and the channel to the small throttle jets (N) behind the throttle valve (A). The air nozzles for idling (J) and the main jet (H) ensure that the quantity of air, required for the fuel-air mixture to be combustible, is correct in relation to the amount of fuel. When these nozzles are fully or partly blocked the engine becomes difficult to start and runs unevenly while idling.

Idling and part throttle

When idling with an open choke valve (B) a specific negative pressure prevails between the throttle valve (A) and the open inlet valve. The fuel is then forced, by the atmospheric pressure, from the float chamber into the low speed system channel and out through the front jet hole (closest to the cylinder). When the throttle valve opens further (to part throttle) both the other jet holes are opened and the engine receives more of the fuel-air mixture. Air passes through the air nozzle for low speed (J) and mixes with the fuel.

Full throttle

When the engine speed increases, the negative pressure in the venturi also increases and fuel is forced up through the main jet (D). Air passes through the air nozzle for full throttle (H) and mixes with the fuel in the main jet. This fuel-air mixture is forced up through the main jet by atmospheric pressure and is sucked into the engine past the throttle valve (A). Note that the idling and low speed systems are no longer functional.

Open choke valve

Full throttle

Closed choke valve

Fuel systems

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Solenoid valve

Some carburettors are equipped with a solenoid valve to prevent afterburning or the fuel flooding into the engine once it stops. When the engine is stopped a solenoid valve presses a tapered needle out towards the main nozzle and the flow of fuel is stopped (A). When the engine starts the solenoid valve pulls the tapered needle away from the main nozzle and the fuel can flow normally (B). If the solenoid valve does not work (no click sound is heard from the valve when the ignition key is turned on and off) the engine can not be started.

Air filter

The engines are equipped with a replaceable high density, paper air filter. Some engines also have an oiled, foam pre-cleaner, located in the outer air cleaner cover. Ensure that the pre-cleaner element is clean and dry, and that the motor is able to breathe through it. Be aware that the air filter may appear satisfactory, but fine dust could have it completely blocked. If the engine seems to lack power or goes irregularly the reason may be that the air filter is clogged. It is therefore important to replace the air filter at regular intervals. Do not use compressed air to clean the paper filter.

Solenoid valve

Fuel systems

Air filter

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In order for a combustion engine to work properly several parts need continuous lubrication. The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing lubrication are the pistons and any bearings that allow things like the crankshaft and camshafts to rotate freely. If insufficient oil or the wrong type of oil is used, the engine will suffer extensive damage, requiring expensive repair.

The engine lubrication system is designed to deliver clean oil at the correct temperature and pressure to every part of the engine. The lubrication of a four-stroke engine differs from a two-stroke engine in the way that oil is stored in the crankcase and is not mixed with the fuel. The engine’s moving parts are lubricated either through a splash mechanism (Splash lubrication) or with a pump (Pressure lubrication). Having a dedicated lubrication system instead of mixing the oil with the fuel has certain advantages. The components of a four-stroke engine last longer and the engine doesn’t produce as much pollution.

Splash lubrication

Splash lubrication is the most common method. The oil is stored in the crankcase at a level just high enough for the crankshaft to touch the oil surface. The oil is splashed onto the bottom of the cylinder, the piston bearing, the crankshaft bearings and the camshaft and its bearings. If the engine is mounted with the crankshaft vertical it is necessary to improve the efficiency of lubrication. Engines of this type therefore have an oil atomiser in the crankcase that is driven by a cam pinion.

Pressure lubrication

An oil pump is used to force oil through channels leading from the crankcase (oil sump) to the lubrication points that are furthest away. The oil reservoir is formed by the crankcase itself as there is no separate oil sump. The oil is then fed through a filter. Pressure lubrication and splash lubrication are often combined to keep the construction as simple as possible.

Lubrication systems

Splash lubrication

Lubrication products

Oil pump

Pressure lubrication

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Positive displacement pump

The engine lubrication circuit is a pressurized system consisting of a positive displacement pump which picks up oil through a filter screen from the crankcase. The oil is pumped through an oil filter (often a replaceable oil filter cartridge), through oil passages in the engine to lubricate internal components, and is returned to the crankcase. A pressure relief valve is used between the oil pump and oil filter to relieve excessive oil pressure by returning excess oil to the crankcase.

Lubrication systems

Positive displacement pump

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The purpose of the ignition system in an engine is to produce a high voltage pulse that generates a spark between the spark plug electrodes at exactly the right moment, i.e. just before the piston reaches top dead centre, the pre-ignition position. In order to make the engine easy to start, and to operate satisfactorily at high speed, the ignition setting must be correct.

The fuel-air mixture is ignited by a spark which results in a sharp rise in pressure in the combustion chamber of the cylinder. This in turn forces the piston down the cylinder. In order to exploit this rise of pressure as effectively as possible, the mixture must be ignited before the piston reaches top dead centre (TDC). The reason for this is that combustion begins around the spark plug electrodes where the flame front then advances at a speed of 10-25 m/s (32.8-82 f/s) and ignites the rest of the fuel-air mixture.

When the engine starts and the flywheel begins to rotate, its magnets pass the ignition module’s two signal sensors. As the magnet passes the first sensor, the charging coil, current is generated and stored in the capacitor. When the flywheel continues to rotate and the magnet passes the second sensor (the discharging coil) another electric current is generated. This current is used to trigger the thyristor, which becomes conductive and thereby discharges the capacitor. A high voltage current is now induced in the coils secondary winding. The current is conducted through the HT (high tension) cable to the spark plug where it travels from one electrode to another generating a spark. The spark ignites the fuel-air mixture in the cylinder.

Right timing

The fuel in the combustion chamber takes approximately the same amount of time to burn no matter what rpm the engine is running at. Remember that the spark only starts the fuel burning. Once we have lit it on fire with the spark, it keeps burning by itself. It takes time for the flame to travel across the cylinder. At low rpm, the piston doesn’t travel very fast, and therefore the spark can occur quite late. The engine speeds up, the piston is travelling faster and if the spark does not occur earlier, the piston will travel too far down the cylinder and the volume in the cylinder will be too large, pressure will be low, and the engine power will be low. The spark must therefore be timed earlier to allow for the increased piston speed.

Ignition systems

Charging- and discharging coil

Pre-ignition position

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Engine knock

Sometimes the fuel tends to auto ignite rather than waiting to catch fire when the spark occurs. There are therefore two explosions, one caused by the compression and the other caused by the spark. This produces a knocking sound from the engine. It also reduces the engine performance and can damage the piston and cylinder.

Spark plug

The purpose of the spark plug is to ignite the fuel-air mixture in the cylinder with a spark. The spark is generated when the current travels across the electrode gap. In order for this to work properly, the electricity must be at a very high voltage when travelling across the electrode gap. Voltage at the spark plug can be anywhere from 20,000 to 100,000 volts. The spark plug must have an insulated passageway for this high voltage to travel down to the electrode, where it can jump the gap and, from there, be conducted into the engine block and grounded.

Ignition module

It requires high voltage to make any spark jump and the ignition module delivers the spark impulses to the spark plugs so they fire at the proper time. The coil is the device that generates the high voltages required to create a spark. The coil is a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. Wrapped around it is the secondary coil. The secondary coil normally has hundreds of times more turns of wire than the primary coil. Current flows from the battery through the primary winding of the coil. The secondary coil is needed to increase the voltage to approximately 10.000 volts. The high voltage from the secondary coil passes from the ignition coil’s large centre terminal along a heavy-duty wire (Coil High Tension Lead - HT Lead).

Electrode gap

On a four-stroke engine the electrode gap should be 0.70-0.75 mm (0.028-0.030 inch). If the gap is too large it puts unnecessary stress on the other components of the ignition system and if it is too small it produces a weak spark, which leads to slower ignition of the fuel-air mixture. If the electrodes are worn the spark plug should be replaced.

Ignition systems

Ignition module

Spark plug

Electrode gap should be 0.70-0.75 mm (0.028-0.030 inch)

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Ignition systems

Heat Range

In order for the engine to work properly, the spark plug must have the correct heat range. If the working temperature of a spark plug is too high, there is a risk that it will be damaged by pre-ignition. If it is too low, oil and carbon will get stuck on the sparkplug, causing ignition disturbance.

Thread length

The spark plug must also have the correct thread length. If the thread is too short it will not fill the full threaded length of the hole in the cylinder head. The unused thread will be coated in soot, which will prevent a spark plug with the correct thread length from being tightened properly. This means that the spark plug gasket will not have sufficient contact area, which will reduce heat dissipation from the spark plug. The result is that the spark plug will overheat and cause pre-ignition. If the spark plug thread is too long it will project into the combustion chamber and this again will result in pre-ignition.

HT - Lead

The high-tension leads are highly insulated to prevent the current taking a short circuit to ground. There is usually one plug wire going to each spark plug.

Flywheel

The flywheel stores energy on the power stroke, and keeps the engine going on its’ non-power strokes through momentum. The flywheel is often made of aluminium and works as a balancing weight for the engine. It also provides cooling air and holds the permanent magnets that are part of the ignition system. The flywheel is connected to the crankshaft and it turns the magnets past the ignition module. The flywheel must be precisely balanced, or it would be damaged. The heavier the flywheel is, the more energy it stores. However engines with heavier flywheels are not designed for acceleration. An extremely light flywheel is needed in order to receive maximum acceleration.

Heat range - cold plug to the left and a hot plug to the right

Thread length

Flywheel

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In many Husqvarna products an electrical system is needed to supply power to different components. In a tractor or rider for instance, electricity is needed to start the engine, turn on the lights, etc. Because of this riders and tractors need a battery to store electricity.

System components

A typical electrical system consists of five main components that together generate, convert and store electricity from the engine’s movement. These components are the starter motor, generator, rectifier, fuse and battery. Electricity is stored in the battery as chemical energy, which is transformed back into electricity when a machine component requires it. The generator transforms mechanical movement from the engine into electricity. The generated electricity is AC, alternating current. The rectifier converts the alternating current (AC) into direct current (DC). The main job of the fuse is to protect the wiring and components from overload. Fuses should be sized and located to protect the wire they are connected to. The starter motor uses electricity from the battery to start the engine.

Battery

The initial source of electricity is a battery, whose most important function is to start the engine. Once the engine is running, a generator takes over to supply the electrical needs and to restore energy to the battery. A 12-volt storage battery consists of layers of positively and negatively charged lead plates that, together with their insulated separators, make up each of six two-volt cells. The cells are filled with an electricity-conducting liquid (electrolyte) that is usually two-thirds distilled water and one-third sulphuric acid. Spaces between the immersed plates provide the most exposure to the electrolyte. The interaction of the plates and the electrolyte produces chemical energy that becomes electricity when a circuit is formed between the negative and positive battery terminals.

Batteries

Fully loaded with the specific gravity of 1.28 gram

Sulphuric acid decreases during discharging and is replaced by water

Water is in majority when the battery is discharged

Electrical systems

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During discharging the concentration of sulphuric acid in the electrolyte decreases, and this is replaced with water. The active coating on the positive and negative plates is slowly converted into lead sulphate. When the battery is charged the concentration of sulphuric acid increases and the amount of water decreases. The lead sulphate on the positive plates is converted back into lead dioxide and the lead sulphate on the negative plates is converted back into lead sponge.

The battery charge level can be checked by using a hydrometer. The specific gravity of the electrolyte should be 1.26–1.28 gram per cubic centimetre at 25 degrees. If it is around 1.20 the battery should be recharged. Checking the charge condition of the battery by measuring the specific weight of the battery acid with the help of an acidimeter is a good method, but does not reveal the complete story about the condition of the battery. Special battery testers are a good complement.

Charging the battery

Connect the earth cable on the tester to the battery’s negative pole and the other cable to the positive pole. A meter on the battery tester indicates the voltage in the battery (fully charged approximately 13 volt). The next step in checking the battery is to short circuit it for 10 seconds and note where the pointer on the instrument stops. The scale shows the condition of the battery either with colour codes or in plain text. When this load test indicates that the battery is poor (despite an acceptable voltage) the only option is to replace it with a new battery.

Nickel/cadmium (NiCd) and nickel/metal hydride (NiMH) batteries

These types of battery do not have a liquid electrolyte, yet they are chargeable in the same way as the lead-acid accumulator. Husqvarna’s Auto Mower is equipped with Ni/MH batteries; these have specific charging and discharging curves which means that they cannot simply be replaced with Ni/Cd batteries. Ni/MH batteries have approximately 50% higher capacity and in addition impact less on the environment as the toxic substance cadmium is not included.

During charging water is replaced by sulphuric acid

Battery tester

Short circuit for 10 seconds

Ni/MH-battery curve

Electrical systems

27

Husqvarna has a special battery tester for this type of battery. This first charges the battery and then discharges it in a controlled process that mimics natural discharging when the battery is used. The condition of the battery can be determined once a full cycle i.e. charging and discharging, has been completed. It is not sufficient to just measure the battery voltage. It can, in spite of full voltage, have a reduced energy storage capacity. The battery must not be completely discharged with an external load. The cells can be damaged and even the polarity can change.

Special battery tester for Ni/MH batteries

Electrical systems

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Generator

If a conductor (metal wire) is moved within a magnetic field, electrical tension is created in the conductor by means of induction. The same result will be reached if the magnetic field is moved or if the power of the magnetic field is altered. The magnitude of the tension is in proportion to the speed at which the field is altered, i.e. the faster the alteration, the higher the tension. Furthermore, if the magnetic field changes direction, the polarity of the tension changes. In other words, if one allows the poles in a magnetic field to change places, the direction of the current changes in the conductors. One can accordingly obtain electrical tension with the aid of a magnetic field. This is used in the ignition system by allowing the magnet that is built into the flywheel to rotate past a coil which is wound around an iron core.

Generators are used for charging the battery. The stator consists of a number of coils (copper wire wound around an iron core) that are connected in series. The coils are wound alternately to the right and to the left.

Nearby coils with the opposite polarity create lines of force that run in opposite directions. When the flywheel’s in-built permanent magnets pass the coils their lines of force change direction as the lines always run from the north pole to the south pole. The current generated is therefore called alternating current.

The current and voltage

A stator with two coils

Alternating current

Electrical systems

Generator

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Rectifier

The current that is produced by the generator is alternating current. The current alternates between a positive value and an equally large negative value (sinusoidal curve). In order to charge the battery this alternating current must be converted into direct current. This is achieved by removing the negative part of the sinusoidal curve, using a rectifier (diode). The diode can be regarded as an electronic non return valve, i.e. the electrons can only move in one direction, from positive to negative. The diode consists of an anode and a cathode. When it is connected to an electrical circuit the cathode must always be connected to the positive pole of the battery.

Half-wave rectification

If the diode is connected in series in an alternating current circuit (anode to +) the diode will only allow through the positive half of the current flow (positive part of the sinusoidal curve). The result is a pulsed direct current. The useful resulting voltage is therefore just half the original voltage. This is known as half-wave rectification.

A = Alternating current B = Pulsating direct current

Full-wave rectification

In order to utilise the entire alternating current signal, two or four diodes are connected together. This is called full-wave rectification and the result is a pulsed positive direct current, which charges a battery more effectively than half-wave rectification.

A = Alternating current B = Pulsating direct current

Alternating current is produced by the generator

The diode consists of an anode and a cathode

Full-wave rectification

Electrical systems

30

In full-wave rectification one half of the alternating current signal from the generator in the current circuit will work as a direct current circuit through two of the diodes in series with the battery.

The negative potential goes through the first diode directly to the negative pole of the battery and on from the battery through the third diode back to the generator coil.

During the second half of the alternating current signal the generator current switches polarity. The diodes 2 and 4 then let through the current, which charges the battery.

Fuse

Most electrical circuits contain at least one fuse. The fuse prevents damage to conductors or electrical components if an overload occurs in the electrical system. An overload means that the amount of current passing through a wire exceeds the amount that the wire is designed to carry. This results in a heat build-up that can damage the wire or the connected electrical component.

Fuses contain a special type of wire (conductor) in a protective casing, which has a lower melting point than the connecting wires. The size of the conductor is calibrated very carefully so that when the rated current is reached, enough heat is generated to melt the conductor and break the circuit, preventing damage to the wire or electrical component.

When a fuse is blown, it must be replaced before the circuit will work. A blown fuse must be replaced with a fuse of the same amperage. It is especially important that a fuse is fitted between the rectifier and the battery, to prevent damage to the charging coils of the generator if there is a short-circuit in the diode. If this happens a high current surges through the diode and causes a short circuit in the generator coils, which can lead to expensive repairs.

Fuse prevents damage to the conductor and other electrical components

First part in a full-wave rectification course

Second part in a full-wave rectification course

Electrical systems

31

Starter motor

The starter motor converts electricity to mechanical energy in two stages. Turning on the ignition switch releases a small amount of power from the battery to the solenoid above the starter motor. This creates a magnetic field that pulls the solenoid plunger forward, forcing the attached shift yoke to move the starter drive so that its pinion gear meshes with the engine’s crankshaft flywheel (ring gear). When the plunger completes its travels, it strikes a contact that permits a greater amount of current to flow from the battery to the starter motor. The motor then spins the drive and turns the meshed gears to provide power to the crankshaft, which prepares each cylinder for ignition. After the engine starts, the ignition key is released to break the starting circuit. The solenoid’s magnetic field collapses and the return spring pulls the plunger back, automatically shutting off the starter motor and disengaging the starter drive.

When the starter is not in use, the drive unit is retracted so that its pinion is disengaged from the flywheel. As soon as the starter is activated, the forward movement of the solenoid plunger causes the shift yoke to move the drive in the opposite direction and engage the pinion and flywheel. The pinion is locked to its shaft by a clutch that unlocks if the engine starts up when the flywheel begins turning the pinion faster than its normal speed. By allowing the pinion to spin freely for a moment, the clutch protects the motor from damage until the drive is retracted.

Starter motor

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32

Earth terminal

The most common electrical problem is not to have an earth terminal. If the battery is not earthed, electricity will not be supplied to any component. If a component is not earthed, other electrical items will work but not that component.

Wiring schematics

Electronic circuits are presented in schematic form. A schematic is really a map showing the path the current takes through the various components. Each component is represented by a symbol, usually with either a label or a value (or both). Some general conventions apply to all schematics. The layout of a schematic is designed to show the function, usually with signal progressing from left to right. The actual layout of the circuit will be quite different. All points on a line are electrically identical. This includes all branches of a line.

Earth terminal symbol

Electronic circuits are presented in schematic form

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Drive systemsAll Husqvarna engine powered applications have a system for transmitting the power generated by the engine to movement on one or more attached components. A transmission is a speed and power changing device installed at some point between the engine and driving wheels. Depending on an application’s engine type and area of use the method for transmitting power differs. A transmission can either work with or without gears (manual or automatic). The reason for using gears in an application is that the transmission allows the gear ratio between the engine and the drive wheels to change. You shift gears in order to let the engine run at the speed (rpm = revolutions per minute) where it has its best performance.

Direct drive

The simplest form of drive system is to bolt the attachment straight to drive shaft. However, damage can result if the attachment is brought to a sudden stop. This can be avoided by fitting a shear pin between the drive shaft and the attachment. An example of a direct drive system is lawn mower blades. If the blades hit a solid object the shear pin is sheared in two, preventing damage to the drive shaft.

Electric clutch

Some models of garden tractors and lawn mowers have a drive system that is equipped with an electric clutch. The purpose of this clutch is to stop the mower blades without stopping the engine. A manual switch operates it electromagnetically.

The electric clutch allows the engine and the transmission to connect and disconnect, both when starting up and during gear shifts. Friction plates route the rotation of the engine crankshaft to the gears, and then to the wheels. In a manual transmission, the clutch is disengaged when the pedal is pressed down. The pedal works the thrust pad, and presses the levers in the middle of the clutch cover. Doing all this lifts the pressure plate away from the clutch plate. The flywheel, which is turned by the crankshaft from the transmission shaft is then disconnected. When the clutch pedal is lifted, springs force the pressure plate and clutch plate against the flywheel. The clutch plate friction linings allow it to slide before becoming engaged. The sliding results in a smooth start instead of a jolt.

Direct drive

Electric clutch

Position of the electric clutch

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Drive systems

Belt transmission

Belt transmission is a common type of transmission in tractors and riders. The belt transmission is based on one or more toothed belts or V-belts that run over pulleys of various diameters. It provides excellent scope for varying the gear ratio and shaft spacing. In addition, it does not require close tolerances to work. The problem with a belt transmission is the risk of slipping, caused by stretching of the belt or contamination of the belt with oil or water. Slipping can be eliminated by using toothed belts instead of V-belts. This is important in applications such as cutting decks that have several blades driven by the same belt, as it prevents changes in the relative positions of the blades.

V-belt transmission

A V-belt transmission can be used as a clutch. By tensioning or loosening the belt the drive can be easily engaged and disengaged by means of a tensioning roller mounted on a lever. The mechanism can be operated manually or, as illustrated, by means of the vacuum in the engine intake manifold. A hose carries the vacuum through a valve to a vacuum actuator. If the vacuum valve is closed the drive belt remains slack and the engine can rotate freely. If the vacuum valve is open to the vacuum actuator it activates a diaphragm inside the actuator, which is connected to a wire that pulls the lever on the tensioning roller back, engaging the drive.

Continuously variable transmission

This transmission type works so that the engine drives a belt that drives a transfer shaft on which a round disc is mounted. At right angles to this disc runs another disc that has a rubber rim. This disc can be moved along its shaft by means of a control on the handlebar. The shaft in turn drives the wheels of the application via a gear transmission. The beauty of this transmission is that the application can be driven at any chosen speed between full speed forwards, when the disc is at one end of its travel, and full speed in reverse, when the disc is at the other extreme. This type of transmission is found in smaller rider models and snow blowers.

Belt transmission

V-belt transmission

Continuously variable transmission

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Drive systems

Transaxles

A transmission is a speed and power changing device installed at some point between the engine and driving wheels. There are two types of transmissions; manual and hydrostatic (automatic). With a manual transmission, the gears are shifted manually while with a hydrostatic transmission, the mechanism does the shifting. The reason for using gears in an application is that the transmission allows the gear ratio between the engine and the drive wheels to change. The gears are shifted in order to let the engine run at the speed (rpm = revolutions per minute) where it has its best performance.

Manual transmission

What causes the transmission to shift in a manual gearbox? It is shifted by shifter forks, also known as sliding yokes. Shifter forks are connected to a cam and shaft assembly. The cam assembly is kept in the selected gear by spring loaded steel balls that jump through notches (in the cam assembly) and hold the shifter forks in that gear. The shafts (of the cam and shaft assembly) go through the housing and are fastened to shift levers. The shifter forks move the synchronizers that engage the gears to the shafts they ride on.

Hydrostatic transmission

A hydrostatic transmission consists of a variable-displacement pump and a fixed or variable displacement motor, operating together in a closed circuit. In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank. As well as being variable, the output of the transmission pump can be reversed, so that both the direction and speed of motor rotation are controlled from within the pump. This eliminates the need for directional and flow (speed) control valves in the circuit. Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation. The charge pump is normally installed on the back of the transmission pump and has an output of at least 20% of the transmission pump’s output.

Transaxles

Hydrostatic transmission

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Torque converter

Just like with the manual transmission, the hydrostatic transmissions need a way to let the engine turn while the wheels and gears in the transmission come to a stop. A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as in idling, the amount of torque passed through the torque converter is very small, so keeping the tractor or rider still requires only a light pressure on the brake pedal.

PTO - Power Take-Off

PTO (Power Take-Off) is an auxiliary gearbox that is driven through main transmission to provide power to drive hydraulic pumps and other auxiliary equipment.

Torque converter

PTO - Power Take-Off

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Power steering

The power steering is basically a hydraulic torque motor, controlled by the steering wheel. It is mounted in the steering column, with its stator section in the power steering housing in the front section of the machine’s frame. When the power steering has no hydraulic pressure provided by the hydrostatic transmission’s pump, the machine can still be steered. This is due to the steering shaft being mechanically attached to the sprocket wheel on the power steering’s rotor section (the output shaft).

Hydraulic lift

The lift cylinder is a double action hydraulic cylinder, and is connected to the lever housing’s shaft. The control valve is a slide valve, with the slider connected to the lever that is in the lever housing to the rear. The valve block is where pressure supply and exhaust occurs, with two hoses used to supply hydraulic oil for the lift cylinder. On the cylinder’s piston side, a throttle is fitted to the nipple on the hose. A mechanically controlled return valve is between the slider and throttle. Its purpose is to retain oil to prevent the cutting unit from lowering when the lever is not activated.

Hydraulics

Power steering

Hydraulic lift

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ChassisThe chassis represents the machine component on and around which all other parts are secured. Irrespective of the type of machine (tractor, rider, lawn mower) it must be sufficiently stable to withstand the maximum stresses without, for example, the axle bearings and other mount points changing. A plate chassis is often reinforced by adding folds and bends as well as welding reinforcement pieces on to meet these strength demands. On tractors, which are intended to be equipped with different attachments, the mounting points are given extra reinforcement.

The chassis of a tractor

By carrying the tractor’s front axle in the centre and close to the machine’s roll centre, the wheels can move vertically without causing too much sideways lean. Driving over, for example, an uneven lawn can be done without problem. With the help of the roller on the front edge of the mower deck the blades do not go down into the ground.

The chassis of a rider

Rider models feature a chassis with articulated steering. The engine and transmission are mounted on the rear section, which makes it into a complete driving and steering unit. The steering is controlled by chains and wires, which despite being relatively long do not affect precision.

Articulated steering

The articulated steering gives the machine an extremely small turning circle at the same time as the unmown area becomes very small. In addition to the wheels being able to move horizontally, they can also move vertically around a centrally located carrying point. The machine can then easily follow all types of uneven surfaces.

Tractor

Rider

Articulated steering

Chassis

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Micro switches

Riders and tractors feature a number of micro switches positioned to prevent personal injury in the event of e.g. careless handling. For example, the switches cut the power when the driver is not sitting in the seat or give an indication when the grass collector is full. It is therefore important when trouble shooting the electrical system that all the micro switches are localised and checked to ensure they work as intended before the remaining electrical system is checked.

Safety handle

The rotating blades on lawn mowers and cultivators as well as the feed roller on snow blowers constitute a substantial accident hazard. To minimise the risks, these machines are equipped with a safety handle, which has the task of stopping the rotating blade or roller as soon as the handle is released.

Slipping clutch device

Some lawn mower models have a slipping clutch device between the engine and the cutting blade to prevent the engine stopping at the same time as the blade. In doing so the need to constantly restart the engine, for example, when emptying the grass collector, is avoided without impairing safety.

Micro switches

Safety handle

Slipping clutch device

Protective equipment

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