GOVERNORS Why a governor 1s required The speed and horsepower capability of any internal combustion engine is regulated by the volume of air that can be retained within the engine cylinders and the volume of fuel that can be delivered and consumed during the engine power stroke. More than likely you have a driver's license, so you are aware of the fact that when you drive a car or truck equipped with a gasoline engine, you determine the rate of fuel supplied to the engine by manipulation of the gas or throttle pedal. Regardless of whether the engine is carbureted or fuel injected, throttle movement controls the flow of air into the engine cylinders and thus the desired fuel flow. Therefore, a mechanical or electronic governor assembly is not necessary on a gasoline engine. Nevertheless, some gasoline engines in industrial and truck applications are equipped with a governor to control the maximum speed and power of the engine/vehicle. In addition, some models of passenger cars are equipped with an electronic ignition cut-off system to control the maximum speed of the vehicle. Remember that a governor is not a "must" with a gasoline engine as it is with a diesel engine. Why then does a diesel engine require a governor assembly? The main reason has to

do with the fact that the throttle pedal controlled by the operator does not regulate the airflow into the diesel engine but controls the fuel flow. Current gasoline engines in passenger cars have electronic controls for both the ignition and fuel systems and are designed to operate at air/fuel ratios that allow the engine to comply with existing exhaust emissions standards. Through the use of an exhaust gas oxygen sensor, the air/fuel ratio is in closed loop operation (oxygen sensor returns a system operating condition signal back to the ECM to complete the circuit). The oxygen sensor monitors the percentage of oxygen in the exhaust gases leaving the engine. The ECM will either leans out or enriches the air/fuel mixture to try and maintain a stoichiometric air/fuel ratio, which is between 14.6 and 14.7 parts of air to one part of fuel (gasoline) by weight. Due to the fact that there is no throttle to restrict the flow of air a diesel engine at an idle speed runs very lean, with air/fuel ratios being between 90 and 120:1 depending on the specific model of engine in question. Under full-load conditions with turbo-charged engines, this air/fuel ratio is approximately 25 to 30:l. Governors are needed in diesel engines because there is no fixed position of the fuel control rod at which the engine will maintain its speed accurately without a governor. During idling for example, the engine speed without a governor would either drop to zero or would increase continuously until the engine races, and runs completely out of control. The latter possibility results from the fact that the diesel engine operates with an excess of air at all times and effective throttling of the cylinder charge does not take place as the speed increases as happens in gasoline engines. If a cold engine is started, and it is permitted to continue idling with a corresponding amount of fuel injected, the inherent friction in the engine as well as the transmission resistance of parts driven by the engine such as, the generator, air compressor, fuel injection pump etc. decrease after a certain length of time. As a result, if the position of the control rod were to remain unchanged without a governor the engine speed would constantly increase and could rise to a level at which the engine would ultimately destroy itself. For example, let's assume for instructional purposes that a given four-stroke-cycle diesel engine is designed to produce 400 bhp (298 kW) at 2100 rpm full load speed. To produce this power, the fuel system may be designed to deliver 185 cu mm of fuel from each

injector into each cylinder for every power stroke. The same engine at an idle, (no load), speed of 600 rpm, may require a fuel delivery rate to each cylinder of only 18.5 cu mm per power stroke and the engine would produce approximately 40 bhp (30 kW). If the engine/vehicle is stationary and the throttle is placed into a WOT (wide open throttle) position, the fuel control rod will immediately, barring any throttle delay mechanisms, go to the full fuel position. The engine does not need to receive full fuel, (185 cu mm), to accelerate to its maximum no-load speed. In fact the engine can be accelerated with very little additional fuel being supplied to the cylinders, because with no load on the engine, it only has to overcome the resistance to motion of the engine internal components, and other accessory driven items that need more horsepower to drive them at this higher speed, such as alternators, compressors etc.. In addition, if the engine has very little additional load from what it had at an idle rpm, the faster rotating flywheel will store enough inertia (centrifugal force generated at the higher speed) to keep the engine turning over smoothly at this higher no-load speed. Once the engine obtains this higher no-load speed, in this example, say, 2250 rpm, the same amount of fuel (or slightly more) that was supplied at idle will basically maintain this higher speed. However, on a diesel engine, remember that operation of the throttle controls the fuel flow and not the airflow as happens on a gasoline engine. Therefore, by opening the throttle to a WOT position in this engine, we actually deliver 185 cu mm of fuel to the engine cylinders. This is 10 times more than we did at idle speed; but all we need to maintain this higher no-load rpm is basically the same volume of fuel that we used at idle (18.5 cu mm) at 600 rpm, or slightly more. If we generated 40 bhp (30 kW) at 600 rpm, at WOT we might need to develop an additional 10 to 15 hp (7.5 to 11 kW) to handle the increased power requirements of the various accessory items such as a fan, air compressor, or generator. We certainly do not require the 400 bhp (298 kW) rated power output of the engine under this operating condition. Without a governor assembly, a WOT position grossly overfeeds the engine in this high-idle no-load example by about 10 times its needs. Since we know from earlier discussions that the diesel engine always operates with an excess air supply, we have sufficient air to burn this full-fuel delivery rate. The result will be that with 10 times more fuel than necessary, the engine rpm will continue to climb in excess of a safe operating speed. Under such an ungoverned overfueled condition most diesels will quickly self-destruct as a result of valves striking piston crowns and connecting rods punching through the engine block as well as possible crankshaft breakage. When a load is applied to a diesel engine, more fuel delivery is obviously required to generate the extra heat energy to produce the higher horsepower required. In our simplified example, this engine can produce 400 bhp (298 kW) at 2100 rpm WOT full-load operating conditions. It is only under such a condition that this engine needs its 185 cu mm of fuel delivery to each cylinder. A governor senses engine speed and limits fuelling to only what the engine requires to maintain a selected speed. This prevents the diesel from over-revving and running away under all operating conditions. Without a governor the engine would rapidly accelerate, faster than 1,000 rpm per second, to self-destruction. Governor Classifications Limiting speed (LS) (min-max, UK) A limiting speed (LS) governor sets the engine idle speed, defines the high idle speed, and permits fuelling between those parameters to be controlled by an operator (driver). Limiting speed governors are the most common in commercial vehicle applications and one of their advantages is to make the diesel engine respond to accelerator input in much the same manner as the SI engine responds to throttle control. A governor classified as limiting speed will in most cases provide excess start-up fuel, define a torque rise profile, define droop curve and be capable of no-fuelling the engine for shutdown. A mechanical limiting speed governor is sometimes known as an automotive governor. Variable speed (VS) (all-speed, UK) A variable speed (VS) governor sets engine idle speed, defines high idle and any speed in the intermediate range depending on accelerator pedal position. A given amount of accelerator pedal travel will correspond to an engine rotational speed; as engine loading either increases or decreases, the governor will manage fuelling to attempt to maintain that engine speed. Hydro-mechanical variable speed governors were common in many Mack Trucks and Caterpillar applications and others where PTO (power takeoff; where theengine is used to drive auxiliary equipment) management was a consideration. From the driver perspective, the VS governor takes a little getting used to. Most of today's electronic management systems can be toggled to either LS or VS mode. A governor classified as variable speed will usually provide excess start-up fuel, define a torque rise profile, define droop curve, and be capable of no-fuelling the engine for shutdown. Isochronous Isochronous governing is only required when driving a generator in which application the engine must respond instantly to load changes with zero droop (no rpm fluctuation when engine load changes) or the (electrical) frequency will alter. However, the term is being used to describe an option in diesel engine electronic management systems. In this instance, isochronous governing mode would be used to manage PTO fuelling while stationary and one OEM uses the term to describe engine fuelling at an electronically managed, default (when critical input signals are lost) rpm.

Basic mechanical governor operation The mechanical governor assembly uses two main components: a set of engine-driven flyweights and a spring. The force of the spring is designed to move the fuel control linkage to an increased setting under all operating conditions. The centrifugal force generated by the engine-driven flyweights is designed to decrease the fuel control linkage setting under all operating conditions. When the engine is stopped, the force of the governor spring is therefore attempting to place the fuel control rack/s into a full-fuel position. On some engines, the governor is arranged so as to provide excess fuel for start-up purposes, whereas on some turbo-charged engine models, a mechanical adjustment device limits start up fuel to half-throttle to minimize exhaust smoke. The diesel engine draws only air in during the suction stroke. During the compression stroke this air is heated to such a high temperature that the diesel fuel injected into the engine toward the end of the compression stroke ignites of its own accord. The fuel is metered by the fuel injection pump and is injected under high pressure through the injection nozzle into the combustion chamber. Fuel injection must take place: • in an accurately metered quantity corresponding to the engine load, • at the correct instant in time in terms of engine rotational position, • for a precisely determined period of time, and • in a manner suited to the particular combustion process concerned. Maintenance of these conditions is the function of the fuel injection pump and the governor. The quantity of fuel injected into the engine during each plunger lift is approximately proportional to the torque of the engine. This fuel delivery is adjusted by turning of the pump plungers, each of which has an inclined helix machined into it. As the plungers are turned their effective stroke is varied. The plungers are turned by means of the control rod acting through either a set of gear teeth or some other transmission linkage. In a motor vehicle the control rod is connected to the accelerator pedal through the governor and a linkage. When the accelerator pedal is pressed down, the pedal travel is converted to corresponding control-rod travel. The governor operates dependent on the rotational speed of the engine, (mechanical governor), or on the intake manifold pressure (pneumatic governor). In both cases, the governor varies the amount of fuel injected into the engine and thereby regulates the engine speed. Functions of the Governor The basic function of every governor is to limit the high idle speed, i.e., it must ensure that the speed of the diesel engine does not exceed the maximum value specified by the manufacturer. Depending on the type of governor, further functions can be the maintenance of certain specified speeds, e.g. the idle speed, or speeds within a particular rotational speed range or the entire range between low idle speed and high idle speed. 1. Maximum-speed regulation. When an engine operating at maximum full-load speed has the load removed from it the governor must not permit the engine speed to rise higher than the maximum no load speed set by the manufacturer. The governor accomplishes this by drawing back the control rod in the shutoff direction. The range between maximum full load and maximum no load is designated as the maximum-speed regulation. The greater the speed droop, the greater is the increase in speed from max full load to max no load. 2. Intermediate-speed regulation. If required by the intended application of the governor (for example, in vehicles with an auxiliary drive), the governor can also maintain constant, within certain limits, various speeds between the idle and maximum speeds. Depending on the load therefore, the speed would only fluctuate between any full load speed and the corresponding no load speed based on the governor droop at the selected speed within the performance range of the engine.

3. Low-speed regulation. The governor in highway applications must also be capable of regulating low idle speed to keep the engine from stalling while cutting back on fuelling as the engine warms up. Mechanical governors are the oldest and most universally used diesel engine governor. This governor relies on centrifugal force and a control spring to regulate the movement of the fuel control mechanism. The governor is mounted at the rear of the injection pump housing. The flyweight assembly is mounted on the injection pump camshaft and turns whenever the camshaft turns. The governor is completely enclosed to permit splash lubrication of the working parts, using oil from the injection pump. An operating lever shuts off fuel delivery to the engine by moving the Control rack to the stop position. Although mechanical governors use flyweights to sense engine speed and to move the control rack, all governors operate on the same basic principle. They all have a means of sensing engine speed and controlling the fuel rack movement. Mechanical governor linkage movement under various engine speeds is described in more detail in the following paragraphs Starting the Engine With the engine stopped, the speed control lever is moved into the low idle position and then advanced slightly. The starting spring will then pull the control rack to the excess fuel position. At the same time the tensioning lever will move up against the full-load stop, which moves the guide lever, knuckle, and thrust sleeve forward. The flyweights then come to rest against the thrust sleeve (innermost position). While the starter is cranking the engine, the injection pump begins supplying excess fuel to the engine. Once the engine starts, the centrifugal force produced by the whirling flyweights overcomes the starting spring tension (even before idle speed is reached). Engine speed increases until the flyweight centrifugal force and the governor main spring are balanced. Engine Idling When the engine is idling, the governor functions automatically. At this speed the governor main spring is almost free of tension, and has only a slight effect on the governor linkage. This means that even at low speeds, the flyweights can swing outward with very little resistance. As the control lever and guide lever move, the fuel control rack also moves, increasing governor main spring tension. Since centrifugal force and spring tension are relatively low at idle speed, the torque capsule in the tensioning lever is only slightly compressed. Because the gap between the knuckle and the tensioning lever is greater at low speeds, the tensioning lever will contact the supplementary idling spring and will result in the desired speed regulation. Engine at Medium Speed Moving the speed lever above the idle position causes the control rack to move to the maximum fuel delivery position, and the tension lever to move to full-load stop. The injection pump delivers more fuel to the engines resulting in increased speed. As soon as the centrifugal force exceeds the governor main spring force (as determined by the speed control lever position), the governor linkage moves the control rack to a position where the centrifugal force is just equal to the spring force. In this way, the governor maintains a lower fuel delivery rate, which results in a steady engine speed Engine at Maximum Speed Governor operation at maximum speed is similar to medium speed operation, except that the tension lever stretches the governor main spring to its maximum length. The fully stretched main spring causes the tension lever to move against the full-load stop with greater force, and the control rack to move into the maximum fuel delivery position The torque lever is now compressed and will remain this way until engine speed is reduced enough to slow the flyweight centrifugal force. This moves the fuel control rack into a position where it will provide adequate torque reserve. Once full-load maximum speed id reached, governor response will regulate fuel delivery between full-load and high idle to handle any load as long as there is no overload.

Activating the engine shut-off lever moves the stop device, which then moves the control rack to shut off the fuel supply to the engine. This movement takes place independently of the flyweight and speed control lever positions. The stop device has a supporting lever that is Coupled to the shaft and shut-off lever by pressure springs. This lever continues to pivot until the control rack is in the stop or no fuel delivery position. At this point, the supporting lever stops moving, the pressure springs become tensioned, and the shut-off lever reaches the limit of its travel.

Governor Speed Droop Every engine has a torque characteristic curve corresponding to its maximum loading capacity. A certain maximum torque is associated with every speed. If the load on an engine is removed with no change in the position of the control lever, the engine speed may increase within the control range by only a certain permissible amount as determined by the engine manufacturer. The increase in speed is proportional to the change in load, i.e., the greater the reduction in load, the greater the increase in speed. Conversely of course, when the

engine is idling and a load is applied, the speed will decrease somewhat, hence the designation of this characteristic as "speed droop”. The speed droop of the governor is generally related to the maximum full-load speed and is related as a percentage of full load speed, it is typically 5 to 10% in mechanically controlled on-highway truck engines. Generally more stable behaviour of the entire control circuit (governor, engine, and driven machine or vehicle) can be attained by a fairly large speed droop. With the nominal speed set to a constant value, the actual speed varies within the speed-droop range as the load on the engine is changed (resulting, for example, from a change in the slope of the road). Hydraulic Governors Hydraulic governors are used on many marine, industrial, and power generator applications. Hydraulic governors regulate the fuel supply

indirectly through oil pressure. Pressurized oil from the engine's lubricating system is supplied to an auxiliary pump in the governor. The auxiliary pump then develops the oil pressure needed to actuate the governor mechanism. The oil pressure is maintained in the annular Space between the undercut portion of the pilot valve plunger and the bore in the ball head. At any given throttle setting, the force of the governor spring is opposed by the centrifugal force created by the revolving flyweights. When the two forces are equal, the land on the pilot valve plunger covers the lower opening in the ball head, producing a constant speed condition. If engine load increases and engine speed decreases, the weights are forced inward by the spring, allowing the pilot valve plunger to uncover the lower port in the ball head. Pressurized oil now enters the cavity at the lower end of the power piston and forces the piston and the floating lever upward this movement is transferred through the terminal lever to the fuel rod, and ultimately to the injectors.

Torque control.

The charts above depict various “torque control” strategies for diesel engines. Engines have a normal fuel requirement throughout their operating range. As the non turbo-charged engine accelerates past the point where it can ingest sufficient quantity of air to burn the injected fuel the governor must cut back fuelling or the engine will smoke. (Chart 12). Without torque control the engine would be over-fuelled. Positive torque control is shown in chart 13 as the control rod is drawn back slightly towards no fuel as the engine accelerates past 1500 rpm. The use of torque control allows us to take advantage of the diesels natural lugging ability as is shown in chart 14. Without torque control, in order to reach the appropriate fuelling level at rated speed, fuelling would have to be diminished through the torque curve of the engine to prevent smoking at rpms higher than peak torque rpm. With a turbo charged diesel engine the fuel requirement changes, see chart 15. The dotted line represents the fuel requirement of the engine. Because of manifold boost the fuel requirement to take advantage of the engines torque capabilities is much greater, (the apex of lines c1 and c2). Therefore the turbo-charged diesel must have negative torque control, (additional fuelling), to peak torque and then positive torque control, (reduced fuelling), from peak torque to rated speed. The de-accelerating, (because of applied load), diesel then has gradually increasing fuelling as it lugs down to peak torque. This is the torque rise profile of the engine and is controlled by the design of the governors’ weights and springs. As you can see from chart 15 the engine receives more fuel per cycle at peak torque. This occurs because at peak torque there is sufficient real time for combustion of this amount of fuel to be completed.

At rated speed this time window is reduced therefore fuelling must be reduced or incomplete combustion would be the result. The engine horsepower, (work X time), however, continues to climb from peak torque to rated speed, this is because of the increasing number of power strokes per second. Consequently the diesel engine produces its’ peak torque or turning effort, (pressure on the crank-shaft to turn), at a relatively low rpm because there is sufficient real time to burn this fuel load. It produces peak horsepower, (its’ ability to perform work), at rated speed because although the fuel load per cycle is less there are more cycles per second. The diesel turbocharger also plays a role in this. The turbocharger is set up to deliver an almost constant flow of air between peak torque and rated speed, therefore the amount of air charge ingested at peak torque is naturally larger because the valves are open for a longer real time window than at rated.

Pneumatic governors Pneumatic governors cannot be used on turbo-charged engines and therefore will not be found on any modern highway trucks. They use a modified air intake system, which uses a throttle plate to sense the amount of air being drawn into the diesel engine. This plate creates a venturi effect and creates a vacuum source, which is then used to control the movement of the fuel control rack or rod. The movement of the rack controls the rotational position of the pump plungers in their barrels and regulates the fuel delivery. Common governor terms Deadband: term used to describe the sensitivity of a governor. It is the speed window around set speed where no fuelling correction is made by the governor. Droop: transient (of short duration) speed variation from set speed when engine load changes. Droop curve: expressed as a percentage of high idle speed, the droop curve is the difference between high idle speed under load (rated speed) and high idle without a load. Governor cutoff: speed at which governor cuts off fuelling.

High idle: (WOT or top engine limit). The maximum no-load speed of an engine. Hunting: rhythmic change in engine speed often caused by unbalanced fuel delivery in multi-cylinder engines. Hydro-mechanical governing: refers to engines that are governed without the use of computers (ECMs). Idle: any no-load running speed of an engine but usually refers to low idle, the lowest speed the engine is designed to run at, usually with no input from the speed control mechanism. Overrun: the inability of a governor to keep the engine speed below the high idle speed when it is rapidly accelerated. Over-speed: any speed above high idle. Peak torque: rpm at which the engine develops peak torque, often located at the base of the torque rise profile. Rated speed: the rpm at which peak power is achieved from a diesel engine. Road speed governing; any governor system in which engine fuelling is moderated by a predetermined road speed value. Sensitivity: ability to respond to maintain a set rpm without rpm fluctuation as load changes. Speed drift: where engine speed rises above or below set speed often in surges. Differentiated from hunting by the fact it is not rhythmic. Stability: ability to maintain set rpm.