Engine characteristicsT Alrayyes

Geometrical properties for reciprocating engines

• Typical values of these parameters are rc = 8 to 12 for SI engines and rc = 12 to 24 for CI engines;

• R = 3 to 4 for small- and medium-size engines, increasing to 5 to 9 for large slow-speed.CI engines.

Cylinder geometry

Piston speed

• The piston velocity is zero at the beginning of the stroke, reaches a maximum near the middle of the stroke, and decreases to zero at the end of the stroke.

• where N is the rotational speed of the crankshaft.

• The instantaneous piston velocity Sp is obtained from-

Piston speed

• Average piston speed for all engines will normally be in the range of 5 to 15 m/sec

• Engine operate in this range for the following reasons.

1. this is about the safe limit which can be tolerated be material strength of the engine components (Piston accelerate then decelerate)

2. because of the gas flow into and out of the cylinders. Piston speed determines the instantaneous flow rate of air-fuel into the cylinder during intake and exhaust flow out of the cylinder during the exhaust stroke. Higher piston speeds would require larger valves to allow for higher flow rates. In most engines, valves are at the maximum size with no room for enlargement.

Ratio of Bore to stroke

• Square Engine are when B=L• Most modern automobile engines are near square, with some slightly over square and some

slightly under square

• Under Square: if the stroke length is longer than the bore diameter• These produce strong torque at low to mid range rpm's

• longer stroke usually means greater friction, a weaker crankshaft and a smaller bore means smaller valves which restricts gaseous exchange.

• In general, a longer stroke leads to higher thermal efficiency through faster burning and lower overall chamber heat loss. (less surface area in the combustion chamber)

• Over square: stroke length is less than bore diameter• This decreases friction losses but increases heat transfer losses.

• wears less, and can be run at a higher speed. power does not suffer, but low-end torque does.

• Very large Engines are always under square, with stroke length is 4 times the bore diameter

Compression ratio

• Compression ratio are limited by engine materials,technology, and fuel quality.

• Compression ratio has an effect on the combustionquality

• Engines with superchargers or turbochargers usually have lower compression ratios than naturally aspirated engines.

• The figure how values of rc increased over time to the 8-11 range used on modern spark ignition engine

• This limit of 8 to 11 is imposed mainly by gasoline fuel properties

Variable compression ratio

• Variable compression ratio is technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase fuel efficiency while under varying loads.

• Higher loads require lower ratios to be more efficient and vice versa.

• Variable compression engines allow for the volume above the piston at 'Top dead centre' to be changed

Work• The work is the output of any heat engine. The work is generated

by the combustion chamber of the cylinder.

• The work is a result of the gas acting through a cylinder.

• Because engines are often muticylinder, it is convenient to analyse engine cycles per unit mass of gas within the cylinder with a specific volume v and specific work w

Work• The work that is generated by the gas is called indicated work.

• Work delivered by the crankshaft is less than indicated work due to mechanical friction and parasitic loads of the engine.

• Parasitic loads include the oil pump, supercharger, air conditioner compressor, alternator, etc

• Gross indicated work per cycle wgross Work delivered to the piston over the compression and expansion strokes only (Area A and Area C).

• Pumping work wpump is the work transfer between the piston and the cylinder gases during the inlet and exhaust strokes (Area B + Area C)

• Net indicated work per cycle wnet Work delivered to the piston over the entire four-stroke cycle

• Wpump is negative for naturally aspirated engines

• Wpump is positive for engines with turbochargers or superchargers

WOT, naturally aspirated Engine

Part Throttle, naturally aspirated Engine

WOT, Supercharged or Turbocharged engine

Brake work and mechanical efficiency

• The mechanical efficiency is ratio of brake work at the crankshaft to indicated work at the combustion chamber

Mean effective pressure

• The pressure is continuously changing in an engine during a cycle.

• The average mean effective pressure is defining as:

• Mean effective is a good parameters to compare engines for design and output because it is independent of engine size and or speed• If the torque is used: big engines will always look better

• If power is used: speed becomes very important.

• Various mean effective pressures can be defined by using different work terms

• Brake mean effective pressure:

• Indicated mean effective pressure

• Indicated mean effective pressure can be divided into gross and net:

• Pump mean effective pressure (which can have a negative value):

• The following equation relate some of the previous definition:

• Friction mean effective pressure:

Torque

• Crankshaft output Torque is related to work by

Power• Power is defined as the rate of work of the engine

• If n = number of revolutions per cycle, and N = engine speed, then:

Torque and Power• Torque are function of Engine speed

• At low speed, torque increases as engine speed increases

• At certain speed the torque will reach its maximum, Maximum brake torque (MBT)

• The torque after that start to decrease. This because at higher speed there is not enough time to draw a full charge of air.

• Maximum torque happened usually between 3000 to 3500 rpm

• Power is also function of speed• Power increases as engine speed increase until really high speeds (6000 to

7000 rpm) . Power start to decrease after.

• The reason for the decrease in power is the high friction as speed increases

• Many modern engines have a maximum torque in the 200 to 300 Nm range at engine speed usually around 4000 to 6000 rpm

• The point of maximum torque is usually called Maximum torque speed MBT

DYNAMOMETERS

• Dynamometers are used to measure torque and power over the engine operating ranges of speed and load.

• They do this by using various methods to absorb the energy output of the engine, all of which eventually ends up as heat.

• Fluid or hydraulic dynamometers: absorb engine energy in water or oil pumped through orifices or dissipated with viscous losses in a rotor-stator

• Eddy current dynamometers use a disk, driven by the engine being tested, rotating in a magnetic field of controlled strength. The rotating disk acts as an electrical conductor cutting the lines of magnetic flux and producing eddy currents in the disk.

• Electric dynamometer

• absorbs energy with electrical output from a connected generator. In addition to having an accurate way of measuring the energy absorbed, the load is easily varied by changing the amount of resistance in the circuit connected to the generator output.

• Many electric dynamometers can also be operated in reverse, with the generator used as a motor to drive (or motor) an unfired engine.

• This allows the engine to be tested for mechanical friction losses and air pumping losses, quantities that are hard to measure on a running fired engine

Air to Fuel and fuel to air ratio

• Energy input to the engine comes from the combustions of a hydrocarbon fuel

• Air is used to supply the oxygen needed for the chemical reaction

• Air to fuel ratio (AF) and fuel to air ratio (FA) are used to describe the mixture ratio

Air to Fuel and fuel to air ratio

• The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct.

• The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct.

• the stoichiometric AFR for many gasoline-type fuels is very close to 15:1

• Combustion is possible between 12 to 18. AF less than 6 is too rich to sustain combustion and AF greater than 19 is too lean

• AF input change depending on the operating conditions at the time (accelerating, cruising, starting….etc)

• CI engines have AF input in the range of 18 to 70, this is outside the limits where combustion is possible.

• Combustions occur in CI engine, unlike SI engine, has a very non-homogeneous air-fuel mixture, with reaction occurs only in those regions where combustible mixture exist, other region either are too lean or too rich

• Equivalence ratio is defined as following:

• Lambda ratio:

𝝀 =𝟏

𝝓

Specific fuel consumption

• Brake Specific fuel consumption

• Specific fuel consumption

• Indicated Specific fuel consumption

Specific fuel consumption

• Other examples specific fuel consumption parameters can be defined:

Fuel consumption

• There are many factors affecting brake specific fuel consumption

• Speed is one of the major factors. Initially as speed increase, this will reduce BSFC until a point then it will start to increase again• At low speed a longer time per cycle allow for a great heat loss

• For higher speed friction start to dominate

• Compression ratio is another factor that effect BSFC. Decrease with the increase in compression ratio due to higher thermal efficiency.

• Equivalence ratio: BSFC is at its lowest at equivalence ratio= 1

Engine Efficiencies

• Combustion efficiency 𝜼𝒄 the Fraction of fuel that is burns: typically between 95 to 98%

• Heat input to the engine can be defined as

• Thermal Efficiency 𝜼𝒕:

Thermal efficiency

• Thermal efficiency can be given as indicated or brake whetherindicated power or brake power is used.

• Mechanical efficiency can be given by:

• Engine has an indicated thermal efficiency between 50 to 60%

• Engine has a brake thermal efficiency of 30%

Thermal efficiency

• Fuel conversion efficiency

Volumetric Efficiency

• The maximum amount of air that can get into the cylinder will govern the performance and the power in the cylinder

• The maximum amount of air entering the cylinder usually is less then the amount of air expected to fill the displacement volume:• Short cycle time available

• Flow restriction

• Intake manifold

• Intake valves

Volumetric Efficiency• Volumetric efficiency can be presented by

• Typical value of volumetric efficiency for an Engine at WOT is in the range of 75% and 90%

Emissions

• The four main engines exhaust emissions that must be controlled are • Nox

• Carbon monoxide

• HC

• PM (Solid particulate)

• There are two methods used to present the amount of pollutants • Specific emissions

• Emission Index

Specific Emissions

Emissions Index

2. Indicated work

1. Brake work

3. Brake mean effective pressure

5. Friction Mean Effective Pressure

4. Indicated Mean Effective Pressure

6. Power lost to friction

7. Brake work per unit mass of gas in the cylinder