Cylinder Pressure in a

Cylinder Pressure in a

Petrol Engine The combustion process creates energy within the cylinder in the form of heat from the burning fuel/air mixture. Due to the enclosed nature of the

Petrol engine figures – Diesel figures are

approximately double

cylinder, this heat energy creates a pressure rise in the cylinder above the piston. This pressure, applied over the piston area, in turn, creates a force pushing down on the piston and turning the crankshaft via the connecting rod, thus producing torque at the crankshaft. The pressure in the cylinder is a shown plotted against cylinder volume in the diagram. This is known as an indicator diagram.

Cylinder Pressure in a Diesel Engine The torque at the crankshaft is a function of the cylinder pressure and crankshaft angle; the maximum torque is produced when the connecting rod and crankshaft main/big-end bearings are at right angles (i.e. 90 degrees crank rotation from TDC position) Note that at TDC, any pressure on the piston produces no work as there is no turning moment (torque), just a force pushing down on the bearings.

Ideal Combustion The ignition and fuel settings of an engine are set by the manufacturer at the optimum position to achieve the best compromise of performance, economy and

minimal exhaust emissions. With respect to combustion, it is important that the maximum cylinder pressure and energy release occur at the correct angle. Damage to the engine can occur if this happens to early or late in the engine cycle. An example is early ignition; this causes engine ‘knock’ and damages the piston if allowed to occur for any significant period of time.

Pinking This is a characteristic noise caused by pre-ignition or early ignition of the fuel/air mixture. Early ignition causes an early pressure rise that is applied to the piston at TDC. At this

crank angle, no engine torque can be produced and this means that all the combustion energy is applied directly to the engine mechanical components (piston crown, bearings etc.), causing

them to generate the ‘pinking’ noise. Although the noise is quite subtle, the forces are massive and cause considerable damage to the engine.

Advanced Ignition When ‘pinking’ occurs, the combustion energy precipitates through the engine components causing damage. In addition, heat is generated that is not dissipated normally and this causes excessive temperature of engine components

(pistons, valves and valve seats for example) and consequent heat related damage.

Retarded Ignition Over retarded ignition causes incorrect timing of the energy release from the fuel that, in turn, means less energy to do work and therefore more energy to dissipate via the cylinder boundaries. This causes an increase in engine temperatures, damages components and reduces overall engine efficiency. This excess energy also has to be rejected via the exhaust and this causes increased exhaust gas temperatures that can damage exhaust valves and seats, as well as exhaust gas components (catalytic converter).

Diesel Combustion Combustion in a diesel engine begins very rapidly as the fuel is being injected into the combustion chamber and heated. This causes a rapid energy release that generates the

characteristic ‘diesel’ engine noise. For this reason,

a simple diesel engine is noisier than the equivalent petrol engine. The combustion process is most rapid in a direct injection diesel engine and due to this, combustion losses are minimal and these are the most fuel efficient type of internal combustion engine seen in road vehicles.