Intake (A): The descent of the piston produces a depression which sucks the air (fuel injected engine) - or mixture if carbureted - by the open intake valve. The gasoline is injected (common fuel injection engine operating in homogenous mode). The valve is closed again only when the piston is already traveling up because the fresh charge column, carried by its inertia, continues to flow into the cylinder.
Compression (B): The mixture is compressed by the upwards movement of the piston, the two valves being closed (except on fuel injected engines operating in stratified charge mode where the fuel is injected towards the end of the compression stroke). The temperature and the pressure at the end of compression reach respectively more than 400°C and 10 to 15 bar.
Combustion (C): The mixture is ignited by the spark plug. The combustion produces a strong rise in temperature and pressure. The flame front can propagates at more than 100 m/sec with a temperature of 2000°C or even 2500°C. The pressure usually reaches some 60 bars and forcefully pushes the piston back. The exhaust valve starts to open before the end of the expansion stroke to decrease the pressure in the cylinder and to facilitate the return of the piston.
Exhaust (D): The new upwards movement of the piston pushes the combustion gases in the port opened by the exhaust valve and expels them out of the cylinder. The pressure in the latter is just slightly over atmospheric. Towards the end of the exhaust stroke, the inlet valve starts to open, the exhaust one being closed again completely only after the beginning of the intake stroke. This overlap of the valves opening duration is useful because their full lift requires some delay. It also allows a better filling, particularly at high revs.
According to theoretical chemical considerations, the combustion of one gram of gasoline requires 14.7 grams of air (stoichiometric proportion). If the air/fuel ratio is lower than 14.7:1, the mixture is known as "rich"; the emissions of CO and unburnt residues are increased, the combustion chambers tend to get clogged and the oil on the cylinder walls can be washed off. Conversely, if the air/gasoline ratio is higher than 14.7:1, the mixture is known as "lean". Then the flame propagation speed is slowed down and the combustion might be prolonged during the entire exhaust stroke, causing abnormal thermal stress, particularly on the exhaust valves. The acceptable range lies between air/fuel ratios of 12:1 and 15:1.
Admission (A): The descent of the piston produces a depression which sucks the air by the open inlet valve (naturally aspirated engine). However, most of the diesels are turbocharged today and in this case the air charge is forced under pressure into the cylinder.
Compression (B): The air charge is compressed by the upswing of the piston, the two valves being closed. The temperature and the pressure at the end of the compression stroke reach values of 500 to 700°C under 50 to 60 bars because the compression ratio (ratio of cylinder + volume of combustion chamber at the BDC and TDC) of a diesel is much higher than that of a spark ignition engine. The injection of the fuel starts at the end of the compression and the fuel ignites spontaneously after a lag which one endeavors to reduce to the minimum. During this delay, the fuel continues to be injected and the more fuel in the chamber at the moment it finally ignites, the higher the rate of pressure increase. The ignition lag decreases with the temperature at the end of compression and this is why the Diesel engines run rough after cold starts.
Combustion (C): The injection still continues until a score of crankshaft degrees after TDC. The temperature rises to about 2000 °C. A pressure which may exceed 160 bars drives the piston towards the bottom dead center (BDC). The exhaust valve starts to open at the end of the expansion to decrease the pressure in the cylinder and to facilitate the return of the piston.
Exhaust (D): The new upwards movement of the piston pushes the combustion gases in the port opened by the exhaust valve and expels them out of the cylinder. The pressure in the latter is just slightly over atmospheric. Towards the end of the exhaust, the inlet valve starts to open, the exhaust one being closed again completely after the beginning of the intake stroke only. This overlap of the valves opening duration is useful because their full lift requires some delay and it allows also some scavenging of the combustion chamber to expel the residual exhaust gas, especially on turbocharged engines.
According to theoretical chemical considerations, the combustion of one gram of diesel fuel requires 14.4 grams of air. However, and in spite of improved injections techniques, the fuel micro-droplets cannot be pulverized sufficiently finely: a presumably spherical droplet of 9 microns in diameter still includes more than 70'000 billion molecules! One is thus constrained to adopt a combustion under an important excess air coefficient relatively to the theoretical value of 14.4:1, this in order to obtain acceptable fuel specific consumption and thermal stresses as well as low exhaust smoke emissions.
On a diesel, contrary to a spark ignited engine, the fresh charge of air remains constant whatever the load is. Only the volume of injected fuel is varied accordingly to the load. There is thus no intake throttle. The coefficient of excess air for combustion decreases with the load and the minimum acceptable value of this coefficient limits the mean effective pressure and the output torque. Turbocharging makes it possible to increase the mass of the fresh charge of air and to burn more fuel with an identical or even higher excess air coefficient. A pressure charged diesel can still have a sufficiently high compression ratio so that its thermodynamic efficiency hardly diminishes, contrary to the spark ignition engine which is subjected to the detonation and pre-ignition limits.