Phases of Diesel

Phases of Diesel

Fuel Combustion The most important phase of controlled combustion is when fuel is being injected into a burning mixture. This must

be at a rate that

Controlled combustion in cylinder

maintains an even combustion pressure onto the piston throughout the critical crankshaft rotational angles. This gives maximum torque and efficient fuel usage, because temperatures remain controlled and the heat lost to the exhaust is minimised. The low temperatures also help to keep nitrogen oxide emissions (NOx) to a minimum.

Flame Spread The speed of flame spread in a diesel engine is affected by the air charge temperature and the atomisation of the fuel. These characteristics are shared with the delay period. A sufficiently high air charge temperature, of at least 450°C, is a minimum requirement for optimum ignition and combustion.

Delay Phase The delay phase or ignition lag for diesel fuel combustion lasts a few milliseconds. It occurs immediately on injection as the fuel is heated up to the self-ignition temperature.

Ignition lag or delay

The length of the delay is dependent on the compressed air charge temperature and the grade of fuel. The air charge temperature is also affected by the intake air temperature and the engine temperature.

Diesel Knock A long delay period allows a high volume of fuel to be injected before ignition and flame spread occurs. In this situation diesel knock is at its most severe. When a diesel engine is cold, there may be insufficient heat in the air charge to bring the fuel up to the self-ignition temperature. When ignition is slow, heavy knocking occurs.

Cold Start Devices To aid starting and to reduce diesel knock, cold start devices are used. For indirect injection engines, starting at lower than

Glow plug

normal operating temperatures requires additional combustion chamber heating. For direct injection engines, cold start devices are only required in frosty weather.

Initial Delay An initial delay, known as injection lag, occurs in the high pressure fuel lines. This occurs between the start of the pressure rise

Pipes and

and the point when pressure is sufficient to

injectors

overcome the compression spring force in the injectors.

Diesel Fuel Injection Timing Ignition of the fuel occurs in the combustion chamber at the time of injection of fuel into the heated air charge. The

Ignition timing

injection point and the ignition timing are

mark

therefore, for all purposes, the same thing. Injection Timing Diesel engine injection timing is

equivalent to the ignition timing for petrol engines. Injection timing must fall within a

Diesel pump

narrow angle of crankshaft rotation. It is

being timed on

advanced and retarded for engine speed and load engine conditions. Injection timing is set by accurate positioning of the fuel injection pump. Incorrect timing leads to power loss. An increase in the production of nitrogen oxides (NOx) when too far advanced or an increase in the hydrocarbon (HC) emissions, when too far retarded also occurs.

Particulates Another exhaust gas constituent is particulate emissions. These result from incomplete combustion of the fuel. Particulates are seen as black carbon smoke in the exhaust under heavy load or when fuel delivery and/or timing is incorrect. White smoke may also be visible at other times, such as when the injection pump timing is incorrect. It also occurs when compression pressures are low or when coolant has leaked into the combustion chambers.

Direct and Indirect Injection Direct injection (DI) is made into a combustion chamber formed in the piston crown. Indirect injection (IDI) is made into a pre-combustion chamber in the cylinder head. Direct injection engines are generally more efficient but the indirect types are quieter in operation. The internal stresses in the engine are very high. Direct injection produces a higher detonation stress than indirect injection and therefore the smaller engines tended, until recently, to be the indirect type.

Electronic Control Recent developments in electronic diesel fuel injection control have made it possible to produce small direct injection engines. It is probable that all new

Common rail injection

designs of diesel engine will be of this type. Diesel engines are built to withstand the internal stresses, which are greater than other engines. Diesel engines are particularly suitable for turbocharging. This improves power and torque outputs.