The behaviour of fuel as it is injected into a diesel engine cylinder is critical to the functioning and performance of the diesel engine.

Diesel Combustion Process Explanation (Ricardo)

When fuel is injected into a diesel engine cylinder the combustion process can be viewed as four rapid, successive processes. The overall timeframe is measured in thousandths of a second.

Pressure curve in the cylinder during the four phases.png

The pressure curve in the cylinder during the four phases

(i) Ignition Delay

Points 1 to 2 on the curve show the compression curve. Fuel injection commences at point 2 on the curve but the fuel does not ignite immediately. The droplets pass through the hot dense air charge, vapourising as they are exposed to the surrounding high temperature air. Ignition occurs at point 3 when the fuel vapour reaches its auto ignition temperature.

(ii) Uncontrolled Combustion

When ignition occurs all the fuel already injected combusts leading to a rapid rise in pressure and temperature shown from point 3 to 4 on the curve. If ignition delay is excessive a large quantity of fuel will ignite rapidly during this phase, leading to excessive rise in pressure

(iii) Partially Controlled Combustion

The fuel continues to be injected from point 4 to point 5. The combustion process is dependent on the quantity and rate of injection.

(iv) After Burning

Injection ceases at point 5 and the remainder of the fuel burns. Combustion should be completed before exhaust occurs.

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Further Reading

Wright, A.A. & BP Shipping Limited 2000, Exhaust emissions from combustion machinery, Institute of Marine Engineers, London. (p 6-15)
Kuiken, K. 2017, Diesel engines for ship propulsion and power plants from 0 to 100,000 kW, 3rd revised edition, Target Global Energy Training, Onnen, The Netherlands. (p 192-195)
Inside the Cylinder of a Diesel Engine – by Harry Ricardo