4 stroke engine cycle is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle.

Compression-ignition (C.I.) engines burn fuel oil which is injected into the combustion chamber when the air charge is fully compressed. Burning occurs when the compression temperature of the air is high enough to
spontaneously ignite the finely atomised liquid fuel. In other words, burning is initiated by the self-generated heat of compression

Engines adopting this method of introducing and mixing the liquid fuel followed by self-ignition are also referred to as ‘oil engines’, due to the class of fuel burnt, or as ‘diesel engines’ after Rudolf Diesel, one of the many inventors and pioneers of the early C.I. engine. Note: in the United Kingdom fuel oil is known as ‘DERV’, which is the abbreviation of ‘diesel-engine road vehicle’.

Just like the 4 stroke engine cycle , the C.I. engine completes one cycle of events in two crankshaft revolutions or four piston strokes. The four phases of these strokes are (i) induction of fresh air, (ii) compression and heating of this air, (iii) injection of fuel and its burning and expansion, and (iv) expulsion of the products of combustion.

Induction stroke With the inlet valve open and the exhaust valve closed, the piston moves away from the cylinder head.

The outward movement of the piston will establish a depression in the cylinder, its magnitude depending on the ratio of the cross-sectional areas of the cylinder and the inlet port and on the speed at which the piston is moving. The pressure difference established between the inside and outside of the cylinder will induce air at atmospheric pressure to enter and fill up the cylinder. Unlike the petrol engine, which requires a charge of airand- petrol mixture to be drawn past a throttle valve, in the diesel-engine inlet system no restriction is necessary and only pure air is induced into the cylinder. A maximum depression of maybe 0.15 bar below atmospheric pressure will occur at about one-third of the distance along the piston’s outward stroke, while the overall average pressure in the cylinder might be 0.1 bar or even less.