The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected. This is in contrast to igniting the fuel-air mixture with a spark plug as in the Otto cycle (four-stroke/petrol) engine. Diesel engines are used in aircraft, automobiles, power generation, diesel-electric locomotives, and both surface ships and submarines.

The Diesel cycle is assumed to have constant pressure during the initial part of the combustion phase (V₃ to V₂ in the diagram, below). This is an idealized mathematical model: real physical diesels do have an increase in pressure during this period, but it is less pronounced than in the Otto cycle. In contrast, the idealized Otto cycle of a gasoline engine approximates a constant volume process during that phase.

The image shows a p-V diagram for the ideal Diesel cycle; where p is pressure and V the volume or v the specific volume if the process is placed on a unit mass basis. The idealized Diesel cycle assumes an ideal gas and ignores combustion chemistry, exhaust- and recharge procedures and simply follows four distinct processes:

1→2 : isentropic compression of the fluid (blue)

2→3 : reversible constant pressure heating (red)

3→4 : isentropic expansion (yellow)

4→1 : reversible constant volume cooling (green)

The Diesel engine is a heat engine: it converts heat into work. During the bottom isentropic processes (blue), energy is transferred into the system in the form of work W_in, but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant pressure (red, isobaric) process, energy enters the system as heat Q_in. During the top isentropic processes (yellow), energy is transferred out of the system in the form of W_out, but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant volume (green, isochoric) process, some of energy flows out of the system as heat through the right depressurizing process Q_out. The work that leaves the system is equal to the work that enters the system plus the difference between the heat added to the system and the heat that leaves the system; in other words, net gain of work is equal to the difference between the heat added to the system and the heat that leaves the system.

Work in is done by the piston compressing the air (system)

Heat in is done by the combustion of the fuel

Work out is done by the working fluid expanding and pushing a piston (this produces usable work)

Heat out is done by venting the air

Net work produced = Q_in - Q_out

The net work produced is also represented by the area enclosed by the cycle on the P-V diagram. The net work is produced per cycle and is also called the useful work, as it can be turned to other useful types of energy and propel a vehicle (kinetic energy) or produce electrical energy. The summation of many such cycles per unit of time is called the developed power. The W_outis also called the gross work, some of which is used in the next cycle of the engine to compress the next charge of air.

 Efficiency = (net work output / heat supplied)

In a Compression Ignition engine, for the given compression ratio what will happen to the thermal efficiency, if net work output is increased????