Diesel cycle

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 (

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 idealized Diesel cycle assumes an ideal gas and ignores combustion chemistry, exhaust- and recharge procedures and simply follows four distinct processes: 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

During the constant pressure (red, isobaric) process, energy enters the system as heat

During the top isentropic processes (yellow), energy is transferred out of the system in the form of

The net work produced is also represented by the area enclosed by the cycle on the p–V diagram.

is also called the gross work, some of which is used in the next cycle of the engine to compress the next charge of air.

The actual thermal efficiency will be significantly lower due to heat and friction losses.

The Otto cycle by comparison has both the heat addition and rejection at constant volume.

Comparing the two formulae it can be seen that for a given compression ratio (r), the ideal Otto cycle will be more efficient.

However, a real diesel engine will be more efficient overall since it will have the ability to operate at higher compression ratios.

Furthermore, the ideal Otto cycle formula stated above does not include throttling losses, which do not apply to diesel engines.

These advantages also make the diesel engine ideal for use in the heavy-haul railroad and earthmoving environments.

"Diesel" model airplane engines have variable compression ratios.

Some 19th-century or earlier experimental engines used external flames, exposed by valves, for ignition, but this becomes less attractive with increasing compression.

(It was the research of Nicolas Léonard Sadi Carnot that established the thermodynamic value of compression.)