Diesel cycle

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The Diesel cycle is the thermodynamic cycle which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897. It is assumed to have constant pressure during the first part of the "combustion" phase (V2 to V3 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. The idealized Otto cycle of a gasoline engine approximates constant volume during that phase, generating more of a spike in a p-V diagram.


The Idealized Diesel Cycle

The image on the left shows a p-V diagram for the ideal Diesel cycle; where p is pressure and v is specific volume. The ideal Diesel cycle follows the following four distinct processes (The color references refer to the color of the line on the diagram.):

  • Process 1 to 2 is isentropic compression (blue)
  • Process 2 to 3 is reversible constant pressure heating (red)
  • Process 3 to 4 is isentropic expansion (yellow)
  • Process 4 to 1 is reversible constant volume cooling (green)[1]

The Diesel is a heat engine: it converts heat into work. The isentropic processes are impermeable to heat: heat flows into the loop through the left expanding isobaric process and some of it flows back out through the right depressurizing process, and the heat that remains does the work.

  • Work in (Win) is done by the piston compressing the working fluid
  • Heat in (Qin) is done by the combustion of the fuel
  • Work out (Wout) is done by the working fluid expanding on to the piston (this produces usable torque)
  • Heat out (Qout) is done by venting the air

Maximum thermal efficiency

The maximum thermal efficiency of a Diesel cycle is dependent on the compression ratio and the cut-off ratio. It has the following formula under cold air standard analysis:

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