Automotive engine calculations and cycles

The internal combustion engine that powers most road vehicles is analysed using thermodynamic cycles, which idealise the sequence of pressure and volume changes that occur inside a cylinder. The most common road engine follows the four-stroke sequence: intake, compression, power and exhaust. During intake the piston draws in an air–fuel mixture (or air alone in a diesel); compression raises its pressure and temperature; combustion drives the piston down on the power stroke; and the exhaust stroke expels the burnt gases. One full cycle takes two revolutions of the crankshaft, and a rotating camshaft opens and closes the valves in time with the piston.

Two idealised cycles describe the dominant engine types. The Otto cycle models the spark-ignition petrol engine, in which combustion is treated as happening at almost constant volume after the mixture is ignited by a spark. The Diesel cycle models the compression-ignition engine, in which air is compressed strongly enough that injected fuel ignites without a spark, with combustion modelled as occurring at roughly constant pressure. The efficiency of the Otto cycle rises with the compression ratio, which is one reason diesel engines, running at higher compression, tend to be more fuel-efficient.

The Carnot cycle sets an upper limit on the efficiency any heat engine can reach between two temperatures, and no real engine matches it. Automotive calculations use these models, together with figures for fuel energy content, to estimate work output, fuel consumption and efficiency, while accepting that friction, heat loss and incomplete combustion make real engines fall short of the ideal.

Frequently asked questions

What are the four strokes of a four-stroke engine?

Intake, compression, power (combustion) and exhaust. The sequence takes two crankshaft revolutions to complete one cycle.

How does the Diesel cycle differ from the Otto cycle?

The Otto cycle uses a spark to ignite a petrol–air mixture and models combustion at constant volume; the Diesel cycle compresses air until injected fuel self-ignites and models combustion at constant pressure.

Why is the Carnot cycle important?

It defines the maximum possible efficiency of any heat engine operating between two temperatures, providing a theoretical benchmark that real engines cannot exceed.