Engine cycle

Engine cycle
Idealised P/V diagram for two stroke Otto cycle

Two-stroke

Main article: Two-stroke cycle

This cycle has one power stroke for every two strokes of the piston (up-down) and exhaust of the exhaust gases and charging of the cylinder happens at much the same time.

The steps involved here are:

1. Intake and exhaust stroke: Exhaust is released and air and vaporized fuel are drawn in.
2. Compression stroke: Fuel vapor and air are compressed and ignited.
3. power stroke: piston is pushed downwards by the hot exhaust gases.

Four-stroke

Main article: Four-stroke cycle

Idealised Pressure/volume diagram of the Otto cycle showing combustion heat input Qp and waste exhaust output Qo, the power stroke is the top curved line, the bottom is the compression stroke

Engines based on the four-stroke ("Otto cycle") have one power stroke for every four strokes (up-down-up-down) and employ spark plug ignition. Combustion occurs rapidly, and during combustion the volume varies little ("constant volume").[6] They are used in cars, larger boats, some motorcycles, and many light aircraft. They are generally quieter, more efficient, and larger than their two-stroke counterparts.

The steps involved here are:

1. Intake stroke: Air and vaporized fuel are drawn in.
2. Compression stroke: Fuel vapor and air are compressed and ignited.
3. Combustion stroke: Fuel combusts and piston is pushed downwards.
4. Exhaust stroke: Exhaust is driven out. During the 1st, 2nd, and 4th stroke the piston is relying on power and the momentum generated by the other pistons. In that case, a four cylinder engine would be less powerful than a six or eight cylinder engine.

There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. The diesel cycle is somewhat different.

Diesel cycle

Main article: diesel cycle

P-v Diagram for the Ideal Diesel cycle. The cycle follows the numbers 1-4 in clockwise direction.

Most truck and automotive diesel engines use a cycle reminiscent of a four-stroke cycle, but with a compression heating ignition system, rather than needing a separate ignition system. This variation is called the diesel cycle. In the diesel cycle, diesel fuel is injected directly into the cylinder so that combustion occurs at constant pressure, as the piston moves, rather than with the four stroke with the piston essentially stationary.

Five-stroke

Engines based on the five-stroke cycle are a variant of the four-stroke cycle. Normally, the four cycles are intake, compression, combustion, and exhaust. The fifth cycle, which was added by Delautour,[7] is refrigeration. Engines running on a five-stroke cycle are claimed to be up to 30% more efficient than equivalent four-stroke engines.

Six-stroke

The six stroke engine captures the wasted heat from the four-stroke Otto cycle and creates steam, which simultaneously cools the engine while providing a free power stroke. This removes the need for a cooling system making the engine lighter while giving 40% increased efficiency over the Otto Cycle.