Fuel-air cycle and actual cycle losses

Air-standard assumptions

• The working fluid is air only, treated as a perfect gas with constant specific heats.
• Combustion is replaced by heat addition from an outside source, and exhaust by heat rejection.
• All processes are reversible; compression and expansion are isentropic.
• No heat loss, no leakage, no valve flow loss.

Fuel-air cycle

The fuel-air cycle uses the real composition of the charge (air + fuel + residual gas) and the following:

• Specific heats of the gases increase with temperature ($c_v$ rises, so $\gamma$ falls).
• Dissociation: at high temperature CO$_2$ breaks into CO and O$_2$, and H$_2$O into H$_2$ and O$_2$. This absorbs heat, so the peak temperature falls.
• The number of moles changes during combustion.
• Fuel quantity affects the heat released (rich mixtures cause incomplete combustion).

Because of these, the peak temperature and pressure of the fuel-air cycle are lower than those of the air-standard cycle, and the efficiency is about 80-85% of it.

Composition of cylinder gases

Burnt gases contain CO$_2$, H$_2$O, N$_2$ and excess O$_2$ (lean mixture), or CO, H$_2$ and unburnt hydrocarbons (rich mixture), plus residual gas from the previous cycle.

Losses in the actual cycle

1. Time loss (finite combustion time): combustion is not at constant volume, so peak pressure is lower and falls after TDC.
2. Heat loss to cylinder walls and head during compression, combustion and expansion.
3. Exhaust blowdown loss: the exhaust valve opens before BDC, so some expansion work is lost.
4. Pumping loss in the induction and exhaust strokes.
5. Incomplete combustion, leakage past rings and friction.

The actual indicated efficiency is only about 75-85% of the fuel-air cycle efficiency.