5.5 DEVIATIONS.

The thermal efficiency deviates from the thermodynamic values because of several reasons which are as follows:

  1. Variation of air-fuel ratio and mixture distribution.
  2. Heat losses associated with
    1. Heat addition to the air or air-fuel mixture in intake system.
    2. Heat transfer in the engine.
    3. Variations of specific heats with temperature.
    4. Dissociation of combustion products.
    5. Incomplete combustion.
    6. Incomplete mixing of fuel and air.
  3. Time losses arising from.
    1. Piston motion in the time required for the combustion.
    2. Spark timing.
  4. Gas exchange processes involving.
    1. Valve operation and timing.
    2. Throttling.
    3. Supercharging.
    4. Exhaust back pressure.
    5. Fluid friction.
    6. Leakage.
    7. Exhaust loss due to opening of exhaust valve before BDC.

The factors that modify the ideal diesel cycle is similar in nature; excepting that fuel is injected directly into combustion chamber. Time loss: Time loss increases pumping work and reduces work in expansion by the motion of piston during combustion. The losses can also be classified as:

Exhaust loss: To reduce pumping work during the exhaust, the exhaust pressure is relieved by the opening of the exhaust valve before BDC so that much of the combustion products self scavenge. However there is friction at the valve and exhaust manifold. The loss associated in overcoming these friction is the cycle loss due to premature valve opening and is the exhaust loss.