Explain how the actual diesel cycle deviates from the air standard diesel cycle.
Answers
Answer:
Very good answers but unless you understand entropy, you won't understand a T-S diagram anyway. Therefore to put it into layman's terms for people who don't either already have, or are aspiring too, a degree in engineering, the difference is in the rate of combustion.
Petrol burns faster than diesel, so in a spark ignition (Otto) cycle, the fuel/air mixture, when ignited by the spark just before top dead centre in the stroke burns very quickly, so quickly that the piston is assumed not to move during the process - hence constant volume combustion.
In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air. As the piston continues to move closer to top dead centre, the mixture temperature reaches the fuel’s auto-ignition temperature, causing ignition of some premixed quantity of fuel and air. The balance of fuel that had not participated in premixed combustion is consumed in the rate-controlled combustion phase. The piston is moving down during the delayed combustion and therefore the pressure in the combustion chamber remains constant as the volume increases - hence constant pressure combustion.
Of course this is theory. In practice it is not possible to obtain perfect isentropic expansion or compression as heat is transferred to the cylinder walls and removed by engine coolant. A petrol engine does not have instant combustion so it cannot have pure constant volume combustion and the rate of combustion in a Diesel engine cannot always coincide with the descent of the piston to produce pure constant pressure combustion, especially in a variable speed engine.
On a slow speed engine it's possible to attach an engine indicator, which will produce a actual P-V diagram, which should be a good approximation of the pure cycle curve. Integrating the curve will give you an indication of power developed, hence the term Indicated Horsepower (IHP), another method is to drive a brake (or more likely a dynamometer these days); hence the more common term today BHP (Brake Horsepower). The difference between IHP and BHP is the internal engine losses.