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P-V diagrams & Engines

Gases undergo changes that will eventually cause them to return to the original state. An ideal gas undergoing these changes has the properties shown below:

  • Isovolumetric changes between a & b and c & d
  • Isobaric changes between b & c and d & a

P-V Diagrams & Engines

Thermal Efficiency: net work output ÷ heat input

Actual efficiency of the engine will be lower than the value of thermal efficiency alone, due to frictional losses within the engine. The efficiency of a car = approx. 30%

Petrol Engine: Otto Cycle

Otto CycleFour Stroke Engine

Diesel Engine:

  • Higher thermal efficiency that petrol engines
  • Heavier than petrol engines
  • More noise and incomplete combustion (pollution)

Both Engines: power output: area of p-V loop x no cylinders x no cycles per sec

maximum energy input: fuel calorific value x fuel flow rate