Engine Cycles
Physics: AQA A Level, topic 11.2.4
Four-Stroke Petrol and Diesel Engine Cycles
Unsurprisingly, there are four stages in the four-stroke petrol engine cycle. The diesel engine cycle is very similar.
Stage 1 - intake
- Fuel sucked into cylinder as piston moves down.
Stage 2 - compression
- Fuel and air compressed by piston moving up.
Stage 3 - power
- Fuel ignited by spark and piston driven down.
Stage 4 - exhaust
- Waste gases expelled from cylinder by piston.
Diesel engine cycle
- The diesel engine cycle is similar to the petrol engine cycle but operates at higher pressures because of the need to ignite the fuel which is injected (at pressure) at the top of the compression stroke.
Power in Engines
There are many types of power and efficiencies to deal with when studying engines.
Input power
- Input power = calorific value of fuel × fuel flow rate
Indicated power
- Indicated power = (area of p−V loop) × (no. of cycles per second) × (no. of cylinders)
- This can sometimes be more easily calculated as:
- (Area of p−V loop) ÷ (time for one cycle)
Output or brake power
- Output or brake power:
- Torque × angular velocity
- P = Tω
Friction power
- Friction power = indicated power - brake power
Efficiencies in Engines
There are many types of power and efficiencies to deal with when studying engines. It is important to study them to be able to compare engines.
Overall efficiency
- Overall efficiency = brake power ÷ input power
Thermal efficiency
- Thermal efficiency = indicated power ÷ input power
Mechanical efficiency
- Mechanical efficiency = brake power ÷ indicated power
Test yourself on Engine Cycles
Four-Stroke Petrol Engine
Which of these statements are true about four-stroke petrol engines?