Thermodynamics - Engine Cycles Flashcards
What is the system in a four stoke engine
The gas inside the cyclinder
What makes a cyclic process useful
Work done by the system is greater than the work done on the system
What is a stroke
Single movement of piston, up or down
What is a 4 stroke engine
Burns fuel every 4 piston strokes
How many revolutions is 4 piston strokes
2 revolutions
What are the 4 strokes of a piston in a 4 stroke petrol engine
1 - induction
2 - compression
3 - expansion
4 - exhaust
Describe the induction stroke (petrol)
- Piston moves down from top of cylinder
- volume of cylinder, gas and fuel increases
- air is drawn in through inlet valve
- pressure of gas in cylinder remains constant, just under atm
Describe the compression stroke (petrol)
- inlet valve is closed
- piston moves up and does work on the gas
- this increases pressure and decreases volume
- just before piston finishes stroke, spark plug produces spark which ignites gas mixture
- pressure and temperature increase rapidly at an almost constant volume
- this is an adiabatic change
Describe the change is P, V and T in the compression stroke
Before spark: P increases, V decreases
After spark: T and P increase, V stays almost constant
Describe the expansion stroke (petrol)
- gas mixture expands and does work on the piston, moving it downwards
- higher temperature means more work is done as gas expands than to compress it, so there’s a net output if work
- just before end of stroke, exhaust valve opens and pressure reduces to atm
Describe the exhaust stroke (petrol)
- piston moves up cylinder
- burnt gas forces out exhaust valve
- pressure remains almost constant, just above atm
Describe the induction stroke (diesel)
- only air is pulled into cylinder
Describe the compression stroke (diesel)
- air is compressed to a very high temperature to ignite diesel fuel
- just before stroke ends, diesel is sprayed into cylinder through fuel injector and ignites
What strokes are the same for a petrol and diesel engine
Expansion and exhaust stroke
How is the diesel indicator diagram different to the petrol one
There is no sharp leak at the start of the expansion stroke
Why is petrol gas mixture ignited just before end of compression stroke
- so there’s enough time for ignition to occur before end of stroke
- so maximum amount of pressure is output on piston
What are pressure-volume diagrams for engines
Indicator diagrams
What can indicator diagrams be used for
Calculating output power and efficiency of engine
What are the two types of indicator diagrams
Theoretical and actual
What 4 assumptions are made for theoretic indicator diagrams
1 - same gas is taken continuously. Gas is pure air wirh adiabatic constant 1.4
2 - pressure and temperature changes are instantaneous
3 - engine is frictionless
4 - heat source is external
What is the adiabatic constant for pure air in theoretical diagram
1.4
Describe the 4 processes in petrol theoretical diagram
A-B: gas is compressed adiabatically
B-C: heat is supplies, volume is kept constant
C-D: gas expands adiabatically so cools
D-A: system is cooled at constant volume
Describe the 4 processes in diesel theoretical diagram
A-B: gas is compressed adiabatically
B-C: Heat is supplied, pressure is kept constant
C-D: gas expands adiabatically and cools
D-A: system is cooled at a constant volume
Describe the compression stroke in a diesel engine
Adiabatic compression
Work done on the air so temperature increases
