Engine cycles (A-level only) Flashcards – AQA A-level Physics (7408)

What are the four strokes of a petrol engine cycle, in order?

1. Intake
2. Compression
3. Power
4. Exhaust

What enters the cylinder during the intake stroke of a petrol engine?

An air–fuel mixture.

What happens to the air–fuel mixture during the compression stroke?

It is compressed and ignited by a spark near the end of the stroke.

What drives the power stroke in a petrol engine?

Combustion raises the pressure, and the expanding gases drive the piston.

What leaves the cylinder during the exhaust stroke?

Combustion products.

What is the idealised cycle for a petrol engine called?

The Otto cycle.

Which two processes are modelled as adiabatic in the Otto cycle?

Compression and expansion.

How are heat addition and heat rejection modelled on an Otto cycle pressure–volume diagram?

Both occur at constant volume.

What does ideal heat addition in the Otto cycle approximate?

The rapid combustion of the compressed air–fuel mixture.

How does ignition in a diesel engine differ from ignition in a spark-ignition petrol engine?

In a diesel engine, fuel ignites when injected into hot compressed air; a spark-ignition petrol engine uses a spark.

How does combustion produce the power stroke in a diesel engine?

Combustion raises gas pressure, and the expanding gases drive the power stroke.

What are the four strokes of a diesel cycle, in order?

1. Intake
2. Compression
3. Power
4. Exhaust

How are compression and expansion modeled in the ideal diesel cycle?

Both are adiabatic processes.

Under what condition is heat added in the ideal diesel cycle?

At constant pressure.

Under what condition is heat rejected in the ideal diesel cycle?

At constant volume.

What does constant-pressure heat addition approximate in the ideal diesel cycle?

Combustion as fuel is supplied. It does not mean pressure stays exactly constant during combustion in a real engine.

What does an engine indicator diagram plot over a cycle?

Cylinder pressure $p$ against cylinder volume $V$.

How do compression and expansion paths differ in their volume direction on an engine indicator diagram?

They move in opposite volume directions.

What does the net area enclosed by an engine indicator diagram represent?

The net work done by the gas per cycle, not necessarily the useful shaft work.

Besides compression and expansion, which processes can contribute work to an actual engine indicator diagram?

Intake and exhaust.

How do theoretical and measured engine indicator diagrams differ?

A theoretical diagram predicts behaviour using idealised processes; a measured diagram records actual cylinder pressure against volume.

What two effects can cause a measured engine indicator diagram to differ from a theoretical one?

Combustion takes time and heat is transferred to the surroundings.

For identical cylinders, how is indicated power calculated from an indicator diagram?

$P_{\mathrm{indicated}}=(\text{area of }p\text{–}V\text{ loop})(\text{cycles per second per cylinder})(\text{number of cylinders})$

How many crankshaft revolutions correspond to one cycle per cylinder in a four-stroke engine?

Two revolutions.