Engines

Almost all cars and light aircraft currently use what is called a four-strokecombustion cycle to convert petrol (Chemical Energy) into motion (Kinetic Energy).

The four-stroke cycle was invented in 1867 and is comprised of:

1. INTAKE STROKE:a. The stroke commences with the piston moving downward.b. The inlet-valve opens carrying the petrol mixture with it.c. The exhaust-valve closed.

2. COMPRESSION STROKE:a. The stroke commences with the piston moving back up to

compress the gases into the space above the piston.b. The inlet-and exhaust -valve closes.c. The compression of the fuel produces an immense increase in the

efficiency of an engine contributing to power output.

3. COMBUSTION STROKE:a. The stroke starts with the passing of the spark.b. Both valves remained closed.c. The gas in the combustion chamber being ignited by the spark

burns rapidly.d. The heat generated causes the gases to exert a high pressure on

the piston that forces it to descend to its original position.

a

b c

a

bb

Piston

c

Engines

4. EXHAUST STROKE:a. The exhaust stroke begins with the opening of the exhaust valve.b. The ascending piston forces the burnt or exhausts gases out of the

cylinder through the exhaust port.c. This stroke, which ends with the start of the Intake stroke.

Newton’s Third Law:For every action there is an equal and opposite reaction.

b b

a

d

Spark plug

a

b

b

EnginesTurbine engines work in a similar concept to a four stroke engine:

EnginesGas turbine engines have three parts:

1. Compressor:a. Compresses the incoming air to high pressure.

2. Combustion area:a. Burns the fuel and produces high-pressure, high-velocity

gas.3. Turbine:

a. Extracts the energy from the high- pressure, high- velocity gasflowing from the combustion chamber

1. Gas turbine engines have a great power- to-weight ratio compared to pistonengines. That is, the amount of power you get out of the engine comparedto the weight of the engine itself is very good.

2. Gas turbine engines are smaller than their counterparts of the same power.3. Gas turbines operate at high temperatures, making designing and

manufacturing gas turbines a tough problem from both the engineering andmaterials standpoint.

4. Gas turbine engines are also more expensive compared to a reciprocatingengine of the same size and tend to use more fuel when they are idle.

1. Turbine engines have a higher compressibility of air and hence betterperformance. This is especially true at high altitude.

2. Turbine engines have different variations to produce the thrust, forexample turbo- jet, turbo-fan and turbo-prop.

3. Piston engines can only rely on the propellers to produce the thrust.

Compressor CombustionArea

Turbine

Engines1. Turbo-jet engines:

a. Consist of a compressor, combustion and turbine section.b. They function by accelerating a relatively small amount of air to very

high speeds to produce the reactive force required to move theaircraft.

2. Turbo-fan engines:a. They are turbo- jet engines with a large fan in front.b. The final turbine stage therefore drives a shaft that makes its way

back to the front of the engine to power the fan.c. All modern jet aircraft have Turbo-fan engines. All of Cathay’s aircraft have

different versions of the Turbo-fan engines.

3. Turbo-prop engines: Turbo-prop engines are similar to turbo-fan butinstead have a conventional propeller at the front of the engine.