Piston Engines
Piston Engine Basics
An aviation piston engine is similar in many respects to the engine found in your car. The engine produces power by burning air and fuel under pressure in an enclosed cylinder, the expansion of the fuel air mixture as it burns causes a piston to move down the cylinder turning the crankshaft and producing a rotary motion, some of this rotary motion is used to drive the other pistons when they are not producing power and some is used to turn the propeller of an aeroplane or rotor system of a helicopter.
The 4 stroke piston engine operates on the principle of 'The Otto Cycle', there are four phases to the Otto Cycle these are
- Induction
- This is where the fuel and air mixture is drawn into the cylinder.
- Compression
- This is where the mixture is put under pressure.
- Power
- This is where the mixture is ignited and work is done on the piston forcing it down the cylinder.
- Exhaust
- This is where the burnt mixture is expelled from the cylinder.
The fuel/air mixture is allowed into and out of the engine by a system of valves, these valves are opened and closed at the correct points in the cycle (described below).
Four Stroke Piston Engine Operations
The induction stroke is used to draw in a fuel and air mixture into the combustion chamber (or cylinder). The mixture is drawn into the cylinder throught the inlet valve which is open during the induction stroke. During the induction stroke the piston travels down the cylinder increasing the volume and decreasing the pressure inside. This decrease in pressure allows the mixture to flow into the empty cylinder.
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The compression stroke allows the fuel/air charge to be compressed ready for burning. During the compression stroke both the exhaust and inlet valves are closed.
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The power stroke occurs as a result of an electric charge being provided to the spark plug(s) in the cylinder, this cause the mixture to ignite and as it burns it expands increasing the pressure in the piston forcing it down the cylinder. The linear motion of the cylinder is converted into rotary motion by the cranskhaft. The crankshaft in turn drives the other pistons around and is used to drive the flywheel and ultimately the load (propellor or rotor gearbox).
Example Content
The exhaust stroke allows the burnt gasses to exit the cylinder. The exhaust valve is open during the exhaust stroke and as the piston rises up the cylinder it pushes the spent air/fuel charge out of the cylinder. The flow of exhaust gases out of the cylinder creates a lower than atmospheric pressure in the cylinder allowing the next fuel and air charge to begin being drawn.
Example Content
Textron Lycoming Service Instruction 1094D recommends a maximum temperature change of 50°F per minute to avoid shock cooling of the cylinders.
Example Content
Web References
Lycoming
Carburettors
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