The Piston Helicopter Engine
The Piston Engine
Piston engines are the most common type of engine to be found in modern, light helicopters. Normally there will be 4 or 6 cylinders in a horizontally opposed configuration.
Pistons move back and fourth inside the cylinders. Inside each cylinder, fuel is mixed with air and ignited. The energy produced by the combustion of the fuel air mixture causes the gases to expand and drive the piston down into the cylinder.
The piston is connected by a connecting rod "con-rod" to a drive-shaft which is forced to turn due to the movement of the piston.
Piston engines may operate on either a two stroke cycle or a four stroke engine cycle.
The Four Stroke Engine Cycle
A complete cycle of the four stroke engine comprises four strokes of the piston moving within the cylinder. This cycle is also known as the "Otto cycle" after its inventor in 1876.
The four strokes are:
Combustion (or Expansion) (or Power)
During induction, the fuel and air mixture is sucked into the cylinder through an open intake valve (on the right) as the piston moves from the top of the cylinder to the bottom of the cylinder.
The exhaust valve (on the left) is closed.
Early in the compression stroke, the inlet valve closes and the fuel/air mixture is trapped in the cylinder. The piston then moves back up to the top of the cylinder.
This compresses the mixture and causes the temperature and pressure of the fuel/air mixture to rise.
As the piston reaches the top of the cylinder and completes its compression stroke, the fuel/air mixture is ignited by a spark from the spark plug. This causes combustion which causes the gases in the cylinder to expand.
As the piston has passed the top of its stroke, the expanding gases force it back down the cylinder.
This is called the power stroke as the heat energy provided by the combustion process is now converted into mechanical energy.
Just before the piston reaches the bottom of its stroke, the exhaust valve will open.
As the piston returns to the top of the cylinder again, the burned gases are forced out of the cylinder and into the atmosphere through the exhaust manifold.
As the piston nears the top of its stroke and while the last of the burned gases are being expelled, the inlet valve opens in preparation for the next induction stroke.
In one complete Otto cycle, only one of the four strokes provides power - but the crank-shaft has rotated two times.
Engine manufacturers increase the power of the engine by adding more cylinders. This has the added bonus of making the engine run smoother. Each cylinder will have the power stroke occurring at different positions during the rotation of the crankshaft to try to even out the power impulses.
This is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (bottom dead centre BDC) compared to the top volume of the cylinder when the piston is at the top of its stroke (top dead centre TDC).
The compression ratio is designed to suit the type of fuel used. If the compression ratio is too high, the fuel may ignite early and excessive wear will occur.
Both the inlet valves and the outlet valves must open and close at the correct times in relation to the movement of the piston. The timing of the valve operation is controlled by a camshaft.
The camshaft only rotates at half the speed of the crankshaft. The camshaft operates rocker arms and pushrods that push to relevant valve open. When the camshaft releases the pressure, a spring returns the valve to the closed position.
A typical helicopter piston engine speed in flight is 2700 revolutions per minute (RPM). Each inlet valve and exhaust valve opens once during the four strokes of the Otto cycle. I.e. once in every two revolutions of the crankshaft.
This means that at 2700 RPM, each valve will open and close 1350 times per minute = 22 times per second. This is a very short time to get the fuel/air mixture into the cylinder and exhaust the burnt gases again.
To increase the efficiency of the fuel/air mixture induction, the inlet valve opens before the piston reaches top dead centre (TDC). This allows maximum time to induce the fuel/air mixture into the cylinder. It is referred to as valve lead.
Similarly the exhaust valve opens before the piston reaches bottom dead centre (BDC) on the power stroke.
It is worth noting that for a very short time at the start of the induction stroke, the exhaust gases are still exiting through the open exhaust valve while the fuel/air mixture is being forced into the cylinder through the open inlet valve. This period of overlap when both the inlet valve and the exhaust valve are open at the same time is known as valve overlap.
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