So, let's describe our engine stages at combustion space between piston top and cylinder head.

From Bottom Dead Center (BDC), piston, full of fresh air, moves up cylinder liner until it covers up air intake port. The air intake process stops.

The piston moves further up. It then covers up exhaust port on cylinder liner. The exhaust process stops.

The piston moves further up. The air inside combustion space is compressed and becomes hot. The piston has nearly reached Top Dead Center at this point.

Highly atomized fuel is then injected into combustion space. The fuel burns rapidly causing an explosion inside combustion space. The explosion causes a tremendous rise in pressure and piston is pushed down towards BDC.

As piston moves down, exhaust ports uncover about 50 degrees from BDC. Exhaust gases are thus led out from cylinder. The pressure inside cylinder drops immediately.

The piston moves further down. At 35 degrees from BDC, it uncovers air intake port. Fresh air is then led into cylinder.

The piston then reaches BDC. Because of momentum created from force of explosion, piston reverses in direction and moves upwards towards Top Dead Center (TDC).

The process then repeats itself.

In some designs, exhaust gases are removed through an exhaust valve, located at cylinder head and very similar to 4-stroke engines. This type of scavenging is called uniflow scavenging. The timing of valve opening and closing will be controlled by a camshaft, push rods, rocker arms or other similar devices.

Well folks, why do we want to squeeze all strokes into 2 cycles? That could be discussed further...

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The fuel is supposed to burn so fast as to explode. So there is no point in injecting it more than it can burn. If fuel is not able to catch fire, it will remain in piston as unburnt fuel.

So timing of fuel injection will stop slightly after piston has reached top dead center.

The explosion inside piston is driving force for engine. It will create momentum to turn engine for next 2 cycles.

The next cycle is exhaust stroke. In this stroke, all products of combustion is driven off from piston top. The exhaust valve(s) will have opened to allow combustion gases to escape. In an ideal engine, these include carbon dioxide gas, water vapor and excess air.

However, in actual practice, combustion may not be so perfect. Carbon particles, unburnt fuel particles, lubricating oil particles, sulphur dioxide, carbon monoxide, and ash may also be discharged. As piston will not be able to reach the cylinder head perfectly, not all exhaust gas can be driven off completely. A small quantity of these exhaust products will remain inside cylinder even as next cycle starts.

Once exhaust gases have been driven off, next stroke starts. This is air intake stroke that we started off with. The cycles repeat itself as engine runs continuously.

Obviously more exhaust gas can be driven off, and more oxygen content cylinder can contain, more efficient combustion of fuel can be achieved.

This can be achieved by supercharging and cooling air intake.

Well folks, that should boost up your power!

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