Once piston moves away from combustion space, pressure drops. The next stage of operation depends on design of engine. These can be either 2-stroke or 4-stroke designs.

Regardless of type of design, spent exhaust gas is first driven out, and then new fresh air is drawn back into combustion chamber.

After this, rotating crankshaft drives piston to compress fresh air inside combustion chamber. The piston acts as a reciprocating compressor at this stage.

The compression of air causes latter to become hot - hot enough to ignite finely distributed fuel particles.

At this moment, fuel is sprayed in at high pressure. The tiny sprayed fuel particles form a mist inside combustion chamber.

What do think will happen when you have heat, fuel and oxygen? A fire! Each tiny particle of fuel burns rapidly, and an explosion occurs.

The cycle starts again, and crankshaft turns continuously, pistons move continuously, and engine runs.

How does engine know when to spray fuel, let in air, compress air, and exhaust spent combustion product?

Well folks, start your engines.

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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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