This series shows a cut-away diagrams of a Two-Stroke Petrol engine. This engine design is the simpler mechanically of two and four stroke as it minimises the number of moving parts which must be kept in sync. The description "two stroke" comes from the fact that the engine fires (burns fuel) on every upward stroke (travel of the piston from bottom of the cylinder to the top), thus there are two strokes for every ignition of fuel, and upward and a downward stroke. The first stroke moves from bottom to top, where compressed air and fuel ignite and begin the second stroke where the piston is forced back downwards by the explosive force of the fuel igniting.

petrol-2-stroke-bdc

STROKE 1A: PISTON AT BDC

In the diagram the piston is at BDC (Bottom Dead Centre - meaning it is at the lowest point of travel within the cylinder). The air, petrol and oil mixture within the crankcase is forced into the cylinder and exhaust gases are driven out.

petrol-2-stroke-instroke

STROKE 1B: THE INSTROKE

The piston moves upwards and covers the air intake and exhaust ports closing them.

The charge of fresh air is compressed to about 5:1 (20%) of its original volume. The act of compressing the air heats it tremendously.

This happens on every upward stroke of the piston.

petrol-2-stroke-tdc

STROKE 2A: PISTON AT TDC

Just as the piston reaches the uppermost portion of it's travel at Top Dead Centre (TDC) the spark plug is fired causing combustion of the compressed mixture within the cylinder. At the same time petrol and oil vapour is being drawn into the crank-case in preparation for the next stroke.

petrol-2-stroke-near-bdc

STROKE 2B: PISTON NEARING BDC

During the downward stroke the exhaust port opens, and the cylinder is swept clean of burnt fuel by fresh air from the inlet port. The petrol, oil and air charge within the crank-case is compressed during this stroke in preparation for the next stroke.

This entire cycle is repeated for every revolution of the crank shaft.

Thanks to Dr Gary Zimmer who pointed out the compression ratio was way out for the engines described here. I appreciate it when people who discover errors on the pages point them out so they can be corrected!