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Two stroke petrol engines

Two stroke engines have been used in a limited number of classic cars, but are nearly exclusive to smaller engine uses, such as motorcycles and hand held machinery.

They work on one powered stroke for every two strokes, rather than every four strokes as in the four-stroke engine described above. In essence, there is only a compression stroke and an expansion/combustion stroke. Therefore, this requires a slightly different approach for both intake of the fuel-air mixture and the expulsion of the exhaust gasses.

The following is the most common approach to a two-stroke operation:

  1. Stroke one: As the piston moves into the cylinder, the air fuel mix is added, allowing the mixture to be fully compressed at the end of the stroke. At this point the ignition takes place, starting the second/return stroke.
  2. Stroke Two: as the piston is driven back by the ignition, creating the power, the exhaust is expelled near the end of the stroke via a side valve.

Two-stroke engines, although powerful for their size, are less efficient than four stroke engines, partly due to the inlet and exhaust processes having to take part during the stroke, meaning the full stroke is not optimised. This can make two stroke engine thirstier and more polluting than four-stroke engines.

One other key difference between two stroke and four stroke engines is lubrication. Since the operation of the two-stroke uses the crankcase as part of the channel to pump the air-fuel mix into the cylinder, it cannot be used to carry oil, like in its four-stroke counterpart. To accommodate for this, oil would be added to the fuel.

Outside of the obvious emissions increase this also created some issues for early two-stroke cars and motorcycles. When the throttle was open, the engine was always receiving oil, but if off the throttle, such as long down-hill sections, the engine could be starved of essential lubrication.

Engine History for cars and motorcycles


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