The Two-stroke Engine: Crankcase Compression Type

  • G. P. Blair


It is assumed that the reader of this chapter has a basic understanding of the principles of operation of the two-stroke cycle engine and of the many mechanical variations which can occur on this simplest of all engine themes. It cannot be assumed that the reader, unless he is actively involved in the design, development or research of two-stroke engines, will be aware of the many problems which beset the progress of this type of power unit. For example, as the working cylinder has a compression and combustion process taking place within it upon every crankshaft revolution, it follows that some 50% of the cycle will be occupied by that firing process. It follows equally that, as combustion produces exhaust gas from the original air and fuel, the remaining half of the cycle must be used to remove the exhaust gases and replace them in the working cylinder by fresh air and, either simultaneously or eventually, fuel. As the working cylinder pressure varies from about one to many tens of atmospheres during this action, clearly the cylinder filling and emptying process is very dynamic and is conducted in an unsteady manner. Rather than use my own words at this point, I will quote the quite classic words of Hopkinson1 given in 1914:

‘The performance of 2-cycle internal combustion engines is determined very largely by the efficiency or otherwise of the process of charging. In the course of less than one quarter of a revolution the products of combustion resulting from the previous explosion have to be replaced as far as possible by the fresh charge of air, or of gas and air, which is blown in through the inlet valves and drives before it through the open exhaust ports the exhaust gases. Inevitably some mixing occurs and some of the fresh charge or of the scavenging air passes away to the exhaust and is wasted. On the amount of this waste depends very largely the performance of the engine. Its economy suffers to the extent of the waste of fuel.’


Specific Fuel Consumption Exhaust Pipe Brake Specific Fuel Consumption Scavenge Efficiency Crank Angle 
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Copyright information

© Elsevier Science Publishers Ltd 1990

Authors and Affiliations

  • G. P. Blair
    • 1
  1. 1.The Queen’s University of BelfastBelfastNorthern Ireland, UK

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