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What Are Opposed Piston Engines?

written by: Raunekk • edited by: Lamar Stonecypher • updated: 1/10/2010

An engine with a special type of arrangement, opposed piston engines are supposed to be known as the most stable engines in the market today.

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    Opposed Piston Engine is a type of diesel engine which has two pistons working in the same cylinder. Technically, opposed piston engine is just a variation in the design of conventional engine. Each of the cylinders of the engine has two pistons, one at each end. The main advantage of opposed piston arrangement over others is that they have a higher power to weight ratio.


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    As mentioned earlier, in an opposed piston engine, there are two pistons at both the ends of the cylinder. The cylinders of opposed piston engine are generally longer in size than those of the conventional engines. The arrangement of cranks is also such that both the pistons move towards and away from each other simultaneously. Moreover, the system works on a two stroke cycle and a uniform method of scavenging. In opposed piston engine the combustion chamber is the space left between the two pistons when both are at inner dead centre positions. It is this place between the pistons wherein the fuel injection valve, air starting valve, pressure relief valve and indicating cocks are fixed.

    Most of the opposed piston engines have two crankshafts, one for the upper piston and other for the lower one. Both the crankshafts are arranged as trunk piston engines and through a series of connected gears. However, the earliest opposed piston engines used to have just one crankshaft in their design. Such arrangement would have three cranks, one at the center which is attached to the lower piston with connecting rod and cross-head. The other two cranks are arranged on the same line as that of the center crank and are connected with the top piston with connecting rods, tie rods and crossheads. The exhaust and scavenge ports at the top and bottom of the cylinder, operates because of the reciprocating motion of the piston. Other equipments such as supercharger, air box etc are attached similar to any conventional diesel engine.

    opposed 2  

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    The air fuel mixture is pushed into the space in between the pistons. The ignition of the mixture pushes both the pistons downwards, leading to power stroke. The ignition is usually provided using a spark plug. As both the pistons move downwards, one of the pistons opens the outlet valve, which pushes the gas out of the exhaust, whereas the other piston opens the inlet valve, pushing in the fresh gas mixture. The compression stroke then takes place and the cycle repeats itself.


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    Advantage - Better Power to Weight Ratio

    The main advantage of opposed piston engine is that unlike conventional engines, where the stresses generated due to firing loads are transferred from the cylinders to the bedplates of the engine, no stresses are transferred and thus it have an excellent power to weight ratio. Moreover, the arrangement of opposed piston engines provides a higher degree of balance than the conventional engine.

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