What is an opposed-piston engine and how are the cranks arranged? What advantages and disadvantages do these engines have?

An opposed-piston engine has two pistons working in the same cylinder, which is much longer than normal. The cranks are arranged so that movement of the pistons towards each other takes place at the same time, as does movement away from each other. The opposed-piston engine always works on the two- stroke cycle with the uniflow method of scavenging. The combustion chamber is formed in the space between the heads of the pistons and the small exposed section or belt of the cylinder left between the pistons. The fuel injection valves, air starting valve, cylinder pressure relief valve and pressure-indicating cock are fitted to the cylinder in way of the belt left between the two pistons when they are at their inner-dead-centre position.
Opposed-piston engines may have two crankshafts, one at the top of the engine for the upper pistons and one in the conventional place {or the lower pistons. Engines with two crankshafts are arranged as trunk-piston engines for both upper and lower pistons. The two crankshafts are connected through a train of gears.
Another form of opposed-piston engine has one crankshaft. For each cylinder there are three cranks: the centre crank is connected to the lower piston through a connecting-rod and crosshead, and the two outside cranks, which are in the same line and opposite to the centre crank, are connected to the upper piston through connecting-rods, crossheads and tie or side rods. Movement of the pistons uncovers and covers the exhaust ports which are in the top of the cylinder and the scavenge ports which are at the bottom of the cylinder.
A third variation of the opposed-piston engine uses eccentrics for the upper piston instead of the two side cranks.
The advantage of the opposed-piston engine over other types of engine is that no firing loads are transmitted from the cylinders to the bedplates holding the crankshaft bearings. In consequence of this they may be constructed to lighter scantlings and therefore have a good power to weight ratio. Another advantage
is that a high degree of balance may be more easily achieved with opposed- piston engines than with’ conventional types.
Their disadvantage is the amount of headroom they require in comparison with other engines of equivalent power and rotational speed.

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