In engines of this type admission of air is effected by ‘air piston’ controlled inlet ports, and rejection of products of combustion by ‘exhaust piston’ controlled exhaust ports. The motion of the two sets of pistons is controlled by either two crankshafts connected through gearing, or by a single crankshaft with the ‘top’ bank of pistons transmitting their motion to the single crankshaft through a crosshead-siderod mechanism. By suitable offsetting of the cranks controlling the air and exhaust pistons asymmetrical timing can be achieved.
It is evident that this system displays the same favourable characteristics as the exhaust valve in head system, but at the expense of even greater mechanical complications. Its outstanding advantage is the high specific output per cylinder associated with two pistons. However, the system is now retained only in large low speed marine, and smaller medium speed stationary and marine engines. In high speed form it is still employed for naval purposes such as in some fast patrol vessels and mine searchers, although its use in road vehicles and locomotives is discontinued.
