DOXFORD-J Type Engine

Dual & Opposed Piston Engine

A single acting, opposed piston engine of this type which was the last opposed piston model produced by the company. Various cylinder sizes up to 760 mm were marketed; it had a combined stroke of 2180 mm and a speed of 119 rpm. Engines were built with form 3 to 9 cylinders.

Two pistons are fitted per cylinder. The lower or main piston is oil-cooled and has a long shirt fitted; it uncovers the scavenge ports at the bottom of its stroke. It is connected via normal cross-head and connecting rod to the centre crank for the unit. The upper or exhaust piston is inverted in the top of the liner. It also has a skirt fitted, is water cooled and uncovers the exhaust ports at the top of its stroke. This piston is connected by a beam to two side-rods with cross-heads and is connected with each of the two outer cranks of the unit. Two pistons moving in opposite directions give good primary balance. Because all the gas load is transmitted directly to the crankshaft, no tie bolts are required. The bed-plate is also of lighter construction and is manufactured of fabricated steel plates.

The crankshaft is of built-up construction. It has three cranks per unit. The causes bending and high stress. The cylinder liner is in three parts bolted together, the centre combustion chamber being of cast steel. This contains all the valve pockets.

The fuel system works on a common rail; the fuel pumps, which are operated by a chain driven crankshaft, maintain the system under pressure. Two injections are fitted per cylinder, and are controlled by a cam-operated mechanical timing valve. The engine operates with through scavenge and pulse turbocharging