REASONS for designing an engine with 12 cylinders for fire apparatus, motor-trucks and motor-coaches are set forth by the author. Among them were the requirement for 225 hp., a speed range of 200 to 3000 r.p.m. with little torsional vibration and torque-reaction effect, and economy of space. The design adopted has its cylinders in two rows of six each, disposed at an included angle of 30 deg. The statement is made that it can be installed in the space occupied by a 150-hp. six-cylinder engine.
Advantages claimed for setting the cylinders at this angle are that the engine can be made narrow, so that all cylinders and the crankcase can be cast in one block; that the accessories can conveniently be placed outside; and that the synchronism of impulses that causes torsional vibration can be avoided.
Vertical valves are operated from a central over-head camshaft by rocker arms that carry rollers at one end and are split horizontally at the other end. A set-screw is provided to adjust the split portion for valve-stem clearance without itself coming into contact with the stem. The camshaft is driven by a double-faced silent chain running over one idler and the water-pump sprocket, the adjustment being designed so that it will not change the timing of the camshaft. Fuel-pumps are operated by cams at the front end of the camshaft.
Factors of safety in the engine are said to be higher than in other heavy-duty automotive engines. The first engine of this design was tested for 4000 hr. with only two valve grindings; negligible wear on the working parts, including the chain; and the replacement of only one small part, the design of which has since been revised.