Mack Announces New Thermodyne Engine
To be revealed for the first time at the International Fire Chiefs Convention and Show at New Orleans, this month, is the first of a new series of engines produced by the Mack Manufacturing Corporation, for its line of Fire Apparatus. These engines are of an entirely new design which has been in process of development in Mack laboratories for several years, the name Thermodyne having been adopted to reflect the thermodynamic advances achieved in its combustion system.
Combined in the new engine are high output, together with exceptionally heavy duty, durability and reliability. The result is high power and torque per cubic inch, with high output sustained over a wide range of speed.
No departure has been made from the conventional four-stroke cycle, nor from the established carburetor and spark ignition principle. These engines are of the same six-cylinder layout to which Mack has adhered for many years. The complete series will range from 145 to 180 horsepower. Their novelty lies in the arrangement of the combustion chamber, valve and porting whereby the maximum turbulence is combined with extremely even rate of combustion and smooth pressure rise, together with high volumetric efficiency, most favorable spark plug position and clean combustion and scavenging.
Structurally the engine is novel so far as fire apparatus is concerned, in that the crankcase and cylinders are cast in a single block. This practice is fairly common in small engines, but has heretofore presented such problems to the foundry that it has not previously been attempted on engines of this size.
Full advantage of this construction has been taken in imparting extraordinary rigidity to the entire engine, together with the utmost compactness, while full length water jackets have been used to effectively assist in cooling the crankcase oil.
The crankshaft has seven main bearings 3 1/2 inches in diameter and 3-inch pins, and like previous Mack crankshafts, is fully counterbalanced by twelve counterweights forged integral with the shaft.
The combustion chambers employed are considerably offset with reference to the bores so that a large part of the bore is covered by the flat deck of the cylinder head with minimum clearance above the top of the pistons, and the bulk of the combustion chamber volume in the form of a D-shaped domed cavity at one side and above the bore. Both valves open into this cavity and the spark plug is located at a focal point so that the flame propagation is both rapid and even throughout the volume. Furthermore, this combustion chamber shape insures high turbulence. The surface exposed to the flame is at a minimum and cooling is at a maximum.
Controlled temperature cooling continues to be a feature of Mack engines. The thermostatic control being on the same cold-circulation principle used in the past. In addition, on fire pumpers, an auxiliary intercooler is incorporated in the radiator, whereby water from the fire pump is by-passed through a copper coil located in the bottom tank, so that the auxiliary cooling is accomplished without mixing of water, without prejudice to anti-freeze solution.