Desirability of a Large-Bore Engine 290008

COMPARISONS are made of the respective characteristics of the large-bore short-stroke engine and the small-bore long-stroke engine in connection with the argument of the author that the former engine best fulfills the requirement that an engine must be a good product that is easily produced. He chooses the L-head type of engine for purposes of illustration, since this type is within the scope of the experience of all automotive engineers.

When consideration is being given the specifications of a new engine, the first problem to be met is the determination of length. Usually a certain length is set arbitrarily, but this circumscribes the designer at the outset and, for some unaccountable reason, a new project is thus compromised rather than to change the preconceived idea of what the length of wheel-base must be. But there is abundant evidence in the industry as to what happens when a designer starts to “crowd,” and the author asserts that, if engines are to represent something more than so much cast iron equipped with plumbing, sufficient space for the engine-at least enough to permit proper consideration for each function-must be allowed. He states also that the large-bore short-stroke engine demands elbow room for itself and its parts, and then discusses how large the cylinder bore should be.

After considering the subjects of valve cooling and engine stability, cylinder-block construction is analyzed and a method of construction is described whereby the block is cast on end. By this method each half of the block is a separate casting but a simple liquid assembly of the two is made by fastening together the two molds in their respective flasks before pouring.

The advantages and disadvantages of the two types of crankshaft are discussed, as is also the proper distribution of crankshaft material to secure correct balancing; and the effects of counterweights are analyzed.

In conclusion, the author says that the large-bore design offers a good engine that has a maximum performance over the longest period, and that can be produced most easily and at lower cost. It has also better cylinder-blocks, better crankshafts for less cost, and presents a greater opportunity for further development than does the small-bore long-stroke engine.

It is stated by one discusser that, to get water close to the exhaust-valve seat, valve ports are often crowded so that it becomes necessary to make a D-shaped port, and that this is not as satisfactory with regard to the seat itself remaining perfectly round under operating conditions as is the concentric type of port. Other critical analyses, with diagrams, are submitted in relation to bore-stroke considerations, one conclusion reached being that an increase in bore-stroke ratio for engines of the same power increases the bearing pressures. Crankshaft stiffness and weight considerations, as well as the advantages of the short-stroke engine, are treated also.

Engine smoothness and factors that limit engine length are discussed, and the relative detonation tendencies of the small-bore and the large-bore engine are considered.