Firepower efficiency: Why Chrysler went with hemispherical heads

In this 1904 Belgian Pipe Co. engine, a twin camshaft layout was adapted to pushrod actuation of overhead valves. Air passages were tortuous, due to sharp bends, but the central spark plug allowed short, uniform flame travel, providing quick, even burning of fuel.

In recent years, there has been so much conversation about “high-compression” engines that some people have been led to believe they are a recent invention. Actually, high-compression engines are not new. All Chrysler Corporation cars have had high-compression engines since the first 1924 Chrysler, and most people will recall the famous 1928 Chrysler Red Head engine with its then unheard of compression ratio of 6.2:1. The rest of the industry followed this lead, and compression ratios have steadily increased since that time until reaching their present maximums of about 7.5:1. Recently, many people have been asking how much higher compression ratios may be expected to go in the future.

Compression ratio is a measure of how much the fuel-air mixture in the cylinder is [compressed before being] ignited by the spark plug. A compression ratio of 7.0:1, for example, means that the mixture is compressed to one-seventh of its original voume before being ignited.

Engine designers are constantly trying to increase compression ratios because this is one of the most direct ways of increasing the efficiency and power of an engine. Unfortunately, the compression ratio can be raised only so far before a point is reached at which, with available gasolines, the fuel mixture in the cylinder head begins to explode wastefully instead of burning smoothly. This is the phenomenon-familiar to every motorist-known as detonation, or knock; and it not only reduces efficiency and power but is also very hard on the engine.

delage hemispherical head engine

The heart of an engine is the combustion chamber — the compartment in the cylinder head where the fuel mixture is burned. For a number of years at Chrysler Corporation, steady improvement has been made in the design of the combustion chamber. These improvements, individually small, have, with the steady increase in octane value of the fuel, added up to a large increase in engine power and economy. But we became convinced that much more could be made available by a major improvement in combustion chamber design. The engineering research which resulted from this recognition led to the adoption of one of the most radical advances in an American production automobile engine in decades—the hemispherical combustion chamber—in the 180 bhp Chrysler FirePower engine introduced last January.

1951 Hemi Firepower

The hemispherical combustion chamber, shaped like the inside of a dome, has long been recognized by automotive engineers as the ideal for internal combustion engines. It has been used extensively in aircraft and racing car engines, where cost is [not as important], but never before have automakers been able to work out a satisfactory engine design using this combustion chamber and the overhead lateral inclined valve arrangement for a volume-produced American passenger car.

Left: Using four different valve and combustion chamber designs in a single-cylinder test engine, the L-head design required a compression ratio fo 10:1 at 1200, 8.1:1 at 3600 rpm, to equal the efficiency of the FirePower combustion chamber at 7:1. F-head and regular overhead valve setups could not match hemispherical layout's efficiency.
Right: FirePower excels, when all use a compression ratio of 7:1. The full-throttle octane requirement was almost identical for each.

During the development of this engine, every type of combustion chamber shape was investigated, but . Throughout all of this test work, the hemispherical combustion chamber consistently developed the highest efficiency of all the many designs tested. In other words, this chamber was able to put to work more of the heat energy available in the fuel than could any other production passenger car engine in America. This may be seen in the graph where the thermal efficiencies of four different chamber designs, operating at the same compression ratio, are compared.

The practical importance of such differences in thermal efficiency may be more apparent if these differences are considered in terms of compression ratio, illustrated by the other graph. Here are shown the compression ratios to which the L-head, F-head, and conventional overhead designs would have to be increased in order to have thermal efficiencies equal to that of the hemispherical chamber at compression ratio of 7:1. The less efficient designs will require higher ratios ranging up to 10.5:1 and will consequently demand much higher octane number fuels.

bristol hemi diagram

Equally important to the high performance of the FirePower engine is the exceptional breathing capacity of this hemispherical chamber design. The cross-section of a FirePower cylinder shows the many features that are conducive to high volumetric efficiency, or breathing. The valves are not crowded together, nor are they surrounded closely by the combustion chamber walls. Both ports are ideally streamlined with a minimum of directional change. The complete separation of the ports, together with the wide space between the valve seats, assures that the incoming charge picks up a minimum of heat from the hot exhaust. In addition, the flow within the cylinder is not restricted by any barriers or tortuous passages.

ardun heads

The hemispherical chamber has another remarkable characteristic. At 7.5:1 compression ratio, it does not require the high-cost, premium grades of fuel which must be used in other conventional OHV V-8 engines at the same compression ratio. In other words, the hemispherical chamber does not require such a high octane rating of the fuel.

The modern Lea-Francis engine uses pushrods and twin camshafts to avoid the complications of placing camshafts in the head. This layout has also been used by Riley for many years. The short pushrods hold reciprocating weight at a minimum, improving efficiency.

We at Chrysler Corporation are in favor of increasing compression ratios as fast as higher-octane fuels become commercially available. In the interest of economy and savings to the consumer, we intend in the future, as has been our consistent practice, to use the highest compression which will permit smooth operation with regularly available fuels. But at the same time we intend to pursue all other avenues of increasing engine performance and economy.

The outstanding performance and economy which more than 60,000 Chrysler FirePower engines are now giving to their owners attest to the great advances which can be made in other ways than just by increasing compression ratio. And the hemispherical combustion chamber not only makes this the most efficient engine available today (in terms of pounds of fuel used per brake horsepower hour) but also makes it better able to take advantage of the better fuels which will be developed in the future.

original FirePower hemi drawing


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