Recently I have made several posts about the significance in North American engine development of the 1949 overhead valve V8 engine in the Cadillac cars. Prior to that time Cadillac had been using a “flathead” or side-valve engine design. On July 12, 2012, I posted a critique of the side-valve engine design in performance cars. This posting hopes to present the advantages of the overhead valve engine design.
I have prepared the following sketch of a typical overhead valve engine arrangement. It should be noted that I am an engineer not a draftsman as demonstrated in my sketch.
Typical Overhead Valve Engine Arrangement
Note that the intake valve and the exhaust valves are located across from each other. This crossflow arrangement helps to both draw the air/fuel mixture into the cylinder and to expel the exhaust gases.
On the power stroke, the exhaust valve always opens a few degrees before the piston reaches bottom dead center (BDC). This provides the benefit of using the rather low remaining power stroke cylinder pressure to help to expel the exhaust gasses. The detonation of the air/fuel mixture causes an explosion that exerts pressure on the wall of the combustion chamber and thus drives the piston downward on the power stroke. If the exhaust valve opens while there is still some residual pressure in the combustion chamber pressure, then the exhaust gases will rush toward the exhaust valve because the pressure in the exhaust system will be less than in the combustion chamber or the cylinder.
On the upward exhaust stroke, obviously the exhaust valve is open and the intake valve is closed. While the upward movement of the piston pushes the exhaust gases out through the exhaust valve, it also establishes an air movement pattern that flows toward to the exhaust valve. This air flow pattern goes from left to right in my sketch or away from the intake valve. The air movement will assist in establishing an air flow pattern that will assist in bringing the air/mixture into the combustion chamber once the intake valve opens. The intake valve will open before the piston reaches Top Dead Center (TDC), but by then most of the exhaust gas is gone, but the air flow pattern remains until the exhaust valve closes. This improved air flow pattern in the overhead valve engine is much better than the air flow pattern in the side-valve engine.
The overhead valve engine design also concentrates the ignition of the explosion of the air/fuel mixture in the area of the combustion chamber that is directly over the piston. This helps to harness the power produced by the explosion.
Therefore the arrangement of the overhead valve design has much more volumetric efficiency than the side-valve design. This advantage is important with today’s high revving engines. For these reasons, I think that the introduction of the Cadillac overhead valve V8 engine, while not the first overhead valve engine by any means, it was an important step in the development of high horsepower North American V8 engines.