What are the Advantages of Sleeve Valves ?
World War II, the Bristol
Engines Co. built 57,000 Hercules Engines - a 14 cylinder, sleeve valve, air cooled radial.
Why did they use the sleeve valve concept instead of the American practice
of overhead poppet valves ?
There were several reasons :
- Increased Volumetric Efficiency
The large port openings, in combination with supercharging, allows much greater filling of the cylinder during the intake stroke.
- Higher Compression Ratio
Pre-ignition is often caused by the intake charge coming in contact with the hot exhaust valve.
With the sleeve valve design, the combustion chamber is formed by the piston, cylinder head, sleeve wall and the spark plug. There is less chance for pre-ignition to occur.
For this reason the compression ratio can be raised by one entire number, this delivers more power for the same engine displacement.
- Less Power to Operate the Valves
A traditional radial uses a great deal of power to operate the valve train, The inertia load, spring load and gas load must be overcome to open the intake and exhaust valves. At full power, approximately 4000 pounds is required. This force must be exerted on 28 valves - 1200 times per minute !
With a sleeve valve, the sleeve is in constant motion, at half the speed of the crankshaft. The inertia load is greatly reduced by the smooth sleeve motion. There are no gas loads or spring loads to be overcome.
So, the power available to the propeller is greatly increased.- Reduced Installed Drag
Overhead valves increase the engine diameter of a traditional radial. By eliminating the overhead valves, the diameter of a sleeve valve engine is smaller for a given displacement. As a result, the diameter of the installed engine and cowling is smaller, creating less aircraft installed drag.
The tradeoff to these advantages was increased manufacturing costs. By the end of the war, Pratt & Whitney, Rolls-Royce and Lycoming were all working on there own sleeve valve engine design.
However, with the coming of the jet engine, the days of large reciprocating engines were numbered.