iverson said:
There were no belts strong enough.
I believe much the same problem was found with early marine steam turbines in destroyers - the turbines spun too fast, and the shafts needed multiple props on them to absorb all the power (couldn't use larger props due to hull clearance and draught issues).
Not exactly. Propeller sizes are limited by tip speeds, particularly in an incompressible fluid like water. If the tip speed exceeds the local speed of sound, you get cavitation around the propeller. Cavitation causes huge losses in efficiency and destructive shock waves that rapidly erode the propeller. Given the same output shaft speed, multiple small propellers can transmit the needed power at a much lower tip speed than a single large propeller.
The critical-altitude issue is different. While cars are usually supercharged to make more power by operating above sea-level pressure, airplanes generally are not (except for Reno racers). In military aeroengines, supercharging is used to provide the equivalent of sea-level power at higher altitudes. Ideally, you'd like to maintain constant full-throttle power at all altitudes. Turbochargers can come close to this ideal, but, in practice, all aircraft engines had to be optimized for a narrow range of tactically important altitudes.
An engine that is designed to deliver maximum full-throttle power at sea level is set up to burn fuel in dense, sea-level air. Above sea level, it starts to lose power. An engine that is supercharged to deliver maximum full-throttle power at a critical altitude of 12000 or 30000 feet can't deliver enough air to maintain that power above the critical height. Nor, less obviously, can these altitude engines deliver full power below critical altitude.
Power is limited by the amount of fuel that you can burn, and the amount of fuel that you can burn is limited by the amount of air that you can mix with it. To burn sea-level amounts of fuel in an internal combustion spark-ignition engine at altitude, you thus have to pack more, thinner air into the same space. You have to raise the compression ratio. The highly successful "over-compressed" high-altitude engines that the Germans developed in WW1 achieved this by significantly raising the compression ratios of otherwise conventional engines. This made it possible to fly well above the altitudes that Allied fighters could reach. But it also introduced a problem: the over-compressed engine would not run and might even fail at lower levels. To reduce power to a safe level, the pilot had to throttle back or use a decompression fitting on the engine that released pressure from the head.
Why high-compression engines had problems at sea level was not fully understood until Harry Ricardo and Sam Heron discovered detonation post-war. Detonation occurs when compression heats the fuel/air mixture until ignites on its own, without a spark. The resulting shock waves can wreck engines.
Since supercharging is just a more efficient way of raising the compression ratio in the engine, you have to be careful not to overboost an engine so much that you raise the effective compression ratio to the point where detonation starts. A supercharger that will compress the air enough to give maximum, full-throttle, sea-level power in the thin air at 30000 feet will raise the compression ratio enormously if run at sea level. Sea-level air is something like three times as dense as that at 30000 ft. If you ran the high-altitude engine at full throttle under those conditions, detonation would destroy the engine in no time. So the high-altitude pilot had to throttle back lower down. No engine produces its best power throttled, so the power of the high-altitude engine drops significantly when run at low level. Worse still, the supercharger machinery is just dead weight at sea level, the impellers further throttle the airflow, and extra friction and moving parts consumes additional power.
For this reason, low-altitude fighters usually had single-speed, single-stage superchargers that delivered minimum boost.
IIRC all single-stage Merlins fitted to production Spitfires (the ASR.IIc was a later conversion and the III never got beyond prototype) were single-speed and vice versa; the only two-speed, single-stage engines the Spitfires got were the Griffons fitted to the MkXII (and derivative Seafires), while the speeds required for chasing V1s were gained by allowing a more open throttle and consequent higher boost pressures rather than altering the gearing per se.
True. Gearing was for multi-altitude flexibility. The low-level Merlins were single-speed because they were intended for use in airplanes that operated at a single level--low-level. The superchargers were "cropped"--given smaller impellers that delivered lower volumes of lower pressure air--so that the engines could run at full throttle in dense, low-altitude air.
For low-level operation, the Germans and Americans also used water injection to lower the charge temperature, delay the onset of detonation, and allow higher boost for brief periods--usually during take off and landing or emergencies. In the USAAF it was called ADI (Anti-Detonation Injection). In Germany it was called MW-50, for the methanol that was added to the water as antifreeze. The British do not seem to have used water injection to any great extent, perhaps because the design of the Merlin's supercharger provided a measure of detonation protection. In war-time Merlins , the supercharger was located between the carburetor and the engine's induction manifold. So the vaporization of fuel in the carburetor cooled the charge significantly.
For high altitudes, the Merlin used a two-stage supercharger with an intercooler between the stages. This lowered the total end-to-end temperature rise for a given level of supercharging.
American-made fuel was, however, the key to Allied high-altitude performance. Tetraethyl led and high-quality American crude oil produced gasolines with very high octane numbers. High octane fuel resists detonation and allows more boost and thus more power. German fuel never approached the detonation resistance of US fuel. So German engines tended to be of larger displacement for the same power. Direct fuel injection may also have helped to maximize the detonation resistance of engines running on lower quality fuel.
