Richard Cownie
, grew up w/ stacks of Aviation Weekly
Answered 11 months ago
Answered 11 months ago · Author has 1K answers and 869.5K answer views
The Allison engine was a clever, elegant design with relatively few parts, designed by a fairly small company which didn’t have the engineering and manufacturing resources to quickly fix all the design, manufacturing, and quality-control problems.
Technically, it didn’t have a two-stage supercharger which would allow it to give high power at higher altitudes - whereas the Merlin had a number of different variants with highly-optimized superchargers designed by Stanley Hooker, who was head and shoulders above everyone else in the world in that particular field of engineering. The favored US solution was to use a General Electric turbocharger in aircraft designed for high altitude missions - but turbocharger installations were bulky and complex, and managing the interactions of turbocharger, intercooler, and engine across a wide range of altitude and weather conditions was a hard problem that Allison, GE, and aircraft manufacturers didn’t really figure out until too late.
The Merlin was a complex engine, sometimes described as “a watchmaker’s nightmare”. But Rolls-Royce had the expertise and the resources to make it work well enough, and (together with Ford UK and Packard) to produce it with high quality in high volume. Britain more or less went all-in to make the Merlin work, and thank God it *did* work, even as early as mid-1940 when Merlin-engined Hurricanes and Spitfires defeated the Luftwaffe. In the USA, on the the other hand, a larger proportion of engineering resources went into radial engines, and the Allison was underdeveloped.
TL; DR you get what you pay for, and the Allison wasn’t given enough priority and resources to become fully mature and robust across a range of applications.
[Update: a book review here goes into more detail about problems with uneven fuel-air mixture due to the Allison’s manifold design; and Allison’s corporate policy of not spending their own money to make design changes, and avoiding major design changes.
“Whitney does point out that during WWII, Allison had in place a policy of not using its own corporate funds to develop or improve its engines. Allison did make some efforts to improve the V-1710 as the war progressed, but it also had in place a policy of making as few changes as possible to the basic engine; as a result, thorough re-designs to correct persistent problems were never done and so later upgraded versions of the V-1710 were unable to reach the same level of reliability and performance as the Merlin (and later, the Griffin) engines.
The V-1710's manifold had 4 pipes feeding 3 cylinders (unlike the common plenum chamber design of the Merlin) and so was inherently prone to providing poor air-fuel distribution into the cylinders, leading to this problem of premature detonations in the cylinders whenever the engine was stressed to produce high power under unfavorable circumstances (e.g., in combat at 30,000 feet). The later two-stage supercharger was also a tacked on afterthought lacking all of the refined features of the Merlin's two-stage supercharger (chiefly, a backfire screen and an intercooler - the two-stage supercharger V-1710 used only anti-detonation injection to prevent detonation inside the supercharger) and was also prone to failure.“]
