Quote:
Originally Posted by justme262
Any one know why the German engines were so lightly boosted? Was it a design decision or the quality of the fuel they had?
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My idea is that it is most directly a result of their large displacement. Power developed is more or less a function of the amount of fuel burned, you "boost" (use higher pressures to inject more oxygen) to allow more fuel to be burnt. In a bigger displacement engine you simply do not need greater boost, you still get the power - the larger volume engine can take more air (=can burn more fuel) without having to resort to heavy supercharging. Even 87 octane could probably take as much as 1.6-1.7 ata, and a result practically lineally increased power output, but what's the point if the stress is too much for the engine components and the engine will have service life of 20 hours..?
Take for example the early Merlin and DB 601 at their normal 87 octane ratings.
33 liter x 1.3 ata boost pressure = possible 42.9 air volume
27 liter x ca. 1.65 ata boost pressure (roughly equivalent to +6.5 lbs/sqinch) = 44.55 air volume
So you can see they probably take in about the same amount of air, the Merlin simply makes up for smaller displacement with supercharging.
Now compression ratio was also different (6:1 on merlin, 6.7 on the DB, and later DB engines increased this to 8.5:1, the Merlin stayed the same), meaning that a DB piston is doing relatively more work, all things equal. In the end you get ca. 1000 HP on both engines, though the Merlin uses a bit more fuel for that.
The reason for that is two-fold: a, Higher CR of the DB b, the strong supercharger on the Merlin consumes more power from the engine than the DB, and that power is taken from the engine - which provides it by burning fuel (which goes to 'waste' ie. driving the s/c and not the propeller). c, More accurate fuel metering of direct fuel injection, as Crumpp noted.
Keep also in mind that power outputs are NOT primarly limited by boosts and the quality of fuel. Typically the fuel allows for higher boost pressures than used in service, but it is limited by the manufacturer because the engine components can't take the stress of the so developed power with an acceptable engine life. For example very early Merlins using 100 octane were limited to +9 lbs, but later ones could take +18 and much greater output on the same fuel.