Quote:
Originally Posted by TomcatViP
More boost does not mean more power : 0+0x12 = 0 
Temp might be the limiting factor to consider (tht's where MW50 (with water) was so important for German eng with their poor quality materials). Early war Merlins shld be weaker than their late counterparts. Hence drawing a comparisons with later Merlins even at the same boost level is risky
~S! |
Firstly, I don't understand your logic at all.
A Merlin III with the R.M.2.S. rating could achieve +6¼ psi boost at 15500', which would deliver about 1000 bhp.
A Merlin XII with an improved supercharger intake elbow and AVT40 carburettor could achieve +9 psi boost at 15750', which would deliver 1150 bhp.
More boost = more power.
Alternatively, compare the takeoff power of Merlin III engines at different ratings.
The R.M.1.S. rating was +6¼ psi boost, 3000 rpm, giving 880 bhp at sea level.
The R.M.2.S. rating was +8¼ psi boost, 3000 rpm, giving 1000 bhp at sea level.
The engines were physically identical, with the extra power allowed by the use of 100 octane fuel.
More boost = more power. Indeed, you can see that an extra +2 psi is worth about 120 bhp, whilst an extra +2.75 psi is worth about 150 bhp.
To a first order approximation, you can see that running a first generation Merlin at +21 psi absolute gives about 1000 bhp unthrottled if the supercharger gear ratio is about 8.5.
So in round numbers, that would be roughly 50 bhp for every extra psi boost.
Now, this is very rough and ready stuff, but it's quite a good first order guess; a Merlin 66 on 150 grade fuel gives a little over 2000 bhp in MS gear unthrottled at +25 psi boost.
1000*(25+15)/20 ~ 2000.
Basically what we're saying here is that the amount of power produced by the engine is proportional to its air consumption, which is limited by the physical size of its intake & exhaust valves.
Supercharging increases the air consumption by increasing the charge density in the intake manifold.
P*V = roh*R*T
So actually the error associated with drawing a comparison between early and late Merlins is smaller than you might perhaps expect.
Thermodynamically, the piston engine at the heart of the Merlin doesn't change much after the ramp head combustion chamber was discarded c.1938. The vast majority of its power development came from improvements to the supercharger. Valve timing was only changed for the prototype R.M.17.S.M engine.
Mechanically there were considerable changes devoted to improving life at high power, but they didn't greatly impact upon the thermodynamics of the machine.
Indeed, although the Griffon is
mechanically very different from the Merlin, if you take the Merlin model developed by Hooker et al and plug in Griffon numbers then you'll find that the agreement is impressive, because
thermodynamically they're extremely similar machines.
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ICDP,
My high speed testing of the Spitfire II was indeed conducted with CEM disabled. I was actually trying to tackle the prop pitch change controversy at the time, rather than to investigate the performance of the aeroplane itself. But I was struck by just how much faster the Spitfire II was than the Spitfire I, and I
suspect that what's happened is that 1c have done the same thing with the early Merlin that they did with the later Merlin when the Mustang III was introduced to IL2, namely increased the boost without changing the FTH appropriately.
However, it's very difficult to be sure at this stage given the various bugs and our lack of knowledge of the atmosphere model etc.