|
George:
Airplanes, unlike cars, operate from a baseline 30” Hg ambient at sea level - - to a baseline ambient of only 12-13” Hg , with pressure ratios varying from 1.2 up to 2.8 to 3.0.
Thus, at constant RPM, the compressor would spend most of its life well off of any “optimum” pre-determined design point.
The variable speed turbo-supercharger compressor helps to mitigate this problem to a substantial degree.
Actually, I think it is quite a bit easier to make a mechanicly driven centrifugal compressor system work efficiently in an airplane than in a car. Power demand, and engine speed are much more consistant in an airplane and even cruise altitudes are fairly predictable, where most operation takes place and economy is most important. (reffering to typical Detroit/Tokyo/Whichita models, not racing applications here)
In an airplane, you simply turn it on when you climb through a given altitude and shut it off when you descend through the same level. In a car, low engine speed would decimate the boost from a mechanically driven centrifugal compressor.
Assuming the engine is already operating at or near the engine’s detonation limited maximum BMEP - - (many aircraft engines are) - - then in order to improve the detonation margin, without reducing power (reduction in CR goes the wrong direction on the power and the efficiency issues) or in order to increase the BMEP - - and remain free of detonation, one will have to do one or more of the following:
1) Reduce the CHTs;
2) Increase the fuel octane;
3) Reduce the induction air temperature.
Add to this list:
4)&N> Increase the fuel octane;
3) Reduce the induction air temperature.
Add to this list:
4) Increase engine speed;
5) Reduce major combustion chamber radius;
6) Reduce/Alter combustion chamber turbulence;
7) Alter combustion chamber wall angles to reduce infrared and shockwave energy reflected into areas of instabillity.
8) Relocate spark plug(s)
Reductions in IAT from the intercooler will buy one LARGE improvements in the detonation limited BMEP (ie, more horsepower becomes usefully available).
That sounds a lot like something I said earlier. Your'e starting to agree with me.
>>However, when you make the blanket statement that a mechanically driven supercharger will result in a slower airplane, we have to part company.<<
I didn’t say that. I said the data I had seen revealed that one particular aircraft using one particular configuration of belt driven compressor ended up, based on actual test, going no faster with the belt driven compressor than it did without.
I felt that was what you were implying by making the statement. I see Walter came to the opposite conclusion. If I am mistaken, then I appologize. I did not mean to put words in your mouth.
It's ironic that in the beginning my intent was to steer this thread away from an argument between personalities and back toward a discussion of the merits and fundamentals. I seem to have had the opposite effect on the argumentative part.
I am still curious to know what boost Rick runs on his otherwise stock engine at Reno.
Rob
|