RE: Jett .76L  
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RE: Jett .76L - 6/1/2007 2:01:16 PM   
Sport_Pilot



Posts: 7874
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline 		The older carburated engines had heated manifolds, but there was a thermostat which shut off the heat when the car is warmed up. This was to prevent the fuel from puddling in the manifold which caused wet fuel droplets to sputter into the intake. This would make the engine run randomly rich and lean making it run rough. Of course with fuel injection there is no puddling so there is often no preheat, though some may have it just to quicken engine warm up.

As far as engine RPM dropping at high altitude and heat, that does happen. Because the engine displacement is to the cube, and the prop drag is the square of its surface area, the engine power drops faster than the prop resistance. On full scale fixed pitched engines you cannot get the engine to over rev with full throttle at high altitude, however you have to watch the tach redline and throttle back to prevent damage to the engine.

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“I Saw Elvis at 1000 Feet” John Force

(in reply to Sobrinof)
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RE: Jett .76L - 6/1/2007 3:40:51 PM   
DarZeelon



Posts: 7343
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: offline 		Sobrinof,


I will not argue with the observations of Sadi Carnot.

In his short life (38 years) he made a strong impact on the study of physics.


But as that tire advertisement goes; "Power is nothing without control'.

Unless you are talking about an absolute speed competition event, if your engine cannot idle properly, it doesn't matter if you can get 2-3% more HP, from forcing cooler, denser air into the engine.

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Dar Zeelon - ISRAEL - ddzeelon@gmail.com
MVVS - Jett - Nelson - Bolly - Mejzlik

(in reply to Sobrinof)
       Post #: 27

RE: Jett .76L - 6/1/2007 9:20:36 PM   
Sport_Pilot



Posts: 7874
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline
quote:

Probably, this is done to assure that the mix gas-air is well done in order to burn the mixture completely in the engine. When doing this, you reduce the efficiency of the engine because of carnot law and reduce also the power output because you introduce less air into the engine as i said before, but surely, the designer of the engine, when balancing both effects, though this was the best solution


Preheating the air with the exhaust increases the efficiency, however the power goes down. If comparing it to the Carnot Cycle the incoming temp is still the ambiant temperature, but because the exhaust is used to preheat the incoming air the exhaust temperature is lower. This is done with the large gas turbine peaking generators the power companies use. It has also been used in economy runs and experments in automobiles. However, it reduces power so much, must be turned off and on for drivability, and then there is the maintenance problem of cleaning the soot in the heat exchanger. As I recall they had to turn it off when idling at stop lights to prevent overheating, off durning heavy passing, but worked great at a steady cruise on the interstate. Cooling the air will only increase efficiency if the energy used for cooling is free. If for example you use an air conditioning system to precool the air then the energy used to cool the air will offset the energy saved and of course does not change the ambient air temp which is still used as part of the system. I recall a system where air was pre cooled cool water from a stream which is free except for pumping costs, then preheated with the exhaust stream. I don't actually recall how well it worked though.

_____________________________

“I Saw Elvis at 1000 Feet” John Force

(in reply to Sobrinof)
       Post #: 28

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