Thermal efficiency and SFC

Id have to work out the physics of it, but if you are reporting thermal efficiency in percent, then you can see that something is missing.

There could be engines of similar thermal efficiency but having widely varying fuel consumption per unit time.

You can however work backward from specific fuel consumption and relate it to thermal efficiency by making a few assumptions.

As I say, I would have to work out the physics, and I hope I havent overlooked the obvious and said something really stupid.

For the common aircooled aircraft engine the thermal efficiency is on the order of 25%. 15% is lost through the cooling fins, 10% through the oil cooler, and 50% goes out the tailpipe. Modern auto engines will be somewhat more efficient than that, using EFI and variable ignition timing and the liquid cooling that keeps temps closer to ideal levels.

35% might be more normal in such an engine. Still, even a 100% efficient engine wouldn't give us a perfect world. A long time ago when I was learning Physics during Aircraft Mechanic's training we did some calculations. A 2300-lb Cessna 172, with a 150-hp engine, accelerating from a stop to 60 mph takeoff speed in an 800-foot run at sea level, was experiencing the equivalent thrust of about 28 HP, IIRC. The rest was lost to things like rolling friction and various forms of aerodynamic drag, and some power was missing due to the fact that the engine, with its fixed-pitch prop, couldn't achieve rated RPM and therefore rated HP in the takeoff run. A constant-speed prop would fix that but would only bring the equivalent power up to about 33 HP. The automobile won't be much better. HP lost in the drivetrain is significant, and most cars are pretty draggy, in spite of the apparently slippery designs. Just look at the chopped-off rear ends of most; there's a lot of turbulence generated there, and that represents wasted energy. The undersides of most cars are anything but smooth. Some years ago I read that the Terraplane automobile of the 1920s had the lowest drag coefficient of ANY automobile ever produced, but it was "ugly" and gasoline was cheap, so it didn't catch on.

Dan

See, I should have Googled instead of relying on fading memory. It wasn't a Terraplane, it was the Rumpler of 1921. The article

says that it held the record forlow drag until 1988, yet another Wiki article says the Tatra (1935) hada lower coefficient yet. There have been even lower-drag cars thatdidn't reach production. See to seewhere your car fits. Dan

Exactly. You can get any car's drag numbers from the EPA, or figure your own by coast down. The only uncertainty is that you won't know the amount of engine power that goes to power steering, charging or some other belt driven thing.

Most of the responses have dealt with inefficiencies outside of the engine. I guess what I am getting at is, if one measures the specific fuel consumption on a dyno, one should be able to convert that to thermal efficiency. Seems to me that if we divide the energy content of gasoline in hp hrs/lb by the sfc, and multiply by 100, we should get the thermal efficiency in percent. Is my algebra right?

But I was talking about SPECIFIC fuel consumption, the lbs of fuel per horsepower hour. If the engines have the same efficiency, how can the SFC be different? Yes two engines with the same SFC but different horsepowers would be burning different lbs per hour, but wouldn't they be burning the same lbs/hr per HORSEPOWER?

Essentially, yes it is. Here's the quickie formula:

Eff = 100*LHV/SFC

Make sure you convert Lower Heating Value into the appropriate units (i.e hp-hr/lb for sfc in lbs/hp-hr).

Great,that is just what I had come up with. Thanks.