Re: Do cooler operating temps reduce mechanical losses? - Saab 900 Bulletin Board - Saabnet.com


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Re: Do cooler operating temps reduce mechanical losses?
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Posted by Ari [Email] (more from Ari) on Tue, 14 Jul 2009 04:19:15 Share Post by Email
In Reply to: Do cooler operating temps reduce mechanical losses?, mag [Profile/Gallery] , Mon, 13 Jul 2009 02:54:45
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Actually, running the engine cooler makes it less fuel efficient. I'm talking about coolant temps here - from a combustion standpoint, hotter is better. But you've got little control of that.

The engine is designed to operate at a given temperature. At that temperature, the clearances are as close to the design targets as possible (tight but not too tight), the oil is thin enough (less drag), and the gasoline is vaporizing, and not condensing on the intake walls and burning poorly.

I'd think that the water pump actually pulls more power from the engine when the thermostat is open - it is pushing fluid through more pipes, with more drag, and the radiator has lots of internal drag. Some folks use 'water wetter' which reduces the surface tension of the coolant and can reduce some drag. But the cooling system is one of those unfortunate necessities. Throwing away heat in a radiator is a waste of energy and aerodynamic drag, but there are no good ways to recover that energy today. As to the waterpump load, you're pretty much stuck with it.

There are two parts to the alternator. One is parasitic drag - some power loss just spinning the alternator unloaded, flex in the belts, heat in the pulleys. Not a lot, but some. The major part of the alternator is the electrical load. Yes, the more current you pull from the alternator, the more energy it sucks from the engine. When the engine is running, only peak current loads come from the battery - the alternator provides most all of the car's electrical needs, and recharges the battery. But for the alternator to produce CURRENT, it consumes mechanical power from the engine.

The cooling fan should only be running when the car isn't moving, or moving very slowly. The cooling fan's job is to replace the airflow over the radiator you'd normally get when the car is moving. Given that one of the greatest power losses in a car is aerodynamic drag, and drag increases with (the square) of speed, the cooling fan is the most energy-efficient way of cooling the engine. Of course, it doesn't get you where you need to go.

So yes, any electrical load - cooling fan, AC clutch, stereo, lights - all suck down engine horsepower. A cooling fan pulls about 25 amps at high speed - at a nominal 13.5 volts, that is about 337 watts, or about 0.43 horsepower.

The internal combustion engine is maybe 30% efficient. Electrical needs, cooling needs, all contribute to the inefficiency. Of course, it takes energy to pull air through an air filter, and even the energy lost to pulling air past a partially open throttle takes a surprising amount of power. There are ways to recover some of this power, for example, it's more efficient to run a small engine at wide open throttle than a big one at closed throttle, as it improve pumping losses. And a turbocharger recovers heat energy that would normally be lost down the tailpipe. But many other ways to save a few percent from the engine adversely impact the life of the engine, or driveablilty, or add lots of weight and cost.

The best way to make the whole system (car) efficient is to use as efficient an engine as possible, and then use it sparingly. Losing 400 pounds of sound deadening, or smoothing the surfaces as much as possible all help.

A buddy of mine did some fuel economy 'races' back in the '80s with a Sonett III. The Ford 4-cylinder in the Sonett was no high-tech engine - it was an old pump engine with a carb. But he was able to get 50 mpg out of the car on a 200 mile race in New England, including the 'hills' in Vermont. Of course, he leaned the mixture out to the point of just about melting the engine down, and a K&N filter for less drag. He ran the narrowest tires he could find at 50 psi. He removed the side mirrors, and taped over the radiator. He cooled the engine by running the interior heat at full. And he drove the car like the accelerator had a hand grenade under it, and stayed off the brakes, period. It was the aerodynamics, skinny tires, and gentle, gentle driving that saved the gas. The minor engine mods saved a few mpg.

posted by 75.11.4...

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