“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, August 06, 2006

More on fuel used to run the engine

I've spent a couple of posts attempting to determine where the heat energy in the cylinders from burning fuel is used. I talked about the air conditioner and about the fuel used to keep the engine turning. I was rather surprised at the amount used for the latter and decided to look into it a bit more.

Several posts back I wrote about the web site of Dr. Steven Dutch, a Professor at the University of Wisconsin at Green Bay. He has an article debunking the fantasy 200 mile per gallon car. To be clear, he doesn't claim that no such vehicle exists or is possible, only that it's not possible to use simple bolt-on parts or additives to achieve this kind of mileage with "off the shelf" cars.

In any case, Dr. Dutch uses several strategies to infer the force required to turn the engine against the friction of the moving parts (probably primarily the pistons in the cylinders I would guess). The one I'm considering is his analysis involving the cranking power of an automotive battery. He concluded that turning the engine over requires 3600 joules per second or 3600 watts.

If that's true, it means that the energy needed to turn the engine for an hour is 3600*3600 or 12,960,000 joules. However, the engine only uses about 25% of the heat energy in gasoline to do useful work, the rest is wasted as heat expelled to the environment. In fact, some of the "useful work" is turning the fan and the water pump to dissipate the heat. In any case, this means I need the energy of 4*12,960,000 or 51,840,000 joules of heat. This is the amount available in 51,840,000/125,000,000 or about 0.41 gallons of gasoline.

Let me repeat that. Dr. Dutch's calculations imply that I consume about 0.41 gallons per hour to turn the engine. My observations on the road lead me to conclude it's about 0.38 gallons per hour. Absolutely amazing that the agreement is so close. And it reinforces my conclusion that a large amount of the fuel burned in a car is used to operate the engine.

No comments: