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Old 12-30-2007, 07:21 PM   #6 (permalink)
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The Mighty Mira - '92 Daihatsu Mira
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Originally Posted by DonEaston View Post
Sorry for the confusion, Mr. Firstlaw- not trolling at all.
Ok then, welcome to!

So, not "underpowered" in the sense of "my vehicle is more powerful than yours", simply in the sense of "is it really economical to try to push a (relatively) heavy vehicle with the smallest engine possible? Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"
The answer is "it depends on how you drive it".

If driven correctly, a smaller engine will ALWAYS be more economical, provided that it is correctly geared and that power produced at peak BSFC is more than the load on the engine at constant desired cruising speed on level ground. This is the case with any car you care to name, geo and kia included, and including my car (660cc engine). The exception might be a 105cc unfaired postie bike, but only if driven at top speed.

For top fuel economy, giving it more than about 65% of available power in an Otto cycle engine at that rpm is wasting fuel. At about that power level, you get more bang for buck, i.e. more power at the wheels for a given quantity of fuel. Diesel is different, wide open throttle (WOT) is the most efficient, so as long as your engine is capable of cruising, it is not undersized wrt fuel economy.

Also, for hill climbing, even the smallest engine can move a truck up a hill, given the correct gearing. (I just know there has to be someone more nitpicky than I am who is about to bring up static friction, but I beat you to it. ) "Give me a place to stand and I will move the earth" - Archimedes. It's all about leverage, not power. Getting there at the same pace you would cruise on the level is a different story however. But this is a site about fuel economy.

Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"
No. The reason for that is the engine map of a petrol engine. They are most efficient at load, and get less and less efficient the closer you get to idle. The reason for this is that in a larger engine, at idle, you have more mass to rotate, more surface to rub against, more air to pump, a more closed throttle plate that is choking off the flow through the engine.

You could get a good idea of what it's like to be an Otto cycle engine by forcing yourself to only breathe through a drinking straw every time you slowed down, and a smaller drinking straw when you stop. It's hard work, and you are putting out next to no power. If your aim was to go somewhere, it would be easier to walk or jog along.

It's not a great analogy, but it might help?

Remember not to confuse it with gearing issues. Low power at low rpm is more efficient than the same power at higher rpm, but it is still inefficient compared to a smaller engine at low rpm with a more moderate power loading.
"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton

Last edited by newtonsfirstlaw; 12-30-2007 at 07:27 PM..
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