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#1
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economy zone for I.C. engines
why do I.C. engines have a particular speed limit which give higher
average then other speed limits |
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#2
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economy zone for I.C. engines
jas wrote:
> why do I.C. engines have a particular speed limit which give higher > average then other speed limits > Its not that simple. If you just consider the ENGINE in any car by itself, it will be most efficient when its at its maximum power output. But in a car, you don't need that maximum power output except during all-out acceleration or driving at the absolute top speed of the car, which never happens. If you put a teeny engine in a car so that its maximum horsepower output was *just* enough to move the car at 70 mph through the wind, then it would be super efficient, but couldn't accelerate fast at all, couldn't go uphill at 70 mph, etc. So you need a n engine with reserve power, which means that most of the time you're running it at way, way less than its maximum theoretical efficiency. When you do THAT, the best way to get economy out of it is to just turn it as slowly as possible which is why overdrive is pretty much standard on every car nowdays. So the real answer is that the "particular speed" that a car is most efficient at is sorta where the increasing efficeincy of the engine as you demand more power from it crosses over the decreasing efficiency caused by wind drag. Car builders put that point around the average highway speeds that the cars will see for best overall results. |
#3
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economy zone for I.C. engines
"Steve" > wrote in message ... > jas wrote: > > > why do I.C. engines have a particular speed limit which give higher > > average then other speed limits > > > > > Its not that simple. It surely isn't. As you suggest, you have to define your terms carefully. |
#4
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economy zone for I.C. engines
is it true to say that only the wind drag is a key factor in deciding
the speed limit or in other words........ a car with more aerodynamic shape wiil have higher speed limit where fuel economy can be achieved |
#5
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economy zone for I.C. engines
"jas" > wrote in message oups.com... > is it true to say that only the wind drag is a key factor in deciding > the speed limit > or in other words........ > a car with more aerodynamic shape wiil have higher speed limit where > fuel economy can be achieved A car with improved aerodynamics will be more economical at higher speeds. At low speed, drag is not much of a factor. As the speed increases, drag increases as a mathematical power function (not linearly). |
#6
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economy zone for I.C. engines
On Sun, 30 Apr 2006 14:09:24 GMT, > wrote:
> >"jas" > wrote in message roups.com... >> is it true to say that only the wind drag is a key factor in deciding >> the speed limit >> or in other words........ >> a car with more aerodynamic shape wiil have higher speed limit where >> fuel economy can be achieved > >A car with improved aerodynamics will be more economical at higher speeds. > >At low speed, drag is not much of a factor. As the speed increases, drag >increases as a mathematical power function (not linearly). > AIUI, mu is linear with speed (notwithstanding tire heating effects), and wind drag is proportional to the square of the windspeed, while actual resistance (power required) is proportional to the cube of the vehicle speed. Correct me if I'm wrong. At low speeds, we mainly deal with thermodynamics and volumetric efficiency. The old saw, "jack-rabbit starts waste a lot of fuel" dies hard, with today's ICE's, because they are well ported, and volumetric efficiency offsets throttling losses, with gasoline engines. With older large carbureted V-8's, all bets are off, of course. :-) Diesel engines don't really have this problem, which explains why they can run at curb idle for hours, and use very little fuel, compared to an equivalently sized gasoline engine. -- -john wide-open at throttle dot info |
#7
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economy zone for I.C. engines
but i could not find solution to my starting question
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