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economy zone for I.C. engines



 
 
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  #1  
Old April 27th 06, 08:42 PM posted to rec.autos.tech
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Default 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  
Old April 27th 06, 10:34 PM posted to rec.autos.tech
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Default 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  
Old April 27th 06, 11:28 PM posted to rec.autos.tech
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Default 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  
Old April 28th 06, 08:47 PM posted to rec.autos.tech
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Default 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  
Old April 30th 06, 03:09 PM posted to rec.autos.tech
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Default 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  
Old April 30th 06, 03:36 PM posted to rec.autos.tech
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Default 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  
Old April 30th 06, 06:36 PM posted to rec.autos.tech
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Default economy zone for I.C. engines

but i could not find solution to my starting question

 




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