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Old 01-03-2013, 01:59 AM   #1 (permalink)
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gas engine efficiency

Is there a theoretical way of finding the point in the engines dyno graph where any engine is producing the most power with least amount of power( gas) used? Or is there a point of optimal efficiency? Yeah I know, key off. Other than that.

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Old 01-03-2013, 02:28 AM   #2 (permalink)
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Here is an interesting discussion pertaining to BSFC (Brake Specific Fuel Consumption).

The BSFC chart thread (post 'em if you got 'em)

For more info : http://en.wikipedia.org/wiki/Brake_s...el_consumption
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Old 01-03-2013, 10:30 AM   #3 (permalink)
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I knew something like this was out there. Brake specific fuel consumption. Thanks so much.
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Old 01-03-2013, 11:35 AM   #4 (permalink)
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The speed of optimal efficiency is usually at the rpm of the torque peak, this occurs because the airflow in the intake is at maximum volumetric efficiency at filling the charge into the cylinder.
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Old 01-03-2013, 11:55 AM   #5 (permalink)
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I had a gut feeling that was true about torque curve and in just reading other stuff found that stated again. So two questions. 1. So in theory you can go slower and use more energy? 2. If you use bolt-on changes, high flow air filters/headers/cat back, that improve torque at a given rpm will we gain efficiency at that same rpm?
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Old 01-03-2013, 01:52 PM   #6 (permalink)
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Actually, the max BSFC typically seems to fall below the peak torque RPM. It seems to be somewhere around 2500 RPM in most cases for gasoline engines, while peak torque RPM varies a lot from engine to engine--in many cases, though, it is over 3000 RPM.

The question of what speed to drive is not really the same question as what the most efficient RPM is, though. When you move a car, you have to use power to overcome friction, air resistance, rolling resistance, to warm up the engine parts, and so on. At anything over in-town speeds (guessing, 40 MPH?) the largest factor is air resistance, which goes up by the square or cube of the speed, depending on what kind of drag you're talking about.

So the slower you can go, the less power needed to push your car through the air. But if you don't need to make much power, you have to throttle the air going into the engine way way down. And that isn't so efficient. So there's a trade-off. But the trade-off seems to generally be weighted so that lower speed is more important than the throttling losses, so for steady-state cruising the most efficient speed is typically "in top gear as slow as you can stand". For me, that is anywhere from 50 to 65 MPH, depending on a lot of factors.

-soD
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Old 01-03-2013, 03:56 PM   #7 (permalink)
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Quote:
Originally Posted by some_other_dave View Post
Actually, the max BSFC typically seems to fall below the peak torque RPM. It seems to be somewhere around 2500 RPM in most cases for gasoline engines, while peak torque RPM varies a lot from engine to engine--in many cases, though, it is over 3000 RPM.
-soD
Yes I got this thanks.

Quote:
Originally Posted by some_other_dave View Post
The question of what speed to drive is not really the same question as what the most efficient RPM is, though. When you move a car, you have to use power to overcome friction, air resistance, rolling resistance, to warm up the engine parts, and so on. At anything over in-town speeds (guessing, 40 MPH?) the largest factor is air resistance, which goes up by the square or cube of the speed, depending on what kind of drag you're talking about.
-soD
Got this too. I have a Jeep. boy to I got this.

Quote:
Originally Posted by some_other_dave View Post
So the slower you can go, the less power needed to push your car through the air. But if you don't need to make much power, you have to throttle the air going into the engine way way down. And that isn't so efficient. So there's a trade-off. But the trade-off seems to generally be weighted so that lower speed is more important than the throttling losses, so for steady-state cruising the most efficient speed is typically "in top gear as slow as you can stand". For me, that is anywhere from 50 to 65 MPH, depending on a lot of factors.
-soD
I'm trying to look at this mileage thing from inside out. I have no doubt that overcoming drag, especially air in my case, is the largest part of work performed. I would then assume that if an engine had a flat torque line instead of a torque curve I would want to gear it to put my most frequent driving speed at the lowest rpm that hits that near peak/peak torque?
So, in theory, must bolt-on's that improve near peak/peak torque improve BSFC?
I know I am missing pieces of this hence the questions.

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