Understanding aero vs throttle pos.

[Kodak]

I read a post the other day (can't seem to locate it) that confused me a bit. Maybe I misread it.

Here is what I know: Aero improvements cannot change engine rpm for a set gear and speed because gear ratios are fixed. Aero improvements can only decrease the required throttle position to maintain that speed, by putting more engine power towards forward motion (versus overcoming wind resistance). One could opt for a taller final drive ratio, but that's a whole different story.

But here's the part that tripped me up: In some situations, the lighter throttle position invites more pumping losses? I can't wrap my head around a situation in which better Cd, which means lighter cruising throttle pos., could have a negative result.

Perhaps pumping losses are increased, but it's still always a net gain - right?

I'll try to see if I can come across the post, but I think I explained it correctly.

[tjts1]

The decreased in aerodynamic drag is a much larger factor than pumping losses. The increased pumping losses are more than offset.

[JRMichler]

Originally Posted by Kodak

Perhaps pumping losses are increased, but it's still always a net gain - right?

Correct, it's always a net gain. I read one paper (don't remember title, when, or where) that said the real world gain for aero improvements alone is about half the total drag reduction. You get the rest of the gain by gear ratio changes.

[Patrick]

And by gearing up you require a larger throttle opening, eliminating the additional pumping losses you originally created.

[Kodak]

Ah! And now it's clear. Thanks for filling in the missing pieces of the puzzle - makes total sense.

[redyaris]

An other way to think about it is that at Wide Open Throatle the pumping loses are the lowest but the fuel consumed is the greatest. Whereas at idle the pumping loses are the greatest and the fuel consumed is the lowest. In spark ignition gas engines the throatle plate controls the mass of air/fuel the combustion chamber sees... to evercome resistance; rolling, aero...

[brucepick]

Originally Posted by JRMichler

Correct, it's always a net gain. I read one paper (don't remember title, when, or where) that said the real world gain for aero improvements alone is about half the total drag reduction. You get the rest of the gain by gear ratio changes.

Makes sense to me.
I would also say that with better aero, you're able to coast more often and for a longer distance. This reduces your rpms to either zero or idle speed, depending on your preferred coasting method. Of course, that's once you've added coasting to your "tool kit". So depending on your route and the hills etc., I believe coasting WITH improved aero can give more benefit than just what you gain from drag reduction alone.

[tjts1]

Originally Posted by Patrick

And by gearing up you require a larger throttle opening, eliminating the additional pumping losses you originally created.

No it doesn't. Taller gearing means lower RPM for a given speed therefore reduces manifold vacuum (higher absolute pressure) and reduced pumping losses. The amount of power to overcome rolling resistance and aerodynamic drag remains the same.

[HAHA]

This should mean diesel or electric powered vehicles will respond better to aerodynamic improvements than gasoline counterparts....

- Unless we use pulse & glide

[brucepick]

Originally Posted by HAHA

This should mean diesel or electric powered vehicles will respond better to aerodynamic improvements than gasoline counterparts....

- Unless we use pulse & glide

Sorry - I don't see the logical path for either conclusion.
Unless the point you're making is that you need to P&G to get the maximum benefit of an aero mod, when driving a gasoline car. I think I'll agree with you on that.

My gas-powered Civic has responded very well to aero improvements and I'd be very reluctant to give them up. In fact, I'm working on a new mod almost "as we speak".

Even if you could show a logic for your double conclusion above, saying that one type would respond "better" to aero improvements would be an apples to oranges comparison.

I could argue that gasoline cars, with their inherent losses of thermal inefficiency, need every bit of help that we can bring to bear.