Understanding aero vs throttle pos.

[dcb]

Originally Posted by tjts1

...Thats why all OBD2 systems use MAF not injector PW to calculate fuel consumption.

if you mean mpg monitors, they use MAF (or figure it out from MAP/RPM/temp) because there isn't an obd2 pid for pulsewidth, and they hope for stoic. Vacuum referenced regulator is good in that it standardizes the fuel delivered for a given amount of open injector time regardless of the manifold pressure.

If you mean OBD2 ECUs use MAF and not injector PW, well, that is because injector PW is an OUTPUT from the ECU ?!?

[aerohead]

Originally Posted by 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.

I believe it was Gino Sovran that published that unless gear-matching was accomplished along with drag reduction,up to 60% of the streamlining benefit could be lost,as the engine BSFC is moved into a less efficient operating regime.Hucho covers this in his book.Chrysler discovered this firsthand in 1934 when they aero-modded the DeSoto Airflow test mule.
I may have just experienced this with the trailer project.Only further,higher speed testing will tell.

[redyaris]

Patric & tjts1
If I accept that Power = Torque x rpm [and I do...] and I accept that the throatle position remains the same at a lower rpm [and I do...] and I accept that the engine pumps the same mass of air at both higher and lower rpm's and therefore produces the same amount of power, then the force provided must be greater for each revolution ie torque is greater at the lower rpm. Which if I am not mistaken is consistent with the throatle position being the same at both higher and lower rpm.
Please correct any errors I may have made with my high school phisycs and math limited understanding.

[Patrick]

Please explain how the engine increased torque with the same amount of air.

Also, please explain how the engine (a positive displacement pump) pulled in the same amount of air at a lower rpm with the throttle plate at the same position.

[redyaris]

Originally Posted by Patrick

Please explain how the engine increased torque with the same amount of air.

Also, please explain how the engine (a positive displacement pump) pulled in the same amount of air at a lower rpm with the throttle plate at the same position.

The volume [displacement] of air is the same for every cycle and at all rpm. What changes is the mass of the air in the cylinder when the intake valve closes. The mass of air is controled by the throatle plate in spark ignition engines, therefore if more torque is produced the throatle plate should have to be more open [less restiction].
To produce the same amount of power the engine must consume the same mass of of air and fuel with the same thermal efficiancey at the lower and higher rpm. So the volume of air is directly proportional to the rpm the mass is changed by the throatle plate.
The only thing that changes is the amount of time [longer] the intake valve is open at the lower rpm so with the same rate of flow [mass] past the throatle plate the longer time would allow a greater mass of air to enter the combustion chamber. The question that I don't know the answer to is; is that enough to raise the torque as much as is needed to produce the same amount of power, I some how doubt it. So unless the time difference is enough then the plate has to be less restrictive [more open] to allow the same mass of air to enter the engine at the lower rpm.

Do I have it right yet?