Request for Data from TheHolyCow

[dosco]

Originally Posted by theholycow

BSFC charts describe how efficiently the engine produces power when you're talking about a fixed amount of work, but don't take into account the different amount of work that you need done at different speeds.

Yes, but units of SFC are lb per hp-hr. Which also means that I should be able to very easily compute my vehicle's fuel consumption (lb per hr) based on engine load and rpm.

I'm not worried about transients, I'm interested in steady state comparisons at different speeds.

The same car at 60mph requires a lot more power to move than at 20mph. There's aerodynamic drag, rolling resistance from the tires and wheel bearings, and drivetrain drag from everything between the transmission and the wheels.

In your example, 60mph would use more volume of fuel but be more efficient.

This is why I'm interested in this subject and data ... there should be a way to connect the BSFC charts to the data measured in operational settings.

Back to my example. The answer is that I don't know (if I knew I wouldn't be asking). If I'm driving with the same throttle angle and rpm but different gears, then I would assume the FI duty cycle would be the same in each case.

If the rpm were identical in both cases but the throttle angle smaller in lower gear, I would assume the FI duty cycle would be longer, as the engine would be operating with a poorer SFC due to pumping losses, etc.

If the rpm were identical in both cases but the throttle angle larger in the higher gear, then I would assume the FI duty cycle would be shorter as the engine would be operating with a better SFC.

The logic I'm using comes right from the "iso-curves" on the BSFC chart that correspond to differing engine loads.

[dosco]

Let me add something that might have been what you were getting at (and is where I've gone wrong)

I could be driving at 2000 rpm, however in 2nd gear I could have a throttle setting of 1/4 and in 5th gear could have a throttle setting of 1/2.

In each case the engine is at 2000 rpm however the difference in throttle angles results in more power with the 1/2 throttle setting versus the 1/4 setting. Which means if I go to my BSFC chart, I would select the SFC corresponding to the correct rpm and load. (I'm assuming prior assertions that "load" corresponds to "throttle setting" are correct. In other words, 50% load is the same thing as 1/2 throttle.)

The next step, to compute the actual fuel consumption, is to multiply the BSFC value by the HP produced by the engine at that rpm and throttle setting.

This is where I go wrong (and to be honest where I came to a quandary several weeks ago, however I'd since forgotten about it)

Question: To figure out HP for a certain throttle and rpm, can I take a WOT dyno reading and multiply it by the fraction representing the throttle setting? In other words, if at 2000 rpm and WOT the hp is 100, can I figure the power produced at the 2000 rpm and 1/4 throttle is 100hp*0.25=25hp?

Sorry I didn't elucidate this earlier, as I mentioned I came to this several weeks ago while poring over the BSFC and BMEP/SFC charts and totally forgot about it.

In which case the lower throttle setting may or may not have a longer FI duty cycle depending on the engine's characteristics (power produced and BSFC ratings for throttle settings (i.e. BSFC isocurve)).

(it sure would be nice to have an BSFC chart for my car rather than the generic one that's floating around out there)

[bobc455]

Originally Posted by dosco

Question: To figure out HP for a certain throttle and rpm, can I take a WOT dyno reading and multiply it by the fraction representing the throttle setting? In other words, if at 2000 rpm and WOT the hp is 100, can I figure the power produced at the 2000 rpm and 1/4 throttle is 100hp*0.25=25hp?

No. If anything you might be able to compare injector duty cycle %, but even then you have a ton of other theoretical issues. (What is HP? Total heat generated? Flywheel output? Rear-wheel output?) I suppose you could subtract WOT duty cycle from idle duty cycle (at the same RPM), then graph that out and figure out your HP output based on WOT DC% - idle DC% or something crazy.

Like I've tried (poorly) to explain before, airflow is NOT a linear relationship to throttle position. I believe it would be an inverse-logarithmic curve, but I'd have to think that one through. To explain simply, the first 1/8" of pedal travel might increase HP by 30%, but the last 1/8" of pedal travel might increase HP by 2%.

Originally Posted by theholycow

This is one very troublesome semantic failure in fuel economy discussions...it's hard to know when someone (or a document or chart) is using load to mean technical engine load (throttle position / vacuum), the amount of weight on a vehicle, the drag, etc...stupid language...

Agreed - Engine "load" has to be one of the most annoying discussion concepts that I've come across.

-BC

[dosco]

Originally Posted by bobc455

No. If anything you might be able to compare injector duty cycle %, but even then you have a ton of other theoretical issues. (What is HP? Total heat generated? Flywheel output? Rear-wheel output?) I suppose you could subtract WOT duty cycle from idle duty cycle (at the same RPM), then graph that out and figure out your HP output based on WOT DC% - idle DC% or something crazy.

Well, in the end the power used is defined by what makes the car 'go.' Right?

I presume the BSFC charts are for the engine, measured at the flywheel.

So if I can figure out the aero drag and rolling resistance, I can figure out the power at the wheels that makes the car 'go.' For any gear, I should be able to correlate a particular FI duty cycle and/or throttle angle to the power applied at the wheels.

Not sure how to account for transmission/mechanical losses, will have to think about that for a bit.

[theholycow]

Originally Posted by dosco

If the rpm were identical in both cases but the throttle angle smaller in lower gear, I would assume the FI duty cycle would be longer, as the engine would be operating with a poorer SFC due to pumping losses, etc.

False. While the engine is operating less efficiently, it's producing so much less power that the inefficiencies do not add up to the difference in power. So maybe you're asking for 50% of the power, and paying 5% pumping losses; you're still using 45% less fuel.

Since aerodynamic drag goes up as a square of speed, often lower speeds are more efficient -- but of course the sweet spot(s) can vary due to BSFC map, gearing, etc.

If the rpm were identical in both cases but the throttle angle larger in the higher gear, then I would assume the FI duty cycle would be shorter as the engine would be operating with a better SFC.

No. At a given RPM, FI duty cycle will move in the same direction as throttle -- wider throttle, higher FI duty cycle; restricted throttle, lower FI duty cycle.

Originally Posted by dosco

I would select the SFC corresponding to the correct rpm and load. (I'm assuming prior assertions that "load" corresponds to "throttle setting" are correct. In other words, 50% load is the same thing as 1/2 throttle.)

This is one very troublesome semantic failure in fuel economy discussions...it's hard to know when someone (or a document or chart) is using load to mean technical engine load (throttle position / vacuum), the amount of weight on a vehicle, the drag, etc...stupid language...

Question: To figure out HP for a certain throttle and rpm, can I take a WOT dyno reading and multiply it by the fraction representing the throttle setting? In other words, if at 2000 rpm and WOT the hp is 100, can I figure the power produced at the 2000 rpm and 1/4 throttle is 100hp*0.25=25hp?

I have no data or theory to back it up, but I am almost sure that the answer is no. That is a great question and I hope someone has a more authoritative answer.

In another thread, someone mentioned that throttle is not linear; so, the difference in power between 2/10 and 3/10 throttle is way more than the difference between 7/10 and 8/10.