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Old 07-08-2008, 08:18 AM   #31
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Originally Posted by monroe74 View Post
In other words, "2000rpm and 20% throttle" will always use the same number of gallons per hour, regardless of gear (and vehicle speed). But if I change gear (while maintaining 2000rpm and 20% throttle), vehicle speed will change. Since I am changing MPH while holding GPH constant, MPG is going to change.
That's the root of what I want to know.

If I'm driving in my development at 20 mph in low gear at 2000 rpm, I'm burning the same volume per unit of time as I am if I'm driving at 60 mph at 2000 rpm in top gear.
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Old 07-08-2008, 08:31 AM   #32
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Quote:
Originally Posted by dosco View Post
That's the root of what I want to know.

If I'm driving in my development at 20 mph in low gear at 2000 rpm, I'm burning the same volume per unit of time as I am if I'm driving at 60 mph at 2000 rpm in top gear.
No, because throttle position will be different.

This morning I actually was thinking about this thread a little as I accelerated, going from gear to gear at WOT. As I entered each gear at the same RPM and same throttle angle, I watched the meter, and it used the same fuel rate.

I'm still waiting for my ELM327 to arrive from China. The PCMCIA USB card I ordered arrived but doesn't work in the laptop I planned to use; I need to try it in another to find out if it's bad or if the laptop is bad.
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Old 07-08-2008, 08:44 AM   #33
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Originally Posted by theholycow View Post
No, because throttle position will be different.
Sure. According the the BSFC charts, I should be burning *more* fuel. Thus the question. You should see it in your data, assuming you can correlate duty cycle, throttle angle, and rpm at the same time.

I would think, though, that one could do it "by the seat of your pants" to get a feel for what's happening.
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Old 07-08-2008, 09:54 AM   #34
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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. You can be burning more fuel while still being more efficient, because you are getting more work done.

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.
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Old 07-08-2008, 10:05 AM   #35
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Originally Posted by dosco View Post
If I'm driving in my development at 20 mph in low gear at 2000 rpm, I'm burning the same volume per unit of time as I am if I'm driving at 60 mph at 2000 rpm in top gear.
As thc said, that's true only if the throttle position is the same.

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As I entered each gear at the same RPM and same throttle angle, I watched the meter, and it used the same fuel rate.(
Yes, that's exactly what I noticed when I had my meter connected. (And just to be clear, we mean fuel rate per unit of time, not unit of distance.)

Here's how I look at it. Fuel rate per unit of time is directly proportional to duty cycle. Duty cycle just means how much time the injectors spend being open, as compared with how much time they spend closed. The ECU is constantly judging what the duty cycle should be, and it's basically taking into account two things: engine speed, and throttle position.

So if those two things are held constant, then fuel rate (per unit time) is also going to be constant, even if you change gears (and, therefore, vehicle speed).

There are some other factors that matter but we're assuming that those are also held constant. I'm talking about things like engine temperature, fuel temperature, air temperature and atmospheric pressure.
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Old 07-08-2008, 10:19 AM   #36
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Quote:
Originally Posted by monroe74 View Post
The ECU is constantly judging what the duty cycle should be, and it's basically taking into account two things: engine speed, and throttle position.
Actually, I'm pretty sure the ECU uses the MAF and O2 sensors to decide. RPM and TPS, in this context, are merely a way for us to predict/express MAF, because we know that air flow depends on RPM and TPS.

We could just talk about MAF, but that wouldn't really help us decide on gears and speed and what not.
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Old 07-08-2008, 10:29 AM   #37
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Originally Posted by theholycow View Post
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.


Quote:
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.
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Old 07-08-2008, 11:04 AM   #38
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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)
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Old 07-08-2008, 11:48 AM   #39
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Quote:
Originally Posted by dosco View Post
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.

Quote:
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.

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Originally Posted by dosco View Post
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...

Quote:
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.
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Old 07-08-2008, 02:23 PM   #40
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Quote:
Originally Posted by dosco View Post
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%.

Quote:
Originally Posted by theholycow View Post
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
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