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Old 07-09-2008, 07:10 AM   #41
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Originally Posted by bobc455 View Post
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.
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Old 07-09-2008, 08:08 AM   #42
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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.
Two types of ECU.

Mass Flow uses MAF, IAT, engine speed, and throttle position to determine where on the map it needs to be. The O2 sensor is just there to tweak the map both long term and on the fly.

Speed Density uses manifold vacuum, IAT, engine speed, and throttle position to calculate an appropriate amount of fuel(or reference a map depending on manufacturer) and the O2 sensor, again, just tweaks the calculation or map.

Throttle position is mostly for ignition timing parameters.

Injector pulse width probably isn't any better of a way to measure fuel usage than using the OBD2 data. Even if the injectors are clogged or not flowing right OBD2 will still be able to calculate the required amount of fuel and how much you are actually using. The ECUs whole purpose in the car is to measure how much fuel is needed so I wouldn't be so quick as to dismiss it. On top of that, if you go by injector pulse width and your injectors aren't flowing right then you just end up with a set of bad numbers.
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Old 07-09-2008, 08:38 AM   #43
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That's a good point about clogged injectors. It hadn't occured to me because I'm mainly measuring in a brand new car. Would a clogged injector be inconsistent, or just flow at a consistently lower rate? Measuring pulse width can be used to determine absolute consumption, but the way I use it is merely relative.

I don't doubt that the ECU measures accurately, but it seems that OBDII isn't designed to communicate it very well.
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Old 07-09-2008, 08:44 AM   #44
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Speed Density uses manifold vacuum, IAT, engine speed, and throttle position to calculate an appropriate amount of fuel(or reference a map depending on manufacturer) and the O2 sensor, again, just tweaks the calculation or map.
Are there any OEMs that use Speed Density? I'm only aware of aftermarket systems that use this.

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Old 07-09-2008, 10:30 AM   #45
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Are there any OEMs that use Speed Density? I'm only aware of aftermarket systems that use this.

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All new Hondas are Speed Density.
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Old 07-09-2008, 11:21 AM   #46
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Injector pulse width probably isn't any better of a way to measure fuel usage than using the OBD2 data. Even if the injectors are clogged or not flowing right OBD2 will still be able to calculate the required amount of fuel and how much you are actually using. The ECUs whole purpose in the car is to measure how much fuel is needed so I wouldn't be so quick as to dismiss it. On top of that, if you go by injector pulse width and your injectors aren't flowing right then you just end up with a set of bad numbers.
My first thought is that the ECU is computing the amount of fuel needed. Personally, I'd like to be able to directly measure how much fuel is actually being used.

I agree that FI duty cycle could be erroneous based on the assumption that the injectors are not clogged, but how would the ECU compensate for clogged injectors? Altering FI pulse width?
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Old 07-09-2008, 11:37 AM   #47
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I agree that FI duty cycle could be erroneous based on the assumption that the injectors are not clogged, but how would the ECU compensate for clogged injectors? Altering FI pulse width?
Well, altering FI pulse width is the only way the ECU can feed more fuel in, I think. The question is really how does it decide to do that, if it calculates the fuel rate based on the injectors not being clogged, and if it actually cares about fuel rate (which I doubt; I think it only cares about whether it needs to increase or decrease), it would do so based on O2 sensor readings.
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Old 07-09-2008, 04:16 PM   #48
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All of the systems I've seen will increase the injector PW (no, there is no other way to get more fuel into the engine) if the O2 readings aren't happy. If the voltage is too low (i.e. lean), the ECM will make a correction by extending the injector PW. If it still is too lean, it will open the PW further, and so on.

A non-wide-band ECM has a "target" O2 voltage of 0.450. However it cannot really ever attain 0.450, so it actually tries to "cross" 0.450 (higher-lower-higher-lower-higher-lower etc.) at a certain minimum frequency (called "cross counts"). Well that's how it was a few years ago, but I suspect it is still the same.

A wide-band system can target a specific air-fuel ratio.

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Old 07-10-2008, 08:04 AM   #49
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Just trying to catch up here. Lots of helpful comments.

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Originally Posted by theholycow View Post
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.
Good point. Thanks for the correction. I bet you're right.

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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...
Yes, another good point. But maybe it's helpful to realize that when vehicle speed is constant, it means that those forces are in equilibrium.

Given a certain rpm, and a certain load (i.e., throttle position), the engine is producing a certain amount of power. If vehicle speed is constant, that means the power currently being produced is exactly equal to the power required to overcome all the forces (i.e., drag, friction etc) the engine is currently working against.

One reason the two things get mixed up is that they balance each other perfectly (given constant vehicle speed). Realizing this might help people not mix them up.
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Old 07-10-2008, 04:41 PM   #50
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Fuel consumption will be constant in any gear at 20% throttle and 2000 RPM only when the manifold vacuum reading is identical, and only for a short period of time.

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