Quote:
Originally Posted by rmay635703
I was at steady state highway travel about 58mph in 4th gear.
The readings were off the scamguage.
The motor was warm and in closed loop
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OK, now we have some clues. But the first thing to bear in mind is an OBD-II MPG gauge is an educated guess, not a precise technical measurement. The reason is that there is no general PID for 'fuel flow' or 'injector pulse width'. Instead, there are basically two paths to a measurement.
MAF -> Guess at operating AFR -> combine with VSS -> display 'MPG'
Combine MAP, RPM, and IAT -> Guess at VE -> Guess at operating AFR -> combine with VSS -> display 'MPG'
Sometimes, the guesses are wrong, that's why you generally have to 'calibrate'. It isn't just tweaking for precision problems in the sensors, but driving style and even common routes.
Now, this could start a whole bunch of seperate arguments, so I want to reiterate, I'm not saying that the measurements are useless. They are not, they have a strong statistical correllation to actual fuel consumption. I'm just saying that when you need a 'fudge factor' you have to take the precision of individual measurements with a grain of salt.
Quote:
Originally Posted by rmay635703
It appears that when my timing at highway speeds (again previous experience) is in the 20's my fe is the same as open loop which also has timing in the 20's
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This is the problem with distinguishing correlation from direct cause and effect. The optimum peak pressure point in the engine is fixed by geometry, it is built into the design. You are looking at timing in DEGREES, but to understand timing, we have to think in terms of TIME.
At it's simplest, this is easy to visualize. When we are at idle, timing generally drops to near zero, this is because the piston is moving slowly. As RPM goes up, we have to pull the timing back in TIME to have the burn hit the same peak pressure point.
But there is another dimension, the burn of the mix is not a fixed interval in time. It is effected a great deal by turbulence, or swirling of the mix, AFR, and by temperature. Temperature may be the biggest factor.
The reason that there is a statistical correlation between economy and high timing numbers is that the style of driving is lower RPM's and lots of time off the throttle. When RPM is low, you have to move back in degrees, because the slower speed of travel on the piston. When you are coasting and letting off the gas a lot, things have to pull back because your effective mixes are lean and the 50% burn is slow. 'Lean' doesn't have to mean air, it can also be exhaust gas.
This is the part of the puzzle I missed initially. Under load, high numbers are bad, but you are striving to drive under low loads.
Quote:
Originally Posted by rmay635703
I have had experiences across the board on different vehicles where fuel economy would drop for no apparent reason and stay there while lightly feathering throttle going down a hill to maintain speed. In those cases however I could let off the gas and wait for the GPH meter to bottom at idle rates then my FE would go back up.
I have sometimes heard of this called detroit fever or a long term correction.
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We could tell for sure with a wideband in your exhaust, but it seems much more likely to me that this is simply a case where the FE reading is just plain wrong. You seem to be describing overrun conditions. In that case, the injectors are often closed and the tailpipe is full of free air. Since the MPG gauge is not looking at injectors, but the MAF or MAP side and then presuming fuel use, it is reporting poor FE when you are not using any at all.
Try setting your scan tool up to show one of the O2 sensors. They are easier to look at as a bar graph than numbers, but when it is oscillating, you are chasing stoich, when the voltage is at the higher end, you are richer than stoich, when it is pegged low, you are lean, and frequently in overrun.
Quote:
Originally Posted by rmay635703
From what I have learned the motor tries to operate at stoich but very rarely does except on the highway at steady state and even then few motors actually operate at true stoich but usually a few degrees below, exceptions being very modern motors. Even light acceleration usually drops you out of stoich.
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Actually, they run at stoich a lot. As much as possible. Again, you can tell by looking at the O2 sensor. The narrow band sensor detects a very narrow range around stoich. It is very accurate at finding stoich because the chemistry actually reverses as it is crossed. If the O2 sensor is oscillating, the ECU is 'chasing it', and you are there. If it is reading high, you are enrichened, usually under fairly aggressive accelleration (when you kick the ECU into "Open Loop"). When it is low, you are lean, typically in coasting and recirc situatations.
It HAS to hit stoich a lot for the CAT to function at all. Cat efficiency plummets as you move off stoich. This is because the high EGT is needed for the cat processes to occur.
Emissions is the ONLY reason engines are built to target stoich. Best Power is richer, Best Economy is leaner.
Quote:
Originally Posted by rmay635703
Also note is was quite cold, many motors go a bit excessive on timeing & A/F corrections when it is cold thus why many suffer MASSIVE FE reductions even on long trips, couple that with "winter gas" and you have a craptaskic combination.
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Think about this for a moment. Small airplanes LOVE cold weather. Because they are underpowered and often run full throttle. When the air is cold, it is denser. More air molecules means you can mix in more fuel, which releases more energy, which translates into faster climb rates.
But when you are in your car, you don't say 'OH BOY, It's cold, I can go *faster*!'. You generally drive at the same speeds. But what happens to the air charge at the inlet?
Just like the airplane, it is denser. So, to go the same speed, you throttle back more. The more you throttle, the more vacuum is created behind the throttle and the lower your efficiency. The cold air gives the engine more performance, but we have no use for it, so we operate the engine at lower effiency to meet our demand on it.
To better understand why, take a look at why high compression engines are more fuel efficient that low compression ones. It is the the same fuel and air, but the top of the V/P graph is taller and sharper, so we get a greater effective change.
But, that is generally not why the timing is "aggressive". Remember, burn rates are highly influenced by temp. It would be counter productive for the ECU to run at the same timing at low IATs that it does at high IATs!
As far as the 'low FE', there are two possibilities, one is that it is actually reporting the inherent loss in efficiency in throttling back more because of the higher air charge. The other is that it is just more wrong than usual about guessing at fuel consumption. Remember, it is primarily looking at the air side, not the fuel side.
Going just on timing is probably a bad idea. For example, when you shut down you car in the cold weather, cooling happened quickly. When you started up again, there was a shift in timing. But there is no reason to believe that the ECU suddenly decided to be more efficient. It is more likely that a temp change in CHT changed burn and swirl. Rather or not this resulted in better or lower true fuel efficiency is hard to say. In general, the cooler engine throttled back would be less efficient, but there are lots of exceptions.
-jjf
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