Theory about why MPG indicator reads high

I haven't tried it with the Prius, but when I switched from BP to Shell many months ago (had to do with the BP spill in the Gulf), I noticed an IMMEDIATE drop in my MPGs in my Charger. no different driving habits, just different gas.

then I realized that Shell uses Nitrogen in their gas as an 'additive' to help clean your fuel system. problem is that Nitrogen doesn't BURN, which would cause your mileage to drop. in my case, about 2 mpgs. they must use a LOT of nitrogen.

I still use Shell with the Prius, but I may on my next fill up, try BP again......just to see if there is a difference. my 'actual' MPG vs what the 'computer' reads isn't that far off. since I started driving it to and from work daily, I started at 52.2mpg average according to the Prius. my 'actual' calculations came up with 51.6. this tank I'm running 54.5, so I'm curious to see what my 'actual' is when I fill up.....probably next week.

with my Charger, I was filling it up at least once a week. with the Prius, I can go two weeks on a tank......and a much smaller tank at that!

 

Give it a try. My hunch: you'll get a bit better mileage, but the in-dash prediction will be as inaccurate as ever.

FYI, our previous '06 Honda Civic Hybrid consistantly underestimated mileage, by a small percent: if it said 5 liter per 100 km, calc'd would typically be 4.9 or 4.8 (better).

 

While I strongly doubt that Prius directly measures tailpipe carbon emissions, ScanGauge and other aftermarket engine monitors do something else with a similar result. If Prius uses the same method, then part of its display error could be attributed to the differing energy densities of these fuels.

ScanGauge cannot directly measure fuel use, because that information is not available on the OBDII port. But airflow information is available, either directly from a mass airflow sensor, or computed from the engine displacement, RPM, and intake air pressure. Because modern engines run at a fixed air:fuel ratio most of the time, fuel use can be estimated from engine air consumption.

The engine ECU adjusts the air:fuel mix with feedback from the oxygen sensor in the exhaust. Since E10 has a different composition than pure gasoline (less carbon, more included oxygen), the input mixture is changed slightly to get the right oxygen level in the output.

ScanGauge doesn't know what fuel is in the tank, so it cannot give accurate MPG readings in the blind. But it can take user refill information to calibrate its readings.

If the Prius uses the same methods, it will suffer the same fuel blend uncertainty. But Prius lacks any user calibration path, so its display would have to assume one particular blend, most likely a summer blend E0 gasoline.

I don't know for certain that Prius computes MPG in this manner. And if it does, this gas vs E10 difference accounts for only part of the display error.

 

The closest gas station to my office sells E0 gas for about a 10-cent premium. They get a lot of motorcyclists, who are even more FUD'ish about alky-haul in their gas than people who drive Priuses.
I've run several tanks through 'my' car, and quite frankly I can't tell any difference at all. I drive varying routes in 'my' Prius (it's a company car) and I gave up on the hypermiling thing about 3,000 miles back. I currently get about 50MPG, hand calculated, tank to tank---50.3 on my last tank. I could and have legged that out to about 58 MPG, but I don't buy the gas for this car, and so I just drive it like.....a car.
So...I figured that if there's not an obvious difference in real-world driving then I would just keep getting gas at the bigs.

Out of semi-scientific curiosity, I did try a tank in one of my motorcycles (09 VN-900) which I drive to and from work, along a repeatable route, same TOD, same driving style etc....
I saw about a 1 MPG delta with the E0 versus the E10 on the cycle...or about 60 cents savings on a tank, which costs about 50 cents more to fill up with the E0.

So yes...theoretically (in my case) you can save a little on E0 versus E10, but it's not worth planning my fuel stops over. E0 is getting harder to find too---and the gas stations in this area that (say that they) sell it are the grungier Mom and Pop places that make me wonder how anal that they are about things like filtration, and tank maintenance.

At any rate, all of this isn't going to affect the MDF calculations, which do have a repeatable bias from what I'm reading about on this forum---regardless of fuel blends, climate, mosquito density, etc.
I think it's just marketing on Toyota's part to take advantage of folks who are too lazy to hand calculate their mileages tank-to-tank....but I'm something of a skeptic, and Toyota needs all of the help with marketing that they can get lately.
Priuses are marketed for their fuel efficiency, so why not fudge high on the MDF?

 

No E10 up here at all, just straight gas. My indicator is still 5-6% generous.

 

does you 09 Vulcan has oxygen sensor or it is open loop system?

 

sorry.

 

The MPG displayed is an estimate based on the counting of strokes for one of the four pistons. That means you've got thousands of them per minute each multiplied by an rough estimate of how much fuel was actually injected into the cylinder.

That leaves a lot of exposure for rounding & truncation errors.

As for the influence of E10, that's a non-issue. Fuel volume isn't any different and the estimate difference existed long before most people were using E10 in their Prius.
.

 

Both bikes are open loop...
(the other is an 06 VRSC-D)
Why?

 

Open loop will meter the same amount E0 or E10 fuel.

Close loop should take into consideration additional oxygen and meter more as E10 has 3.5% oxygen content.

So where the difference comes from? 3.5% lower energy density? or due to E10 having 89 octane number?

 

I do not know.
My Vulcan is an 09 (clearly built with gasohol in mind) based on a largely unchanged 06 design.
It's probably energy density.
The 1.something MPG delta could also be just noise in the data.
Hand calculating MPG is far easier on a bike than it is in a car (you get to fill the tank to more precise degree by sight, rather than waiting for the pump to click off) but my mileages still dither about by 1-3 MPG along the same routes based on environmental variables and the vagaries of my throttle hand.

It's hardly good science...but I've always opined that if you have to look that hard to see the difference---there's no difference.
That's just about where I'm at with the whole E0-vs-E10 thing.

The MDF bias in Priuses is clear, present, and repeatable. I don't know if it's an honest mistake by the folks in Aichi or skullduggery, but I also know that it doesn't seem to be affected by how much alky-haul is present in your fuel.

YMMV.

 

I've been to gas stations where we pump 5.1gal into 4.8gal bike tank, that is before hitting reserve (.5gal). Gas was alot cheaper at that station but they made money by adjusting pumps?

Also the some bike tanks are very sensitive to which way you park bike uphill or downhill, just a thought.

 

I don't buy that. Innocent errors would be reasonably random, ie: one time the in-dash prediction might be high, the next time might be low.

I don't think I've ever heard of a Prius owner reporting calculated mpg (or liters/100km) that was better than the in-dash. And the error is a fairly consistant 5-9%, "in favour of the house".

 

I

 

I didn't notice any change in MFD accuracy when using E5 or E10 since Feb. My MFD average error is about 6.8%. Each tank displays a different error, but since Sep. 2009 when I got the car (and I fuel *everywhere*, don't have a preferred place - I buy where it is cheaper), I have 48 tanks on over 26000km/16000mi and have seen % values all over the place - averaging 6.8%...

There is also the error introduced at the pump. And I don't think that all are selling 1L or 1gal that is *really* 1L or 1gal - actually it will be less...
Plus if the pump is delivering the fuel in the tank really fast, you will get a lot of air - so when you think it is full, it might be actually not *that* full - so another source for error...

I think the error is also due to a cheap-ish measurement method used to reduce overall instrument cluster/sensors cost. To make more accurate measurement, you need more accurate instrumentation that, for the average users, i.e. not people on PC and on the fuel economy forum , is practically useless. 5-7% error is not something to really to be worried about. And since it is repeatable and worldwide the same, I don't really care about it anymore. It is not a defect, it is by design so.

I have unfortunately not checked what was the MFD error on my previous car, a VW Polo...

 

I have a spreadsheet of all the data from my Insight-I. Over almost 60,000 miles, the onboard lifetime average reads 84.3mpg. My hand calculations from gallons pumped in is 83.8mpg. So Honda's number is 0.6% higher than the hand calculation (which is the direction one would expect if some of the pumps exaggerated volume pumped).

So I have to suspect that the 6% error people are seeing with the Prius is built in by Toyota. If it were simply an inaccurate method for measuring mpg, we would expect to see scatter of the numbers both high and low rather than the fairly consistent 6% high that people are reporting.

I won't have data for my new Prius until I've run a couple of tanks through it. So I'm trying not to be too happy with the good mpg I am seeing on it.

 

There might be some truth to the OP's idea.

The O2 sensors always make sure that the mixture is stoichiometric. Most ECU's have adjustment tables to correct the fuel injector duration to correct for changes in injector flow characteristic (from fouling, dirt, etc.).

E10 has less oxygen demand than E0, so more will have to be injected to burn all the O2. The ECU will probably see this via the O2 sensor and think the injectors are fouled a little bit. It'll open them longer to compensate, based on the airflow (measured very accurately).

More fuel is flowing than the ECU thinks, so when it thinks 5 gallons have been fed, more than 5 have been actually been fed. Mileage reads high relative to actual based on pump volume.

Energy content of E10 is about 3.5% lower than E0. But, the real question is O2 demand (per gallon). I'm sure O2 demand is lower for E10, but I'll have to work out the math to see how different it is.

 

Answer is about 2.85% difference. Gasoline has a stoichiometric ratio of about 14.7 (14.7 gm air per gm fuel). Ethanol's is about 10 (needs less air per unit mass).

Using a fuel density of 0.74 for gasoline and 0.789 for ethanol (and assuming no deltaV-mixing), E10 has a fuel demand 2.85% higher (by volume) per unit of air.

So, if the meter reads 50 MPG, you will calculate 48.6 MPG using pump volume, or a drop of 1.4 MPG.