2010 Toyota Prius FCD Inaccuracy

15", 16", 17" are the diameters of the metal wheel, not the overall diameter of the tyre as fitted. The actual diameters are hard to compute due to the mix of units, but it turns out that the rolling circumference of the larger wheels is slightly smaller than of the 15" wheel, but it's a very small difference.

The tyres available on Gen 2 were 185/65 R15 (US/Japan) or 195/55 R16 (US Touring/European). That means a tyre 185mm wide with sidewall height 65% of tyre width, on a nominal 15" wheel, or a tyre 195mm wide with sidewall height 55% of the width on a 16" wheel. On Gen 3 the options are 195/65 R15 (US II-IV, UK T3) or 215/45 R17 (US V, UK T4, T Spirit). The circumferences are:

185/65 R15: 1952.50mm
195/55 R16: 1950.61mm (-0.1%)

195/65 R15: 1993.34mm
215/45 R17: 1964.44mm (-1.45%)

225/45 R17 would have been closer at 1992.71mm, only off by 0.03%, but the extra width would have been harder to accommodate and reduced economy still further.

There's a size calculator about half-way down this page.

 

When they fix it, will my car actually get better than 50? Or will the gauge just read accurately?????????

 

HSI discrepencies for my recent EBH road trip:

545.6 miles 62.3MPG indicated 56.69 calculated 9.625 gal.

569.3 miles 56.6MPG indicated 50.97 calculated 11.17 gal.

476.2 miles 57.5MPG indicated 52.56 calculated 9.06 gal.

535.2 miles 59.9MPG indicated 54.61 calculated 9.80 gal.

598.2 miles 57.1MPG indicated 51.44 calculated 11.63 gal.

So we have differences of 9%, 10%, 8.5%, 9%, 10%. Doesn't help when your running long highway distances and have gas far between.

Wayne

 

See my MUST READ RESPONSE.

Is it about time we combined these threads?

 

I've always used my pump, fill-up data and calibrated trip meter distance for vehicle mileage. The vehicle MPG indictor is used for short, benchmarks. The reason is I pay for my gas and everything else is 'an opinion.'

Bob Wilson

 

I just sent off a request to the staff to merge the threads.

Buzz

 

A full combination of these threads not only could be "messy and confusing" -- it would contain numerous duplicate posts.

Perhaps because one is a Sticky, we should close the other and provide a final post with a reference?

This is a request for the mods to consider what is best, not a request to do the above.

Other thoughts?

 

That is what I'd do. Folks who want to merge in their thoughts could still quote from the locked thread into the remaining one.

Bob Wilson

 

Yeah. I'm a moderator on an Internet forum and we try to catch duplicate threads early and merge them leaving redirects. In this case though, you're correct, too complicated now. I like your idea of leaving one thread open with a reference in the others.

Buzz

 

My fill-ups for the last five tanks (ignoring my first two) have yielded calculated tank MPG's of: 49.3, 50.2, 48.2,0 46.8, and 50.3. My recollection of the indicated MPG is that it's been around 50 MPG.

I didn't record my indicated MPGs per tank, except this last tank which had an indicated 53.4 versus the calculated 50.3, but in this case at least some of the error is probably due to the fact that I had to pull in the wrong way to the pump and run the hose over the top of the car so my fill was different.

 

i think a merge would be a disaster. i agree its a bit confusing, having posted to close to a half dozen posts all with similar topics, its easy to lose track of what i said and where... but WHAT THE HEY!!!

this is a big deal. in over 70,000 previous Pri Miles, the error has been less than 1.5%

not surprised there is a lot of talk about this. its really perplexing. what happened between 2009 and 2010 that would cause such a LARGE discrepancy.

 

4 fillups so far. Discrepancies for each fillup in order:

Mileage: 1.2mpg, 2.3mpg, 2.9mpg, 3.2mpg
Percentage: 2.2%, 5.1%, 5.4%, 6.4%.

Overall real mpg is 48.9mpg, display showing 51.4mpg.

 

I've started calibrating our ZVW30 indicated versus actual mileage. The first part is to define the standard, the actual mileage, and then look at the dependent variable, the indicated mileage. For our standard:

• same pump - #6 at the Shell station on South Parkway in Huntsville AL
• same fill-up protocol - manually listening for the rising tone when the gas backs up into the filler tube. Pause at least 5 seconds and then top to 'one click' manually
• same fuel - Shell 87 octane, E10
• quantity - at least 1 gallon since pump reports to 1/1000th of a gallon
• temperature - not recorded

There needs to be a model for how mileage is calculated and assuming the distance measurements are valid, there are two methods for measuring fuel consumption: (1) injector timing and summing, or (2) mass flow stoichiometric operation.

Injector timing, as Ken@Japan pointed out, is subject to mechanical delay and then has a linear output. When the injector is first turned on, it takes a finite amount of time until fuel begins to flow, the delay. This means, as Ken@Japan pointed out, a fixed overhead has to be 'taken out' of the fuel measurement.

The second approach is to use mass flow at the 14.7 air to 1 unit of gas ratio. This works when the engine operates in the closed-loop, feedback system range. But this range does not start working until the O{2} sensors are operational, which typically takes the first 45 seconds in my NHW11. At high power settings, the ZVW30 engine begins using cooled, exhaust gas to moderate exhaust temperatures. In contrast, the NHW11 used a richer mixture to avoid burning out the catalytic converter. In this case, one would expect the error to change as a function of power used, something proportional to vehicle speed.

To figure out the calibration curve, we need to know which mechanism, injector timing or mass flow, is being used. I don't know but errors in the data by looking at average speed, gives an indication of which mechanism is being used. I'm still gathering data and only have four samples.

PRELIMINARY INJECTOR COUNTER

Assuming injector timing is used to calculate fuel consumed, the correction curve should be a line with possibly a fixed offset:

The "14.467" constant in the linear trend-line would correspond to a fixed overhead, the delay that Ken@Japan reported for the 1.5L engine. With just four records, I don't see a speed related factor, which suggests mass flow is not being used.

After I get enough samples, the trend-line values will become more and more accurate. I can then solve the inverse function and convert the indicated MPG to true MPG. But I will only be able to do this for my particular vehicle. Others will need to gather their own calibration data.

If we were to gather enough data points, we could do a population study. This would allow us to test different hypothesis of the source of the error. But that is beyond the scope of this study as I'm just using this, my raw data:

	Column 1	Column 2	Column 3	Column 4
0	pump MPG	indicated MPG	avg mph	gallons
1	54.4	61.0	33	2.28
2	54.8	60.2	33	1.444
3	49.8	56.4	35	9.65
4	48.5	55.0	32	9.8

.

NOTE: My data is saved at MyHybridCar.com with notes that allow me to analyze specific variables. Similar functions exist at www.fueleconomy.gov but we are not able to see the individual records of others.

Bob Wilson

 

I'd be shocked if the engine didn't run leaner than 14.7 to 1. I would think that at lower power settings, it would be a whole lot leaner than that, and at full power it may be a lot richer.

 

last tank info in my sig....pretty much in line with all the other tanks ive had after tank to tank variations are considered. look at tank figures verses LT...not much difference.

58.08 verses 64.1 last tank....just about average

 

No. The fuel mixture must be kept at exactly 14.7 to 1 because that's the point where CO2 and NOx formation are at their joint minimums. You get lower CO2 and HC with leaner mixtures but NOx formation goes way up; lower NOx but higher CO2 and HC with richer mixtures. See graph:



(graph from FuelSaving.info - air bleed devices)

Raw NOx, CO and HC have to be at about the same level so that the three-way catalyst works properly, taking oxygen from NOx and adding it to CO to get pure nitrogen and CO2 - while this is a greenhouse gas it isn't immediately poisonous at ground level.

Engine output is controlled by throttling the amount of air entering the engine, and adjusting the fuel level to match. The throttle makes the engine work harder to suck air into the combustion chamber, which is a major source of engine inefficiency at low outputs.

The car actually cycles between slightly lean and slightly rich, to keep the two parts of the catalyst in equilibrium.

Cars can run a lot leaner, and many were adjusted that way before mandatory smog checks. The unacceptable result is a greater level of smog-forming NOx emissions.

Direct-injection engines, including diesels, do, or can, run a lot leaner, but that means they need other forms of catalyst to remove the excess NOx emissions. You may have heard of urea injection, or SCR (selective catalytic reduction), under the trade name 'AdBlue' for the urea fluid. I believe VW's latest 'clean' (Bin 5) diesels simply don't run as lean as earlier diesels - at the cost of greater fuel consumption.

 

Makes sense. I was shocked when folks were suggesting that it used mass flow instead. The injection pulse information is available and mapped, so why would they use a secondary indication (air mass flow) that itself is less accurate and/or precise? Furthermore, once one considers that the calculated fuel flow from that is based on a ratio from another measurement (O2 meter) with probably greater imprecision it becomes a tertiary measurement.

 

With two exceptions, mass flow means only one metric is needed instead of two, ICE rpm and injector timing. One of the things I keep hitting my head against is the limited number, six, and longish latency, ~200 ms., of the Graham scanner. I suspect the scan-gauge folks share the same frustration, hence the use of mass-flow. Also, injector timing requires taking out the dead-time and that can also cause grief.

There are two times when mass-flow appears to have significant errors:

• 0-45 seconds - this is the time it takes for the O{2} sensors to come online. During this period, the ICE runs 'open loop', which is typically brief although heavy acceleration could be significant for a lot of short trips.
• maximum power - in the 1.5L, NHW11 engine, this would be rpms greater than ~3,900-4,150 rpm. The ZVW30 paper discussing cooled exhaust recirculation discusses 'running a richer mixture to protect the catalytic converter from excessive heat.' They are using the cooled exhaust to reduced exhaust gas temperature.

Bob Wilson

 

About how many tanks have you purchased, Dave?

I find it interesting that my variation is settling in, too, but at a much lower level than you are measuring. I see your indicated as about 9% higher than the measured MPGs. For me, the MID numbers are about 5% higher than what I calculate at the pump (57.1 vs. 54.3).