Does anyone know how a car computer calculates MPG? How does it measure fuel flow? Seems like a lot of peeps quote these figures but in reality they’re at least 10% out.

Well at a guess I’d say it’s distance travelled (from the odometer) divided by the fuel used as measured by the gauge from the fuel tank. I can’t see any reason why it would need to be more complicated than that.

Edit Scratch that as fuel pumps are postive displacement types all you have to do is count the number of pump strokes and multiply that by the stroke volume to get the fuel usage.

They vary in accuracy, the one in my ibiza tdi seems to be within 1% when driven gently. The one in my old mondeo was 3% out on average. They’re still a good guide and useful way to make yourself drive more efficiently.
Function is indeed pump usage / mileage, the instantaneous results show that it can’t be done of the tank content.

It doesn’t use the fuel gauge – that’s far too inaccurate. Also the fuel pump doesn’t just pump fuel that’s used, it pressurises the system and the fuel actually injected is taken from the system and the rest is returned to the tank, so you can’t use that either.

I think it knows the flow rate of the injector, and it knows how long it’s opening it for (according to the fuel map) so it therefore knows how much fuel is being let in to the microlitre. However, there are variations in the tolerances of the injectors and the nozzles, so the amount of fuel that you actually get can be slightly different to what the ECU things it’s giving you. Hence the differences. Also your tyres get smaller as they wear, and they are not always inflated exactly the same especially as the pressure in them varies according to both ambient temperature and the carcass temperature, which is affected by how you drive and for how long…

Btw for all those using the brim to brim method to calculate fuel usage, it’s worth noting that whilst the petrol pump on the forecourt is accurate in terms of measuring how much fuel is pumped, they dont’ always stop at the same level. So ‘brim full’ at one pump is not necessarily the same as ‘brim full’ at another pump. It varies quite a lot apparently.

The only accurate way to measure economy is to log miles vs litres added over a longer period.

Didn’t old ones (eg Mk2 Golf GTI) use the vacuum in the inlet manifold as a measure of engine load and hence fuel burn? Hence why it was so inaccurate.
(Could be totally wrong.)

You used to be able to get a fuel economy gauge that was in fact an inlet manifold pressure gauge with MPG numbers written on it, yes.

That only worked if you were in top gear on a flat road and it wasn’t windy, I think.

Yup, early ones were just load gauges. Newer ones keep track of actual fuel usage by knowing injector open duration and mapping against distance travelled. They’ll never be completely accurate as I suspect they take a few liberties like “sample every 10 seconds” rather than measuring each individual event, just to save on data storage, or simply keep a running average. But technically they could make it absolutely accurate and measure to the millilitre the amount of fuel used.

Given the cars ECU accurately controls the amount of fuel injected into each cylinder several thousand times a second you’d think they’d be very accurate?

Maybe, as above, its the ‘brimming’ methods thats inaccurate?

One ‘feature’ of the newer Passat is that the ECU samples things (everything) a lot more frequently that previously. The instantaneous fuel monitor is a lot more responsive than on other cars.

The biggest variation is probably the distance travelled figure mind.

Given the cars ECU accurately controls the amount of fuel injected into each cylinder several thousand times a second you’d think they’d be very accurate?

It doesn’t have to be that accurate with the amount of fuel injected. If it injects too much, the driver will just lift off. Other feedback mechanisms will control the mixture/turbo pressure etc for emissions.

Actually it controls the fuel entering the cyl engine only about 70 times a second on even the fastest turning road engines at max rpm, but the point stands 🙂 Though there are developments towards multiple squirts per cycle which would push it closer.

It doesn’t have to be that accurate with the amount of fuel injected. If it injects too much, the driver will just lift off. Other feedback mechanisms will control the mixture/turbo pressure etc for emissions.

I’m not so sure what you’re getting at there – the ECU calculates everything, including the mixture, the ideal mixture, the required boost level, whether the amount of knock in the engine is acceptable, and modern FBW engines even figure out whether you’re pressing the throttle too fast and only open the throttle as fast as they want so they can control emissions. The calculcations are incredibly accurate and fast.

The pump fuel cannot provide an accurate gauge of fuel used. For example, on a petrol car, the pump pushes far more fuel than is used, in a constant loop at a regulated pressure, out past the metering device and back to the tanks. This moving volume of petrol means problems such as fuel vaporisation are eliminated from modern cars.

The fuel is burnt at a stoichiometric ratio i.e. the perfect blend of oxygen and fuel to give a burn that produces the correct balance of power and/or economy. The primary measure of achieving this is the lambda sensor which measures the exhaust oxygen, in a feedback loop to the injectors that meter the fuel via timing, as fuel pressure is constant. This loop has other inputs, such as throttle position and the coolant temperature sensor.

So as the engine control module knows the rpm of the engine and opening period of the injector, it can calculate consumption. As molgrips mentions this is not 100% accurate, because the volume of petrol being delivered by the injectors isn’t known exactly; best values have to pre-programmed by the car manufacturer.

I was thinking about measuring fuel economy on my old fully mechanical Passat. The best idea I could come up with was accurate flow meters in the out and return fuel lines. That could’ve been fairly accurate, no?

Supply and return flow meters are what we use at work for large diesel engines. It’s considered to be more accurate than using the calculations from the injector signals.

An accurate flow meter in the fuel line would work esp when averaged over a reasonable interval.

My Mark IV Golf’s ECU seems to vary MPG accuracy based on driving style – consistently about 10% too high on motorway journeys, but about 20% too low in lots of stop / start town stuff.

I use a Garmin Ecoroute HD in my car, it plugs into the diagnostics port (OBDCII) and sends engine data including fuel consumption to my Garmin GPS via Bluetooth.
The results are pretty close to those I get filling the tank up and checking the mileage.

The Engine Control module knows exactly how much fuel is going into the engine. It calculates fuel usage through injector pulsewidth and flow rate (this is corrected using the oxygen sensors in the exhaust to control to the correct air fuel ratio). The amount of fuel added by the purge system is also added to this number. Then a correction factor is added (which will be unique to each manufacturer and engine type) which is the bit that makes the trip computer less accurate. This is to give the trip computer an optimistic reading, making the car appear to be more efficient than reality. Most people will quote their trip computer number as gospel, so this works in a manufacturers favour. This correction factor will be around 3% for some cars. The only bit the ECM does not know is the tyre circumference – a predetermine number is used, but this does not compensate for wear/pressure/after market fitment etc etc. So, inaccuracy is caused by tyre circumference and the correction factor – could be 7-10% on a bad car. Any other calculation the driver does is more likely to contain error (for instance, how accurate is the fuel pump used to fill the tank?) Should be within government tolerances (probably 0.5%), but is it?

If I was sad enough – no, really, I’m not 😀

The gauge in my Toyota can be adjusted 3% either way so, if after exhaustive testing of fuel used over miles done, I found it to be inaccurate, I could recalibrate it [within the 3%]

Uplink – what car?

My Prius can recalibrate the speedo for new tyres using the built in GPS (even new ones of the same size) and I think it also adjusts automatically as they wear as well as being able to trigger it when you change.

Now that’s joined up thinking – well done Toyota.

Then a correction factor is added (which will be unique to each manufacturer and engine type) which is the bit that makes the trip computer less accurate. This is to give the trip computer an optimistic reading, making the car appear to be more efficient than reality. Most people will quote their trip computer number as gospel, so this works in a manufacturers favour.

Nahh, my dads BMW is about 7% worse than its actual trip figures, either individual trip or overall figure. You’re right it’s based on an assumption about tyre diameter, based on original specification, but that’s as far as I’d agree. The fuel pump used to fill the tank is MUCH more accurate than a successively sampled, averaged onboard computer. The problem is you never fill the tank to the same place, the pump never stops at the same level and unless you brim it you’ve no idea how much you put in.

At the end of the day, it doesn’t really matter.

Uplink – what car?

It’s on an Auris T180 but it works with others

It something like switching the ignition on whilst hold a button down for a couple of seconds gets you to another menu to adjust it – I’ll dig out the info for you when I get home if you need it

Nah it’s ok – the procedure for my Prius is different and it’s in the manual.

Oh and another thing. Doesn’t petrol expand a lot with ambient temperature? But it’s the mass rather than the volume of fuel that’s important, and we only ever measure volume…

Uplink – what car?

My Prius can recalibrate the speedo for new tyres using the built in GPS (even new ones of the same size) and I think it also adjusts automatically as they wear as well as being able to trigger it when you change.

Now that’s joined up thinking – well done Toyota.

And there is me thinking all the unneccessary fluff has an environmental impact at the manufacturing point, and the complication makes the car less economic to repair in old age and more likely to be scrapped ‘prematurely’. Neither of which is environmentally sound. Might get more mpg between charges if they drop all the complication. Not least the mass of two engines.

And there is me thinking all the unneccessary fluff has an environmental impact at the manufacturing point

And there’s you being quite wrong and completely uninformed 🙂