Trying to calculate fuel cost

[v8vroom]

Hi all,

I own a 42L tank in my car but due to inconsistencies of the range, I base my fuel cost on the fuel used during my journey.
My car is a 2000 Jaguar XJ8. The car still reads the fuel used, even if the tank is empty so I am assuming the ECU does not need the fuel tank to calculate what is used.

Here is my question:
The car is assuming petrol is being used, therefore petrol is burnt (in my case, it was 6.8 imperial gallons.) As I was using LPG on this journey, do I add 20% on top due to the efficiency of LPG?

[Gilbertd]

I assume you are looking at the petrol fuel gauge and trip computer? The trip computer calculates how much petrol it uses from the fuel trims. It knows the fuel pressure, it knows the injector pulse duration and the fuel trims plus or minus the norm so from that it can calculate how much fuel you have used. Some cars also use this to drive the fuel gauge, if you started off with a full tank and it has calculated that you have used half of that, it tells the fuel gauge to read half full. This will be reset at some point where it will look at the fuel gauge sender. The reset may be caused by opening the fuel filler flap, removing the cap or after a certain number of engine starts.

You can also use the trip computer to find out how well the LPG system has been calibrated. As said, the mpg figure that it displays will take into account the fuel trims. If the LPG system calibration is spot on, the fuel trims and hence the displayed mpg figure will stay the same as the fuel trims do not alter. If the LPG system calibration is out, the petrol fuel trims will alter to get the mixture correct so will have an effect on the displayed mpg figure. If it shows the mpg is getting better, appearing that you are using less fuel, then the LPG system is set rich which causes the fuel trims to go negative to counteract the richness. If it shows worse, it is set lean.

As for calculating fuel cost, you can work out approximately what the mpg should be on LPG by taking into account the fact that Propane has 80% the energy density of petrol. So if it does 20 mpg on petrol, it will do 80% of that, around 16 mpg, on LPG. You can work out an LPG mpg figure by distance travelled divided by number of litres of LPG you have used, which will give you a miles per litre figure, multiply that by 4.54 to give miles per gallon.

[LPGC]

It depends on how the fuel computer works.

You know it doesn't go on actual fuel used... it doesn't go on measuring petrol level in the tank or physically measure the amount of fuel flowing to the engine.

Some work on the basis of fuel flow can be determined from manifold pressure x rpm with a fiddle factor on the rpm (to account for how the engine takes shallower of deeper breaths according to the cam profile), or airflow from a Maf reading.

Most seem to work on the basis of (in effect) totting up all the petrol injector pulses to find the length of time petrol injectors are being pulsed for during a set length of time. Since petrol injectors will flow a certain amount of fuel if open for a certain time it can work out the flow rate to the engine. But using this method the accuracy can be affected if the injectors flow a different amount to expected (because say the nozzles in the injectors are a bit bigger or smaller than average or fuel pressure is a bit lower/higher than expected). A well tuned LPG system will see petrol injector pulse length the same when running on LPG as when running on petrol... But if the petrol injector pulse lengths increase because the LPG system is calibrated lean this will be reflected in the fuel computer showing more fuel being used than if the LPG system is calibrated correctly or (especially) if the LPG system is calibrated rich. An LPG system is calibrated at many points, it is possible for an LPG system to be calibrated rich at some load points, lean at others and spot on at others.

Assuming the LPG system is calibrated spot on. Then since the correct air fuel ratio for LPG is higher (at 15.4:1) than the correct ratio for petrol (14.7:1) we should be burning less weight of LPG than weight of petrol (about 5% less weight of LPG than weight of petrol). But per litre LPG (0.51kg/L) is lighter than neat petrol (0.74kg/L) by about 35%. Could do a rough estimation of the 35% - the 5% to get 30% more LPG than the fuel computer shows. But it's not as straight forward as that because weight for weight (not litres for litres or volume for volume) LPG has a higher calorific value than petrol by around 10%, which brings the 30% down to around 20%. So we could use ballpark of 20% but it won't be as straight-forward as that because the result of calculations depends on which order we do the calculations in (and I haven't properly got my thinking cap on today... head is on other matters lol)... what I'm getting at is for example if we start with 100 and add 10% we get 110 but if we start with 110 and subtract 10% we get 99, or if we start with 100 and subtract 10% we get 90 but if we start with 90 and and 10% we get 99. And still not as straight-forward as that because there can be losses and gains in other aspects of engine efficiency when we swap fuels... such as losses associated with manifold vacuum / pumping losses (which are likely to be lower on LPG than on petrol) and the more complete burn of fuel on LPG can help with efficiency.

Often been said that one way you can tell if an LPG system is calibrated well is if you're running on LPG keep your foot in the same place on the throttle the instant fuel economy figure shouldn't change.. There is quite a lot of truth in that but that still doesn't necessarily mean that if you were accelerating/decelerating/holding a constant speed with the constant throttle position and unchanging road and environment conditions (temperature, humidity, rain, wind, etc) the car will keep accelerating/decelerating/hold the same speed when you switch fuels, because with constant throttle opening the torque from the engine may change. Some vehicles with fly-by-wire throttles will notice the torque change (or maf flow rate or manifold pressure) and notice that the driver didn't intend on changing power output (because they didn't move the throttle) and self compensate by adjusting the fly-by-wire throttle to keep torque output the same even though the driver didn't compensate by moving the throttle.

Edit - there is another factor too. Since for a given airflow the engine will make very slightly less torque on LPG than on petrol, the driver may need to (probably subconciously because the difference is so subtle) increase throttle position slightly to maintain the same speed. The increased throttle position will mean petrol injector pulse length is also increased and that will mean the fuel computer will read slightly less mpg on LPG than on petrol anyway... So the 20% (roughly calculated above) that the mpg reading might over-read when running on LPG compared to correct mpg reading when running on petrol (assuming correct on petrol) can become less than 20% over-read on LPG. Might expect somewhere between 0 and 20%, so maybe around 10% over-reading mpg on the fuel computer when running on LPG. All assuming the fuel computer works as per 'totting up pinj' divided by distance travelled and some/all of the other assumptions mentioned above.

Quite often find that engines need less air to idle running on LPG than when running on petrol. This could be counter-intuitive because the difference in stoch ratios 15:4 vs 14.7 should mean the engine should need more air to maintain same idle speed when running on LPG than when running on petrol (might expect airflow to increase slightly not decrease). I've done the sums in the past and totting everything up (so including allowing for stoch ratios and calorific values, etc) the lower air use does point to a real efficiency gain when idling (and probably at low engine loads) when running on LPG versus petrol. It would be more difficult to ensure other variables such as the car slowing down or speeding up very slightly were not skewing the results if we tried to do similar comparisons when actually driving, at idle the underlying conditions don't really change so it's easier to do this kind of comparison.

Post crossed with Gilbert's... I've stripped the plenum off an engine since I started typing this, type during fag breaks lol.