Fuel Consumption

Consumption Display Correction

It is the ECU software that estimates fuel consumption as it calculates all individual injections (multiple per stroke) anyway. Based on injection quantity demand it will calculate injector drive time, taking conditions like rail pressure and fuel temperature into account. It cannot measure fuel flow directly as this would require expensive equipment – tracking fuel flow + return flow accurately, all temperature compensated.
In reality most drivers notice some trip computer offset vs. actual consumption calculated at gas station fill-up. Usually the displayed value is too optimistic. The offset may depend on driving conditions and fuel quality. Often enough it is roughly constant for an individual driver so it is still useful information.

We offer fuel consumption display correction by patching ECU software.
Simple solution is to derive a constant factor.

E.g. indicated 6.6 litre/100km vs. 7.0 actual → correction factor = 7.0 / 6.6 = 106.1 %.
After modifying and reprogramming ECU ROM, fuel consumption CAN value (ECU output, [mm³/s]) will be adjusted accordingly, to be picked up by combination meter computer. This one calculates all trip values, finally sends display control commands to center display over serial connection.

Injecting complex ECU subroutine f(torque, temp, rpm, …) and/or table data is also possible, requires measurement data in the first place, though.

DPF Active Regeneration

As you may have noticed, active DPF regeneration has a big impact on instant fuel consumption. All injection parts are included in CAN consumption value, therefore post-injections are noticeable in center display when it is set to current consumption mode.
Collected CAN data showed ~300 % actual fuel consumption at idle when active regeneration is in progress, mainly due to post-injections and intake air throttling in order to heat up the DPF, compared to normal idling with regeneration off.

Idling at 800 rpm, coolant temperature ~90°C both times:

Item Unit Normal Active Regeneration
Injection Amount mm³/st 3.3 12 1
Fuel Consumption mm³/s 88 319
cm³/s 0.088 0.319
litre/h 0.32 1.15

1) Calculated from CAN value, SSM2 item “Final Injection Amount” would indicate 4.0 mm³/st (!). Logged data suggests this SSM2 parameter matches CAN consumption value as long as post-injections are not occurring (DPF active regeneration). To get fuel consumption, best way is to parse the CAN fuel flow value instead.


Other Brands

Mazda CX-5 (SkyActiv-D 2.2)

Fuel consumption algorithm and correction works similarly.


13 responses to “Fuel Consumption

  1. Interesting post, like all of your posts. 😀 but how is it possible to patch the ECU software?


    • Not sure what you’re after. We’ve developed our own tools for patching as well as the actual changes. However we won’t produce and sell end user (Windows) software, within near future at least. As for now, we do personal services (using our Linux stuff, the ECU won’t care anyway).


  2. 09 cars have only trip average consumption displayed, later also provide instantaneous. Is this functionality in ECU, instruments, or central display, and can it be added to earlier cars?


  3. why are there no news?
    is this project dead?


  4. Hi Is this site still in use mrdatwn


  5. We need some updates!!!


  6. That’s great new Richard, do you mind if I ask – what type of aircraft? Smaller single-engine or commercial?


  7. Hi, guys. I own a 2010 impreza diesel and was very satisfied with it until now when the fuel economy took a dive. Its consumption went from 6,5 l to over 10l/100 km. I’ve spoken with local service but they have no idea what could be the reason for that kind of car behavior until they do a full set of tests. Could you experts point me into the right direction, where to look? The performance of the car stayed the same and is stil not changed, no oil burning detected either.


    • Hi, never heard of this huge economy drop behaviour before, sorry.
      I would suggest data logging on a test drive, ideally comparing the results with a proper working Subaru diesel in same conditions.
      For example parameter “injection amount” should be around 3 to 5 [mm³/st] during warm idle with electrical consumers and air condition off. Would be interesting if the engine constantly needs more fuel in all or only some operating modes…
      Does real consumption roughly match center display value?


    • Hi ROK, I have a 2011 Forester and the same problem Sudden fuel consumption increase no other symptoms. I suspect the DPF burner is stuck on but service people can’t locate a problem.


  8. Thank you for your reply, stek80.
    I would like to share the rest of the story with you.

    I actually bought a used car. What I didn’t know was, that engine was not behaving properly. There was a big lack if torque under 2000 rpm, so I had to drive the car at high revs, what by itself meant high consumption.

    The problem was solved by a software update on the vehicle ECU. After that, the car’s behavior was totally different. Why such a difference after an update, no one could really explain, not even the service tech guy, who performed the update.

    Perhaps the engine self-learning ability was the reason. The service manual (yeah, I managed to get one…) stated clearly, that misguided engine self-learning could be the reason for improper behavior of engine. I cannot, however surely pinpoint anything. The software update helped, anyway.

    Oh, and about the center display value: it showed 5.8l/100km, when in fact it was 10,5l.


  9. Need to update this blog, DIESEL FTW!!


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