Category Archives: Math

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 amount 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 [liter/100km] vs. 7.0 actual → correction factor = 7.0 / 6.6 = 106.1 %.
After modifying + reprogramming ECU ROM, fuel consumption CAN value [mm³/s] (ECU output) 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
liter/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.

Formulas