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Comment by roman_savchuk | Hacker News Reader

When I was reverse engineering car ECUs, one of the interesting little projects was finding fuel consumption display calibration data and calculation algorithm, on 2008+ Subarus. And then I did some measurements trying to get accurate real world consumption, and "sync" the dashboard with it. And while there was ~3% error IIRC, I think it has more to do with getting the most accurate average consumption over the car's lifetime, than fooling the customer on purpose. The calculation is based on the effective injector pulsewidth (ECU can't count actual fuel flow))), and as injector performance deteriorates over time, you get less and less optimistic values with more miles.

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> ECU can't count actual fuel flow

Thanks, this is pretty interesting.

It's the first non-bullshit explanation I've seen for why cars tend to display inaccurate average MPG on the dash. I always assumed there was a fully accurate measure of fuel flow into the engine and there was something else broken (or dishonest) about that calculation.

This has come up in the press occasionally and the manufacturers never admit that they don't have a totally accurate measure of flow: https://www.edmunds.com/fuel-economy/your-fuel-economy-gauge...

> ECU can't count actual fuel flow

That doesn't seem right. Flow sensors exist and could be fitted to the fuel line to the engine for a precise measurement of how much fuel is consumed.

So I'm assuming manufacturers just estimate fuel flow in the ECU because a flow sensor would be additional cost for negligible benefit?

> because a flow sensor would be additional cost for negligible benefit?

Exactly. It's basically unnecessary given the other sources of information that the ECU has at its disposal.

It knows the target fuel pressure (some cars even have fuel pressure sensors) and it knows the flow characteristics of the injectors with respect to pressure and pulse width. That alone is enough for a decent calculation of fuel mass flow.

In addition to this, the ECU is also calculating the mass air flow into the engine, either directly via a MAF sensor or indirectly via manifold pressure, VE, and RPM (known speed density).

When it goes to target a specific air/fuel ratio, this is all the information that is really required. However, since injectors tend to deteriorate (clog, stick, etc.) over time, this initial calculation is usually “trimmed” by means of feedback from the oxygen sensor(s).

At the end of the day, there’s a fair amount of information available to the ECU which can be used to refine the fuel flow calculation.

The fuel consumption calculation on my 335i is usually within 0.5 MPG. However, my 2002 Maxima was about 3 MPG off.

Never saw a separate fuel flow sensor in an automotive application, but fuel injector is technically an orifice flow meter in this calculation method (pressure differential is constant and you calculate how much fuel is flown through orifice by measuring time it is open). Sure, when orifice size is reduced by lacquer depositions or dirt it will cause measurement error. But it will also cause increasing fuel trims (to the point when CEL goes on) and bad driving performance when fuel trims don't apply

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