Maxidot display

[Tech-Yeti]

The electronics reduce the rail pressure during overun, it's called "de-energizing" so more fuel is returned to the tank, it does not cut off the fuel altogether.

[Llanigraham]

Sandy, why does it need fuel if there is no power being produced?

All that is happening is that the pistons are going up and down because they are being "driven" by the wheels

[Sandy.Rogers]

Dunno G (ask me one on computers)

I thought that the engine would be on 'tick over' as it does when the vehicle halts at traffic lights etc?

PS I would use quote but it doesn't appear to be working on this laptop at the moment!

[weasley]

When you are going downhill with your foot off the throttle, it is gravity and momentum than are pushing you along - there is no need for the engine to do anything, so in this age of fuel economy and emissions, the injectors stop injecting and no fuel goes into the combustion chamber. The engine is still turning because the car is moving forwards, turning the wheels which turn the gearbox which turns the engine, but all the engine is doing is sucking, squeezing and blowing air. (There is a myth that the compression stroke is what causes engine braking, but this is (mostly) not true - happy to have this debate in another thread - it always causes some strong defence of both sides of the argument!)

Anyway, since the injectors are not injecting, the fuel that is pumped to the rail just gets recirculated back to the tank (actually this is happening to some extent all the time, even when you are using fuel in the engine). The ECU monitors road speed or throttle position; if you push the throttle you obviously need to accelerate so need some fuel so the ECU switches the injectors back on and you're off. If you are coasting to a stop, when the engine speed reaches just above idle the ECU will fire up the injectors again so that the engine won't stall when you de-clutch and roll to a stop. In fact if you de-clutch when rolling down a hill and 'coast' the ECU will also light up the injectors to keep the engine idling - this is why it is more efficient to coast downhill in gear with no throttle than to de-clutch and let the engine idle (amongst other reasons why this is a better technique).

So, back to the Maxidot. I'm pretty sure that the average mpg reading only takes account of driving time, so it ignores idling data; it therefore only gives you a driving mpg figure, not a figure for overall fuel use. This is part of the reason why the Maxidot mpg can differ from measured (tank-brim to tank-brim) mpg and maybe why it shows "----" since these data are not being recorded.

Edited 7 May, 2012 by weasley

[threeshoes]

So, back to the Maxidot. I'm pretty sure that the average mpg reading only takes account of driving time, so it ignores idling data; it therefore only gives you a driving mpg figure, not a figure for overall fuel use. This is part of the reason why the Maxidot mpg can differ from measured (tank-brim to tank-brim) mpg and maybe why it shows "----" since these data are not being recorded.

I think you'll find that it gives you overall mpg, including time spent stationary. If you do as I do and switch off the engine at traffic lights you will see an improvement in the calculated average mpg (I don't think that it's just my imagination . . .)

Chris

[Sandy.Rogers]

Cheers Weasley - very informative, you learn something every day

[Transfesa]

I think you'll find that it gives you overall mpg, including time spent stationary. If you do as I do and switch off the engine at traffic lights you will see an improvement in the calculated average mpg (I don't think that it's just my imagination . . .)

Chris

Hi all,

We don't have MPG here in Spain, but we have L/100Km instead. I've noticed that when going downhills or coasting, it shows 0.0 L/100Km but when stopped at iddle (or going very very slowly) the measurement unit changes to litres per hour (L/h). While idling it shows about 0.6-0.7 L/h in the instant consumption. It's a clever idea, since while you are stopped consumption expressed in L/100Km should be infinite. You can check the total (or partial) consumption in L/100Km keeps growing slowly in this scenario, depending on the distance travelled prior to stopping.

I have also noticed with my DSG that, when braking to a full stop, if you raise the brake pedal just before stopping you can note the engine pushing again as it starts again the ignitionto keep the engine idling and prevent stall.

Kind regards.

[EDIT]: I've noticed Maxidot actually shows litres per hour while idling or running really very slowly in the instant consumption.

Edited 8 May, 2012 by Transfesa

[weasley]

I think you'll find that it gives you overall mpg, including time spent stationary. If you do as I do and switch off the engine at traffic lights you will see an improvement in the calculated average mpg (I don't think that it's just my imagination . . .)

Chris

I'll admit this was an educated guess; my last BMW had this explained in detail in the handbook and I'd be surprised if BMW were alone in this approach, which will, after all, report a higher average mpg (thus make the driver more happy).

[threeshoes]

I'll admit this was an educated guess; my last BMW had this explained in detail in the handbook and I'd be surprised if BMW were alone in this approach, which will, after all, report a higher average mpg (thus make the driver more happy).

Well, I had a 320d Touring (E46) from new in 2003 and was continually disappointed with the difference between the computer mpg and my calculations based on what I put into the tank — the computer was about 5% optimistic. But my current E91 is more realistic and pretty accurate and I've decided that I prefer it that way

Chris