How does car's fuel economy indicator work?

[Racebred]

Does anyone know how the fuel economy indicators in your cars work? I have a specific question with respect to this: Why is it that when you throttle-off, and are in a low gear, cruising to almost a halt, the economy shoots up to max? It runs counter-intuitive, as when the throttle is off, the fuel usage is minimal?

In older cars, say 20 years back, I remembered observing a mercedes' needle. Whenever it rolls to almost a halt, the needle shoots up to max, and when the car is idling, the needle stays up. I believe this was how all the old-generation indicators work.

Then about 10 years back, I noticed in my E46 manual that specifically stated that "the needle will come back to rest at 0 when the car is stationary". It's like a "new" feature built in (which is why they have to specifically trumpet the new feature) so that they can over-ride the phenomenon of the needle staying at max when the car is stopped, because it runs counter to logic. However as the car slows down, I still notice the needle moving upwards until the "new feature" cuts in and brings the needle back to rest.

Now, in the new genertion cars, the needles may not behave like that anymore, but in my digital readout, I notice that the numbers are higher when I throttle off cruising in neutral. And then when I give it a slight throttle, the number comes back down. Same phenomenon.

So, qn is, does the car use more fuel when you throttle off? Shouldnt be as the engine loading is really low, right? Is this actually a quirk of how they calculate fuel consumption? Where on normal operations the numbers are fairly accurate, up till the moment you throttle off and slow down. any ideas?

 

[phil]

Re: How do car's fuel economy indicator work?

it works off vacum. so at idle vac is less, so fuel consumption is higher. With higher vac, it is more efficient. Something like that anyway

 

[Breyton]

Re: How do car's fuel economy indicator work?

Maybe cos they measure by 'fuel consumed per unit distance' (XX litre / 100km)? This would result in 'higher' consumption as you slow to a stop, since fuel is still being consumed even though the car is travelling less distance or stationary. i.e. when the car is stationary, fuel used divided by zero distance = infinity....

If they switch it the other way round and measure distance travelled per unit fuel (XX km / litre), then it would drop to zero when the car is not moving.

I may be waaaay off base here.....

 

[R2D]

Re: How does car's fuel economy indicator work?

Hi RB..

the fuel consumption calculator works by an algorithm that uses the fuel injector pulse width, vehicle speed and RPM into consideration. the older Merc's use a vacuum gauge to measure manifold vacuum.

If you have a L/100km calculation, travelling at 90km/h at a particular gear, the algorithm is able to show you some approximation of instantenous fuel consumption as it knows how much fuel it is squirting. When you coast to a stop, the needle goes higher as the vehicle speed drops and the gearbox also drops gears to get ready for acceleration. At a full stop, the OBC cuts the display so as not to show an obscene figure. if you reset your OBC at a top up, the BMW OBC average consumption goes up to 39.5L/100km before slowly going down.

cheers

 

[TripleM]

Re: How does car's fuel economy indicator work?

Why is it that when you throttle-off, and are in a low gear, cruising to almost a halt, the economy shoots up to max? It runs counter-intuitive, as when the throttle is off, the fuel usage is minimal?

That is what it means lor, when u r throttle off, the fuel economy at highest coz no throttle input. Wot u smokin?

 

[Racebred]

Re: How does car's fuel economy indicator work?

Hi RB..

the fuel consumption calculator works by an algorithm that uses the fuel injector pulse width, vehicle speed and RPM into consideration. the older Merc's use a vacuum gauge to measure manifold vacuum.

If you have a L/100km calculation, travelling at 90km/h at a particular gear, the algorithm is able to show you some approximation of instantenous fuel consumption as it knows how much fuel it is squirting. When you coast to a stop, the needle goes higher as the vehicle speed drops and the gearbox also drops gears to get ready for acceleration. At a full stop, the OBC cuts the display so as not to show an obscene figure. if you reset your OBC at a top up, the BMW OBC average consumption goes up to 39.5L/100km before slowly going down.

cheers

thanks for the detailed explanation. appreciate it.
However, I tried this experiment. Reset the consumption indicator, put on neutral and cruise. So fuel injectors should be at idling rev squirts, which is not very high. The cruising is for a long distance, with speeds coming down from say 90-60, so not exactly crawling speeds yet. Immediately the average consumption becomes very bad. I engage gear while cruising, and gently step on it to bring the speed up to constant 90. Then I noticed the average consumption improve immediately.

Qn is, is this a quirk of consumption calculation? I tend to think the actual fuel used is truly lesser while cruising in neutral, whereas the indicator shows otherwise.

Coz if the indicator is to be believed, does this mean we actually use up more fuel crusing at neutral than if we keep a constant speed, constant throttle?

 

[Racebred]

Re: How does car's fuel economy indicator work?

That is what it means lor, when u r throttle off, the fuel economy at highest coz no throttle input. Wot u smokin?

nb..... ok what i meant is "economy indicator". smart la.

 

[SHAMELESS]

Re: How does car's fuel economy indicator work?

phil is right for once.....R2D also damn smart hee

 

[R2D]

Re: How does car's fuel economy indicator work?

Well, when you are at idle and coasting, your manifold holds less vacuum compared to if you are in gear and coasting down. If you are in over-run (i.e. in gear and decelerating), the ECU usually cuts fuel off to a really small trickle or cut it off entirely as it uses the momentum to drive the engine. this feature reduces consumption. Only when revs drop to 1000r-900pm or so does the fuel come back on again to keep it going. So actually, given the conditions, you do save fuel if you coast in neutral compared to in gear. This occurs as you actually coast further given the same starting velocity. The engine braking causes your overall coasting distance to be shorter so you got to spend fuel covering the difference in distance.

As for the Qn "should every body coast in neutral?" I would think not. The reason is that your brakes are not quite as effective comapred to in-gear coasting which develops higher vacuums and thus better braking assistance. Also, if for whatever reason you need to accelerate to avoid a nasty situation, you reaction would be faster. The last reason is less wear on your drive train with sudden bursts of rotational acceleration.

cheers