VW Polo speed limiter

John.G

Above has a speed limiter, I notice that the engine still injects fuel when coasting or going down hill with foot removed from the accelerator, when not engaged the fuel is cut off at anything above ~ 1200/1500 RPM.

Does cruise control work in a similar manner? and is this some form of safety measure when either of these modes are engaged?.

colm_mcm

They have a thing where you can’t rev them too high in neutral. That’s all.

Cee-Jay-Cee

You're going to have to explain better what your trying to say as it doesn't make much sense.

How do you know fuel is cut above 1200/1500rpm?

When you are coasting down hill with your foot off the accelerator, the gearbox controls the revs of the engine. If you put the car in neutral and coast down hill, yes the revs usually sit a little higher than at idle speed which I think is to ensure that the engine doesn't cut out and leave you in a position of having no brake assist or acceleration if you need it.

I don't understand what you mean by speed limiter as nothing you are referring to has anything to do with speed.

John.G

I know the fuel is cut off because the fuel consumption displayed number goes to zero, the same as it has done in all the fuel injected cars that I have owned for the past twenty years or so,

Coasting is probably not the best choice of words as I have never put a car in neutral while the car is in motion, what I mean is that if I remove my leg from the accelerator with the car in motion and if the engine revs are ~ > 1200/1500 RPM then the fuel is cut off, certainly in top gear, if I then depress the accelerator fuel injection is again resumed or if the engine revs drop below the above level.

The speed limiter is exactly as I described, you just press a button on the steering column and the max speed is then limited to the speed you engage it at, you can then incrementally increase/decrease the speed setting with another button, if you are going downhill it flags a beeping alarm if you exceed the set limit by a few kms/hr. As I said while in speed limiting mode the fuel injection is not interrupted and just wondered is this some safety feature and ditto with cruise control as I have never owned a car with cruise control.

garv123

If the fuel is cut off, whats keeping the engine running.. think about it..

Toyotafanboi

garv123 wrote: »

If the fuel is cut off, whats keeping the engine running.. think about it..

If it's in gear, momentum will keep it going.

Del2005

John.G wrote: »

I know the fuel is cut off because the fuel consumption displayed number goes to zero, the same as it has done in all the fuel injected cars that I have owned for the past twenty years or so,

Coasting is probably not the best choice of words as I have never put a car in neutral while the car is in motion, what I mean is that if I remove my leg from the accelerator with the car in motion and if the engine revs are ~ > 1200/1500 RPM then the fuel is cut off, certainly in top gear, if I then depress the accelerator fuel injection is again resumed or if the engine revs drop below the above level.

The speed limiter is exactly as I described, you just press a button on the steering column and the max speed is then limited to the speed you engage it at, you can then incrementally increase/decrease the speed setting with another button, if you are going downhill it flags a beeping alarm if you exceed the set limit by a few kms/hr. As I said while in speed limiting mode the fuel injection is not interrupted and just wondered is this some safety feature and ditto with cruise control as I have never owned a car with cruise control.

Cruise control and speed limiters are totally different. Cruise control you set a speed and the car will maintain that +/- a few km/h. Speed limiter doesn't maintain anything it'll just warn you when you reach the set speed and slow down the acceleration, you still need to use the throttle, and if you keep going it'll keep accelerating just slower than usual.

All fuel injected cars on overrun will stop fuel going to the engine. What you are doing is confusing the speed limiter slow down/warning while you are telling the car to go faster with a car in overrun, it won't stop pumping fuel if you are telling it to speed up. On cruise control if the car goes too fast it'll stop the fuel as it's in overrun, with speed limiters it won't stop the fuel as you are telling it to go faster.

Leonard Hofstadter

John.G wrote: »

I know the fuel is cut off because the fuel consumption displayed number goes to zero, the same as it has done in all the fuel injected cars that I have owned for the past twenty years or so,

Coasting is probably not the best choice of words as I have never put a car in neutral while the car is in motion, what I mean is that if I remove my leg from the accelerator with the car in motion and if the engine revs are ~ > 1200/1500 RPM then the fuel is cut off, certainly in top gear, if I then depress the accelerator fuel injection is again resumed or if the engine revs drop below the above level.

That's what's happens in pretty much every car ever since multi-point fuel injection became a thing back in the 80s.

John.G

Leonard Hofstadter wrote: »

That's what's happens in pretty much every car ever since multi-point fuel injection became a thing back in the 80s.

Yes, I am quite aware of that, hence my query as to why the the fuel injection does NOT SEEM to be interrupted in speed limiting mode with the accelerator not pressed, it works exactly as expected in normal mode ie if accelerator not pressed and RPM > 1200/1500, then the fuel consumption number indicates 0 l/100kms and as soon as the accelerator is again depressed then fuel injection recommences.

kirving

John.G wrote: »

The speed limiter is exactly as I described, you just press a button on the steering column and the max speed is then limited to the speed you engage it at, you can then incrementally increase/decrease the speed setting with another button, if you are going downhill it flags a beeping alarm if you exceed the set limit by a few kms/hr.

It seems as though the system is a Speed Limiter by name, but not by nature. I mean I understand how it works, by limiting the throttle input when above a certain speed, but not actually braking the car by itself.

By convention, they probably all work like that, but a true "limiter" would brake the car downhill. I think the name is confusing things a bit.

John.G wrote: »

As I said while in speed limiting mode the fuel injection is not interrupted and just wondered is this some safety feature and ditto with cruise control as I have never owned a car with cruise control.

Same behaviour on Cruise Control too?

Is the car manual or Auto? Petrol or Diesel? Each will work quite differently.

It may depend on a number of things really.


To be perfectly honest, not all of the below may be strictly correct, but some ideas:
Limit manifold vacuum which would slow the car down more quickly, so actually puts in fuel to keep some turbo pressure.

Keep some heat in the engine, as it's still warming up, or you have the heater on. Pumping air will cool it quickly.

Can the car, or certain models of the same car brake itself? Under Speed Limiter or Cruise Control, the care may be keeping itself in a "prepared" state.
to apply the brakes at any moment. It may need instant power (electrical or vacuum), so it doesn't want to allow the turbo to slow as it may need to be spooled quickly.

It may see the overrun (going downhill) as an efficient time to charge the battery a bit.

John.G

Its petrol.

No, it doesn't brake the car which I find perfectly acceptable as it bleeps a alarm, I don't have cruise control but I wouldn't expect this to brake the car either which I would be pretty sure is the case.

It acts as a poor mans cruise control in that if you keep the accelerator depressed then it will maintain a steady (maximum) set speed and you can see the fuel consumption changing with the road conditions.

Overall, it does exactly what it says on the tin.

Cee-Jay-Cee

Just because the fuel economy gauge drops to zero doesn't mean that no fuel is being used, fuel is still being injected into the engine, however it is a very low amount and the computer just zeros out to show that. The injectors however do not stop working at any stage while the engine is switched on and running despite what the gauge/computer says.