How do speed limiters work ??

paulgolding

Torque Junkie
Points
67
Location
Thanet
Car
vw t4 caravelle 2.5
I have a speed limiter on the new car . . So, basically I set a speed and it won't let me go any faster. If i put my foot down it will only accelerate to that speed.

Does it limit the petrol being used ??

I put up the instant MPG computer. Surely with my foot to the floor the mpg would be dreadful, but instead of that when it reached the speed set the mpg was really good . . as if it was only using the exact amount of fuel needed.

If this is so then surely by setting it to 56mph and driving down the motorway, fuel consumption will be at it's best ??
 
You'll find that the accelerator pedal simply sends pedal position signals to the ECU. My car is the same - no throttle cable as such. It's really a variant on cruise control except that it simply will not allow any more air/fuel in than necessary to maintain the preset speed. You can accelerate up to that speed as hard as you like in any gear.

It will not in any way affect your fuel economy compared to normal driving. Even if you drive foot to the floor with the limiter set at 30mph!!

Some systems do have a kickdown override.

Personally I don't like speed limiters, preferring to use cruise control if I want to regulate my speed.
 
56 is not the best speed. it was the one chosen by the society of european motor manufacturers years ago as a benchmark. Back then with not catalysts to poison or burn, the manufacturers leaned off the mixture ridiculously to coincide with the exact rpm that occured at 56 in fifth gear (few 6ths back then).

That is a very old benchmark now, and the optimum speed varies widely between car given different gearing, torque curves etc.

The euro combined figure is now the accepted std. As an objective number, forget it. You'll never equal the official figures unless you drive the official journey.

Treat them as indices. ie. car A from manufacturer A says 36mpg combined.

car B from manufacturer B does 41.

All you can conclude is the B will use less fuel than A if driven on similar journeys.

Which, to be honest is a much fairer way to test.

WRT to speed limiters you might as well use the cruise control for that exercise and give your right foot a break. Although my extensive epxerience with cruise control is that unless you have one of the very sophisticated adaptive systems (and a good one of those at that) it's not that much use in the UK really. I do use mine (which is a slick but dumb speed maintaining system) but not all that often to be honest.
 
re: 56mph . . . really ?? never new that. Thanks for info. Yeah, agree, might as well use cruise control. I'm just playing with new toys at the mo !! lol

The cruise control on this is better than the bm. The bm was ok, but if you set it for say 70mph and then hit a steep hill it would slow slightly, then realise and accelerate to compensate. With the pug, it stays exactly at the speed you set it at, accelerating perfectly to keep the exact speed. Quite impressive.

I agree though, it can be annoying in this country. You plod along and all of a sudden come up to some slow person in fast lane and have to brake . . or you cruise past someone and just as you get past they speed up slightly. . Aaarrggh !!
 
Pug cruise controls are very well integrated if mine's anything to go by. Occasionally it can overshoot if you accelerate hard with the switch/lever and then let go. I put this down to the last few 'ounces' of turbo boost to be used up and vented. But it's only by a few mph and it does settle imediately afterwards. Your setup may be a bit more forgiving being a newer generation ECU.
 
You know you've arrived when your 11 year old son pleads with you to slow down.

Makes a difference from the more usual "go faster Dad, go on, boot it."

:)
 
The cruise control in my BMW works brilliant, and its 20 year old!!! I use it all the time, it doesn't overshoot, nor hesistate on hills. It also saves me loads on petrol, which I put down to it being drive by wire. Driving with cruise I get 16 mpg on the way to work, 11 driving at the same speed manually. Its back roads. I can see 30 on the motorway with cruise. My speed warning beeps and shows messages-till I hit 150ish when it cuts the ignition or fuelling or something. Basically your face bounces off the steering wheel, pretty much the same as hitting the rev limit in a lower gear.
 
lol, have to admit ive never tried cruise control at that speed, and come to think of it ive never had the speed warning thing turned on with cruise either. cruise does funny things, like on a ford mondeo it deactiveates the traction control when cruise is enabled. thats weird:amazed:
 
Speed Limiters,

with OBD-2 Control usually now are available in three different flavours;

1]Spark = does as advertised, curbs ignition spark advance at a pre-determined setting or parameter. Which feels like much like the ETC restriction in operation.

2] Fuel
Cut-Out, at a set speed or rpm. This causes the injectors to fire every second pulse instead of every pulse usually, and can be set 'soft' or 'hard.' in the PCM/ECM depending on factory settings.

Soft for example is when the RPM fuel cut-off limit is at say 6,500 and the fuel re-enable is at 6,499rpm. This allows the car to run against the limiter quite gently.

Where as Hard would be if the RPM fuel cut-off is at 6,500 and the re-enable is set at 6,450 or even 6,400 the vehicle starts jerking at the limiter which definately wakes you up and doesn't do much good to the drive-train as it's being banged quite violently against drive and over-run.

3]ETC speed/rpm limiter.
The ETC's are now mostly combined with Cruise-Control and Traction Control (along with taking over the job of idle control and thus making the IAC or stepper motor redundant). This is the third option for limiting speed and is in more and more widespread use.
Again they can be set to control either rpm or speed.
 
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