Rev limiter

[Graham]

Hi all,

Is there a way of introducing a rev limiter?

For example, when I am towing at say 60 mph, my revs are generally around 1800 rpm.
However when I approach a long steep hill, pulling a heavy caravan, the auto-box, kicks down, and the engine revs up.

Now I really don't want the engine to rev up to say 3800 rpm, just to maintain my 60 mph cruise.
I would be quite happy if the rev limiter could be set to say, 2800 rpm, then of course, let the speed drop away accordingly, until the top of the hill.

I would rather let the speed drop away to say 45 mph, on the inclines, than have the engine revving it's bollocks off at 3800 and keeping the 60 mph set cruise speed.

I know I could simply reduce the cruise speed to say 45 mph, but that's a fiddle, and I would rather set it at say, 60 mph, and leave it.

Any ideas ???

Gra

 

[Doodle]

In a word, not simply. You'll likely be looking at reprogramming ECU's to add new functionality - the cruise control is intended to hold a set speed regardless and you're trying to override it.

Trying to set a rev limit outside of the native electronics will probably be less than successful, and just keep kicking out errors as the two systems fight each other.

 

[Shayne]

I couldn't say for certain Gra but i think you could have your ECU re-mapped to limit revs . I believe most modern vehicles have revs limited out of the factory and its a common modification for boy racers to UP the rev limit so i don't see why the opposite can't be achieved .

Following what Doodle says (we posted at same time) perhaps this will be of interest ?
http://www.frsport.com/Bee-R-RevLimiter-Launch-Control-Universal-S13-S14-S15-AE86_p_15270.html

 

[AndyCook]

surely its good to keep revs higher on a hill when towing?
keeps the engine fan turned faster and waterpump turning faster to pump coolant round and cool the engine more

 

[Doodle]

I don't think that unit will work - it is designed to cut the spark rather than the fuel supply (because cutting fuel on tuned turbo motors is not a good idea). I guess you could modify it to cut the fuel pump but I'm not sure if that would be a good idea either under the circumstances.

Explaining this to the insurance company might be entertaining too.

 

[Shayne]

I'm out of my depth here but as the issue seems to be with towing rather than just general driving would i perhaps spark some ideas from those in the know if i mentioned a torque limiter ?

 

[Beau]

Just because the engine is revving a lot doesn't mean it'll destroy the engine... There is actually more strain on the engine when in a taller gear going up a hill than if it was in a gear bellow revving more. On the plus side, the water pump is spinning faster which cools the engine down more.

I wouldn't be so quick to mess around with the rev limiter, it's set there for a reason, best thing to do when approaching a hill is to knock the Overdrive off and therefore you can control the revs a lot easier than planting your food down to maintain speed. It all comes down to driving style I reckon

 

[fast but dim]

Just because the engine is revving a lot doesn't mean it'll destroy the engine... There is actually more strain on the engine when in a taller gear going up a hill than if it was in a gear bellow revving more. On the plus side, the water pump is spinning faster which cools the engine down more.

Good point, when I'm towing I try to keep the revs around 2000 rpm for hills ( manual 120 ).... My lorry driving grandad used to say labouring an engine is as bad as over revving.

 

[Graham]

Many points taken in, thanks.

However, when I start a long hill climb, I often have the auto-box doing what is supposed to do, and the cruise control also doing what it is supposed to do, ie, maintain my set cruise control.
However, on hills, I am quite happy to forgo the set cruising speed, and allow the speed to drop, to whatever speed the engine, (at 2600 or 2800 revs) will pull the combination along at.

I am really looking for an easy way to prevent the electronics in the cruise control from trying to maintain a set 60 mph, up hills, where the engine would rev at 3600 rpm, I would be happy to allow the speed to drop back to, for example 45 mph, and not ask the engine to rev its balls off at 3800.
For me, 2600 or 2800 would suffice, for the duration of the hill climb.

I guess it may be to optimistic to "drive by revs control " rather than "drive by speed control"

So I could set my "revs control" to say 1800 rpm, and the engine would hold “1800” for example, and then trundle along at whatever speed the road incline / decline permits.

Gra

 

[Doodle]

Effectively you want a engine speed governor. Relatively common on old diesels, but no easy way of incorporating that into a modern electronically-controlled one that I can see without re-hacking the ECU.

 

[Graham]

That's the thing doodle.

If there was a way of looking into the ECU, I guess one might be able to find the engine speed input.
This will no doubt be a voltage input from a crankshaft sensor.

I guess the higher the the crankshaft speed, then the higher the voltage.
If there was some way of externally adjusting this set point, perhaps by way of a potentiometer, then this might achieve my goal?

Gra

 

[Shayne]

Is it possible your clutch is on the way out and the revs increase because its slipping ?

 

[Ian Rubie]

Does a 120 have a traditional throttle cable or is it electronic from the pedal? If it has a throttle cable, can you rig up an old fashioned choke knob to control engine speed the same as 80s have?

Ian

 

[silvercruiser]

I reckon it will be easier to sell and get the amazon Graham

Il get my coat

 

[Graham]

I reckon it will be easier to sell and get the amazon Graham

Il get my coat

.
.
Thats next.
My 120 is a manual, so no clutch to speak about in the traditional manner.


Next is Amazon 2003 face lift.

But for now, I can't part with this 120 series.
It owes me nothing, and runs extreemly well,

Gra

 

[Doodle]

That's the thing doodle.

If there was a way of looking into the ECU, I guess one might be able to find the engine speed input.
This will no doubt be a voltage input from a crankshaft sensor.

I guess the higher the the crankshaft speed, then the higher the voltage.
If there was some way of externally adjusting this set point, perhaps by way of a potentiometer, then this might achieve my goal?

Gra

Getting into and finding the values will be the easy bit (comparatively).

The hard bit will be working out a way of translating the value you set into a holding point for the engine speed, what other bits of the electronics system are affected by the values you're tinkering with (and there *will* be some), how they are affected and how to stop them throwing a fit and/or shutting things down because you're asking the system to behave in a way it isn't designed to.#

To give you some idea, it took the guy who remapped my Ducati several months to work out how to break into the ECU and modify it, and that was just to change the fuelling tables which is a pretty easy job as you're not trying to make the system do something it isn't meant to.

 

[Shayne]

Well my 90's a manual and it sure took some high revs to get me to the top of the hill when the clutch gave up the ghost while towing a good 3 tonne .

 

[cir287]

Not sure what way the auto box is controlled on the 120 but it might be an easier option to figure out an over-ride to get it to hold a gear rather than down change automatically.
Dont think any of this will be easy as you are probably trying to reinvent the wheel. as in, no one has probably done it before.