Over-revving a diesel engine?

What happens if a diesel engine is over-revved? Particularly a 2.5TDI

5-cylinder Audi one.

I know with petrols you get all sorts of crazy valve bounce and all that - is it the same with diesels? And if anything bad were to happen, would you notice it straight away, or within the next 50-60 miles or so?

Also, a question about limiters on diesel engines, like the 5-pot Audi

2.5TDI one - is it feasible that the limiter might be set to an amount of revs slightly higher than the beginning of the red zone on the rev-counter?

The reason I ask is that I put my foot down on a slip-road in 3rd, and changed up a bit late - it might have been in the process of hitting the limiter. but it was definitely 300-400rpm into the red zone. It still runs and performs absolutely fine, but I'm a tincy bit worried.

Peter

Reply to
AstraVanMan
Loading thread data ...

rev-counter?

The rev limiter is almost certainly set higher than where the red section starts, probably where it ends :-) There'd be no point in having the red zone if you could never get to it (well not unless changine down gear.. ;-) You will notice if you hit the limiter, it will sound funny :-)

Reply to
scott

rev-counter?

Don't get paranoid, as said elsewhere in this thread, it'll be fine. My (albeit limited) experience of diesels is that they just stop revving any higher when you get to their governed limit. As tested in Peugeot 205 1.7 Diesel, 306Dturbo and 306HDi, Mercedes benz 108D, 310D and 311CDi.

On a lighter note, WOT at max rpm in 3rd in my works vans is ~50mph. One can allegedly carry on this way for half a mile or even longer with no apparent ill effects. In fact you should, you're supposed to.

Douglas

Reply to
Douglas Payne

You cant "really" overrev a diesel because its diesel pump pumps only the fuel needed for a specific RPM. Its how the pump works.

If you put your foot down halfway it revs at FULL power up to say half revs and goes no faster. Same happens at full throttle, the fuel will be cut off at the highest RPM the pump was calibrated to see.

Unless you are a bit "spirited" and knock it down into *too* low a gear entering a bend/braking???

Anyway all that happens is the same as a petrol engine. It buzzes a bit and hopefully off you go again! If you bent any valves or anything you would be able to hear something at tickover at least.

Reply to
Burgerman

Wide Open Throttle (c:

Audi weighs considerably less and has considerably better aerodynamics I suppose.

formatting link
Heh, if we're going to get all like that on each other, my mother's 306 HDi does an indicated 80 in 3rd (heh, hope she hasn't discovered usenet). The Audi probably does more than 70, you're just too much of a girl (c:

I think my Carlton 2.6i petrol Auto does about 110 in 3rd

Douglas

Reply to
Douglas Payne

Ah, that's ok.

Nope, didn't do that, I was accelerating on a sliproad (joining the A40 westbound from Swakeleys roundabout near Uxbridge if anyone knows it, or cares for that matter!) and I was paying a bit too much attention to how much smoke I was chucking out the back rather than looking at the dials and out the front. I think I helped clear it out a bit :-). It was just one of those "shiiiiiit, I should really have changed up by now" moments.

Talking of tickover, and this is completely unrelated, but the other day I noticed a slight (and by slight, I do mean very very slight) variation in the tickover revs. IIRC the engine was properly warmed up. Any idea what could cause this - a sensor that may need replacing perhaps? Like I said, it's only very very slight, barely noticeable in fact, so probably not worth worrying about. Don't know if it could be due to the fact that it's long long overdue an oil and filter change (7000 miles since the last one, heh). Anyway, I don't want this to become like the other Peter's "pinking" thread :-)

Peter

Reply to
AstraVanMan

What's WOT??

And 50mph in 3rd? My Audi's doing around 70 IIRC!!!

Peter

Reply to
AstraVanMan

Heh, probably right - it's probably a bit more, possibly nearer 80, but I get all scared for the welfare of my engine if the needle goes into the red zone even the slightest amount, and there's really no need to anyway!

I remember the day I bought the Audi I couldn't drive it as I couldn't get through to the completely useless s that Direct Line are to change the insurance over, so my mate Paul who came with me to pick it up had to drive it back for me, going via the services at the end of the M180 to meet with Tim and Dervy. We went for a bit of a drive and popped into a McD's where Paul told me my car did 110mph in 3rd gear, and for a brief split second I wasn't quite sure whether to be angry or to just not believe him. Common sense quickly resumed control :-)

Peter

Reply to
AstraVanMan

Oh ok I didn't realise this. Is there any reason why they don't do this for petrol engines, or why they do it for diesels?

Reply to
scott

I'm never sure when the optimum time to change gear is (assuming you want to get up to a certain speed as quickly as possible). I think my max power is at 5500 rpm and my red zone starts just after 6000, so I normally change at

5500 if I'm trying to accelerate as fast as possible. I guess the optimum time to change is when the torque at the wheels in the next gear becomes higher than the gear you're currently in.

Anyway, the only time I've hit my rev-limiter is when flooring it with one wheel in the mud, it was a bit like flooring it in neutral!

Reply to
scott

Bullshit. I can quite happily hit the highest RPM the Megane will do at about half throttle.

Cheers, Andy

Reply to
Andy Laurence

Most diesels work this way - half throttle gives half revs. Funky drive-by-wire systems are a bit different, so perhaps this explains it...

Reply to
DervMan

Paul over revved it by changing down to third with about 110 showing on the speedo, so you'd have noticed it by now, surely? :)

Well, not really in my experience. You'd notice it then and there. A diesels' governor (limiter) is there to protect the engine from catastrophic damage. Lucy's governor, the '94 Mondeo TD, was at 4,750 rpm. Power was waning from around 4,500 rpm, but I used to rev out in second and third during the weeks before the MOT. :)

rev-counter?

Absolutely, yes, but there's no need to worry about it.

Don't be - the governor is there to protect, the redline is an advisory (disclaimer: on most cars I know!).

Reply to
DervMan

Lol - Fiat Bravo 1.6, 105 indicated in third, 15 miles on clock... ooops...

Reply to
Tim S Kemp

In news: snipped-for-privacy@uni-berlin.de, Douglas Payne decided to enlighten our sheltered souls with a rant as follows

My 500 SEL does 137 mph in 3rd.... bit more in 4th... (auto)

Reply to
Pete M

Heheheh, now THAT's cool.

Douglas

Reply to
Douglas Payne

I ran in my Hyper 21 8 Port to the spec's Hobao gave in the manual :) Anything that revs to 38,000rpm deserves to be broken in gently :) However, there is no rest now i have discovered Byron 20%, 16% oil race fuel ;) S'crazy fast now :D

Reply to
Dan405

Oil changes don't effect the running, they just wear out your motor if you don't bother...

Reply to
Burgerman

Because a diesel has the "throttle" wide open at tickover. There is no throttle plate! The amount of fuel injected depends on RPM and accelerator position... Think of it as an RPM controlled fuel flow.

Petrol engines work a bit different... They CAN be RPM controlled via the pump but this makes them run lean... A diesel "cant" run len, or if you prefer they always run lean. The fuel is injected into the cylinder, and the heat ignites it. It is ALWAYS rich near the injector, (neat fuel!) so burns as it sees the oxygen further away.

Reply to
Burgerman

Because electronic systems overide the "real effect" of how diesel engines work, to giove a "nicer" response.- Your half thottle is really FULL throttle as this is what the computer control does to make it nicer to drive. But the RPM limit of the pump remains. It is not a "rev limiter" as such.

Reply to
Burgerman

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.