Why do diesels have lower rev limiter?

[paddyhasneeds]

Sorry to show my lack of engine knowledge here, but why do diesels always have a much lower rev limiter than petrol cars?

[davepoth]

Higher compression = longer stroke, which means higher piston velocities, which means more stress on the crankshaft. It's the crank stress that normally limits engine speed.

[Marf]

Physical limitation of the fuel. Diesel oil burns slower than petrol which in turn limits the max RPM of a diesel engine.

[eldar]

Marf said:

Physical limitation of the fuel. Diesel oil burns slower than petrol which in turn limits the max RPM of a diesel engine.

This is the answer. Plus the risk of scouring lubrication oil and using it as fuel...

http://www.youtube.com/watch?v=5zx3qKX_Pno

[paddyhasneeds]

eldar said:

This is the answer. Plus the risk of scouring lubrication oil and using it as fuel...

http://www.youtube.com/watch?v=5zx3qKX_Pno

WTF is going on there?!

[Globs]

paddyhasneeds said:

Sorry to show my lack of engine knowledge here, but why do diesels always have a much lower rev limiter than petrol cars?

It's just an idle control, you need enough revs to keep the vacuum pump + oil pump + alternator working.

ETA:
Sorry, misread the question.
I think the limiter is set lower because the conrods etc are heavier.
I think the reason they do not rev like petrol cars is that the timing is fixed (compression ignition), whereas when a petrol engine revs up the spark advances more and more - allowing a much faster rev.

Edited by Globs on Sunday 15th May 19:11

[Marf]

paddyhasneeds said:

eldar said:

This is the answer. Plus the risk of scouring lubrication oil and using it as fuel...

http://www.youtube.com/watch?v=5zx3qKX_Pno

WTF is going on there?!

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the piston rings do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

Edited by Marf on Sunday 15th May 16:38

[Egg Chaser]

Marf said:

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the rings in the engine do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

It can also happen if the bearings in the turbo fail, meaning oil is drawn in through the intake and burnt.

[The Trig]

Marf said:

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the piston rings do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

Blocking the intake does not always work as sometimes the blocking media will also be injested !

[paddyhasneeds]

Marf said:

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the piston rings do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

Presumably that just comes down to bad luck? As a diesel owner there's nothing you can do other than follow the maintenance schedule?

[Dangerous Dan]

co2 extinguisher is your friend. liberal application to intake and problem (hopefully) solved.

[paddyhasneeds]

Oh crap I thought I'd put this in General Gassing. Sorry.

[gog440]

Marf said:

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the piston rings do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

Edited by Marf on Sunday 15th May 16:38

And if this happens then you get what I saw the other day which is a huge amount of smoke over all 3 lanes of the m1 and a load of pissed off people stood around their freelander which was pumping smoke out of the exhaust and all out from under the bonnet. They hadnt worked out that they might have saved their engine if they had managed to stall it.

[Perra]

Marf said:

paddyhasneeds said:

eldar said:

This is the answer. Plus the risk of scouring lubrication oil and using it as fuel...

http://www.youtube.com/watch?v=5zx3qKX_Pno

WTF is going on there?!

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the piston rings do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

Edited by Marf on Sunday 15th May 16:38

I didnt know this could happen, thanks for the info.

[philthy]

Egg Chaser said:

Marf said:

Diesel engines can run on pretty much any oil, including its own oil supply.

If for whatever reason the rings in the engine do not seal the combustion chamber from the bowels of the engine, it can start to run on its own oil and will continue to do so until either the engine explodes, the oil runs out, or you block the intake.

It can also happen if the bearings in the turbo fail, meaning oil is drawn in through the intake and burnt.

Iveco artic tractor unit, London bound M20, it suddenly started to accelerate, and nothing I did made any difference. I realised I had one shot at pulling it up, and stood on the brakes. It stopped and stalled.......just!
fking scary! The smell of the brakes was horrendous!

[ean21]

I came across this just last weekend. There was a van at the side of the road with smoke billowing out of the bonnect and the exhaust. I pulled up and jumped out with a fire extinguisher because i thought it was on fire, but then I realised that it was absolutely revving it's nuts off. There was smoke and oil spraying out of the exhaust in a plume about 5 metres long. The owner was trying to disconnect the battery, I stopped him and jumped in the driver's seat and stalled it.

I assumed it was a problem with the throttle linkage, but now I realise it was probably running on its own oil.

[Mikeyplum]

For more info just YouTube "Runaway Diesel"...

There is a cracking one on there of a Defender revving it's tits off!

There is also a good, sort of, tutorial where this yank guy strips down an engine which was victim to the turbo seals going and "running away"... should see the state on the internals!

[Parrot of Doom]

gog440 said:

And if this happens then you get what I saw the other day which is a huge amount of smoke over all 3 lanes of the m1 and a load of pissed off people stood around their freelander which was pumping smoke out of the exhaust and all out from under the bonnet. They hadnt worked out that they might have saved their engine if they had managed to stall it.

I saw a similar thing a few weeks back. Everyone had to crawl past the poor woman, wondering what was wrong with her Rover 25:

[Mikeyplum]

doogz said:

Mikeyplum said:

For more info just YouTube "Runaway Diesel"...

There is a cracking one on there of a Defender revving it's tits off!

There is also a good, sort of, tutorial where this yank guy strips down an engine which was victim to the turbo seals going and "running away"... should see the state on the internals!

Ooft, it says that's it's first start since it was rebuilt.

Maybe he should leave that to the professionals next time!

I'd love to see his face as he turns the key on the next "first start"...

[manic47]

Egg Chaser said:

It can also happen if the bearings in the turbo fail, meaning oil is drawn in through the intake and burnt.

I have some experience of this.