Of course, sensible owners buy a chain driven diesel... or get their cambelts and gubbins changed at safe intervals and don't buy cars with plastic pulleys that disintegrate when they are spoken to unkindly.

:-)

Edited by madf on 11/02/2012 at 13:10

If you don't understand that an unloaded engine at maximum speed is much more highly "stressed than a loaded engine at maximum speed then do everyone a favour and pipe down!"

As an ex-diesel manufacturer's service inspector and test engineer, I really am going to enjoy taking you to task on this one!

Your understanding of how diesel engines are governed and controlled is sadly lacking.

A diesel engine running at maximum unloaded speed, ie with the speed control or accelerator held at maximum, sometimes called "high idle" is running under the control of the governer, whether mechanical or electronic, and is being supplied with just enough fuel to maintain that speed. The exhaust temperature, fuel consumption and what you call "stress" or load on the engine is way down low and should not harm the engine in any way and is part of diesel engine makers test procedure.

It's producing a lot of revs but very little power.

The maximum load on the engine components occur at the rpm that corresponds to maximum power, the exhaust temperatures, fuel consumption, engine load and as you call it "stress" will also be at maximum.

So now it's your turn to pipe down.

The maximum load on the engine components occur at the rpm that corresponds to maximum power, the exhaust temperatures, fuel consumption, engine load and as you call it "stress" will also be at maximum.

I'm talking about mechanical stress. That is generally at a maximum at fast idle.

Thermal stress is generally maximised at peak output. This can be generally be disregarded for very short duration excursions, since piston crowns, exhaust valves and turbines can withstand short duration heat soak.

Didn't your training as a test engineer extend to torque flux and higher order torsional vibrations?

The gobbledegook theory is what determines why an engine is the way it is for a particular application. Any monkey can be taught to follow some simple recipe and swing a spanner. The gobbledegook theory is tested and verified or discarded in engine test cells.

Ever been in one?

So if revving an unloaded engine is so benign, why do so many engines have electronic limiters to prevent free revving above 2500 RPM or so?

Because it is hard on the engine that's why.

Edited by unthrottled on 12/02/2012 at 12:19

Sounds like bull. A camshaft is not a load-carrying part of the engine and would have no way of knowing about the loading.

Its like unthrottleds version of a 'rap battle' going on in here.

I don't particularly want to engage in lengthy discussions on this topic but for what it's worth, I'll add my contribution as one who earned a living at one time testing diesel engines.

Some of the above views represent an over simplification of the factors which can become very important if an engine is allowed to reach high speeds when unloaded. A multi cylinder internal combustion engine is not like an electric motor. Motive forces are provided by gas expansions which occur sequentially but not in a coplanar field - disregarding radial engines which don't suit automotive applications. Despite the fact that primary and secondary balance forces and moments can be either offset (4 cyl with Lanchester shafts) or - very preferably - eliminated by configuration (straight 6, flat 6, V12 etc), resonant oscillation is a very real danger to an engine.

Much work is therefore done in engine development to both avoid and to damp resonances which may occur in a useful part of the engine's operating map. Torsional crank oscillation is a particular danger to any non-coplanar multicylinder engine and most designs will incorporate a damper at the free end of the crankshaft to limit the amplitude of such oscillations. The designer of the spring/mass/damper system which is in effect the engine's crankshaft assembly will assume that the other end of the crankshaft is also damped by the presence of a load. This, after all is what we have engines for.

Running any engine (and especially a diesel) at speed off load is potentially very damaging and high levels of torsional oscillation can cause catastrophic engine failure - usually a crank web. The MOT "smoke test" is crude, non-representative (no load) and potentially damaging. Engine failures during testing are thankfully rare due to safety factors in design, but the test has the potential to over stress several engine components if a resonance occurs.

I reset the governor to 3500 crank rpm when my car goes in for testing.

659.

Edited by 659FBE on 12/02/2012 at 23:00

I accept that my last post was not very well constructed in that engine balance was "lumped in" with torsional oscillation - the two things are separate. In the preamble, the major point being made was that due to the automotive engine's cylinders being non-coplanar, dynamic torsional cyclic stresses would be set up in the crankshaft. I did make the point that TV dampers are fitted to nearly all (decent) engines, even those in perfect primary and secondary balance such as straight 6s.

I am not prepared to discuss the details concerning the resetting of diesel governors - without specialised knowledge and some test gear, the dangers are obvious.

659.

Edited by 659FBE on 13/02/2012 at 15:26

And sometimes a very blinkered person fails to on move with the times and lives with 20th century technology .Or should we all go back to living in caves.

A real engineer embraces new technology and moves forward .

Edited by Collos25 on 19/07/2012 at 11:10

A reving a diesel engine under no load conditions will not be damaged in the least going up a hill fully loaded will.

Ask yourself why is this test being carried out your quote "putting good cars of the road" is non sensical its putting cars of the road that do not meet present day emission standards or do you want to go back to the London smogs.

There is now a procedure called "fastpass"which does not require maximum revs.The old test is only used if you fail this one.

"fastpass" does mean you rev to the limit, you gust do it once, and so long as the reading is below 1.50 it passes.. so you are wrong..ok

".. why. because there would be to much fuel and not enough air.. by putting so much fuel into the engine it just chokes it, "

Sorry you really need to read up about how fuel injection works.

And how diesel engines work.

And modern drive by wire throttles..

and you really need to answer a question, why do scania stop their engine from over revving out of gear,

Which is what the governor on your car's engine does! Note the quote above where it says to gradually increase revs to the governed limit or until it becomes apparent that the governor is not working. Nowhere does it say; "floor it and see if it goes bang".

They're quite deliberately <i>not</i> overrevving it. If the engine is damaged by running at the governed maximum unloaded revolutions as set by the manufacturer, it's the manufacturer that needs a swift education with the clue stick rather than the MOT lads.

And now the shipping forecast: Dogger, westerly, force 5 increasing. Teacup, hurricane force 12......

I am sorry but anyone who says

"if i throw a load of fuel over a fire some would flow away.. why. because there would be to much fuel and not enough air.. by putting so much fuel into the engine it just chokes it,"

Obviously understands nothing about fuel injection systems.