I think they are still supposed to take the engine to the limiter (the more knowlegable testers don't) but get to max revs gradually. If the reading is less than 1.5 it passes without further abuse.

I took my C5 fitted with DPF a few years ago.

Although it had covered 150k miles, the inside of the original exhaust tail pipe was still silver, no soot at all.

The tester kept revving & revving it, more than the 6 times he was supposed to.

The test sheet showed 0.01, I think he thought his test equipment was faulty, no fast pass then.

>>the purpose of the limiter is to protect the engine in case of a failure such as burning its own oil if overfilled...

The limiter will not stop the engine over revving in those circumstances. The limiter controls the flow of fuel to the engine, if the engine's running on some other source (sauce?) then it's liable to rev to destruction.

The "rev limiter" or maximum speed governor as it is more correctly described will not protect a diesel engine from running away if it burns its own oil. (Or stray butane if it lives in the bottom of a boat).

The only sanction the governor has is to control the engine fuelling. In car applications, a 2 speed govenor is generally fitted which is designed to hold the required idle speed over a range of engine loads, and to limit the maximum engine speed as previously detailed. Overspeed from either oil ingestion or running down a hill is beyond the control of the governor.

A maximum speed limit is set for any diesel in order to protect the rotating and reciprocating engine components. As most of the forces generated through rotation vary as the square of speed and because diesel engine components are heavily loaded, a governor is essential. Bearing and component loadings can be exceeded very quickly.

The "MOT" smoke test is unrepresentative as the governor will reduce fuelling as the speed limit is approached. Engine load is required to ensure that maximum fuel and therefore the highest smoke level is reached. Observe a laden lorry climbing a hill.

The main problem arising from unloaded maximum governed speed is that of torsional oscillation in the crankshaft. The load on an engine damps the driving end of the crank (the free end incorporates its own vibration damper) and dangerous levels of oscillation can occur when the engine is run at high speeds without load.

659.

Edited by 659FBE on 27/08/2010 at 19:05