OT: Diesel Engines (again)

[Clive George]

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I'll put a datapoint in which you will already know but others might not : PSA HDi fuel pumps are driven off the timing belt, ie from the crankshaft via a belt.

[Cursitor Doom]

Given the extreme PSI generated by the high pressure pump I'm wondering if one could be adapted and pressed into service as a metal cutter? I know they normally do this with water, but the pressures generated by the diesel pump can't be far off being strong enough to cut steel for example?

[The Natural Philosopher]

All of the ones I looked at. The reason being the camshaft rotates at half engine speed and that is the timing required for the injector pumps which are often integrated with the injectors. And if its a single pump, the sensible place to put it is at the cylinder head which is where the camshaft is.

[MrCheerful]

Common rail pumps are not timed

[Andy Burns]

That's for unit injectors, rather than CR injectors.

[The Natural Philosopher]

I wondered that, so common rail is no longer the latest and greatest then?

Latest brood seem to have combined punmps and injectors

[NY]

Did diesel engines at one time have a pump that produced pulses of pressure that corresponded roughly with the times that the injectors were opened, rather than producing a fairly constant pressure which is available whenever the injectors are opened (maybe multiple times per cylinder in rapid succession)?

I suppose it you make the injector pressure-operated (simply a spring-loaded valve that opens while the pressure exceeds a threshold) then a pulsed pump can control its own injector timing, though presumably not to the higher degree of accuracy that an ECU-timed engine can do.

[MrCheerful]

yes, that is how, traditionally, diesel injection worked, the correctly timed pump looked after it all. But electronic injectors can be much more accurate and hence more efficient and give better emissions. Downside is greater expense and complications.

[The Natural Philosopher]

The systems I looked at combine both. Lobes on the camshaft driving the pumps, but the exact timing and duration down to the electronically controlled injectors.

[Vir Campestris]

The rest of the post was great. This paragraph?

Heavily turbocharged petrol engines are just as laggy as heavily turbocharged diesels. But they are much rarer - it's the lower revs of a diesel, and the resulting lower power output, combined with their greater weight, that pushed turbocharging in cars first into diesels.

And AIUI you can't rev a diesel the way you can a petrol because it's burning decane, not pentane - the bigger molecule takes longer to burn.

Andy

[Fredxxx]

I agree about the type of alkane, but are you sure compression ignition has a slower burn than its petrol counterpart. Can you cite any reference because my ears suggest otherwise?

A quick look suggest at modest rpms the advance for diesel examples seem to be circa 10degs and for petrol engines 30degs.

[Vir Campestris]

You may be hearing the induction crack. That is louder at low loads for a diesel. I have no references.

Surely that's not why they're all specced to such low red lines?

I'd be guessing if I said any more.

Andy