OT: Pinking Diesel Engine

[Chris Bartram]

I didn't realise anyone else had done unit injectors in cars, thought VAG were the only one.

[The Natural Philosopher]

It was very common before common rail

[Chris Bartram]

Yes, and even some ECU-controlled ones.

The first VW TDIs with a VE pump (i.e: not the PD or CR engines) needed timing checking after a cambelt change.

The ECU detects injection timinmg with a needle lift sensor in No.3 injector, and can alter the timing, but there's a mechanical adjustment to set as well:

[Johnny B Good]

====snip====

It's not a *bang* but a *burn*! The compressed charge of air provides the heating of the fuel to well above its ignition temperature so it starts burning as soon as it is injected in the same fashion as happens when playing with a can of hair-spray and a lighter. The lighter provides the ignition source in this case whilst the heat of the compressed charge of air does that job in a diesel engine.

The injection timing is just as critical as the spark timing in a spark ignition engine and will occur before TDC at medium to high revs and after TDC at idling and very low speeds. Incorrect injection timing can wreck a diesel engine. Too early and the pistons and conrods will be overstressed, too late and exhaust valves will burn up as well as reduce power output, increase exhaust noise and produce black smoke.

[NY]

If the injection timing is just as critical as with a petrol engine, it makes you wonder how earlier diesel engines managed where the timing was very vague and based on when the *mixture* of fuel and air that was admitted on the induction stroke happened to get to the critical temperature/pressure to detonate, rather than having a precisely timed injection that is spread over part of the power stroke.

[Tim+]

Please supply links to these mythical engines.

Tim

[Tim Lamb]

Quite:-) A cautionary tale....

Power and Machines lab, Hatfield Poly around 1964. Power/fuel consumption test on a single cylinder diesel.

Loading was by weighted friction band on a very large flywheel. Starting off load with 10 or 20 kg weights added in steps. At the end of the test, the instructions called for the weights to be removed in steps and the fuel reduced to avoid overspeeding. There was some discussion amongst the group as to why this was necessary. Without further ado the lad from Africa stepped forward and turned off the fuel supply.

We stood watching as the flywheel slowed and slowed and then.... reversed direction flinging weights around the lab!

[NY]

Sorry, I thought someone earlier in this thread had referred to them - fuel injected into the inlet manifold as air was being sucked/blown into the cylinder and then the mixture was detonated once the inlet valve closed and the pressure got to the critical level during compression.

It was news to me, too - so if I've misunderstood then apologies. Are we now saying that all diesel engines (excluding hot-bulb model aircraft engines, perhaps) get their ignition timing by timing (electrical or mechanical) of fuel injection?

[Tim Lamb]

I had a quick dig.. Wiki offers the Field Marshall but it is not clear how ignition was achieved. There is a link to continental tractors which refers to hot bulb and vapourising plates but again does not clarify matters. I suppose for a single cylinder it matters not as there will not be a manifold:-)

[NY]

Enlighten me. Why would the engine go into reverse when the fuel was turned off? Or was it just a single convulsive jerk as the piston nearly reached TDC and then compression pushed it backwards slightly to BDC?

It shows the importance of instructions like these explaining *why* an apparently unnecessary stage (removing the load gradually) is needed, rather than just expecting enquiring minds to accept instructions unquestioningly.

[The Natural Philosopher]

No, nearly all diesels are injected straight into the cylinder or a port at equal pressure.

The timing of the injection is what determines the timing of te explosion, with some inherent delays.

Initially injection pumps driven mechanically. then mechanical pumps with solenoid operated injectors for more precise timing, and finally high pressure electric pumps and solenoid injectors.

However it's been a long time since diesel that are purely mechanical have been made for car use.

So timing is now controlled electronically.

[The Natural Philosopher]

All (vehicles) made today.

Model aircraft diesels work as you describe, and I am sure that pre-war diesels on some tractors did, but they may have been TVO or paraffin type engines with spark plugs. Its a long time ago.

[Dave Plowman (News)]

The injection pumps and injectors on these early engines were precision engineering. At least as accurate as the average distributor - and much more expensive.

[Clive George]

Anybody sensible is saying that, yes :-)

[Tim Lamb]

In message , NY writes

I think it was bound to stop and then reverse on a compression stroke as you suggest.

We probably learned quite a lot on that occasion. The lecturer, who had been busy elsewhere, took it very well and there were no disciplinary consequences.

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[Tim Lamb]

In message , "Dave Plowman (News)" writes

Yes. Cars have the drivers right foot to control speed. Agricultural tractors need to maintain a reasonably constant speed without attention from the driver who may be doing other things. Hence the *governor* part of the injector pump.

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[Dave Plowman (News)]

Pretty certain all diesels with mechanical pumps had governors.

[Tim Lamb]

In message , "Dave Plowman (News)" writes

Pass:-)

Overspeed protection certainly. Wiki brings up lots of examples but doesn't rush to mention automotive applications.

Stationary engines or tractors where relatively constant PTO speeds are important, they are crucial.

All diesel cars I have driven slow when climbing hills:-)

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[NY]

Was it an overspeed governor (the equivalent of a speed limiter on a modern car) or did it maintain a constant speed without pressing on the accelerator despite changing conditions (the equivalent of a cruise control)?

[Dave Plowman (News)]

Overspeed.