Cranks
- Thread starter Fingers
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I'm intrigued. I always thought that the sudden ignition and uncontrolled explosion of the fuel caused by a mechanical Injector injecting all at once caused the "diesel rattle". Hence why newer diesels are able to be much more quiet using multiple injection events prior to the main Injection event. (pilot injection) So what is the "Diesel Rattle" really Fingers?
Could it be a timing or injector related problem, where there is ignition prior to the optimal point? That would put a tremendous stress on any internal parts. It would also explain piston failures on LBZ's as well.
Could it be a timing or injector related problem, where there is ignition prior to the optimal point? That would put a tremendous stress on any internal parts. It would also explain piston failures on LBZ's as well.
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The rattle comes from the immense pressure wave generated during combustion. That's why older diesels (with one injection event) rattle like crazy.... The Duramax has pilot injection so the pressure wave is more linear throughout the power stroke. Every engines crankshaft has angular velocity....
The speed of the crank varies in all engines as it rotates. The energy to compress the air charge, accelerate the piston, and a host of other things has to come from some place.
Diesels, especially at idle, have drastic and multiple variations in crank speed.
The sources are many, but the injection event is one of the major ones. To have the fuel ignite at idle, it must be injected fairly early. Thus bucking the piston on the way up, slowing the crank a great deal more than say a gas engine. Then again, gas engines don't need that much timing at idle. They rattle too when you add that much.
Then on the power stroke, a diesel charge burns fairly quickly in the rich air environment and stops burning abruptly. I didn't show it on the pressure chart, but the pressure will actually fall to or below the adiabatic curve at idle. Usually well before 90° ATDC.
Pilot injection softens the blow by injecting an early, but small amount of fuel to warm up the chamber for the main charge which comes way later than it would in a non-pilot scheme. The change in crank speed is not nearly as abrupt.
Diesels, especially at idle, have drastic and multiple variations in crank speed.
The sources are many, but the injection event is one of the major ones. To have the fuel ignite at idle, it must be injected fairly early. Thus bucking the piston on the way up, slowing the crank a great deal more than say a gas engine. Then again, gas engines don't need that much timing at idle. They rattle too when you add that much.
Then on the power stroke, a diesel charge burns fairly quickly in the rich air environment and stops burning abruptly. I didn't show it on the pressure chart, but the pressure will actually fall to or below the adiabatic curve at idle. Usually well before 90° ATDC.
Pilot injection softens the blow by injecting an early, but small amount of fuel to warm up the chamber for the main charge which comes way later than it would in a non-pilot scheme. The change in crank speed is not nearly as abrupt.
So with that being said^, would it be "detrimental" to the health of the crank to turn off pilot injections at higher rpm?
I don't think so
"Diesels, especially at idle, have drastic and multiple variations in crank speed."
You would have a lot more broken cranks I would think. Guys have been shutting off the pilot since the beginning of EFI.
"Diesels, especially at idle, have drastic and multiple variations in crank speed."
You would have a lot more broken cranks I would think. Guys have been shutting off the pilot since the beginning of EFI.
Well, lets think in terms or harmonics.
If we are still getting those variations at higher RPM, then we are approaching the frequency domain of the crank's first harmonic. I'm thinking in terms of 2000 RPM here.
With common rail injection, the timing of the injection events(s) can be most anywhere. I wonder if we have, through timing, found the harmonic for the crank by accident.
I need an economical way to measure crank speed variations.
If we are still getting those variations at higher RPM, then we are approaching the frequency domain of the crank's first harmonic. I'm thinking in terms of 2000 RPM here.
With common rail injection, the timing of the injection events(s) can be most anywhere. I wonder if we have, through timing, found the harmonic for the crank by accident.
I need an economical way to measure crank speed variations.
You have this almost exactly backwards. Look at the stock tunes for some Duramaxes, then some LS motors.
The Duramaxes run 0 to 5 degrees timing. I've run as little as -5 degrees without issue. I never run more than 0 BTDC because it just wastes fuel at idle.
Now check some gassers. They run massive timing (25 to 40 degrees BTDC) at idle. This is due to the low cylinder pressures from the high vacuum at idle-- the fuel burns very slowly with that little air. This slow burn means it is a smooth event, and quiet.
This has been true for ages. Think back to distributors: most have a vacuum advance. So at idle, they throw in extra timing. The opposite of diesels.
This is absolutely correct. Being a compression ignition engine, you can't spray fuel in too early (at idle/low load/low rpm) cause the fuel actually cools the air mass, in SOME cases inhibiting ignition. PW is extremely small at idle, timing at 0 or atdc produces the smoothest idle.
I have found through tuning my lb7 that, spraying fuel after TDC, idle quality is improved.
My LS2 runs 22 degrees at idle.
IIRC my LBZ is the same. I don't know where I ended up leaving it but I'm sure I'm running more timing then stock.
I use a RoadRunner ECU. It lets you adjust any part of the tune in real time. It is super handy.
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