darrenjlobb's Adaptronic Managed Compound Blown Derv Bus Build

[Ruan]

Can you get away with a small pre-injection to get some fuel in early before the main injection/combustion event, or are you running too much pressure to get away with it without pre-ignition?

Yeah, it does help massively at lower engine speeds to keep combustion stable, shorten the ignition delay and keep a smooth pressure rise in the cylinder:



That spike in the red line without pilot is the classic diesel "CLANK" you always hear - it's the rapid pressure rise in the cylinder from the fuel all combusting very quickly - rather than a stable burn.

However at higher RPM (>~3500rpm) you're fighting the ignition delay... There's not the compression/heat at the angle you'd need to get the fuel in early enough to have it help the main combustion - by the time the compression comes, it's already too late and the main injection needs to start going in to allow for the ignition delay. This is why almost all OEM engines will shut the pilot off at high RPM/quantity.

Any small improvements in shortening the ignition delay from the pilot injection pre-heating the combustion chamber will instantly be wiped out by the fact that solenoid injectors aren't quite fast enough at very high RPMs to do two injection cycles without negatively affecting the main injection profile - that is time between the current hitting the coil, the needle lifting and fuel starting to flow. Again at high RPM, the time is so short, you can't do anything about it.

Simply getting the fuel in within a decent window is the key to getting good quality combustion at high RPM, rather than it going out the exhaust as noise and heat.

You can't/won't get "pre-ignition" based upon changes in pressure, because the injection angle (+ duration of the ignition delay) is the angle at which combustion starts... You're waiting for combustion to start, rather than preventing it from starting too early from external influences (pressure, heat, cylinder hot spots etc). If it's starting too early (pressure rise is too fast, loud dieselly rattle) then you need to back off the timing.

Large, well controlled nozzles FTW basically!