Thursday, Jan 26, 2006 at 09:09
Thanks Captain, but was simply adding yet another "theory". I did say "in gaseous" form without any real idea how that might occurr.
I still believe these engines were dramatically over fuelled. Unlike (for instance) an older disco 300tdi which has a screw stop fuel limiter and a pressure regulated tubo boost (wastegate) the ZD30 has electronic control over both fuel and turbo boost.
Now I know as much about electronics as I do about chemistry, but I do have some understanding of air/fuel ratio. In a turbo diesel, power doesn't come solely from boost, it comes from a combination of fuel then boost. When the fuel injection to the cylinder has reached the calibrated maximum, boost should also be at its maximum, full engine torque output should be reached and exhaust gas emissions within required parameters. There is no such thing as "overboosting", earthmoving diesels do not usually have any control over the turbo, it will only boost to whatever maximum injection is. You want more power, turn up the fuel!
My contention remains that if the electronics controlling the amount of injection goes out of control, but the turbo boost is controlled, a serious overfuelling situation can exist. I accept that this should be accompanied by clouds of black smoke, something which has not been reported except at the time of failure where different sorts of smoke emmissions were noticed. However, the one I followed towing a horse trailer up a steep
hill was belching out volumes of black smoke, unable go get over 15kph or so, had not expired at that stage (I stopped the driver to ask if I could help and he wanted to keep going). He had not particularly noticed the smoke! If the turbo boost is not controlled, more fuel will create more turbo boost (reports of up to 25psi have been noted) which whilst developing more engine torque at the correct air fuel ratio will also lead to extreme EGT, a situation which can quickly destroy the turbo itself.
How any engine copes with any degree of over fuelling would have to come back to (a) cooling and (b) fragility of the engine components to withstand the overdesign stresses. Its one thing to over fuel/overstress for 5 mins. ascending a
hill, but make that several hrs. on end, high speed towing a load and its a different matter. ( THe RR Spitfire engine had an emergency supercharge boost to be used briefly in combat situations only) Overheating has not been an issue (from reports). Inherent strength of the engine is an unknown factor. (unlike the repution of the 4.2 Nissan)
It was commented widely that the high sulphur diesel could be affecting the performance of the injectors. These multi hole pintle type have been in use almost since the inception of the diesel engine, are reliable and able to give many thousand hours of service. At the moment of injection the initial combustion should take place within the piston top depression. This engine would have the injection timing controlled electronically through the ECU, perhaps another avenue of thought.
The mystery still unfolds.
kesh
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