Quote:
Originally Posted by leathermang
That is a very imaginative description... but I just can't go with it.
The idea that the slowing down of the exhaust gas due to the cranking speed being slow making a vacuum effect as far back as the end of the exhaust pipe just does not fit my intuition of the physics.
First, those two , or even just one, mufflers would sure mix up the pulses.... a dampening effect which would preclude an actual backwards movement being available at the end of the exhaust pipe... I can see a lessening of the exhaust speed...but not a reversal needed to give this effect... remember that other pistons are sending gas into that confined stream also... even if one could do that the combo of several feeding that tube would surely.
Also, this supposed valve opening which occurs after top dead center... ignores the fact that we are dealing with the two strokes of the crank in that bore which have both valves closed ... Compression AND Power.. so that does not fit either..
I agree with the concept of inertia creating a scavenging effect... but given the whole situation I think slow cranking or idle would only take that away.... as compared to going into the other side of the realm... (of vacuum. )
I think your mental concept of the valve overlap comes from gas engines....where a hot cam can cause that kind of effect at certain rpms...
Nice try though....
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Sorry you don't like it!
I don't know the particular valve timing events for a MB diesel engine (I should look it up...) but there is most certainly an overlap period. In fact, a diesel could tolerate a much "hotter" cam than a gasoline engine for three reasons:
1. There is no throttle, so the scavenging effect will not be spoiled at light loads... the intake vacuum will not attempt to keep the exhaust in the cylinder.
2. There is no gasoline in the incoming fresh air charge, so there is no possibility of raw fuel going out the exhaust during the overlap ruining efficiency and raising emissions.
3. The exhaust has a lot of excess air (unlike a gasoline engine), so some exhaust dilution of the incoming air will not cause misfiring the way it does in gasoline engines.
V-6 gasoline engines are rarely used as marine engines anymore because they had chronic problems of aspirating sea water in through the exhaust exactly because of the phenomenon I described.
If there is any restriction in the intake, the negative pulses in the exhaust will be amplified.
At cranking speed, the air in the exhaust will be moving at low velocity and under low pressure. It will behave as a nearly incompressible fluid. Any suction generated at an exhaust port will be transmitted throughout the system.
If I can get two helpers tomorrow, I'll try an experiment. One to hold the "stop" lever, one to crank the engine, and I'll hold my hand on the tailpipe.