Quote:
Originally Posted by Madact
The use of the word "explosion" was hyperbole / non-technical-definition there (sorry for the imprecision) - the rest still applies though. Exhaust gas flow is one of those problems for which you don't need to consider laminar flow conditions at all.
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I realised that you didn't mean a real explosion, but still, it doesn't help to think about it in that way because it should operate smoothly,
The flow in the pipe should not be turbulent, maybe "laminar" is not the best description for a tube but there is certainly a boundary layer against the edge and the main flow goes down the centre, unless the pipe is curved in which case it will be off centre and you need to worry about keeping the flow attached to the inside of the curve - it is laminar flow.
I dug out this diagram:
EVO - exhaust valve opening takes place during the power stroke, long before the end of the stroke, red line is the pressure in the exhaust port which rises smoothly and then starts to drop again just before the end of the power stroke because by the end of the power stroke most of the exhaust gas has left the cylinder. Dark blue is cylinder pressure. Half way between BDC (end of power stroke, start of exhaust stroke) and TDC (end of exhaust stroke) the cylinder pressure reaches atmospheric despite the fact that the piston is rising so should be compressing the gasses left in the cylinder, the inertia of the exhaust gasses in the manifold have sucked the gasses out of the cylinder leaving the piston with almost no work to do, they continue to drop so that by IVO (inlet valve opening) there is a strong vacuum in the cylinder to pull the new fuel air mixture into the cylinder even though the piston is still on it's way up. Cyan line is inlet port pressure which meets the exhaust port pressure before the end of the exhaust stroke and then there are a few waves on the exhust port caused by wave reflections in the exhaust manifold one of which is timed to give a little extra suction just before the exhaust valve closes - those little waves are the effect of the wave timing, the creation of the large vacuum is the result of suction from the exhaust gas inertia and venturi effect between header pipes. Remember that EVO, IVO and EVC are events that take time, not instant events as indicated by the lines on the graph.