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 Heat wave, we need cool flow!

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Trevor Amos



Number of posts : 873
Registration date : 2010-08-13

PostSubject: Heat wave, we need cool flow!   Wed Jul 23, 2014 11:53 pm

Hot, over-stressed piston crowns will always benefit from the lower temperature cooling transfer streams flowing over them, and in compromised air-cooled cylinders can mean the difference between survival or not. Particularly so if heat production approaches dissipation potential. With increasing use of forged pistons, and the general high rates of thermal expansion that forging material specification imposes over cast alternatives, it makes absolute sense to encourage the beneficial of that cooling flow to cling to the piston crown.
The so called Coanda effect depends upon the fact that air cannot interpose between the piston crown surface and the scavenge gas flowing over it. Atmospheric pressure pushes at the outside of the stream, even if that flow is curving away from the direction of the flow source. Once air enters detachment occurs the effect is lost. What should be aimed for is attached flow from case entry into the transfer ducts, up and around the inner duct wall and out onto the piston crown. If you are very clever,or lucky, and manage to match appropriate piston speed to scavenge velocity then this effect can yield meaningful benefits. Flow density and local pressures all play their role here. Paradoxically, short circuiting of cool scavenge flow out of the exhaust port will also take with it heat from the piston crown. Which is just about the only positive that can come from this negative event that reduces potential engine torque!
Appropriate angling of the upper and lower transfer port exits, to at least approximate a tangent to the piston crown, will provide for flow to adhere and minimise interference to that that flow. Just as the walls of the radial port exits taper, so must the axial outlets be the same. Only this way can the exiting streams of mixture be controlled and directed to do the most effective job of scavenging the cylinder of burnt residues and retain as much new gas as is possible. Merely providing a port and leaving the outcome to chance will never make any sensible power. The passage of high speed transfer mixture from the crankcase and underside of the piston, going through a 180* curve and twist, must be as turbulent free as is possible to achieve. Priority must always be given to SMOOTH gas flow, no acute turn angles or sharp edges anywhere! The inclined lower floor of the inner transfer duct will allow for a generous, smooth free flowing curve to guide the mixture into the cylinder with a minimum of fuss. Flow attachment to the duct inner wall can only be successfully achieved in a turbulent free environment. This in turn will benefit the mass movement of fresh mixture throughout this limited time frame. With more gas in there is the potential for more power out, what`s not to like?
Rising pressure in the crankcase after inlet closure will be much earlier in the angular rotational cycle of a reed valve than a piston port engine. So with the transfer ports still closed static pressure (potential energy and charge density) will build much more in the ducts in readiness to be exploited by the later events at transfer opening, scavenging, then combustion. With flow underway, the tapering profile of the duct will see static pressure reduce and velocity pressure increase. If this column of in-going mixture`s velocity exceeds that of mean piston velocity then scavenging losses to the exhaust port could be severe. Conversely, if it lags behind the piston movement by too much then the cylinder may not be fully purged of hot, burnt gas before the exhaust port closes. Neither scenario is desirable, but are eminently achievable by the careless and unwary, hitting the sweet spot needs careful thought and planning.
Piston motion in both the up and downward direction performs the dual and mutually reversing roles of having to pressurise one chamber and de-pressurise the other. If the transfer ports remain open for too long at an inappropriate rpm rate then gas flow-speed falls and reverse flow takes place. Continual changing of pressure sign, negative to positive and back, is responsible and can considerable. If the atmosphere is regarded as one chamber and the crankcase as the other, with a carb somewhere in between, some of the strange and unwanted effects that bedevil carburation might be explained? Pressure waves of continually converting sign can even reverse to atmosphere whilst the piston is travelling upward! Piston port engines suffer this more than the self controlling reed valve alternative. Common sense might suggest that this is illogical and can`t be possible, but it happens all the same! And to add another layer of complication, when the piston eventually stops at bdc, pipe diffuser action augments pressure wave activity.

Sorry, but there are no free lunches where high speed racing two stroke engines are concerned. The unsteady gas and thermo-dynamics are far more complex than their mechanical simplicity might imply. And all of these events have to be completed in just one crank revolution.
Over-riding all of the best intentions of Bantam tuners and competitors alike is the following caution and it is doubly relevant for us: The effective torque spread should always be able to cover any gaps in the range and number of gears available whilst factoring in the machine and rider`s collective power to weight ratio!

Trevor




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