Superposition and the exhaust pulse
So what is an exhaust wave superposition of particle velocity, simply stated and within the context of a two stroke engine, it is where a residual pressure wave (reflected initially from the piston skirt of the closing port) is in turn reflected from the rear cone, coincides with the new pulse exiting the exhaust port at its opening. The sum of the new and residual pulses provides a large wave amplitude that moves down the exhaust duct to the header pipe and beyond. The larger this amplitude then the deeper the depression around bdc which can initiate the fullest, mass flow of fresh mixture from the transfer ports. What happens to this mixture is in turn dependent, in large measure, on the transfer port directional control that has been initially designed in. Get that wrong and a significant amount of fresh mixture will be irretrievably lost to the exhaust pipe.
The lowish exhaust duration of 190* creates these large residual pressure ratios over a wider band range, and work well with an appropriately designed pipe to use true resonance over a wide rev band and provide for reasonable peak power. It is eminently feasible to design a pipe that works with the power band where the tuned length resonates with the port over a wider power range. The positive implications for a gear ratio limited engine such as a Bantam are easy to appreciate, so, what`s not to like then?
The above scenario is of course centred on the max torque and power within the performance curve that you select or are able to achieve with your engine, either side of that rpm point and things can change, sometimes quite dramatically. For instance, when you throttle off, the airflow collapses, as does pipe resonance and temperature. Recovery from this situation is not a problem with a 6 speed box, just drop a couple or so gears and the revs are quickly back in the zone; you can`t do that with a Bantam.
All two stroke race engines are blowdown deficient at peak rpm and in the over rev area, based on the premise that, if you double the rpm you halve the time the port is open. If you have transfer duration timings in the optimum range of 128/130* then an exhaust period of 190* will probably not be sufficient to achieve the power expected. Extending exhaust timing to improve blowdown will therefore reduce the opportunity for a superposition to occur within the exhaust waves; problem one!
Unquestionably then, the biggest problem is that the exhaust pipe has rigid, fixed dimensions that can only create a depression over a certain period in time, but the rpm, hence time, is constantly changing as we accelerate up to and through the power range.
This depression has its optimum effects move in relation to bdc as the rpm varies, moving from before bdc then to after bdc with rising rpm. The ideal scenario being that we have a superposition at exhaust opening and maximum depression occurring around bdc. The max depression is restricted to the limits set by a 34% header length and a 66% mid-section pipe dimensions, neither of these percentages of the tuned length are set in stone but are a firm guide for success. The superposition resonance effect is set more by the port timing and the pipe`s tuned length, so scope is there for juggling the numbers to achieve a specific end effect.
Sizing of a pipe`s belly diameter is down to the effective energy recovery within the pipe, and in the case of a Bantam that energy level is very finite so going too big on diameter can be counterproductive. The available BMEP from your engine spec determines that usable energy level, so at around 7-8 bar the typical Bantam engine has little to spare and enough energy must remain after traversing from cylinder to reverse cone to create a meaningful, returning, plugging pulse of sufficient magnitude for the next phase to be effective.
Diffuser action is largely, but not exclusively responsible for transfer mixture to move into the cylinder and the case does play a role, particularly early in the power range where the pipe is not working to optimum effect.
At initial port opening transfer mixture is at zero velocity, and it is the pressure difference between case and cylinder that initiates movement. The mixture begins to accelerate to its maximum velocity, when the two pressures equalise velocity is at its maximum. The cylinder starts to re-pressurise and the case drops below atmospheric, both act to begin to slow down transfer flow. However, the transfer mixture has sufficient inertia that keeps it flowing for a while until natural forces or the port edge stops flow altogether.
You might have noticed that there is no mention of flow reversal at transfer port opening, or of the collapse of torque at around 2/3 of peak torque rpm. Both of these events impact on the above but to include it all will make the article longer than I have time to adequately devote to it however, the general thrust of what I was trying to put across is unaffected. Another article can be forthcoming to include the omitted functions, should it be required?
As has already been mentioned it is the diffuser that does the major work of moving mixture in and around the engine, and it is the steepest angled part of the diffuser which does that work. What you don`t want is excess energy being used when the transfer ports are still closed or about to close. So from about half open until half closed is the optimum position for the steepest part of the diffuser to extract the maximum quantity of fresh mixture from the case into the cylinder from a maximal port area. Stroke and con rod length significantly affect the period of time the piston stays at and accelerates away from bdc thus affecting specific time area of the transfer ports and can be used to beneficial effect.
The general approach today is to have a short relatively shallow first and last cones, of a three cone arrangement and the steepest and longest cone residing in the 40-60% part of the tuned length. The shorter cones can be adjusted depending on where you want the start of diffuser depression action to be the most effective, individual engine characteristics will decide that for you. But remember, the depression curve will always move from the start to late in that cycle with increasing rpm in the engine`s power range. As general rule, the earlier it happens the less scope there is for over rev power, so by moving the steep diffuser out may gain front side power but will lose as much past peak rpm, the choice is yours to make.