THOUGHT FOR THE 2nd DAY!
Based upon the assumption that in one single revolution (or cycle) of an engine at a particular rpm, the total mass of gas that enters the engine must go around and through the engine, then, that mass must eventually exit.
Specific time areas can be calculated for all of the ports and then related to the potential bmep produced for any engine speed. We can also assume therefore that the higher the bmep produced the greater will be the trapped charge mass. After combustion has completed, the greater the mass of exhaust gas and cylinder pressure the more urgent the need for a progressively larger or, more efficient blowdown STA becomes to rid the cylinder of the exhaust gas in the available time frame, as rpm rises so the time element diminishes! Any remaining gas in the cylinder by about 2/3 exhaust port height will be so depleted to be unable play any useful part in the exhaust process. The exhaust duct floor can be raised to reflect this and out flow, and returning plugging pulse efficiencies can be significantly improved.
Accepting also that with all things being just about equal (they never are of course), and that the quantity of exhaust gas produced is proportional to the power produced, then an 8bar engine makes roughly half the amount of exhaust gas that a 16bar engine makes at their respective peak torque rpm.
One prime example of this are the shapes and profile of the transfer ducts. If they have straight sides with no inner radius then their overall flow capabilities will nowhere near match the imagined flow profile of the STA of the port windows.
Just to digress here for a moment to get us all in agreement as to what time-area and angle-area and STA actually are, the following interpretation may help.
With the port area measured, rotating the crankshaft for the port`s linear height will give a certain angle-area for the port in the cylinder.
Adding rpm will assign a certain time element to that angle which then gives us time-area (TA)
Dividing that TA value by the cylinder swept volume plus the combustion chamber volume at TDC will give us specific time area (STA).
STA is a pretty good indicator for how effectively an engine can breathe, and approximately at what rpm it will eventually run out of breath! However there are certain caveats to all of that, you can have all of the STA possible but until the pressure ratios across the transfer ports have equalised there can be no mixture flow from the crankcase. Even with an adequate pressure ratio and suitable STA a stationary column of gas has a lot of inertia so needs time to establish motion and then accelerate up to maximum velocity. A longer and heavier column of gas, as in the case of inlet and transfers, will be much slower off the mark. It could be argued that the heavier, initially sluggish column might provide a `ramming` effect later on, yes, it just might, but will never make up for what was missing in the first place!
Again, you may have what looks on paper an ideal set of STAs, but if the overall flow coefficients are poor then performance will reflect this and will also be poor, it`s all about optimising flow capability, the internal aerodynamics if you like, and not just about holes in the bore!
What we are seeking from our engine is often in conflict with what the weakest link in the power creating chain is capable of producing, all too often though that link is unknown? As is so often the case with home tuned engines, a chronic miss-match of port timings and angle-area always gives disappointing results. I once looked at a well turned out engine that had the exhaust port of an outright race motor, the transfers of a sports bike and the inlet of a commuter bike and ran a 9:1 comp ratio! The power it failed to make was far too predictable, but all was eventually corrected I`m glad to say and is today a very successful engine.
No matter how much the internal ratios, from the crank to rear wheel, are juggled around there are still only three gears available with which to achieve an optimum track performance. A strong emphasis therefore must be placed on achieving a wide, working power range. This can be maximised down at 190* duration of the exhaust port, where a superposition of wave energy combines with the residual pressure exiting the port. The addition of this extra pressure means that the wave front moving down the header pipe and into the diffuser has a much enhanced amplitude, and this creates a larger depression around BDC. In turn this can pull a larger mass of mixture into the cylinder. It also means that there is a much better port/pipe resonance over a wider range than can be achieved at much longer exhaust port durations up at 200*.
Where does this additional wave energy come from in a superposition situation, and what is a superposition anyway? Where you have two waves meet coming from different directions and pass through each other, their two energies combine to produce a spike. It is this spike that combines with the original exhaust wave to produce the enhanced wave exiting into the header pipe. The original plugging pulse from the tailpipe arrives at the exhaust port just before closure, some of the this pulse is reflected from the piston skirt, returns to the tailpipe and in turn is reflected back again to hopefully combine with the exiting pressure pulse. It will be of a lesser magnitude but will retain enough energy to assist in enhancing power potential via the process explained.
With the short comings of home engineered Bantam engines that may also have limited subsequent development it seems that a BMEP of 150psi is unlikely to be realised, this being the case then long exhaust timings are not necessary. Indeed it is the required blowdown STA that will decide what is achievable at the top end of the engine`s rpm potential. By way of a sobering comparison, the Aprilia engine has BMEP of over 230psi!
If the blowdown STA has been correctly calculated for realistically achievable power within the rpm band required, and the recovery time period from the transfer reverse flow at port opening is such that mixture inflow is maximised then the conditions are set to also maximise cylinder pressure after combustion which then forces the piston down. We do want all the power we can achieve, but not necessarily all the revs we can achieve?
It is this set of conditions, all relating to the blowdown pressure bleed off, that ultimately determines how much cylinder pressure is later achieved, this in turn creates torque, and when combined with engine rpm, that creates power.