Blowdown Lowdown

Blowdown Lowdown (plus extra bits)
The blowdown value is very important, by comparison, the exhaust value is very unimportant with certain aspects of blowdown critically important! Emphasis therefore is placed upon the blowdown time-area and not on total exhaust port time-area. Exhaust gas flow and pressure pulse values are established during blowdown which the pipe then seeks to manipulate as dictated by its various dimensions and pressure differences.
Now that`s a radical statement you won`t find in any of the 20thc two stroke tuning reference books currently available. Some popular material totally ignores blowdown altogether, but as we are now in the 21stc we have to reassess what we think we know to enable meaningful progress to be made.
When there is sufficient cylinder blowdown there is no remaining exhaust gas to flow before the transfer ports open, but this a condition that almost never ever exists. As a consequence of the enforced blanket GP ban, the most significant two stroke development work at a `factory` level has been in the Karting world. One area here which has seen the most intensive investigation is cylinder blowdown integrated with corresponding reduction of exhaust duct dimensions and profile which so far has been shown real possibilities to yield extra performance.
In the blowdown time-area, time is between the opening of the exhaust port and transfer ports, and area is the open area of the exhaust port at transfer opening. However, as rpm increases real time available for flow decreases, double the revs and you halve the available time, with the area remaining the same. How much exhaust gas flows through that limited port opening during the time period available not only depends on the driving pressure and time-area but increasingly on the flow coefficient, (coefficient of discharge). Time-area is a mathematical concept and so remains undefined until the engine is actually running, but the flow capability is very much a physical one. Angle–area is a true constant and is defined by the geometry of the engine design elements namely, stroke, rod length and port design, and so is the sum of port open area x port open angle. The more of the circumference of the cylinder wall that is utilized the greater angle-area achieved but the wider you go with the port, the greater will be the amount of fresh charge that is lost to the exhaust port through short circuiting. The double whammy here is that fresh charge will in turn cool the exhaust gas and slow it down, as a consequence the very careful pipe calculations you made will be compromised. More significantly, that short circuited mixture thus energy potential cannot then be used to make power. A typical two stroke power band relies very heavily on utilising pressure fluctuations in the exhaust pipe to best effect.  
My dog eared, 1960s copy of The Ladybird Book of the Two Stroke Engine states “The exhaust port opens and a short time later the transfer ports open”, that’s it, further expansion on that statement is not offered. Which is pretty much the situation with the popular tuning reference literature for the next 15 years or so, with emphasis almost exclusively on exhaust port timings. It was not uncommon to see durations reach the dizzying heights of 205* and more, with scant reference to the effects of blowdown. The concept of needing to equalise high cylinder pressure with lower crankcase pressure by means of cylinder blowdown, and, that scavenge flow cannot begin until that equilibrium is established, appears yet to be acknowledged. A high case pressure will assist in the equalising of pressure, so might it be said that with a high case pressure you can have less blowdown? That might fly in the face of modern trends where case pressure has systematically reduced from the stratospheric numbers of the 60s-70s. It is usually assumed that the opening of the transfer ports to be the end point of blowdown but in reality we have no simple way of determining when the two pressures are actually equal, and in any case that point will vary with changing rpm and varying pressure ratios.
Blowdown consists in the main of four principle factors: exhaust port timing, partial exhaust port open width/area, transfer timing and the largely ignored but absolutely crucial flow coefficient.
Mass flow from the cylinder is proportional to the blowdown exhaust port open area, and again to the flow coefficient. Twice the open area will provide for twice the flow, but only if the flow coefficient remains consistent. We need therefore, as much mass flow as possible during blowdown to extract all of the combustion junk from the cylinder before the scavenging cycle begins. Blowdown should be completed by the time the transfer ports first open, but flow in an engine does not happen instantly due to gas inertia per sq. mm of open area and pressure ratio. It takes a certain amount of time to get flow underway, after the initial weak movement from zero the gas has to accelerate up to its maximum velocity and for the pulse to achieve its maximum amplitude at the local speed of sound. That in turn depends in large part upon the upper port profile, is it straight across or curved down with large corner radii or sharp edges and crucially the whole duct geometry profile leading to the outlet at the header junction.
It is quite possible that a powerful Bantam engine (that is a relative observation of course) could achieve exhaust gas velocity at Mach speed where there is maximum pressure over the smallest outlet open area. However, this will very rapidly decline as the exhaust gas falls off the cliff edge of the piston crown into the gaping abyss of the duct volume, all the while expanding, losing precious energy creating chaotic turbulent eddies, vortices and flow reversals towards the floor of the duct adjacent to the piston skirt. It is never a wise move to sacrifice so much energy in engine ducts after putting so much effort into creating it in the first place, energy which is then lost before the pipe ever sees it. Abandoning old ideas which are firmly entrenched is always tricky but tying duct geometry to blowdown is crucial to achieving the sort of power that in reality the engine can make.
The ideal situation would be of course to have all of the burnt gasses discharged from the cylinder before the transfers begin to open, so from that standpoint it is difficult to have too much blowdown. But in trying to achieve that you can arrive at too high an exhaust port, which in the case of a Bantam engine will cause an unacceptable reduction of power at lower rpm. Here then is the first contradiction of blowdown, at low rpm blowdown will be far too great and consequently there will be a loss of fresh charge straight to the exhaust port. At rpm leading to peak torque and beyond, blowdown becomes time limited so may continue after the transfers open. If the blowdown phase does not allow cylinder pressure to fall below that of the crankcase the remaining exhaust gas will reverse flow into the transfer ducts at their first opening. Only when cylinder pressure has dropped sufficiently can the transfer phase begin; but what is actually being transferred first: the exhaust gas that filled the transfer ducts! Only after the transfer ducts have cleared away this rubbish can fresh combustible mixture begin to flow. This effect can be summarised as: insufficient blowdown angle-area will eat doubly into the transfer angle-area.
To maximise outflow from the cylinder during blowdown the flow needs to attach the duct roof (the Coanda effect), separated flow always creates a lot of extra turbulence, with the duct floor still too far away at this stage. By moving the duct floor upwards and re-profiling, the energy sapping turbulence beneath the main flow is reduced and some velocity and directional stability is restored. This is important as a lot of hot spent gas can be trapped within the tumbling maelstrom and is never ejected and this contaminant returns to the cylinder by exhaust pipe action.  Modern thinking suggests that as the exhaust pressure event is all but over by the time the exhaust port is about two thirds open, the duct floor can then be raised to the benefit of gas flow. So having the lower part of the duct window effectively narrower with less port area in the lower portion, it becomes less inviting for transfer mixture to short circuit straight out into the exhaust. The piston skirt, crown and rings are given a little more time to cool off and pick up some lubricant from the extra cylinder wall. Having the exhaust duct floor higher reduces turbulence in the outgoing exhaust flow during the blowdown period. It also reduces the cross section of the duct, so the volume of over-scavenged fresh charge is contained effectively within a longer column so there will be less mixing contact at the interface between the two gasses and a greater chance of the old junk leaving and the good stuff remaining. The benefit of this is seen when the exhaust pipe sends a plugging pulse back to the cylinder and the raised duct floor returns and guides the fresh charge, of greater purity waiting in the duct, up over the piston edge into the cylinder inducing less turbulence and improving efficiency. Imagine a sort of ski jump in reverse and you`ll get the basic concept.
There is a considerable contraction of fluid flow creating very turbulent, constraining conditions around the edges and through any opening that leads from a large area, an exhaust port is a classic case. The circumference of a 64mm bore Bantam is 201 mm and for a 54mm bore, 170 mm, a single exhaust port at 68% of bore diameter for each is, 43.5mm and 36.7mm respectively with just 6.8mm difference. They will initially be very much smaller when taking into account the port`s corner radii and with the piston set just at transfer port opening. The mass of combustion gas produced has to flow through that restrictive slot in a miniscule time frame at peak torque rpm from bore areas of 3217 sq.mm and 2290 sq.mm respectively, a 40% difference. A bridged port represents two such contractions with four corner radii and the web itself is a major blowdown flow disruptor. That opening of course only represents part of the port window in the cylinder bore, what lies beyond in the duct may well ultimately determine the overall flow rate. I firmly believe that in 2021 it can safely be stated that most exhaust ducts are way too big, and in all probability most Bantams are the same!
Blowdown is today still the least understood and reported on of all of the events occurring in a two stroke race engine, perhaps because it is also the briefest, but everything depends on the initial flow behaviour through the opening formed by the port/duct ceiling and the edge of the piston crown. Every opportunity therefore must be given to allow the exhaust gas to escape as quickly, smoothly and completely as possible, so when embracing the concept of raising the duct floor, the whole port window to bdc is becoming increasingly irrelevant.
 Am I alone within the Bantam world in thinking that so many of our race engines, even class of 2021, still reflect their vintage heritage, along with maintaining outmoded design concepts and narrow, restrictive and restraining “Bantam Lore” thinking? It is all the more puzzling when there is such an enormous amount of state of the art techno information out there for the finding, interpreting and where appropriate, applying.
If a bantam engine is making less than 10bar bmep then it doesn’t need up to a 200* exhaust timing. Just to put a bit of context here, a 125 engine running at peak torque at 11,000rpm with a crank power of 25hp is making just 8.5bar. Similarly, a 186 engine running at 10,000rpm and 30hp at the crank is making just 7.2bar, 10bar for the 125 would equate to 31 crank hp; don`t think we are quite there yet?
Many Bantam cylinders are angle-area deficient in their transfer ports and have poor duct design and make their power by multiplying their limited torque with as many revs as possible. They are also of course woefully deficient in blowdown angle-area, this is often compensated for, perhaps without realising it, by extending timing taking the exhaust port up in height to arrive at timings towards 200* in duration. The consequence of this is that blowdown will continue after the transfer ports open, further reducing the transfer angle-area and pre-heating the waiting fresh mixture. With exhaust timings down towards 190*duration there will be strong wave superposition at exhaust port opening, providing for a desperately needed, wider power band than could otherwise be achieved. If you want to take advantage of this augmentation then that more or less fixes the timing around 190*, and with only a three gear option available you would be unwise not to do so!
Superposition occurs where the residual pressure sitting at the exhaust opening point has a new returning pressure pulse added to that residual and the enhanced pressure pulse exits down the duct into the header. When two waves interact and combine the resulting wave is larger than the combined original waves, the sum of their magnitude could best be described as “The whole is greater than the sum of the parts”. It follows that the larger the amplitude of this wave in the pipe`s diffuser the greater the depression around BDC to promote the biggest mass transfer flow over a wide band width. If you can arrive at an appropriate pipe design, its natural resonance will increase peak power and not just band width. This enhanced primary pulse may also lead to a stronger reflected pulse. However, these waves can also react destructively combining in such a way as to make the conjoined wave smaller than the original ones. For example, this effect can occur when a returning wave from the end of the rear cone catches up with existing waves and the two collide at the junction of a reducing cone and the whole collapses into a confused shock wave of greatly diminished amplitude. Suction waves of strong negative amplitude returned from the diffuser are particularly at risk for they travel at a far slower amplitude than pressure waves from the rear cone and are particularly at risk of being overtaken and nullified.
As a slightly left of field observation, a smaller exhaust duct and header diameter will lower the Helmholtz effect of the whole exhaust system, and that could well promote a more enhanced bottom end power. Not everyone may realise that the Helmholtz frequency and the wave frequency together determine the exhaust pipe frequency, and as they say that, every little bit helps, it is well worth thinking about?
Trevor.