Conceptual Design Analysis of a Bantam Race Engine
Part 2, with emphasis on exhaust port and duct design and construction
The conflicts between design and reality all crash together within the construction of a Bantam race barrel!
With the transfer ducts fixed in place on the original barrel casting and awaiting some final small detail cosmetic internal milling, I had a dimension across them which could provide an inside diameter for the water jacket. I had of course made a number of appropriate drawings but the physical components in the hand can often be more telling and informative. By skimming each duct outer edge in the lathe I could have a jacket i/d of 117mm with a wall thickness of 4mm (8 gauge) giving an outer diameter of 125mm, with an initial length of 80mm, all nice convenient numbers. Trimming the duct outer walls also gave a touch more clearance for water to flow around the outer duct walls to help keep the transfer mixture cool and dense. The jacket itself was rolled up from an off-cut from a sheet of 4mm (8 gauge) boiler plate. There is no particular significance to using boiler plate other than it was quality steel, available for free and of the correct size. The guys in the fabrication department of the family engineering firm where I worked produced a beautifully round tube of astonishing accuracy for me which was seam welded then after was gently persuaded back into a circular profile. What those skilled engineers can produce by cutting, roll bending, welding and finishing are little short of true sculptural works of industrial art, I`m so envious of their talents! The lower fin of the original bantam barrel was skimmed on its diameter to provide just a few thou clearance to the inside diameter of the water jacket. The top fin under side, after skimming, engaged in a suitable register in one end of the tube thus creating a suitable datum for linear measurements. A weld prep was machined at the same time in readiness for the final assembly, again the Forum drawings and photos illustrate the whole arrangement. This set up enabled the two parts to be hand slid apart at will. Importantly, all clamping forces from cylinder head and outer water cap were projected vertically downwards through their respective wall thickness to the crankcase face thus ensuring that none of the sealing joints were mechanically stressed at all.
With the barrel able to be slid out the jacket at will I had tremendous scope for arranging a prefabricated exhaust duct to fit between the barrel stub fins and jacket inner face, I also had a dimension to work with between the two in which to install the duct. I had already decided that some of the full duct length and inner profile would be external to the 125mm water jacket outside diameter and incorporated within the pipe header location stub. The main design considerations here being the whole duct length, its exit area and the optimum transitional profile and volume from port to pipe. Some thought must be given to the effects of exhaust duct reverse flow during the period of piston motion between transfer ports closing, up until exhaust closure. It seemed somewhat counter-productive to markedly improve the conventional transfer ducts and then ignore the exhaust duct transfer reverse flow potential of the plugging pulse.
Before any of the duct design and construction could begin the actual exhaust port shape and dimensions in the liner had to be decided upon. To a certain extent, the transfer ports have an influence on the size of lower part of the ex-port, too wide and the transfers become cramped and narrower losing vital area, smaller and narrower and STA of the transfer port would suffer. Clearly there was going to be fine balancing act between these two conflicting elements of design parameters, throw into the mix the crucial requirement of adequate cylinder blowdown at peak torque rpm and it all gets far more complicated.
There are two main options for exhaust port design, the traditional single outlet or the bridged version, there is also the triple port arrangement but for the Bantam engine there are a lot of installation and construction obstacles to overcome if considering using such a set up. Bridges clearly are an obstruction to flow, and run extremely hot, so having two bridges disrupting blow down flow as opposed to just one would be problematical. I did read in an SAE paper that after much testing and flow bench work a bridged port will flow no better than 90% of its un-bridged counterpart, one can only imagine that the triple port will fare even worse. There is no point in going to all the considerable trouble of constructing a triple port set up if the flow co-efficient is inferior to the alternatives. During the last gasp of the 250 GP two stroke era Yamaha abandoned their factory triple port exhaust set-up in favour of the Honda style single bridge and promptly had a competitive engine once more. During any re-design process it is always wise to keep in mind that we Bantam dabblers, would be foolish in trying to create a GP, 15 bar level of power, as nice as that would be, but merely trying to improve an existing race design to hopefully lift the bmep level a digit or so higher. It is that reality which should inform the design process where appropriate and realistic decisions can be made from that standpoint.
I dug out my old drawings and notes from the 1990s, and earlier, ostensibly to re-acquaint myself as to the sequence of analysis as to the pros and cons of a single port compared to the bridged port. After doing some more sums, and re-doing some of the old for both options, the balance was tipped in favour of the bridged port. However, the design and subsequent construction of a single port duct would have been much easier and far less stressful than the bridged version proved to be. The decisions made at this initial stage of assessing the alternatives have to be the correct ones, for none of the options can easily be retro fitted, so you really only get one choice, one chance!
During the initial assessment and elimination process, I looked first at the traditional single port. It makes sense in a race engine to max out the width, hence blowdown area. With a 54mm bore and a `traditional` 70% width that made for a 38mm chordal width dimension. Modern competition pistons and rings are now very reliable, although I always try to opt for the cast option for pistons finding the forged alternatives softer and expand more. The port height, with the 54mm stroke, of the port is determined by timing and the con rod length. For this illustrative article I have opted for a 193*duration and a rod length of 110mm, this combination provides for a 26.75mm high port where the lower edge is at bdc. I have picked on the 110mm rod as a sort of average, I know rods are used from 104 up to 120mm centre lengths in 54mm stroke race engines. Indeed, back in the 80s the very first 54mm stroke Bantam engine I built had a 116mm Bultaco con-rod fitted, together with a Wiseco Suzuki TR 250 piston. Using a 115mm rod with all the other data remaining the same reduces the exhaust duration to 192.44*.Yamaha, in their old RD based TZ racers, used 72% for exhaust port width along with generous corner radii. I opted for top corner radius of 10mm and the bottom 12mm, if I was going for 72% width, or greater, then a top edge radius of 100mm and lower of 80mm will give the piston ring and piston top lands an easier time, certainly the 64mm bore engines need careful consideration, bigger diameter rings are inherently more failure prone. 50 cc engines with 40mm bores can live with 75%, and more, port widths with no apparent reliability issues.
Cylinder blowdown is the most critical of any of the gas flow situations in the entire engine, and with a single port we are forced into a corner where the need for a realistic transfer STA can compromise the available time/area of the cylinder blow down. A few sums can serve to illustrate the problem. The exhaust port in our example has a total area around 912sq.mm. However, if the duct roof angles down from the port top edge by say 20*, then gas flow over the piston and port edge will be initially impeded by the duct roof. So to arrive at a realistic port flow area the total area must be multiplied by the cosine of that 20*declivity (.94) to the duct roof. So the effective area of the port is 912 x.94= 857 sq.mm, which in turn equates to a diameter of 33 mm. While it may only be those first few degrees of initial port opening that are impeded they do however represent a significant proportion of the area of that important blowdown phase. Indeed, it is not until the 10mm corner radius is fully exposed that the port can begin to flow to its full blowdown potential.
Selecting a suitable timing for the transfer ports will to a large extent determine the blowdown area available. My records show that at the design stage I chose a height of 12.5mm which gave a timing of 64* bbdc and with the exhaust timing of 193*, a blow down angle of 32.5 degrees. I must have been ultra-keen back then for I also calculated that blow down area as 495 sq.mm equating to a diameter of 25 mm. Now, if you accept that current thinking suggests that the exhaust event is all but over a little after blow down has ended then most exhaust ducts and outlets are way oversize. Consequently the duct has excessive volume impairing high velocity flow, as a result fresh mixture and old gas mixing is inevitable, thus costing power?
More than 30 years ago Gordon Blair published the first of his two influential books, `The Basic Design of Two Stroke Engines`. One particular section is of great significance and relevance for today and it is one I would like to quote here. With the knowledge of the exhaust port flow area, the empirical calculation equivalent describes the ideal initial pipe diameter.
“In a racing pipe where maximizing the plugging pressure gives the highest trapping efficiency, there seems little point in having the first header pipe diameter much more than 1.05 times larger than the exhaust port area. In unsteady gas flow, the larger the pipe the lower the pressure wave amplitude for the same mass rate of gas flow.”
Subsequent development work, mostly conducted in Europe has now advanced the suggestion that for a single exhaust port the duct outlet diameter need be no more than 90% of that port flow area. In our example of the single port the effective flow area is 857 sq. mm. It follows that 90% of 857 gives an area of 771.3 sq.mm which is an equivalent diameter of 31.3 mm; using the 1.05 value provides for a 33.8 mm port diameter.
You may have wondered why I convert areas into equivalent diameters, I often feel that complex areas can be tricky to visualise, but a diameter is easily analysed and compared. Let`s face it, not everyone enjoys doing the maths which can become tedious, and whilst it may be considered somewhat perverse, I do enjoy the numbers!
I next looked at the bridged exhaust port option, at face value there were many potential advantages over the single port but also some disadvantages, not the least of which was actually constructing the thing. The bridge would impede blowdown flow as would the radii at the top corners adjacent to the bridge sides, it would also run hot, but then cast iron would resist distortion far better than an aluminium race barrel such as the RS Honda. To justify its selection the blowdown area would have to be significantly more than the single port could offer to compensate for the obstruction to exhaust gas flow from the bridge and loss of STA. The lower portion should not compromise the size or position of the A transfer ports. To achieve this the lower portion would have to be a lot narrower than the upper and profiled with a negative radius to avoid the top corner of the A transfer port. If I was doing this now, I would have also arranged for the bottom of the port to be a few mm higher than bdc. The higher exhaust port floor and duct, when taken directly all the way to the outlet diameter, makes for an improved blowdown outflow coefficient, hence stronger exhaust pulses which the diffuser can use to greater effect, less mixing of spent and fresh mixture in their contact area, less short circuiting of transfer mixture lost to the exhaust pipe and, an improved flow coefficient for the returning plugging pulse mixture; so what`s not to like, the dyno would show its approval!
Just as I had with the single port I got the speculations transcribed into sketches, then to more accurate drawings. This process does not demand particularly detailed images, in many cases a few lines and curves will suffice but what is important is that dimensions are included. Referencing angular and linear details at a later date is much easier with accompanying numbers.
I am not a fan of offset exhaust ducts, even more so when a port bridge divider is included, just where do you position it and avoid compromising one side or the other? Air cooling around one side of the barrel duct is minimal with very short fins shrouded by the offset, a very far from ideal situation. There is always significant asymmetry to the passage wall lengths so the conflicting velocities of gas flow streams just create turbulence which saps vital pressure wave energy right at the start of the exhaust event, and energy that you have worked hard to create in the first place! So yes, another example of not being able to make up for later what you lost at the beginning. The obvious way to resolve most of the barrel issues was to straighten the duct; but the frame`s single downtube is in the way!
After a lot of sketching, measuring, adjusting and finally getting a reasonable plan view assembly vision drawn out I came to the conclusion that if the frame tube was scalloped out and plated, with the reverse side suitably reinforced then a straightened duct which terminated just beyond the water jacket 125mm o/d then angled away at that point with the header pipe diameter to be used, it would all just fit. A short multi angled exterior stub and flange which also projected downward to accommodate the exhaust pipe completed the exterior barrel design work, as far as the exhaust duct was concerned. The actual bridge for the duct was made from the outer portion of one of the fins that were bashed off the original barrel, a couple of location grooves were cut into the barrel and duct then a suitably shaped piece of fin was eventually brazed into place. At the same time some of the old duct floor, which was now too low, was built up with braze. Using a segment of old fin for the bridge had the advantage of it being similar material and had a nicely tapered section with a radius on its trailing/ leading edge face.
I next looked at the bridged exhaust port option, at face value there were many potential advantages over the single port but also some disadvantages, not the least of which was actually constructing the thing. The bridge would impede blowdown flow as would the radii at the top corners adjacent to the bridge sides, it would also run hot, but then cast iron would resist distortion far better than an aluminium race barrel such as the RS Honda. To justify its selection the blowdown area would have to be significantly more than the single port could offer to compensate for the obstruction to exhaust gas flow from the bridge and loss of STA. The lower portion should not compromise the size or position of the A transfer ports. To achieve this the lower portion would have to be a lot narrower than the upper and profiled with a negative radius to avoid the top corner of the A transfer port. If I was doing this now, I would have also arranged for the bottom of the port to be a few mm higher than bdc. The higher exhaust port floor and duct, when taken directly all the way to the outlet diameter, makes for an improved blowdown outflow coefficient, hence stronger exhaust pulses which the diffuser can use to greater effect, less mixing of spent and fresh mixture in their contact area, less short circuiting of transfer mixture lost to the exhaust pipe and, an improved flow coefficient for the returning plugging pulse mixture; so what`s not to like, the dyno would show its approval!
Just as I had with the single port I got the speculations transcribed into sketches, then to more accurate drawings. This process does not demand particularly detailed images, in many cases a few lines and curves will suffice but what is important is that dimensions are included. Referencing angular and linear details at a later date is much easier with accompanying numbers.
I am not a fan of offset exhaust ducts, even more so when a port bridge divider is included, just where do you position it and avoid compromising one side or the other? Air cooling around one side of the barrel duct is minimal with very short fins shrouded by the offset, a very far from ideal situation. There is always significant asymmetry to the passage wall lengths so the conflicting velocities of gas flow streams just create turbulence which saps vital pressure wave energy right at the start of the exhaust event, and energy that you have worked hard to create in the first place! So yes, another example of not being able to make up for later what you lost at the beginning. The obvious way to resolve most of the barrel issues was to straighten the duct; but the frame`s single downtube is in the way!
After a lot of sketching, measuring, adjusting and finally getting a reasonable plan view assembly vision drawn out I came to the conclusion that if the frame tube was scalloped out and plated, with the reverse side suitably reinforced then a straightened duct which terminated just beyond the water jacket 125mm o/d then angled away at that point with the header pipe diameter to be used, it would all just fit. A short multi angled exterior stub and flange which also projected downward to accommodate the exhaust pipe completed the exterior barrel design work, as far as the exhaust duct was concerned. The actual bridge for the duct was made from the outer portion of one of the fins that were bashed off the original barrel, a couple of location grooves were cut into the barrel and duct then a suitably shaped piece of fin was eventually brazed into place. At the same time some of the old duct floor, which was now too low, was built up with braze. Using a segment of old fin for the bridge had the advantage of it being similar material and had a nicely tapered section with a radius on its trailing/ leading edge face.
Before work on the prefabricated duct could begin the actual ports that would eventually in the liner had to be drawn out. The first was a plan of the radial position and gas stream directions of all of the ports, ie, bridged exhaust and five transfer ports. This brought in the headache that the transfer ducts and ports taper in section from the crankcase to the port in the bore. Indeed this taper applies also through the wall thickness of the liner where the ports on the o/d are wider that the port at the actual bore. The axial element of the transfer ports are further complicated by their individual and differing inclination angles between the A,B pairs and single C port openings. More about the liner and porting details in a later instalment, if I get that far. Again there are photos and drawings on the Forum site which can help inform any interested and inquisitive enthusiasts.
The centre line through the exhaust port to the o/d of the water jacket indicated where the tapering walls of the exhaust duct would project to and where the water jacket could be breached to then blend with the tapering duct. A final transition from the oval section at the jacket to the final circular outlet to pipe junction is taken care of in the length of the external exhaust stub /pipe joiner.
The liner exhaust port sides were machined parallel to the duct axis along with the original barrel wall. This gave a 12mm of wall parallel to the duct bridge centre offering then minimal obstruction to the initial exit of high velocity gas flow. The combined bridge length and liner wall thickness was 16mm, so with the taper of the bridge those 4mm length extra would seem to offer no measurable hindrance to flow.
As this latest installment has grown to 3600 words I felt it wise to post it now before the whole thing gets unmanageable. More to come on the exhaust topic. What I have thought a little strange is the 500+ hits on the engine topic but far, far less in the topic describing that same engine?
Cheers for now, enjoy your chocolate eggs, Trevor
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Sun Apr 09, 2023 9:25 am