Delta-T

Delta-T

Delta-T…The difference in temperature between two measuring points, in either time, and or, position. For there to be any temperature differences there has to be a heat input, some of which develop prior to and during the onset of combustion. It remains a difficult and challenging proposition to satisfactorily cool two stroke race engines and that is particularly so in air cooled examples like our Bantams.
 Way back in the formative years of Bantam racing during the early 1960s, it was a given that any prospective engine builder/tuner would get hold of one of George Todd`s 12:1 cylinder heads, adjust the squish band to piston crown clearance of around .032” and that was the combustion chamber taken care of, along with very effective head cooling. For most of us that was a fit and then forget sort of exercise. That 12:1 static compression ratio remains a constant even if the engine is not running, simply being the ratio of cylinder volume between bdc and tdc, with no reference at all as to how much combustible mixture is actually trapped at tdc. The quest has always been to remove excess heat in the most efficient and reliable manner whilst factoring in that heat is energy which can be mutually converted to advantage. When viewing the clever Todd head design closely, the number of largely uninterrupted fins rooted directly onto the hottest area, immediately over the chamber roof, shows that a lot of thought went into the effectiveness of the design in creating the shortest, largely uninterrupted heat path to atmosphere. Lots of cool air flowing ostensibly parallel with machine direction could traverse both flanks of those five central fins and thereby remove a lot of heat, and in doing so reducing the thermal loading to more manageable proportions. In order to transfer heat into the atmosphere there needs to be a large amount of surface area to radiate the heat and a substantial volume of air is required to pass over that surface. There is a very strong case for ducting air closely towards and over the engine fins. The faster the wind blows, the faster heat goes!
We are not helped by the fact that air is not a particularly effective conductor of heat, being used as an insulator in a lot of applications. Air is a gas, and as such is not very dense, water has greater density so consequently has far better thermal conductivity. Even petrol has about twice the specific heat capacity of air, so a richer air/fuel mixture allows for a tad more heat energy to be absorbed and not transmitted to the engine cooling system and internal components.
 Keeping hot two strokes alive has always been an immense challenge, especially so back then when compelled to use an air cooled cast iron barrel, however, it did present a rapid, but at times painful, learning curve into engine survival techniques. Today there is readily available an aluminium alternative head and barrel combination which thankfully offers a step change in cooling potential.  
I guess it could be stated that our engines are powered by heat, but not to be able to adequately manage that heat leads to excessively high internal temperatures which can have destructive consequences. It is then the task of the cooling system to dissipate that heat into the atmosphere. The threshold of heat dissipation in an iron engine is low but somewhat better in an aluminium one, with water cooling superior in every respect. Thus when the engine is thermally compromised by the fin`s limited ability to quickly and adequately reject heat, the problems begin.
  Returning to the 1960s; it was during this period that Yamaha intensified their racing engine development program, one of the priority areas to investigate and analyse was cooling. The 1963 version the 125 RA55 single cylinder engine rolled out with a greatly reduced compliment of cylinder and head fins, the air gap between each barrel fin being increased from 7mm to 10mm, as a result cooling improved to cope with increasing power output. So what is happening here then, it may seem at first sight an unlikely proposition to reduce fin area to try and improve engine cooling? Cooling depends almost entirely on how much air passes through the entirety of the fin spacing’s per unit of time (introducing the speed element) and how much heat is picked up during that process. Air is a lazy medium and always takes the path of least resistance, so why should air try to pass between tightly packed and strangely angled fins when it can just go around, over or under. In doing so only the extreme ends of the fins get cooled which might be fine when pootling around at 60mph (88ft/sec) but not much good at continuous, intense race speeds.
Whilst you charge around the circuit, the air speed of the bike is not replicated in the speed of air passing through the engine fins, it takes pressure to force air through the limited gap between the fins. There is also a stagnant boundary layer on all surfaces subjected to fluid flow, even on piston crowns during combustion, this layer resists being accelerated away and in doing so offers resistance to air flow. There is always kinetic energy associated with fluid motion, high velocity air first striking the leading edges of the fins decelerates, converting then to pressure energy. If every tiny scrap of that air velocity was fully converted to pressure energy (stagnation pressure) it could be calculated to be about .455kn/m^2 (9.5lbs/^2ft). Subtract then all of the air that spills away from the cylinder and head and all of that which doesn`t make it through the confined fin spaces and sundry obstacles there is not a lot of truly effective cooling going on at all.
Yamaha realised this fact and increase the fin spacing to 10mm, quite a significant percentage increase from 7mm, and more cooling air could then pass between the fins and importantly, right down to their roots. So effective was this revised configuration that the RD56 disc valve 250 twin with just this fin spacing went on to take the world championship, despite Honda producing a 6 cylinder engine to try to keep up. In its final stage of development the RD56 produced around 54/55 hp, fast forward to the mid-2000s and the Aprilia 125 produced just such a power output!
  Bantams have the big handicap in that their racing road speed is relatively modest so the potential for increased cooling air flow remains at that modest level.   Assuming the machine frontal area remains substantially the same along with the drag coefficient, if you wanted to double your speed you will have to make four times the power, don`t see that happening any time soon?  
Using one of the one of the popular aluminium head/barrel pairings I have, and for the purpose of this article, put the fin spacing’s under some rudimentary analysis. I used a series of twist drill shanks to try and determine the diameter that could actually reach close to the roots of the 8 fins and at a multitude of locations. A 6.25mm drill shank could bottom out in about 20% of probing’s, in others that drill would not slot between the fins at all. Head fins fared somewhat better and an 8mm drill comfortably fitted everywhere. It may be that the examples I have are one-offs and all others are an improvement on this situation. Not a terribly scientific or definitive exercise of course but a practical one which does give rise to some food for thought and not a little anxiety?
Whenever the engine has a high compression ratio, a lot of ignition advance or both, a large percentage of combustion heat is conducted out from the combustion chamber into the cylinder and head fins. Reduce one or both of these and more heat ends up in the exhaust pipe, that extra heat will increase the pulse wave speed and make the pipe appear shorter, that may be beneficial or not. When the engine becomes thermally compromised by the fin design`s inability to adequately cool itself, excessive heat input must be reduced by limiting the static compression ratio.
  There are other areas of engine function that produce varying amounts of heat prior to combustion, and during normal running. Friction, the force that opposes motion, be it linear or rotational, is an obvious one and losses increase at the square of the speed whilst at the same time heat increases dramatically. Pistons and rings are the obvious candidates with the big bore Bantam engines suffering much more than their 125 counterparts.

  Chains are another primary source of friction heat and therefore significant power loss. When QUB were developing their 500cc single competition engine, the original transmission drive was by chain to a separate AMC style gearbox. So much power was lost to chain friction heat that the oil inside its case became smoking hot, according to Prof Blair that loss was estimated to be around 6kw (8hp) a high percentage of that heat was absorbed straight into the crankcase! The subsequent engine redesign to unit construction and gear primary drive saw an immediate power increase, confirming the previously calculated power losses. Staying with the big 500 engine, the cylinder barrel featured just 6 barrel fins for cooling the huge 91mm bore and 76mm stroke cylinder and the final power produced from the engine was close to 68hp. But surprisingly, there appeared to be few cooling problems even when running towards 8,000rpm a piston speed of 20.26mtrs/sec, not a high number by today`s standards. A 54mm stroke engine running at 11,000rpm has a piston speed of just 19.8mtrs/sec, the 58mm stroke engines will have to rev to 10,250 to achieve the same piston speed, but all other loading are considerably higher for the 64mm bore engines with their chunkier engine components and increased piston/ring friction.
Chains and sprockets are, with the exception of choosing a suitable overall gear ratios, one of those areas that has little consideration given to them. However, in terms of efficiency there are certain sprocket sizes that should be avoided, Bantams applications seem to contravene all of these rules! A tooth count of 15 is the lower limit and is set by “chordal action” or variation in link speed.  A chain flexes only at its joints, and can`t bend into a true circle, the effective shape of a sprocket is a polygon. For instance, a six tooth sprocket is a hexagon, 8 teeth is an octagon and so on, the higher the tooth count the closer the approximation to a circle. The full action of links and rollers reacting with sprocket teeth is a little tricky to visualise, as the link goes around the teeth the middle of each link has a lower speed to the two ends which are further away from the centre of rotation. As the moving chain engages a tooth, a sharp change in velocity is required as the link alternates between engaging the tooth and when leaving. These sharp impacts generate a lot of friction, thus heat, so the lower the sprocket tooth count the higher the excessive friction becomes, and friction also means wear. I dread to think of the sum total all of the losses associated with a Bantam primary chain flailing around with a clutch body wobbling from side to side going in and out of alignment plus the snatching back and forth from power on and off. Factor in then the chain wear and stretch and things go downhill pretty fast.

Petrol in its liquid state cannot itself burn, it is the vapour given off when combined with oxygen in the air that can burn when some form of ignition is provided. To enable the petrol in a combustion chamber to release enough vapour many times per second, as is demanded by a high revving race engine, something pretty radical and aggressive has to happen. The auto ignition temperature of petrol, where the fuel has sufficient energy to break its carbon-hydrogen bond structure and be oxidised, is about 220*c, bonds between atoms are in themselves a store of energy. To achieve this the fuel/air mixture has to be compressed enough to build up sufficient heat to break the chemical bond of the fuel. Next, and as soon as high tension ionises the gas molecules between the plug electrodes converting from non to conductive, a bridge is formed and current starts to flow, a small intense spark ignites the mixture between the spark plug electrodes and ignition proper is initiated. This small flame front super heats the mixture, vaporises it and so the process of combustion burn gets underway, a high degree of turbulence in the combustion chamber is essential in encouraging a rapid burn rate.
The foregoing is an oversimplification but does serve to illustrate the point that there are significant heat inputs to an engine other than just combustion, and those combined with excessive combustion heat can then overwhelm the ability of the cooling system to cope. It is always wise to bear in mind the fact that a cooler running engine produces more power than a hot one, and making and maintaining power is the name of the game, however air cooled engines always run much hotter than water cooled ones!
1983 saw the publication of a supplement to the original BRC `s tuning manual, it took the form of a pamphlet (booklet) of contributions by the club`s then technical committee. This committee consisted of George Harris, Dave Hunter and Mick Scutt, inspiration came from the writings of Gordon Jenkins and from their individual knowledge and experiences and the whole thing was pulled together by Brian Ing. A lot of the content has been superseded by the advancement of general two stroke technology but there was one article that is as relevant today as the day that its author, Mick Scutt, wrote it and I have copied it here just as it appeared originally.
                                                                    Scuttie`s Dictum

“If the compression ratio of an engine is too high then it will suffer from persistent detonation and overheating problems that cannot be overcome by retarding the ignition or by richening the mixture.
Excessive retarding or richening reduces power output dramatically but a lower C.R. than optimum will cause only a minor loss in power.
The optimum ratio for an engine can only be found by experiment but it is not worth putting a great deal of effort into this as the gains are really very small when approaching the optimum.
The C.R. should not be experimented with until the engine is running reliably so that small changes are easily noticed. It is all too easy to raise the ratio to just over the optimum so that any other small changes to the engine or fuel will bring on overheating problems.”

When I first saw Mick`s piece I was so impressed by it that I got my wife to protectively laminate the page at her school along with the title of “Scuttie`s Dictum” and I had it prominently displayed in my workshed. You could read a dozen books and not find better advice!
Power is heat is power, the big variable here is the dynamic compression and the quantity of heat the static comp pushes into the engine cooling system. BMEP is directly tied to the delivery ratio, trapping efficiency and scavenging efficiency, improving the input value of these elements will potentially mean more power and that means more heat. The more efficient the breathing, the greater the quantity of combustible mixture is trapped at TDC, so when pushing the threshold of overheating the static comp needs to be reduced.  Certain aspects of playing with comp ratios can be mutually cancelling, reduction slows the burn rate but at the same time the gas expansion rate is less so there is more energy remaining at exhaust port opening for the pipe to do good things with. Too restrictive an exhaust tailpipe diameter can also significantly overheat an engine, particularly so if the engine temperature is already rising towards danger, a mil too large will hardly be noticed, but too small could well be a mil too far!
I often feel that trying to understand two stroke behavior is at times like trying to navigate a mine field, in the dark with tracer bullets pinging everywhere and having no idea as to the direction you`re stumbling along in, be it towards friend or foe.

Trevor.