2 STROKE versus 4 STROKE

I hope you don't mind, but I have the habit of thinking of the two-stroke engine as a "Single Cycle" (SC), and the four-stroke as a "Double Cycle" (DC). As it happens, the guy who invented the DC (Nicolaus Otto) also preferred the 'cycle' reference base. As a multi-discipline engineer, who tends not to separate out those disciplines, I find it rather silly to 'divide the indivisible', if you see what I mean.

Clearly, if an SC engine provides power on every cycle, and a DC on every other cycle, the SC is more efficient. But the issue is far from as simple as that. Indeed, there are many issues to do with IC engines which are very far from "cut and dried". A serious researcher would be well advised to tread very carefully through the minefield of information, opinion, experience and even written studies of IC engine technology.

One of the best, but also the worst, sources of information is the racing arena. Many serious engineers have indicated to me a very clear aversion to this particular avenue, which is a pity, because by applying a little common sense, it is easy enough to separate the fact from the fiction. Listen to race engineers talking about creating a "Still air chamber" to feed their carburettors, and you have to wonder if they are sane. On the other hand investigate why in British Super-Bike racing, a 750cc DC engine is held equivalent to a 1000cc SC, and you can be sure that you will get very sound information. Why? Because if there were any 'nonsense' involved, the collective wisdom of the team engineers would be down on the association like the proverbial ton of bricks. As I say, it only takes a little common sense.

So then, it would seem that my earlier statement was wrong! A 750cc DC is able to deliver the same power as a 1000cc SC engine?

In fact, what we are seeing here is the result of practical application: A very good example of "cause and effect", muddling the issues. Using conventional technology, the DC engines are able to achieve significantly higher flywheel speeds. Ergo, more power. (So if anyone needed proof that flywheel speed is a significant factor – there it is). However, design an SC engine which will match the flywheel speeds of the DC, and the racing rules will have to be changed.

So are there any SC engines that can match the speeds of the DC? Not exactly (I leave the VLB out of this discussion – for now) but take a look at the huge locomotive versions of the SC diesel engines and you will begin to see where the problem lies. These multi-thousand HP loco giants employ conventional lower cylinder (SC style) air input systems, but use top of cylinder (DC style) exhaust valves. The purpose is to improve gas-flow, and the improved efficiency of the engine is unarguable.

Unfortunately, their designers missed the trick of arc initiated adiabatic combustion, so they have to employ very high compression (unnecessary load). They also squirt the fuel into the cylinder at ignition time (seriously stretching the burn time, and so limiting the cycle speed) and, of course, they are so huge, that direct comparisons are difficult. However, if you apply rational factors to the equations, you will find that they are closing the SC / DC speed gap. Furthermore, they are amongst the most efficient piston type diesel-fuel converters around.

Ask any race engineer what is the most controversial and mysterious area of engine preparation, and you will most often be told – "gas-flow". Tuned exhaust systems and the VECTIS (my own contribution) have unarguably provided substantial improvements to the external gas-flow, and hence to the overall efficiency of IC engines. In addition, though, every engine designer (race or road) wants to tell us how they have 'tamed' the gas-flow inside the cylinder. (Let me tell you that the 'inside the cylinder' situation is the biggest information minefield of them all). I'm afraid that, as ingenious and scientifically admirable as most of these feats of engineering are, all that they are really saying is that they have managed to "paper over the cracks". The conventional SC engine strangles itself with its 19th Century adherence to "simple is beautiful" technology. I don't have any argument with the principle - I absolutely loathe engineering sophistication - but "simple is best" only if it works! The fixed nature of the SC engine gas-flow severely limits the efficiency of the engine. On the other hand, the crazy "in at the top & back out at the top" gas-flow characteristics of the conventional DC engine isn't really that much better. It is better, but it is nowhere near good enough. I have watched highly skilled race engineers contouring the piston crowns and head geometry with the skill and understanding of a brain surgeon - or perhaps I should say painting over the defects with the skill of a master artist.

So to conclude the issue: The SC engine would be the most efficient, if the gas-flow could be as good or better than in, and if the cycle speeds could match or exceed those of, the DC engine. Which is why the SC VLB engine has simply been designed without the cracks.

John Allen