supercharger

Supercharging a 2-stroke engine is a lot more complicated than a 4-stroke. Simply forcing air into the intakes of the 2 model engines used in model cars will not work. This is because, given no other problems, the piston port valve system on these engines means that port timing can not be altered and as standard there is always a considerable time during which both intake and exhaust ports are open. Forcing the combustible mixture into the intake would only result in a great deal of it going straight out of the exhaust. What is needed is a new method of either shutting the exhaust port early, or having a different intake port controlled by a disc valve or similar, running off the crankshaft to keep in time with the engine. Shutting the exhaust port early is not a practical solution because exhaust must be expelled and there really is no other way of getting rid of it. Also, the crank case becomes a sealed unit containing combustible mixture, BETWEEN the intake and the cylinder, thus negating the effect of trying to force air into the intake to increase density in the cylinder.
Here are some interesting supercharger calculation results I did:

We assume the engines are revving to 25,000 RPM
Each engine in the Twin21 has a displacement of 3.5cc = 7cc in total (considering both engines)
2-stoke engines induct on every cycle, that is 25,000 times per minute
Therefore they require 25,000 x 7cc of air per minute for normal aspiration
25,000 x 7cc = 175,000cc/min = 2920cc/sec

So the engines suck a pretty colossal 3 litres of air per second under normal aspiration.

To achieve a supercharger boost pressure a charger must supply in excess of this volume per second. Simple calculations using a large computer fan in a sealed duct, attached to the drivetrain by a geared-up toothed belt drive show the fan would have to spin at around 1000RPM to achieve a useful boost pressure. This is within the capability of the drivetrain when the engines are spinning at 25,000 RPM, so in theory the supercharger is completely plausible - IF all the above problems described could be solved!

It has also been pointed out to me by a number of people that the engine is already being charged by the tuned exhaust system used. This was not lost on me when I was thinking about the workings of a supercharger. It is because the exhaust is designed to be a resonant cavity, such that the exhaust pulses are magnified in amplitude but contracted in duration, that the simultaneously open intake and exhaust ports do not cause much more combustible mixture to pass unburned through the engine. This pressure wave (sound wave) generated in the tuned pipe comes close to equalising the pressure imbalance at the exhaust port, so it is energetically favourable for most of the combustible mixture to remain in the cylinder. The sound amplification in the tuned pipe means there is far less energy coming from the pipe's outlet, so the engine sounds quieter too - effectively the tuned pipe is also a silencer.
Now if somehow the inlet port was under greater pressure than normal, the tuned pipe's effect would be diminished. The problem with resonant cavities is that they do not resonate at all frequencies (or it wouldn't be resonance!) - meaning that they have a decent impact on performance only in a short interval of engine speeds. It is not possible to make a resonant chamber exhaust which resonates at all useable engine RPMs - UNLESS it has on the fly adjustable dimensions. Practically this is plausible, by using a telescopic pipe. Of course then there is the problem of sealing the sliding joint and everything else. Interesting idea though.

So, I am not going to try to make a supercharger for these engines. BUT, should I ever have the fortune to see an OS 40 Four-Stroke for sale 2nd hand, or a couple of FS26's, I might rethink things! As it is, this project has totally cleaned me out at the moment.