Ideal for Trials
Rotax had gained experience with this motor in Enduro machines and it had a reputation for superb reliability. The main attraction of the 250cc air-cooled single cylinder twostroke engine was the innovative reed-valve induction system which, in theory, would appear to be ideal for the slow-speed world of trials. The inlet timing would be controlled by a spinning disc valve on one end of the crankshaft, which would have a cut away in it; when this opening paired up with the hole in the crankcases it would allow the airfuel mixture into the combustion area.
The four main functions of a two-stroke engine: induction, compression, power and expulsion are carried out in two movements of the piston, hence the name ‘Two-Stroke’. On a conventional engine the mixture enters the crankcases first as the piston goes up, sucking it in, and then as it goes down it forces the mixture through the transfer ports up to the top side of the piston. As the piston goes back upwards it sucks a fresh charge into the crankcase, and at the same time it compresses the first charge, the spark plug ignites and pushes down the piston. The burnt air/fuel mixture is expelled out the exhaust port at the same time as the second air-fuel charge goes up through the transfer ports.
The rotary valve system is unique in the way it controls the intake of the air-fuel mixture into the engine. The spinning rotary valve is in reality a very thin metal disc with a cutaway that opens and closes the intake passage controlling the flow between the carburettor and the crankcases of the mixture.
What attracted SWM to the Rotax engine was the ability to be able to fine tune the timing of the ‘open and close’ function of the spinning rotary disc. If you can get this timing correct it can create a form of ‘super charging’ in the motor which could be found to be ideal for a trials engine. One problem with the rotary valve system is that when the mixture enters the crankcases with the piston going to the top it stops, and when it starts returning to the bottom, if the disc valve window is open it can be forced back to the carburettor. The secret is to get the disc-valve timing correct so it can be made to open and close at exactly the correct time.
With this all set to give the optimum performance it can actually increase performance above normal, conventionally aspirated twostroke engines. The down-side is that it makes it less efficient at low engine revolutions if the disc-valve timing is not correct.