2 stroke engines in racing motorcycles
reed-valveAlthough two stroke engines have long since been updated with four stroke racing bike engines, two stroke engines provide a lightweight and suitable solution for racing bikes, motocross and dirt bikes.
Earlier on, the two stroke engines in racing bikes were quite popular, but they have since then have been replaced with the four stroke version of the engine.
The two stroke engine was a much simpler design, therefore provided a low cost solution for racing teams.
The two stroke bike engine operates in two strokes, instead of the normal four stroke Otto cycle.
The stages in a two stroke engine are:
1. Power/exhaust stroke: This is the stroke that occurs right after the ignition of the charge, forcing the piston down. After sometime, the top of the piston goes over the exhaust port, with a large amount of the pressurized gases to escape. The downward movement of the piston continues to compress crankcase containing the air, fuel, oil mixture. The top of the piston having passed the transfer port, the charge that is compressed enters the cylinder with any exhaust being forced out.
2. Upon entering the cylinder, the air fuel mixture makes the piston move up, compressing the charge in the cylinder. This results in a vacuum being drawn in the crankcase, with drawing in more air, fuel and oil. The spark plug now ignites the compressed charge, and thereafter the cycle begins again.
The main variations of the two stroke engine types are found in the method of the fuel intake; this depends on the various different types of the cylinder arrangement.
The Reed valve type of configuration delivers power through a wider RPM range than the regular piston and port types, which is generally a suitable arrangement for dirt bikes, as a dirt bike course demands more power over the difficult jump sections.
A rotary inlet valve type of engine configuration has a thin disk attached to the crankshaft, which spins at the crankshaft speed. The fuel-air mixture is made to pass through the disk, with the disk having a section cut, opening if it passes the intake pipe, closing otherwise.
The advantage of the rotary valve arrangement is that it asymmetrically arranges the two stroke mechanism’s intake timing, not possible with piston port type of arrangements.
Cross flow engines are configured to house the transfer ports and exhaust ports on opposing sides, with on top of the piston being the deflector, directing the fresh intake charge on to the upper side of the cylinder.
The residual gas is pushed down the deflector’s other side, out of the exhaust port. The 235 hp
2.6 L V6 crossflow till date has some of the highest output, coming from a low weight engine.
For motocross bikes, such as the 125 SX, bought out in 2007, being ultra light is a key feature. The two stroke engines are competitive, and preferred by racers who do not want to indulge in complicated four stroke machinery.
This bike has a displacement of 124.8cc with bore and stroke 54/54.5, with a TVC power valve.
This 6 gear, premix fuel run two stroke has a front suspension of WP USD 48 mm, with a total weight being about 89.5 kg. Bikes such as this one have a multi-disc hydraulically operated clutch system, with an ignition through a Kokusan digital magneto CDI.
The basic advantage over such two stroke dirt bikes over their four stroke counterparts is that they provide a much more hassle free, yet powerful transmission solutions.
The 250 SX 2007 is a two stroke bike that is powerful enough to give the four stroke category bikes a run for their money. Added to this it is light, making it ideal for motocross racers.
The single cylinder, two stroke engine calls for easy maintenance, and the fantastic suspension, with PDS geometry means that it is a beast on the tough dirt circuit.
