This year, for the first time in many decades, we find ourselves without a two-stroke engine in the Grand Prix classes of motorcycle road racing. For lovers of diversity in engineering in general and engines in particular, the class of 2012 Grand Prix bikes are perhaps a disappointment. Having seen the two-stroke bikes regulated out of existence with the demise of the 500 cc and 250 cc World Championships, 250 cc four-stroke single-cylinder engines have come to replace the 125 cc two-stroke engines that had been ubiquitous in the smallest Grand Prix class for many years. Limited to an 81 mm bore, they have thankfully not simply been tuned motocross engines, as many had feared that the might be. There are two main engine constructors, Honda and KTM, with Ioda developing an engine for its own machine, and these will be joined by the British GE effort in 2013.
The Honda in particular has strayed furthest from what everyone perhaps expected these engines to be. It has the airbox and inlet on the front of the engine and the exhaust port pointing backwards in the bike, a layout not commonly used for motorcycles. One might expect that the exhaust pipe would exit straight towards the rear of the motorcycle, coming out underneath and at the rear of the seat unit. However, Honda sweeps the exhaust forwards in the motorcycle, and then down, allowing a silencer to sit just behind and below the engine.
By comparison, the KTM and Ioda engines are much more akin to a conventional motorcycle layout, with the inlet on the rear of the engine and the exhaust port on the front. As per conventional practice, the KTM and Ioda engines bring the exhaust down the front of the engine. We will have to wait to see what GE chooses to do here.
Why does the Honda bring the exhaust around the front of the engine, having gone to the effort of designing an engine with the exhaust port on the rear of the cylinder head? Well, it is probably because it is an easy way to package the necessary exhaust length. The straight-run approach from the exhaust port to the back of the bike may not have given enough length for the exhaust to tune properly without having to design a contorted pipe. If the engines require a silencer, then having this mass low on the bike is probably another reason not to use an underseat exhaust exit.
It seems that the exhaust packaging could be more easily achieved if the cylinder orientation was conventional. If we consider the exhaust alone, this is probably true. However, the small cylinder requires only a small airbox, and this can easily be housed above and ahead of the engine. Giving the inlet a 'straight run' into the airbox will help maximise pressure at the inlet port, which is good for engine performance. It is probably the case that the design of the exhaust and the orientation of the cylinder are dictated by considerations of engine breathing.
Fig. 1 - The KTM Moto3 engine has a conventional cylinder and exhaust layout
Written by Wayne Ward