I loved two-stroke Grand Prix motorcycle racing, especially the top 500 cc class. The bikes were light and difficult to ride, and I never had any illusions that I would be able to tame one of them. The four-stroke class has brought us machines with an incredible amount of power, and engines that are more closely related to those in their road-going cousins. As such, the bike manufacturers turned away from two-strokes largely because of emissions.
The four-stroke engines are more complicated than the two-strokes. There are lots of valves with camshafts and springs that need to be controlled, which is not conducive to a cheap engine formula for motorsport. It was perhaps inevitable therefore, especially during the global economic downturn, that engine numbers would be limited. This year, that limit is five. Use of further engines brings with it penalties, so preserving the engine in all circumstances is very important.
Anyone who closely follows MotoGP (which is the top class in the latest four-stroke era) will be aware of the complaints about tyres this year. Some very good riders are really struggling to understand the limits of their tyres, and so are making more mistakes than normal. The result is lots of crashes, and whether these are of the frantic cartwheeling variety, with riders being tossed about like dishcloths, or simply sliding off into a corner, the motorcycle gets damaged.
There is no allowance for crashes – whether they are due to tyres, weather or unwarranted ambition on behalf of the rider – on the number of engines allotted to a rider, so the engine needs to be protected in such circumstances. Damage can come in many forms, and the obvious cause of impact damage is probably the easiest to counteract. However, the gravel trap, where many motorcycles come to rest after unseating their rider, presents a particular hazard to an engine, as it can ingest large chunks of hard debris through the inlet and exhaust.
Now that engine life in MotoGP needs to be preserved at all costs, what we have seen is for the end of the exhaust to be equipped with a ‘screen’ which is designed to prevent gravel going into the exhaust system. One has to remember that there are some pretty strong pressure waves moving in both directions along the exhaust, and it is not certain that gravel in the exhaust will always be ejected from the tailpipe.
While the protective screen does its job, it will also inevitably interact with the exhaust pulses, necessarily weakening them and affecting the nature of the reflected wave. A compression wave meeting the open end of a pipe is reflected as a rarefaction wave, while one meeting the closed end is reflected as a compression wave. A compression wave meeting a partly open end of a pipe is likely to still reflected as a rarefaction wave, but this is probably much weaker in magnitude than for a fully open pipe. Consequently its action in helping the gas exchange process is diminished.
Written by Wayne Ward