Have you ever thought why the 2-stroke engines are always louder than the 4-stroke engines? It is often seen that the 2-stroke motorcycles can easily be identified by the loud and peculiar sound they produce. The reason behind the same is as interesting and mysterious as the 2-stroke engines are in itself. There are more than one reasons behind the loud characteristic of a 2-stroke engine, we will try to explain all of them in this article in detail.The first and foremost reason behind the high decibel sound produced from the 2-stroke petrol engine is the basic functioning of it. As compared to the 4-Stroke petrol engine which produces power in 4- cycles, the 2-stroke is characterized by producing power in two complete cycle of crankshaft. In simple terms a two-stroke engine fires at every cycle as compared to its counterpart, which fires at every alternate cycle. This means at a given RPM a 2-stroke engine fires twice than the 4-stroke engine, producing not only almost twice the power of a 4-stroke engine but also double the sound of a 4-stroke engine.
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Another reason lies with the simple structure of the two-stroke petrol engine, the 2-stroke engine is known for its simple design hence it is devoid of complex engine parts as opposite to a 4-stroke petrol engine. In typical four-stroke petrol engine there is Valve Train, which includes Input and Exhaust Valves. On the other hand the 2-stroke engine comes devoid of any valve; the Exhaust Port of the engine is regulated by the piston against the dedicated Exhaust Valve given in a 4-stroke engine.
The Exhaust Valve present in a 4-stroke petrol engine provides better sound insulation and it acts as a muffler for the sound wave generated at the combustion chamber. Opposite to this the simple design of a 2-stroke engine has no exclusive valve train or valve system in it. The Exhaust Port present on the wall of the Block gets covered by the Piston and it gets opened and closed by virtue of the movement of the Piston itself. This whole setup is effective yet crude in form doesn’t work as good as a 4-stroke engine as far as sound muffling is concerned.
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The Exhaust system of a 2-Stroke engine is another reason for the loudness of it, the Exhaust Chamber plays vital role in functioning of a 2-stroke petrol engine. The Two-Stroke engine works on pressure wave theory and hence highly dependent on the exhaust gasses for the efficient fuelling. The importance of the Expansion Chamber cannot be overlooked in a 2-stroke engine, it must be designed in such a way that the exhaust gasses get enough space to expand. Overall volume of the gasses must also be calculated before designing the expansion chamber of a 2-stroke motorcycle.
A properly designed Expansion Chamber of a two-Stroke machine is generally big enough to accommodate substantial space on a motorcycle; if you try to put a muffler on it to suppress the sound decibel the exhaust system will become huge. You can take the infamous “Bazooka” CatCon silencer of Yamaha RXZ (5-Speed) for example. Yamaha had put the Catalytic Convertor and some muffler to silent the exhaust of the RXZ, resulting a big Bazooka like silencer with subdued exhaust notes. However, one point is to be noted here that the capacity of the engine featured on the Yamaha RXZ was only 132cc and you can easily imagine how big a silencer could be if it would featured on a motorcycle with a capacity of 250cc or more.
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No wonder noise pollution was being taken as one of the major factors for banning the 2-Stroke petrol engines apart from the stringent emission norms. It is not that the manufacturers were unable to meet the emission norms related to the 2-Stroke technology; it was moreover easier/ feasible for them to move towards cleaner 4-Stroke technology. However, company like Honda is still trying to get the puissant 2-Stroke machine back with cleaner technology; it seems unlikely to see a 2- Stroke modern motorcycle with a Bazooka Silencer (pun intended).
We love the two strokers with all its shortcomings and certainly not consider the mesmerizing sound of it as a SHORTCOMING. Who wouldn’t like to see the 2-stroke motorcycles on our roads with all the modern gadgetries, better and cleaner technology?
By: Farhan Kashif