Until legislation effectively outlawed them, 2-strokes were the best sellers in most market segments and dominated international motorcycle sport.
Love them or hate them, 2-strokes play a huge part in the classic motorcycle scene. The versatility of the 2-stroke has brought them many fans over the years. The simple engine has been used for everything from paratrooper mobility to Grand Prix winning machines.
The single biggest advantage the 2-stroke has over its 4-stroke cousin is its simplicity and fewer moving parts. Its disadvantage, and the problem that eventually saw its demise for street bikes, is the amount of pollutants emitted when running.
As the name suggests, a 2-stroke completes its operating cycle in two strokes of the piston. To achieve this, the 2-stroke must accomplish some tasks simultaneously.
The initial phase is the induction phase. The piston will uncover the inlet port (piston ported types) as it travels upward in the cylinder. However, at the same time the piston is moving upward to uncover the inlet port, it is also pushing burnt gases out of the exhaust port. The new charge of unburnt fuel will be collected in the base of the engine during the induction phase.
When the piston moves back down inside the cylinder, the fresh charge is compressed in the crankcase (primary compression) until additional cylinder ports are exposed by the piston's travel which allows the fresh charge to be transferred to the combustion chamber.
The cycle begins again as the piston moves back to the top of the cylinder. This vertical movement of the piston compresses the fresh charge and allows the next intake phase to begin. Once the piston has reached a point near TDC (Top Dead Center), a spark is produced and the piston travels back down inside the cylinder during the power stroke.
From simple piston ported 2-strokes of the early engines, engineers have modified the various operating phases to increase power and efficiency of them. Various types of induction systems have been tried, including read valves (cylinder and crankcase mounted), rotary disc valves and direct feed to the crankcase.
Exhaust
The exhaust systems have also been developed over the years to increase the power band–as many of the early 2-strokes suffered from a narrow power-band (some with as little as 500 rpm!). Included in the exhaust modifications are expansion chambers and variable height port systems.
2-stroke engines dominated many forms of motorsport until being outlawed by changes in the governing body's regulations. In particular, the 500-cc Grand Prix racing class was dominated by two strokes from the 70s until being replaced by the 4-stoke MotoGP™ machines in 2002.
The same applies to the world of international MX and Supercross where 2-strokes dominated for many years before being replaced by 4-strokes from 1998 on.
Of all the famous and infamous 2-strokes motorcycles, two stand out for being bad: the H1 and the H2 Kawasakis triples. Described by many journalists at their launches as terrifying, unrideable death traps, these bikes were an instant success for their brute power, and an instant failure for the handling and braking capabilities.
All of the rockers from the 60s would argue that the scooter is the ugliest form of a motorized cycle, but then along came the "sports moped." Yamaha had the FS1E, Suzuki the AP50, but possibly the ugliest of all 2-strokes was the Harley Davidson Topper.
In fairness the sports mopeds were forced onto the manufacturers by legislation (primarily in the UK) restricting both the engine size and performance of machines for younger riders. But none of the early sports mopeds won prizes for outstanding aesthetics!
1. 2-Strokes; The Good, The Bad, and the Very Ugly!
Love them or hate them, 2-strokes play a huge part in the classic motorcycle scene. The versatility of the 2-stroke has brought them many fans over the years. The simple engine has been used for everything from paratrooper mobility to Grand Prix winning machines.
The single biggest advantage the 2-stroke has over its 4-stroke cousin is its simplicity and fewer moving parts. Its disadvantage, and the problem that eventually saw its demise for street bikes, is the amount of pollutants emitted when running.
2. Operating Principles
As the name suggests, a 2-stroke completes its operating cycle in two strokes of the piston. To achieve this, the 2-stroke must accomplish some tasks simultaneously.
The initial phase is the induction phase. The piston will uncover the inlet port (piston ported types) as it travels upward in the cylinder. However, at the same time the piston is moving upward to uncover the inlet port, it is also pushing burnt gases out of the exhaust port. The new charge of unburnt fuel will be collected in the base of the engine during the induction phase.
When the piston moves back down inside the cylinder, the fresh charge is compressed in the crankcase (primary compression) until additional cylinder ports are exposed by the piston's travel which allows the fresh charge to be transferred to the combustion chamber.
The cycle begins again as the piston moves back to the top of the cylinder. This vertical movement of the piston compresses the fresh charge and allows the next intake phase to begin. Once the piston has reached a point near TDC (Top Dead Center), a spark is produced and the piston travels back down inside the cylinder during the power stroke.
3. Development
From simple piston ported 2-strokes of the early engines, engineers have modified the various operating phases to increase power and efficiency of them. Various types of induction systems have been tried, including read valves (cylinder and crankcase mounted), rotary disc valves and direct feed to the crankcase.
Exhaust
The exhaust systems have also been developed over the years to increase the power band–as many of the early 2-strokes suffered from a narrow power-band (some with as little as 500 rpm!). Included in the exhaust modifications are expansion chambers and variable height port systems.
4. The Good
2-stroke engines dominated many forms of motorsport until being outlawed by changes in the governing body's regulations. In particular, the 500-cc Grand Prix racing class was dominated by two strokes from the 70s until being replaced by the 4-stoke MotoGP™ machines in 2002.
The same applies to the world of international MX and Supercross where 2-strokes dominated for many years before being replaced by 4-strokes from 1998 on.
5. The Bad
Of all the famous and infamous 2-strokes motorcycles, two stand out for being bad: the H1 and the H2 Kawasakis triples. Described by many journalists at their launches as terrifying, unrideable death traps, these bikes were an instant success for their brute power, and an instant failure for the handling and braking capabilities.
6. The Ugly
All of the rockers from the 60s would argue that the scooter is the ugliest form of a motorized cycle, but then along came the "sports moped." Yamaha had the FS1E, Suzuki the AP50, but possibly the ugliest of all 2-strokes was the Harley Davidson Topper.
In fairness the sports mopeds were forced onto the manufacturers by legislation (primarily in the UK) restricting both the engine size and performance of machines for younger riders. But none of the early sports mopeds won prizes for outstanding aesthetics!