TECH: ARE TWO-STROKES COMING BACK?
Why two-strokes might be the engine of the future.
The last mass-made, highperformance two-stroke motorcycles disappeared from production the best part of 20 years ago, defeated by emissions legislation. But the two-stroke cycle could be the key that gives the internal combustion engine a new lease of life.
In January the man in charge of Formula surprised many by suggesting the top tier of car racing was considering turning to two-strokes. Pat Symonds, F’s chief technical officer, told the Motorsports Industry Association that F is considering its next generation of engine rules, saying: ‘I’m very keen on it being a two-stroke. Much more efficient, great sound from the exhaust, and a lot of the problems with the old two-strokes are just not relevant anymore.’
While the idea of two-strokes is intrinsically linked with great clouds of fragrant blue smoke for most of us, the reality is that many engine designers see strokers as a genuine hope for the future of internal combustion. Jamie Turner is Professor of Engines and Energy Systems at the University of Bath. ‘What we’re really talking about is the two-stroke cycle,’ he said. ‘When we mention two-strokes people tend to think of 500cc GP bikes, Aprilia RS250S, things like that. That’s not really why they’re gaining interest now.
‘The big problem with the fourstroke cycle is throttling loss. A fourstroke engine is actually only an engine for half of its lifetime. The other half, it’s a scavenge pump. Between one set of compression and expansion strokes and the next, we have exhaust and intake strokes, and that’s when it’s just pumping gasses in and out.” Because a four-stroke, spark-ignition engine needs precisely the right air/ fuel ratio, it has a throttle (usually the familiar butterfly valve in the intake) to restrict how much air can get in. That means the pumping part of the cycle sucks up a lot of energy – the word ‘throttle’ derives from the fact it restricts the engine’s ability to breathe. Two-strokes don’t spend half their time working as a pump. Traditional two-strokes use crankcase scavenging where the intake air is sucked into the crankcase by the upward movement of the piston during the compression stroke and is then forced into the cylinder via a transfer port when the piston moves down during the power stroke.
However, efficient future two-strokes don’t work on that basis. Instead they’ll have a turbocharger, supercharger or both to pump air into the cylinder.
Problems solved
Direct fuel injection could solve the traditional two-stroke problem of unburnt fuel escaping into the exhaust port during the period when both the intake and exhaust are open. However, they still need to operate with a very
‘A lot of the problems with the old two-strokes are just not relevant anymore’