The lure of the two-stroke
You only have to look at the likes of eBay, Bike Trader or any classic motorcycle magazine to realise that two-stroke motorcycles remain hugely popular.
Even if the authorities and manufacturers have officially turned their collective backs on stinkwheels, the lure of two-stroke still remains huge for many who grasp just what the concept has to offer – more power per cubic centimetre than a comparable four stroke engine. And if you doubt that fact, then check the statistics from when MotoGP went over to four-strokes in 2002.
In order for the new four-strokes to achieve comparable lap times to the supposedly outdated two-strokes the ‘powers that be’ had to ‘gift’ the four-stroke engine designers a two-fold increase in engine capacity! Yes, two-strokes are that much better... well possibly.
The two-stroke engine exploits the negative pressure or vacuum beneath its rising piston to draw in fresh air/ fuel mixture, whereas the four-stroke gets its fix by inhaling through an open exhaust valve as its piston descends. Strokers utilise both sides of the piston to push compressed air/ fuel up through transfer ports at the same time as the piston is descending on its firing stroke. Therefore the stroker engine is actually doing multiple jobs all at the same time, which naturally overlap whereas the four-stroke effectively has clearly delineated parts of Otto cycling happening one after the other.
The two-stroke engine produces power every time the piston moves downwards. The four-stroke only produces power every other time the piston moves downwards; the first descent being the induction phase with power only produced during the ignition phase.
Another benefit of the two-stroke motor is its lack of weight. In its simplest form its only moving parts are piston/rings, conrods, and crankshaft assembly. In comparison the four-stroke also has piston/rings, conrods, and crankshaft assembly plus rotating camshafts, reciprocating valves and cam chains, all of which require energy to drive them. Oh and, valve springs that sap a substantial amount of power to compress. With all of this being spun, rotated, open and closed, there’s a lot of friction which, in turn, creates heat meaning more energy being squandered etc. Perhaps not too surprisingly, two-stroke engines tend to run significantly cooler than four-strokes.
In theory the two-stroke should be more efficient, as it’s always doing something to create power. But several fundamental issues combine to prejudice a normally aspirated, non-catalytically equipped stroker. Firstly, two-strokes are wastrels when it comes to fuel. Because the incoming fuel/air combination is moved into the combustion chamber via the transfer ports, some of this unburnt mixture can find its way out of the exhaust port and into the atmosphere. Swiftly following on from this environmental no-no is the combustion process itself, which automatically burns some of the oil from inside the engine. This happens simply because the incoming fuel will always wash some of the lubricating oil away from the crankshaft assembly and lift it right beneath the spark plug. Both of these consistent and perpetual occurrences obviously aren’t good for the planet.
The huge attraction with two-strokes for riders, engineers and designers is the potential horsepower that can be liberated. As we stumbled into the 21st century, Honda’s 125cc RS125RS (pictured) was delivering 44bhp… that’s 352 bhp per litre. Today’s 250cc, four-stroke Moto3 replacements are only now coming up with 48 horses from twice the engine capacity. That’s how good two-stroke engines are in terms of power output but manufacturers, sporting governing bodies and, to a degree, governments, all view the two-stroke engine as the devil incarnate.
That said, there is renewed interest in two-strokes from engineers who can see beyond the hype, spin and false prophets; the two-stroke may yet live again commercially. Fuel injection is an obvious route with its computer controlled accuracy and it should see a dramatic reduction in emissions. Similarly, charge trapping valves can stop unburnt fuel being forced down the exhaust pipe.
Variable compression ratio cylinder heads actuated by pneumatics are proving effective and there have been successful trials of two-stroke engines running a pair of opposing pistons in the same cylinder augmented by electric fan supercharging.
The commercial two-stroke motorcycle’s days might be over, but that’s no reason not to celebrate them.