TORQUING POINT: A modern stroker?

Alan Dowds muses…

It feels like a lifetime since we last saw a two-stroke road bike taking pride of place in a dealership, but the wait for a resurrection might be closer than we think… 

If you want to get a big reaction from bikers on a social media post, there’re a few easy wins. Mentioning Valentino Rossi is a great start. Drop in a pic of a wild crash. Or if you want to cause apoplexy all round, post a positive story about an electric motorbike.

The real gold standard though, is to mention a bike with a two-stroke engine. Whether it’s a Honda MT-5 or a Yamaha RD500LC, the replies will go into meltdown quicker than a cheap cast piston in an old race-tuned Suzuki RG250 Gamma.

Because everyone loves a stroker. It’s not hard to see the appeal: the engines are light, have great performance for the capacity, sound amazing – and have a whole extra dimension to their appeal: they actually SMELL good. Add in the particular power delivery of a tuned two-stroke – sudden increases in power over a narrow rev range, or the famous ‘power band’ – and you have a unique set of charms.

Which makes a recent Kawasaki post on its UK and USA social channels very interesting indeed. The firm teased a return of the two-stroke, with the sound of a two-stroke engine revving and the catchphrase: ‘We heard you. #2stroke.’

There’s absolutely no other official info from the big K at the moment – but Japanese bike magazine Young Machine has speculated that there will be a new two-stroke bike from Kawasaki – and it will be a road bike rather than a dirt bike like the KTM 300 EXC two-stroke which has been on sale in recent years. Big if true, as the kids like to say.

Two-stroke road bikes essentially had disappeared by the mid-1990s. Emissions rules meant they couldn’t be sold for road use, which makes sense from a health point of view: two-stroke exhaust fumes smell great but are, of course, properly poisonous. Would you really want your kids or grandchildren to be breathing that stuff in when sat in their pram waiting to cross the road? 

The problem is twofold: the total-loss lubrication system, and the amounts of unburnt fuel that are lost out of the exhaust from a conventional two-stroke. Those engines are almost laughably primitive when you look at them: the crankshaft, big-end, small-end, piston and cylinder are all lubricated by a few squirts of oil which then burns along with the fuel. The fuel/air mix is sucked into the bottom of the engine by the action of the piston moving up, then squeezed through some transfer ports into the combustion chamber when the piston moves down. The new fuel/air pushes the exhaust gases out of a hole in the side of the cylinder, then there’s a quick spark as the piston gets to the top of the bore, you have a juicy power stroke and it all continues once again.

Unburnt oil, atomised fuel, soot, half-burned petrol, carbon monoxide – this mess of organic chemistry tumbles into the exhaust pipe and out into the atmosphere, making for that amazing scent – but with genuinely poisonous impacts to the local environment. Catalysts and clever design can help, but once you add all that complexity in, a four-stroke engine begins to seem like an easier option. And none of that improves the poor fuel consumption of a two-stroke, or the low service life of those imperfectly lubricated internal parts.

Is there a way to get the benefits of a stroker, without those downsides though? Well yes, and there are massive, powerful, economical two-stroke engines all over the world – but they’re powered by diesel and live inside ships and factories. Two-stroke diesels use superchargers to blow fresh air into their cylinders, rather than crankcase/piston pumping, and some designs use camshafts and poppet exhaust valves. That allows them to use a conventional four-stroke style lubrication system for the cylinders and bottom end: oil is kept out of the combustion chamber by the piston rings. Direct injection of precise amounts of fuel straight into the combustion chamber gives more accurate burning, which improves fuel consumption and cuts emissions. 

You can use intake ports in the cylinder wall, fed directly from the supercharger, and when the piston opens the ports up, fresh air blasts in, blowing out the exhaust gas through the poppet valves in the cylinder head. The piston moves back up, fuel is injected straight into the combustion chamber, the spark fires, the piston moves down and the cycle repeats, with a power stroke every revolution of the crankshaft. Below the piston, you have a normal crank/conrod design with pressure lubrication.

So, a possible option for a clean petrol-powered two-stroke. But again, once you make a cylinder head with camshafts and valves, you’re halfway to a four-stroke, and it’s simpler to go with the flow and do that, rather than bother with the supercharger, the R&D, the direct injection and the faff. After all, four-stroke development hasn’t exactly resulted in rubbish engines: specific power outputs of 215bhp/litre from the latest 999cc superbike motors isn’t far off the 248bhp/litre from something like a Suzuki RGV250 VJ22 in 1996 – and that’s with Euro 5+ exhaust emissions.

But what if you had some supercharger tech lying about the place – and also had a potential market with a strong affinity for a two-stroke engine? Can you think of a company like that? Hands up!

Of course, it’s Kawasaki, the firm making those saucy stroker social posts. Its H2 supercharged range has been out for a decade now, so the engineers have a lot of experience with blowers on bikes. It has a massive two-stroke road bike heritage, going back to the H1 Mach III 500 triple in the 1960s up to the gorgeous KR-1S 250 parallel twin race-rep in the early 1990s. One complication is that the centrifugal supercharger design used on the H2 doesn’t work well at slow speeds, unlike ‘positive displacement’ blowers used on big two-strokes. So, to allow start-up and slow speed running, they might need an electrically-driven supercharger – as seen on the new Honda V3 engine showed off at the Milan Show last year. But, if anyone could come out with a new 2025-friendly two-stroke technology for the road, it’s them.

Now, it’s fair to say that the audio on the social posts sounds like an old-school KX250 single rather than some sort of new-fangled supercharged stroker. But the good folk at Young Machine magazine in Japan are guessing that the new bike might be a retro-style multi-cylinder roadster design, with echoes of the old two-stroke triples that still resonate in moto-legend. 

Whether they’re right, or if the social media stramash will end in the slight anti-climax of a neo-KX dirt-bike with a conventional two-stroke single, remains to be seen. Fingers crossed though…