The Boost Era is finally upon us

Next year could go down as the moment when forced-induction bikes – those boosted by superchargers or turbos – finally made the shift to the mainstream.

Kawasaki has confirmed that it’s going to launch the third supercharged model in its range at the EICMA show in Milan next month. Having whet our appetites with the exotic H2 and the forged-from-unobtainium H2R, its next boosted machine will be something a bit more down-to-earth; a sports-tourer.

And within weeks it could well be joined by more turbocharged concepts or even production bikes. Suzuki is ploughing forward with the parallel twin, turbocharged XE7 engine and in just the last few weeks we’ve seen patents for a turbo’d V-twin Suzuki cruiser and a supercharged V-twin naked sports bike from Honda.

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Kawasaki's teaser video for what it thought to be a supercharged tourer due in 2018

Why boost?

No, it’s not as sexy as talking about BHP but there’s no denying that the emissions angle is the main driving force behind forced induction.

You only have to look at the trends in car design – where engineers already have to meet tougher emissions standards – to see that turbos and superchargers are fast becoming the norm, even on mundane models, where they can be allied to smaller-capacity engines to offer a combination of better performance and improved economy when compared to their naturally-aspirated predecessors. Even firms that have traditionally spurned forced induction – notably Honda and Ferrari – have given in. The adoption of turbocharged engines in F1 racing since 2014 came at the request of engine manufacturers who believed that the previous, non-turbo motors didn’t reflect the technologies in their production vehicles – effectively admitting that the era of the naturally-aspirated internal combustion engine in mainstream production, the dominant design for more than a century, was coming to an end.

What’s more, emissions limits that are making it increasingly difficult to build road-legal, high-revving, normally-aspirated engines play into the hands of supercharged or turbocharged ones.

Although Kawasaki’s numbers show that the H2 pumps out more CO2 than the 2017 Ninja ZX-10R (198g/km vs 162g/km), that figure doesn’t tell the whole story. In America, where more complete emissions data are published by the EPA, it’s possible to see that in terms of hydrocarbons the Ninja H2 is cleaner-running than the 2017 ZX-10R, even though it’s an older design and making more power.

And that’s despite the fact that the American market ZX-10R is cleaner than the European one. Its peak power is down (188PS vs our version’s 200PS, with peak power coming at 11,500rpm instead of the European bike’s 13,000rpm). The American market H2 is also fractionally down on power (198.5PS vs 205PS), but the key point is that both in America and Europe the peak comes at only 11,000rpm.

High revs – needed to get big power from normally-aspirated, small capacity engines – are at odds with emissions requirements. To achieve them, you need lots of valve overlap. That means the time when both the inlet and exhaust valves are open simultaneously. You need a lot of it at high revs to give the cylinder time to fill with fresh intake charge – you’re basically opening the inlet valves long before the exhausts have closed. But at low revs the same long overlap means that there’s a chance for unburnt fuel to escape into the exhaust, harming emissions.

Variable valve timing would be another solution, but while it can reduce valve overlap, and hence emissions, at low revs, it won’t add significantly to a bike’s overall performance in the way forced induction does.

What’s more, a forced induction engine can offer the performance of a larger-capacity motor with the fuel economy of a smaller one.

In Europe bikes will have to meet even stricter Euro5 limits by the beginning of 2021. Those 2021 models will already be under development as you read this, and it’s a certainty that at least some will be using forced induction achieve their targets.

Turbos and superchargers. What are they and what’s the difference?

First of all, they’re both superchargers. The turbocharger – or turbo-supercharger – simply takes its drive from a different source.

Let’s start with the basics. Suck, squeeze, bang, blow. That’s the way a four-stroke engine works. It sucks in a combination of air and fuel as the piston goes down on the intake stroke, it squeezes the whole lot (usually compressing it into a space around 11 times smaller than its initial size) on the compression stroke. That makes it burn more fiercely when the spark plug sets fire to the whole lot at the top of that compression stroke. As it burns, the result is a load of exhaust gas that expands with a significant force, pushing the piston back down again in the power stroke. And finally there’s the exhaust stroke – the ‘blow’ bit – as the piston rises again, pushing out the last remains of the exhaust gas before the process starts all over again. Rinse and repeat, many times every second.

A supercharger – whether engine or exhaust driven – works by compressing the intake air before it even enters the cylinder. That means a cylinder of a given capacity – say 250cc like those on the H2 – will be able to accommodate more gas than it would if it were normally-aspirated. The maths of exactly how much air gets into the cylinder is complex, and of course it varies depending on supercharger speed and the pressure it attains, but the effect is similar to having a larger cylinder.

On the down-side, that means the compression ratio in the engine needs to be lower. While a normally-aspirated motor might have a compression ratio of 11:1 or 12:1, that would be too high if the air entering the cylinder was already pre-compressed. The H2’s compression ratio is 8.5:1. If air entering the H2’s cylinders was already compressed by the supercharger to be 1.3 times as dense as ambient air, compressing it by a further 8.5 times would result in the sort of 11:1 compression ratio that you might expect to find in a normally-aspirated engine (in fact it’s a bit more complex than that, but you get the gist.)

So, in short, the H2’s engine, while only 1000cc, is working like a 1300cc, and its peak power and torque figures reflect that. But it’s not working very hard to do it, allowing the emission to be much lower. Increasing the boost and adding a freer-flowing exhaust without the emissions-cutting kit means massive power increases are possible – as reflected in the H2R’s 300-odd horsepower output from the same basic engine design.

So, what’s the difference between a supercharger and a turbo? Well, the engine-driven supercharger, as used on the Kawasaki, is powered by the engine’s crankshaft via belts, chains or gears. That means the speed that its compressor wheel spins at – peaking at around 130,000rpm – is directly related to how high the engine is revving. That’s good in terms of response because it means the increase in boost pressure is gradual and directly connected to the engine’s speed. It should lead to a predictable throttle response.

On the down-side, an engine-driven supercharger saps power at the same time as it makes it. The engine is working to spin the compressor and to squeeze all that air, which sucks away horsepower. That’s why a lot of engines that are designed with peak performance and fuel economy in mind (ie efficiency both in terms of how much fuel they use and how well they convert that fuel into power) use turbochargers instead.

Turbos, or exhaust-driven superchargers, work on the basis that the gases that result from burning air and petrol together will expand to a fill a larger space than the engine’s cylinder. That means that even after the exhaust valve is open and the piston is purging the ‘spent’ gas after burning it, it’s still expanding. That means it’s carrying energy, which on a non-turbo engine is just blown out of the tail pipe.

Turbochargers take that ‘waste’ energy from the still-expanding exhaust gas, and use it to spin a turbine (hence ‘turbo’ charger). That turbine is connected to a compressor via a shaft, and can easily spin it to the sort of speeds needed, say around 200,000rpm.

That’s great, because it means you can get the supercharging effect ‘for free’ without spending any horsepower generating it. But it also has disadvantages, mainly in the fact that the turbo will only start to spin at those high speeds once you’ve opened the throttle and started getting some serious gasflow through the engine. Which means there can be a delay between asking for full power and actually getting it – that’s what ‘turbo lag’ is, and on a bike it’s not good news. The solution tends to be a smaller, lighter turbo that spins more easily, but that in turn begins to put a limit on how much power can be made, while turbocharged engines might also need a higher compression ratio to improve their off-boost performance – great, apart from it then limits how much boost can be added when the turbocharger is blowing.

What’s next?

We’ve already mentioned that Honda is working on a supercharged V-twin naked bike, and the firm appears to have firmly nailed its flag to the supercharging pole because that’s just one of several such machines its patented so far.

Others include a supercharged parallel twin derived from the NC750, and a supercharged inline four-cylinder engine.

Kawasaki is also firmly in the supercharging camp. It’s spent millions developing its own supercharger technology – the H2’s kit is all made in-house rather than bought in from other specialists – and has been open about the fact there are more supercharged machines coming.

First will be the sports tourer that’s being launched next month at EICMA. It’s going to use the ‘Balanced Supercharged’ engine that was first revealed two years ago at the 2015 Tokyo Motor Show. That has a more advanced supercharger than the one used on the H2, with variable vanes on the intake to carefully control airflow into the supercharger and optimise its boost at all speeds. The idea is to maximise low-end torque rather than make pointless increases in top-end power.

Kawasaki has also confirmed it has smaller-capacity supercharged bikes coming, with a strong likelihood that one will be a four-cylinder of between 600cc and 800cc. Will the new sports tourer be that smaller-capacity bike, or another 1000cc machine like the H2? Japanese sources currently believe the latter, with a smaller model appearing further down the line.

Hinting at future supercharged models, Kawasaki has shown two drawings; the SC-01 Spirit Charger and the SC-02 Soul Charger. The SC-02 is a naked bike, and one that Kawasaki hinted would be a ‘downsized’ model, presumably using the rumoured 600-800cc engine. The Sprit Charger design showed a high-end, café-racer-style supercharged bike, using a Balanced Supercharged version of the H2’s engine and aimed at a well-heeled clientele.

Kawasaki has also covered all its bases in terms of trademarks; it’s applied for rights to the names S2 and S2R, R2 and R2R, and E2 and E2R. Any or all of them could be future supercharged models.

On the turbocharging front, Suzuki is the standard-bearer. The Recursion concept bike – a 588cc parallel twin making 100hp and loads of torque thanks to its turbo – was shown in 2013. It was followed up in 2015 with the XE7 engine, which is the production-ready turbo twin that’s expected to eventually find its way into a whole range of machines. That engine could eventually become the spiritual successor to the evergreen SV650 V-twin, which has been powering everything from the SV to the Gladius and V-Strom over the years. More recently we’ve also seen the turbocharged V-twin engine that the firm appears to have planned for an M1800R-style muscle-cruiser.