Art of suck and squeeze: The future of turbo and superchargers

FIVE years ago it seemed that the motorcycle industry was standing on the brink of a forced-induction revolution. In 2013 Suzuki whipped the covers off its turbo-boosted Recursion concept bike and Kawasaki first revealed the supercharged four-cylinder engine that would power the H2 two years later.

But after half a decade the floodgates to boosted bikes still haven’t opened. While there’s been a bit of movement – the debut of the cheaper, more practical Kawasaki H2 SX – it’s barely a trickle and certainly not the tsunami that seemed so certain back in 2013.

So what’s happened?

First, we need to look at the two technologies and the reasoning behind using them.

Turbos and superchargers: the basic idea

Whether a bike uses an exhaust-driven turbocharger or an engine-driven supercharger, the basic idea is the same. By compressing air before it enters the cylinders, more air and fuel can be forced into them. That means when the spark plug ignites the mixture you get a bigger bang and more power.

That’s all great, and when it comes to an engine that’s designed for power and nothing else – in a drag bike, for instance – that’s about as far as the argument for forced induction goes. But when it comes to road bikes, more nuanced reasons come into play.

In recent years the shift towards forced induction, and particularly turbos, in cars has been a very clear trend. While power remains one of the aims, the reasons behind the shift are largely ones of fuel economy and emissions.

By adding boost, car makers have been able to reduce the capacity of their engines. They spend the vast majority of their time making a fraction of their potential peak power – just humming down motorways or trickling around towns – and a smaller engine can be more efficient in those circumstances than a large one would be.

And those car makers don’t just have your fuel bills at heart. For years, in Europe in particular, there’s been an obsession with CO2 (carbon dioxide) emissions, and that’s led to emissions rules and tax regimes that penalise high CO2 outputs. By adding a turbo or two, car makers can cut their CO2 emissions (at least at the sort of small throttle openings that the tests use) and get impressive fuel economy figures.

And that’s one of the reasons for the slow take-up of the technology in bikes. CO2 limits on two wheelers haven’t reached the strict levels imposed on cars. Even when the limits are tightened, as they surely will be, bikes are much lighter and smaller-engined anyway. That means they need to burn less fuel to move them, and burning less fuel is the most straightforward route to reducing emissions of all.

However, as emissions rules get ever tighter, the same reasons that pushed the car industry to turn to turbos will put increasing pressure on bikes, perhaps prompting their manufacturers to look to the same solutions.

What’s the difference?

While turbos and superchargers aim to achieve similar results and both use compressors to squeeze more air into an engine’s cylinders, the differences are greater than their similarities.

A supercharger is the simpler design. Its compressor is driven by the rotation of the crankshaft, either via a belt or gears.

That means the effort of compressing the air saps the engine’s power directly, but it also means that the speed of the supercharger is directly related to the engine’s revs. In other words, there’s no (or very little) ‘lag’ between opening the throttle and getting boost.

The compressor of a turbo is very similar to that of some superchargers, but it’s not driven directly from the engine.

Instead a turbo’s compressor is connected, via a common shaft, to a turbine that sits in the exhaust flow. The more exhaust gas that flows, the faster the turbine spins and the more air is compressed by the turbo’s compressor section.

But it’s actually even cleverer than that. One issue with compressing the fuel-air mixture before it enters the cylinder – either with a turbo or a supercharger – is that when that mixture burns there’s much more exhaust to get rid of. In fact, the exhaust gasses are still expanding when they exit through the exhaust. On an engine with a mechanically-driven supercharger this additional expansion is wasted power, but on a turbocharged motor the expanding gas keeps working, driving the turbine, even after it’s exited the cylinder.

Problems with turbos

While a turbo is more efficient than a supercharger, maximising the energy that’s recovered from the fuel that the engine burns, it isn’t without problems.

The first, and best known, is lag. Because it relies on the movement of exhaust gas to get the turbine spinning, which in turn moves the compressor wheel, there’s a distinct delay between opening the throttle and getting boost.

It’s an issue that’s been largely overcome in cars; fly-by-wire throttles and clever engine management, allied to lightweight turbochargers means that lag is often nearly imperceptible. But doing the same in bikes is much more difficult for several reasons.

One is that bike engines have lighter flywheels than cars, so they respond to throttle inputs faster, and that expectation of instant response makes even a small amount of lag frustratingly annoying. And because we use our hands, not our feet, to adjust revs, we’re much more likely to notice any disconnect between the movement of the throttle control and the engine’s response.

In cars, the solution to turbo lag is often to use two small turbos rather than one big one. On bikes, though, that’s not such a viable choice; adding another turbo means more weight, more plumbing, more complexity.

Ah. Complexity. That’s another of the turbo’s Achilles’ heels. The turbo itself might be fairly small, but it represents a mere fraction of the kit needed to get one working. At the very least you need all the pipework, both on the exhaust side and also to route the compressed air from the turbo back to the engine’s intake. There, you’ll need a pressurised airbox. You’ll also need pipework to take fresh air to the turbo, via a filter, in the first place.

Since compressing air makes it get hot, you’ll ideally have an intercooler whether you’re using a turbo or a supercharger, but with a turbo it’s even more vital. Because the turbo is driven by hot exhaust gas, some of that heat is also transferred to the compressed intake charge. So you’ll need an intercooler, which could be air-cooled – although they tend to be bulky – or water-cooled, which is smaller but requires yet more plumbing…

Over and above all that, a forced-induction engine that makes significant power will need to be built more heavily than a non-turbo engine, simply to cope with the strain.

Combine the turbo, the intercooler, the associated plumbing and the fact that the engine needs to be heftily-made in the first place, and the overall package ends up being surprisingly big. Indeed, a small-capacity turbo engine, as an entire package, could easily end up being bigger and heavier than a larger-capacity, naturally-aspirated engine giving similar performance.

Problems with superchargers

Superchargers are simpler than turbos and don’t suffer the same lag issues, and they’re also potentially easier to attach to an engine in the first place.

Because there’s no need to plumb them into the exhaust system, a supercharger can be positioned closer to the engine’s intake, or virtually anywhere provided it can still get drive from the engine.

Most supercharged bike designs opt for positioning similar to that chosen by Kawasaki for the H2 and H2 SX. Slotting the supercharger behind a transverse-four engine sites it in a spot that’s often relatively uncluttered, while putting it near both the engine’s intakes and the conventional spot for an airbox, cutting down on pipework needed both to get air to the supercharger and to get it from the supercharger to the engine.

It’s also a relatively protected spot, particularly when compared to the places designers are forced to hang exhaust-driven turbochargers. On a conventional transverse engine, a turbo – which needs to be sited as close to the exhaust ports as possible to reduce lag – ends up being right in front of the engine. That means putting an expensive, intricately-machined and carefully-balanced piece of equipment right in the place where it’s going to get a regular soaking with water and road grime, not to mention being pelted with whatever grit the front tyre picks up.

But that doesn’t mean superchargers are ideal. Unlike a turbo, as we’ve already said, they draw power from the engine rather than getting it ‘free’, and if you want to get serious performance from them, you still really need an intercooler to reduce the heat of the air being fed into the engine. Kawasaki does without that intercooler on the H2, preferring to stick with the simplicity and lighter weight of a non-intercooled arrangement. But it’s not idea when it comes to making maximum power.

What about cost?

Money might make the world go round, but it’s another huge sticking point that’s stopping the forced-induction revolution from happening.

Even without taking research and development costs into account, or any of the myriad other changes a bike might need to cope with a forced-induction engine’s outright power, a turbo or a supercharger, along with its plumbing and the intercooler that you really need to maximise its potential, will easily suck up £1000 in terms of components alone.

On a mass-market bike, where every last penny counts when it comes to competing with its rivals, amortising its R&D costs and providing a sliver of profit to the manufacturers and dealers, that’s a crippling amount of money. And that’s before we even consider the huge amount of additional testing and development needed to get a relatively novel technology – in bike terms – to the market.

More than anything else, this cost, which applies to both superchargers and turbos, but probably even more to the latter given the extra peripheral kit needed to fit them, is what’s likely to be delaying the onset of a forced-induction bike era.

Are they really necessary?

All these downsides to both turbos and superchargers brings us to ask whether they’re really necessary. And the answer that most bike firms seem to be agreed on is ‘no’.

Or perhaps ‘not yet’ would be more accurate. At the moment, and particularly in the wake of the VW Dieselgate emissions scandal that’s rocked exhaust emissions testing regulations to the core around the world, there’s a lot of uncertainty over what shape future limits could take. Where previous limits, particularly in Europe, have focussed heavily on CO2 – making turbo engines increasingly popular in cars – future limits could take a more wide-ranging approach, potentially encouraging development in other directions. And above all this hangs the overarching topic of electric vehicles; is it really worth spending a lot on new combustion engine technology at all?

Bikes lag a long way behind cars in terms of emissions limits, and their low weight and relatively small engines mean they have a huge advantage in terms of economy. And while there are hints of future forced induction on two wheels, they don’t seem much more distinct than they did five years ago.

Sure, we’ve got the Kawasaki H2 and H2 SX, and the upcoming turbocharged Suzuki twin is expected to reach production in in 2019, some six years after the original Recursion concept bike was shown. But apart from the occasional patent application there’s little sign of other companies following suit.

Perhaps what’s really lacking is a conclusive argument in favour of either technology. All the Japanese bike firms are well aware that they tried turbos before – in the 1980s. That short-lived era of the Honda CX500 Turbo, Yamaha XJ650 Turbo, Kawasaki GPZ750 Turbo and Suzuki XN85 seemed to prove, fairly conclusively, that traditional technology – or just using a larger-capacity, normally-aspirated engine – was more effective than adopting boost. Today’s Kawasaki H2 might be mighty impressive in 300hp, track-only H2R form, but the road-legal version is barely more powerful than a ZX-10R or ZZR1400.

We’ll have to wait and see whether Suzuki’s upcoming turbo twin changes the dynamic, but at the moment the balance still seems weighed heavily in favour of natural aspiration over any sort of forced-induction engine on two wheels.