Forcing the issue

by Simon Hargreaves

So we've got all the power we can use. What do we do now?

With traction-controlled streetbikes making more power than a ten-year old World Superbike, and with pressure to reduce emissions and fuel consumption, where do engines go from here?

One answer is more torque. For a given size of engine it could be just as powerful as a bigger engine but at lower rpm, thus cleaner and more efficient – more time to fill the cylinders, longer burn, better scavenging, lower mechanical losses. Be nice to use, too.

But since modern bike engine architecture was established in the mid-80s, the peak torque from a given capacity has remained static. A 1992 Suzuki GSX-R1100 N made 75 lb.ft. A BMW S1000RR, two decades later, makes 80 lb.ft. A 1992 Ducati 916 made 70 lb.ft; a 2013 Panigale is 88 lb.ft: 26% more torque from 30% extra cc. Power has increased – by making the torque faster, ie at higher rpm. But turning force at the crank is the same.

The reason is because the work an engine can do is governed primarily by the mass of air it pumps. The theoretical maximum is a reference figure set by the cubic capacity of the cylinder multiplied by air density. When an engine's actual air mass use is expressed as a percentage of the maximum, it's called its volumetric efficiency. A road bike VE is around 90% at peak torque, because air has to be drawn through an airbox, into a throttle body, down an intake and past a couple of valves with only a fraction of a second to fill the cylinder, much the same way as it did 20 years ago. In fact with emissions restrictions, engine designers have done well not to lose torque.

The easiest way to make an engine torquier is make it bigger ('no substitute for cubes') – pick almost any model with history and chances are it was a smaller capacity a few years before. But it's cheating, because we still have the same fuel consumption and emissions issues. What we want is to somehow, magically, make a small engine as torquey as a big engine.

The key is air density. It's possible to get well over 100% VE by forcing air into the engine. Increasing air density in the combustion chamber increases the heat, speed and ease of combustion, making more torque for a given capacity.

Ram-air, introduced in the early 1990s, does this to a limited degree. So does nitrous oxide, by lowering air temperature (which increases its density) and releasing oxygen during combustion. Bit impractical though.

Turbos and superchargers force air into the engine. Both have pros and cons but turbos, driven by exhaust gas and tending to suffer from lag, lack the fine part-throttle control needed by a road bike. A supercharger, driven from the crank, effectively matches normal engine response and, with modern materials technology, can be shrunk to a suitable size for packaging on a bike. It's an ideal solution.

Which suggests supercharging could be the way forward for road bike engine development, as suggested by the 'concept' engine Kawasaki had on display at the Tokyo show last year.

All they need to do now is make them eligible for racing...