Hybrid future | Getting under the skin of Kawasaki’s pioneering new hybrid powertrain

Hybrids may have become commonplace in cars over the last 25 years, but the difficulties of incorporating the idea into motorcycles mean Kawasaki’s achievement in creating the world’s first ‘strong hybrid’ production bikes – the Ninja 7 Hybrid and Z7 Hybrid – is an impressive feat of engineering.

• Related: The future of motorcycling and alternative fuels

The term ‘strong hybrid’ means the bikes can operate in pure electric mode, rather than simply adding an occasional boost like some other hybrid systems. It’s the set-up used on many cars, albeit with differing marketing terms for the technology. It’s not a plug-in hybrid, which is an altogether different thing.

This meant it was essential that the internal combustion engine could be disengaged and reengaged at will without making the bike more complicated to operate than a conventional design. 

The 57.2bhp, 451cc parallel twin and six-speed box are similar to existing, manual bikes, but with the addition of a 9kW (11.8bhp) motor. This is located above the gearbox and permanently connected to its input shaft via a chain, upping the outright power to 67.6bhp when both powertrains work together.

With that permanent connection between the electric motor and the transmission, Kawasaki needed an automated clutch, controlled by the ECU, to disengage the engine from the transmission when operating in pure electric mode. 

From there it’s logical to add automation to the gearshift as well, again putting it under the control of the electronics while still allowing a manual mode with shifts controlled by buttons on the bars. The transmission itself is conventional, but the shaft where the foot lever would normally be attached is instead operated by an electric motor and several reduction gears.

The addition of an automatic transmission also helps with fuel economy, allowing the ECU to time shifts to prioritise efficiency, particularly in ‘Eco-Hybrid’ mode when auto shifts are selected. The rider can still override the ECU’s gear choice and use the buttons to change ratio if desired.

Because both the electric motor and the engine drive the input shaft to the transmission, switching between the two or mixing the power of both can be seamless without affecting the gear selection.

The electric part of the system uses a 48V lithium-ion battery mounted under the seat and wired to a converter that drops the voltage to 12V for the standard electrical components on the bike, such as the ECU, and an inverter to convert the battery’s DC output to AC for the motor, both combined into a single unit to save space. 

The main electric traction motor, which is the same liquid-cooled, synchronous AC motor used in the all-electric Ninja e-1 and Z e-1, also works as a generator to harvest electricity when you decelerate. 

The engine, meanwhile, is fitted with a starter-generator instead of a normal starter motor and separate alternator. It fires-up the engine seamlessly since it’s permanently engaged, unlike a conventional starter, and provides power to top up the 48V battery when the engine is running.

Although the initial range of Kawasaki hybrid bikes are mechanically identical to each other, there’s little reason why the system couldn’t be scaled to bring hybrid benefits to larger, higher-performance bikes. Some of Kawasaki’s earliest hybrid motorcycle patents, for instance, were based on a four-cylinder design, but with an otherwise-similar layout of the transmission and electric motor.

Kawasaki hybrid riding modes

There are several options available to the rider, all controlled by toggle switches on the handlebar. Eco-Hybrid has selectable automatic or manual transmission modes and the engine idle-stop system activated. The bike pulls away on electric power only, with the petrol engine starting once you’re on the move.

Sport-Hybrid relies on the rider to shift gears via buttons on the handlebar. The engine idle-stop system is deactivated so the engine is always running and in full-power mode. E-boost function is available for five seconds at the press of a dedicated button. 

The EV mode, as you would expect, uses electric power only with the petrol engine turned off. You only get the auto transmission using first to fourth gears. This is for slow speeds only and the bike must be in fourth gear or below and travelling at under 15 mph to engage.

And finally, walk mode delivers electric power only for forward and reverse gears at extremely slow speed. This has been developed to make an easier job of parking the bike.

Kawasaki’s hybrid motorbike engine in detail

• Smooth sailing: Since the ECU has control of both the electric motor and the petrol injection and ride-by-wire throttles, it can feather the power of both during upshifts or blip on downshifts to make progress as smooth as possible.
• Auto action: The automated clutch is hydraulic, with a dedicated oil pump and solenoid valve that engages and disengages it at the command of the ECU.
• Semi-automatic: Gears are shifted by an electric shift motor, controlled by the ECU. In automatic modes, the ECU decides on the shift points. In manual mode the bar-mounted buttons tell the ECU to trigger up- or down-shifts.
• Chained to the rhythm: The electric motor is permanently connected to the transmission’s input shaft by a chain, and can drive the bike at low speeds (in first to fourth gears, with automatic shifts) without any help from the petrol engine.
• Clever starter: A starter-generator allows the engine to be stopped and started seamlessly. It’s permanently engaged to the engine, unlike a conventional starter that engages only when you hit the button, and once the engine is running it becomes a generator to help charge the battery.