Inline six-cylinder with turbo from Kawasaki: Monster Turbo - 2.1 liters, 375 hp, 6 cylinders

Kawasaki is probably a household name. It's also probably no surprise that Kawasaki builds a wide range of motorcycles. Engine displacements above 1.8 liters are a rare exception. And turbocharging engines is neither a good idea nor easy to implement for motorcycles for many reasons. However, Kawasaki has experience and success with superchargers in two-wheel drive systems.

All the more surprising are the recently introduced new inline engines with up to 6 cylinders from Kawasaki, with up to 2.1 litres displacement and turbo.

In 2015, Kawasaki launched the Ninja H2. It was the first production motorcycle with a supercharger. The spiral-wound supercharger sits atop the transmission and produces a legal 200 hp from a 999 cc four-cylinder engine. The Z H2 makes this engine available in a naked bike.

Correct, there was a brief phase of motorcycles with turbos. In the Roaring 80s, Honda, Yamaha, and Kawasaki ventured into turbocharging engines between 500 and 900 cc. The problems of turbo lag, which are familiar from cars and the subsequent steep increase in torque, are even harder to deal with on two narrow tires than on four wide ones. And the significantly delayed response is more of a danger than just an annoyance for vehicles that often lean. Added to this was, and still is today: the turbo is located in front of the engine in the exhaust tract and has to channel the charged air around the engine in a very tight path towards the mixture preparation, which can take up hardly any space on a motorcycle.

Anyone stumbling over the other big numbers on these engines should be caught here. Six-cylinder engines with displacements of around or over 2 liters are by no means rare or unusual in modern motorcycles. BMW has an inline six-cylinder engine with just under 1.7 liters in its lineup, while Honda powers the Gold Wind with a 1.8-liter boxer. And even engines with displacements over 2 liters and only two cylinders are "almost" normal for Harley-Davidson.

However, power outputs significantly over 300 hp are still the exception, and are not street legal. In the H2R—the motorcycle from the movie Top Gun II: Maverick—Kawasaki allows supercharged power from 999 cc, but the licensing regulations do not permit it. Kawasaki could therefore power motorcycles with these new engines, and it wouldn't necessarily be alone.

The images of the engines, however, reveal features that are atypical for motorcycles. First, the enormous crankcase with integrated auxiliary components is striking. On a motorcycle, the starter, alternator, oil system, and so on would be designed more peripherally, allowing for more flexible packaging of the drive. Second, the unusual dual ignition system is striking. While this is nothing new in motorcycle and car engines, two spark plugs, each with an ignition coil, placed directly next to each other, does not help with a more harmonious ignition of the mixture.

Fittingly, Kawasaki is designing the new engine with four and six cylinders with bores of just 73.4 and 76 millimeters, respectively. These diameters don't actually require dual ignition. However, in the context of the stroke-to-bore ratio, the large crankcase makes sense, because with a stroke of 76 millimeters for the six-cylinder and 59 for the four-cylinder, the engines are tuned more for torque than speed. Fittingly, peak outputs of 200 and 375 hp are available at just 8,500 rpm.

These are unlikely to be engines for two-wheelers, and the maximum flight altitude of 7.6 kilometers also rules out most cars.

The solution: For the first time in its 129-year history, Kawasaki is building engines for civil propeller aircraft. While Kawasaki emphasizes that the new engines are somehow "related" to the aforementioned supercharged engine of the H2R, there's no other indication of this. However, these new engines fit into Kawasaki's strategy, as the use of hydrogen as a direct or indirect fuel via e-fuel is planned, albeit not until after 2023. Conventional combustion engines are scheduled to enter test runs—sorry, test flights—in 2025.

For now, only the 6-cylinder engine is permitted for direct propeller drive, as it has an integrated gearbox that limits the drive speed to a maximum of 2,800 rpm. The four-cylinder engines are reportedly intended to either function as generators for electric propulsion or, with appropriate attachments, power smaller helicopters.

This also explains the enormously large individual turbochargers, which increase the power of the four-cylinder engine by 70 percent, from 117 to 200 hp, and help the six-cylinder engine from 240 to 375 hp, a 56 percent increase. Such turbos require constant speeds to build up optimal pressure.

Depending on their size and design, propellers in civil aviation can rarely rotate faster than 3,000 rpm, as otherwise the propeller tips would scrape the sound barrier and lose a lot of efficiency. Hence the reduction to a maximum of 2,800 rpm on the Kawasaki engine.

Also on the agenda, but still a long way off: Kawasaki plans to build aircraft engines with a 9-liter displacement distributed across 12 V-cylinders and a turbocharged output of around 1,000 kilowatts, or 1,360 horsepower. Further development will see the inline 6-cylinder engines displace up to 4.5 liters and deliver 608 turbocharged horsepower.

And despite this sheer power, Kawasaki aims to reduce aviation fuel consumption by 30 to 50 percent with these modern engines. The problem with current engines is that some of the basic designs – usually constructed as boxer engines – are very old and were first approved in the late 1950s.

However, since certification as an aircraft engine is expensive, the basic models are still used today. In the case of the Lycoming engine, which powers many Cessna aircraft, it's a pushrod boxer, the simplest and most inefficient way to control a valve train. And Kawasaki wants to score points with the new, significantly lighter engines.