Prechamber Combustion for Future Honda Superbikes?

[Hugh Janus]

Honda’s new patent shows a CBR1000RR fitted with a prechamber combustion system. (Japanese Patent Office/)

Making high-revving, high-powered superbikes that squeak past the latest emissions laws while churning out ever more power is becoming increasingly difficult, and it looks like Honda is turning to the lessons it’s learned in building Formula 1 race engines to achieve that target. A new patent application filed in Japan shows a version of the latest CBR1000RR Fireblade fitted with an unusual version of the prechamber combustion system that’s become the norm in current era hyper-efficient Formula 1 cars.

As the name implies, prechamber combustion relies on fuel being injected into a chamber above the main combustion chamber near the spark plug. (Japanese Patent Office/)

Prechamber combustion is one solution to the problem of getting an air-fuel mixture close to the ideal 14.7:1 stoichiometric ratio to burn efficiently in the harsh environment of an engine’s combustion chamber. In theory that ratio—14.7 parts air to each part fuel—gives ideal combustion, resulting in the cleanest, most efficient burn and the most power. In practice, though, it’s hard to achieve that ratio and burn all the mixture in the combustion chamber. A richer mixture with more fuel will consume more of the available oxygen, resulting in extra power but also inevitably resulting in some unburned fuel being wasted into the exhaust. Rich mixtures also reduce cylinder temperatures, allow the use of higher compression ratios, and reduce the chance of detonation, where the mixture explodes rather than burning in a controlled way, thus potentially damaging pistons and valves.

Unlike the F1 systems, Honda’s bike application utilizes two injectors per cylinder for more efficiency. (Japanese Patent Office/)

In F1, where both the total amount of fuel used in a race as well as the fuel flow are limited by regulations, wasting gas by using a rich mixture isn’t an option. Every last ounce of power needs to be extracted, and prechamber combustion is the way to do it. It means the fuel is injected into a chamber above the main combustion chamber, where the spark plug also sits. Holes between the prechamber and the main chamber allow the fuel through more slowly than the initial injection, so when the spark plug fires there’s a rich, easily ignited mixture in the prechamber and a leaner mixture in the main combustion chamber. Jets of flame from the prechamber then shoot into the main chamber, providing a more complete burn than you’d get from a spark alone.

Honda’s new prechamber idea for bikes is similar but because it’s unfettered by restrictive F1 regulations, should be more effective and cheaper to produce. In F1, for example, a key rule is that only one fuel injector is allowed per cylinder, but in Honda’s bike engine design, there are two. We’ve already seen that Kawasaki is working on a dual-injection engine, combining conventional indirect fuel injection—where the injector fires into the inlet tract—with direct injection. Such systems offer the efficiency benefits of direct injection but address some of the cost and wear issues related to pure DI engines. Honda ups the game again by adding prechamber combustion.

A rotating tube in the prechamber driven by the cam chain acts as a door between the two chambers. (Japanese Patent Office/)

One of its injectors is completely conventional, firing into the intake tract just below the throttle butterfly. The other is located in the prechamber. Cleverly, Honda has isolated the prechamber from the main combustion chamber with a rotating tube that acts like a rotary valve. Driven by the cam chain, this rotary valve solves one of the problems of prechamber combustion, which is that it’s difficult to completely purge exhaust gas from the prechamber during the exhaust stroke. Honda’s rotary valve arrangement opens a wide “door” to the prechamber during the exhaust and intake strokes, making sure all the burned gas is replaced with fresh charge. This “door” then closes during the compression stroke, allowing the second fuel injector to add more fuel to the prechamber without mixing any of it into the much leaner mixture in the main chamber.

Related Content: Ask Kevin: Is Direct Injection Coming for High-Performance Streetbikes?

Finally, at the point of combustion, the valve exposes small holes between the prechamber and the main combustion chamber, directing focused jets of burning mixture into it to ensure a complete burn of the leaner mixture in the main combustion chamber. The result should be more power for less fuel, while still allowing the use of higher compression ratios without the risk of detonation.

At combustion, jets of burning mixture are directed through holes from the prechamber into the main combustion chamber. (Japanese Patent Office/)

Although there’s no indication from Honda’s patent that its system will reach production, there’s a growing interest in dual-injection engines that combine direct and indirect injection, particularly for high-revving motorcycle applications. Direct injection (where fuel is fired straight into the combustion chamber) is hard to apply on its own to a bike engine, particularly a high-revving one, as massive fuel pressure is needed to atomize the fuel into the air in the combustion chamber fast enough. Dual injection, with one direct injector sited near the spark plug to enrich the mixture near it and one conventional injector supplying a lean mixture to the rest of the intake charge, gives most of the benefits of direct injection in terms of reducing how much unburned fuel escapes into the exhaust, but at a lower cost.

The arrangement allows for a better purge of exhaust gases in the prechamber and a more complete burn in the main chamber. (Japanese Patent Office/)

When Euro 6 emissions rules are brought into force in Europe, many manufacturers believe that direct or dual injection will be needed to meet them, though at the moment the exact standards and their date of introduction have yet to be established. The question now, with many countries already looking to ban or scale down the use of internal combustion engines, is whether this sort of technology will even reach production before the industry adopts a more mainstream shift toward electric power.

Source

[Pedro]

I love this.