Works best under pressure
Some cars have had direct fuel injection since the 1950s but it’s only just getting to bikes. What can we expect? And why the wait?
Honda’s recent patents for a directinjected version of the Africa Twin suggest that new bikes may soon have high pressure fuel blasted straight into their combustion chambers, rather than dribbled in upstream. This will make engines more powerful and economical. If the technique – variously called petrol direct injection (PDI), gasoline direct injection (GDI) and fuel stratified injection (FSI) – can be made to work on bikes, it’s a big deal. Euro6 emission regulations are still being hammered out, but if they’re in line with cars, we’ll need PDI to maintain power while getting cleaner. PDI has several advantages over current port injection systems. With PDI, fuel is so finely atomised by super-high pressures that it instantly mixes with the air and then cools the combustion chamber and piston top. This allows higher compression and leaner fuel/air mixtures – a recipe for more power, better emissions and, of course, fuel economy.
Some downsides
The main downside is that pressure. PDI systems run at around 200 Bar (2900 psi), and future designs are expected to reach 500 Bar (7250 psi). Current bike injectors run at around 4 Bar (58 psi). Other disadvantages include increased particulate emissions, something that the future, higher-pressure systems promise to solve, and a tendency for PDI engines to suffer a build-up of carbon deposits on the intake valve seats, in part because they’re no longer being washed by fuel in the intake charge. But PDI has proved a huge hit on four wheels. More than half of the new petrol cars sold in Europe and the USA are PDIS, giving 15% mpg improvements and a boost in performance. Diesels have been on the PDI bandwagon for years and are now almost all direct-injected.
Early adopters
The reason diesels adopted the technology first hints at why bikes are last to get it – the main sticking point is getting fuel to atomise and mix with the intake air fast enough. So low-revving diesel cars are easier than higher revving petrol cars, which are easier than sportsbikes. The first PDI bikes are likely to be lower-revving models, such as Honda’s Africa Twin. Honda isn’t the only firm working on a direct-injected bike – Kawasaki have got something similar on the drawing board, but it mixes direct and port injection to get the best of both worlds. The idea is that the engine management could switch between the two, or blend them together, depending on the engine’s demands. Kawasaki’s patent shows the idea on a supercharged four-cylinder engine, hinting that the H2 might be an early beneficiary. This isn’t a new idea though. Toyota and Ford already make dual-injected car engines and the results are impressive. On bikes the idea could be particularly useful, eliminating the problem of getting ecient atomisation at high revs.