Gasoline Direct Injection

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Recently Angelo Triantafylos posted a note on the CW site:

“Why do motorcycles not have direct injection? Doesn’t the fuel injector inside the combustion chamber help cool the intake charge, creating better volumetric efficiency?”

Direct injection has been a subject of debate for at least the last 80 years. During World War II British aero-engine engineers injected fuel into the “eye” of centrifugal superchargers, sure that the cooling effect of the evaporating fuel would, by “shrinking” the fuel-air mixture being compressed, reduce the blower work required. Large piston aircraft engines of that time consumed hundreds of horsepower-driving superchargers.

Only a few hundred miles to the east, equally studious and well-educated German engineers were certain that their practice of direct cylinder injection, filling engine cylinders with pure air and then spraying metered fuel right into the combustion chambers, gave superior results. Who was right?

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In commentaries written in English after the war, injecting fuel into the supercharger was praised while direct injection decried as proof that if there was a more complicated way to do something simple, German engineers would find it.

GDI and Better Fuel Distribution

When the Wright R-3350-23A engines powering American B-29 bombers had early and chronic difficulties with fuel maldistribution (some cylinders operating rich, others running lean enough to backfire or even set the whole induction system on fire internally), a crash program was initiated to develop German-style direct cylinder injection that would guarantee every cylinder received the same fuel flow. The last roughly 6,700 wartime-production R-3350 engines were delivered and flown with Bendix direct injection in place of the original carburetor system.

In a 1976 conversation with the late Professor Gordon Blair, I got an inkling that thinking on this subject was changing. He referred to some instrumented testing he had done in which, as he put it, he somehow neglected the increase in charge volume caused by the evaporation of fuel into a vapor.

Increase in charge volume? I thought evaporating fuel was supposed to shrink charge volume.

GDI and Emissions

Recently we have seen major automakers adopt GDI as a performance-increasing and emissions-reducing technology, based upon getting more air into engine cylinders by avoiding the volume increase caused by fuel evaporation. The cylinders are filled with just air, and the fuel is added later by injection directly into the combustion chambers. How does this decrease emissions? When fuel is injected at the intake ports or above the intake bells by a “showerhead” injector there is often “wall wash”—liquid fuel sliding along the port walls, some of which manages to escape complete combustion once in the cylinder.

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At a MotoGP event one year I asked Yamaha engineer Masao Furusawa the same question Mr. Triantafylos asks: Why no GDI on motorcycle engines? His answer was itself direct: “Because at MotoGP engine rpm there is too little time for the injected fuel to evaporate.” Mr. Furusawa retired early in 2011.

GDI and Engine Speed

On takeoff power, those WWII-era large aircraft piston engines operated at quite low engine speeds, 2,700 to 3,000 rpm, providing much more fuel-evaporation time than was then available in an 18,000-rpm MotoGP engine.

What happened next shows that GDI development has been able to raise its rpm capability. In March 2014 the FIA, Formula 1′s ruling body, made this rule:

5.10.2 There may only be one direct injector per cylinder and no injectors are permitted upstream of the intake valves or downstream of the exhaust valves.

At the time, F1 engines were limited to 15,000 rpm but that has subsequently been raised to 18,000, supposedly in response to spectator preference for a “shriekier” exhaust note.

GDI and the Future

Many production automobiles now have GDI fuel systems. The Motus motorcycle, introduced in 2014 and no longer produced, was originally to have featured GDI, but the economics of the 21st century prevented that. Peak power at only 7,700 rpm made successful GDI operation practicable.

Thus the F1 experience shows that GDI has been developed to work to 18,000 rpm, at least with a Formula 1 budget behind it. That means there is still some chance we could one day see it on a production motorcycle engine.

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