~ Auto Buzz ~: ESSAY: Having It All If even a basic little Hyundai has GDI and cam phasers, why not motorcycles?

When I recently flew to California for the Cycle World Ten Best celebration, the car I rented rubbed my nose in two technologies currently little used in motorcycles—direct fuel injection (GDI, or Gasoline Direct Injection), and widely variable cam timing by means of hydraulic cam phasers. Cars are utilities to me—I take an interest in their technologies but haven’t given much thought to which maker’s name is on the sheet metal. So I climbed into what Budget issued me—a 2015 Hyundai Accent—had a look around and started it up. The stand out aspect of this little car’s performance was its unusually powerful lunge away from lights. That made me think it had cam phasers—like Ducati’s recent Multistrada 1200 with Testastretta engine. Normally small auto engines have compromise cam timings. They are not too GSX-R-like because that would weaken bottom-end torque too much. Nor are they too Harley-like because that would mean the engine would run out of breath on top. But with cam phasers, you can combine the benefits of both. What I was feeling was the bottom-end torque-boosting effect of short valve overlap away from stoplights, with the ability to carry on strongly to peak revs as the cam phasers shifted to longer and longer valve overlap as the little engine (1.6 liters, or about 96 cubic inches) revved up.

When I could, I opened the hood to verify that the bulges indicating cam phasers were present in the cam covers. Then I noticed the letters “GDI” on the plastic engine cover. This is injecting the fuel directly into the combustion chambers instead of spraying it into the intake airstream just under the throttle butterflies. This increases the air delivered to the engine because there is no fuel vapor in the intake ducts competing for space with the airflow. It also results in a cooler charge because the heat required to evaporate the fuel comes from the combustion chamber rather than from the walls of the intake duct. The cooler charge allows a substantial increase in compression ratio—in this little Hyundai Accent it is 11:1—and the torque boost that goes with it. That may not sound like much to sport motorcyclists who are used to 12:1 compression, but there is a difference. Car engines are required to pull from low rpm, which makes them more likely to knock. Before the coming of GDI, a common compression ratio for small auto engines was 9.5:1. That makes 11 a big number. Variable valve timing (VVT) has been used from time to time in motorcycle engines, but usually in the form of a single step change rather than with timing varying smoothly with rpm. Why would Ducati take this step (see DVT explained here: VIDEO: TECH UPDATE: Why Variable Valve Timing)? First, Ducati clientele is especially willing and able to pay for new technology, and second, Ducati management saw how much people liked the bottom-end torque (which resulted not from VVT but from conservative fixed valve timings). That hearty bottom end greatly increased the appeal of Diavel among riders used to the usual cruiser torque.

Why no GDI so far on motorcycles? When you inject fuel directly into the combustion chamber you give yourself less time in which to evaporate the fuel. That requires a specialized high-pressure injector that produces smaller fuel droplets. You must also take care that very little fuel actually reaches the cylinder walls, where it mixes with the thin layer of oil there and may be swept into the crankcase by the oil scraper ring (such oil dilution was a problem even with some of the first port injection adopted on motorcycles). Clearly, GDI is capable of operating at the peak rpm of a small auto engine (6300 rpm in this case), and even higher because its use has been made mandatory in the new, greener, polytechnic Formula 1. As always, there’s more to come.