Compared to conventional carburetion, direct fuel injection (DI) is more than merely an alternative method of getting fuel into the mariner's engine. In carbureted two-stroke motors, the fuel and oil mixture enters the cylinder from the bottom of the piston. The mixture travels up and around the piston skirt to reach the top piston ring. Because the oil is thinned by fuel, enough oil passes around the bottom ring to move up and lubricate the top piston ring.
Conventional two-strokes also carry a fuel penalty because the exhaust port is partially uncovered by the piston as it moves up on its compression stroke. So some of the intake charge is pushed out the exhaust along with the remains of the previous cycle's burnt gases. Emissions take a hit for the same reason.
With direct fuel injection (DI), the exhaust port is completely covered by the piston before the fuel is squirted directly into the combustion chamber, so no fuel is wasted. But DI can pose some lubrication challenges. Oil is still drawn in from the bottom of the cylinder but fuel is injected from the top. Since the fuel and oil don't mix, the oil isn't thinned sufficiently to move up and lubricate the top piston ring. That's a recipe for engine disaster-as one manufacturer has discovered.
Yamaha's HPDI system gets around this problem with a new, patented bottom piston ring that's made with a taper for oil bypass every 30 degrees. The 12 tapered areas around the ring allow sufficient oil to reach and lubricate the top ring.
DI two-strokes have a potential for combustion fine-tuning that's similar to today's computer-controlled fuel-injected auto engines'. HPDI takes advantage of this potential with a sophisticated engine management system to elevate the two-stroke to new levels of efficiency. These outboards, like their automotive counterparts, use multiple sensors to transmit data such as crank position, throttle position, timing, rpm, water temperature, air temperature, atmospheric pressure and the amount of oxygen in the exhaust to the computer. Yamaha uses an industry exclusive O2 sensor to continually monitor the oxygen content of the exhaust gases. The computer analyzes all this data and automatically adjusts the ignition timing and fuel mixture to each individual cylinder for maximum power and fuel efficiency, and fewer emissions during the next combustion cycle.
