Boat Outboards 2013By Michael Vatalaro
Published: April/May 2013
The past decade has seen an amazing array of changes and improvements in outboard engines. If you haven't shopped for one lately, you're in for a nice surprise.
Different Strokes For Different Folks
You'll notice all of these manufacturers are trying to design an outboard that "performs like a two-stroke," which is precisely the reason that two-strokes haven't gone away, just cleaned up their act. Direct injection (DI) is the innovation that allows a two-stroke to run cleanly and efficiently. By separating the fuel injection from the intake of air into the cylinder and, more importantly, waiting until the piston has risen far enough up the cylinder bore to block the exhaust port before injecting the gas, a DI two-stroke doesn't blow any unburned gas right through the cylinder and out the exhaust like its carbureted cousins. And because this process is electronically rather than mechanically timed, the engineers can tweak the performance with software rather than wrenches.
While Mercury offers Optimax DI two-strokes and Yamaha still has their "High Pressure Direct Injection" two-stroke engines in their lineups, Evinrude has chosen to offer only DI two-strokes, called E-TEC. Karl Sandstrom, product manager of Evinrude Outboards, says that using computers to control engine timing, and the smallest details of when and how combustion happens, allows them to adjust for maximum efficiency under different operating conditions. "Forty percent of an outboard's duty cycle is at or near idle speed, either trolling or motoring in no-wake zones," he says. "With the computer power we have, the engine can act like a very small outboard when all that power isn't needed, then switch to homogeneous mode when it is."
At idle, he says, the computer can tell the engine to inject only a tiny amount of gas, far less than needed to create a combustion event that would fill all the space above the piston, which is typical. Instead of a big bang, you get a little pop, enough to keep the piston moving (stratified combustion). But hit the throttle and the engine switches to homogeneous (all the same) mode, and enough fuel flows to make the combustion event fill the entire space. This fuel-saving strategy has even made its way into newer-model cars.
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