Low-emissions two-stroke outboard motors have been available since 1996, so it would seem like this technology is old-news. Unless you’re a boating Rip Van Winkle, like my neighbor—who has decided that this is the year he’s going to replace his chalky old runabout and its trusty V-4 90 hp motor. Perfectly satisfied with this package, he’s been motoring about in a cloud of two-stroke fumes, oblivious to a decade of advances in outboard technology.
There are plenty of old outboards prowling around out there, and a lot of boat owners like my neighbor. If you’ve been out of the outboard loop, it may be time to get up to speed with a quick tutorial on the modern two-stroke outboard.
Two-stroke outboard motors with DFI (direct fuel injection) were developed in response to an EPA mandate requiring a 75 percent reduction in exhaust emissions between 1998 and 2006. Since the days of Ole Evinrude, almost all outboard motors used a traditional two-stroke powerhead, which was lighter and less expensive to manufacture than a four-stroke engine. Unlike the four-stroke engine, the two-stroke does not have moving valves to seal the combustion chamber and control the flow of intake and exhaust. In a two-stroke engine the rising and falling piston covers and uncovers ports in each cylinder through which a fuel/air mixture flows into, and exhaust flows out of, the combustion chamber.
A traditional two-stroke produces very high exhaust emissions because the fresh charge of fuel and air, streaming from the crankcase through the transfer ports and into the combustion chamber, arrives while the exhaust port is still open. Some of the unburned fuel mixture escapes with the exhaust, creating the cloud of hydrocarbon emissions we all used to take for granted. In fact, the unburned fuel often escaped by design, to ensure that all of the spent exhaust was “scavenged” from the combustion chamber. This was especially evident at low speeds when up to 40 percent of the fuel entering a traditional two-stroke motor escaped unburned. At its most efficient speed, generally 4000 to 4500 rpm, a traditional two-stroke may still expel eight percent of its fuel with the exhaust. To get a visual, check out this video of How Two Stroke Engines Work.
This had to stop, according to the EPA. One alternative was direct fuel injection (DFI), which had been used on four-stroke engines in some World War II aircraft and early fuel-injected automobiles. By aiming a fuel injector directly into the combustion chamber, fuel delivery cam be timed to occur only when the moving two-stroke piston has closed the intake and exhaust ports. Exhaust can still be scavenged from the cylinder by fresh air being pumped through the transfer ports from the crankcase, along with the very small amount of oil required to lubricate moving parts.
In a two-stroke marine engine, the DFI solution presents two significant challenges. At 6000 rpm each rotation of the crankshaft takes a hundredth of a second, and at that engine speed the moment that both the intake and exhaust ports are closed is only 0.0019 of a second. So most of the fuel required for full power must be delivered into the combustion chamber in that fraction of a second. And to ensure good combustion, the fuel has to be atomized into tiny 100-micron droplets – really just a fog.
Various technologies have been developed to accomplish this task; either by used of a high-speed solenoid injector, an injector that combines fuel and compressed air pressure, or a very high-pressure fuel pump feeding injectors. In each case, the fuel injectors are placed in the cylinder head, close to the spark plug. The nozzle end of these new injectors is designed to withstand the harsh environment of the combustion chamber. Modern computer controls, used to time injector duration, are also a key part of any DFI system.
Each of these DFI systems significantly reduced exhaust emissions and also improved fuel economy, most dramatically at low speeds where the traditional two-stroke was least efficient. At trolling speed, a DFI motor consumes 70 percent less fuel than the carbureted or EFI motors previously offered. Economy is 30 to 40 percent better at cruising speed, and about the same to 10 percent better at full throttle. Two-stroke outboards with DFI are also easier to start, quieter, and offer smoother operation than the traditional two-strokes they have eventually replaced.
At the same time DFI outboards were being developed, new four-stroke motors also appeared on the market. The new four-strokes also significantly reduced emissions to meet the current EPA mandate, and offered much-improved fuel economy and smoother operation. However, a higher-horsepower four-stroke outboard was heavy, often 100 pounds or more heavier than a DFI two-stroke of similar power. This gave the DFI two-stroke a performance advantage, especially on bass boats and other high-performance boats.
Over the years, further development has narrowed the weight difference between a four-stroke and a DFI two-stroke outboard. But from 25 to 300 horsepower a DFI two-stroke remains the lightest option at each horsepower rating. It is also usually less-expensive than a four-stroke. At low speeds, a DFI two-stroke typically produces lower exhaust emissions and better fuel economy than a four-stroke, a consideration for anglers that spend a lot of time trolling, or boat owners faced with long no-wake zones. Until recently, a few “traditional” two-stroke models were still available as outboard manufacturers were allowed to average two-stroke emissions with those of much cleaner models in a product line, but the most-recent EPA regulations capped emissions for all outboards and those old-technology two-strokes are no longer manufactured for sale in North America.