Four-stroke engines have benefited from decades of refinements in the automotive industry, and designing an emissions-friendly four-stroke outboard would be a relatively straightforward process. Much of the technologies developed for the cars we use today could be incorporated into outboard engines to some degree, but there are definite downsides. When compared to a two-stroke engine, generally, a four-stroke engine of equivalent horsepower will have twice the components and many more moving parts. Things such as camshafts, timing chains and gears, and an oil sump simply do not exist in a two-stroke engine. This also means that a power-equivalent four-stroke engine will almost always weigh more and be larger in size -- a very negative trait in an outboard-powered boat. Furthermore, the added mechanical components in every four-stroke engine increase operating friction, adding to parasitic power losses which must be overcome. But the most important difference is in how these two engine designs principally function; the two-stroke engine produces power on every cycle of the piston, which is every revolution of the flywheel. The four-stroke engine must use a separate, non-power cycle for expelling burned gases out of the combustion chamber. Because of that required exhaust gas purging cycle, a four-stroke can only produce power on every other revolution of the flywheel.
There were indeed challenges to overcome with the four-stroke engine platform for an outboard engine application. Even so, this became the platform of choice for most of the industry. It was viewed as being the easiest to develop for future emissions requirements, and it was a common belief at the time that the two-stroke engine would be incapable of matching the emissions performance of four-stroke designs. It was also a misconception that emissions laws actually outlawed the two-stroke engine altogether. This of course, was false - the EPA regulations stipulate maximum allowable limits for hydrocarbons (unburned fuel) and nitrogen oxide (NOx) as well as carbon monoxide (CO). The total hydrocarbon emissions reported by any particular engine or type of technology did not matter to the EPA; just the maximum emission limits that they regulated. Put simply, the EPA regulates emission levels, not engine technology.
And while it seemed that the industry would, as a whole, move toward four-stroke outboards, there was an exception. It might be difficult to believe, but the technology that was to be the genesis of what would be the emissions-compliant two-stroke outboard engine was developed over 90 years ago, in 1925. This is direct fuel injection or as it is known commonly in today’s times as DFI. Its history might suggest that direct injection is not much different than the fuel injection systems that became common in the 1980s. However, the opposite is true; direct fuel injection offers important advantages over intake manifold injection systems, and those advantages would open the way for the new generation of emissions-compliant, high-performance two-stroke engines.
2006: Engine testing in Key Largo, Florida. The famous Evinrude E-TEC test boat rigged with a Yamaha four-stroke for performance comparison testing. This boat is always a favorite. |
By contrast, direct fuel injection delivers gasoline at very high pressures directly into the combustion chamber after the exhaust port is closed by the piston, which offers an extremely precise delivery of fuel, resulting in more power and fuel economy. By injecting the fuel directly into the combustion chamber, the inefficiencies of passing a charge through the intake manifold and valves are bypassed. That means combustion will be much more complete, and thus, leaving less byproducts of combustion – a much ‘cleaner’ engine than the traditional two-stroke of previous years. The downside to direct fuel injection is that it is a more complex system than ordinary automotive-style fuel injection. Thus, a system had to be developed for use in two-stroke outboard engines that was accurate enough to minimize emissions yet not be super expensive or unwieldy.
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