All of these powerplants – which seldom go by their full name of two-stroke cycle engines – have the same basic mode of operation, with a complete combustion cycle in each up-and-down motion of the crankshaft. They draw in fuel, compress it, create power at ignition and exhaust the combustion byproducts every 360 degrees of crank angle. This feature makes them very desirable, since in theory they can deliver twice as much power as a four-stroke cycle engine with an equivalent displacement. In actual practice it doesnt work that way, since the combustion process isnt fully efficient; exhaust gases wind up in the fuel-air mix for the next revolution, and some fuel may be expelled before combustion is complete.

Despite these drawbacks, Stihl Co., known for its chain saws and other outdoor power tools, reports that a two-cycle engine can generate 20 percent more power with the same displacement as a conventional four-stroke, and at 30 percent lower weight. Some of the weight savings comes from there being no need for a separate oil tank, because the lubricating oil is mixed into the fuel to circulate and burn in the engine. For superior horsepower in a small, lightweight package, the two-cycle engine is certainly hard to beat.

Nevertheless, the times are changing and two-cycle engines are under serious scrutiny. Because most two-cycle engines expel fuel and lubricant out the exhaust pipe, they are serious polluters. Until recently, the U.S. government did not seriously challenge two-cycle exhaust emissions, and their usefulness as lightweight energy sources made them seem untouchable. No more! Starting next year, the federal Environmental Protection Agency plans to enforce new regulations that it says will cut down on emissions from lawn and garden equipment, portable generators, personal watercraft and other equipment. The new rules aim to cut hydrocarbon emissions from small spark-ignition engines by 35 percent.

One approach to the emissions problem is to use direct in-cylinder (DI) fuel injection, as opposed to the traditional fuel/oil induction process. In a draft paper on the subject, EnviroFit International Ltd., Fort Collins, Colo., reports that it has been field-testing a DI retrofit kit on motorcycles in Asia, based on technology from Australias Orbital Corp. This has reduced exhaust emissions by 88 percent, and carbon monoxide (CO) emissions by 72 percent. As a bonus, fuel efficiency rose 32 percent, since nearly all the gasoline burns.

Last year, the Arizona Republic newspaper took note of the proposed changes, and went on to describe another possible solution. Freescale Semiconductors, based in Austin, Texas, has developed an integrated circuit that measures emissions from two- and four-stroke engines and makes adjustments to control them. The potential applications, it said, range from scooters, mopeds and garden tractors to chain saws, leaf blowers and other lawn equipment, to recreational vehicles.

In North America recreational boating has traditionally been a major two-cycle engine market, particularly for outboard motors. This is changing too, Tom Marhevko, vice president of engineering standards at the National Marine Manufacturers Association in Chicago, told LubesnGreases. North American two-cycle engine production is dropping and four-cycle engines are taking their place. Although the rest of the world is seeing growth in the two-cycle engine market, and the two-cycle oil market following suit, the economic slowdown hurt U.S. boat manufacturing and sales. Manufacturers reduced production while boat dealers worked off existing inventories. Now boat manufacturers have begun to rebuild inventories. Its similar to the phenomenon seen in the automobile industry, where dealer inventories were drawn down while manufacturers operated at reduced output.

Fuel choices are undergoing change as well, Marhevko said, noting that ethanol-containing fuels are now more common. Some marinas offered ethanol-free gasoline but it is getting more difficult to obtain, so 10 percent ethanol (E10) is finding its way into marine fuels. When these fuels get wet (boating, marinas), the water and ethanol tend to separate from the gasoline and settle to the bottom of the tank. In many boats, the tank is made of aluminum and the water/ethanol combination corrodes the tank. Ethanol-containing fuels also may be incompatible with the fueling system (gaskets, seals and lines) causing failures.

With the push to increase ethanol content in gasoline, there is concern that these problems will continue and even become more severe. Fortunately, a lot of work is being done to deal with higher ethanol levels in the automotive area, which can see fuels up to E85. Marhevko is confident that engine technology will be available to overcome such problems.

Overall, E10 or greater fuels deliver less energy per pound so there is a loss of fuel economy when using them. However, ethanol-containing fuels do result in emissions improvements. And Marhevko pointed out that outboard engine duty-cycles are changing. After years of demanding more engine horsepower, boaters now are going at lower average speeds and for shorter duration, he said. That may signal a change in how people drive their boats, general belt-tightening and fewer hours on the water.

Marhevko indicated that marine engine oil formulation changes may be coming. The traditional water-cooled engine lubricant is ashless and contains an extremely effective dispersant. Thanks to emissions mandates how-ever, more exhaust systems will be equipped with sensitive catalysts. Direct fuel injection will eliminate the classic fuel/oil mix and could precipitate oil modifications. Currently, there are engine test projects under way to evaluate oil/fuel/catalyst compatibility.

The good news is that NMMAs venerable TC-W3 marine engine oil standard is still good to go and seems to function just fine with direct injection. Currently, NMMA lists 138 products as licensed to the TC-W3 standard, and given the number of two-cycle boat engines already out there, the need for TC-W3 will be ongoing.

According to Ed Callis, vice president of business development and technology for Spectrum Corp. in Port St. Lucie, Fla., there are an estimated 12 million marine engines in use in the United States, most of them two-cycle engines. This makes it unlikely there will be any rapid change in demand for TC-W3 engine oils. Still, the industry produces about 200,000 new engines annually and most of these are now four-cycle models, so at some future time the two-cycle market is going to be affected.

Air cooled two-cycle oils differ from water cooled products in that they contain a detergent material which is designed to minimize high-temperature deposits in the engine. These deposits can really mess up engine opera-tion by causing ring sticking and loss of compression. Deposits also lead to spark plug fouling, which is a major headache in air cooled engines. Since two-cycle engines operate at higher temperatures than four-cycle engines, deposit control is critical.

For both air cooled and water cooled oil formulations, the main change lately has been a move towards API Group II base oils, from traditional Group I stocks. This is the result of closures of Group I plants and the wider availability of Group IIs nationwide. There is also some movement towards synthetics and biobased stocks.

Callis pointed out that there is a potential price to pay for these changes. Traditionally, Group I heavy neutral and bright stocks were used to formulate two-cycle oils. They provided good lubricity as well as deposit control and solvency. If carbon deposits formed, they were sufficiently soft that they didnt result in a great deal of ring sticking.

By contrast, Callis said, Group II and Group III base stocks as well as synthetics such as polybutene are more thermally stable and form much harder deposits. This can result in more ring sticking. Additionally, some blends use a very light neutral base stock to offset polybutene thickeners, which otherwise would result in a significantly heavier viscosity oil in the engine and cause more problems in bearings such as reduced lubricity and increased temperatures.

The basic chemistry of two-cycle additives has not changed. However, the use of some esters or even a return to bright stock to improve lubricity is occurring. Bright stock is in tight supply, and as time goes on and more Group I refineries are retired, it will be harder still to obtain.

As with so many things, nothing is constant but change. Two-cycle engine oils will have a continuing place in the global market, and as technology improves, some of the shortcomings with regards to emissions will be resolved. Meanwhile, consumers will continue to use them and enjoy those days on the water, and grumble a bit about the days in the yard.