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Solutions - Two-Stroke Engine Modifications



Solutions to emission problems of off-road recreational vehicles may include:

  • Substituting four-stroke engines for two-stroke engines to improve fuel economy and reduce emissions.
  • Introducing fuel injection to replace carburetors in new two-stroke machines
  • Improving fuel atomization in two-stroke engines for leaner burning mixtures.
  • Treating exhaust to reduce emissions from two-stroke engines.
  • Substituting rotary four-stroke engines for two-stroke engines to maintain power-to-weight ratios of the two-stroke engine.

Prospects for lowering emissions of two-stroke engines to levels 75 percent below earlier technology have already been demonstrated in personal watercraft, primarily by use of fuel injection and variable exhaust ports.

Atomizing carburetors and catalytic converters for two-stroke engines appear to be the only practical after-market solutions. Other approaches emphasize refinements to newly manufactured two-stroke equipment, substituting four-stoke engines for two-stroke engines in new machines, or adopting newer versions of four-stroke rotary engines.

Direct fuel injection

Electronic fuel injection

Fuel injection, which has replaced carburetion, increases the performance of the engine. Electronic fuel injection provides better distribution of fuel into the combustion chamber. Because fuel injection allows for a more accurate amount of fuel being used, it produces better fuel economy, lower emissions and more power. There are three generations of the fuel injector. The first generation injectors are mechanical actuated by a pulsating the pressure from the high-pressure fuel pump.


  • Inject even distribution of fuel into cylinder to prevent excessive wear and optimize power output from engine
  • Correct ratio of fuel to oxygen
  • Adjust for temperature and speed of car to optimize engine performance
  • Reduce emissions

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To learn more about fuel injection, see:

David E. Cole, Fuel injection, Discovery Channel School, original content provided by World Book Online:

Orbital Engine Corporation, an Australian technology company specializing in proprietary combustion engine technologies, has developed an air-assisted direct injection fuel system that makes engines more fuel efficient and substantially reduces exhaust emissions. This technology may be applied to snowmobile engines. Orbital claims that its Tohatsu outboard cuts fuel consumption by up to 60 percent and exhaust emissions by about 80 percent compared with a conventional two-stroke outboard.

Orbital claims to have designed a range of new automotive engines (2, 3 and 6 cylinder variants using a mix of natural and externally blown aspiration) that offer advantages over conventional four-stroke engine technology of comparable performance.

The application of OCP direct fuel injection to the two-stroke engine overcomes traditional deficiencies enabling exhaust emission levels to achieve ultimately California Ultra Low Emission Level (ULEV) status. 

Carburetor Modifications

Atomizing carburetors and catalytic converters for two-stroke engines also hold the promise of an after-market solution to snowmobile emissions.

Atomized Fuel Technologies of Apple Valley, Calif., has tested snowmobile engines equipped with atomizing carburetors and catalytic converters (as an alternative to electronic fuel injection). Although it does not yet offer a commercial product, AFT intends to offer kits for retrofitting engines of major snowmobile manufacturers with atomizing carburetors and catalytic converters at a cost of about $750. AFT carburetor and catalyst technology was fitted to a Polaris 550 snowmobile engine, and test by an independent testing laboratory (California Environmental Engineering). The outcome, compared to baseline tests against a stock engine, showed a hydrocarbon reduction of 72.5 percent, and CO reduction of 82 percent. NOx emissions were increased by 7 percent.

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Two-Stroke Snowmobile Engine Emissions Reduced Over 80% Advanced Fuel Technologies, Inc., Apple Valley, CA, (AFT) announced in April 2000 that independent tests confirm that AFT achieved a breakthrough that should allow snowmobiles as well as other two-stroke powered vehicles, to be operated in an environmentally acceptable manner. In a news story published April 24, 2000, in Business Wire., AFT's claims its two-stroke engine reduced emissions by more than 80 percent. 

Catalytic Converter

A catalytic converter reduces exhaust pollutants produced by an automobile engine. The engine's combustion process gives off carbon monoxide and other harmful chemical compounds. A substance called a catalyst in the converter helps change these pollutants into safer substances. The catalyst in most converters is a blend of the metals platinum, palladium and rhodium. The catalytic converter is installed in an automobile's exhaust system. As the exhaust gases pass through the converter, the catalyst causes carbon monoxide and other pollutants to change to oxygen, nitrogen, water, and carbon dioxide.

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To learn more about catalytic converters, see:

William H. Haverdink, Catalytic Converter, Discovery Channel School, original content provided by World Book Online:

A promising technology that would improve two-stroke engine efficiency and reduce emissions is the Smart Plug, which, in effect, transforms a catalytic converter into the equivalent of a spark plug. The Smart Plug is a hollow cylinder with a precious-metal "catalyst" on a ceramic rod running down the middle, and a nearly-sealed bottom end, where a small amount of fuel-air mixture can enter from the area of the combustion chamber of the engine.

The catalyst has a tiny wire inside, allowing it to be pre-heated. When the catalyst is sufficiently heated and the engine's fuel-air mix is pumped in, courtesy of the starter motor, the catalyst sets fire to the small amount of charge inside its body, and the flaming charge then blasts out the holes, into the combustion chamber, and the piston goes down.

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An explanation of the Smart Plug can be found on line at the Montana State University Tech Link Center website: Catalytic Ignition Replaces Spark Plugs.

Read a story about the Smart Plug titled A clean machine? Solution to snowmobile pollution may be on park's horizon, in the Jan. 9, 2000, issue of the Bozeman Daily Chronicle.