Engineering News

Small engines face big

Engine makers look for new ways to meet emission

Chicago--Ready or not, the $79 discount-store
lawnmower is in for some changes. New emission regulations are forcing engine
manufacturers to crank out a new breed of cleaner engines for lawnmowers,
snowblowers, chain saws, and other outdoor equipment.

Though the proposed Environmental Protection Agency (EPA) regulations aren't scheduled to take effect until August of 1996, engine manufacturers already are scrambling to meet them. The toughest task, they say, will be to clean up the commodity engines used on mowers costing $100 and less. "There isn't much room for extravagant changes on a $100 walk-behind mower," notes George Gatecliff, chief research engineer for Tecumseh Research Laboratory in Ann Arbor, MI.

Still, the EPA and the California Air Resources Board (CARB) are set to make small-engine manufacturers comply with the 1990 Clean Air Act. At the federal level, the EPA has proposed emission regulations for engines used on automobiles, boats, and outdoor power equipment, among other items.

In outdoor power equipment, they call for reductions in nitrous oxides (NO), hydrocarbons (HC), and carbon monoxide (CO). The proposed regulations are particularly stringent for small engines. Non-handheld engines under 225 cc, for example, require HC and NO to measure 12 g/BHP-hour. In 1990, such engines averaged 37.7 g/BHP-hour, say engine manufacturers. That's a reduction of approximately 70%.

The regulations directly affect such companies as Tecumseh, Briggs & Stratton, Kohler, and other small engine manufacturers. But they also profoundly affect many original equipment manufacturers, such as Toro, which produce engines for their own equipment.

To meet the regulations, engineers are implementing a raft of design changes, including improvements in cooling, carburetion, ignition systems, oil consumption, engine timing, and air-flow control.

Key among those changes is improved carburetion. In the past, most small engines ran a comparatively rich fuel mixture as a way of ensuring good performance across a wide operating range. Richer mixtures, however, raise undesirable emissions. As a result, engineers must now find a way to make carburetors more precise. Key improvements involve finding more precise ways to drill, ream, and polish metering holes to remove burrs and imperfections.

Engineers also are abandoning "bump-compression relief" to help owners start their outdoor power equipment. In bump-compression relief, manufacturers form a tiny metal bump on the exhaust cam. The bump opens the exhaust valve when the user yanks on the starter cord, thus easing the pull-start process. After the engine starts, however, the bump keeps bumping and more hydrocarbons escape outside the exhaust port.

Now, bump-compression relief devices are giving way to mechanical-compression relief devices. Most use a protruding pin that retracts after the engine picks up speed. "Using mechanical-compression relief, the exhaust valve isn't open at an inappropriate time," Gatecliff says. The upside is fewer emmissions. The downside: higher costs.

Design improvements involve a heavy dose of finite element analysis and computational fluid dynamics. Using those computer tools, engineers analyze air-flow control and structural deformations related to engine sealing.

In some cases, manufacturers have decided that re-design is not cost effective. Kohler,


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