Researchers at Southwest Research Institute and the California Air Resources Board recently completed a project to modify existing small off-road engines to meet proposed 1999 ARB emissions regulations. They wanted to show that compliance could be attained without the need to redesign base engines. Four high-sales-volume, ARB-certified MY1997 engines—handheld two- and four-stroke and nonhandheld side- and overhead-valve engines—were selected and baseline emissions tested using applicable ARB test procedures. Appropriate emissions control strategies—including air/fuel ratio optimization and catalytic aftertreatment—were applied to the engines, with all four engines meeting ARB's original 1999 Tier 2 emissions standards after development. Researchers said that the cost of the additional hardware (catalyst) required to meet the standards was low. The size of the engine packages would remain the same or slightly increase, although with careful catalyst packaging in the muffler increases would be minimal. The weight of the engine package may also increase slightly. For more information, circle 78.
Engineers at The Queen's University of Belfast and Federal-Mogul Technology have investigated the effect of bore/stroke ratio on a special-purpose two-stroke engine of modular design. The engine allowed four combinations of bore and stroke to attain a common swept volume of 400 cm3, with bore/stroke ratios of 0.8, 1.0, 1.2, and 1.4. Factors that could affect engine performance—such as the exhaust, intake, and ignition systems; port timings and time areas; and cylinder and crankcase compression ratios—were kept the same. The most important observations were that engine power was greatest with bore/stroke ratios of 1.0 or 1.2; combustion efficiency tended to decrease with increasing bore/stroke ratio; mechanical efficiency tended to increase with increasing bore/stroke ratio; and specific consumption tended to rise with increasing bore/stroke ratio. The investigation concluded that a square or slightly over-square bore/stroke ratio produces an engine with the best brake performance. For more information, circle 79.
NGK Spark Plug researchers attempted to reduce emissions and improve fuel economy of a small utility engine by employing a lean air/fuel ratio under partial load conditions in the most frequent operating range. Since NOx emissions increase at lean air/fuel (A/F) ratios, they tried to reduce emissions by retarding spark timing to 7 CA° from a MBT condition while minimizing power degradation. However, this caused combustion instability, which was later avoided by adjusting spark timing with respect to engine speed. CARB emissions tests using their technique resulted in CO and THC+NOx emissions below 1995 CARB standards. Researchers expect that application of the technique could be extended to meet year 2000 requirements. Fuel economy was also improved up to 6.9% during the tests. Remaining tasks include investigating durability problems at high temperatures, transient dynamic characteristics, and emissions as the engine ages. For more information, circle 80.
Emissions of total HC, CO, NOx, and combined HC and NOx from small utility engines were used by researchers at the University of Wisconsin-Milwaukee to judge the effect of 0 to 50 vol% ethanol addition to HC fuel, using factory air/fuel ratio carburetor settings. Emissions from two 9.3-kW (12.5-hp) side-valve and one overhead-valve four-stroke engines were analyzed. Primary results indicated that increasing the ethanol concentration in gasoline was effective in reducing CO emissions from engines designed to operate on nonoxygenated fuels, but ineffective in reducing combined HC and NOx emissions. This has implications on the strategy of blending a specialized highly oxygenated lawn & garden fuel. Secondary results indicate evidence that the overhead-valve engine exhibited lower HC and CO emissions, but had no discernible effect on NOx and negligible effect on combined HC and NOx emissions. This has implications on the shift from side-valve to overhead-valve engines in small-engine applications.
For more information, circle 81.
SAE Off-Highway Engineering June 2000
