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US10 Capable Prototype Volvo MG11 Natural Gas Engine Development: Final Report: December 16, 2003-July 31, 2006
C. Tai (au)
The objective of this Nat. Renewable Energy Lab. (NREL) project is to develop a low emissions natural gas engine. The emission targets for this project are 0.27 g/kW-hr (0.2 g/bhp-hr) of NOx and 0.013 g/kW-hr (0.01 g/bhp-hr) of particulate matter. To meet the objective, a chemically correct combustion (stoichiometric) natural gas engine with exhaust gas recirculation (EGR) and a three-way catalyst (TWC) was developed. In addition, a Sturman camless Hydraulic Valve Actuation (HVA) system was used to improve efficiency. A Volvo 11 liter diesel engine was converted to operate as a stoichiometric natural gas engine. Operating a natural gas engine with stoichiometric combustion allows for the effective use of a TWC, which can simultaneously oxidize hydrocarbons and carbon monoxide and reduce NOx. High conversion efficiencies are possible through proper control of air-fuel ratio. Adding EGR lowers combustion chamber temperatures, which can improve efficiency, reduce the tendency to knock and lower engine out NOx emissions. A camless HVA system by Sturman Industries was applied to the engine. Using the HVA system can reduce pumping losses at light loads through either an early intake valve closing or a late intake valve closing (Miller cycle). The HVA system can also enable the use of high compression ratio pistons, where a lower effective compression ratio can be used at the high loads through either an early or a late intake valve closing, and higher compression ratios can be used at light to medium loads. Pistons with a 14.3:1 compression ratio were used on this engine. Emission tests were run at the 13-mode steady state test points and at three additional points (15% load at the low, intermediate, and high engine speeds). The results show a weighted 13-mode NOx emission level of 0.005 g/kW-hr, which easily meets the emissions target of 0.27 g/kW-hr (0.2 g/bhp-hr). The high compression ratio pistons and variable valve timing available from the HVA system improved efficiency by 6.1% over a fixed valve timing cam based system using the weighted 13-mode results. Further efficiency improvements may be possible through reduced losses of the HVA system and through improvement in light load combustion by using the HVA system to increase in-cylinder motion (swirl). Illus.
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