Chang, Y., Wooldridge, M., and Bohac, S., "Extending the Dilution Limit of Spark Ignition Combustion via Fuel Injection during Negative Valve Overlap," SAE Technical Paper 2016-01-0671, 2016, doi:10.4271/2016-01-0671.
Using exhaust gas recirculation (EGR) as a diluent instead of air allows the use of a conventional three-way catalyst for effective emissions reduction. Cooled EGR can also reduce fuel consumption and NOx emissions, but too much cool EGR leads to combustion instability and misfire. Negative valve overlap (NVO) is explored in the current work as an alternative method of dilution in which early exhaust valve closing causes combustion products to be retained in the cylinder and recompressed near top dead center, before being mixed with fresh charge during the intake stroke. The potential for fuel injection during NVO to extend the dilution limit of spark ignition combustion is evaluated in this work using experiments conducted on a 4-cylinder 2.0 L gasoline direct injection engine with variable intake and exhaust valve timing. The results demonstrate fuel injection during NVO can extend the dilution limit, improve brake specific fuel consumption (BSFC), and reduce CO and NOx emissions. Specifically, 80 CAD of NVO with start of fuel injection at top dead center allowed the use of 32% total EGR (internal and external) and resulted in the best BSFC of 278.7 g/ kWh; an improvement of 22% compared with the BSFC of 358.3 g/kWh for the production version of this engine at the same speed and load condition (1800 RPM and 3 bar BMEP) without EGR. The optimized case of SOI = 360 CAD bTDC and NVO = 80 CAD, resulted in slightly higher CO emissions of 3865 ppm and significantly lower NOx emissions of 281 ppm, compared with the CO and NOx emissions for the stock production version of the engine of 3230 ppm and 986 ppm, respectively.