Chang, J., Kalghatgi, G., Amer, A., and Viollet, Y., "Enabling High Efficiency Direct Injection Engine with Naphtha Fuel through Partially Premixed Charge Compression Ignition Combustion," SAE Technical Paper 2012-01-0677, 2012, doi:10.4271/2012-01-0677.
More stringent emissions standards along with higher fuel economy demands have obliged auto makers to develop technical solutions that exploit synergistic features from gasoline and diesel engines. To minimize NOx and soot trade-off, diesel powertrain has been developed to adopt increasingly complex and expensive technology such as extremely high pressure fuel injection systems, low pressure EGR, and variable valve timing. These attempts are associated with promoting Partially Premixed Charge Compression Ignition (PPC-CI) combustion via increasing mixing time and ignition delay. Alternatively, PPC-CI combustion can be achieved easier by using fuels with higher resistance to auto-ignition than conventional diesel fuel. Previous work has demonstrated the possibility of reducing the cost of future diesel after-treatment systems by using gasoline-like fuels. In this study, we start with a 0.5-liter single-cylinder direct injection spark ignition (DISI) engine and demonstrate that fuel economy can be improved significantly by running it in PPC-CI mode. Naphtha, less processed refinery stream in the gasoline boiling and carbon number range, but with lower Octane Number, has been run in a 12:1 compression ratio single-cylinder engine with DISI fuel system and shallow oval-type bowl piston. Both light and heavy Naphtha were successfully run in PPC-CI mode with regular valve events and intake charge boosting at six engine running conditions representative of a typical urban driving cycle including idle. Very low NOx level was achieved through high EGR and advanced injection timings to segregate the fuel injection from heat release. When compared to a Stoichiometric SI operation with optimal valve timings, 19% weighted cycle average fuel consumption reduction was achieved. This result demonstrates that an engine equipped with a low cost DI system could offer noticeably better efficiency through PPC-CI combustion, especially when run with Naphtha, which can provide lower CO₂ emission in a refinery process. Although further work is needed to develop a practical engine, high efficiency, reduced system cost and overall CO₂ footprint benefits can be achieved by matching the fuel and the combustion system.