Naphtha Fuel on a Light Duty Single Cylinder Compression Ignition Engine with Two Different Compression Ratios 2016-01-2302

Gasoline-like fuels have been recently identified as good candidates to reduce NO_X and particulate emissions when used in compression-ignition (CI) engines. In this context, straight-run naphtha, a refinery stream directly derived from the atmospheric crude oil distillation process, was identified as a highly valuable fuel. In addition, thanks to its higher H/C ratio and energy content (LHV) compared to diesel, CO₂ benefits are also expected when using naphtha in such engines.

In a previous study, wide ranges of Cetane Number naphtha fuels (CN 20 to 35) were evaluated to optimize CI combustion, with different bowls and nozzle designs. CN 35 naphtha fuel has been selected for its better robustness and lower HC and CO emissions.

The purpose of the current study is to investigate the potential of CN 35 naphtha fuel on a light duty single-cylinder compression-ignition engine as well as the minimum required hardware modifications needed to properly run this fuel. Two different compression ratios: CR16 (stock piston) and CR17.5 were evaluated. The hydraulic flow rate of the nozzle was increased for naphtha to compensate for its lower fuel density vs. diesel. After optimization of the injection strategy, the results were compared to those obtained with a reference diesel fuel.

Basic thermodynamic investigations in single injection without EGR confirm that CN 35 naphtha is more resistant to auto-ignition than diesel. This leads to a longer air-fuel premixing duration, particularly at low load operation, enabling lower soot emissions. Increasing load and then in-cylinder pressure and temperature tends to significantly decrease the low CN impact. By the optimization of combustion modes at NO_X target, the premixed combustion with naphtha leads to better fuel consumption and lower particulate emissions than diesel, for the same levels of noise. Moreover, global CO₂ emissions are reduced by approximately 7% compared to diesel. Compared to CR17.5, CR16 enables an earlier combustion phasing, as well as a higher degree of fuel stratification. This enables to limit HC and CO emissions at low loads and a better fuel consumption with CR16.