An Investigation of High Load (Compression Ignition) Operation of the “Naphtha Engine” - a Combustion Strategy for Low Well-to-Wheel CO2 Emissions 2008-01-1599

A computational and experimental study has been carried out to assess the high load efficiency and emissions potential of a combustion system designed to operate on low octane gasoline (or naphtha). The “naphtha engine” concept utilizes spark ignition at low load, HCCI at intermediate load, and compression ignition at high load; this paper focuses on high load (compression ignition) operation. Experiments were carried out in a single cylinder diesel engine with compression ratio of 16 and a common rail injector/fuel delivery system. Three fuels were examined: a light naphtha (RON∼59, CN∼34), heavy naphtha (RON∼66, CN∼31), and heavy naphtha additized with cetane improver (CN∼40). With single fuel injection near top dead center (TDC) (diesel-like combustion), excessive combustion noise is generated as the load increases. This noise limits the maximum power, in agreement with the CFD predictions. The noise-limited maximum power increases somewhat with the use of single pilot injection. Operation at peak loads comparable to conventional light duty diesel power levels requires a “split combustion” approach that utilizes a widely-separated pilot and main injection. The engine results show good performance and efficiency at low speeds across medium to high loads. High speed operation of split combustion is limited to engine speeds of < 2700 rpm due to insufficient ignitability of the naphthas even when cetane improver is added. While the large amount of premixed combustion at low and medium loads results in low NO_x and PM emissions, high (diesel-like) emissions are observed at high load with the split combustion approach. Thus, while the approach yields promising peak load levels and high efficiency operation speed and emission considerations limit the utility of this approach at present and further research is required to improve its attractiveness.