Compression Ratio and Derived Cetane Number Effects on Gasoline Compression Ignition Engine Running with Naphtha Fuels 2014-01-1301

In the context of stringent future emission standards as well as the need to reduce emissions of CO₂ on a global scale, the cost of manufacturing engines is increasing. Naphtha has been shown to have beneficial properties for its use as a fuel in the transportation sector. Well to tank CO₂ emissions from the production of Naphtha are lower than any other fuel produced in the refinery due to its lower processing requisites. Moreover, under current technology trends the demand for diesel is expected to increase leading to a possible surplus of light fuels in the future. Recent research has demonstrated that significant fuel consumption reduction is possible based on a direct injection gasoline engine system, when a low quality gasoline stream such as Naphtha is used in compression ignition mode. With this fuel, the engine will be at least as efficient and clean as current diesel engines but will be more cost effective (lower injection pressure, HC/CO after-treatment rather than NOx).

In a previous publication, it was demonstrated that a 0.5liter single-cylinder DISI engine of a compression ratio (CR) of 12 could be run in CI mode using a heavy cut of Naphtha with a derived cetane number (DCN) of 41. Over a range of operating points covering the FTP cycle, average fuel consumption was improved by 19% compared to running the base engine in SI mode on 91 RON gasoline. In this study, both light and heavy Naphtha from one of Saudi Aramco's refineries were successfully run in GCI mode with regular valve events and intake charge boosting at six engine running conditions representative of a typical urban driving cycle including idle. The presented work highlights the potential for improvement and the limitations of such a configuration.

In this paper, the effect of increasing the CR on this type of operation is investigated. Three different pistons were designed to increase CR from 12 to 13 and 14 respectively with a diesel type bowl feature. Light Naphtha and Heavy Naphtha cuts with a DCN of 31 and 41 were run over the same operating points covering the FTP cycle with low soot and NOx with each piston. Fuel economy improvements over SI operation vary from 15% with CR12 to 26% with CR14 with light naphtha. Heavy Naphtha did not benefit as much from the increased compression ratio and the greatest fuel economy improvement in this case was 22% at CR13. Hence, the efficiency of a DISI engine could be significantly increased by fueling it with Naphtha with partially premixed combustion.

The load range of these engine/fuel configurations was assessed across engine speeds from 1000RPM to 3000RPM. The maximum load achieved was 10bar NMEP with light Naphtha at CR12, however it did not provide good combustion stability at loads below 6bar NMEP. At all three CRs, Heavy Naphtha allows operating with good combustion stability on the load range below 2bar NMEP but fails to reach loads above 7bar NMEP on the high end due to high pressure rise rates.