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In the current study a single cylinder Spark Ignited engine has been operated in Controlled Auto Ignition mode with centrally mounted spray guided direct injection, employing negative valve overlap to achieve the conditions required for auto-ignition. The injector is a type known in the market and utilises air at elevated pressure to assist preparation of the fuel and delivery of the fuel spray into the combustion chamber. Operation of the injector enables a high degree of control for fuel and air delivery, particularly with regard to stratification of fuel, temperature, air and turbulence within the chamber. Engine test data show that variation in injection parameters at a fixed engine condition yields high authority over the CAI combustion process. A range of combustion phasing is achieved from 355 to 375 degrees ATDC for the timing of 50% Mass Fraction Burned, whilst the range of maximum pressure rise rate is 70 to 450 kPa / degree.

Orbital have been developing their stratified combustion process (Orbital Combustion Process OCP) for direct injection gasoline engines over the last 15 years, with successful production releases of the system in both the marine and automotive 2-stroke applications in 1996. This paper discusses how the same basic qualities of the air-assist fuel system and combustion process have been applied to automotive 4-stroke engines. The inherent qualities of the air- assist fuel system in combination with careful design of the combustion chamber has enabled high charge stratification with late injection timings and very stable combustion over a wide range of operating conditions. Experimental test data from a 4-cylinder, 16 valve 4-stroke development engine demonstrates the ability of this low pressure system to operate at very lean air/fuel ratios, with part load fuel economy improvements of up to 34 % at an operating condition equivalent to a vehicle speed of 40 km/hr.

A single-cylinder, 500cc research engine was tested under Spark-Ignition (SI) and Controlled Auto-Ignition (CAI) operation with similar load and speed conditions. Camshafts with low-lift and short duration, run with a negative valve overlap, were used to obtain CAI at wide open throttle. Two different camshaft profiles were tested in order to get a wide span of loads at 1200 and 2000rpm. The SI engine was Port Fuel-Injected (PFI) while the CAI engine was tested with both PFI and an Orbital Air-Assist Direct-Injection (DI) system. To reduce the high Indicated Specific Nitrogen Oxide (ISNOx) emissions at λ=1, 10% External Exhaust Gas Residuals (EEGR) was applied to the SI engine. EEGR reduced ISNOx emissions and there was slight reduction in ISFC. However, when the engine was tested in CAI mode, both ISNOx and ISFC were lower than the SI engine.

With the ever increasing desire to improve the thermal efficiency of the internal combustion engine, the combination of gasoline direct injection with engine downsizing and intake charging may offer the greatest potential for maximizing the efficiency of these engines. Although the comparative benefit of direct injection alone may be reduced when compared to the downsized and charged engine directly, the combination of the two technologies has the potential to offer substantially larger total fuel consumption reduction in automotive applications than either technology independently. The Orbital developed air-assisted spray guided combustion system has unique properties which are shown to be desirable given the increased demand of boosted engine operation. The decoupling of fuel metering and direct injection events promotes large dynamic range of fuel metering and, when combined with variable fuel metering differential pressure, results in turn down ratios in excess of 24 to 1.