Experimental Investigations to Extend the Operating Load Range of a Homogeneous Charge Compression Ignition Engine through Fuel Modifications

Homogeneous charge compression ignition (HCCI) is a potential contender to replace conventional diesel combustion due to higher thermal efficiency along with near-zero oxides of nitrogen (NO_x) and soot emissions. Commercial adaptation of HCCI strategy in automotive engines demands addressing problems associated with narrow operating load range and higher unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. This article intends to address these problems through fuel modifications. A production light-duty diesel engine used for agricultural water pumping applications is modified to run in the HCCI mode through suitable modifications in the intake system. To improve external mixture preparation with low volatile diesel fuel, a high-pressure fuel injection system and a fuel vaporizer are utilized in the intake manifold. The results obtained show that the engine could run only up to 40% of rated load or 2.12 bar BMEP in the HCCI mode with diesel, beyond which it knocks severely owing to early ignition. To address the problems associated with diesel and extend the operating load range, highly volatile and low reactivity gasoline and gasoline-butanol blends with an ignition improver additive are investigated as alternative fuel options. With gasoline and 5% ethylhexyl nitrate (EHN), the operating load range is extended up to 60% of rated load or 3.19 bar BMEP, along with higher brake thermal efficiency and lower HC and CO emissions. Blending 50% butanol with gasoline helps to extend the high load range by up to 65% of rated load and also reduces unburned emissions but poses misfire problems at low loads. Overall, this article demonstrates the feasibility of extending the operating load range and reducing the unburned emissions in HCCI operation through fuel modifications. Utilizing exhaust gas recirculation and intake air preheating along with fuel modifications would help to further extend the load range in HCCI.