Experimental Investigation of Close-Loop Control of HCCI Engine Using Dual Fuel Approach

Homogeneous Charge Compression Ignition (HCCI) offers great promise for excellent fuel economy and extremely low emissions of NO_x and PM. HCCI combustion lacks direct control on the "start of combustion" such as spark timing in SI engines and fuel injection timing in CI engines. Auto ignition of a homogeneous mixture is very sensitive to operating conditions of the engine. Even small variations of the load can change the timing from "too early" to "too late" combustion. Thus a fast combustion phasing control is required since it sets the performance limitation of the load control. Crank angle position for 50% heat release is used as combustion phasing feedback parameter.

In this study, a dual-fuel approach is used to control combustion in a HCCI engine. This approach involves controlling the combustion heat release rate by adjusting fuel reactivity according to the conditions inside the cylinder. Two different octane fuels (methanol and n-heptane) are used for the study. Port fuel injection technique is used for preparing homogeneous mixture of methanol, heptane and air using two separate injectors for methanol and heptane. Close loop control of combustion phasing is attained by instantaneous variation of fuel ratio of methanol and n-heptane while maintaining the injected fuel energy constant. Total fuel energy injected is used to control the IMEP of the engine. It is found that controller is able to keep close track of the reference combustion phasing using PID control by changing the fuel ratio. PID control of combustion phasing and IMEP using dual fuel is achieved and is successfully demonstrated in the HCCI engine in the study.