Modeling and analysis of an HCCI engine during thermal transients using a thermodynamic cycle simulation with a coupled wall thermal network.

Abstract

This computational study addresses the unique characteristics of the strong coupling that exists between the thermal condition of the engine structure and the combustion in a Homogeneous Charge Compression Ignition (HCCI) engine, with particular emphasis on the effects of thermal inertia and possible control strategies to compensate for the thermal non-equilibrium that occurs. The engine modeled is a single-cylinder HCCI engine with a re-breathing exhaust valve configuration that utilizes a large amount of hot residual to increase thermal energy of the air-fuel mixture for auto-ignition and to dilute it for preventing rapid heat release rate as well as to keep burned gas temperature low for NO_x control. The in-cylinder combustion and heat transfer, the gas exchange process through valves, and thermal inertia of the engine structures are considered simultaneously in order to fully investigate the HCCI engine transient behavior. A system level engine model including original combustion and heat transfer models developed for the HCCI engine was developed for this purpose. The original contribution of this study is the addition of a thermal network model that tracks the behavior of the engine's thermal boundaries during transient operation. The combustion and performance of an HCCI engine were found to be very sensitive to the engine thermal conditions including intake air temperature, residual level and coolant temperature. In particular, the transient wall temperature excursions from steady-state values were shown to play a great role in determining the combustion characteristics by reducing or enhancing the wall heat transfer. A stable steady-state HCCI operating range was defined and optimized for the best fuel economy by controlling the residual level, and possible shifts of the operating limits due to thermal transitions were studied. An original method was proposed to modulate the role of thermal inertia on auto-ignition during transients by compensating for thermally non-equilibrium wall conditions to enhance robust control of ignition timing in transient operation. A variable valve system was used for that purpose to control combustion phasing by optimizing residual level. The results were improved fuel economy while complying with knock and misfire limits.