A Methodology for Assessing Thermal Stratification in an HCCI Engine and Understanding the Impact of Engine Design and Operating Conditions.

Abstract

HCCI is a promising advanced engine concept with the potential to pair high thermal efficiencies with ultra-low emissions. However, HCCI has so far been demonstrated only over a narrow operating range due to a lack of control over HCCI burn rates. While there is an emerging consensus about the critical role of thermal stratification on HCCI burn rates, there was a gap related to availability of a method to rapidly assess the impact of engine design or operating conditions on thermal stratification in a practical HCCI engine. The objectives of this research are to develop a novel post-processing technique for studying thermal stratification in a fired, metal HCCI engine, and use the proposed technique to understand the impact of operating conditions on the in-cylinder unburned temperature distribution. The technique is called the Thermal Stratification Analysis (TSA) and it uses the autoignition integral coupled to the mass fraction burned curve to determine a distribution of mass and temperature in the cylinder prior to combustion. The technique is then validated by comparing the TSA results to predictions from CFD simulations and experimentally measured unburned temperature distributions in an optical engine. A large amount of data was collected and processed with the TSA to determine the effects of engine design and operating conditions on the in-cylinder unburned temperature distribution and HCCI burn rates. The results show that the thermal width increases with a higher internal residual gas fraction, increasing intake temperature, advancing combustion phasing, increasing the maximum TDC temperature, and increasing the in-cylinder swirl. Finally, an innovative method for active control of the thermal stratification and HCCI burn rates with a glow plug is proposed. The results show that the glow plug is able to control combustion phasing and, more importantly, broaden the temperature distribution and lengthen the burn duration a considerable amount. The glow plug improves some of the emissions characteristics slightly and the combustion efficiency as well. The main drawbacks of using a glow plug in HCCI are the efficiency penalty associated with the energy consumed by the glow plug and the observed increase in the cycle-to-cycle variations.