Maximizing Power Output in Homogeneous Charge Compression Ignition (HCCI) Engines and Enabling Effective Control of Combustion Timing

Homogeneous Charge Compression Ignition (HCCI) engines are one of the most promising engine technologies for the future of energy conversion from clean, efficient combustion. HCCI engines allow high efficiency and lower CO2 emission through the use of high compression ratios and the removal of intake throttle valves (like Diesel), and allow very low levels of urban pollutants like nitric oxide and soot (like Otto). These engines, however, are not without their challenges, such as low power density compared with other engine technologies, and a difficulty in controlling combustion timing.This dissertation first addresses the power output limits. The particular strategies for enabling high power output investigated in this dissertation focus on avoiding five critical limits that either damage an engine, drastically reduce efficiency, or drastically increase emissions: 1) ringing limits, 2) peak in-cylinder pressure limits, 3) misfire limits, 4) low intake temperature limits, and 5) excessive emissions limits. The research shows that the key factors that enable high power output, sufficient for passenger vehicles, while simultaneously avoiding the five limits defined above are the use of: 1) high intake air pressures allowing improved power output, 2) highly delayed combustion timing to avoid ringing limits, and 3) using the highest possible equivalence ratio before encountering ringing limits. These results are revealed by conducting extensive experiments spanning a wide range of operating conditions on a multi-cylinder HCCI engine.Second, this dissertation discusses strategies for effectively sensing combustion characteristics on a HCCI engine. For effective feedback control of HCCI combustion timing, a sensor is required to quantify when combustion occurs. Many laboratory engines use in-cylinder pressure sensors but these sensors are currently prohibitively expensive for wide-scale commercialization. Instead, ion sensors made from inexpensive sparkplugs are proposed for sensing combustion timing. Ion sensing, however, is unreliable under certain HCCI conditions. The dissertation presents two strategies for improving the usefulness of ion sensors in HCCI engines: 1) the use of tiny fractions of metal-acetate fuel additives that expand the useful range of ion sensors, and 2) the use of ion sensors for detecting excessive ringing that must be avoided in HCCI engines. These two innovative research efforts make ion sensors viable for sensing combustion characteristics across the full range of HCCI operation, making them effective for use in engine control systems.In summary, this Ph.D dissertation addresses two important technical challenges facing HCCI engines: power output limits, and difficulty in sensing combustion characteristics for control applications. The strategies proposed in this dissertation research bring HCCI engines closer to widespread commercialization allowing vehicles to operate with significantly higher efficiency and with cleaner emissions

Iván D Exploring Strategies for Reducing High Intake Temperature Requirements and Allowing Optimal Operational Conditions in a Biogas Fueled HCCI Engine for Power Generation

This paper evaluates strategies for reducing the intake temperature requirement for igniting biogas in homogeneous charge compression ignition (HCCI) engines. The HCCI combustion is a promising technology for stationary power generation using renewable fuels in combustion engines. Combustion of biogas in HCCI engines allows high thermal efficiency similar to diesel engines, with low net CO 2 and low NO x emissions. However, in order to ensure the occurrence of autoignition in purely biogas fueled HCCI engines, a high inlet temperature is needed. This paper presents experimental and numerical results. First, the experimental analysis on a 4 cylinder, 1.9 L Volkswagen TDI diesel engine running with biogas in the HCCI mode shows high gross indicated mean effective pressure (close to 8 bar), high gross indicated efficiency (close to 45%) and NO x emissions below the 2010 US limit (0.27 g/kWh). Stable HCCI operation is experimentally demonstrated with a biogas composition of 60% CH 4 and 40% CO 2 on a volumetric basis, inlet pressures of 2-2.2 bar (absolute), and inlet temperatures of 200-210 C for equivalence ratios between 0.19-0.29. At lower equivalence ratios, slight changes in the inlet pressure and temperature caused large changes in cycle-to-cycle variations, while at higher equivalence ratios these same small pressure and temperature variations caused large changes to the ringing intensity. Second, numerical simulations have been carried out to evaluate the effectiveness of high boost pressures and high compression ratios for reducing the inlet temperature requirements while attaining safe operation and high power output. The one zone model in Chemkin was used to evaluate the ignition timing and peak cylinder pressures with variations in temperatures at intake valve close (IVC) from 373 to 473 K. In-cylinder temperature profiles between IVC and ignition were computed using Fluent 6.3 and fed into the multizone model in Chemkin to study combustion parameters. According to the numerical results, the use of both higher boost pressures and higher compression ratios permit lower inlet temperatures within the safe limits experimentally observed and allow higher power output. However, the range of inlet temperatures allowing safe and efficient operation using these strategies is very narrow, and precise inlet temperature control is needed to ensure the best results