New correlation between fuel octane index and HCCI combustion provides basis for more robust control strategies
A team of researchers in the US and Australia has developed a new correlation between the octane index (OI) of a range of refinery stream fuels and Homogeneous Charge Compression Ignition (HCCI) combustion phasing.
The behavior of the new model is much improved compared to the original OI model—particularly in the low intake temperature range and for fuels with high aromatic and high ethanol content. The new octane index correlation can be used for designing robust HCCI control strategies, capable of handling the wide spectrum of fuel chemical compositions found in pump gasoline, the researchers said in their paper published in the International Journal of Engine Research.
Homogeneous charge compression ignition (HCCI) is a novel form of combustion capable of attaining diesel-like efficiency with low emissions of nitric oxides and soot. A lean, homogeneous mixture of fuel and air is compressed until it auto-ignites, and the combustion process is driven by chemical kinetics. Because an HCCI engine runs un-throttled, lean and with high compression ratio (CR), it achieves far better thermal efficiency than a conventional spark ignition (SI) engine. The homogeneous mixture of air and fuel ensures that soot formation is negligible during the combustion event, and low in-cylinder temperatures mitigate NOx formation.
Favorable in-cylinder thermal conditions are required to achieve timely auto-ignition and enable stable HCCI combustion. In contrast to SI and compression ignition (CI) engines, there is no direct trigger for HCCI. Hence, combustion control has historically been the main barrier that spurred intense research around the globe. Several academic and industrial research teams have been able to demonstrate control of combustion phasing and mode transitions in the laboratory. However, uncertainties encountered in the real world create additional challenges and represent a significant obstacle on the path to commercialization of this very promising technology.
One source of uncertainty over the life cycle of the vehicle stems from variability of fuel composition, depending on the refinery streams that are used to produce pump gasoline in various regions of the United States. Large variations in chemical composition have been reported, particularly for the volumetric content of aromatics and olefins. Variations in chemical composition obviously affect chemical kinetics of real fuels. Hence, deep understanding of the interplay between fuel properties and HCCI combustion is a necessary prerequisite for developing robust control strategies and reliable HCCI engines.—Lacey et al.
The researchers studied 17 gasoline fuels with varying chemical compositions, including 0%, 10%, 20% and 100% ethanol, as well as several blends that featured high aromatics or high olefins.
Fifteen fuels from the matrix were blended using gasoline refinery streams with varying chemical composition in terms of the volume percentage of aromatics, olefins and saturates. The fuels covered a wide range of RON, MON and sensitivity.
A single-cylinder research HCCI engine was operated over a range of intake temperatures with each fuel. For each fuel, the team conducted a sweep of intake temperature at a consistent load and engine speed. They tracked combustion phasing given by the crank angle of 50% mass fraction burned to assess the sensitivity of auto-ignition to fuel chemical kinetics.
The experimental results provided the data for predicting the HCCI combustion phasing from the given properties of a fuel.
Chevron and the General Motors Corporation have provided the technical and financial contributions toward this project. Chevron provided funding and fuel samples, while General Motors donated the engine hardware and contributed to the planning of experiments.
Joshua Lacey, Karthik Kameshwaran, Sakthish Sathasivam, Zoran Filipi, William Cannella, Peter A Fuentes-Afflick (2016) “Effects of refinery stream gasoline property variation on the auto-ignition quality of a fuel and homogeneous charge compression ignition combustion” International Journal of Engine Research doi: 10.1177/1468087416647646