EGR and Intake Boost for Managing HCCI Low-Temperature Heat Release over Wide Ranges of Engine Speed 2007-01-0051
Reaching for higher loads and improving combustion-phasing control are important challenges for HCCI research. Although HCCI engines can operate with a variety of fuels, recent research has shown that fuels with two-stage autoignition have some significant advantages for overcoming these challenges. Because the amount of low-temperature heat release (LTHR) is proportional to the local equivalence ratio (ϕ), fuel stratification can be used to adjust the combustion phasing (CA50) and/or burn duration using various fuel-injection strategies. Two-stage ignition fuels also allow stable combustion even for extensive combustion-phasing retard, which reduces the knocking propensity. Finally, the LTHR reduces the required intake temperature, which increases the inducted charge mass for a given intake pressure, allowing higher fueling rates before knocking and NOx emissions become a problem.
However, the amount of LTHR is normally highly dependent on the engine speed. For low engine speeds, the amount of LTHR can become so great that the intake temperature required to maintain CA50 falls below ambient. Conversely, if the engine speed is too high, the LTHR goes away completely. In this work PRF90, PRF80, and PRF70 fuel blends were tested. Although the engine-speed range with an acceptable amount of LTHR shifted with PRF-number, each fuel could only be operated with LTHR over a fairly narrow speed range. Three techniques for managing the amount of LTHR over wide ranges of engine speed were then investigated. It was found that combining a fairly reactive fuel (PRF60) with various amount of EGR is a very effective way to accomplish a nearly constant amount of LTHR and eliminate the need to adjust the intake temperature as the engine speed is changed. Also, intake boost adjustments were found to be effective for controlling the amount of LTHR. Furthermore, by adding toluene to the PRF-fuel blends, it was demonstrated that the high engine-speed sensitivity of LTHR can be reduced by changes to the fuel composition.