Increasing the HCCI Autoignition Reactivity of Gasoline Using Conventional Ignition Improvers.

The application of HCCI (or CAI) combustion processes offers substantial benefits for increasing the efficiency of gasoline-fueled automotive engines. Although HCCI engines have been developed that can operate on regular gasoline over some portions of the speed-load range, there are challenges because of gasoline’s low HCCI autoignition reactivity. To overcome this HCCI/CAI engines typically retain large amount of hot residuals or high intake temperature to increase temperatures sufficiently for autoignition. As a result, charge densities are much lower than those of typical engines, and the fresh fuel/air portion of the charge is limited. These factors limit the loads that can be reached with HCCI. Previous works have shown that these problems can be substantially reduced by the use of a low-octane gasoline or an appropriate PRF blend. The higher reactivity of these fuels significantly reduces the heating requirement. Moreover, these studies also showed that the combustion phasing for a more reactive fuel could be retarded farther with good stability, which allows higher charge-mass fuel/air equivalence ratio (φm) without knock. The combination of higher φm and higher charge density from low-octane fuels has been shown to have the potential to significantly increase the load limit compared to conventional gasoline. Since these fuels are not readily available, this study explores the use of conventional ignition improvers, including 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), to enhance the autoignition of the regular gasoline in an HCCI engine at naturally aspirated and moderately boosted conditions (up to 1.8 bar absolute) with a constant engine speed of 1200 rpm. The results showed that both EHN and DTBP are very effective for reducing the intake temperature (Tin) required for autoignition and for enhancing stability to allow a higher φm. On the other hand, adding additives can also make the gasoline too reactive at certain conditions, so significant exhaust gas recirculation (EGR) is required to maintain the desired combustion phasing. Thus, there is a trade-off between improving stability and reducing the oxygen available when using ignition improvers to extend the high-load limit. The effect of the additives on NOx emissions was also studied. The results showed that the NOx emissions increase with increased EHN concentration. This work indicates that conventional ignition improvers can effectively enhance the HCCI autoignition reactivity of conventional gasoline at naturally aspirated and modestly boosted operations, offering significant benefits for HCCI/CAI engines.