Investigating the Effects of Reformed Fuel Blending in a Methane- or n-Heptane-HCCI Engine Using a Multi-Zone Model

Given the advantages of ultra low NO_x emission and high thermal efficiency at part load, HCCI engines might develop a significant niche in the engine world, provided that a suitable HCCI combustion control mechanism can be found. The problem is that HCCI occurs in a narrow operating range bounded by severe knock and misfire limits. Acceptable combustion behavior can be lost due to minor changes in speed, load, temperature or other variables. One approach to control HCCI combustion is to use a variable blend of base fuel and reformed fuel which can be adjusted on a cycle-by-cycle basis to control the combustion behaviour. Developing this control technique requires researchers to be able to optimize the settings and predict the effects of the many variables that affect HCCI ignition and combustion.

This paper describes a computational modeling study on the effect of base fuel/reformed fuel blends on HCCI engine combustion with a very high octane base fuel: natural gas or a very low-octane base fuel: n-heptane. A physics-based, multi-zone chemical kinetic model was developed to simulate HCCI combustion and predict engine performance parameters such as indicated mean effective pressure (imep). The study shows that the quantity of RG, (CO and H₂), has strong effects on the combustion behaviour of HCCI engines with both high-octane and low-otane base fuels. For high-octane, CNG-fueled HCCI engines, adding RG advances ignition timing, primarily because of its effect on thermodynamic properties during compression rather than chemical kinetic effects. For low-octane, heptane-fueled HCCI engines, adding RG delays ignition timing and slows combustion, primarily because of its effect on auto-ignition chemistry. The capability of controllinig HCCI ignition timing through adjusting RG replacement of the base fuel provides an opportunity to develop practical HCCI engines with a wider useful operating range.