Study on single-fuel reactivity controlled compression ignition combustion through low temperature reforming

Abstract

To achieve wider reactivity control, single-fuel reactivity controlled compression ignition (RCCI) through low temperature reforming (LTR) was proposed and experimentally explored. An LTR system was established on an optical compression ignition engine to investigate the impact of LTR on engine performance, where LTR products of the same fuel (n-heptane) as in-cylinder direct injection was routed into the intake port. The quantitative online gas chromatograph (GC) measurements indicate that LTR of n-heptane can produce numerous species with different chemical structures. The higher reforming temperature can lead to the higher fuel conversion rate. Optical engine experiments show that the ignition timing is retarded significantly via LTR. A two-stage low temperature heat release (LTHR) arises at the reforming temperature of 423 K (LTR-423 without LTR). CH2O-PLIF images suggest that the HCCI combustion of unreformed n-heptane cause the first LTHR because of its earlier occurrence time than in-cylinder direct injection and the homogeneous distribution of CH2O fluorescence. Meanwhile, the chemical calculation of CH2O formation manifests that the contributions of each reaction pathway to CH2O formation are different in each stage of LTHR. At the reforming temperature of 523 K (LTR-523 with LTR), prior to LTHR, weak PLIF signals were also captured, emitted by aldehydes and ketones LTR produced in the reformer. In the subsequent LTHR stage, formaldehyde develops at a relatively low speed due to LTR. In the HTHR stage, high-speed images show that LTR leads to a smooth combustion and reduces soot formation. The effect of LTR products on mixture reactivity was analyzed by the constant volume ignition delay model. The calculation results manifest that the reactivity of each LTR product is different, influenced by both thermal dilution effect and chemical effect. The combined impact of LTR products on mixture reactivity depends on both the chemical structure and the concentration in the mixture. The thermal dilution effect of LTR products contributes more for the final decline of mixture reactivity LTR caused. Finally, although the reactivity just decreased under current LTR conditions, it is believed that LTR has the potential to achieve further reactivity control of RCCI combustion to accommodate changes in the operating conditions of engines by changing LTR conditions (temperature, equivalence ratio, residence time, fuel type, feeding water, etc.).