Effects of spark assist on HCCI combustion

Jacek Hunicz,Paweł Kordos,Michał Gęca,Alejandro Medina

doi:10.19206/ce-116894

Abstract

HCCI (homogeneous charge compression ignition) combustion is initiated by compression temperature and is independent of spark discharge. However, spark discharge can be applied under certain conditions to achieve hybrid combustion, where combustion by flame propagation is followed by auto-ignition of the unburned mixture. Spark assist can be applied to improve combustion stability at low loads or to reduce pressure rise rates under high load regime. In the current study variable spark ignition timing was applied for stoichiometric HCCI combustion, achieved using negative valve overlap technique. Under investigated conditions increase of nitrogen oxides emissions, due to flame propagation, was not observed. To provide more insight into combustion evolution, double Wiebe function was fitted to experimental heat release rates. It was found that only less than 10% of mixture was burned by flame propagation, even for very advanced spark discharge.

Highlights

Homogeneous charge compression ignition (HCCI) is an innovative combustion system, which can be applied in piston engines
A gasoline engine with a fully variable valvetrain and direct fuel injection was used for the analysis of spark ignition effects on residual effected HCCI combustion
The engine was run as autonomous HCCI and spark assisted HCCI with variable spark timings

Summary

Introduction

Homogeneous charge compression ignition (HCCI) is an innovative combustion system, which can be applied in piston engines. Air-fuel mixture is prepared early, is well premixed before combustion occurs. The combustion is initiated by temperature during the compression process. Auto-ignition is spontaneous and appears in multiple sites within the combustion chamber. It results in low temperature combustion, as unburned and burned mixture zones, typical for other combustion systems, do not appear [6, 9, 13]. Volumetric combustion of premixed charge produces extremely low concentrations of nitrogen oxides as well as particulates. HCCI engines can be equipped with standard three-way catalytic converters, despite lean combustion. HCCI engines offer higher thermal efficiency than current spark ignition engines [13]

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