This paper presents the emission characteristics of a HCCI engine operation using n-heptane. The experiments were conducted in a single cylinder Co-operative Fuel Research (CFR) engine equipped with an air-assist port fuel injector. The effects of intake temperature, air/fuel ratio, compression ratio, turbo-charging, and EGR rate on exhaust emissions were explored. The analysis of the exhaust gases included oxides of nitrogen (NOx), nitrous oxide (N2O), carbon monoxide (CO), total hydrocarbon (THC), and soot. The hydrocarbon species present in exhaust gases and their concentrations at several operating conditions were also characterized. The strategies to obtain low HC, CO and NOx emissions are presented and discussed. The approaches to effectively retard HCCI combustion phase without deteriorating combustion efficiency are examined.
It was found that HCCI combustion produces extremely low soot and NOx emissions. In comparison, relatively low THC and CO emissions can only be obtained with advanced combustion phasing. Both THC and CO emissions increased significantly when the combustion phasing was retarded, which leads to low combustion efficiency. The application of EGR showed superior characteristics over changing intake temperature, compression and air/fuel ratio in retarding HCCI engine combustion phase.
The hydrocarbons and oxygenates present in exhaust gases and their concentrations at several operating conditions were also characterized. Unburned n-heptane was the main hydrocarbon species present in the exhaust gases, followed by alkenes, carbonyls and other oxygenates. With retarded combustion phase, the emissions of alkenes and carbonyls tend to increase significantly, reflecting the quenching of reactions prior to complete oxidation in HCCI combustion.
A unique NOx emission phenomenon was observed. With retarded and incomplete combustion, NOx emissions were found to increase in spite of the fact that reduced combustion temperature was expected. The increased formation of N2O and presence of a large quantity of intermediate HC species observed simultaneously may contribute to this phenomenon.