Investigation of Causes and Preventive Measures for Destructive Abnormal Combustion (Super-Knock) in High-Performance Spark Ignited Gasoline Engines, 03-R8049Printer Friendly Version
Inclusive Dates: 04/01/09 Current
Background - The spark ignition (SI) engine has been known to exhibit several different abnormal combustion phenomena. These abnormal combustion processes are generally attributed to SI engine knock (end-gas knock at higher engine loads) and preignition (rumble), which is generally caused by hot-spot ignition. Most of these abnormal combustion issues have been addressed with improved engine design or control schemes as well as engine fuel specifications (i.e. octane number). However, in the next generation of SI engines where turbocharging and direct injection technology are employed to increase brake power at low engine speeds for improved efficiency a new phenomenon of abnormal combustion, described as "superknock," has been exhibited. Because of the violent nature of superknock, damage to engine components can occur within only a few engine cycles. However, because superknock events occur only sporadically and in an uncontrolled fashion, the causes for this phenomenon have not been successfully explained and solutions to suppress it have not been developed.
Approach - SwRI is conducting this project to investigate the causes of and potential preventive measures for superknock. First, a study of the superknock phenomenon from a fuels and combustion chemistry standpoint is being conducted to explore whether superknock is influenced by fuel chemistry. With the support of a major fuel supplier, several gasoline-like engine fuels with vastly different chemical compositions, but similar octane number and volatility, have been selected to investigate the fuel chemistry effects on superknock. Then, as a possible measure to suppress superknock, the potential benefits of operating the engine with various levels of exhaust gas recirculation (EGR) is being explored. From previously conducted research projects, it is known that EGR reduces the likelihood of gasoline engine knock. EGR has also been shown to be very effective in reducing chemical reaction rates and influences the autoignition characteristics of gasoline fuels. Finally, EGR has been shown to reduce in-cylinder temperatures, leading to a reduction in the preignition potential of SI engines. Based on these factors, it is believed that moderate to high levels of EGR have the potential to reduce the incidence of superknock.
Accomplishments - Besides the identification and selection of suitable test fuels, a large engine manufacturer has offered its support for this project in the form of engine hardware and information on how to operate the engine to initiate superknock repeatedly but without causing engine damage. The turbocharged V-6 GDI engine has been installed in SwRI's engine laboratory and will be used to conduct the experimental tests for this project. Initial engine tests without EGR have been conducted and the fuels testing phase will be initiated in the near future.