In a paper to be presented this week at WCX SAE World Congress Experience, researchers from KAUST (King Abdullah University of Science & Technology) show that probability of a pre-ignition event relies on (a) the likelihood of precursor generation (from fuel impinging the liner); (b) the likelihood of precursors being held back in cylinder (related to exhaust back pressure); and (c) the reactivity of bulk mixture (related to in-cylinder temperature).
A fuel injector can be heavily damaged by a pre-ignition event (bottom); a new injector is shown above. © 2019 Eshan Singh
Pre-ignition in modern engines is largely attributed to oil-fuel mixture droplets igniting before the spark timing. Researchers have also found pre-ignition events to be triggered by high hydrocarbon emissions from the previous cycle as well as late spark timing in the previous cycle. Additionally, an ideally scavenged engine was not found to be limited by pre-ignition. These observations point to a significant role of residuals in triggering pre-ignition events.—Singh and Dibble (2019)
A single occurrence of pre-ignition—when fuel ignites prematurely in the engine cylinder to release a mistimed kick of energy—can cause major damage. Pre-ignition is sporadic and unpredictable. It is more prevalent in the downsized, turbocharged engines that car makers are now producing for improved fuel efficiency while maintaining power output.
Pre-ignition remains a bottleneck to further downsizing and improving engine efficiency.—Eshan Singh, lead author
In the study of pre-ignition, the team systematically changed engine parameters, including air intake temperature and exhaust back-pressure, thereby establishing the conditions most likely for pre-ignition. For example, the team observed more pre-ignition events with a higher exhaust back pressure: this occurs when the engine cylinder has less effective emptying of burnt fuel between combustion cycles.
The researchers combined all their observations to build a picture of the pre-ignition phenomenon. The work unravels the pre-ignition event is divided into several steps, Singh said.
A pre-ignition event originates during the previous fuel combustion cycle, when fuel injected into the cylinder interacts with oil on the cylinder liner forming an oil-fuel droplet. At the end of each combustion cycle, the oil-fuel droplet is usually expelled from the cylinder; however, from time to time, the oil-fuel droplet remains and may trigger pre-ignition in the following cycle, the team showed.
Automakers need to break this chain of events to suppress pre-ignition, Singh said.
Stopping the fuel and oil from interacting, or completely exhausting all the burnt mixture from the engine, are two points where the event chain can be broken. Preventing the injected fuel from hitting the oil-coated cylinder liner is one approach.
We are looking at practical methods to reduce pre-ignition by changing the injection strategy. We have filed a patent that uses injection strategies to provide pre-ignition-free operation without compromising engine power.—Eshan Singh
Other routes to break the chain of events demand deeper understanding of pre-ignition, Singh added.
Singh, E. and Dibble, R. (2019) “Mechanism Triggering Pre-Ignition in a Turbo-Charged Engine,” SAE Technical Paper 2019-01-0255 doi: 10.4271/2019-01-0255