Researchers at KAUST’s Clean Combustion Research Center have developed a method to predict and to avoid undesirable combustion events in advanced engines, such as knocking and super-knock. In a recent study, published in the journal Combustion and Flame, they report that their predictions implicate the composition and temperature of the initial mix of fuel and air.

Advanced low-temperature-combustion (LTC) strategies of downsized and boosted engines are capable of offering ultra-low emissions and higher efficiencies. In these engines, combustion is chemically driven by the autoignition process with no direct means to control the ignition timing and combustion rate. The autoignition process and its timing are highly sensitive to the fuel types and operating conditions, including the intake temperature and pressure, thermal and compositional inhomogeneities of the mixture, the amount of exhaust gas recirculation (EGR), and cooling.

As such, these engines are prone to suffer from pre-ignition, a higher possibility of knock, and even super-knock, which is characterized by high-pressure peaks and oscillations leading to a severe structural damage. Therefore, a reliable prediction of such abnormal ignition phenomena is of critical importance.

—Luong et al.

Minh Bau Luong, a postdoc in the research group of Hong Im at the Clean Combustion Research Center, led the research to find ways to overcome the higher propensity of these engines for abnormal combustion events, including super-knock, a detonation event generating extreme, and damaging pressure oscillations.

The key hypothesis of this study was that the occurrence of engine knock is determined by the way the composition and temperature of the fuel/air mixture is distributed at the onset of ignition.

—Hong Im

In previous work, Im had developed a theoretical formula to predict fuel ignition behavior based on a metric called the Sankaran (Sa) number, which relates flame speed and propagation to the temperature gradient.

The significance of the Sa-based metric is that the prediction of combustion modes is based purely on the initial conditions.

—Minh Bau Luong

Quantitative visualization of pressure contours to depict different knock intensities (l-r): normal autoignition process, mild-knock and super-knock. © 2020 KAUST

In their latest work, the team has extended the theoretical formula to consider temperature and concentration fluctuations of the fuel/air mixture. Using this formula, the researchers first predicted knock occurrence and its strength based on the mixture distribution at the onset, and then ran direct numerical simulations to check if the prediction was right.

The simulations confirmed the team’s initial hypothesis, proving that the conditions at the onset of ignition determine the occurrence and strength of engine knock events.

The study suggests that knock events would be suppressed if there were temperature and concentration fluctuations of the fuel/air mixture within the engine cylinder.

Thermal and compositional stratifications of the in-cylinder fuel/air mixture can provide a smooth, sequential combustion process under high-load conditions.

—Minh Bau Luong

The next step is to modify the ignition criteria to determine knock probability based on the information available in larger, ‘device scale’ simulations, or experimental measurements from engines, said Im.

Resources

• Luong, M.B., Hernández Pérez, F.E. & Im, H.G. (2020) “Prediction of ignition modes of NTC-fuel/air mixtures with temperature and concentration fluctuations.” Combustion and Flamedoi: 10.1016/j.combustflame.2019.12.002