Researchers from Ghent University in Belgium have assessed the applicability of five different pressure-based knock metrics for use with methanol fuel in heavy-duty engines. Their findings, published in a paper in Fuel, highlight the need for dedicated, fuel-specific, knock threshold values.

The heavy-duty industry typically uses a high cetane fuel to power their vehicles, currently typically fossil diesel. Switching to renewable fuels such as methanol requires a major changeover in engine technology. Due to the high octane number of methanol, the diesel engines will have to be replaced by more suitable spark-ignited engines.

An accurate knock control system will then be needed to accommodate the high power density requirements of this sector. However, current pressure-based knock detection methods were primarily developed for gasoline fuel operation. Blindly copying these methods for a fuel which has vastly different knock properties can lead to suboptimal engine operation with an efficiency and power loss as result.

—Suijs et al.

For the study, the team conducted experiments on a single cylinder port fuel injected spark-ignited CFR engine. They established knock threshold values for both gasoline and methanol for the MAPO (maximum amplitude of pressure oscillations), AE (average energy) and MVTD (minimum value of third derivative) methods.

The results showed that the performance of the single peak metrics MAPO and IMPO (integral of modulus of pressure oscillations), and the time-averaged metrics AE and HRR (heat release rate) remained identical between methanol and gasoline operation.

For the MVTD method however, the suggested low sampling rate of 1 sample/°ca is insufficient for methanol combustion since the fuel by nature already shows a narrow pressure peak.

Overall, they found that the HRR method outperforms the others through the addition of a noise constraint. The calculated knock threshold values, however, are drastically different between both fuels.

Compared to gasoline, the thresholds for methanol were 1.25, 2.40 and 5.45 times higher for the MAPO, AE and MVTD methods respectively.

Resources

• Ward Suijs, Stijn Broekaert, Thomas De Cuyper, Sebastian Verhelst (2024) “The sensitivity of pressure-based knock threshold values to alternative fuels: A comparison of methanol vs. gasoline,” Fuel, Volume 362 doi: 10.1016/j.fuel.2023.130850