https://orcid.org/0000-0002-9113-9774
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ABSTRACT
To clarify the mechanism of destructive knock combustion and piston ablation in heavy-duty diesel engines operating at low temperatures, CONVERGE simulation and wall temperature measurement were performed. The results show that as the initial temperature (intake valve closing instant) decreases from 380K to 307K, the spray wet-wall ratio increases from 12.8% to 22.2%. The fuel film evaporates slowly at low temperature before the hot flame, so the ignition is seriously delayed until the fuel injection ends, and the free-jet autoignition shifts to wall-attached combustion. However, the fuel film evaporation reverses after hot flame, and the maximum evaporation rate at 307K is about 54 times that at 380K. The wall temperature rises sharply to 350-450°C during wall-attached combustion. As the initial temperature decreases, the maximum pressure rise rate increases from 0.9MPa/°CA to 8.3MPa/°CA, and the pressure waves repeatedly oscillate along the end-center-end path in the cylinder. The highest pressure at the monitoring point is nearly an order of magnitude higher than the average, and the oscillation amplitude is as high as 6-10MPa, which far exceeds the failure threshold. The piston whose strength has been greatly reduced after thermal shock quickly undergoes ablation damage under strong pressure shock.
GRAPHICAL ABSTRACT
Highlights
Knock mechanism of heavy-duty diesel engines at low temperatures are clarified.
Fuel film evaporates slowly before hot flame at low temperatures but then reverses.
Free autoignition shifts to wall-attached combustion with decreasing temperature.
Pressure oscillation amplitude exceeds 6MPa with intake air temperature below 15°C.
Local high temperature and pressure shock together cause piston damage.
Acknowledgement
This work is supported by the National Natural Science Foundation of China (Grant Nos. 52176098), and the Project funded by China Postdoctoral Science Foundation (Grant No. 2021M700013). Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation and China Postdoctoral Science Foundation. The authors would like to thank the editors and reviewers for their valuable comments on this research.
Disclosure statement
No potential conflict of interest was reported by the authors.
Nomenclature
| CVCC | = | constant volume combustion chamber |
| FFT | = | Fast Fourier transform |
| HRR | = | heat release rate |
| HTI | = | high-temperature ignition |
| HTR | = | high-temperature region |
| LTI | = | low-temperature ignition |
| PRR | = | pressure rise rate |
| RWT | = | return water temperature |
| SOI | = | start of injection |
| Pinj | = | injection pressure |
| dtinj | = | injection duration |
| Lw | = | wall distance |
| Tw | = | initial wall temperature |
| PPmax | = | maximum pressure oscillation amplitude |
| φ | = | equivalence ratio |
| init_T | = | initial temperature in the cylinder |