Investigation of Combustion Processes in Miniature Internal Combustion Engines

Abstract

In-cylinder pressure measurements, high-speed combustion imaging, and a regime diagram analysis are used to develop a general understanding of the combustion process occurring in miniature engines and how it is affected by scale. Cylinder pressure is measured in three different size miniature engines using a small fiber-optic pressure sensor with minimal impact on engine operating characteristics. A net heat-release analysis based on the pressure measurements suggests that a two-stage combustion process occurs at low engine speeds and equivalence ratios close to 1. Comparing the heat release rates between different sized engines shows that the observed phenomena are also scale dependent. Preliminary analysis shows that this behavior cannot be explained solely on the basis of heat transfer. The pressure traces are also used to analyze cycle-to-cycle variation of indicated mean effective pressure (IMEP) as functions of engine speed and equivalence ratio. The high-speed images suggest that combustion begins in the vicinity of the glow plug in a turbulent premixed mode but that a delayed secondary ignition may occur near the walls in the smallest engines. A combustion regime analysis shows that miniature internal combustion engines operate in the “flamelet in eddy” regime whereas conventional-scale engines operate mostly in the “wrinkled laminar flame sheet” regime.

Keywords:

• Chemiluminescence
• Combustion
• Miniature engine
• Regime diagram

ACKNOWLEDGMENTS

This research work was supported by the Army Research Office through MAV MURI Program (Grant No. ARMY-W911NF0410176) with Technical Monitor Gary Anderson. The authors also thank Minor Appleman of the Naval Surface Warfare Center Carderock Division (NSWCCD) and Vince Castelli formerly of NSWCCD for partial support of this work during its early stages.

Notes

¹Experiments are inherently dangerous and must be performed in appropriate facilities by appropriately trained personnel. It would be impractical for the authors to describe herein all of the hazards involved in such activities, so it is not done. Further, no warranties are made regarding the safety of the methods/procedures described herein nor do the authors assume any responsibility for damages of any kind to person or property that might result from attempting to reproduce them.

²“Glow fuel” is modeled as a mixture of methanol and nitromethane only.

³Oil is included as a component of the fuel but it is assumed to be inert in the flame speed calculations.