Effects of Dilution and Flammability Changes on Mixture Reactivity in a Natural Gas Internal Combustion Engine

ABSTRACT

Changes in the flammability characteristics of a combusting mixture and its ‘dilution’ levels – characterized, respectively, by the mixture’s equivalence ratio and residual gas fraction – can have significant impacts on the nature of combustion in internal combustion engines. These combustion changes can manifest as variations in combustion phasing and repeatability, and affect engine efficiency, emissions, and performance.

The current work uses experimental, high-speed, in-cylinder CO₂ concentration data from a single-cylinder, natural gas-fueled engine to track compositional variations in the combusting mixture; and then uses a chemical kinetics solver to isolate the effects of flammability and dilution changes on the mixture’s ‘reactivity,’ which is characterized by its laminar flame speed (LFS) and ignition delay (ID). It is found that LFS changes are driven strongly by the trapped equivalence ratio, while ID changes are driven primarily by dilution and pressure changes. Additionally, the effects of LFS changes on the overall combustion phasing are found to be more pronounced than those from ID changes, as the former influences both the combustion initiation and propagation processes.

KEYWORDS:

• Diluted combustion
• engine stability
• cyclic variability

Nomenclature

Table

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

¹ In two-stroke engine jargon, ‘trapped’ quantities refer to properties of the mixture that is trapped in the cylinder after all ports have been closed either by valves or the piston edge. This is an important qualification because two-stroke engines suffer from significant short-circuiting of fresh air.