Direct-Injection Constant-Volume Combustion for Ignition Delay Measurement*

The temperature dependence of ignition delay provides insight into the combustion mechanisms of heavy liquid fuels. In particular, negative temperature coefficient (NTC) behavior is predicted by chemical-kinetic models of hydrocarbon combustion. Experimental data at a wide range of conditions is necessary to validate chemical-kinetic models and examine the effects of fuel additives. This investigation explores the use of a small-scale direct-injection constant volume combustion chamber (CVCC) to obtain combustion pressure traces and ensuing ignition delays. A single-hole injector ensured near-homogeneous mixing between fuel and air. Temperature sweeps from 625 K to 875 K were conducted for iso-octane at 5 bar, n-heptane at 5 and 10 bar, and a 20% mixture of n-butyl acetate in heptane at 5 and 10 bar. A global equivalence ratio of 1.0 was maintained during all tests. Results showed strong NTC behavior for octane, but little for heptane. The observed ignition delays were similar in magnitude to those predicted by models in simulation. The butyl acetate/heptane blend showed longer ignition delays compared to heptane alone, and some NTC behavior was observed at similar temperatures. Overall, these results suggest that the direct-injection CVCC is a valuable platform for combustion experiments and can be used to evaluate diesel additives.