Effects of Fuel Composition on Critical Equivalence Ratio for Autoignition

Abstract

In a companion study, the critical equivalence ratio (ϕ) for several fuels (defined as the minimum equivalence ratio at which a fuel can autoignite) was examined in a motored engine as a metric to predict the tendency of a fuel to produce incomplete combustion due to an overly lean charge, leading to excessive emissions of total hydrocarbons (THCs) and carbon monoxide (CO). Higher ignition quality fuels (i.e., higher cetane number fuels) were shown to have leaner critical ϕ, but a question remained regarding fuel compositional effects on critical ϕ. In this work, to address that question, blends of n-dodecane/toluene and n-dodecane/isooctane were prepared to have the same derived cetane number (DCN) as n-heptane. Critical ϕ measurements for the two blends and n-heptane revealed that critical ϕ could vary between fuels with the same DCN. The n-dodecane/toluene blend was found to have a leaner critical ϕ than the n-dodecane/isooctane blend under low compression ratios or simulated EGR. It was concluded that the 11% greater n-paraffin content of the n-dodecane/toluene blend compared to the n-dodecane/isooctane resulted in more low-temperature heat release (LTHR) and a leaner critical ϕ. The critical ϕ of the low cetane number FACE (fuels for advanced combustion engines) fuels was also determined. A linear correlation with an R2 coefficient of 0.949 was observed between critical ϕ and n-paraffin mass content of the low cetane number (CN) FACE fuels. This result corroborated the conclusion from the DCN parity blends that the critical ϕ of a fuel is governed by the fraction of more reactive components (n-paraffins), which increases LTHR. The critical ϕ measurements of the low cetane number FACE fuels were compared to CO and THC emissions from a light-duty turbodiesel engine operating in an advanced combustion mode with the low CN FACE fuels. It was concluded that the relationship between low CO and THC emissions and a lean critical ϕ is only present when early SOI timing produces an overly lean fuel–air charge, but not when late SOI produces an overly rich mixture. These results suggest that a fuel can be blended to have a low ignition quality, which is desired for high-efficiency advanced combustion operation and with high n-paraffin content to reduce CO and THC emissions.