Second law analysis of waste cooking oil biodiesel versus ULSD during operation of a CI engine

Abstract

Waste cooking oil (WCO) biodiesel is a sustainable option compared to ultra-low sulfur diesel (ULSD) due to its relatively low cost and carbon cycle recovery potential while not competing with food resources. Research on this fuel finds limited studies with little to no information regarding a second law heat release analysis and contrasting findings with respect to NOx emissions. This effort rectifies these issues by performing experiments with ULSD and WCO biodiesel on a single cylinder compression ignition engine running at constant speed (2000 rpm) and various loads (0, 25, 50, 75, and 100% load). The combustion timing of WCO biodiesel was normalized to match the peak in-cylinder pressure timing of ULSD. Generally, WCO biodiesel demonstrated shorter ignition delays and, thus, quicker periods for the mixture formation while requiring more fuel (and greater CO2 emissions) to match the load requirement. Subsequently, this cuts the premixed event short and induces diffusion-dominated combustion above that of operation with ULSD. WCO biodiesel primarily resulted in cooler peak temperatures, but longer periods of high temperatures that appear later in the combustion cycle where work extraction is more difficult; hence, lowering the 2nd Law efficiency. As a result, availability losses due to heat transfer increase, but this warmer in-cylinder environment mitigates losses due to entropy production. The emissions analysis highlights a more complete combustion of WCO biodiesel with lower CO and HC emissions. NOx emissions are closely tied with in-cylinder temperatures and the amount of diffusion burn; hence, increasing or decreasing based on combustion behavior.