A detailed comparison of ethanol–diesel direct fuel blending to conventional ethanol–diesel dual-fuel combustion

Abstract

Dual-fuel combustion is a well-known measure to enable the combustion of low-reactivity fuels (LRF) in compression-ignited engines with high thermal efficiency through a pilot injection of a high-reactivity fuel (HRF). In most cases, the LRF is introduced into the intake manifold and therefore premixed with the air before entering the combustion chamber during the intake stroke (premixed charge operation, PCO). In this work, this approach is investigated for bioethanol-diesel dual-fuel combustion using external and internal exhaust gas recirculation (EGR) to improve emissions and engine efficiency. In addition, PCO is compared to an alternative concept in which bioethanol and diesel are blended shortly upstream of the high-pressure pump (premixed fuel operation, PFO) at variable mixing ratios. The results show that higher ethanol shares of up to 70% can be achieved at low engine load when using PCO, while at medium and high load, the maximum energy share of ethanol is higher with PFO. While PCO is limited by engine knock, PFO rather suffers from the reduction in cetane number. In PCO, external and internal EGR allow for a reduction of unburned hydrocarbons (up to − 82%) and carbon monoxide (up to -60%), while nitrous oxide emissions are simultaneously lowered by up to − 65%. Both with and without EGR, PFO shows low emissions of unburned hydrocarbons and carbon monoxide (similar to conventional diesel combustion) and a significant reduction in nitrous oxide and soot formation. Brake thermal efficiency (BTE) drops in both modes compared to conventional diesel combustion, in PCO operation due to unburned and partially unburned fuel and in PFO due to increased friction in the high-pressure fuel pump caused by an increased fuel flow.