Synergies between aliphatic bio-alcohols and thermo-chemical waste heat recovery for reduced CO₂ emissions in vehicles

The impact of aliphatic (C1–C5) bio-alcohols for on-board thermo-chemical exhaust waste heat recovery, H₂ production and hence fuel savings for a gasoline direct injection (GDI) engine has been investigated with iso-octane as reference. Gibbs free energy analysis was conducted to establish which reaction pathways for fuel reforming were dominant and how these reactions are influencing H₂ and CO yield. To obtain engine exhaust temperature and mass flow maps for several steady-state engine operation conditions, an engine model was created in AVL CRUISE M and validated with real GDI engine data. A methodology was developed to couple the thermodynamic reactor results and engine model results to compute waste exhaust heat recovery by accounting limited available exergy in exhaust and optimising reforming reactor's operational parameters to maximise exergy recovery and reforming efficiency in the reformer. Under steady state engine operation, iso-pentanol was more effective overall due to its high hydrogen content by weight (8.3% and 4.3% higher than methanol and ethanol respectively), high energy density of 34.6 MJ/kg (42.2%, 22.3% 13.2% and 4.3% higher than C1-C4 alcohols respectively), high reforming endothermicity and the versatility for heat recovery under all engine operation conditions with heat recovery of at-least 2% of engine power even under low-load conditions. Subsequently, the engine model was evolved into a full AVL CRUISE M vehicle model and was studied over a duty-cycle to evaluate the fuels further. Iso-pentanol and iso-propanol were most effective concerning average heat recovery over the duty cycle. For the high-speed regions of the cycle, up-to 9% of waste heat recovery is observed for C4 and C5 bio-alcohols. In terms of vehicle's carbon emissions, iso-pentanol, n-butanol, and ethanol exhibited maximum CO₂ reduction of 8.4%, 7.7% and 7.1% respectively. The results presented herein indicates versatility of bio-alcohols for cleaner and more efficient powertrains.