Real-world fuel efficiency and emissions from an urban diesel bus engine under transient operating conditions

Until recently, heavy-duty vehicle (HDV) engines were subjected to pollutant emissions type-approval tests, which were developed on engine test benches under standardised driving cycles. However, these results do not reflect engine behaviour under real-world driving conditions. In this context, experimental measurements were conducted on a diesel Euro V bus under urban off-cycle conditions in Madrid (Spain). The main objective was to create efficiency and emissions engine maps by combining transient-state engine data obtained directly from portable emission measurement system (PEMS) and on-board diagnostic (OBD) data acquisition systems. The engine maps were used to evaluate engine performance in real-world conditions, and to create a model that allows for second-by-second prediction of the engine fuel consumption and emissions. Owing to the variability of transient-state engine data, this work proposed an engine map development method, consisting of grouping the measured data into grids by engine speed and torque ranges, and then averaging them to obtain a single value per grid. The results showed that the Euro V diesel bus engine operates in urban off-cycle conditions with a brake thermal efficiency (BTE) of 41%, a brake-specific fuel consumption (BSFC) of 205 g/kWh, and a carbon dioxide (CO2) energy-emission factor of 637 g/kWh. The NOx energy-emission factor was 80% higher than the levels in the Euro V Standard. This work demonstrated that the grid engine maps model has the potential to predict second-by-second fuel consumption and CO2 emissions, with a relative total error of less than 5%. Therefore, this approach could be useful for accurately simulating engine or vehicle performance for any operation scenarios at a microscopic level, provided engine torque and engine speed data from the bus are available.