A Dynamic Test Bench for the Assessment of Common Rail Fuel Injection Systems Impact on CO ₂ Emissions over the WLTP Cycle 2019-01-0292

The internal combustion engine technological development is today driven by the pollutants and carbon dioxide (CO₂) emission reduction targets imposed by law. The request of lowering CO₂ emission reflected in a push towards the improvement of engine efficiency, without sacrificing performances and drivability.

The latest generations of Diesel engines for passenger cars are characterized by increasing injection pressure levels (250 MPa for the current production). Enhancing the injection pressure has the drawback of increasing the energy needed to pressurize the fuel and thus the high-pressure fuel pump energy request. A small but not negligible quantity of fuel has to be burned in order to provide this energy, generating a contribution in CO₂ emission. In this frame, the injector back-flow represents a significant energy loss for the fuel injection system and for the whole engine.

The energetic analysis of the overall fuel injection system of a modern passenger car is therefore interesting.

In this research an experimental test bench was developed in order to energetically assess the behavior of a common-rail fuel injection system over the Worldwide harmonized Light vehicles Test Procedure (WLTP) driving cycle. The hydraulic and mechanical energy related to the fuel pump were determined, along with the corresponding CO₂ emission. With the proposed test procedure is possible to split the energetic cost into the contributions from injected fuel and back-flow, allowing a comprehensive energetic assessment. Two different configurations of the fuel injection system were tested, differing in the fuel injector type. A standard, latest generation low-leakage injector model was tested and compared to a zero-leakage injector, featuring a particular architecture that should ensure very low back-flow rates.

The results showed that the fuel injection system of a passenger car engine in the power range 55 - 65 kW/l absorbs 142.8 kJ of mechanical energy over the WLTP cycle, being responsible of 1.18 g/km of CO₂ emission (1.3% of the total emission).

The low-leakage injectors mounted on the same fuel injection system showed a 24% energy reduction and 0.29 g/km saving of CO₂. In the analyzed setup, the low back-flow benefits are partially balanced by a pump volumetric efficiency reduction due to the lower operating flow-rate.