Water Injection System Application in a Mild Hybrid Powertrain

Artikel i vetenskaplig tidskrift, 2020

The potential of 48V Mild Hybrid is promising in meeting the present and future CO2 legislations. There are various system layouts for 48V hybrid system including P0, P1, P2. In this paper, P2 architecture is used to investigate the effects of water injection benefits in a mild hybrid system. Electrification of the conventional powertrain uses the benefits of an electric drive in the low load-low speed region where the conventional SI engine is least efficient and as the load demand increases the IC Engine is used in its more efficient operating region.
Engine downsizing and forced induction trend is popular in the hybrid system architecture. However, the engine efficiency is limited by combustion knocking at higher loads thus ignition retard is used to avoid knocking and fuel enrichment becomes must to operate the engine at MBT (Maximum Brake Torque) timing; in turn neutralizing the benefits of fuel savings by electrification. Water injection suppresses engine knocking and enables operating at stoichiometric air-fuel ratio. In addition to that, the injection of water reduces flame temperature, giving room to ignition advance towards MBT timing. Ignition timing close to MBT results in higher thermal efficiency.
Operating a downsized engine installed with water injection in a hybrid powertrain architecture can be beneficial in exploiting both technologies and cumulatively a high efficiency powertrain can be produced. A 48V compact passenger car simulation model, developed in GT-suite, was used to simulate the fuel efficiency using experimental results. A 1.5 L, downsized-turbocharged SI-engine was operated with and without water addition during the experiments. Fuel consumption benefits of approximately 0.5 to 1% were observed in stationary to aggressive drive cycles.