Water Injection System Application in a Mild Hybrid Powertrain 2020-01-0798
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 downsized SI engine equipped with water injection system, 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 has gained popularity in a hybrid system architecture. However, the engine efficiency is limited by combustion knocking at higher loads, and fuel enrichment becomes must to operate the engine at MBT (Maximum Brake Torque) timing otherwise ignition retard is used to avoid knocking; in turn neutralize the benefits of fuel savings by electrification. Water injection suppresses engine knocking while operating at stoichiometric air-fuel ratio. Additionally, 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.
A 48V compact passenger car simulation model, developed in GT-suite, was used to simulate the fuel efficiency benefits over drive cycles, using experimental BSFC (Brake Specific fuel Consumption) data. A 1.5 L, downsized-turbocharged SI-engine was operated with and without water addition during the experiments. Fuel consumption benefits of 1% to 2% were observed over different drive cycles.