Modeling Analysis on Combined Effects of VVT/VCR Engine Technology to Reduce Fuel Consumption of Light-Duty Parallel Hybrid CNG Trucks

Potential fuel consumption (FC) improvements of a parallel hybrid, light-duty, compressed natural gas (CNG) truck are numerically predicted using the combined benefits of a variable valve timing (VVT) and variable compression ratio (VCR) engine. A CNG hybrid electric vehicle (CNG-HEV) simulation model is developed in commercial software based on conventional delivery trucks in the Japanese market. The hybrid powertrain model includes a pre-transmission e-motor, high voltage battery, and production-typed 3.0 L CNG engine. Power splitting between e-motor and engine is controlled by a rule-based control strategy. Under the JE05 drive-cycle, engine maps such as torque, fuel and flow rate, and engine friction were measured under different intake valve timings in the 4-cylinder, twin-turbocharged spark-ignition engine with customized pistons to reach a compression ratio (CR) of 17. For VCR operations, the performance maps were recorded under CR14, CR15, and CR17 using standard valve timings. Simulated FC of the CNG-HEV under the JE05 driving cycle is improved by 10.9% and 15.7% using standard and optimal intake valve timings, respectively, against a conventional powertrain vehicle. By combining optimal VVT/VCR engine operations with a selected battery sizing, the predicted FC of the CNG-HEV could be achieved up to 18.2% against the conventional powertrain.