Compressed natural gas (CNG) is a popular fuel for commercial delivery trucks as an alternative to diesel and gasoline due to its abundant resource availability and steady price. Besides, due to its high hydrogen-to-carbon ratio, CNG vehicles emit cleaner emissions than gasoline- or diesel-powered cars. However, a significant issue of the CNG vehicles is their short driving ranges because natural gas contains low physical density. To race toward a carbon-neutral society, hybrid electric vehicles (HEVs) equipped with CNG engines can improve the vehicle driving ranges and contribute to greenhouse gas emission reduction from the transportation segment.In this study, potential fuel economy improvements of a light-duty CNG truck were numerically investigated using a combined variable valve timing (VVT) and variable compression ratio (VCR) engine with a parallel hybrid-electric powertrain. A CNG-HEV simulation model was developed in commercial software based on a conventional delivery truck platform available in the Japanese market. The hybrid powertrain model includes a pre-transmission or P2 electric motor, a high voltage (HV) battery, and a CNG engine. The vehicle model uses a rule-based control strategy for the power split between the e-motor and engine.&nbsp; The engine is a 3.0 L production-type, 4-cylinder turbocharging, spark-ignited engine with customized pistons to reach a compression ratio (CR) of 17. Engine performance maps such as brake torque, fuel flowrate, air flowrate, and engine friction were measured under a standard intake valve timing (intake valve closure timing or IVC90) and two retarded valve timings (IVC100 and IVC108). For the VCR operations, the performance maps were recorded under CR14, CR15, and CR17 using the standard IVC90. A generic speed-torque-efficiency map of the electric motor and open-circuit voltage and internal resistance of the 1.9 kWh battery were input to the baseline HEV model (baseline HEV with the generic e-motor and the 1.9 kWh battery).The simulation results show that the actual vehicle speed follows the target speed reasonably well under the JE05 driving cycle (gross vehicle weight over 3,500 kg). Under the baseline case (standard payload 1,000 kg and IVC90 engine), the simulated fuel economy (FE) of the CNG-HEV is improved by +10.9 % against a conventional powertrain vehicle (CV). +10.9 % and +14.4% FE can be obtained when the hybrid truck is equipped with a 2.1 kWh and 2.4 kWh battery, respectively. FE is enhanced by +15.7% when the baseline HEV is fitted with the IVC100 engine. The FE improvement is enhanced by the IVC100 engine because the spark timings can be advanced under the retarded IVC, therefore improving the engine's thermal efficiency. Using the IVC108 engine achieves a 14.7% FE improvement for the baseline HEV against the CV. The -1.0% drop in FE (IVC108 versus IVC100) is due to reduced engine volumetric efficiency when the IVC timing is too retarded. By combining optimal VVT/VCR engine operations with a selected battery sizing, the predicted FE improvement of the CNG-HEV can be obtained up to 18.2% against the CV.

Prediction of Potential Fuel Economy Improvements of an Electrified Natural Gas Truck Equipped With a Vvt/vcr Engine

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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