Numerical Methods on VVA and VCR Concepts for Fuel Economy Improvement of a Commercial CNG Truck

Natural gas has been used in spark-ignition (SI) engines of natural gas vehicles (NGVs) due to its resource availability and stable price compared to gasoline. It has the potential to reduce carbon monoxide emissions from the SI engines due to its high hydrogen-To-carbon ratio. However, short running distance is an issue of the NGVs. In this work, methodologies to improve the fuel economy of a heavy-duty commercial truck under the Japanese Heavy-Duty Driving Cycle (JE05) is proposed by numerical 1D-CFD modeling. The main objective is a comparative analysis to find an optimal fuel economy under three variable mechanisms, variable valve timing (VVT), variable valve actuation (VVA), and variable compression ratio (VCR). Experimental data are taken from a six-cylinder turbocharged SI engine fueled by city gas 13A. The 9.83 L production engine is a CR11 type with a multi-point injection system operated under a stoichiometric mixture. For minimizing optimal valve strategy selections and engine testing procedures, a one-dimensional engine model is developed in GT-Power software using experimental data and engine specifications provided by a project partner. The model is built using the same theory as of spark-ignition engines. Knock prediction is based on the Shell model, and a spark timing optimization logic is coupled to the model. In-cylinder pressure, rate of heat release, brake mean effective pressure, and maximum brake torque spark ignition timings are well reproduced, as compared with that of 12 experimental operating points under engine speed and load variations. In order to build a baseline brake specific fuel consumption (BSFC) map in the driving cycle, the simulation model is used to generate 51 BSFC points, as proposed in the JE05 cycle under speed-Torque changes. From the baseline engine model, the average fuel economy of the heavy-duty natural gas truck is 4.14 km/L. 0.5 % and 2.43% of simulated fuel economy improvements are found when the engine is operated under VVT and VVA mechanisms (fixed lifts), respectively. Significant fuel economy improvement is achieved at about 6.31% under VCR engine operation compared with the baseline engine model.