Overview of the Safety Issues Associated with the Compressed Natural Gas Fuel System and Electric Drive System in a Heavy Hybrid Electric Vehicle

Abstract

This report evaluates the hazards that are unique to a compressed-natural-gas (CNG)-fueled heavy hybrid electric vehicle (HEV) design compared with a conventional heavy vehicle. The unique design features of the heavy HEV are the CNG fuel system for the internal-combustion engine (ICE) and the electric drive system. This report addresses safety issues with the CNG fuel system and the electric drive system. Vehicles on U. S. highways have been propelled by ICEs for several decades. Heavy-duty vehicles have typically been fueled by diesel fuel, and light-duty vehicles have been fueled by gasoline. The hazards and risks posed by ICE vehicles are well understood and have been generally accepted by the public. The economy, durability, and safety of ICE vehicles have established a standard for other types of vehicles. Heavy-duty (i.e., heavy) HEVs have recently been introduced to U. S. roadways, and the hazards posed by these heavy HEVs can be compared with the hazards posed by ICE vehicles. The benefits of heavy HEV technology are based on their potential for reduced fuel consumption and lower exhaust emissions, while the disadvantages are the higher acquisition cost and the expected higher maintenance costs (i.e., battery packs). The heavy HEV is more suitedmore » for an urban drive cycle with stop-and-go driving conditions than for steady expressway speeds. With increasing highway congestion and the resulting increased idle time, the fuel consumption advantage for heavy HEVs (compared with conventional heavy vehicles) is enhanced by the HEVs' ability to shut down. Any increase in fuel cost obviously improves the economics of a heavy HEV. The propulsion system for a heavy HEV is more complex than the propulsion system for a conventional heavy vehicle. The heavy HEV evaluated in this study has in effect two propulsion systems: an ICE fueled by CNG and an electric drive system with additional complexity and failure modes. This additional equipment will result in a less reliable vehicle with a lower availability than a conventional heavy vehicle. Experience with heavy HEVs to date supports this observation. The key safety concern for the electric drive system is the higher voltages and currents that are required in the electric drive system. Faults that could expose personnel to these electric hazards must be considered, addressed, and minimized. The key issue for the CNG-fueled ICE is containment of the high-pressure natural gas. Events that can result in a release of natural gas with the possibility of subsequent ignition are of concern. These safety issues are discussed. The heavy HEV has the potential to have a safety record that is comparable to that of the conventional vehicle, but adequate attention to detail will be required.« less