UC Davis Institute of Transportation Studies

Natural gas vehicles have been favored by U.S. air quality agencies for the cleaner burning properties of natural gas. However, the climate consequences of a switch to natural gas vehicles for long distance, heavy-duty applications has been less clear. The radioactive forcings of short-lived methane leakage must be weighed against any long term benefits of emitting less CO2. The scientific literature reports a variety of results and conclusions, thus policy makers often find it hard to make science-based, sound decision-making. But there is inherent natural variability in the system and virtually any result can be justified based on a given choice of input values. Some scholars deal with this variability with probabilistic distributions of inputs to produce probabilistic distributions of results. But this treatment of uncertainty might not resound with decision makers, who might prefer the simplicity of one single estimate. In this study, we attempt to tackle and communicate uncertainty in a simplified way in which a transparent base case scenario is modified one input at a time to determine the distinct parameters that are critical to assessing the climate impact of natural gas as a transportation fuel. Instead of focusing on a specific number, this analysis shows what makes a natural gas fuel a better option versus a bad option, so policy makers and agencies can focus on promoting these best practices among the interested parties. We utilize Argonne’s GREET1 2014 model to test sensitivities for the life cycle carbon intensity of natural gas versus diesel fuel under a range of scenarios for upstream and in vehicle methane leakage, fueling and storage technologies, and operational performance of various kinds of class 8 engines. We evaluate the relative importance of engine technology, natural gas fuel storage choice, upstream methane leakage (i.e., well-to-tank), and vehicle methane slip (i.e., tank-to-wheel). We find that: 1) Upstream methane leakage contributes between 7 and 11% and vehicle methane leakage (i.e., methane slip) contributes between 5 and 9% to the total carbon intensity of natural gas in long haul trucks; 2) Variability factors include whether natural gas fuel is stored as compressed (CNG) or liquefied (LNG) and whether a natural gas vehicle uses spark ignition or compression ignition. Natural gas engines typically being spark ignition, which are around 10% less efficient than compression ignition diesel engines; 3) If no efficiency penalty is assumed (as in the case of the currently out of market HPDI model), NGVs offer a climate advantage compared to diesel only if well-to-tank methane leakage remains under 5%; and 4) CNG storage is more sensitive to leakage than LNG storage. This analysis allows us to identify the most important strategies to reduce the carbon intensity of NGVs.