Pathways to Near-Zero-Emission Natural Gas Heavy-Duty Vehicles

California has the most stringent air quality and climate protection policies in the nation. Nonetheless, the state is not on track to meet smog reduction requirements and will have to generate greenhouse gas (GHG) emission reductions earlier than currently projected to maximize the benefit of GHG reductions on global climate change. Although California is working to harmonize its air quality and climate protection planning, policymakers face significant challenges in their efforts to devise a “pathway” that can simultaneously help the state achieve NOx and GHG reduction targets. Compared to the 2010 “Business as Usual” scenario — which includes all adopted emission control measures for the South Coast — NOx levels must be reduced by 65 percent and by 75 percent to meet the 2023 and 2032 ozone standards, respectively. An 80 percent reduction of GHG emissions is targeted for 2050, relative to 1990 levels.

Heavy-duty diesel trucks are the largest contributors to the nitrogen oxide (NOx) inventories of the South Coast and San Joaquin Valley ozone non-attainment areas. They are also major producers of toxic air contaminants and GHGs. In order to meet federal deadlines for attainment of ozone and fine particulate matter (PM2.5) standards, these regions must expeditiously phase in heavy-duty vehicles (HDVs) that emit at, or below, the equivalent of “zero-emission” battery-electric vehicles when accounting for pollution from base load electricity generation. This constitutes a NOx reduction of approximately 90 percent below the current federal heavy-duty engine standard of 0.2 grams per brake horsepower-hour (g/bhp-hr).

Since the advent of the first Low-Emission Vehicle (LEV) regulations in 1990 and the passage of California Assembly Bill 32 (AB 32) in 2006, the California Air Resources Board (ARB) has attempted to address the state’s air quality and climate goals by requiring the development of motor vehicles that do not directly emit NOx or other criteria pollutants. Policymakers have emphasized policies that compel the manufacture and/or purchase of vehicles that emit zero emissions at the tailpipe. These “technology forcing” requirements, which direct manufacturers to phase-in sales of battery-electric vehicles or large transit fleets to buy zero-emission buses, have not resulted in the commercialization of zero-tailpipe emission vehicles as quickly as air quality regulators hoped or planned for. Zero-emission vehicle mandates have, however, spurred tremendous innovation by the manufacturers of internal combustion engines, emission control equipment, and advanced vehicle drive trains. The resulting technological transformations have dramatically increased the menu of options available to policymakers to meet the state’s air quality and GHG reduction goals.

Given the sector’s dominant contribution to California’s ozone pollution and toxic air contaminant problems, the ARB and other air quality regulators are keenly focused on dramatically reducing emissions from the state’s heavy-duty vehicle fleet. With ozone reduction deadlines looming, strategies that quickly reduce NOx emissions from this sector are essential. Fortunately, an alternative to diesel in heavy-duty vehicles has already found a substantial foothold in the market and is poised to achieve the NOx and GHG emission reductions necessary to push California toward attainment of its air quality and climate protection objectives. The use of heavy-duty engines powered by natural gas (NG) offers a unique, viable, and complementary pathway to help meet California’s aggressive reduction goals for NOx and GHG emissions. It also supports a variety of other state and national goals, such as reducing the public’s exposure to toxic diesel exhaust and reducing the nation’s dependence on foreign energy sources. Natural gas-fueled trucks, buses, and off-road equipment can serve as a key element of California’s smog reduction and climate mending programs while dramatically decreasing the mass of cancer-causing chemicals in our air.

Today’s commercially available natural gas engines already emit NOx at levels well below the current (2010) federal heavy-duty engine standard. Heavy-duty vehicles fueled by natural gas are also recognized by the ARB as a method of reducing GHG emissions. This provides a low-emission, low carbon baseline upon which engineers have begun to apply a suite of well understood technologies that have been utilized to improve the emissions performance and fuel efficiency of conventionally-fueled engines. In the near term, utilization of technologies such as optimized compression ratios, enhanced three-way catalysts (TWC), integration of electric and hydraulic hybridization, improved aerodynamics, and low-pressure storage are expected to help reduce NOx emissions from natural gas heavy-duty vehicles by 75 percent. Engine manufacturers have already begun to integrate these technologies into natural gas heavy-duty engines and are thus expected to bring product to market in the next few years with NOx emissions less than 0.05 g/bhp-hr. At the same time, advancements in engine and vehicle design will dramatically increase fuel efficiency, thereby reducing GHG emissions.

Furthermore, research indicates that heavy-duty natural gas engines are on a trajectory to be certified at a NOx level of 0.02 g/bhp-hr, an emissions level so low that it equates to the power plant emissions that would result from charging an electric vehicle of comparable size. These technologies, which are currently being developed, include advanced aftertreatment and waste heat recovery, lean burn plus lean NOx emissions traps, integration of zero-emission miles technologies, further refinements in reducing friction and parasitic energy losses, and widespread utilization of renewable and hydrogenized natural gas. Integration of these technologies will increase the likelihood that California can meet smog reduction requirements and also help heavy-duty natural gas engines meet the 2050 goal to reduce GHG emissions by 80 percent.

Heavy-duty natural gas engines are well along the path to achieve a 90 percent NOx reduction from the existing heavy-duty engine NOx standard while also becoming increasingly fuel-efficient to reduce GHG emissions. Widespread deployment of these near-zero and power plant emission-equivalent heavy-duty natural gas vehicles (NGVs) are needed to meet tough air quality and climate protection goals. To realize these benefits, supportive public policies and public-private partnerships are needed that continue to encourage the development, demonstration, and deployment of critical natural gas-fueled heavy-duty vehicle technologies.

California’s policymakers can encourage this development with three actions: 1) Implement policies that fund the research, development, and demonstration of these crucial pathway technologies; 2) Support modifications to the state’s already robust air quality incentive programs that promote the commercialization of near-zero and power plant emissions-equivalent heavy-duty vehicles; and 3) Develop new and innovative requirements for the use of pathway technologies throughout the state. Coupled with continued promotion of zero-emission technology, particularly in those sectors of the vehicle population in which the most progress is being made, an air quality plan that encourages the rapid development and massive deployment of near-zero and power plant emissions-equivalent NGVs can propel California down the path to a cleaner, more climate-friendly future.