By Dr. Raj Shah, Dr. Vikram Mittal and Nabill Huq
As the world continues to transition from nonrenewable and pollutive energy sources to ones that are sustainable and clean, it is critical to innovate across all aspects of our infrastructure and industries. The difficulty with transitioning to newer types of fuels is that the industries that primarily rely on older and dirty fuel types are usually stubborn to change, resulting in slow progress. The trucking industry, in particular, has been slow to adapt its fuel technology. The trucking industry plays a monumental role in transporting goods and resources over roadways and employs the use of heavy-duty trucks (HDTs) for long-distance transportation.
In the United States, the trucking industry was worth $791.7 billion, employing over 947,000 truck drivers in 2019. The size of the trucking industry thus corresponds with the large consumption of fuel, in which 55 billion gallons of fuel (39 billion gallons of diesel and 16 billion gallons of gasoline) were consumed by trucks in 2015. Heavy-duty vehicles account for only 10% of the global vehicle stock but are responsible for 46% of the total greenhouse gas emissions from road transport according to The International Council on Clean Transportation.
The trucking industry first developed during World War I, where the established fuels used by most trucks were diesel and gasoline. These fuels have been used ever since, and the adverse effects of the resulting emissions are now well established. In 2006, the environmental and health effects of the particulate matter of diesel exhaust were chronicled by the California Air Resources Board, prompting advancements in diesel fuel and stricter regulations on the emissions of diesel exhaust. The U.S. Environmental Protection Agency (EPA) ordered petroleum refiners to reduce the sulfur content of highway diesel fuel from 500 parts per million (ppm) to 15 ppm, a 97% reduction, starting in June 2006. This cleaner-burning fuel helped to reduce smog-causing nitrogen oxide emission by 2.6 million tons every year according to CARB.
Another area of advancement made with diesel fuel is fuel efficiency. While other vehicles have made tremendous gains in fuel efficiency, the gains for diesel trucks have been much more modest. From 1966 to 2018, the fuel economy of medium- and heavy-duty trucks only improved from 5.6 miles per gallon (mpg) to 7.5 to 9.5 mpg. Some additional gains have been realized through better vehicle aerodynamics; however, the long vehicle bodies necessitate a large amount of drag. New drivetrain technologies are showing increasing promise, especially related to hybrid systems. For example, the Shell Starship truck uses an electric hybrid system coupled with advanced aerodynamics to achieve 8.9 mpg. Larger gains can be achieved from a hydraulic hybrid truck, which can achieve fuel economies of more than 10 mpg. Overall, great strides have been made to lower the adverse effects of traditional diesel fuel over the past few decades.
However, significant larger gains can be achieved through further innovation of fuel by developing diesel with a cleaner and renewable source of energy—biomass. The trucking industry has implemented this cleaner biodiesel by creating blends of it with traditional diesel. Biodiesel can be created from a multitude of different sources, such as soy and animal fat. The convenience with using biodiesel is that it can be used in both old and modern diesel engines without alterations, thus requiring little restructuring to current trucking infrastructure.
Compared to traditional diesel, biodiesel based on soy and animal fat blends shows reductions in particulate matter, total hydrocarbon and carbon monoxide emissions, with higher reduction in blends with a higher biodiesel component. The main disadvantage of using biodiesel is that less energy is stored in biodiesel when compared to equal amounts of traditional diesel, resulting in a lower range of travel for HDTs. The travel range of a truck is an important property for the trucking industry because of the long-distance nature of travel. For example, pure biodiesel (B100) will only travel 92.4% of the distance that an equal amount of pure traditional diesel (B0) can travel. Fortunately, this disadvantage is remedied by creating biodiesel blends that find a good balance of fuel efficiency and lowered emissions. Biodiesel blend B20 (6% to 20% biodiesel) is a common blend that only gives up 1% to 2% mileage per tank when compared to traditional diesel.
Another recent development in diesel technology is renewable diesel, which has properties similar to biodiesel. Renewable diesel and biodiesel are both produced using biomass sources but differ in the method that they are processed. Chemically, renewable diesel is a hydrocarbon that is processed similarly to traditional diesel using techniques like hydrotreating, gasification, and pyrolysis; meanwhile, biodiesel is a mono-alkyl ester produced via transesterification. Due to renewable diesel’s hydrogenated nature, it burns cleaner than biodiesel. Despite the benefits of renewable diesel, its viability in a competitive fuel market depends on regulations that favor cleaner fuels. The largest producer of renewable diesel, Neste, only sells renewable diesel in the United States where a low-carbon fuel standard is set in place and the costs for the company are subsidized by the U.S. Blender Tax Credit. While biodiesel and renewable diesel technology provide a good compromise between performance and harmful emissions for HDTs, there is still a lot of fuel technology to explore for the prospect of a zero-emission future.
One of the biggest contenders for transforming the trucking industry to zero-emissions is battery-electric trucks. This has been made possible through the continuous advancements in lithium-ion technology driven by the commercial sector. The energy densities of modern lithium-ion batteries are 2-3 times that of rechargeable batteries from 20 years ago. Trucks powered purely by lithium-ion batteries have fewer moving components and systems to maintain when compared to diesel-powered engines. A promising example of an upcoming battery-electric truck is Tesla’s all-electric semi-truck. It has better performance in acceleration, with reportedly 3-5 times faster acceleration than the average diesel semi on a full load. In addition, Tesla’s semi-truck has better uphill hauling at which it can maintain a speed of 60 miles per hour (mph) on a 5% uphill grade, while a diesel version can only go 45 mph under the same conditions.
Despite these improvements, implementing battery-electric trucks into the trucking industry will require massive investments in the restructuring of current infrastructure and still comes with a host of disadvantages. For one, recharging stations would have to be prevalent across the U.S. before battery-electric trucks can be feasible. Additionally, current battery-electric trucks cannot match the range of travel of diesel trucks. Tesla’s battery-electric semi-truck has a maximum range of approximately 500 miles, compared to the approximate 800-mile range of conventional trucks. Another disadvantage is the longer time associated with recharging the batteries versus refueling a tank of fuel. Batteries also are not expected to last the lifetime of the HDT itself, thus requiring replacements and increasing demand for raw materials such as lithium, cobalt and nickel. Like the automobile market, many of these disadvantages can be overcome by simply using battery-electric semi-trucks for short-haul transport, which accounts for about half of American trucking.
The other major contender in fuel technology for zero-emission trucking is hydrogen fuel cells. Like battery-electric trucks, hydrogen fuel cell trucks are also driven purely through electric motors. The only substance that is emitted from the tailpipe is H2O, making this fuel very environmentally friendly. The convenience of using hydrogen fuel is that fueling up a tank is a similar process and takes a similar amount of time to diesel fueling. One company at the forefront of hydrogen fuel trucking, Nikola Motors, boasts a fueling time of fewer than 20 minutes for their hydrogen fuel semi-truck. Nikola Motors also touts traveling ranges from 500-750 miles on a full tank of fuel, making it comparable to diesel trucks as well. If all short-haul diesel trucks were to be switched into hydrogen fuel trucks or battery-electric trucks, 30% of all NOx emissions from the medium- and heavy-duty truck sector would be eliminated.
Currently, this fuel technology is still in the developmental phase and has several challenges that must be addressed before it can be fully implemented into the trucking industry. The largest issue is related to storage, since hydrogen must be stored at high pressures and is extremely volatile. Compared to gasoline, liquid hydrogen fuel has 2.6 times more energy per unit mass but requires about four times the storage volume. These tanks must be either maintained at -253°C or pressurized to 5000-10000 psi. Getting away from traditional storage tanks, a promising technique for storage is related to metal hydrides, where the hydrogen is reversibly bonded in a metal matrix. Several challenges exist especially as related to the weight and reversibility of the metal hydride. Another critical issue is that hydrogen trucks would require a new network of hydrogen fueling stations, which is a massive investment. Promisingly, Nikola Motors has committed to building a network of 700 truck-stop-sized hydrogen fueling stations across the U.S. and Canada by 2028.
Another consideration is that to achieve the zero-emission fuel goal, the method in which hydrogen fuel is produced must also be considered. Currently, the primary source of hydrogen is through the partial combustion of methane, a process that carries a large carbon footprint. Renewable-based hydrogen fuel technologies such as solar electrolysis will greatly increase the value of hydrogen fuel trucks. Fortunately, hydrogen fuel cell-based trucks have garnered the attention of new startups and veteran companies alike, in which their investments will help grow and improve hydrogen fuel technology.
The trucking industry has come a long way since its World War I roots. For a long time, the effects of the emissions coming from diesel trucks and other vehicles were unknown. It was only when these harmful exhausts were documented that real change came for fueling technology. Traditional diesel became much cleaner, and now the trucking industry has incorporated biodiesel and renewable diesel to lower its reliance on nonrenewable sources like petroleum. Futuristic fuel technology like battery-electric and hydrogen fuel cells are slowly being incorporated into the trucking industry and becoming a current-day reality. The trajectory of the progress toward zero-emission trucking is encouraging. The continued investment to evolve our fueling infrastructure will be a slow, but fruitful endeavor for the betterment of the health of ourselves and the environment.
Dr. Raj Shah is a director at Koehler Instrument Company, a leading manufacturer of petroleum testing instruments, and an active ASTM member for the past 25 years. He has held numerous leadership positions within various ASTM committees and is a three-time recipient of the ASTM award of Excellence. Contact him at [email protected].
Dr. Vikram Mittal is an assistant professor at the United States Military Academy in the Department of Systems Engineering. He earned his doctorate in Mechanical Engineering at the Massachusetts Institute of Technology, where he researched the relevancy of the octane number in modern engines.
Nabill Huq is a chemical engineering student at Stony Brook University, where Dr. Shah is the chair of the advisory board of directors, and Huq is also a part of a thriving internship program at Koehler Instrument Company.