Increasing truck transport demand [1] and greenhouse gas emissions [2] have garnered the interest of many to research alternative powertrain technologies for heavy-duty vehicles (HDVs). Objectives to reduce and mitigate emissions across all sectors have been set by many governments regarding the transportation sector. Conventional HDVs disproportionately represent the on-road carbon dioxide (CO₂), nitrogen oxide (NO_x), and particulate matter (PM) [3]. Medium and heavy-duty vehicles account for approximately 23% of the greenhouse gas (GHG) emissions in the United States [4]. HDVs also account for approximately 40–60% of the NO_x and PM emissions [3]. Climate change, pollution, and the resulting health impacts are some of the major concerns with the rise of these combustion emissions [5]. Therefore, the proposal for lower emission and zero-emission HDVs has been made.

A vehicle is classified as heavy-duty if it has a gross vehicle weight rating (GVWR) greater than 26,000 lbs [6]. The GVWR is the maximum loaded weight of the vehicle, which is the weight of the vehicle in addition to its payload. HDVs can be further classified into two classes, as seen in Figure 1. Class 7 vehicles include transit buses, tow trucks, and furniture trucks. Class 8 vehicles include semi-tractors, fire trucks, and dump trucks. Class 7 vehicles have a GVWR of 26,000 lbs to 33,000 lbs, and Class 8 vehicles have a GVWR of greater than 33,000 lbs.

Conventional HDVs use fossil fuels and an internal combustion engine (ICE) which produce energy to power their movement [7]. Most HDVs today use an ICE that utilizes diesel called a compression-ignition engine due to its greater energy efficiency than gasoline. However, many kinds of emissions are released during diesel combustion, such as CO₂, NO_x, and PM. Lower emission strategies in HDV technology consist of increasing the efficiency or reducing the pollution of the conventional diesel truck [8], using alternative fuels which produce fewer emissions [9], or using a hybrid-electric powertrain that stores some of the energy it uses within batteries [10]. These strategies work by reducing fuel consumption or by reducing their tailpipe emissions. Examples of lower emission strategies include reducing the rolling resistance of the tires, which increases the fuel savings of the vehicle and using alternative fuels such as compressed natural gas or biodiesel, which produce fewer emissions [11]. However, zero-emission vehicles require a different powertrain as any vehicle with an ICE will result in tailpipe emissions.

The two types of zero-emission vehicles that will be discussed in this review are battery electric vehicles (BEVs) and hydrogen fuel cell vehicles (HFCVs). These vehicles propel themselves using electricity and do not require the use of an ICE. BEVs and HFCVs both convert the chemical energy stored in active materials into electrical energy within the electrochemical cells. Batteries differ from fuel cells because batteries have the active material stored within the system, while fuel cells have the active materials continuously fed into the system. BEV batteries are often composed of lithium-ion cells due to their high energy and power density [12,13,14]. On the other hand, HFCVs often use proton exchange membrane fuel cells (PEMFC) due to their high-power density and cold-start capabilities [15].

Both solutions offer zero tailpipe emissions, however, their well-to-wheels (WTW) emissions are still high in many countries because of fossil fuel energy generation. WTW emissions are the emissions released from two main stages: well-to-pump, and pump-to-wheels [16]. Well-to-pump emissions are the emissions released from the production of energy and the transport of it to the consumer. Pump-to-wheels are the refueling and tailpipe emissions generated when the vehicle is being refueled and used, respectively. Therefore, if the energy to generate the electricity or hydrogen to power the vehicle was utilized fossil fuels, the well-to-pump emissions would be significant. However, if the sources of energy used to generate the energy in these zero-emission vehicles were renewable and not fossil-fuel related, they would theoretically have zero emissions [17].

In recent years, many researchers have been working to either improve the traditional diesel powertrain or develop battery and fuel cell electric powertrains for HDVs. With that, there have also been some works that provided a comparison between different powertrains or analysis of their impacts based on economic, environmental, and social aspects. Zhao et al. [18] proposed several non-electrification efficiency-improving technologies as well as hybrid powertrain technologies to reduce heavy-duty truck fuel consumption. It was found that improvements in engine efficiency, aerodynamic drag, and rolling resistance would result in 6–13% in fuel savings, while hybridization was found to achieve 16% fuel saving. Sen et al. [19] compared traditional diesel, biodiesel, compressed natural gas, hybrid, and battery electric HDVs based on life-cycle emissions, costs, and externalities, to gain insights on how alternative fuel technologies could be employed in HDVs. The results showed that overall, battery-electric HDVs outperformed all other types of trucks. Assuming that electricity could be produced from renewable energy sources in the future, the use of a battery-electric powertrain would significantly improve the life cycle cost of trucks as well as the air quality through emissions reduction. Shin et al. [17] conducted a case study using a conjoint survey with 1049 people and a mixed multiple discrete-continuous extreme value model to analyze consumer preferences between battery electric vehicles and fuel cell vehicles. The results indicated that consumers preferred battery-electric vehicles over fuel cell vehicles. In [9], the authors presented a summary of market diffusion studies for alternative fuels and powertrains in HDVs. The methods, main findings, and policy recommendations in each study were compared and synthesized to identify the strengths and weaknesses of each type of powertrains. It was found that there was a high degree of uncertainty regarding which HDV powertrain was superior compared to the rest. Smallbone et al. [20] evaluated the potential impact of several HDV powertrain technologies, including conventional diesel engines, hybrid, battery-electric, hydrogen fuel cells, and free-piston engine generators. 

Internal combustion engines use transport fuel and combustion to propel their movement. There are two major types of internal combustion engines: compression-ignition (CI) and spark-ignition (SI) [21]. Most conventional heavy-duty vehicles use CI engines while passenger vehicles use SI engines. Both use the four-stroke combustion cycle of intake, compression, combustion, and exhaust. However, their difference lies in the way they ignite their fuel and the type of fuel they use, which results in differences in efficiencies. CI engines use the heat from the compression stroke to ignite the fuel, while the SI engine uses spark plugs [22]. As well, CI engines use diesel fuel which has 10–15% more energy content than gasoline [23] which is used in SI engines. Compression values of diesel engines are higher, which leads to higher efficiencies as the likelihood that air molecules will react with the molecules in the fuel is greater. The greater energy density and the higher efficiencies make it the most popular fuel engine to date for HDVs.

A simplified conventional diesel powertrain configuration involves the engine, clutch, and transmission. The transmission converts the torque output from the engine to rotations in the axles. This is a result of engines running at higher rotations per minute than the average vehicle’s wheels. The clutch allows for the disconnection between the transmission and the engine. The engine is also connected to an alternator which transfers energy to a battery. The battery is then connected to electrical accessories and is used to start the engine alongside the starter.

Conventional HDV ranges can vary depending on the existing payload and fuel in the truck. The average mileage per gallon of fuel for each truck is around 6.5 mpg [24]. As well, a truck can typically carry around 150–300 gallons of fuel. The range that this allows for is around 975–1950 miles without any stops to refuel. However, this range is highly dependent on the topology, the traffic, and the fact that truckers would not consume 100% of the fuel at each run. The average flow rates are 20–30 gallons per minute for a diesel pump results in a refueling time of a 300-gallon capacity truck of around 10–15 min [25].

Diesel HDVs release three major types of emissions: carbon oxides, nitrogen oxides, and particulate matter [26]. There are other components, such as sulfur oxides and organic compounds that are released, but they are not analyzed in this report. Emissions from HDVs tend to vary widely depending on the weight, driving cycle, age, terrain, injection timing, and after-treatment of the vehicle and its engine [27]. According to a study by Seo et al., an HDV with a GVWR from 26,000 lbs to 88,000 lbs can emit CO₂ from 29 g/km-ton to 119 g/km-ton, respectively [28]. The Canadian National Railway, on average, estimates that a heavy-duty truck will emit around 63.8 g/km-ton of CO₂ with an average shipment of 16 tons (35,200 lbs) [29]. Regarding NO_x emissions, the average truck will emit around 7 g/mile, which, compared to a conventional gasoline car, is greater by a factor of 10 [30]. To combat emissions, after-treatment system technology can be installed, which processes some of the exhaust gases into non-harmful substances. For example, selective catalytic reduction technology uses the reactions between ammonia and NO_x through a catalyst to form nitrogen and water vapor [11]. This aids in reducing the amount of NO_x that is released into the environment.

The current infrastructure for diesel HDVs is highly prevalent and it is available in most, if not all, countries. As of 2019, nearly all the HDV market runs on conventional diesel fuel [9,31]. The demand for alternative fuels is increasing for HDVs, but the alternative fuels markets are still in the very early stages. Regarding energy efficiency, diesel engines, from tank-to-wheel, on average, are around 23% energy efficient [32]. This means that out of the available energy at the fuel tank, only 23% of it is used as actual work, with the rest being lost due to heat and friction. The specific energy of diesel fuel is around 42.9 MJ/kg [33].

3. Battery Electric Powertrain in Heavy-Duty Vehicles