Powertrain Alternatives

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Abstract: The internal combustion engine has been a prime mover of the industrial economy for more than a century. The development of alternative powerplants is now driven by climate change considerations and depletion of fossil fuel resources. Low carbon powertrains widely envisioned by policymakers include battery electric vehicles and vehicles powered by hydrogen and liquid electrofuels. However, these powertrain options can reduce GHG emissions only in conjunction with carbon neutral electricity generation, and their manufacture can be very resource intensive.

Internal Combustion Engines

Prime Movers of Economy

Most conventional powerplants used for light- and heavy-duty highway vehicles and nonroad machinery applications have been based on reciprocating internal combustion engines (ICE) powered by petroleum fuels—primarily gasoline and diesel. A complex refining and distribution infrastructure has been developed over many decades to handle these liquid hydrocarbon fuels.

Diesel engines, characterized by the highest thermal efficiency among all ICEs, power the vast majority of commercial applications, Figure 1, such as heavy trucks and light commercial vehicles, nonroad machinery in the construction and agricultural sectors, and higher horsepower marine engines. They are also used in stationary applications for power generation, along with natural gas engines.

The importance of the diesel engine for the industrial economy cannot be overestimated. Diesel-powered machinery enables resource extraction in the mining and energy sectors, food production in the agricultural sector, and the global and local distribution of products, components, and materials by ship, rail, and truck. It has been argued that if diesel trucks suddenly stopped running, the economy would be brought to a grinding halt within just a few days [4132].

Spark ignited (SI) gasoline engines, on the other hand, can be an attractive substitute for a diesel engine where the benefits of a diesel engine such as higher efficiency, durability and low speed torque are either not sufficient to offset the higher cost of the diesel engine or offer no tangible advantages [5299]. There are numerous examples in the onroad vehicle and nonroad equipment sectors where the need to comply with emission regulations has increased the cost of diesel engines, while gasoline engines are able to meet the requirements with a relatively inexpensive emission control system.

At the macroeconomic scale, a balanced coexistence of diesel and gasoline powertrains is desired because of the petroleum refining process. Crude oil cannot be refined into diesel or into gasoline fuel alone—both diesel and gasoline fuels are produced together. If all engines were powered by only one fuel—either diesel or gasoline—the other fuel would see little demand and could become a waste byproduct.

Gasoline engines have been predominantly used in light-duty powertrains, including passenger cars and light trucks. In some markets—particularly in the EU and in India—light-duty diesel vehicles had also been a popular choice, Figure 2 [3118]. The market share of diesel cars had been driven by their superior fuel economy, fuel taxation that favored diesel, and by climate change policies that relied on diesel fuel. The light-duty diesel penetration data in Figure 2 is close to its all-time high—the market share of light-duty diesels has declined after 2015, when US regulatory authorities discovered and exposed the illegal emission defeat strategies used on a massive scale by Volkswagen and other manufacturers in their diesel engines, and a number of European cities imposed restrictions on the operation of older diesel vehicles. In less than a decade, by Q2 2022, the EU market share of new diesel vehicles declined to 17.3%.

Further common applications of the gasoline engine include small utility equipment and nonroad engines under about 37 kW (50 hp) that, even if they are run continuously, use a relatively small amount of fuel. Other examples with larger engines include nonroad equipment that accumulates only a relatively small number of hours annually. In the United States, spark ignited gasoline engines have been commonly found in commercial vehicles up to Class 6 and to a lesser extent in Class 7, Figure 3 [5298].

As also apparent from Figure 3, a relatively minor population of engines have been fueled by alternative fuels such as natural gas—compressed (CNG) or liquefied (LNG)—and liquefied petroleum gas (LPG). In the period spanning the late 20^th to the early 21^st century, the popularity of alternative fuels has gone through numerous cycles. After about 2010, increased supplies of shale gas in the United States have driven down natural gas prices in North America and made alternative fuels more attractive once again, with potential applications ranging from light-duty vehicles to low speed marine engines.

Alternative Combustion Concepts. While alternative combustion engine concepts have seen development since the early days of the internal combustion engine, current interest in these ideas has focused on their potential to deliver increased thermal efficiency. An example alternative engine concept that continues to attract attention is the two-stroke opposed piston engine. The rotary Wankel engine is another example concept that had been used in some commercial vehicles. Alternative combustion engine concepts also include the gas turbine and external combustion engines, such as the Stirling engine. These various alternative powerplants have found some specialized uses, but are not serious contenders for the reciprocating internal combustion engine in most of its many applications.

Drivers for Change

The development of alternative powerplants—driven for several decades by better performance, increased thermal efficiency, and reductions in pollutant emissions—is now largely motivated by the desire to reduce the reliance on fossil fuel energy. There are at least two reasons to do so:

The former consideration—reduction of carbon emissions—has dominated government policies and mainstream media narratives. On the other hand, the ongoing developments in energy markets suggest that the global economy may stagnate due to energy scarcity even before it is seriously affected by climate related phenomena. While the relative importance and timing of the environmental (climate) and economic (resource depletion) factors remains debatable, both point to essentially the same objective—a rapid and substantial reduction in fossil fuel consumption.

Two main approaches have been considered to decarbonize the transport sector and reduce the use of fossil fuels:

The benefits and challenges of these low carbon powertrain options are discussed in more detail in the following sections. It must be emphasized, however, that the key challenge in moving away from fossil fuels—often marginalized or omitted entirely from the public policy debate—is energy supply. Petroleum based fuels had served for decades as a highly efficient, cheap, and abundant energy source. The necessity to move away from petroleum may require the adoption of less efficient mobility technologies—on a lifecycle basis—while energy supply to the global economy becomes increasingly constrained. An in-depth analysis of the energy transition process suggests that replacing the existing fossil fuel powered system using renewable technologies, such as solar panels or wind turbines, will not be possible for the entire global human population [5227]—there is simply not enough time or resources to do this by the current target set by the world’s most influential nations. What may be required, therefore, is a significant reduction of societal demand for mobility services and for other resources of all kinds, including energy. The role of energy in the economy was discussed under Fossil Fuels and Future Mobility.

Acknowledgements

Ameya Joshi of Corning pointed us toward several pertinent studies on life cycle analysis of vehicle emissions and provided valuable feedback on this paper.