Zero emissions, or ZE, is an easy concept to understand. It describes a vehicle with a propulsion system that emits nothing from a tailpipe. No particulate matter, no nitrogen oxide, no carbon dioxide—nothing adding to air pollution or global warming gases. It’s a precise definition, at least in theory.
When it comes to ZE vehicles, though, the reality is far cloudier. One way or another, ZE is going to be part of trucking’s future, as regulatory and social pressures mount around the world to eliminate mobile sources of greenhouse gases and airborne health hazards.
But this is still just the early days of ZE vehicle development. With the growing likelihood that trucks are going to be pushed into the vanguard of mobile ZE applications, it’s time to take a close look at what’s quickly gone from simple concept to a far more nuanced subject.
What started as a rich person’s fad with Tesla and other electric cars is now being asked to take on a utilitarian role where reliability, practicality and cost rule. Is ZE technology ready to stand up to those demands? Will ZE trucks radically change the industry’s basic equipment, or will it become a good fit only for carefully chosen niche applications?
And what about alternative truck technologies that deliver what’s sometimes called near-zero emissions? Are they part of a long-term solution, or just a bridge until ZE systems become cost-effective and practical?
Since the entire discussion revolves around vehicle emissions, we need to start with some basic definitions. Diesel engines, which power almost all medium- and heavy-duty trucks, produce two categories of emissions: those that affect ambient air quality and those that contribute to global warming.
The important air quality emissions are particulate matter (PM) and oxides of nitrogen (NOx), which in large concentrations like those found in many urban areas pose serious health risks. Carbon emissions from vehicles in the form of carbon dioxide (CO2) are one of the principal greenhouse gases (GHGs) implicated in damaging the Earth’s ozone layer and causing a steady increase in global temperature predicted to bring catastrophic environmental changes.
For the past 25 years, federal emissions standards focused on improving ambient air quality with a series of stepped reductions in PM and NOx that reached its final stage in 2010. Or final at least for now. The PM and reductions to ultra-low 2010 standards required diesel engine aftertreatment systems with high costs and complexity, as well as removing sulfur from diesel fuel.
With ambient air emissions addressed, the U.S. Environmental Protection Agency (EPA) turned its attention to GHG, calling for an overall 20% reduction in heavy-truck CO2 emissions between 2014 and 2018. For diesels and any other internal combustion engines, CO2 is directly related to fuel consumed, so in effect EPA mandated that trucks become more fuel efficient. Improved fuel controls and combustion efficiency along with aerodynamic aids have delivered the required fuel efficiency gains.
Which brings us to 2019. Ultra-low ambient air emissions and a 20% reduction in CO2 for trucks are milestone achievements, but large urban areas like Southern California are still struggling to meet clean air standards, and concern over global warming is only increasing pressure to escalate carbon reductions. Prodded by regional regulatory bodies like the California Air Resources Board (CARB), EPA responded in 2016 by proposing what it is calling Phase 2 of its GHG regulations. Starting in 2021, it would require heavy-duty tractors to cut CO2 tailpipe emissions by another 24% by 2027 and vocational HD trucks to drop CO2 production by 16%.
The focus on PM and NOx has not gone away either. With CARB leading the way in calling for even lower ambient air tailpipe emissions, late last year EPA released a proposed “Cleaner Truck Initiative” that would cut diesel NOx from its current 0.2 g/bhp-hr. level to 0.02. Details on a timetable for the reduction aren’t expected until 2020, when EPA says it plans to issue a final rule.
Despite deep reductions in diesel tailpipe emissions, trucks are still identified as a major source of both ambient air and GHG emissions. And that is the impetus behind growing calls for ZE trucks, calls that range from cautious proposals to fund pilot projects to those that want to spark a rapid and nearly total conversion of all commercial vehicles.
ZE is Electric
No matter what the gross vehicle weight, all ZE trucks are electric vehicles with an electric motor, or motors, rather than a diesel or other internal combustion engine. There are some potential advantages to electric propulsion beyond zero emissions. Electric motors have fewer moving parts than a diesel and don’t require multi-speed transmissions, which should reduce maintenance costs and improve reliability. They are also quieter, a major advantage in urban applications.
While ZE relies on a single propulsion technology—electrification—there are three different ways to store electrical “fuel” for vehicles, each with its own substantial drawbacks at this early stage of ZE development.
The simplest approach uses batteries. The first commercially available battery-powered trucks are starting to come onto the market, although in very limited numbers. All established truck manufacturers, as well as some highly publicized startups, are showing prototype battery trucks that range from Class 1 vans all the way up to Class 8 tractors.
Advances in battery technology since 2010 have reduced the cost of vehicle storage batteries by 80% and improved energy density by nearly 50%, but the battery arrays needed to provide a practical range for a truck are still costly and bring a large weight penalty compared with diesel. Depending on the application, a battery-powered truck must also be recharged daily, if not more frequently.
Rapid recharge stations, which are expensive, can top off batteries in an hour or two. Less costly plug-in facilities might require eight or more hours, depending on the size of the battery pack. In both cases, trucks have to be scheduled for extended downtimes. And pending legislation to exempt battery weight from GVW calculations, reduced payloads for battery ZE trucks are an issue.
Fuel cell technology produces onboard electric power through a chemical process fueled by hydrogen, potentially mitigating the weight and range issues of batteries. Hydrogen is a plentiful non-carbon fuel, and in liquid form hydrogen refueling times are similar to diesel. Nikola created a major publicity coup by announcing that it would offer a fuel cell Class 8 tractor. And far more quietly, Toyota has partnered with Kenworth to field test fuel cell tractors. Vehicle manufacturers in China are also working on developing fuel cell systems that can be mass-produced at lower cost.
Even with large investments in research over the last 20 years, vehicle fuel cells are still a work in progress. Current hardware costs are high but might reasonably be expected to come down with production volumes. However, building a fuel cell compact enough and rugged enough for daily duty in a working truck requires more development work. While the onboard technology is getting closer to a practical form, refilling truck hydrogen tanks presents a more serious challenge. An extensive electric grid already exists to bring power to battery recharging stations, but no similar infrastructure exists for delivering hydrogen to accessible refueling stations.
The third approach to electrifying a truck generates electric power onboard to augment a diesel or other traditional engine. Stored in a small battery pack, the generated electric energy can be used to drive a truck’s wheels under limited circumstances such as start-up or within urban areas, with the diesel taking over for longer range. Alternatively, a small generator driven off the diesel or driveline can electrically power accessories like air conditioning and hotel services to reduce parasitic loads and improve fuel efficiency. Compared to a plug-in battery technology, both hybrid approaches reduce the battery weight penalty and eliminate range limitations. Also, both are market ready.
The drawback to hybrids is they still require exhaust aftertreatment systems in addition to a separate electrical motor or generator and storage batteries. That adds both cost and complexity, as well as a weight penalty compared to a simple diesel powertrain.
Just to confuse things a bit more, there’s another category of trucks that are near-ZE. Essentially natural gas propulsion, they have exceedingly low air quality emissions, even today meeting the EPA’s proposed 0.02 g/bhp-hr. NOx limit. While lower than diesel, NG trucks still have GHG emissions, at least when measured at the tailpipe. Companies like Clean Energy, however, point out that using renewable natural gas made from waste goes beyond ZE to deliver an actual reduction in overall CO2 creation. The company also argues that such a near-ZE truck approach is already well developed and available today to address emissions concerns.
The winner will be…
There are many competing voices championing the different approaches to addressing truck emissions, each with a different perspective or agenda. While that may be causing some confusion among fleets about future truck purchases, one thing should be clear at this point—the societal pressure and political will are in place to eliminate truck emissions. The only questions are how and when.
Putting aside the timetable, the scenario for a transition to ZE trucks has started to become clear over the last few years. While the industry has relied on using a single power source—diesel—for decades, there will be no such silver bullet in the push for ZE. Instead, each of these technologies will find the right fit in different applications. The challenge for fleets will be to choose the right one for their particular operation.
Painting the big picture, Dr. Mihai Dorobantu of Eaton’s Vehicle Group divides ZE truck adoption into two categories.
Urban areas like Southern California and other regions with ozone issues are most concerned about NOx as an air quality and health problem. Diesel-powered trucks involved in city delivery and drayage produce a large amount of NOx, especially when they operate under low-load and low-speed conditions. Along with buses, these types of trucks typically run under 200 miles a day and return to a garage or terminal. Such characteristics address the range and recharging limits of the plug-in truck. Given the relative simplicity of a battery electric powertrain, these urban NOx-focused trucking applications will likely find plug-in ZE trucks cost-effective even if they’re not required by regulation, according to Dorobantu.
“Total cost of ownership drives [new technology] adoption,” said Mike Roeth, executive director of the North American Council on Freight Efficiency. “There’s a real chance that [battery plug-in trucks] will actually save fleets money. We’re optimistic that medium-duty box trucks and Class 8 day cabs will transition more quickly than many think.”
The second category Dorobantu sees is long-haul trucking, where the emissions focus is not air quality but rather CO2. “Long-haul transport accounts for 7% to 8% of the total energy consumed,” he said. “Today petroleum is the only source for that energy, so long-haul trucks are a significant source of carbon.”
Hydrogen fuel cells are the right ZE application for long haul, both for acceptable range with ZE and for the evolution already underway in how electric power is generated. The need to reduce overall carbon emissions is pushing electrical utilities to develop renewable sources like solar and wind power.
The problem is that unlike current generating plants that can scale production to changes in demand, such sources are intermittent generators of electricity dependent on environmental conditions. Creating hydrogen with that intermittent power generation provides a buffer to store it during peaks in generation, according to Dorobantu. “Hydrogen fuel cells are a long-term solution to long haul decarbonization and to the new intermittent generation [of electricity] with renewable sources.”
While Dorobantu believes fuel cell truck technology will soon be priced competitive with diesel, the real barrier that makes it more of a long-term ZE technology is building a completely new infrastructure for hydrogen fueling.
Right now, it seems that the most practical way to distribute hydrogen for truck fuels is as a cryogenic liquid.
“Those liquid hydrogen plants are expensive and difficult to permit,” said Erik Neandross, CEO of the clean technology consulting group Gladstein, Neandross & Associates. “There’s no clear picture at this point on how all of that will happen.”
The hybrid approach to electrifying trucks is sometimes seen as a bridge to trucks that are 100% ZE all the time. There’s some truth to that. Long haul, in particular, could use mild hybrid systems for fuel economy gains while hydrogen technology and infrastructure mature. But it may also play a longer-term role in certain applications.
One possibility for hybrids is emissions reduction in vocational trucks, trucks that aren’t a good application for full electrification. A hybrid transmission—one that combines an electric motor/generator with a multi-speed automated transmission—could at low speeds provide ZE operation and eliminate torque breaks while reverting to diesel at higher speeds where NOx emissions are lower, according to Dorobantu.
Another possible application for hybrid technology could use a small displacement engine possibly running on natural gas or propane as an onboard generator for an electric drivetrain, Neandross said. A smaller engine wouldn’t need an active aftertreatment system.
The combination would also reduce the amount of fuel carried onboard as well as require fewer storage batteries, further reducing weight while increasing range and eliminating plug-in recharging. “That could optimize each piece of the puzzle to meet both operational and environmental requirements,” Neandross explained.
The bottom line is trucks will have many ways to reach zero emissions—or at least approach that absolute. Determining which path best fits a particular application will be complicated, but it’s far from impossible.
The far larger challenge is plotting a reliable timeline for that transition. The technologies are evolving along a steady predictable path, but there are a multitude of regulatory forces that make it difficult to predict when ZE will tip over into trucking’s mainstream.
In a recent study titled “Reinventing the Truck,” consulting firm IHS Markit proposed two scenarios on ZE adoption, with a timeline out to 2040. The market scenario, which it called the Rivalry model, looked at adoption driven by market conditions, and the Autonomy model considered a stricter regulatory environment.
Largely because diesel engines continue to make significant gains in fuel economy, electric trucks face an increasingly difficult total cost of ownership target, according to Matt Trentacosta, one of the study’s authors. In the Rivalry model, the study sees diesel remaining the dominant truck powertrain for some time, accounting for 66% of all new U.S. truck sales in 2040. However, that’s down from nearly 80% today as it forecasts the electric truck market share to grow at 15% a year.
Initially, the IHS market model sees medium-duty, plug-in trucks accounting for almost all of that growth as payload and range are less of an issue there, especially in Classes 4-5. Based on expectations of steady fuel efficiency improvements, diesel is expected to remain competitive with all alternative fuels, including electric, in the long-haul heavy-duty segment. Because of the potential fuel efficiency benefits, some portion of that long-haul diesel market may include hybrid powertrains.
Fuel cells still have too many unknown costs to meet the diesel cost-of-ownership benchmark, but they could be expected to see higher adoption rates in the last decade of the study’s 2018 to 2040 market model, according to Trentacosta. Near-ZE with natural gas would also see some market-driven penetration in the short term as it performs well compared with diesel costs, he added.
The Autonomy scenario posits a far more aggressive approach from regulators aiming to force a shift from fossil fuels to non-carbon-producing alternatives. With some European cities and regions already proposing outright bans on diesels within city centers, this model sees more rapid adoption of ZE trucks in that market. Similar proposals are being considered in Southern California and other areas in the United States, though they seem to favor incentives over bans. With regulation overruling TCO, the adoption rate for ZE is unclear under the Autonomy model, but it anticipates natural gas with its near-zero tailpipe emissions finding only small adoption rates.
While either scenario is plausible, it’s more likely that it will be rapid ZE technology improvements, in combination with regulation and incentives, that will drive the change to electric powertrains for trucks. That means a mix of technologies to match a variety of applications, with TCO motivating adoption of ZE for some fleets and regulation the deciding factor for others.
In either case, zero emissions are part of trucking’s future.