Outlawing Diesel Trucks Makes No 'Green' Sense
No year in California would be complete without banning more stuff or pretending the world could run on pixie dust and unicorn farts, and 2022 was no exception. Among the latest targets California’s green czars have identified for elimination are diesel trucks, including the kinds that transport goods across long distances. These heavy-duty vehicles are known in the trucking industry as Class 7 trucks (gross vehicle weight between 26,001 and 33,000 pounds) and Class 8 trucks (gross vehicle weight greater than 33,000 pounds). Cal Matters has the details:
The California Air Resources Board held its first public hearing on rules that would ban manufacturers from selling any new fossil-fueled medium-duty and heavy-duty trucks by 2040. The new rules would also require large trucking companies to convert their fleets to electric models, buying more over time until all are zero-emission by 2042. The move is part of the state’s wider strategy to end its reliance on fossil fuels and cut planet-warming emissions.
The article notes that the weight of electric truck batteries could necessitate relinquishing thousands of pounds of cargo weight, requiring more trucks and drivers on the road. It also explains that California is ill-prepared for the transition to electric trucks because of the lack of charging infrastructure and generating capacity. Still, a number of manufacturers have already introduced Class 8 electric vehicles to the market, including Freightliner, Volvo, Kenworth, Nikola, Tesla, and Lion Electric. Undoubtedly, more will follow suit.
A serious question that should precede such a major decision is, does it make sense to deploy electrically powered trucks on a large scale over diesels, especially for long-haul use? Assuming the consequent increase in electric power demands are met and recharging infrastructure is built – hardly a small feat – there are still a number of other factors to consider, such as recharge time, range (on a full charge), economics (including battery replacement and cargo displacement due to battery size and weight), energy efficiency, and environmental impact. Proponents of electric vehicles concede that impact is sensitive to the way in which electricity is generated.
Big diesel trucks can carry 300-gallon fuel tanks and have an average range of over 2,100 miles. Refilling a diesel tank takes relatively little time compared to battery charging, which is prohibitively slow with standard electric charging. A fundamental problem with battery charging is the state of charge approaches full charge inverse exponentially. That means the battery achieves a partial charge quickly, but charging decreases proportionally to the state of charge, and a full charge can take many hours.
As a result, high-power fast direct current charging (DCFC) has been developed to mitigate the delay, but it is expensive and still not widely available. Recharge time is dependent on the truck range and charger power, but as an example, Kenworth states its T680E battery has a range of 150 miles and takes about 3 hours to recharge using DCFC. Bigger batteries with longer range and greater weight are available, but the charge time increases as well. Class 8 electric trucks currently fall short of the long-haul performance of diesels, both in range and delivery schedule.
The cost of achieving greater range in big trucks is heavy – literally. Here are typical range, power, and weight combinations:
- 235 miles (480 kWh) = 6,600 pounds
- 275 miles (565 kWh) = 7,768 pounds
- 350 miles (750 kWh) = 10,300 pounds
A diesel day-cab may weigh about 15,600 pounds, while a comparable electric day-cab with approximately 200 miles of range weighs about 22,000 pounds. A cab with 350-mile range weighs about 29,000 pounds without a trailer. In other words, a Class 8 electric cab with a fraction of the range and significantly longer refueling/recharge time is almost twice as heavy as a comparable diesel cab.
Since the premise for electric vehicles rests on reducing CO2 emissions, the next question to answer is, how well do they perform in that regard? A good first step is to measure the fuel efficiency of the two competing end-to-end models, i.e., from the fuel to the vehicle powerplant. Diesel efficiency is easier to assess, since the fuel is in the tank, and there are no real losses from tank to engine.
That leaves measurement of the efficiency of the diesel engine itself. According to a 2014 article, the typical diesel is able convert 52 percent of fuel energy into motion, but a recent breakthrough at the University of Wisconsin demonstrated a combination diesel-gasoline engine that runs cooler, pollutes less, and increases efficiency to 59.5 percent. The Department of Energy has set a goal of 55 percent brake thermal efficiency for big diesels, and Cummins has reported that it has achieved that goal. (The term “brake” refers to the motor’s/engine’s net power output, after internal losses such as friction, and “brake-specific” means relative to the engine’s net power.)
Now let us compare that to the electric vehicle model, whose total fuel efficiency, ηTotal , can be computed in the following way:
ηTotal = ηGeneration ∙ ηTrans-Distrib ∙ ηCharge ∙ ηDischarge ∙ ηMotor
- ηGeneration = power generation fuel efficiency
- ηTrans-Distrib = transmission and distribution efficiency
- ηCharge = battery charging efficiency
- ηDischarge = battery discharging efficiency
- ηMotor = electric motor efficiency
Charging and discharging efficiencies are dependent on temperature, state-of-charge, and current draw, but each can be approximated as 90 percent. Transmission and distribution efficiency is about 94 percent, and motor efficiency is around 95 percent. The combined result gives us a total fuel efficiency of electric truck motors from the power plant fuel source to the vehicle motor’s output as
ηTotal = 0.72∙ηGeneration.
Natural gas combined cycle generators are the most efficient electric generators. General Electric has achieved an astounding 63.9 percent efficiency with its natural gas combined cycle plants. Coal and ordinary gas-fired generators are much less efficient, with numbers between 36 percent and 40 percent. As a result,
0.72∙0.36 ≤ ηTotal ≤ 0.72∙0.64,
0.26 ≤ ηTotal ≤ 0.46,
which means the end-to-end fuel efficiency of electric vehicles is between 26 percent and 46 percent with carbon-based power generation. This is significantly less than state-of-the-art diesel engine fuel efficiencies in the 52 percent+ range.
An alternative comparison would be to calculate brake-specific CO2 mass per unit energy for electric truck motors and compare it to the accepted corresponding values for diesel engines. According to the U.S. Energy Information Administration, in 2021, carbon-based electrical generation produced about 1.338 pounds of CO2 per kWh, which is equivalent to 607 g CO2/kWh (relative to power at the generator output). Converting kWh at the generator to kWh at the motor output is equivalent to dividing 607 g CO2/kWh (generator) by 0.72, giving us a carbon-generated brake-specific 843 g CO2/kWh. Including carbon-free generation methods – currently about 38 percent of total generation – the average brake specific CO2 of electric trucks is diluted down to about 523 g CO2/kWh.
According to 2017 standards cited here and here, heavy diesel trucks must achieve an average brake-specific CO2 of 617 g CO2/kWh, which suggests that, the current average brake-specific g CO2/kWh for electric trucks is only 15 percent less than it is for diesels. It should be recognized, however, that brake specific CO2 is only a part of the analysis. The complete life-cycle fleet-CO2 output is a more accurate picture.
That's because enormous amounts of CO2 are generated in the production of electric vehicle batteries, and the increased battery weight, as noted above, means electric trucks carry significantly more overhead weight. Consequently, cargo pounds per kWh expended is reduced, and more trucks are required to transport the same amount of goods. Considering these additional factors, the advantage of electric trucks over their diesel counterparts is essentially nonexistent. U.S electric power generation would have to get significantly more carbon-free for electric trucking to live up to its billing as “green.”
To make matters even worse, the safe design of lithium ion batteries relies on cobalt as a key component, and the electric vehicle industry is experiencing public relations heartburn because of the awful labor practices and environmental problems of cobalt mining in Africa. Considering the additional upfront costs of the trucks themselves, their relatively short range, and the expansion in power generation and charging infrastructure that would be needed to support electric trucking on a large-scale, the present case for major investment in heavy-duty electric trucks by the long-haul trucking industry is hollow. Whether or not California ultimately bans diesel trucks, the other 49 states would be wise not to follow its lead.
Note: The efficiency analysis to compute ηTotal was kindly provided by engineering Professor Sage Kokjohn of the University of Wisconsin.