Molly Ayala
Several factors contribute to the absence of diesel-electric trucks, despite the existence of diesel-electric trains and buses. Firstly, weight and space constraints in trucks make it challenging to accommodate both diesel and electric motors, which are heavy. Secondly, diesel engines and electric motors operate in a similar power band of low rpm and high torque, making diesel engines less suitable for traditional hybrid setups. Gas motors, on the other hand, can complement electric drive at high speeds and rpm. Additionally, diesel powertrains are well-established, reliable, and predictable, making them a preferred choice in the logistics industry. Furthermore, the weight of electric motors and batteries in hybrid trucks reduces payload capacity, which is a significant consideration in the trucking industry. While some companies are experimenting with diesel-electric prototypes and alternatives, the adoption of diesel-electric trucks has been slow, with concerns about weight, complexity, and the need to balance cargo load and efficiency.
| Characteristics | Values |
|---|---|
| Power-to-weight ratio | Diesel and electric motors are heavy, and the smaller platform of a pickup truck does not allow much room for both |
| Efficiency | Diesel motors already operate at an efficient rev range, so there is little to gain by decoupling the motor from the drivetrain |
| Hybrid setups | Diesel engines and electric motors have a similar powerband of low rpm, high torque applications, so gas motors are more suitable for traditional hybrid setups |
| Public perception | The general public perceives cars and trucks as major pollutants, and will vote for cleaner emissions, but diesel-electric trucks are yet to be prototyped |
| Weight | Trucks are capped by weight, and adding another 1000lb+ in motors and batteries reduces payload capacity |
| Complexity | Diesel powertrains are simple, well-understood, reliable, and predictable, whereas electric motors are complex |
| Cost | Batteries are more expensive, with Tesla batteries costing around $15,000 |
| Regenerative braking | Electric motors offer massive regenerative braking potential, which is beneficial for downhill driving with heavy loads |
| Maintenance | Electric motors do not require transmission, reducing maintenance costs |
| Fuel economy | Fuel economy would increase with diesel-electric trucks |
| Emissions | Emissions would decrease with diesel-electric trucks |
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What You'll Learn
- Diesel engines and electric motors have a similar powerband of low rpm, high torque applications
- Diesel powertrains are reliable, predictable, and well understood
- Electric motors are heavy, leaving less room for other components
- Electric vehicles are viewed as the biggest pollutants
- Electric motors are more efficient, but also more expensive
Diesel engines and electric motors have a similar powerband of low rpm, high torque applications
Diesel engines and electric motors share a similar powerband of low rpm, high torque applications. The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor can output the most power, i.e., the maximum energy per unit of time. This usually means that maximum acceleration can be achieved inside this band. Diesel engines and electric motors are both known for having high torque at low rpm, which may leave high rpm with little torque.
The largest "low-speed" diesels, such as large generators on land and marine diesels at sea, turn at only hundreds of RPM or even below, with idling speeds of 20-30 RPM. Electric motors are unique in many ways, especially concerning the power band. The exact characteristics vary greatly with the type of electric motor. The maximum torque of a universal motor occurs at a zero rotation rate and falls for higher RPM. For induction motors, the maximum torque is usually just below the synchronous RPM, sinking to zero at this RPM.
Diesel engines are rarely ripping at high rpm. They are almost always down low, chugging along at a fairly stable RPM. A typical transmission and transfer case will be much cheaper (and probably lighter) than the large electric motors needed to pull HD trucks. The technology is available for trains and boats because the immense size allows for huge diesel turbines and generators.
There are several advantages to diesel-electric heavy-duty pickup trucks. A diesel engine powering a generator could stay in its optimal RPM range almost the entire time, significantly increasing efficiency. Instant torque from electric motors would be excellent for towing, and there would be no need for a transmission, so no transmission losses or maintenance costs. There would also be massive regenerative braking potential.
Despite these advantages, there are several reasons why diesel-electric trucks are not common. Firstly, diesel motors and electric motors are both heavy, and the smaller platform of a full-size pickup truck doesn't allow much room for both. Secondly, semis already operate at single-digit mpg (like 3-5), so there's not much to gain by decoupling the diesel motor from the wheels. Finally, diesel powertrains are extremely reliable, well-understood, and predictable, and logistics typically favour the safe, predictable bet.
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Diesel powertrains are reliable, predictable, and well understood
Diesel engines are well-suited for forced induction setups due to their wide ignition limits and the absence of fuel during the compression stroke. This means that knocking, pre-ignition, or detonation cannot occur, and a lean mixture caused by excess supercharging air inside the combustion chamber does not negatively affect combustion. They have a high compression ratio, which greatly increases the engine's efficiency.
Diesel engines have been used in the trucking industry for a long time, from dump trucks to pickup trucks and utility vehicles. They are also used in first responder vehicles due to their reliability and durability. Diesel engines are also commonly used for construction equipment and agricultural machinery, where fuel efficiency, reliability, and ease of maintenance are important.
While there are companies trying to implement electric semi trucks, the transition to electric powertrains is still ongoing. Diesel engines are deeply integrated into our daily lives and continue to be a popular choice for many industries due to their reliability and predictability.
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Electric motors are heavy, leaving less room for other components
One of the main reasons why diesel-electric trucks are not common is the issue of weight and space. Electric motors are heavy, and when combined with the weight of diesel engines and other components, the overall weight of the truck becomes a significant challenge. The weight of the electric motor reduces the available payload capacity, which is a crucial consideration for truck operators who need to maximise cargo loads.
The weight of the electric motor also affects the power-to-weight ratio, which is essential for the performance and efficiency of the vehicle. A heavy electric motor can reduce the overall power available and impact the truck's ability to handle steep grades and aerodynamic efficiency. The available space in a truck's platform is limited, and accommodating both a diesel engine and an electric motor, along with other necessary components, presents a design challenge.
While some companies have attempted to create diesel-electric prototypes, the weight of the electric motor remains a concern. The electric motor weight can be offset by eliminating the transmission, torque converter, transfer case, differentials, and driveshafts, as seen in some diesel-electric designs. However, the overall weight of the vehicle still needs to be carefully managed to ensure it adheres to weight regulations and does not compromise the truck's performance and efficiency.
Additionally, the weight and size of the battery packs required for diesel-electric trucks can be substantial, further adding to the weight and space constraints. Improvements in battery technology are being made, but currently, batteries contribute significantly to the overall weight of electric vehicles, and finding sufficient room for them in a truck's design can be challenging. The cost of batteries is also a factor, as they can be expensive, impacting the overall feasibility of diesel-electric trucks.
Despite the challenges posed by the weight and space requirements of electric motors, some companies are exploring alternative solutions. For example, Wrightspeed has developed a prototype that uses an electric drive train with a battery pack supplemented by a small turbine engine generator. This design targets municipal utility vehicles with specific route requirements, demonstrating that innovative approaches can be explored to address the limitations imposed by electric motor weight and space constraints.
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Electric vehicles are viewed as the biggest pollutants
Electric vehicles (EVs) are often viewed as a planet-saving technology, but some argue that they are not as environmentally friendly as they are marketed to be. EVs are about 30% heavier than gas-powered vehicles, which causes the brakes and tire treads to wear out faster, releasing tiny, often toxic particles into the atmosphere. EVs also require charging, and the power sources that charge their batteries commonly emit carbon dioxide. According to the Institute for Energy Research, most electric vehicles are charged using power generated by fossil fuels. The power demand of large-scale EV adoption may require major upgrades and expansions to the electricity grid.
The manufacturing of EVs also has environmental impacts. For example, the production of EV batteries can be energy-intensive and rely on carbon-emitting power. Some studies have shown that making a typical EV can create more carbon pollution than making a gasoline car due to the additional energy required to manufacture an EV's battery. Additionally, the extraction of materials for EV batteries, such as cobalt, can have extreme human and environmental costs. The disposal of EV batteries and their components also presents a risk to the environment as they can release problematic toxins, including heavy metals, into landfills or groundwater.
However, EVs have zero tailpipe emissions, and research shows that over the lifetime of the vehicle, total greenhouse gas emissions associated with manufacturing, charging, and driving an EV are typically lower than those associated with a gasoline car. The amount of carbon pollution generated by charging EVs varies depending on how local power is generated, such as using coal or natural gas versus renewable resources like wind or solar. As more renewable energy sources are used to generate electricity, the total greenhouse gas emissions associated with EVs could be reduced further. Recycling EV batteries can also help reduce the environmental impact of EV manufacturing by lowering the need for new materials.
While there are valid concerns about the environmental impacts of EVs, it is important to recognize that they are not the biggest pollutants. The perception that cars and trucks are the biggest pollutants may be due to their visibility compared to other sources, such as cargo ships or cruise ships. Additionally, the weight and space constraints of pickup trucks may make it less feasible to implement diesel-electric systems, which are more commonly found in trains and buses.
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Electric motors are more efficient, but also more expensive
Electric motors are more efficient but also more expensive than their diesel counterparts. Electric motors are heavier than diesel motors, and the smaller platform of a full-size pickup truck does not allow much room for both. Electric motors are more efficient at low rpm, high torque applications, while diesel engines are more efficient at higher speeds. Diesel engines have a narrow range of rpm where they are most efficient, whereas electric motors have a wider range.
Electric motors can be more efficient because they do not require a transmission, which can result in power loss. They also have regenerative braking capabilities, which can reduce brake wear and tear. Additionally, electric motors can be equipped with steering and control mechanisms, such as variable-speed drives, that can further enhance their efficiency.
The efficiency of electric motors can be improved by using materials such as copper, which has lower resistance losses, and optimizing the coil quality and design. However, these improvements come at a cost, as copper is more expensive than other materials. The manufacturing process for high-efficiency motors also tends to be more costly due to the use of premium materials and design modifications.
The higher cost of electric motors can be offset by their lower life cycle costs, resulting in shorter payback times. However, for vehicles with very low annual running hours, the payback period may be longer. The weight and size of the batteries required for electric motors can also be a challenge, especially for large vehicles like trucks.
While electric motors offer higher efficiency, the adoption of diesel-electric trucks may be hindered by the higher upfront cost of electric motors and the challenge of fitting both diesel and electric motors in the limited space of a truck platform.
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Frequently asked questions
Diesel motors and electric motors are both heavy, and a full-size pickup truck doesn't have enough space for both.
A diesel engine powering a generator could stay in its optimal RPM range almost the entire time, significantly increasing efficiency.
Diesel-electric trucks would have lower payload capacity and would be more complex and expensive due to the addition of motors and batteries.
Yes, a company called Wrightspeed has prototyped an electric drivetrain with a battery pack supplemented by a small turbine engine generator for power on demand. Another startup called Edison Motors is also working on a diesel-electric prototype.
