Hybrid electric trucks are very different from HEV cars

[ This document explains why it is hard to transfer auto hybrid technology to trucks.  They are entirely different animals — medium-duty trucks weigh up to 10 times more, have up to 10 times the horsepower, and a far longer life-expectancy, and therefore medium-duty truck hybrid technologies need to be 10 times more durable. Hybrid batteries for medium-sized trucks are far behind batteries developed for autos, which are mass produced.  Trucks are usually custom-built for their specific purpose, and therefore don’t have the same economies of scale as mass-produced cars.

Hybrid electric trucks are only suitable for medium-duty trucks that stop and start a lot, mainly delivery and garbage trucks. 

Alice Friedemann   www.energyskeptic.com  author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts:  KunstlerCast 253, KunstlerCast278, Peak Prosperity]

NRC. 2008. Review of the 21st Century Truck Partnership. National Research Council, National Academy of Sciences. 131 pages.

Heavy-duty hybrid Vehicles  

Despite the emerging presence of hybrid electric technology in the passenger car industry (Toyota Prius and Honda Insight/Civic), heavy-hybrid technology for commercial trucks and buses needs significant research and development (R&D) before it will be ready for widespread commercialization.

There is a common perception that investments in passenger car (light-duty [LD] vehicle) technology benefit Heavy-Duty (HD) trucks.

This is not entirely true. First, LD vehicles (including trucks) fall into Classes 1 and 2a, which contain passenger cars, light trucks (such as the GMC/Chevy 1500 series pickup truck), minivans, and most SUVs. HD trucks are everything else—all vehicles that exceed 8,500 lb GVW, which are Classes 2b–8. This group of vehicles is very diverse and includes tractor-trailers, refuse and dump trucks, package delivery vehicles (e.g., UPS and FedEx), buses (e.g., city transit, school, shuttle, para-transit/demand response).

Heavy duty trucks are very different from autos:

  • A heavy-duty truck weighs 2–10 times more
  • Heavy-duty trucks have 2 to 10 times the horsepower
  • Burn 3 to 4 times more fuel per mile driven
  • The life expectancy and duty cycles for HD vehicles are about 10 times more demanding than those for light-duty vehicles.

Therefore, heavy-duty hybrid technologies and solutions must be about 10 times as durable as those being developed for light-duty hybrid applications.

HD truck and LD vehicle technologies and corresponding investments in them leverage each other only at the most basic level.

Bringing complex commercial products, such as HD hybrid propulsion systems, to market can cost $500 million to $1 billion per company and take as long as 10 years.

Comparison of Heavy-duty and Light-Duty vehicles  

Heavy Duty Light Duty trucks & cars
Weight 8,500-200,000 lbs < 8,500 lbs
Peak horsepower 150-600 70-300
Continuous horsepower 100-600 25-60
Annual mileage 20,000-250,000 8,000-20,000
Expected lifetime 400,000-1,000,000 miles 150,000 miles
Purchase price $60,000-$250,000 + $12,000-$40,000
Number of configurations Millions Thousands
Fuel of choice Diesel Gasoline
Fuel consumption 5-15 MPG 14-40 MPG


Industry/market characteristics that are considered barriers include low truck market volumes, high R&D costs, challenging reliability requirements, minimal technology crossover from cars, and razor thin margins in the trucking industry.

2.9. Component-specific barriers

Energy Conversion Technology Barriers

For hybrid electric propulsion systems, most components were not designed or optimized for use in on-road HEVs. Electric components can be costly because precision manufacturing tools are needed to produce the components, and production volumes are low. A new generation of components is needed for commercial and military HEVs. Electric motors, power electronics, electrical safety, regenerative braking, and power-plant control optimization have been identified as the most critical technologies requiring further research to enable the development of higher efficiency hybrid electric propulsion systems. The major barriers associated with these items relate to weight and cost reduction.

The major barriers to introducing hybrid electric drive units for HD trucks include system (life cycle) cost, system reliability, and system durability. Safety concerns and system complexity as they relate to maintenance are also issues. The rigorous duty cycles and demands placed on HD vehicles necessitate a high degree of component reliability. In the lower volume market of heavy hybrid vehicles, cost reduction will be a challenge.

Power electronics. The barriers for introducing improved power electronic systems for truck applications are the cost, complexity, reliability, and the operating environment. Current power electronic converters and motor controllers that meet size and weight requirements are not rugged or reliable enough for 500,000-mile vehicle lifetimes and harsh trucking environments.

Other barriers are thermal management systems for fast, energy-efficient heat removal from device junctions and components, control of electromagnetic interference generated when the devices are switched, and achieving a low-inductance package for the power inverter. Generally, silicon operates too cold for efficient heat removal, and silicon carbide is a preferred technology for more efficient heat removal. The task of packaging power electronics to satisfy the multiple extreme environments and ensuring reliable operation with proper function is a barrier. (The packages that are available are generally not suitable for vehicle applications.) Additionally, there are no domestic suppliers for high-power switch devices. This must be corrected.

Power Plant and Control System Optimization Barriers. Most components used in today’s hybrid vehicles are commercially available. However, they are not optimized for on-road heavy hybrid performance. Electric components can be costly to produce and have low production volumes. Hybrid propulsion components are high weight and high volume. Integrated generator/motors need higher specific power, lower cost, and higher durability.

Safety risks may be higher for prototype HEVs that have not been subjected to rigorous hazard analysis.

Heavy-duty hybrid trucks will have improved fuel economy and potentially significant reductions in emissions. An HEV seeks to recover as much of the braking energy as possible to recharge the battery. If the battery system has insufficient ability to be rapidly charged, the friction brakes will be used and significant energy will be lost to heat.

The equipment must have a payback period of less than 2 years and be sufficiently rugged and durable to perform reliably during the full design life of the truck in bad weather to be successful.  So far hybrid trucks have been held back by the high costs, inability to meet the $50/kW goal, no significant progress toward achieving the desired reliability target of 15 years design life for the hybrid propulsion powertrain equipment, and the limited energy storage capacity of hydraulic accumulators constrains the usefulness of hybrid-hydraulic technology in heavy-duty trucks primarily to those with significant start-stop duty cycle requirements, such as refuse trucks (Gray).

The ideal electrical energy storage system for heavy-duty hybrid trucks would have the following characteristics:

  1. High Volumetric Energy Density (energy per unit volume)
  2. High Gravimetric Energy Density (energy per unit of weight, Specific Energy)
  3. High Volumetric Power Density (power per unit of volume)
  4. High Gravimetric Power Density (power per unit of weight, Specific Power)
  5. Low purchase cost
  6. Low operating cost
  7. Low recycling cost
  8. Long useful life
  9. Long shelf life
  10. Minimal maintenance
  11. High level of safety in collisions and rollover accidents
  12. High level of safety during charging
  13. Ease of charging method
  14. Minimal charging time
  15. Storable and operable at normal and extreme ambient temperatures
  16. High number of charge-discharge cycles, regardless of the depth of discharge
  17. Minimal environmental concerns during manufacturing, useful life, and recycling or disposal

Unfortunately, every commercially viable battery technology being pursued must trade-off compromises of these attributes.

[My note: as I explain in Who Killed the Electric Car?, every time a battery is tweaked to improve, say, #1 (volumetric energy density), it could harm one or more of the other 16 parameters. It can take months of testing to find out which, if any, of the other properties were changed.   This is why it takes about 10 years to bring a new battery to market.]

The optimal electrical energy storage system for a given application will highly depend on the weighted values of these attributes as they relate to the specific application.

Battery-only electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in HEVs (PHEVs) have distinct requirements. An EV developer might place the highest priority on an energy storage system that has the highest energy density or specific energy, to assure maximum range between charges for a given size of system. The instantaneous power available would likely be less important than mileage or range to the EV developer, but the priorities would be reversed for the HEV developer. Systems with higher energy capacity also tend to have higher available power for acceleration but with more mass than is desired for HEV applications.

The EV developer might also interpret system safety and environmental concerns differently from an HEV developer. Because a battery-only vehicle usually has a much larger battery than an HEV, and because it carries more electrical energy and caustic chemicals on-board, it may carry higher battery-related safety risks than an HEV with a smaller battery. However, the HEV includes an internal combustion engine (ICE) that carries additional safety risks associated with its energy storage system (i.e., gasoline fuel tank) that drivers of conventional ICE-based vehicles have lived with for many years.

Because regenerative braking is a primary method to charge the battery in an HEV, the efficiency is critically important to an HEV’s performance characteristics. Because the electric motor is also used significantly to assist the internal combustion engine during acceleration, specific power and power density will become important considerations. PHEVs have battery energy storage characteristics that can have more in common with either typical EV or HEV requirements, dependent on whether the PHEV powertrain design is dominated by the electric motor or by the internal combustion engine.

Battery Technology for heavy-duty applications

Unique challenges exist for the application of energy storage components in heavy-duty hybrid trucks, including batteries, ultra-capacitors, hydraulic accumulators, or flywheels. Light-duty EVs and HEVs focus on energy capacity for long battery range, or rapid power charging and discharging capabilities for acceleration and braking energy recovery, or a combination of both.

It is currently impractical for heavy-duty vehicles and trucks to carry sufficiently large battery packs or electric power sources (e.g., fuel cells) to provide the required power levels for an all-electric powertrain.

Therefore, vehicle manufacturers and researchers are focusing on hybrid powertrains based on diesel-electric architectures that require batteries with high power capability to assist in vehicle acceleration, rapid charging, and efficient recovery of braking energy.

The charge rate and level of charge acceptance needed to maximize the capture of braking energy in a heavy-duty vehicle is much greater than the comparable requirements for a light-duty vehicle, due to the difference in vehicle mass and inertia. A popular way to reach the higher power capacity required for heavy-duty truck applications is to over-size the battery. For light-duty hybrid vehicles there are storage systems available with sufficiently high charge rates that avoid the need to over-size the battery. Over-sizing the energy storage system to obtain the necessary power capacity is undesirable in several regards including the unnecessary expenses of additional mass, volume, and heightened environmental and safety concerns.

The additional mass in the heavy-duty vehicle makes them less practical as battery-only EVs due to the required battery size for reasonable performance, given the current state of the art. DOE EV efforts are only being made for cars and light-duty trucks because of this.

Battery life is critically important to avoid the replacement of the energy storage system before the end of the useful life of the vehicle which would represent a very significant repair/replacement cost and increase the recycling challenges. The need to replace the energy storage system once in a vehicle’s life would more than double its effective cost. Therefore, the goal of achieving battery lifetimes that match or exceed that of the vehicle may be necessary for owner acceptance in large volume production. Capacity Goals. The FreedomCAR energy capacity goals of 300 Wh and power capability goal of 25 kW for 18 seconds may be appropriate for the anticipated battery-only range of a light-duty HEV, but they may fall short of the needs for heavy-duty HEVs, unless two or more of the target battery packs are used for the application.

Safety remains a significant issue for Li-ion battery systems. Overcharging, fast charging, fast discharging, crushing, projectile penetration, external heating, or external short-circuiting, can cause the battery pack to heat up. If heat generation exceeds heat dissipation capability, thermal runaway can occur. Elevated temperatures can cause leaks, gas venting, smoke, flames, or even “rapid disassembly” to occur. Intelligent monitoring and control of the charging and discharging processes is being developed to manage many of the concerns associated with thermal runaway. However, vehicle collisions and projectiles that can cause the battery case to be breached are inspiring the need for new construction materials that are less prone to mechanical and thermal issues.

It is clear that the capabilities needed for heavy-duty use may differ significantly from light-duty applications. it is important to recognize that heavy-duty trucks experience a much wider range of driving cycles than passenger vehicles or light-duty trucks. For example, a Class 6 urban delivery van experiences typical driving cycles that are much different from those of Class 8 long-haul commercial trucks. Because large numbers of accelerations and braking decelerations associated with truck applications such as delivery vans or refuse trucks are well-suited to demonstrating the advantages of hybridization, most of the 21CTP-funded development of hybrid trucks has been focused on these applications.

Adding batteries will make trucks heavier.  A fully loaded tractor-trailer combination can weigh up to 80,000 pounds. Reduction in overall vehicle weight could enable an increase in freight delivered on a ton-mile basis. Practically, this enables more freight to be delivered per truck and improves freight transportation efficiency. In certain applications, heavy trucks are weight-limited (i.e. bulk cargo carriers), and reduced tractor and trailer weight allows direct increases in the quantity of material that can be carried. New vehicle systems, such as hybrid power trains, fuel cells and auxiliary power will present complex packaging and weight issues that will further increase the need for reductions in the weight of the body, chassis, and power train components in order to maintain vehicle functionality. Material and manufacturing technologies can also play a significant role in vehicle safety by reducing vehicle weight, and in the improved performance of vehicle passive and active safety systems. Finally, development and application of materials and manufacturing technologies that increase the durability and life of commercial vehicles result in the reduction of life-cycle costs.

Making trucks lighter

The principal barriers to overcome in reducing the weight of heavy vehicles are associated with the cost of lightweight materials, the difficulties in forming and manufacturing lightweight materials and structures, the cost of tooling for use in the manufacture of relatively low-volume vehicles (when compared to automotive production volumes), and ultimately, the extreme durability requirements of heavy vehicles. While light-duty vehicles may have a life span requirement of several hundred thousand miles, typical heavy-duty commercial vehicles must last over 1 million miles with minimum maintenance, and often are used in secondary applications for many more years. This requires high strength, lightweight materials that provide resistance to fatigue, corrosion, and can be economically repaired. Because of the limited production volumes and the high levels of customization in the heavy-duty market, tooling and manufacturing technologies that are used by the automotive industry are often uneconomical for heavy vehicle manufacturers. Lightweight materials such as aluminum, titanium and carbon fiber composites provide the opportunity for significant weight reductions, but their material cost and difficult forming and manufacturing requirements make it difficult for them to compete with low-cost steels.

Vehicle Corrosion. Many lightweight materials and light weighting approaches cannot be used in commercial vehicles because of significant corrosion and maintenance issues. Corrosion is a significant contributor to the cost of maintenance of heavy vehicles. Research is needed to develop materials that are resistant to both general and galvanic corrosion. Low-cost, durable coatings are needed.

Accidents involving large trucks and buses create significant delays on our highways, particularly in congested areas. During these delays, there are increases in fuel usage due to travel at low speeds and while sitting in traffic at idle. There is a corresponding increase in tailpipe emissions during these times. In some cases, the accidents involve vehicles carrying hazardous materials, creating an even more dangerous situation, and in certain cases, potential issues related to national security. Of course, accidents also contribute to costs associated with lost work time by commuters. Indeed, highway congestion, even in the absence of an accident, is a serious problem in the United States and in many large cities around the world. The Texas Transportation Institute (TTI) tracks congestion data for the 85 largest cities in the United States (http://tti.tamu.edu/). According to TTI, in 2003, in the combined total of the 85 cities, there was travel delay of about 3.7 billion hours, associated with which there was excess fuel consumption of 2.258 billion gallons of fuel. Elements contributing to congestion include heavy traffic, highway construction and repair, and roadway incidents including accidents (Texas Transportation Institute, 2007, Table 2).

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