The case for Class 8 hybrids

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  • Image: Payback Analysis from Fleet Viewpoint.jpg
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  • Image: Mild vs. Heavy Hybrids table.jpg
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  • Image: WalMart dual-mode hybrid.jpg
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  • Image: SR traction motor torque vs. IPM machine.jpg
Image: Coca-Cola class 8 hybrid electric.jpg

During a 13-month study conducted by NREL with Coca-Cola, the Class 8 hybrid-electric tractors—equipped with a parallel hybrid system from Eaton—demonstrated 13.7% higher fuel economy than the conventional tractors, resulting in a 12% reduction in fuel costs for the hybrids.

The conventional wisdom is that Class 8 vehicles have been so well optimized for their vocation that there is little opportunity for generating significant benefits with hybrid technology. The high power levels and long ranges required of these vehicles make a pure battery-electric implementation impractical.

Series-electric powertrains have performed well in transit bus applications; however, for line-haul truck applications, series powertrains fall short in efficiency compared to conventional mechanical powertrains that have been well optimized for operation over a relatively narrow speed and load range.

If there is potential for hybridizing these vehicles, a parallel hybrid solution appears to be the best approach, according to Vern Caron of Caron Engineering Inc. But success requires careful attention to the business case and solutions to a number of difficult engineering problems.

The business case

The business case for hybridizing heavy vehicles is not based on the large percentage improvements in fuel consumption often cited for light and medium vehicles. Instead, it results from the extremely high annual mileage and high fuel consumption typical of these vehicles. Even with relatively small gains in fuel economy, there is potential value in applying hybrid technology to large trucks because of the large number of miles traveled with relatively high fuel consumption.

Of course, caution should be used in making estimates citing the value of hybridization. Those skilled in this technology will immediately volunteer that actual gains in fuel economy vary widely and are hugely dependent on the operating duty cycle of the vehicle. For this study, Caron focused on the merits of implementing hybrid Class 8 vehicles.

A great deal of effort has been put into modeling and simulation to evaluate the benefits of various hybrid architectures and to assist with sizing hybrid drivetrain components. Some of the better known packages are Autonome, Advisor, Cruise, Raptor, and GT drives. Many of these packages are overlays on MATLAB/Simulink.

Heavy-vehicle OEMs, their drivetrain suppliers, and several of the national labs including Oakridge and Argonne are heavily involved in vehicle performance modeling and simulation. Depending on the assumptions made, results sometimes differ. However, there is good evidence that improvements in the following areas can provide a 15 to 20% improvement in fuel economy for a heavy hybrid:

• Regenerative braking energy recapture

• Anti-idling, including hotel load management

• Accessory drive optimization

• Engine operating regime optimization

The problem is determining whether it is possible to provide such a system at an acceptable profit margin, considering the following assumptions: fleets expect payback in 24 to 36 months, typical cost of capital is prime plus 2%, and residual value is a key factor.

In a payback analysis, tax credits can be factored in, if available, to permit a higher upcharge. An acceptable upcharge can be passed down the supply chain to arrive at an acceptable cost of components at the Tier 1 supplier level. To back calculate acceptable supplier component costs, typical parameters are:

• Major fleets expect to purchase vehicles at a 20% discount from list price.

• Dealers typically obtain vehicles at a 25% discount from list price.

• Dealers typically set list price at 2.2 x cost.

• Suppliers typically expect a 30 to 40% gross margin on high-value-add components.

• Pass-through components with little value-add may have a margin of 10%.

• An add-delete analysis must be included. Depending on whether supplier or competitor components are being replaced, this will either result in additional value added or cannibalization of existing products.

Generally low-volume, high-tech products will often make for a difficult business case. Given the need to meet quarterly profit expectations, management is often reluctant to provide the required engineering investment for success or lose interest when immediate profitability is not forthcoming. However, companies with more astute management realize that long-term profitability will not result from selling commodity products.

Hybrid architectures for heavy vehicles

A simple way to architect a parallel hybrid is to add a motor/generator between the transmission and the engine. Depending on the size of the machine and the associated energy-storage element, the system can potentially provide functionality such as recapture of braking energy (regeneration), anti-idling, and engine-off power takeoff operation.

If only a single electric machine is present, it cannot be counted upon to provide motive power to the vehicle since there will be times when there is no source of electrical energy available to power the machine. Because the diesel engine must be counted upon to provide motive power over the entire operating range, requiring a conventional transmission, these systems are often referred to as transmission-centric hybrids. Since the electric machine is located forward of the transmission, a permanent magnet motor (with its typically narrow constant power range) may be successfully used in this application.

Transmission-centric parallel hybrids have shown some success in medium-duty applications and especially in vocations such as bucket trucks and refuse packers. In these applications, which feature heavy stop-and-go operation or idle operation, fuel-economy improvements on the order of 40% are often cited. However, when these same systems are applied in Class 8 applications, improvements of 3 to 6% are typical.

Occasionally, various post-transmission and electric axle concepts surface. There are a number of possible configurations, including motor mounted on frame in center of a two-piece driveline, individual motors mounted at wheel ends of a pure electric axle, motor on carrier of single axle, motor on forward carrier of tandem axle, and electric-powered tag or pusher axle.

Proponents of other e-axle configurations intended for mild parallel hybrid use have yet to show that these systems provide equivalent or better functionality than pretransmission hybrids. Advocates of these post-transmission hybrids often cite ease of retrofit as a primary advantage.

Another alternative is a heavy hybrid system, such as a dual-mode system, so designated because of the ability to operate in either series mode (at lower vehicle speeds, e.g. up to 50 mph) or parallel mode (at highway speeds). These systems employ several large e-machines, typically designated as a generator and a traction motor. These large electrical components permit elimination of some of the traditional mechanical components, including automatic transmission, clutches, and starters.

With greater availability of LNG and CNG, there is strong interest in making use of gas in Class 8 operations. One option is the use of Cummins engines with Westport conversions. Another interesting option may be the use of a new class of economical turbine engines currently under development. These engines are multifuel capable and lightweight. When coupled with a heavy dual-mode hybrid system, they are capable of direct drive operation at highway speeds with a single reduction transmission (no shifting necessary). Their high-speed operation is nicely suited to generation operation in series mode.

Sizing components for a heavy hybrid

A good procedure for sizing a heavy hybrid system is to begin with an understanding of the conventional vehicle. Specifically, one needs to understand the torque and power requirements under worst-case grade and load conditions. It is generally known that only about 37% of the energy in a gallon of fuel is converted to power at the crankshaft; the remaining energy is lost as heat.

Some of the mechanical power actually produced is also lost in operating accessory components such as the engine cooling fan. Other losses can include tire rolling resistance, aerodynamic or drag losses, potential energy losses due to climbing and descending grades, and kinetic energy losses resulting from vehicle acceleration and deceleration.

OEMs have performance expectations for vehicles depending on their vocation. To understand the performance of a hybrid system, such as a dual-mode system, it is useful to over-plot the power curves for the baseline conventional drivetrain, as well as the proposed hybrid system on the requirement curves, and to check performance at specific operating points, such as startability on a 15% grade at 80,000 GCW.

The energy in a fully loaded Class 8 truck is on the order 3.9 kW·h. This can be a useful parameter in sizing the battery packs for the vehicle provided that 1) the battery packs have adequate power density and 2) the e-machines have sufficient power capability to capture the available kinetic energy.

For a Class 8 truck, a maximum performance stop (250 ft/76 m) requires approximately 5 s and develops a deceleration level of approximately 0.5 g. Extensive data indicate that the vast majority of stops occur at 0.1 g or less. Even at a 0.1 g deceleration rate, it is difficult for even the relatively large e-machines used in heavy hybrid systems to recapture the available energy.

Because of grade requirements, it is generally not practical to downsize the diesel engine power rating in dual-mode systems. However, it may be possible to switch to a lighter-duty engine with the same power rating (e.g., switch from a 15-L to a 13-L engine). The traction motor should be sized to provide the same power level as the diesel engine can provide to the wheels. The traction motor should be sized to provide peak torque roughly equivalent to the torque available from the conventional drivetrain when operating in second gear. A two-speed transmission should be provided that permits operation at best BSFC (brake specific fuel consumption) in high gear at 60 to 65 mph (97 to 105 km/h). The generator should be sized to provide 5% more power than the traction motor power rating. This should not exceed the engine power rating.

Concerning energy storage, the battery packs for a heavy hybrid should be sized with the following considerations in mind:

• The batteries should be able to absorb the peak power that can be supplied by the e-machines for 5 s.

• The batteries should be able to absorb the continuous power available from the e-machines for at least 30 s and more ideally, indefinitely.

• The batteries should be able to provide peak power to the traction motor for vehicle acceleration for at least 30 s and ideally for 1 min.

• The batteries should be able to hold sufficient energy to handle overnight hotel loads if that is a requirement. Typically, this amounts to approximately 12 kW·h of energy.

Ultracapacitors are another energy storage option. They have good power density but relatively poor energy density. They are a good match for applications with a large amount of stop-and-go operation, particularly at lower speeds. Good applications are transit bus hybrids and refuse applications. For heavy line-haul applications, the low energy storage makes them less attractive. For example, they do not provide a good solution for overnight hotel operation.

The possibility of mating ultracapacitors with batteries has been considered. The objective is to couple the high energy density of batteries with the high power density of ultracapacitors. To accomplish this, it is usually necessary to add a dc-dc converter to convert the highly variable voltage at the capacitor bank to a relatively constant voltage at the battery pack. Depending on the situation, it may be simpler and more cost-effective to achieve the higher power density simply by increasing the size of the battery pack.

E-machine considerations

Available electric machine technologies include interior permanent magnet (IPM), ac induction (ACI), and switched reluctance (SR). Arguments can be made in favor of each of these technologies. IPM machines provide the best power density and efficiency but are costly and have a narrow constant power range relative to other technologies. This makes them less than ideal for use as traction motors unless they are used in a pretransmission application.

ACI machines are the lowest in cost because they have been in high-volume production for many years. They have reasonable constant power ranges making them suitable for traction motor applications. However, they are less efficient than other technologies.

If SR technology becomes popular, it has the potential to be the lowest-cost technology because it is so simple. These machines make use of simple concentrated stator windings and have rotors that are merely stacks of laminations. SR machines have efficiencies nearly as good as for IPM machines and exhibit good efficiency over a broader operating band. They can operate at very high speeds, limited by bearing speeds, switching frequencies, and rotor lamination strength. SR machines can be produced with wide constant power bands. Ratios of peak to base speed on the order of 5 or 6 to one are easily possible.

Often cited drawbacks of SR machines are audible noise and torque ripple. The audible noise amounts to a supercharger-like whine that may be objectionable in luxury sedans but is not a problem in heavy-duty applications where turbochargers are common. In fact, the largest noise issue with SR machines in heavy-vehicle applications is that users often complain of tire noise because the machines are so quiet. As for torque ripple, this may be an issue with very low inertia systems in light-duty applications but is certainly not an issue in heavy-duty applications that commonly deal with diesel engine torque ripple.

Hybrid outlook

There is potential long-term merit in exploring heavy hybrids based on fuel-economy improvements alone. Simulation results are promising, but the case has yet to be made in real-world operations.

It is likely that the ultimate value of heavy hybrid systems will lie with their potential as enablers for more advanced technology. For example, as mentioned earlier, such systems could enable new engine technology such as multifuel turbines.

Heavy hybrids can also provide cost benefits by significantly increasing foundation brake life and ultimately reducing maintenance costs. Significant drivability enhancement can aid in driver retention and improve safety by reducing driver workload. Fleets, OEMs, and suppliers are eager to participate in this “green revolution,” but there are significant challenges that will take time to overcome. Development of economical, reliable energy storage (batteries) is foremost in this lineup.

As with other initiatives, such as electronic engine controls and electronic stability controls, this journey will likely take 10 to 15 years. This is not a market to jump into lightly, but for those companies in it for the long haul, it’s a journey worth making.

This article is based on SAE International technical paper 2012-01-2063 written by Vern Caron of Caron Engineering Inc.

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