Green Industry Special : Hybridising long-haul trucks

From a fleet perspective, hybridising long-haul trucks can make a huge difference in fuel-efficiency and CO2 emissions. It’s time Indian OEMs got cracking in this direction, an AVL expert tells Eliot Lobo. The payback prospects are promising.

Diesel-electric hybrids hold out manifest advantages in city applications, where the characteristic stop-start duty cycles of light- and medium-duty distribution trucks and buses allow for regenerative braking. Equally importantly, the systems available come packaged in sizes and weights that are acceptable for these types of vehicles.

But the add-on costs remain prohibitive for most operators, which explains why uptake – worldwide – has been dependent on government subsidies, and thus overwhelmingly restricted to the city bus segment. And so, while fuel-efficiency gains of up to 30 percent are claimed, and in a few cases even achieved, the overall reduction in fleet fuel consumption and, hence, in CO2 emissions is negligible.

From an environmental perspective, hybridising heavy duty long-haul trucks will have a far bigger impact, at least in the advanced markets, both at the vehicle and the fleet level. It’s easy to see why — in the US commercial vehicle park, for example, Class 8 vehicles are outnumbered by medium-duty trucks 55:45 percent, but consume more than four times as much diesel as the medium-duty trucks do.

Heavy duty trucks typically cover very high mileages – in Europe, they routinely do 300,000 km and more in a year – and thus even a 5 percent reduction in fuel consumption in this class will lower the fleet-average fuel consumption, and CO2 emissions, far more significantly than a 30 percent reduction in city buses or distribution trucks, RaimundEllinger, system design manager for transmissions and hybrids at AVL, points out.

“Long-haul trucks typically operate at a constant average speed, and therefore have not hitherto been considered for hybridisation. It’s more the city applications that gain significantly from the direct hybrid potentials — the stop-start possibility, regenerative braking, and boosting,” he says. “Nevertheless, if you look at the impact on the overall numbers, even a small improvement in the long-haul truck has a higher impact than a big improvement in a medium-duty or city application.”

In the long-haul segment, Ellinger says, the hybrid system should be designed to enable the optimisation of the full powertrain, more specifically in combination with a downsized engine, or by “downspeeding” the existing engine and modifying the transmission ratios. Either strategy would force the engine to operate at a higher load in steady-state mode, and therefore in a better efficiency range.

Without hybrid assistance, however, the lack of “residual” power in this operating range would take toll of driveability. This is where the electric motor really comes into play, delivering a torque boost in “dynamic situations” such as acceleration.

“By combining a hybrid with downsizing and downspeeding, a fuel efficiency improvement of up to 9 percent is feasible,” Ellinger tells this correspondent. “We’ve demonstrated this for real-world applications and duty cycles in the US, Europe, and Japan.”

In Europe, AVL sees the possibility to downsize the 12 litre engine class common in the heavy duty segment to between 8 and 9 litres. The downsized engine, though, still has to be able to deliver the full-load power of the 12 litre engine, since the electric motor can only function as long as the battery has an appropriate state of charge and thus cannot serve as a constant power supply. What it can do is deliver short boost interventions, enhancing driveability and, thereby, driver acceptance as well.

Depending on the application and the performance figures of the engine, it might be possible to both downsize and downspeed, but in many cases, Ellinger explains, it would be sufficient to just downspeed to obtain a “6–7 percent” fuel efficiency improvement.

“Today in the long-haul application the engine typically operates at at a constant 1,300–1,350 rpm. We would propose to reduce this to 1,200, maybe even 1,150 rpm. At the typical road load point at 90 km/h constant speed, you improve fuel efficiency by at least 5 percent just by shifting to a higher load in the engine map. And instead of 205 g/kWh of CO2, you’re now putting out 190–195 g/kWh,” he says.

The benefit of downsizing/downspeeding in combination with a hybrid is “robust” across vehicle loads and duty cycles, he points out. “If you run fully loaded, or even slightly overloaded as they do in India sometimes, the effect of the downsizing and downspeeding is smaller but the hybrid effect is bigger because you have more acceleration/deceleration phases. If you operate at a completely flat point, and, let’s say, you are half empty or empty, the hybrid effect is not that big but then you have a huge benefit from the downsizing/downspeeding. So in average operation you always get a 7–8 percent fuel efficiency improvement.”

The Western truckmakers have already been working on developments of this nature for some years. . But for India, where the bulk of truck engines are of the 6–7 litre class, Ellinger suggests that OEMs might find it worth looking into hybrids based on the topmost ratings of existing engine platforms instead of investing in the (costly) development of larger engines to help them move up the power spectrum.

By this strategy they could achieve higher outputs (by choosing the highest rating) from engines they already have in their portfolio while simultaneously enhancing the fuel-efficiency of those engines by running them at higher loads. Thus, for example, an engine like the 6.7 litre Cummins ISBe (up to 300 hp) could be combined with a hybrid in applications for which the 8.9 litre ISLe (280–400 hp) might otherwise be specified.

This would not be possible for applications that need all of 400 hp, of course, because the ISBe then would have to be beefed up and “charged” beyond the limits of what its design might allow. But for an Indian OEM that is looking at an 11- or 12 litre engine for that rating, would it make sense to choose an ISLe-hybrid instead? Absolutely, he says.

Nevertheless, it’s important to consider that in India, long haul doesn’t mean the same thing as it does in the West. For one, mileages are lower, and two, starting and stopping are much more frequent than in typical long haul operations in the US or in Europe. Where the rubber meets the road, does a hybrid really hold a value proposition for a long-haul truck in India?

“I would say in general you could get a fuel-efficiency improvement in the same range of 7–8 percent, but you then need to look at how much you are actually saving on fuel in terms of cost, because it’s really a question of total cost of ownership. Hybrids still suffer from a high additional cost. In Europe we expect that a long-haul operator will be able to recover his investment in three years — in the medium term, not today.”

This expectation, Ellinger adds, is based on projections of how component costs for electric motors and batteries could come down and assumes a fuel cost saving calculated on a mileage of 200,000 km a year. In any case, the payback has to be within the first life of the truck, which in Europe lasts four years for operators who log that kind of mileage.

In India a truck like Tata’s Prima would have a similar first life with the bigger fleetowners, though the average period of first ownership is closer to seven years. A 7–8 percent fuel efficiency improvement calculated over 100,000 km per year for that duration and factoring in inevitable increases in the price of diesel should translate into a reasonable payback period. Continuously improving “infrastructural boundaries” will allow for higher mileages, making hybrids that much more attractive.

As pressure mounts in the US and Europe for commercial vehicle manufacturers to reduce fleet CO2 emissions, the spotlight is now on the long-haul sector. For manufacturers hard at work improving aerodynamics and reducing rolling resistance, hybridising their powertrains adds considerably to the benefits they can gain with all the other measures, Ellinger says. “With improved vehicle boundaries, the benefits from a hybrid can even increase since, for example, the regeneration from braking is even improved with a more efficient vehicle.”

He is categorical that Indian OEMs must “think in this direction” now. “Hybrid technology for on-highway trucks may be something for 2020, but if you want to have products available then, development has to start right now in order to build up the supply chain and define all the boundaries they will need to be ready for by then.”

Interview - RAIMUND ELLINGER, SYSTEM DESIGN MANAGER FOR TRANSMISSIONS AND HYBRIDS, AVL

Truck OEMs in Europe have targeted the local distribution application for hybridisation. In India we don’t have that yet. Does it make sense for Indian OEMs to look at hybrids for both short-distance and long-haul? Would that make the development more economical?

My advice would be to look at all applications and see how you can cover as many of them as possible with a limited number of electric drives. So, what would be the most appropriate e-motor family that could serve both, long-haul truck and city bus? At the moment, the low production numbers of these motors are the primary reason for their high cost, but if it were possible to leverage an e-motor across different applications to increase production numbers, that would bring the cost down.

Does each OEM have its own e-motor size and specification? Isn’t there a drive towards standardisation?

I think there will be, of course, but maybe not industrywide. An e-motor for a commercial vehicle is still seen as a component that comes from a supplier. Suppliers like ZF are heavily investing in this area, and they, of course, want to standardise their product portfolio.

Talking about scale, one supplier could then make a lot more motors, bringing the cost down…

In passenger cars, on the other hand, there is the opposite trend in which a lot of OEMs are thinking of in-sourcing their e-motor development. They see this as a core in-house capability for the future. On the commercial side there are diverse interests. Of course there are Tier-1 suppliers like Cummins, for instance, for whom it would make sense to offer hybrid packs, while an OEM like Daimler, which builds its own engines, and transmissions as well, might tend to want to in-source this component too.

Is it really possible to leverage a limited e-motor range across bus and long-haul truck applications?

It could be possible to have a common family across medium-duty and heavy-duty, for instance, where of course the power demands are different. What I’m suggesting is to look closely at the portfolio of the OEM, and at the performance requirements of the different applications, and then define, based on the OEM’s existing engine portfolio, the most appropriate e-drive portfolio that serves the requirements of all those applications.

So standardisation really isn’t something that is easily done?

On the engine side, we see the emergence of modular engine families with different capacities to serve the full range of applications. And these could now be supplemented by, let’s say, two motors of 40 kW and 80 kW that use the same technology, maybe it’s just the length of the active material that differs. You can thus build up a powertrain family.

In that case, does it make sense to have fewer combustion engines and more e-motors instead?

That could be one consequence. With hybrids you can start to think about reducing the complexity, the variabilities that you have on the engine side, and instead use the e-motor to give you the required flexibility at the powertrain level. You could, for instance, stay with a wastegate turbocharger instead of using a variable turbocharger, or have a single turbo instead of a two-stage charger. This again will depend on the application and on the requirement.

What would you achieve?

Today the combustion engine must serve a lot of requirements. On one hand, the low-end torque demand for driveaway, performance, and driveability; on the other, the full-load power demand. With an e-motor you could tune the combustion engine for peak power, and design the e-drive to support all the low-end, dynamic modes. Reducing the dynamic demands on the combustion engine also offers some advantages from a controls perspective. For the Indian boundary conditions this is definitely an interesting technology to consider — you take your existing engine and, instead of upgrading it, you hybridise it to serve the future needs of the application.

Are there existing components that you could propose, or do all of them need to be developed from scratch?

There are e-drives available from ZF and Siemens for heavy duty applications. But we as AVL could also support the Indian industry here if there is an interest in localising such products. We could even help in the development of the electric motor itself. Localisation, of course, is one important means of getting costs down so that hybrid systems become cost-attractive here in India. For this the supply chain will have to be developed and AVL can support the industry in this as well.

What do you think of a system that does away with a geared transmission altogether, like Meritor’s multi-mode hybrid for Class 8 trucks?

With the e-drive the main advantage is that torque control is much easier and much more accurate, and the torque response is much quicker than that of a combustion engine. The first thing I would do if I have a powerful electric drive on board is to electrify the auxiliaries. By electrifying the fan drive, compressors, and steering pump, you can improve fuel efficiency by 2 percent in the worst case and 5 percent in the best. This is over and above everything we’ve talked about.

What effect does hybridising a truck powertrain have on regulated emissions?

Hybridising allows you to reduce engine transients such as acceleration peaks to a minimum. We all know that NOx has a strong dependency on engine load, so being able to operate the engine in a steady state at lower loads has a significant benefit on NOx emissions. However, currently the certification is done based on an engine test and is fixed in terms of the engine speed and load points that must be certified; it doesn’t really take account of the vehicle or the powerpack. But now, in the US at least, they are discussing how to include this hybrid pack in the certification. This is necessary when it comes to regulating CO2.

On passenger cars we have clearly shown that you can significantly reduce NOx with a diesel hybrid. There, the NOx reduction benefit could be even bigger than the fuel-efficiency benefit.

Is it significant enough to allow you to simplify your very expensive aftertreatment system?

This depends on the application. Also there you could maybe stretch your vehicle weight limit to which you comply with the standards without a significant aftertreatment system. It always depends on vehicle weight and engine displacement, and this defines which kind of engine load… We at AVL have been able to achieve Euro VI in a wide range of applications based on raw emissions and, of course, a particulate filter. But stretching the envelope, coming to Tier 2 Bin 2 [for light duty vehicles] in four years, I think there are possibilities to harness the benefits of the hybrid for reducing emissions as well. On the commercial vehicle the situation may be different unless the certification procedure changes.

ELIOT LOBO