Hydraulic hybrids are very much a part of the alternative drivetrain equation, notes Chris Mays, Commercial Vehicle – Senior Technical Specialist, Corporate Advanced Engineering at BorgWarner. “Machines already have hydraulics on them, so adding a hydraulic hybrid is somewhat of a logical next step.” Even so, Mays prefers to talk electrification for heavy-duty on-highway and off-highway vehicles because, frankly, BorgWarner makes a range of electric products such as motors/generators with high-voltage hairpin technology, electrified boosting technologies and eFans for thermal management in 48-/24-V applications, to name a few. Mays recently spoke with Truck & Off-Highway Engineering about the drivers for employing hybrid and electric technology in heavy-duty and off-highway, challenges to implementation, and his view on waste heat recovery, fuel cells and more.
Why is demand for hybrid and electric technology growing in off-highway markets?
It really comes down to the benefits, which can be attractive when you look at certain applications. The OEs are becoming more cognizant of that and providing more solutions. Ultimately, it all comes down to payback. With construction machinery, it’s a lot about productivity—how much work can be done within a period of time. And hybridization allows more to be done within a set cycle time. It’s not so much just about the fuel savings. For example with a crane, energy is required to lift a container, move that container and then lower it, and then do the same with the next container. If you can recover that energy when dropping the container, the next one can be lifted a lot faster, so you end up lifting and shifting a lot more containers per hour, resulting in productivity gains. You see that in other applications, too—wheel loaders it’s the ability to move more material per hour. With a backhoe digger, it’s the ability to dig more and move it per hour.
The other driver is maintenance requirements. A lot of the machines—especially larger machines like large wheel loaders—end up with electric wheel motors, which improve torque at the wheel. But they also provide a great benefit in terms of improved braking efficiency and force, so you don’t have to rely on your traditional friction brakes. Because you’re relying on an electric machine, you also don’t have to service them as often, if at all. And a lot of machines are very cyclic—using them a lot and then idling a lot. The ability to shut them down more often helps with that overall noise level as well as fuel savings.
A couple other points also apply. Emissions for off-highway engines are set in terms of the power rating of the engine. If you hybridize, you can move down a power rating—go from say a 70-kW engine to a 45-kW engine, and that moves you into different emission bands, which generally have a slightly higher limit, meaning you don’t have to use as much aftertreatment for the emissions control. That can be a benefit in reducing cost of the machine. One final thing to think about is some of these machines connect to other accessories—like interchanging tools at the end of the jib. If you’re hydraulically connecting, you also need to make a hydraulic interconnection—remove one and fix another one. If you electrify some of those implements, you can make those connections easier.
What are the best applications in off-highway for electrification?
Anything where it’s repeatable and cyclic, where you’re doing the same job all day long—lifting and lowering; boom swinging. Wheel loaders seem to be one of the first applications in hybridizing—same thing, when you’re driving into a pile of dirt and lifting up, accelerating backwards, and then slowing down and dropping off—those sort of cyclic loads can be very good for hybridization.
The other area to look at is when you’ve got intermittent loads, a vehicle that’s operating at full performance sometimes and then idling or operating at lower load elsewhere—If you hybridize that, that would be an interesting approach where you can reserve some load capacity from parts of the cycle to use in other parts of the cycle and increase productivity without increasing the engine power.
What specific hybrid/electric technologies will make the most impact?
Looking at the types of hybrids at a high level, there’s electric hybrid, hydraulic hybrid and flywheel hybrid. For electric hybrid, power-split architectures make a lot of sense, where you can perform electrically or perform with the engine alone, or you can balance the two and start to decouple the engine speed from the machine speed, and you can decouple the engine speed from the hydraulic pump speed. The engine can then operate at its peak efficiency point, and the hydraulics can operate at their optimal speed as well.
Looking at flywheel hybrids, they’re able to store the energy and discharge it in the same way. With the battery-electric type hybridization, it’s not going to be the same as in passenger cars—it’s not just going to be a battery; you’ll probably end up with a battery and some intermediary device like supercapacitors or ultracapacitors to balance the charge and discharge rate of the battery.
How much does technology development from the passenger car side inform the heavy-duty/off-highway electrification technologies?
The usages are very different and the way they operate is very different. Some of the controls logic about how you transfer power can certainly adapt and apply [across industries] but the actual energy calculations would be unique for the off-highway machines. When you think of on-highway, probably the best application where you’re going to see a lot of carryover is in medium-duty and vocational vehicles rather than heavy-duty.
Another area where you might see some carryover between pass car is going to be 48-V solutions that are coming to play now on mild hybridization. There’s a lot of interest in agricultural and off-highway for some of those 48-V machine solutions. If you’ve got 48-V that’s in high-volume production on a passenger car, then adapting it and using it on a construction or agricultural machine is probably an area you’ll see some transfer.
BorgWarner is looking at 48-V solutions and at ConExpo we had some 48-V technology on the stand. But we don’t necessarily market them as specifically for one part of the sector or not. If it’s a 48-V architecture, as long as it meets the durability requirements, then it can be applied to any sector of the market. But certainly BorgWarner is working hard on 48-V as well as higher-voltage solutions.
Unique challenges for heavy-duty and off-highway applications?
With anything you take from the light-duty market and try to put it on a heavy-duty or off-highway vehicle, the first thing you get challenged on is the life and durability requirements. Passenger car application may see 1000 hours or 150,000 miles; heavy-duty you’re looking at 4000 hours at the low end up to 10,000 hours, so the longer life requirements are the first obstacle we hit. Productivity is another area that’s really important. The machine needs to be available for work a lot more than you’d expect from a light-duty vehicle. Duty cycles are very different, much more varied, a lot more transient with very high transient requirements on an off-highway machine. If you’ve got some battery storage, it’s unlikely to like the high loads, the high charges and discharges all the time, so you have to consider how you’re going to protect the battery. That’s where you see ultracapacitors and supercapacitors being applied in some instances. In other areas, you see flywheels being applied as an energy buffer. The last thing is where they’re used—light-duty tends to be used in a relatively clean environment whereas for off-highway and construction we’ve got some very severe environmental requirements. On top of that, the envelope of operation is from below ground in mining to quite high altitudes for some of the forestry applications, and the temperature extremes can be much higher. Construction equipment can operate in hot temperatures all day long, which can be a burden to deal with, particularly for electrical machines.
Can you discuss connectivity trends as they relate to your electrified powertrain technologies?
With heavy-duty on-highway, you start looking at some of the connectivity for platooning and the challenges it brings. The heavy-duty vehicles operate with an extremely reduced gap than usual—instead of following at say 500 ft they’re following at 100 ft. They’re not getting airflow over the front, because that’s the whole [aerodynamic] benefit, but you end up with the powertrain getting hotter than it used to, so the fan is switching on more, which is bad for fuel economy. Connecting the platoon with regard to thermal requirements as well as fuel saving requirements becomes a challenge. That’s one area we’ve looked at in terms of fan control, considering how to control the fan and communicate the fan and cooling requirements to the platoon distance control. And we’ve got patents on platooning for that area. That’s where we connect more—how our traditional components start talking more to the vehicle, to other vehicles and to the infrastructure at some point.
Outlook for fuel cell electric vehicles in heavy-duty and off-highway?
Fuel cells have always been 10 years out…We’re seeing some solutions come to be, but for big market adoption, there’s still a lot of hurdles to overcome. Fuel cell really enables electric vehicle operation. We’ve already seen electric vehicles for off-highway getting more interest. Fuel cell really extends the range of useful working day for them. There’s still lots of challenges of how to get the fuel to the site. And you’ve got to think why are you doing fuel cell as opposed to other ways of getting the energy. Fuel cell can provide electricity directly but to get the hydrogen you have to put energy in to make it. At the moment, natural gas seems to be the biggest provider for hydrogen, and if you’re taking out the carbon-based fuel and you’re making CO2 to make H2 to make a clean environment, it’s not necessarily the best for CO2. There’s more of a trend to look at things on a well-to-wheels, or well-to-work, basis. Depending on the metrics, hydrogen fuel cells might be slow to be adopted.
BorgWarner is working on Organic Rankine Cycle waste heat recovery. What’s your outlook for WHR technology?
The first application probably will be Class 8 long-haul trucks. As long as you can get payback in two years or less for the system, then there will be a market pull for it. Legislation helps—the greenhouse gas Phase 2 regulation is pushing technology and almost mandates it being taken. Once you get the volume built up, the costs come down, and then you might see waste heat recovery really become adopted widely in those long-haul applications.
If you look at GHG Phase 2 legislation, I think they were portraying about 25% adoption [of WHR] by 2027 in the U.S. market. We not may be as bullish as that but we do see a significant part of the long-haul market ultimately looking for waste heat recovery. As the engine efficiency improves, there is less waste heat available, but modern heavy-duty diesel engines are about 45% brake thermal efficiency (BTE); by physics you’re not going to get much closer to 50%, so there’s still over 50% of the energy to be recovered. You might just choose your waste heat sources better—it might be from the engine coolant, from the EGR cooler, from the exhaust system, from the intercooler—there’s lots of areas of waste heat. And you need to design your system for the sources you’re going to apply it to.
If you look at SuperTruck, the DOE-sponsored program is looking to get to 55% BTE. That’s really going to require waste heat recovery to some extent. Those types of programs came to set the pathway to future legislation. It might be a slow growth curve, but waste heat recovery is going to be with us and I think it’ll provide the benefits, depending on system cost.
What are some of the components that BorgWarner puts into those WHR systems?
We’ve got two business areas that are traditionally closer to some of the components on waste heat recovery. For waste heat recovery, it operates like a mini steam engine—you have to put heat in there to boil a liquid, so we’re looking at boilers. We traditionally have made EGR coolers, which are a stainless steel heat exchanger technology to take heat out of the exhaust for emissions control. Applying similar production methods we have developed exhaust and EGR boilers, or depending on the terminology, EGR evaporator. When you look at the exhaust system you need to bypass the exhaust spoiler sometimes, so bypass flaps and valves is another area in our portfolio that we are developing. And finally, once the waste heat recovery is working through the system, it needs to be expanded to recover the energy. For us, we’re favoring rotary expansion machines so a turbine expander, and that’s closely aligned with what we have in turbochargers. For our first approach on that, we’re looking at electrical turbine expanders—an electrical machine coupled to that—and that plays well with things like eTurbo and eTurbo compound devices we’re developing for other reasons.