Heavy-duty hybrids

  • 27-Feb-2009 04:32 EST
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The Mercedes-Benz Axor BlueTec hybrid is among the first long-haul hybrid trucks.

Vehicles on urban delivery and collection routes and buses are perhaps some of the more obvious beneficiaries of hybrid power systems, where the continual stop/start driving cycles offer many opportunities to recover and recycle braking energy. It is, therefore, hardly surprising that this is where commercial-vehicle (CV) manufacturers have focused their hybrid developments.

Set against the obvious advantages are a number of drawbacks. While CVs are generally large enough to accommodate the battery packs, control systems, inverter, and electric motor that make up most hybrid systems, trucks are designed to carry weight. Adding hybrid systems reduces available payload, while the additional emissions control equipment needed for modern diesel engines has already claimed more space on vehicle chassis. Exhaust particulate filters and catalysts and urea additive dosing systems ensure that chassis space is restricted.

The weight problem may be a less sensitive issue for bus applications, but the hybrid system must still be packaged on the vehicle. For buses, the parallel vs. serial hybrid discussion may also be relevant, which has a bearing on hybrid power requirements and, therefore, scale.

In for the long haul

Hybrid-powered vehicles were widely displayed at the IAA Commercial Vehicle Show at Hannover, Germany, last fall. Although distribution trucks, garbage trucks, and buses were the focus of attention, Mercedes-Benz displayed a concept Axor long-haul hybrid truck, developed with Eaton. It was the first time that a prototype hybrid long-haul truck had been shown.

The Axor BlueTec hybrid adopts a similar layout to other parallel hybrid drive systems and draws on the hybrid system from the Mercedes Atego hybrid light truck. The Axor is powered by a Mercedes-Benz OM 926 7.2-L, six-cylinder diesel developing 326 PS (240 kW) at 2200 rpm and 1300 N·m (960 lb·ft) between 1200 and 1600 rpm. The same diesel engine is used to power Axor 1833 models.

The drive motor/alternator is fitted between the clutch and transmission—the standard format for most parallel hybrid systems. Peak power output from the motor is 44 kW, with a maximum torque of 420 N·m (310 lb·ft). The regular transmission is Mercedes' 12-speed PowerShift automated manual. The Axor hybrid weighs 155 kg (340 lb) more than a regular Axor 1833, according to Mercedes.

Energy recovered from regenerative braking is stored in a water-cooled lithium-ion battery pack with a capacity of 8 kW·h. The system behaves like a "conventional" parallel hybrid in that the vehicle starts under electric power alone, which can propel the vehicle to a set speed before the diesel engine is started. An automatic start/stop system switches off the diesel engine when the vehicle comes to a halt.

Mercedes claims fuel savings of between 4 and 10% compared with a conventional diesel-powered Axor. Although these are far smaller than the 25 to 30% gains claimed for vehicles operating in stop/start traffic, such small gains could make a significant difference to the fuel costs for a fleet of vehicles, each returning around 9.0 mpg and covering some 100,000 mi (160,000 km) per year. Frequently, lighter articulated vehicles carry out long-haul operations mixed with urban operation to reach the final destination.

Controlling downhill speed for a fully loaded vehicle is also a safety matter for long-haul trucks. The hybrid regenerative braking system could be used in place of, or alongside, an exhaust brake or retarder. These are required auxiliary braking systems to help prevent brake "fade" and excessive brake lining wear.

Hydraulic hybrids

Apart from its internal-combustion engine (ICE)/electric hybrid programs, Eaton is also developing ICE/hydraulic hybrids for trucks and buses. The company is developing both parallel and serial hydraulic hybrids, designed for stop/start operations. In both cases, the systems use technology derived from Eaton’s hydrodynamic retarder range. Instead of using the hydraulic fluid simply as a medium to reduce vehicle speed in the hydraulic pump/retarder body, the pumping action is used to pressurize nitrogen gas in a high-pressure accumulator, by transferring the hydraulic fluid from a reservoir to pressurize the nitrogen.

For a parallel hydraulic hybrid, the pressurized nitrogen can then be used to pressurize the hydraulic fluid and in turn drive the hydraulic motor/pump system, feeding power back into the driveline when needed. The system works in a similar way for a series hydraulic hybrid. The braking energy generated by driving the hydraulic drive/pump motor is used to pressurize the hydraulic fluid and in turn nitrogen in the accumulator via a piston. The stored energy can then be recalled to feed back into the hydraulic drive system, assisting the ICE when needed. The engine is used to power the hydraulic drive system, which drives the vehicle. Like serial ICE/electric hybrids, there is no direct link between the ICE and the driving wheels. Eaton claims fuel consumption reductions of up to 70% from the serial hydraulic hybrid system.

A prototype version of the parallel hydraulic hybrid system, called Hydraulic Launch Assist (HLA), was first seen last year at the Waste Expo in Chicago based on a Peterbilt 320 garbage truck. Field trials began with Waste Management of the U.S. at the end of 2008, using four of the Peterbilt garbage truck prototypes.

Eaton claims that the HLA system can recapture some 70% of the kinetic energy during regenerative braking, and that fuel consumption and CO2 emissions savings of between 20 and 30% are possible.

The system offers two operating modes: Economy and Performance. In Economy mode, the energy stored in the accumulator is used to accelerate the vehicle without assistance from the diesel engine, until the pressurized nitrogen in the accumulator has forced all the hydraulic fluid back through the pump/motor. In Performance mode, the accumulator and the diesel engine work in parallel to accelerate the vehicle.

According to Dimitri Kazarinoff, General Manager for Emerging Technologies at Eaton’s Truck Group, the company is currently focused on developing plug-in hybrids as range-extending systems. Other projects include using electric hybrid power for auxiliary drive systems. Examples could include vehicles such as tipper trucks, concrete pumps, or platform lifts working on a construction site.

A platform lift may not travel far by road, but the diesel engine is also used to supply drive to a power take-off (PTO) system, which drives the hydraulic system to raise and lower the platform. The engine could be used for lengthy periods of time in this mode, with the vehicle stationary. Using a hybrid drive system instead, auxiliary systems could be driven electrically and the hybrid battery pack recharged by automatically running the diesel engine for a few minutes each hour, said Kazarinoff. This will reduce both exhaust and noise emissions, potentially important factors on a construction site.

Moving in parallel

Eaton’s European rival ZF is focusing on ICE/electric hybrid systems for buses and CVs. For economic reasons, ZF favors the parallel hybrid, though serial hybrids are under development for city buses.

“From our point of view, [the parallel hybrid] is the most economic solution because it needs only one electric motor,” said Bernd Vahlensieck, who is in charge of the advanced engineering of hybrid drives for ZF’s corporate research and development department. Economics also explain why ZF tries to keep the motor/generator for the system as small as possible.

“This means, in terms of reducing fuel consumption, there may sometimes be more potential in the hybrid system,” said Vahlensieck. “But you would get the last 10% of benefit for too high a cost. If you look at distribution trucks, where we are developing one system for series production, we are talking about 50 kW electric power in such a driveline for a 17-t GVW distribution truck. If we talk about cargo vans, we are talking about powers of around 30 to 40 kW. With more power, we would maybe save another percent or two in a city drive cycle, but this would not be the optimum in economic terms.”

ZF is working on five hybrid projects for CVs and buses at present, although it is not yet clear when these will reach series production. “If we look at second-generation systems, the trend toward more cost-effective systems will be even greater,” said Vahlensieck. It could be some time yet before such systems reach the market.

Since ZF produces driveline components, Vahlensieck believes that auxiliary drive systems that can be driven with regenerated energy are an indirect concern for the company. He also sees limited demand for micro hybrid stop/start systems, even though Mercedes-Benz is offering such systems on some truck models.

“These things might also come for trucks, but at the moment, I do not see pure stop/start systems in trucks and buses,” he said. “We had been developing a mild hybrid system that was quite intelligent and quite cost-effective and space-effective, because it was installation space-neutral, but it did not offer pure electric drive and customers did not accept this system; they wanted a full hybrid.”

ZF is researching hydraulic hybrids, too, but has not yet decided whether to develop them for production. “We see the advantages, especially in terms of cost, but there are also disadvantages,” Vahlensieck explained. “Therefore, there will be a very restricted application field from our point of view for hydraulic hybrids, and we have to decide whether it is worth developing an optimized system for this niche.”

One area that Vahlensieck thinks may be developed is the addition of electric motors to transmission systems: “This is in a very, very early stage of predevelopment. We are trying to prepare for the possible situation when drivelines are more and more electrified and also for the possibility when there will always be an electric motor in the driveline. Then maybe the solutions we develop today are not the optimal solutions for these systems.” He sees these as developments for third-generation hybrids at present.

Vahlensieck is not convinced that plug-in hybrids are the way forward for CVs. One reason is that electric power generation currently produces large CO2 emissions; there would need to be a big shift in electricity generation before plug-in hybrids would offer the CO2 emissions reductions possible from current ICE/electric hybrids.

“Then there are still a lot of obstacles on the way to producing greater battery capacity and higher power from the electric motor,” Vahlensieck continued. “In a cargo van, you cannot go around with 30 kW in a plug-in hybrid; the idea is to go for longer distances purely with the electric motor. Therefore, you will need 50 kW to 60 kW. This would mean higher cost, and the load spectrum of the batteries would change, which would have an impact on the battery life. There are many, many things to consider before this is a solution that can be followed.”

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