Hybrid Powertrains: The Two-Mode Approach

Although some people might think that "hybrid" means "small car" (e.g., the Toyota Prius or Honda Civic Hybrid), in GM's view, it is better, for now, to create hybrid vehicles that can save significant amounts of fuel. Which is why its two-mode hybrid system was developed by GM/Allison.

GM's two-mode hybrid system

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While GM seems to have been inactive in the hybrid arena compared with some of its competitors, it has been slowly gaining experience with a different type of hybrid setup—on city buses. And in the not-too-distant future, this system will be available in SUVs from GM and Chrysler, and even in Mercedes luxury cars.

When it comes to hybrid powertrain technology, Toyota seems to garner the lion's share of credit given its Prius and forthcoming Lexus 400h and Toyota Highlander HV models. Honda is certainly in the game, having brought out the Insight in 1999 (http://www.autofieldguide.com/driven/1101dri03.html), followed up by the'03 Civic Hybrid (http://www.autofieldguide.com/articles/050201.html), and most recently by the Accord Hybrid (http://www.autofieldguide.com/articles/110405.html). Ford has the Escape Hybrid. But General Motors seemingly has little in the way of hybrid technology, with the exception, it seems, of its full-size Silverado and Sierra pickups with the 14-kW electric induction motor (a.k.a., "starter generator") system that shuts off the engine when you come to a stop and which feature four 120-volt/20-amp electrical auxiliary power outlets, so contractors and the like can run electrical tools. Is that all there is at GM?

Hardly. In fact, while much public attention has been focused on passenger cars with hybrid systems, GM announced in the fall of 2003 that it was equipping city buses in the Seattle area with a parallel hybrid system. Generally, that response was met with indifference. Which is rather unfortunate. After all, if you think about buses, they have the ideal starting-stopping driving cycle for hybrid systems. (The reason why there has been criticism of some of the estimated city/highway MPG numbers for hybrid passenger cars is because they don't drive that sort of stop/start, stop/start.) And consider if you have responsibility for maintaining civic equipment like buses. All of that stopping can be troublesome for brakes. But as Tom Stephens, GM group vice president, Powertrain, points out, the GM/Allison hybrid system that is being used on what has been expanded to 18 U.S. cities (for a total 335 buses) has a regenerative braking system, which means that the energy generated during stopping is put into the 600-v NiMH battery pack for the next acceleration and, importantly, the base brakes aren't activated until the bus reaches about 3 mph, so there's less wear of the foundation brakes. There are two 100-kW electric motors used and an electronically variable hybrid transmission that provides an infinite range of gear ratios to the wheels, which permit the sizing of the diesel engine used in the buses to be reduced from a typical 11 liters to 8.9 liters or even, Stephens says, 5.7 liters. All that and a diesel particulate filter result in an emissions reduction, Stephens says, of 90%. Not only is air pollution reduced, but so is noise pollution: the hybrid system results in sound levels analogous to those of a passenger car. Stephens says that based on the typical drive cycle, there is up to a 60% improvement in fuel economy. "If we replaced the 13,000 buses in the nine largest U.S. cities, we would save 40 million gallons of fuel a year," he points out. In late October 2004, GM and Shanghai Automotive Industry Corp. Group (SAIC) announced that the two companies are collaborating on a hybrid bus system for China (as well as on fuel cell development). "What is our hybrid strategy?" Stephens rhetorically asks. "We're trying to target the high-fuel consuming vehicles first. That's why we went after buses and full-size trucks."

But there is more to come from GM in hybrids. The work done on the bus system is going to be parlayed to high volume vehicles, first the Tahoe and Yukon SUVs in the late '07 model year to be followed sometime in '08 by new versions of the Sierra and Silverado pickups. The EP 40/50 hybrid systems from the buses are becoming the AHS2, about which Stephens claims, "it combines the best capabilities of a full hybrid with proven automatic transmission technology to deliver the world's first two-mode hybrid system." 

"Full hybrid" means that the vehicle so equipped can operate with electric power only, the internal combustion engine (ICE) shut off; with the ICE only; or with a combination of the two. The "two-mode" portion is the key differentiator between what GM is doing and what Toyota has done. Stephens says that the "typical" hybrid system is a "single mode" unit. There are the same full hybrid capabilities as those that the system GM has developed provide, but they're achieved in a different manner. The fundamental difference is that the AHS2 uses a more conventional transmission than the ones used in a single mode system. Stephens explains that in the GM system they are using the gearing in the transmission to amplify the torque generated by the engine and the motors. "Think of it this way," he says. "When I took my driver's test, you had to do it with a manual transmission. What do you do when you get a little nervous? You start out in third or fourth gear rather than first, and there was sluggish acceleration. The same engine would go pretty good when you put it back in first. You had a transmission to amplify the torque." And that's what they're doing with the AHS2 system: using gearing to amplify the torque. Stephens says that a mechanical path for power transmission is more efficient than an electrical path: anywhere from about 80 to 97% versus 50 to 75%. This has plenty of ramifications. For example, the size of the electric motors can be reduced, as can the size of the inverter and the batteries. There are reduced mass, reduced cost, and reduced packaging requirements. With regard to packaging, the system not only makes use of automatic transmission technology, but it is being housed in the envelope of a newly developed six-speed automatic transmission. Inside that housing there are, in addition to traditional transmission gears, two hybrid motors coupled to the gears.

Note what GM is doing here. The fundamental parallel architecture used for the buses is being deployed for AHS2, as are the control algorithms. The control hardware, Stephens says, could be taken right out of one of the six-speed transmissions they'll be producing (he estimates that GM will be annually producing nearly one million six-speeds by '08). A phrase that's often heard around GM related to all activities is "run common, run lean." Which is something that is being played out in the hybrid arena.

DaimlerChrysler is in agreement that the two-mode approach is the way to go. That corporation has joined with GM in a cooperative effort. Observes Eric Ridenour, executive vice president of Product Development-Chrysler Group, "This will reduce independent development costs and rapidly advance hybrid technology." Just as GM plans to put the first consumer two-mode hybrid systems in trucks, Chrysler will have them in the Dodge Durango shortly thereafter, in late '07 or early '08. But these systems are not limited to trucks by any means. Dr. Thomas Weber, DaimlerChrysler board of management member with responsibility for Research and Technology and Development, Mercedes Car Group, says, "We will have the design lead for rear-wheel-drive luxury cars." Weber goes on to note that this cooperation will lead to economies of scale that would not be attainable on an individual basis, which will lead, he says, to "cost reductions in favor of our customers." Weber notes that the hybrid powertrain will be tuned specifically to the "brand values" of the vehicles they are applied to, so a Mercedes car with a two-mode system will "feel" like a Mercedes.

Apparently, representatives from GM and DCX were both participating in a conference on the subject of advanced powertrain and, Ridenour explains, they saw a lot of commonality in what they were doing, which led to this agreement. Ridenour goes on to say that they think this allows them to "leap frog" the competition.

From buses to luxury cars, and perhaps someday everything in between. While the end game for the ICE replacement still seems to be the fuel-cell-powered vehicle, the road to that leads straight through hybrids. But the question that comes up, in light of the penetration that Toyota has made in the consumer mindset as regards its leadership position with hybrids, is whether GM isn't late to the game. Stephens first notes that the total market at present is on the order of 0.2 to 0.3% of the entire market, then points out, "All of these technologies are going to be with us for a good, long time." Time—and technology—will tell.