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It was January 2000 when Tim Grewe left General Electric Corp. to take on an almost impossible mission at General Motors Corp.: developing a hybrid-electric powertrain that would work in fullsize SUVs, as well as small, front-wheel-drive cars and city buses.

The assignment followed the failure of GM’s hugely expensive electric-vehicle program and its product, the sporty 2-seat EV1.

That project failed mostly due to lack of a battery pack with sufficient energy to compete with a tank of gasoline, and left the auto maker highly skeptical of investing in hybrid-electric vehicles, which it viewed as an interim step on the way to fuel-cell EVs.

But after GM saw the green marketing benefits being reaped by early hybrid adapters Toyota Motor Corp. and Honda Motor Co. Ltd., it reversed course and chose to target its largest, least fuel-efficient vehicles first, where advanced hybridization would make the most difference.

GMC Yukon Hybrid

The resulting 2-mode hybrid system is an electronically variable transmission (EVT) that marries two electric motors to a 4-speed automatic.

Driven by a 6.0L small-block V-8 with variable-valve timing and cylinder deactivation, the powertrain allows a fullsize SUV to deliver real-world city economy similar to that of a 4-cyl. midsize sedan.

Ward’s test of the 4-wheel-drive ’08 GMC Yukon Hybrid for the 10 Best Engines competition averaged 19 mpg (12 L/100 km) in mixed driving vs. its ’08 U.S. Environmental Protection Agency ratings of 20 mpg (11.7 L/100 km) for combined city and highway driving.

As GM Powertrain’s newly hired chief engineer for rear-wheel-drive 2-mode hybrid systems, one of Grewe’s key resources was retired GM Powertrain senior staff engineer Mike Schmidt, whom he calls the father of the EVT.

Schmidt has spent his entire career working on torque-split transmissions, where two sources of torque are blended together.

“He invented the first city bus EVT and co-invented the adaptation that goes into the Tahoe (and Yukon hybrid) today,” says Grewe. Schmidt’s co-inventors are GM’s Larry Nitz and Alan Holmes and Allison Transmission Inc.’s Don Klemen.

“We looked at every hybrid system and asked what it would take to give fullsize SUV customers good fuel economy and maintain performance,” Grewe says.

It took a big leap of technology to make that happen.

Everything started with the city bus system, Grewe says. “We tried to make a 1-mode hybrid work in a bus, but it didn’t make the technical leaps. It couldn’t give both highway and city performance in that application and would not have sufficient bandwidth.

“We kept searching for a technology solution that would have the capability to power large fuel users and small cars, and that’s where the 2-mode system started. Technically, it’s an input split at low speed and a compound split at high speed.

“We added some components of the transmission, with a clutch to directly transmit the torque, and came up with a system architecture that could take the torque of a 6.0L engine and still get great fuel economy.”

Skeptics scoffed when GM announced its 2-mode hybrid bus in 2003, which started production in October that year. The team then refocused its attention to the fullsize SUV and pickup truck system. At the same time, GM sought capable OEM partners to share the cost.

“GM wanted a global product, believed this technology would be good for the industry and wanted others to use it,” Grewe says.

The former DaimlerChrysler AG came on board in January 2005, and BMW AG followed in September.

“(DC and BMW) have very good engineering groups, and they were thinking about hybrids. We showed them the details behind the 2-mode, and they thought it had enough excellence and bandwidth to become their product as well. The nature of torque-split transmissions married to individual engines does allow you to build your own brand character with each application.”

Both DC and BMW sent engineers to GM’s Troy, MI, Hybrid Development Center to work closely with GM’s engineers.

GM led on the rear-drive truck program; Chrysler and GM worked mostly on the FWD program; GM, Mercedes and BMW worked on the rear-wheel-drive luxury-car program.

The toughest challenge was maximizing fuel economy while maintaining performance.

“That took incredible focus,” Grewe says. “We developed a ‘glide path.’ When we released a certain level of hardware, we had to demonstrate certain levels of economy and performance. And we had to maintain that glide path all the way from early prototypes to production.”

After that was the integration of the high-inertia electric motors with the engine and powertrain, including Active Fuel Management, which cuts off four of the V-8’s cylinders to save fuel under light-load conditions.

“With the ability to have the actual combustion interact with the electric motors,” Grewe explains, “you could hit a resonance and snap the crankshaft or input shaft. You have to write a whole bunch of controls to make sure you’ll never go into that resonance condition.”

Next was security. The HEV powertrain does not have a speed-change device, and there is no reverse gear or reverse clutch.

“So you always have to have the engine torque synergistic with the electric-motor torque. The motor has to cancel out the engine’s forward speed to make the system go in reverse,” Grewe says.

“You have a spinning engine that has to be choreographed with two electric motors. And if one of them turns into Hannibal Lecter and goes insane, you always have to be in a safe place.”

“If you go into the technical aspects of integrating electric motors with 6.0L engines and clutches, you have to make sure they will integrate properly even if one, or even two, fails,” Grewe says.

“We have the processors check each other, a secondary virtual system checks everything else, and can go into operating modes such as reduced engine power or reduced motor power and coordinate them through the electronic throttle control.”

Hardware was another major challenge. The cam phaser is retarded at start-up, and then the oil system pressures it up to full advance to make power. The electric motors are oil cooled.

“We had to develop hydraulic systems integrated with electric motors on the transmission side and cam phasers on the engine side,” Grewe says.

“Most hybrids, just outer-jacket water cool the motors, but that is not enough for a 6.0L SUV engine. We have to direct-spray oil on the motors. Think of them as rotating clutches with high lube at all times. Under certain conditions, they go into a high-flow regime for maximum cooling. That’s an efficiency loss that hurts fuel economy, but if you’re pulling a trailer up a pass, you need that cooling.”

Engine on-off and V-8/V-4 cycles must be transparent, and transmission-mode shifts between compound split and input split must be ultra smooth, “like the world’s best upshift, but done under regeneration,” Grewe says.

“How do you make all that happen so it’s stable and doesn’t get busy? You don’t have a fixed gear-ratio schedule. You can go V-4/V-8 with infinite ratio choices and variable-cam phasing, or in locked-up mode. We had to build a hybrid operating system with an online optimizer to decide the most efficient thing to do, predict where it wanted to go and not waste any fuel getting there.

“It’s a real-time control embedded in the system that looks at motor and engine efficiencies and overall spin losses and decides the right ratio, the best spark profile, etcetera, combined with the variable displacement.”

How does this 2-mode hybrid system differ from 1-mode systems? Grewe says any powertrain has a “sweet spot” where it operates most efficiently. “If I’ve got just one gear relationship between those electric motors and the engine and tires, I can set it at a point, typically around 45 mph (72 km/h) for a 1-mode. But in a 2-mode, you can set it around 28 mph (45 km/h) in the input split and 50 mph (80 km/h) in the compound split and get better city and overall efficiency.

“You have the hybrid capability and the 4-speed capability on top of that, so you can choose whatever combination works best for any scenario.”

Bob Lutz, GM vice chairman-Global Product Development, has quipped the 2-mode system “costs nearly as much as a small car.” That may be an exaggeration, but it very likely costs well more than GM can charge for it right now.

Is there room for future cost reduction, as well as fuel-economy improvement? Absolutely, Grewe says, explaining that once engineers better understand the real-world requirements of the electric motors, they will reduce costs further.

“GM wanted to make the world’s best (HEV powertrain) and be fast to market. If you start in 2004 with a system like this and come out at the end of 2007, that’s a very fast development time,” Grewe says.

Despite the rush, GM has accomplished the mission, Grewe says. “It works and nobody else has figured out how to do it yet.”