Powertrain Developments At General Motors

Gary S. Vasilash

Yes, there's the Volt. But by no means should the new 1.4-liter turbo in the Chevy Cruze be overlooked.

The Chevrolet Volt is getting the lion's share of attention of the vehicles being launched by the venerable brand, primarily because of its innovative powertrain setup. That is, the "Voltec" propulsion system consists of a 16-kWh lithium-ion battery pack (it measures 5.5 ft. in length, running down the middle of the car where the transmission hump might otherwise be found, and weighs 435 lb.), a 111-kW electric drive unit that powers the wheels of the car (during most operating conditions; at high speeds there is the blending of electrical and mechanical power, but by and large this is not the case). And there is a 1.4-liter, 63-kW (a.k.a., 84-hp) internal combustion engine. Power from the engine is inverted so as to provide power to the electric drive unit. So rather than performing as an engine does, say, in a Toyota Prius hybrid, where the engine does most of the wheel turning, the engine in the Volt works primarily (see point four, next column) as a means to power the generator that, in turn, charges the battery.

(The Volt drive unit consists of two motors, three clutches, and a planetary gear set. The motors and gear set are mounted in line with the engine. Two of the clutches lock the ring gear of the planetary gear set or connect it to the generator/motor; the third connects the engine to the generator/motor. There are four driving modes provided. (1) Single-motor electric vehicle (EV) driving, wherein the primary traction motor provides propulsion. The planetary ring gear is locked. The generator/motor is decouple from the engine and the gear set. (2) Two-motor EV driving occurs when the vehicle speed increases. This calls for the deployment of two motors working in tandem. The ring gear is unlocked in this case and coupled to the generator/motor. (3) Single-motor extended range driving occurs when the battery is at its minimum state of charge. At this point the 1.4-liter engine is coupled to the motor/generator with the third clutch. Here, the ring gear is locked and the traction motor, provided with electricity from both the engine-driven generator and the battery through the inverter, provides propulsion of the vehicle. (4) Two-motor extended range combined driving is the mode that some people have pointed out make the Volt a hybrid, not a pure electric vehicle. In this case, the clutches connecting the generator/motor to the engine and the ring gear are engaged thereby combining the engine and both motors output through the planetary gear set to drive the vehicle.)

But in point of fact, while the Volt may be technically fascinating in large part due to its rather unique powertrain architecture, the vehicle that will have a much more profound influence on the fortunes of Chevrolet (and consequently General Motors) is the Cruze, a compact car that is being introduced to the U.S. market, having already had success in both European and Asian markets. The Volt will have limited market penetration. The Cruze promises to be a volume leader in the Chevy lineup.

And while the Volt has had significant engineering resources applied to its powertrain, the Cruze has also benefitted from attention being paid to it. Specifically to the available 1.4-liter Ecotec engine—a turbocharged engine. According to Mike Katerberg, Ecotec 1.4-liter turbo assistant chief engineer, while there is a 1.4-liter engine available for the car in Europe, that's a normally aspirated engine, not the turbo that was developed for the U.S. vehicle. The 1.4-liter turbo is being offered as standard on the Eco, LT, and LTZ models. In an Eco with a six-speed manual, it achieves on the order of 40 mpg on the highway.

Katerberg says that the turbocharged engine is architecturally similar to the normally aspirated one, but there are differences that are predicated on the turbo system. For example, block for the turbo is gray cast iron; the normally aspirated engine has an aluminum block. There are a few reasons why they opted for cast iron rather than aluminum. For one thing, because there are increased loads in the turbocharged engine, they needed the additional strength provided. There is an advantage as regards noise: "Often you have to add ribs and structures in an aluminum block to make it as quiet as an iron block." Aluminum blocks require bore liners, which is tough when there is tight cylinder spacing. "We have very thin-wall technology—walls as thin as 3 mm. GM has made investments in the Defiance Foundry [in Defiance, OH] to make this block for the U.S. market. It is a very precision cast iron block," he says. While cast iron is heavier than aluminum, they calculated that because this is a relatively small-displacement engine, the weight savings weren't that significant ("On big-displacement engines there is still an advantage for aluminum"). However, the block is engineered with hollow-frame construction, which makes the block about 20% lighter than a conventional cast block. And another plus for cast iron is that it is less expensive than aluminum—but then GM needed to make the investment in the foundry, so it isn't as though they got that characteristic for free.

Aluminum is not wholly out of the picture, however. For example, Katerberg points to the aluminum structural oil pan. Half the transmission is bolted to the oil pan, so the oil pan serves as a structural member between the engine and the transmission. What's more, the cylinder head is aluminum. There is an integrated aluminum front cover.

The crankshaft on the turbo is a solid cast part; the normally aspirated section has hollow sections. The connecting rods are forged steel: "Again, because of the loads." The pistons are made with a hypereutectic alloy and designed with a thick crown area and use a special ring pack so that the boost pressure and heat generated by the turbo system can be handled. Piston-cooling jets, mounted at the bottom of each cylinder, are used to minimize piston temperature. The engine's compression ratio is 9.5:1.

There are four valves per cylinder. The exhaust valves are sodium-filled to accommodate the high combustion temperatures of the turbo system. There is variable valve timing. The engine has dual-overhead cams; the camshafts are hollow, which not only reduces engine mass but contributes to the ability of the engine to rev higher and more quickly. Roller-finger camshaft followers are deployed; they are low friction and low mass. The engine has a variable-displacement oil pump that matches the amount of oil needed to engine load, which helps improve engine energy efficiency.

The intake manifold is a composite part. Katerberg says the advantages of the material are low mass and a good surface finish for air flow. The turbocharger is actually integrated into the exhaust manifold. As Katerberg puts it, "It was done primarily for packaging. Partly for weight. Most importantly, for emissions and performance." The turbocharger is sized for low-speed torque, not peak power—although it should be noted that as the 1.4-liter engine produces 138 hp, or about 100 hp per liter, the peak power number is certainly a respectable one. However, Katerberg points out that its 148 lb-ft of torque is reached at 1,850 rpm, then the torque curve is essentially flat.

"The best compliment we get is that it feels like a bigger displacement engine," Katerberg says.