2008 GMC Yukon Two-Mode Hybrid

A hybrid heavy hauler

GMC’s Yukon is a strapping 2500 kilograms unloaded. In other words, it ain’t no Prius.

And yet, Yukons fitted with GM’s new two‑mode hybrid system use essentially the same principle to reduce energy consumption as Toyota’s celebrity-friendly it-car. An electric drive can power the vehicle, by itself or together with the combustion engine. The battery that powers the motor is charged by the engine and also through regenerative braking.

Being as big as it is, though, the Yukon outdoes other hybrids by having two electric drive motors, each capable of 40 kilowatts sustained. A single motor with enough torque to move such a mass couldn’t be accommodated inside the truck. So GM’s engineers, who led a design effort that later included engineers from DaimlerChrysler and BMW as well, used a pair of electric motors—a system GM had previously put into production for its hybrid transit buses. As in the Prius, the Yukon’s motors are actually motor/generators that can produce torque when fed with electricity—or vice versa.

The size of the system was constrained by the need to fit it into the same space as the company’s 6-speed automatic transmission. Inside an aluminum casing are packed not only four fixed gears but (from front to rear) a “planetary” gear set and electric motor, a clutch, a second planetary gear set and motor, and then a final clutch and gear set. In each planetary gear set, a central gear (the “sun”) is spun by the engine or electric motor. The sun is surrounded by planet gears that are in turn surrounded by a ring gear, which drives or is driven by another motor. By varying the speed at which the planet gears spin, you also change the power split between the torque on the sun and that on the ring gear. Being able to split the power arbitrarily like that lets you channel just the right amount of the engine’s power to the wheels; the rest is devoted to charging the batteries.

The “two modes” of GM’s system refers to the different mixes of electric-motor and combustion-engine torque. The first, or input-split, mode is for low-speed and light-load applications from launch through second gear. As in existing single-mode hybrid systems, a planetary gear set splits engine power between the wheels and one of the two electric motors. Acting as a generator, this motor charges the battery, which powers the other motor, which drives the wheels as well. In this mode, the engine-control system alternates among electric-only, engine/electric, and engine-only drive, choosing the option that provides the best performance with the lowest fuel consumption. From rest, energy from the 300-volt nickel-metal-hydride batteries powers the drive motor, which is quickly supplemented by torque from the engine.

The second, or compound-split, mode is for high speeds and heavy loads. It provides electric assist in the third and fourth gears. Here, both electric motors can receive torque from the engine and power from the battery. The second and third planetary gear sets not only split the engine power among the drive wheels and the electric motors, they can multiply all torque to deliver maximum power to the wheels.

Controlling it all are microprocessors. In fact, according to chief engineer Tim Crewe, 70 percent of the total effort involved in designing and testing the system went toward control logic. That logic analyzes hundreds of inputs every 10 milliseconds, including vehicle load, engine operations, battery parameters, and the temperatures in the high-voltage electric components. Depending on vehicle speed and power requirements, one motor-generator may assist the engine, or provide regenerative braking, with the other shut down for greater efficiency. Or both motors can provide boost or braking simultaneously. And the combination of planetary gear sets and electric motors smooths the shifting among modes, so that engine speed can stay constant even as it varies the electric power delivered to the battery or drive wheels.

When coupled with cylinder deactivation, the two-mode hybrid system is expected to improve overall fuel economy by 25 percent, to 10 or 11 liters per 100 kilometers (about 22 miles per gallon). For comparison, the standard GMC Yukon gets 15 to 16 L/100 km (roughly 15 mpg) in the city and 11 to 12 L/100 km (20 mpg) on the highway, according to the U.S. Environmental Protection Agency.

2007 Fiat Siena Tetrafuel

Auto omnivore

Stand on a street corner in any middle-class suburb in Brazil and well over half the cars whizzing by will be flex-fuel vehicles capable of burning various mixtures of gasoline and ethanol.

Among Brazil’s many flex-fuel cars, a version of the Fiat Siena stands out because it runs on any blend of ethanol and gasoline, and also on pure natural gas as well. It burns the natural gas first because it’s Brazil’s cheapest car fuel.

The Siena stores the liquid fuel in a 48-liter tank and the natural gas in two 6.5-cubic-meter cylinders in the trunk. The 1.4-L engine has two sets of injectors, one for liquid fuel and a second for natural gas. But those are the engine’s only major hardware modifications. The engine-control unit (ECU) is the same as the one on ordinary Sienas. It just runs more sophisticated software.

The car’s existing sensors measure the airflow into cylinders, the engine speed and load, and the specific fuel being sent to the engine. The data are fed to the ECU, which uses them to optimize fuel injection and spark timing. Most important, the ECU uses data from the sensor that monitors oxygen in the exhaust to vary the air-fuel mix continuously until that sensor detects no oxygen remaining in the exhaust, indicating complete combustion.