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The Truth About 4x4 Hybrids

Invisible View
Drew Hardin
| Contributor
Posted April 1, 2006

Do they make sense in the 4x4 world?

This see-through illustration from GM shows the mild hybrid componentry used on the Silverado/Sierra pickups. You can see the starter/generator between the engine and transmission, and also how GM has routed power lines to the bed, turning the truck into a generator capable of producing 2,400 watts of 120-volt power. With the truck's 32-gallon fuel tank, it'll run (at idle) as a generator for 32 hours before needing fuel.

Through most of 2005, gasoline prices seemed to rise with no relief in sight. The unstable Middle East, refinery problems, and Mother Nature herself conspired to drive pump-gas costs to unheard-of levels. As this is written at the end of 2005, gas prices are finally starting to fall. But having gone through the rollercoaster that put fuel at or over $3 per gallon at times (or more, if you were "lucky" enough to live in a hurricane-affected area), it seems inevitable that those price levels could-and will-be reached again.

How can you protect yourself from getting gouged at the pump? One way is to follow the fuel-savings tips trotted out every time gas prices climb: proper tire inflation, engine in tune, sparing use of the A/C, and so on. But those will provide incremental gains at best. If you really want to make a difference in your monthly fuel bill, it's tough to ignore the lure of buying a gasoline/electric hybrid vehicle. The 50- and 60-mpg figures trumpeted by the most frugal of the hybrids seem pretty damn attractive compared to the gas hog you've got in your driveway.

Well, maybe. Though there is, right now, a rush to buy these green machines, there's also a backlash-that maybe these hybrid vehicles aren't all they're made out to be.

Hard truth number one: As 4x4 enthusiasts, we're pretty much stuck watching the whole hybrid scene pass us by, at least for now. Most state-of-the-art hybrid technology is being applied to passenger cars and, to a lesser degree, midsize and compact SUVs. Those SUVs are for the most part front-drivers with an all-wheel-drive system designed more for inclement weather than rocky trails or mud holes.

That's not to dismiss the technology. We've sampled nearly all of the hybrid trucks currently on the market and found them to be capable and competent road vehicles. Some, like Toyota's Highlander, are even more fun to drive than their conventional counterparts because the boost from the electric motors makes them more powerful than the gas engine alone.

The electric motors in hybrids don't need to be plugged in because they make electricity by capturing and converting energy that's typically lost during braking, deceleration, and coasting. Here's the regenerative braking system on the Ford Escape hybrid.

But these are road-going vehicles. The only trucks you can buy with a hybrid system and a low-range transfer case are the Chevy Silverado and GMC Sierra hybrid pickups, which are just now going on sale nationwide. You should know, too, that these trucks are equipped with "mild" hybrid systems that (a), return a reported 10 percent fuel economy improvement, not the huge numbers you see with the cars; and (b), are designed to shut off the system's engine start/stop feature-its main source of fuel savings-when low range is engaged. (See the sidebar "Strong or Mild?" for a brief explanation of how hybrid systems work.)Why does it seem the technology is passing us by? Manufacturers are struggling to design a full hybrid system compatible with a vehicle that's often loaded close to its GVWR. All those Priuses and Civic Hybrids you see on the road are typically carrying one, maybe two people. The tiny internal combustion engine/electric motor combo isn't taxed hauling that kind of weight, even going uphill. The full hybrid systems found in the Toyota Highlander/Lexus RX 400h are more powerful, but even those SUVs are rarely as loaded down as a fullsize truck full of ATVs, camping gear, lumber, or other weighty cargo.

Engineers call that sort of use "bandwidth." "It's a challenge to design a full hybrid system for a truck since its bandwidth is so high," said Mike Polom, assistant chief engineer for GM's starter/generator hybrid system. "You've got high expectations for a vehicle like this, to achieve improved fuel economy in addition to meeting the high demands for a truck's utility-work, towing, off-roading. That's something you don't have to do with a Prius."

Plus, many of a hybrid's on-road characteristics just don't fit with the way we use our trucks. If you were crawling along a rocky trail, would you want your engine to shut off every time you lifted off the accelerator? On the other hand, the supplemental torque from an electric motor can add thrills to high-speed blasts around the dunes or on fire roads, but driving like that will drain the motor's batteries without giving them much regeneration time, so you'll inevitably find yourself depending on the internal combustion engine alone. So why pay the weight penalty of having the motor and batteries along for the ride? And really, when you're creeping along at a couple miles per hour or playing trophy truck on back roads, are you that concerned with fuel economy?

How Hybrids Work
Though the system details vary from maker to maker, hybrid drivetrain components in today's trucks can be broken down into two major groups:

Full (or strong) hybrid drivetrains are made up of two propulsion systems: an internal combustion engine and one or more electric motors. Depending on how the system is designed, the engine and motor(s) will work independently or together to improve fuel economy without hurting performance. In some cases, performance is actually enhanced due to the torque provided by the motor(s). These systems shut off the engine when idling and restart it when the accelerator pedal is depressed. (In some cases, engine accessories such as air conditioning will keep the engine running even at idle.) The electric motors do not need plugging in; the internal combustion engine charges the batteries that drive the motor, plus the (normally wasted) energy spent during deceleration, coasting, and braking is captured, converted to electricity, and stored in the batteries.

Mild hybrid drivetrains do not have a second propulsion system beyond the internal combustion engine. Instead they are equipped with a starter/generator system that allows the same stop-at-idle characteristic of a full hybrid and, in some cases, a certain amount of propulsion torque. Typically these vehicles are also designed to make available the electricity stored during coast-down and regenerative braking via power outlets in the cab and bed.

These diagrams, displayed on the dashboard of the Toyota Highlander Hybrid and Lexus RX 400h, present a simplified schematic of how a full hybrid system works. See how the dotted lines and arrows are running from the battery and motors to the wheels? That means the SUV is operating on electric power only (which happens at low speeds). A second line from the engine to the front wheels would indicate the truck was being driven by both gasoline and electricity. In the right-hand diagram the Lexus is either decelerating or under braking, because the dotted lines indicate energy is moving from the wheels to the battery pack.

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