Click for Coverage
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler
Subscribe to the Free

The Truth About 4x4 Hybrids

Posted in How To: Engine on April 1, 2006
Share this
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.

Speaking of long-term plans, Toyota has also developed a hybrid SUV using a hydrogen-powered fuel cell and an electric motor. Several are being tested here and in Japan for their viability, but don't hold your breath waiting for any fuel-cell-powered vehicle.

Speaking of fuel economy, hybrids have gotten some bad press recently for delivering real-world mpg that's lower than their EPA ratings. Not that that's anything new; probably few of you are getting close to EPA fuel economy in your trucks, no matter what powertrain they're running. But when you buy a hybrid car that's supposed to get around 50 mpg, and you're getting 35 to 40, that's a downer.

Why is this happening? Beyond the usual gap between EPA numbers-which are generated in a climate-controlled lab with the vehicle on rollers, not on the road-and real-world mileage, many hybrids are designed to get their best fuel economy in stop-and-start urban driving conditions, which are not replicated very well in the EPA tests. Talk of disappointing fuel economy is one element in a hot debate: What's the true cost of owning a hybrid vehicle? Do the fuel savings offset the premium you have to pay at the dealership? Will the batteries really last? And what will it cost to replace them?

Let's tackle the battery issue first. It's true that if you had to replace the battery pack in today's hybrid trucks, you'd be out several thousand dollars. But that's a very big "if," for three reasons. First, every manufacturer warranties the hybrid components-including the batteries-for anywhere from eight to 10 years.

Second, the makers have logged hundreds of thousands of miles testing the hybrids and their battery packs and have found no premature battery pack failures (at least, none they'd report to the press). Makers are also keeping a close eye on their hybrid owners to check for such a thing, and so far have found no battery issues.

Hybrid drivetrains in trucks and SUVs aren't new; Dodge showed off a CNG/electric hybrid powertrain in the PowerBox, a concept truck unveiled in 2001. Ultimately the "through-the-road" hybrid system used here wasn't further developed for production vehicles, but you can certainly see styling hints of the Durango that was to come.

Third, battery technology, like so many other things, is constantly improving. Paul Williamsen, Toyota's product education manager, told us that the power density available in the Prius battery pack-in other words, the amount of power generated per battery mass-has almost doubled with each Prius generation. "The cost of those batteries also came down 50 percent or better at each threshold," he added. "So should a first-generation Prius owner need to replace the battery pack, it would be more powerful and less costly than the original batteries."

As for the issue of whether fuel savings offsets the hybrid's purchase premium, you have to figure out how many gallons of gas are saved driving the hybrid and compare the value of that fuel to the premium paid the dealer. For example: An AWD Toyota Highlander with the conventional V-6 carries a base sticker of $27,840, while a comparable AWD Highlander Hybrid's base price starts at $34,430, $6,590 over the cost of the gas-only version. The hybrid achieves a combined 29 mpg (per the EPA), while the gas-only SUV gets an average of 21 mpg, an improvement of 8 miles per gallon.

If you estimate the cost of gasoline at $2.20 per gallon (the national average for a gallon of regular at the time of this writing in late 2005 and the number the EPA uses to calculate annual fuel cost estimates), and you figure an average use of 15,000 miles per year, fuel costs for the hybrid model will be about $1,138 annually, compared to $1,571 for the conventional Highlander. Divide that $433 difference into the hybrid's $6,590 purchase premium and it'd take you 15 years to pay off the premium in fuel savings alone. Now, you can fudge that premium down somewhat by taking into account the 2006 federal tax credit for purchasers of gas/electric hybrids. The incentive varies by model; for the AWD Highlander it'll be about $2,200. Subtract that from the purchase premium and the payoff time is cut by a third-down to 10 years. Then again, you'd need to fudge the numbers back up some to account for the difference between EPA mileage and actual mpg. Bottom line: A hybrid will cost you less at the pump, but overall your bank account will be lighter for quite a while, unless gas prices take a big jump.

Toyota's long-term plans include offering a hybrid version of every vehicle it sells, including pickup trucks. The FTX concept truck that was unveiled at the Detroit Auto Show in 2004 was rumored to be a thinly disguised version of the next-generation Tundra fullsize pickup. It was powered by a "large-displacement V-8 hybrid gas-electric engine that pumps out loads of torque while delivering V-6 fuel efficiency," said Toyota.

To Toyota's Williamsen, his company's hybrid customers-including legions of loyal Prius owners-find value in their purchases outside of their bank statement. "Our customers define value and what's meaningful to them beyond having to pencil it out at the other end. I mean think about it: Have you ever heard anyone make a dollars-and-cents argument for an SVT Lightning or a Corvette Z06?" Or, to extend Williamsen's argument, for a locked-and-lifted 4x4? We can all relate to the emotional gratification that comes from owning a cool trail rig. Hybrid owners feel much the same, but in a "green" way that may be somewhat foreign to our way of thinking.

Nick Cappa, DaimlerChrysler's manager of advanced technology public relations, sees the average consumer, and off-roaders in particular, as wary of hybrid technology. "Hybrids make up a half a percent of the industry right now; 100,000 to 200,000 vehicles. It's a speck. Hybrids are usually linked to people who want good fuel economy, who want to be green. But do people really want a 4x4 hybrid? Do they want better mileage off-road? Ultimately it's the customer who will decide if there's a market for a hybrid powertrain in an aggressively modified 4x4, and if they're willing to pay a premium for the system. Based on the size of the market right now, I'd say the answer is 'no,' and that internal-combustion horsepower is still king."

Unlike Toyota, DaimlerChrysler has put just a figurative toe in the hybrid waters, at least until recently. Earlier in the decade, the company made just 100 Ram HEVs-a diesel/electric hybrid on the heavy-duty pickup platform-and sold them to fleet customers. Currently the HEV project is "in limbo," according to Cappa.

That was then. More recently, DaimlerChrysler joined with GM and BMW in a ground-breaking partnership that's cooperating on a new form of hybrid technology called the Two-Mode Hybrid System.

In the not-so-long-term, DaimlerChrysler, GM, and BMW have teamed to develop an innovative Two-Mode Hybrid System that will be applied to '08 models of the Chevy Tahoe/GMC Yukon and Dodge Durango. Inside that transmission case are two electric motors that are electronically controlled to boost power at low and high engine speeds (hence the Two-Mode name). Because the unit is no larger than a conventional automatic transmission, it can be adapted to fit a wide range of RWD, FWD, and 4WD vehicles. GM and DaimlerChrysler estimate the Two-Mode system will deliver a 25 percent improvement in overall fuel economy.

Slated to appear late in 2007 on board the'08 Chevy Tahoe and GMC Yukon, and later on the '08 Dodge Durango, the Two-Mode System houses two 60 kW electric motors inside an automatic transmission. The motors are used to assist the internal combustion engine in two different ways: One mode for low vehicle speeds and light loads, the other to boost performance at high engine speeds and in high-power-demand situations, like towing. As with current full hybrid systems, the Two-Mode will allow the trucks to operate only on electric power at low speeds, and will include a start/stop function that shuts off the engine at idle, a key component to getting high mpg numbers in city driving. Power for the motors will be stored in a 300-volt nickel-metal-hydride battery pack located under the second-row seat.

The electronic controls inherent in the Two-Mode System "will allow you to use our Displacement on Demand technology more aggressively," said Dana Kaplinski, assistant chief engineer for GM's side of the Two-Mode System. "You have more ability to save fuel because you can disable the cylinders across a wider driving range." When mated to an engine with GM's Displacement on Demand or DaimlerChrysler's Multi-Displacement System, the Two-Mode is expected to deliver a 25 percent improvement in overall fuel efficiency.

Another advantage: Because it's being developed by three different companies, they can share the development and engineering costs so that, in theory anyway, the premium for the hybrid system won't be as high as we've seen on other trucks. Officially, "pricing hasn't been released, but we'll be competitive," said Kaplinski.

This schematic from GM shows the differences between the Two-Mode system's operating ranges. The first mode is used at low speeds and enables the SUV to operate on electric power only, engine power only, or some combination of the two. The second mode improves the vehicle's efficiency at higher engine and road speeds and under heavier loads, such as towing a trailer up a grade.

Toyota, too, is working on adapting a full hybrid system to its pickup trucks. A recent news story from Reuters said the company has plans for a "cheaper, smaller" hybrid system, available in 2008, that will slash its premium by half and allow its use on a broader range of vehicles. Toyota's eventual aim is to offer a hybrid powertrain on every vehicle it sells.

But Paul Williamsen pointed out the challenges Toyota's faces. "Our current AWD hybrid system for the Highlander is not a body-on-frame system with driveshafts to both sets of wheels. There is no rear driveshaft; it uses complete electronic control of the rear wheels. That has its advantages; you can instantaneously add torque to the rear wheels, and no internal-combustion engine can do that." But how do you adapt that system to a pickup? Turn the Highlander system around so there's mechanical control of the rear wheels and electric power to the fronts? That's a possibility, said Williamsen, "but that presents some packaging issues. [The front end] is a busy place, and about the worst place to put more mass." Plus, the high amounts of torque from an electric motor "could spin a front hub apart very easily."

If Toyota can release a new truck-based hybrid system in 2008 or thereabouts, and the GM/DaimlerChrysler Two-Mode System stays on schedule, we off-roaders may actually have some viable hybrid choices in the near future. We can only hope that the premium paid for the technology goes down, because we're pretty sure gas prices won't.

PhotosView Slideshow

Ford Escape/Mercury Mariner
Hybrid System Full
Hybrid EPA Fuel Economy (city/highway) 33/29
Conventional EPA Fuel Economy (city/highway) 21/24 (I-4 engine)
Hybrid MSRP $29,140 (Escape AWD), $29,840 (Mariner AWD)
Conventional MSRP $24,285 (Escape XLT AWD), $25,035 (Mariner Luxury AWD)
Estimated 2006 Tax Credit $1,950

PhotosView Slideshow

Chevrolet Silverado/GMC Sierra
Hybrid System Mild
Hybrid EPA Fuel Economy (city/highway) 17/19
Conventional EPA Fuel Economy (city/highway) 15/19
Hybrid MSRP $36,705 (Silverado extended cab 4x4)
Conventional MSRP $35,205 (Silverado extended cab 4x4 with Vortec 5300 V-8)
Estimated 2006 Tax Credit $650

PhotosView Slideshow

Toyota Highlander/Lexus RX 400h
Hybrid System Full
Hybrid EPA Fuel Economy (city/highway) 31/27
Conventional EPA Fuel Economy (city/highway) 18/24
Hybrid MSRP $34,430 (Highlander AWD), $48,535 (Lexus AWD)
Conventional MSRP $27,840 (Highlander AWD V-6 w/third-row seat), $37,770 (Lexus RX 330 AWD)
Estimated 2006 Tax Credit $2,200

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results