ABG First Impressions: Two-Mode hybrid Chevy Tahoe and GMC Yukon
Click on the image for a high-res gallery of the Two-Mode Hybrid GM SUVs
Back in 2004 General Motors first announced plans to partner with DaimlerChrysler and later also BMW to develop a full parallel hybrid drivetrain system. This system, referred to as the Two-Mode hybrid, was adapted from a system developed by GM's Allison transmission division for use on transit buses. Subsequently the three companies opened a joint development center in Troy, MI in 2006 and all three companies have applications planned although only GM and Chrysler have announced specific vehicles. BMW has yet to announce a specific vehicle and likely won't introduce their first application until 2009 or 2010. Early in 2008, Chrysler will release a version of the Dodge Durango/Chrysler Aspen equipped with the Two-Mode system and sometime after that Mercedes is expected to bring forth a hybrid ML class, possibly also equipped with a BlueTec Diesel.
General Motors has announced three vehicles that will get the new system, with the Chevy Tahoe/GMC Yukon being the first one coming this fall. A few months after that the Cadillac Escalade picks up the extra hardware, followed by the Chevy Silverado/GMC Sierra pickups and finally the Saturn Vue/Chevy Equinox. The smaller crossovers will be the first application with a transverse drivetrain in both front and all wheel drive configurations. The GMT900 platform vehicles will also be in available in both rear and all-wheel drive hybrid configurations. With the Tahoe/Yukon hybrids due to come off the Arlington, TX assembly lines in just a few months, GM felt it was time to invite some media to their Milford Proving Ground to sample the new system for the first time.
Continue reading about the first drive in these new hybrids after the jump.
Gallery: 2008 GM 2-Mode Hybrid SUVs
The existing mild hybrid system that GM offers on the Saturn Vue and Aura uses a combined starter and heavy-duty alternator that are belt-driven off the engine. In the Two-Mode system a pair of 60kW electric motors are integrated into a new electronic variable transmission. A 1.8 kWh, 300V nickel metal hydride battery pack supplied by Cobasys sits under the second row seat. The transmission is mated up to a 6.0L V-8 that has variable valve timing and displacement on demand that can shut-off four cylinders under light loads.
The Two-Mode name refers to the two distinct operating modes that are available. The first mode is used under lower-speed light load conditions. Based on conditions and battery charge level, the system can operate in either battery only mode, engine only or combined mode. This first mode is pretty much the same as the other existing full hybrid systems on the market from Toyota and Ford. In this lower speed mode the transmission functions with continuously variable ratios as the power flow is blended between the engine and electric motors. If the battery has power, the truck will pull away from a stop in almost complete silence propelled only by electricity and keep going for about half a mile.
As the charge level of the battery drops, the engine almost imperceptibly fires up and keeps you moving. Unlike the Altima Hybrid that had a distinct shudder when the engine kicked in under light throttle conditions, the only indication of the Tahoe V-8's activity came from the exhaust note and gauge cluster. The exhaust was by no means loud but was mostly noticeable due to the effort GM expended to quiet down the rest of the vehicle. With the capability of running in electric mode, NVH was clearly an issue that needed to be addressed.
One of the biggest sources of noise in traditional setups is the vibration of all the accessories that are mechanically driven off the engines. Two of the biggest loads are the air conditioning compressor and power steering pump and GM has largely silenced these by electrifying them. The air is now chilled by a 300V compressor while the rack mounted electric power steering assist runs off of 42V. The power control module that manages the flow of electricity between the battery and motors has DC power outputs at 300V, 42V and 12V for the various systems on the vehicle.
From the driver seat the primary visual indicators that you're driving a hybrid are in the gauge cluster and center console. The central display features a power-flow indicator like the one in the Altima that lets you and your passengers know whether the vehicle is sending power to the wheels from the battery, engine or both and also when the power is flowing in the other direction. The tachometer now also has an auto stop position just below zero when the engine has shut down. An economy gauge sits to the left of the tach and adjusting your driving to keep the needle pointed straight up will yield the best results.
Where this new system differs from others on the market is the second mode. As speed and load increase, the transmission which features four clutches transitions from variable ratio mode toward one of four fixed gear ratios. When this happens, power flow is diverted completely away from the electric motors and directly through the gears for even greater efficiency. While the motors are about eighty-five percent efficient, power flowing through direct mechanical gearing is more than ninety-five percent efficient. If even more power is needed such as climbing a hill while towing a trailer or boat, the motors can also kick in at higher speeds.
In order to improve the long term durability of the motors, transmission fluid flows through cooling jackets around and through the electric motors to keep the temperature down. In addition the copper windings of the motors are made with extra thick wire to minimize resistance and heat build up.
All this extra hardware adds mass to a vehicle as evidenced by the extra 300 lbs that the Altima Hybrid carries compared to a standard model. GM made a major effort into mass reduction on the hybrid, switching to aluminum for the hood and tailgate as well as other parts. They also added lighter weight front seats that are also thinner than the regular models. That had an added benefit of increasing rear leg-room. New lighter weight wheels also shaved about 5lbs per corner. All together the diet was enough to neutralize the 400 lbs added by the hybrid drive-train and keep the overall weight essentially unchanged.
The most noticeable external difference is in the front clip which has a significantly deeper air dam and is more sealed off. Additional aerodynamic tweaks around the wheel openings, running boards and the rear fascia and spoiler all contribute to a drag coefficient reduction from .37 to .34. Although all of this helps, when you are dealing with a vehicle that has the frontal area of an average garage, it only goes so far.
The calibrations of the 6.0L V-8 have also been optimized to make the most of the hybrid system. The GM V-8s have had a displacement on demand system for several years, but the presence of the hybrid system means that the motors can provide a performance boost when needed. That allows the engine to run in four cylinder mode much more often in medium load conditions than it does currently. The driver information display in the main cluster lets you know whether the engine is running of four or eight cylinders and it's definitely the only way you'll ever know, because the change-over is imperceptible.
The final major mechanical change to the hybrid trucks is a new electro-hydraulic brake system supplied by TRW. The vacuum brake booster normally found on the Tahoe is now supplanted by the new system which uses a high pressure hydraulic pressure accumulator to provide boosted braking. Extra solenoids in the system also control the blending of friction braking and regenerative braking. The regenerative braking can provide up to 0.3-0.4g of deceleration which will cover about 90-95 percent of brake applies in the real world. In the not so distant past, pressing the brake pedal of GM truck resulted in using up half of the travel before any noticeable deceleration occurred. Now it feels like applying the brakes of a race car with decel starting almost as soon as you touch the pedal. It's not grabby, just very responsive.
After a brief drive at the GM proving ground in a pre-production prototype, it was clear that if nothing else the changes done to create a hybrid full-size SUV have resulted in the most refined large truck GM has ever built. The mantra repeated during the presentation was seamless integration. It's incredibly quiet even with the engine running. There are no noticeable transitions between driving modes and it all just seems to work. GM is still doing some tweaking to calibrations, but they are very close.
According to GM, the 6.0L hybrid Tahoe will get about twenty-five percent better fuel economy on the combined EPA city/highway cycles compared to the 5.3L standard vesion and do forty percent better in urban driving. The Two-Mode Hybrid Tahoe/Yukon will be the first hybrid full sized SUV when it goes on sale this fall and the only hybrid with a 6,000 lb towing capacity and eight passenger seating. When asked why not mate the hybrid system with the smaller engine or even the Duramax diesel engine, they gave the usual answer about not discussing future product plans. They did however indicate they are working on a full range of options from the mild hybrid system in the Aura and Vue through the Two-Mode system and on to the plug-in hybrid Vue and E-Flex. This is just the first step and for a vehicle that represents a large segment of the American market it is a very important one. Once production versions are ready later this year, we'll be sure to do a more thorough evaluation and find out how well this all works in the real world.
Reader Comments (Page 2 of 3)
iamsancho99 10:39AM (5/02/2007)
These apartment dwellers need to remember that people in small town and rural America need pickups and SUVs for a variety of uses such as hauling hay and grain and lawn and garden needs. as well as helping their friends move their stuff from one apartment to another. Recreation such as camping, hunting, fishing, and boating call for something with a tow rating or a cargo bed. Have you ever tried to transport a cone with a Prius? I dropped my membership in Sierra a few years ago after listening to other members berate people like me who enjoy spending time in the outdoors. Apparently I should be satisfied with watching it on tv and reading about it in magazines.
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dag_in_va 10:43PM (5/02/2007)
"Now 10% is nothing to sneeze at, but at the same time, it doesn't sound as nice as 25% and 40%."
If your driving is mostly highway, no hybrid is going to buy you much. Their advantage is all in stop-and-go: Better match of engine to varying load and regenerative braking.
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Benson Leung 10:48AM (5/02/2007)
SkiD666... You need to reference that study, or else your credibility is suspect.
Furthermore, even if 50% of people who drive Priuses also own SUVs, that IS more progress than replacing one big SUV with another SUV that is 25% more efficient.
If a person who used to drive around a 15 MPG SUV decides to instead to make the 50MPG Prius his main commuter vehicle, and relegating his SUV for once-in-a-long-while duty when he actually has to haul around 7 passengers, his improvement in efficiency for day to day commuting is 333%.
If a person who owned a big 15 MPG SUV bought a Tahoe Hybrid as their main commuter vehicle, their improvement is 25%.
How much different is it? People switching to smaller size cars (including high efficiency Hybrids) that meet their everyday needs but do not vastly exceed them is MUCH BETTER than buying a vehicle of the excessive huge size, but gets marginally better fuel economy.
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Benson Leung 11:01AM (5/02/2007)
iamsancho99:
I have no problem with people driving an SUV or a pickup truck if they 100% absolutely need it. If you live in a rural area, then I agree, a pickup truck is a necessity in a lot of lines of work.
Recreational uses, fine... but the fact is, most people don't use anywhere near the full utility of a big SUV.
My problem is with single occupancy commuters who go 50+ miles a day from their suburban home to work and back in a big honking SUV... people who absolutely don't need it.
You cannot deny that this (single occupancy commuters) is a huge segment of people who purchase this class of vehicle. For the last 20 years, SUVs have become more and more popular among suburban folks, have become soccermom vehicles... For most of these people, a medium sized car would be better suited for them.
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iamsancho99 11:15AM (5/02/2007)
Applying hybrid tech to heavy vehicles makes good marketing and economic sense. because the fuel consumption is higher, the economic benefit to the customer is greater and after all these are the models that are profitable for Detroit. dont look for Big Auto to try competing with Japan on lowe cost models anytime soon. In fact Japan is or will be manufacturing their lower priced lines in China to save costs.
When all is said and done you have to assume that most car buyers are selecting what meets their needs.
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iamsancho99 11:16AM (5/02/2007)
"many" is usually preferable to "most" or "all'. I beleive that except for the wealthy, people are balancing what they can afford to buy and to operate with what they need. Anyway, $4.00 gasoline will make this all moot soon. we are approaching a long term worsening supply crunch for crude oil that will make operating any kind of car an increasingly expensive thing to do.
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Ted H 1:33PM (5/02/2007)
People are going to buy them anyway, so they might as well improve them.
Posted at 8:27PM on May 1st 2007 by Travis Rassat
Excellent point! People WILL buy SUV's anyway. Now we they still have a chance to improve emissions & fuel consumption relative to the alternative of getting a regular SUV.
People need to get off their high horse and stop saying that other people should buy a small sedan instead of an SUV and do what GM is doing to make SUV owners save fuel anyway.
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Thomas Perry 10:25PM (5/02/2007)
I drive an SUV because many of my co-workers were killed by Deer coming through their windows or running them off the road. I sold my Camaro z28 for a Chevrolet Trailblazer. Three days later I hit two deer on the way to work. Both went under the vehicle instead of over the hood allowed me to keep control.
4 am travels for 20+ miles in South Carolina is not safe for a vehicle with a hood height below the center of gravity on a deer.
Two other times I have had the luxury of clearance enough to drive over items in the road only to see the vehicles behind me hit them. One was a 8 foot ladder and the other was a small pile of wood.
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str8diesel 7:30PM (5/03/2007)
try not to be too cynical about hybridizing an SUV... even though the percent improvement in fuel economy is lower compared to that of a small hybrid car, the overall benefit is quite large. SUV's are popular, and waste a tremendous amount of fuel. any improvement in fuel economy is truly noteworthy. and though i am a greenie who ideally would love to see everyone driving prius's and smart cars, i comprehend reality - the reality being that many people want to and will own and drive SUV's, regardless of how much gas they guzzle. since we can't mandate that SUV's be banned, we have to add a few bandaids to them. it's a step in the right direction. kudos to GM for jumping into this market...
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Chris M 3:35AM (5/04/2007)
Quote: "I drive an SUV because many of my co-workers were killed by Deer coming through their windows or running them off the road."
Reminds me of a photo captioned "Deer vs. Durango", a deer had jumped over the hood of a big Durango SUV and landed right on the windshield, crushing the roof. Bad for deer and Durango. If they had been driving a Miata instead, the deer would have jumped clear over the car...
Quote: "If your driving is mostly highway, no hybrid is going to buy you much. Their advantage is all in stop-and-go: Better match of engine to varying load and regenerative braking."
My, you really don't know beans about hybrids! My Prius routinely get 50 mpg on a 120 mile trip from Modesto to San Jose, over Altamont pass. A good hybrid design will improve fuel milage in all driving conditions, city and freeway. While the Yukon hybrid is no match for a Prius in milage, it will get much better milage than a non-hybrid Yukon in city driving and on the highway.
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cmm 11:28AM (5/04/2007)
Excellent question/comments about the engine displacement. The 6.0L engine in the hybrid differs from the 6.0L used in other GM full size truck applications in that it utilizes late intake valve closing, enabled by the VVT system (sometimes referred to as a modified Atkinson cycle). By closing the intake valve later than normal, the expansion cycle is longer, which gains efficiency but results in a slight loss of power (compared to more traditional valve closing timing) due to reduced charge density. This is partially offset by an increase in compression ratio from the base 6.0L (which is also good for thermal efficiency). In order to maintain a good balance between the performance requirements of the truck (including towing/payload capability) and the fuel economy, the larger displacement makes a lot of sense (including the ability, as mentioned in another comment, to maintain the efficient 4-cylinder mode more often).
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ThriftyTechie 11:17AM (5/06/2007)
First off, of course the hybrid Tahoe is obviously less "green" than a conventional Corolla, Civic or Cobalt, etc.
But if the buyer is committed to buying a large SUV (they've got a boat, have 4 kids, haul large things on a regular basis, etc.) then this Hybrid Tahoe can be a good thing.
Assuming that original Tahoe MPG is 15 MPG
http://consumerguideauto.howstuffworks.com/consumer-guides-real-world-fuel-economy-champions-cga.htm
Assuming that new Tahoe MPG is 18 MPG
Assuming gas is $3 per gallon.
Assuming driving 12,000 miles per year.
Then the hybrid Tahoe owner would save $400/year on gas.
It is rumored that GM will charge a $2000-$4000 premium for the hybrid Tahoe.
If the premium is $2000, this is pretty much a no-brainer for people in the market for a Tahoe. You will do good for the environment for almost no cost over 5 years.
If it's more like a $4000 premium, then this product is simply like the Prius: It's an environmental charity case.
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RF 10:01PM (5/06/2007)
does anyone know when the 2008 Hybrid Tahoe is being released?
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RF 10:16PM (5/06/2007)
does anyone know when this new Chevy Hybrid Tahoe is being released?
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ThriftyTechie 6:54PM (5/07/2007)
According to the Chevy website, the Tahoe Hybrid will be released "Fall 2007". We'll call it November '07, then.
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Joe Alien 6:51PM (5/08/2007)
@NILS: Your comment that this is to ease the consumer's conscious is ridiculous. That's not what GM or any company would use as justification since they need to sell vehicles. And I totally disagree with your notion that it's "the number of gallon's used per vehicle". The true value and comparison should be the gallon's used to move the contents of vehicle, including passengers. It's not up to you to ask or decide who this vehicle is good for, it's up to the overall market. You suggest a soccer Mom with 5 kids use a MiniVan. Why stop there. Afer all that Mom could rent a horse and wagon to get the transportation job done and not use fossil fuels at all. Those who suggest and single out SUVs do so by being their own judge. But who are they to judge?
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JV 12:33PM (5/09/2007)
It seems some people think it's okay to have an SUV only if it's rarely used (i.e, it's the 2nd vehicle, used a couple times per year). Sorry, but I can't afford to buy two vehicles just for one to sit there. I will get one that meets my needs. That's an SUV.
I think GM will take a lot of customers away from their competitors with this product, which will reduce the country's fuel use by a large amount. Note though, that GM's full size trucks are ALREADY more fuel efficient that its competitors, and that includes the new Tundra.
Note too that the worse the fuel economy, the more gas is saved with a small increase in fuel economy. Comparisons about fuel usage should always be done using gallons/xx miles (e.g., gal/100 miles), not with miles per gallon. For example, putting this same type of hybrid system in a city bus that got 10 mpg, but now gets 12 with the hybrid (probably not the right numbers, but the point still stands), saves 3 times more fuel per 100 miles than an increase from 30 to 36 mpg (also a 20% mpg improvement). Despite appearances, the 2 mpg improvement in the bus is much better at saving fuel than the 6 mpg improvement in the car. (This is why GM put its first hybrid in city buses -- and now in full size trucks -- instead of in already-efficient cars.)
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R. Van Sickle 1:03PM (5/10/2007)
I understand that Remy is supplying these motors for the SUV Hybrid. That is very positve with their global technical development on high output alternators and hybrid development. I am looking forward to purchasing this vehicle.
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pedmac2000 8:09PM (5/21/2007)
Hi Sam , in a comment i asked the question if it was Cobasys that was supplying the 2 mode hybrid battery pack on the two-mode system .. and you replied that yes they supplying for mild and the two-mode system . May i ask how you know this . as Cobasys is being very secretive about all this .. i also presumed the same as Gm has been using them as the supplier in the past.. did u see it at the GM TOUR.
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pedmac2000 9:07PM (5/22/2007)
Sam tks ur kind response to my post asking if it was also Cobasys supplying the two-mode 330 v battery pack...but how did u find this out ..
Cobasys is tight liped .. and i cant seem to find out anywhere but here... so kudos to u...but did u find outat the gm tour of the tahoe hybrid?
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