It’s big, it’s bad, and it’s both comfortable and safe. Most surprising of all, it is fuel efficient given its size.
Wayne Gerdes - CleanMPG
- Nov. 26, 2007
Although GM has been developing hybrid drivetrains for automobiles of one sort or another since the mid 60s, only recently have they begun to seriously design a full hybrid to meet the needs of the consumer marketplace. This full hybrid system has come to be known as the GM 2-Mode.
First developed for Hybrid bus applications through Allison Transmission, the Dual-Mode (as it was known before the 2-Mode designation) was mature enough for scaling to a consumer based platform. GM decided their first 2-Mode would be placed in two of their large SUVs -- the Chevrolet Tahoe and the GMC Yukon.
The advantages of the 2-Mode system are three fold. First, a full-hybrid drivetrain offers superior city fuel economy regardless of the size and weight of the vehicle in which it is implemented. Second, the system is enhanced by an advanced fuel management system which features displacement on demand (or cylinder deactivation), Atkinson style intake, and valve train manipulations to close both the intake and exhaust valves to reduce inactive cylinder pumping losses. In concert with the 2-Mode system, previously unavailable fuel efficiencies can be achieved under suburban and highway driving scenarios with a conventionally sized engine. The third and final advantage is realized when using a full-sized engine with the additional power available from the hybrid drivetrain to compensate for any losses due to Atkinsonization. This allows a far larger towing capacity than that of the more typical hybrid with a downsized ICE most are familiar with.
This brings us full circle to the GMC Yukon 2-Mode Hybrid preview. Although a 2-Mode full-sized SUV may never be the first vehicle of choice for most, it is a step in the right direction for those who need a full-sized SUV because it offers a real world size and/or capability advantage over a similarly sized conventional vehicle. Both the 2-Mode Tahoe and Yukon hybrids lay the groundwork for future developments -- including the upcoming Fwd Saturn VUE 2-Mode PHEV-10.
2008 GMC Yukon 2-Mode Hybrid - Basic Specifications
2008 GMC Yukon 2-Mode Hybrid – EVT (Electrically Variable Transmission)
| || |
|Type||6.0L V8 - Gasoline electric hybrid|
|Valve Train||SOHC w/ VVT |
|Transmission||EVT (Electronically – continuously Variable Transmission)|
|Max Power||332 HP @ 5100 RPM|
|Max Torque||367 Lb-Ft @ 4100 RPM|
| || |
|Hybrid System|| |
|Electronics||300-volt (transmission, air conditioning, heater pump, front crash sensor)|
| ||42-volt (power steering)|
| ||12-volt (vehicle accessories)|
|Batteries||300-volt ESS (Energy Storage System) |
| ||12-volt (vehicle accessories)|
| || |
| || |
|Fuel Economy (City/Highway mpg)|| |
| || |
|Basic Performance|| |
|0 – 60 mph||10 seconds (estimated)|
|Trailer towing maximums - 2WD/4WD (lb)||6,200/6,000|
| || |
|Weights and Capacities|| |
|Curb Weight||5,617 pounds|
|Fuel Tank (US gallons)||24.5 gallons|
| || |
|Wheels and Tires|| |
|P265/65R18||18” Aluminum Wheels|
GM’s all-new 2-Mode hybrid drivetrain is a unique assembly consisting of two 60 kW electric motors, three planetary gearsets and four traditional hydraulic wet clutches (C1 - C4). Also shown is the transmission hydraulic pump and input shaft damper, which take the place of a traditional torque converter.
This arrangement allows continuously variable operation while also providing four fixed gear ratios comparable to that of a standard electronically controlled automatic transmission.
The GM EVT can activate any of its four hydraulic clutches to allow power to be transferred via the fixed-gear ratios whenever high load conditions are experienced.
The Hybrid Optimizing System (HOS) constantly receives torque-based data from the powertrain and other vehicle systems, behaving like two transmissions in one — a continuously variable drive for light-load conditions and a fixed-ratio drive for high-load situations.
The HOS achieves what is known as "input-split" mode by locking the first motor so that the output shaft operates at the same speed as the input shaft. This approach provides either high fuel efficiency or high power, but not both. It is for this reason that GM added a second or “compound-split” mode, which adds another electric motor.
All functions of the EVT are controlled by the HOS, which constantly adjusts for optimal transmission behavior (using either variable or fixed ratios) to best accommodate current operating needs. The HOS also incorporates logic allowing the Vortec 6.0L V-8 to take full advantage of its AFM system (Active Fuel Management) so that it remains in V-4 mode as long as possible for maximum fuel economy in low load situations.
2008 GMC Yukon 2-Mode Hybrid - Modes of Operation
Operation of the EVT adapts to many driving conditions. There is no set order to the application of modes or fixed ratios for any given scenario but the chart attempts to show the general interaction of different functions provided by the HOS.
In Mode One (M1), the EVT provides an infinitely variable drive up to 1.70:1, while Mode Two (M2) provides ratios from 1.70:1 to 0.50:1. These two modes include all instances of electric-only (Auto Stop) and electric-gasoline hybrid operation. Depending on driving conditions and vehicle load, a fixed first-gear (G1) ratio of 3.69:1 is available (such as for pulling a large trailer) instead of the variable M1 ratios.
Similarly, a second-gear (G2) fixed ratio of 1.70:1 may be selected by the HOS, if needed, before transitioning to the variable M2 ratios. As road speed increases, depending upon driving and vehicle load conditions, variable-ratio M2 operation can be switched instantly to a third-gear (G3) fixed ratio of 1.00:1, and finally to a fourth-gear (G4) fixed ratio of 0.73:1, which is used mainly for steady-state highway cruising.
During electric-gasoline hybrid operation, the HOS will continually search for the best drive ratio — variable (M1, M2) or fixed (G1-G4) — which will allow the gasoline engine to operate in its most efficient, V-4 mode. Driving the vehicle in reverse is always done in electric-only mode similar to HSD (Toyota) and eCVT (Ford) equipped hybrids, though the gasoline engine may continue to run if coolant temps are low (which infers CAT light off or not), SoC (State of Charge) is low, or either the transmission or battery temperature is too high.
When towing at low speeds and from a stop, the HOS can apply full engine torque as well as electric motor-assist for additional pulling power. This gives the Tahoe and Yukon Hybrid models the greatest towing capability of any hybrid passenger vehicle on the market.
Occasionally, the GM 2-Mode system shuts down the electric motors, allowing the EVT to function as a conventional automatic transmission. Typically, this fixed-ratio operation occurs at highway speeds or when hauling heavy loads, and can actually be more efficient than operating in electric-gasoline hybrid mode under the same conditions. This is because under high-load situations, when the fixed gears are in use, the electric motors can be used to generate electricity — or, if needed, they can be called on to supply additional torque for improved performance.
2008 GMC Yukon 2-Mode Hybrid - Regen and Hydraulic Braking
The Tahoe and Yukon 2-Mode hybrids use a blended regenerative braking strategy to recapture energy that would otherwise be lost to heat either through hydraulic or engine deceleration braking. By using one or both of the EVT’s traction motors as a generator(s), braking energy is converted to electrical energy and stored in the 300V traction battery or ESS (Energy Storage System) for future use to propel the vehicle.
The regenerative brakes are used along with the standard hydraulic brakes to slow the vehicle quickly although only the hydraulic brakes are used from slow speeds to a complete stop. Depending on the amount of braking force applied (such as during mild decelerations) hydraulic braking may not be used at all.
When additional braking is called for, based on a change in the position of and/or the force applied to the brake pedal emulator, the hydraulic braking system will be called on to assist the generator(s) in slowing or stopping the vehicle. The Anti-lock Braking System (ABS)/Electronic Stability Control (ESC) modulator used on the Tahoe and Yukon Hybrids has been adapted to allow this interaction between the hydraulic brakes and the regenerative braking system.
The graph shown depicts how initial deceleration may not even require application of the vehicle’s hydraulic brakes, which are added only when regenerative braking is not sufficient to provide the necessary amount of stopping power. The system does provide feedback in the form of brake pedal resistance, which gives the driver the same feel as would be experienced with a normal hydraulic braking system.
Regenerative braking has the additional benefit of extending the life of the friction materials used in the hydraulic braking system, as well as improving braking performance in the form of shorter stopping distances.
2008 GMC Yukon 2-Mode Hybrid - Impressions
From behind the wheel, the 2-Mode system is very refined with its seamless variable ratio transmission working with the AFM’s ICE-Off to ICE-On, V8 to V4 and ICE-Off again transitions. Although a heavy weight, once the tires were pressed up to 40 psi all around, the Yukon’s FE capability really shone. Remember the Yukon weighs over 5,600 pounds and can EV up to as high as 32 mph (I saw a max of 26 mph in my short time behind the wheel) before ICE-on occurs.
The hybridization, although not what I would consider optimal, is what allows this monster to receive the FE it does around town. Nothing I have ever heard of weighing almost 6,000 pounds can touch 20 MPG in an around town environment yet the Yukon 2-Mode reached its EPA with ease in some of the worst traffic conditions I could have chosen to drive in. With a short initial drive under my belt, I believe GM was forcing the 2-Mode Yukon into a serial hybrid like fashion while in heavy traffic. To describe it more precisely, the Yukon would transition between a V8 start, V4 operation for a few blocks, go to ICE-Off under EV for a few blocks and repeat. Although I do not believe this was the best choice for maximum fuel economy given the inability to control ICE-On to ICE-Off transitions and knowing how to drive in a far more efficient manner in that kind of environment, the GM imposed algorithms worked well.
Because of GM’s AFM allowing V4 operation, a Scan Gauge was all but useless given it could not pick up the 4 cylinder ops and half the fuel use via OBD-II. It was grossly underestimating actual results whenever the AFM system evoked V4 mode. The Yukon does include both a rudimentary i and aFCD from which to swap between so that was a plus.
2008 GMC Yukon 2-Mode Hybrid – Ride and Handling
Because the preview was under heavily trafficked slower speeds, little was learned as to the Yukon 2-Mode’s capabilities in the twisties Because it was a fully loaded luxury vehicle, it was both comfortable and quiet with road isolation almost to good in comparison to other SUVs and P/Us in its class. That alone says quite a bit about the ride.
Making comments about the Yukon 2-Mode’s handling would be a disservice given the traffic conditions driven in.
2008 GMC Yukon 2-Mode Hybrid - Short term results
While in Detroit with temperatures hovering in the mid to upper 30’s, raining for about 75% of the drive and during rush hour with stop light to stop light conditions around town, the 2-Mode proved its worth. It was relatively easy to achieve 25 mpg under those conditions. Where the Yukon 2-Mode really shone however was out on the highway. With the LIVC intakes (late intake valve closing or the more common description, Atkinsonization) reducing pumping losses, the intake and exhaust valves closed during the AFM’s V4 mode, and its superior aerodynamics, I saw numbers I would not have believed achievable from a 6.0L equipped 5,600 pound SUV. The included iFCD would hang in the 36 – 38 mpg range while DWL, DWB at speeds oscillating between 45 and 60 on a somewhat heavily trafficked Detroit highway during rush hour. I did not drive over 60 mph so I do not know what the 2-Mode is capable of beyond that speed but the 6.0L running on just 3.0L with an Atkinsonized intake and 4 cylinders bottled up tight really does show it can hang with any small, non-hybrid SUV and just destroy the larger full sized ones!
With maybe 5 miles of stop light to stop light city and 5 miles of rush hour highway, the Yukon 2-Mode allowed a decent 29.6 MPG round trip for my time behind the windscreen. I took the passenger seat while helping Eddie Alterman - the Men’s Health Journal writer - through the same stop light to stop light traffic stop and crawls while performing a mini-clinic. His drive yielded a nice 24.6 mpg with no highway!
The above was achieved in an automobile with less than 1,000 miles on the odometer, on a road course of our choosing but during the peak of rush hour and in conditions that were far from optimal. To say I was impressed with w/ 2-Mode and AFM would be an understatement. The extremely smooth and imperceptible gear transitions along with a real world capable 25 around town and 30 + out on the highway is almost unbelievable coming from something that can tow its own weight and weighs almost twice what a Prius does!
2008 GMC Yukon 2-Mode Hybrid - Short term conclusions
Unfortunately or not, Japan Inc. and oil prices are probably going to have their way with GM and the 2-Modes. According to GM, the Ni-MH packs are sourced from PanasonicEV (a partner with Toyota on NiMH battery development) and cost more than $2,000 each. Toyota receives possibly as large a NiMH pack from PanasonicEV and builds the Transverter and MGSet’s in-house for less than what GM has to pay for the pack alone!
The price of oil is more than likely a more serious short term concern. With oil running close to $100/BBl and gasoline costing over $3.00 per gallon and still climbing, a 21 MPG combined large SUV makes less sense with each passing day.
With the high cost for the pack and gasoline prices what they are, GM may never recover 2-Mode design and engineering expenses let alone the cost of producing a limited number. No matter what the Tax Credit is worth once released, someone with an income who can afford a $50,000 + SUV is probably going to see the tax credit negated due to AMT.
I am looking forward to a week or more behind the wheel for a full review providing more specific detail, driving technique(s), and an answer to whether or not one can actually be driven to beyond 30 + MPG for lengthy periods of time and distance.
If GM can produce a full size, 5,600 pound SUV that can easily receive 25 around town and 30 + on the highway, imagine what we are going to receive when the Chevrolet Volt arrives
2008 GMC Yukon 2-Mode Hybrid
I would like to thank GM for providing both the GMC Yukon 2-Mode Hybrid and the source material for the 2-Mode’s preview details. I also want to thank both Eddie Alterman and Jonathan Wilde of Men’s Journal for their assistance and the scheduling to make this preview possible. Finally, I would like to thank Sean Welch (Right Lane Cruiser) for his editing skills and patience.