Consumer Reports tests Tahoe Hybrid

Title of the thread should be changed from "Consumer Reports" to "Car and Driver."

Do you think that tranny will hold up for more than 80,000 miles? (Hey, I know people who have had two trannies installed in their Subhumans by 80k). I'm also puzzled by the claims of the CVT being less efficient...

I've got to wonder about the diesel comments in the article. The extra cost of diesel fuel make the comparison questionable.

Do people spending $50K on a vehicle really care about fuel economy enough that going from 12 to 18 mpg is going to wow them?

 

Funny coincidence. I saw a chick driving one just today and I the first thing that came to mind was
"Hey, it looks like GM finally found someone dumb enough to buy one of those..." (Flame suit on for all the SUV devotees here, BTW)
At first glance (to me, at least), the front end looks like it has an aftermarket bodykit on it.

 

The author is assuming that the Prius has a typical CVT transmission with the metal belt and the adjusting pulley ratios. He is of course wrong as the Prius uses a PSD and the smaller mg1 to infinitely variable ratios. The PSD is a planetary gear and as such has the same amount of friction as any fixed ratio manual transmission.

 

I saw the first one in Manhattan a week ago and it was pulled over by a police. Darn, I didn't have my camera.

 

This question really "hits the nail on the head" so to speak!

The so called "Chevrolet Tahoe Hybrid" is like the very overweight person who is trying to loose weight so they go out to eat at a fast food restaurant and ask that their super-size triple chocolate milkshake be made with skim milk to reduce the calories!

Simply unbelievable what some people do or will buy IMHO. :der:

 

They just broadcast a test of Cadillac's version of this vehicle on Motorweek today!
Price was about $70,000. 20 MPG US is not bad for a vehicle that weights 6000 lbs!

If you absolutely positively MUST have a vehicle of this size, I guess this is the one to get.

Most of us can "get by" with much smaller vehicles.

 

I read the keyless starting mention as actually referring to remote starting. I think the Altima also has a keyless ignition a la the Prius.

 

I love the PSD but it is not as efficient as a manual tranny. The gears themselves have about the same friction but the transfer of electricity through the two motors and the controller/inverter is less efficient. At highway speeds its actually worse then some automatic transmissions.

This will be addressed in the next Prius and just better control electronics could yeild a 10% gain there.

The designers of the Prius made a decision to use the PSD over other alternatives that also had issues. The PSD mechanically is far simpler then the other choices. GM went with one of the most complex tranmissons ever put into a consumer vehicle. Depending on how you count its has either two or four clutches, a PSD and a regular automatic tranmission all rolled into one! While I think GM's quality is not nearly as bad as its reputation I still don't want a transmission that complex.

 

One of the advantages is only 28% of the ICE power has to be balanced by MG1. So the power loss runs:

.92*.92 ~= .85% MG2 to MG1 efficiency, ~15% power loss
.28*.15 ~= 4% lost from MG1/MG2
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I would especially like to see the chain gone and replaced by direct gear drive like the Camry.

Personally, I think more than 10% is possible: (1) remove 'built in' drag, (2) 'softer' cruise control, (3) wheel alignment, and (4) better heat management.

Amen about the GM two-mode transaxle. What happened to KISS?

I do find some of the reports of the VW, two-clutch transmissions very interesting. Add a reasonably sized motor, hydraulic or electrical, and it could be a very clever solution.

Bob Wilson

 

I suspect two modes of cruise control would be best: economy (loose) and normal (tight) at the press of a button. Otherwise I doubt Toyota would do it as it would irk the heck out of some customers.

About heat management, what do you have in mind? I'm assuming you would like some sort of temperature controlled grill damper for cold weather efficiency.

Especially with my recollection of Suburban trannies failing in 20,000 miles a decade or so ago. Haven't followed them since then. But I knew several coworkers and family that experienced this.

I would be very nervous about a complex new transmission from this bunch.

 

Most of us can "get by" with RENTING a 20mpg hybrid, on those rare (maybe 1 week out of 100 or less) occasions that you have to carry 6 or more people while towing a 30 foot boat. But since we seldom have 6, and our boat is much smaller, we get by with our 30+MPG hybrid Lexus suv ... which is a little easier to live with type mpg, on a daily basis.

 

Actually year round since the engine coolant likes to be at 70C. Even with the coolant bottle in the North American Prius, the block heater still helps. This suggests heat management could be improved.

I would like to see cooling air brought in only when and where needed. Once the vehicle is parked, other than the 'hot spot' around the exhaust valves and manifold, heat should be preserved. This means a 'sealed' engine compartment, at least for the parts that work best hot: engine block and transaxle.

Bob Wilson

 

Bob, your basic concept is correct, but you made a small but very significant error, unless you are talking about one set of fixed speeds for all of the components. The percentage of torque through MG1 is fixed. The power ratio varies with rpm. That's how the Prius varies the effective gear ratio, by varying the split of power.

You know this, but for new readers, here is a quick recap: The gear ratio through the PSD is fixed. Part of the torque from the ICE goes directly to the drive train, the rest goes to MG1. The portion going to MG1 is used to generate electricity, which can be stored in the battery or used to move the vehicle. Both work the same way, except the battery allows the energy to be used at a later time at the expense of some loss.

Looking at the case where the energy is used directly, the electricity generated from MG1 goes to MG2, where it is used to turn the drive train. This means under normal driving conditions, some of the power gets to the wheels directly from the ICE and some goes around the barn by turning into electricity and then back into mechanical energy. That's what Bob is driving at with his 28% figure. Obviously, it's more efficient to go directly to the drive train than through two conversions.

Why does Toyota do this? Because cars need gears to start, climb hills, and accelerate. But here is the stumper: how can the effective gear ratio change if the PSD has a fixed set of gears? In other words, if the ratio of torque spit between the direct drive and the electrical drive is fixed, how do you change the ratio of power? It's simple. Power is work divided by time, and work is force times distance. With the fixed gear ratio of the PSD, the only variable left to control is speed, and that's how the Prius changes its effective gear ratio. To run in a lower gear, the Prius spins MG1 at higher rpms. The higher rpm means higher power, so a higher percentage of the power is run through the electrical path. To run in high gear, MG1 is kept to a lower rpm, forcing as much power as possible to go directly through the PSD to the drive train, thereby limiting the electrical losses.

To summarize: In the Prius when the ICE is running, part of the power goes directly to the wheels and part gets to the wheels by going through MG1 to MG2. The ratio of this power split is determined by the speed of MG1. The torque spit is always fixed at the same ratio by the gearing in the PSD.

The result of this is that the Prius is more efficient at speeds where less power goes through MG1 and MG2. This is the notion of the "dual-mode" transmission produced by another manufacturer. Dual-mode implies that the electrical path of MG1 to MG2 to the wheels is clutched out at highway speeds, which makes for a more efficient transmission at the expense of higher complexity.

Tom

 

For those that hate SUVs with blind passion the Tahoe/Yukon 2-Modes are pigs at the trough wearing diamond jewelry.

However for the increasing few that might need an SUV I'd make it mandatory that all SUVs must be 2-Modes. They are significatly better for us as a nation and for humanity as a whole than their gasser siblings. There is no way that we could legislate who needs an SUV, we are afterall free to make our own decisions. But if one wants such a vehicle the 2-Modes are so much better than the gasser versions that it could be mandated that the ancient ICE-only technology has to be retired for good.

Three quick data points.
It is better for an SUV owner to give up his gasser SUV in favor of a 2-Mode SUV than it is...
..for a Corolla owner to begin driving a Prius.
..for a Camry owner to begin driving a TCH.
..for a Civic owner to begin driving an HCH.

In fact the 2-Mode SUVs do one phenomenal job that every other hybrid vehicle does as well. They improve the single worst characteristic of driving an SUV ( City driving ) by over 50% such that it is nearly equal to the best drivng characteristic ( Highway driving ). The real benefit here is that City driving in an SUV is the worst characteristic of all vehicles on the road. This technology improves our worst problem. Address the hardest problem first.

In addition in the right hands the 2-Modes have very good upward potential. Over at GreenHybrid.com one of the Mods was given a Yukon 2-Mode to test before launch. In Michigan, in cold weather, he was able to obtain fuel economy in the upper 20 mpg range almost reaching 30 mpg. Now there is a trick that we all need to happen like yesterday.

 

"You can put lipstick on a pig, it's still a pig."

Obama's quote can be applied here as well.

 

That is pretty good. I would like to hear some real world numbers from the CTH. Our Honda Element gets around 20mpg city. (I would not be interested in 'switching up' to something like the CTH)