Prius less efficient at highway speed than two-mode hybrids?

[sola]

Hi All,

I was reading about two-mode hybrids and the Lexus GS450h recently and there is something I cannot fully understand.

Proponents of the two-mode system say that it is better suited for high speed (higway) driving than HSD based hybrids due to some inefficiencies in the HSD system at higher speeds (electrical path efficiency =~ 0.7).

I hear that HSD was somehow modified for the GS450h (added switch for high speed) exactly for this reason.

Is this true? Does this mean that the faster I go with the Prius, the more inefficient it is? As far as I understand it shouldn't be tha case, but it seems to be true that at highway speeds (e.g. 150km/h) the consumption of the Prius is not as good as a comparable diesel, although it beats them in the city.

Is there a difference (in this regards) between the HSD Prius and the GS450?

Thanks,
Andras

[RonH]

<div class='quotetop'>QUOTE(sola @ Jun 10 2006, 11:56 AM) [snapback]269201[/snapback]</div>

Hi All,

I was reading about two-mode hybrids and the Lexus GS450h recently and there is something I cannot fully understand.

Proponents of the two-mode system say that it is better suited for high speed (higway) driving than HSD based hybrids due to some inefficiencies in the HSD system at higher speeds (electrical path efficiency =~ 0.7).

I hear that HSD was somehow modified for the GS450h (added switch for high speed) exactly for this reason.

Is this true? Does this mean that the faster I go with the Prius, the more inefficient it is? As far as I understand it shouldn't be tha case, but it seems to be true that at highway speeds (e.g. 150km/h) the consumption of the Prius is not as good as a comparable diesel, although it beats them in the city.

Is there a difference (in this regards) between the HSD Prius and the GS450?

Thanks,
Andras
[/b]

Just off the top of my head...
running at a constant highway speed, on a flat, straight, windless road, the electromotive gear has little to do and the batteries, inverters, MG's are deadweight. How-some-ever, those conditions are rarely met, the hybrid does its thing when passing, coasting, climbing, descending, etc. I can't imagine what a switch would do as the software pretty much uses the motor only when required. It also would imply there is excess capacity in the engine which is used for passing, climbing, etc. When a conventional drive train is running in those ideal conditions it has a lot of dead weight, too, particularly the excess engine capacity needed to get it up to that speed.

[john1701a]

In theory, it sounds promising. But just like Ron said, that has little reflection on actual real-world conditions.

It's how that system works overall that really counts. Highway cruising only isn't that common. Most people drive some variety of mixed.

[usbseawolf2000]

<div class='quotetop'>QUOTE(sola @ Jun 10 2006, 12:56 PM) [snapback]269201[/snapback]</div>

Proponents of the two-mode system say that it is better suited for high speed (higway) driving than HSD based hybrids due to some inefficiencies in the HSD system at higher speeds (electrical path efficiency =~ 0.7).[/b]

I am seeing this kind of attack on HSD more often now. Let's be clear that Prius (current HSD gen) can not accelerate completely through just the mechanical path. It has to be combination of mechanical and electrical. This is due to the nature of PSD. It will always split the power and the question is how much to which way. Electrical path does have some loss due to the conversion. The point is, this loss is very little because majority (~90%) of power goes through the mechanical path on the highway. Otherwise, Prius' highway mpg would be less than Echo/Scion xA/Yaris/etc. Even during hard acceleration, still majority (~2/3) of the power goes through mechanical path.

The benefit of always having the electric path is torque-on-demand. That's why Prius is so responsive passing on the highway. For GM's two-mode hybrid, to prepare for high speed passing, it will need to go from mode-two -> 4th gear -> 3rd gear -> mode 2 -> 2nd gear. GM claims that the transition will be smooth but how about the power delivery delay??? Since each gear has fixed ratio, as you gain speed, more shifts need to occur. For example, 2nd gear -> mode 2 -> 3rd gear. ICE RPM will also rev up and down therefore it can't stay at the optimal RPM like HSD. Their two-mode hybrid transaxle is basically a souped up 4-speed automatic transmission. It also mean that you will never see Atkinson cycle for this kind of two-mode hybrids.

Ok, enough of micro technical stuffs... For a 1,000 feet up from the sky point of view... Honda IMA adds an electric motor to the engine (ICE). GM's two-mode hybrid upgrades electric motors (4) in the automatic transmission. Only HSD is the true full hybrid, designed from ground up without any legacy/traditional restrictions.

<div class='quotetop'>QUOTE(sola @ Jun 10 2006, 12:56 PM) [snapback]269201[/snapback]</div>

I hear that HSD was somehow modified for the GS450h (added switch for high speed) exactly for this reason..... Is there a difference (in this regards) between the HSD Prius and the GS450?[/b]

There are a lot of evolutional design changes between Pius and GS450h. The first change came with RX400h/Highlander Hybrid where they added a Speed Reduction Unit. Remember Prius' MG2 can make 295 lbs-ft torque however, RX400h MG2 can only make 246 lbs-ft. Prius doesn't have SRU so, 295 lbs-ft is more than enough for 2,890 lbs car. For a heavier SUV hybrid, Toyota added 2.48:1 ratio SRU to multiple the torque to 608 lbs-ft! Camry Hybrid also use SRU to multiply it's 199 lbs-ft MG2 torque (to 494 lbs-ft?).

HSD used in GS450h and LS600h (eCVT L110) has two speed (3.9:1 - 1.9:1) variable reduction gearset to reduce the size of the electic motors, improve torque at low speed and reduce ICE RPM at high speed! This implementation was evolved for high powered HSD. It has nothing to do with inefficiency of HSD on the highway.


<div class='quotetop'>QUOTE(sola @ Jun 10 2006, 12:56 PM) [snapback]269201[/snapback]</div>

it seems to be true that at highway speeds (e.g. 150km/h) the consumption of the Prius is not as good as a comparable diesel, although it beats them in the city.[/b]

Remember, Diesel fuel has more energy than Gasoline.

Dennis

[sola]

<div class='quotetop'>QUOTE(usbseawolf2000 @ Jun 10 2006, 02:36 PM) [snapback]269256[/snapback]</div>

I am seeing this kind of attack on HSD more often now. Let's be clear that Prius (current HSD gen) can not accelerate completely through just the mechanical path. It has to be combination of mechanical and electrical. This is due to the nature of PSD. It will always split the power and the question is how much to which way. Electrical path does have some loss due to the conversion. The point is, this loss is very little because majority (~90%) of power goes through the mechanical path on the highway. Otherwise, Prius' highway mpg would be less than Echo/Scion xA/Yaris/etc. Even during hard acceleration, still majority (~2/3) of the power goes through mechanical path.

The benefit of always having the electric path is torque-on-demand. That's why Prius is so responsive passing on the highway. For GM's two-mode hybrid, to prepare for high speed passing, it will need to go from mode-two -> 4th gear -> 3rd gear -> mode 2 -> 2nd gear. GM claims that the transition will be smooth but how about the power delivery delay??? Since each gear has fixed ratio, as you gain speed, more shifts need to occur. For example, 2nd gear -> mode 2 -> 3rd gear. ICE RPM will also rev up and down therefore it can't stay at the optimal RPM like HSD. Their two-mode hybrid transaxle is basically a souped up 4-speed automatic transmission. It also mean that you will never see Atkinson cycle for this kind of two-mode hybrids.

Ok, enough of micro technical stuffs... For a 1,000 feet up from the sky point of view... Honda IMA adds an electric motor to the engine (ICE). GM's two-mode hybrid upgrades electric motors (4) in the automatic transmission. Only HSD is the true full hybrid, designed from ground up without any legacy/traditional restrictions.
There are a lot of evolutional design changes between Pius and GS450h. The first change came with RX400h/Highlander Hybrid where they added a Speed Reduction Unit. Remember Prius' MG2 can make 295 lbs-ft torque however, RX400h MG2 can only make 246 lbs-ft. Prius doesn't have SRU so, 295 lbs-ft is more than enough for 2,890 lbs car. For a heavier SUV hybrid, Toyota added 2.48:1 ratio SRU to multiple the torque to 608 lbs-ft! Camry Hybrid also use SRU to multiply it's 199 lbs-ft MG2 torque (to 494 lbs-ft?).

HSD used in GS450h and LS600h (eCVT L110) has two speed (3.9:1 - 1.9:1) variable reduction gearset to reduce the size of the electic motors, improve torque at low speed and reduce ICE RPM at high speed! This implementation was evolved for high powered HSD. It has nothing to do with inefficiency of HSD on the highway.
Remember, Diesel fuel has more energy than Gasoline.

Dennis
[/b]

Thanks, I think I understand now the problem better.

So the 2008 Prius will have this advanced HSD, that is why Toyota is promising better acceleration for it. Knowing this, we can be quite sure that the new one will be pretty nimble.

Cool. I just hope they also make it a plug-in. Here in Europe gasoline is so expensive.

Andras

[Denny_A]

<div class='quotetop'>QUOTE(usbseawolf2000 @ Jun 10 2006, 04:36 PM) [snapback]269256[/snapback]</div>

....................................
The benefit of always having the electric path is torque-on-demand. That's why Prius is so responsive passing on the highway. For GM's two-mode hybrid, to prepare for high speed passing, it will need to go from mode-two -> 4th gear -> 3rd gear -> mode 2 -> 2nd gear. GM claims that the transition will be smooth but how about the power delivery delay??? Since each gear has fixed ratio, as you gain speed, more shifts need to occur. For example, 2nd gear -> mode 2 -> 3rd gear. ICE RPM will also rev up and down therefore it can't stay at the optimal RPM like HSD. Their two-mode hybrid transaxle is basically a souped up 4-speed automatic transmission. It also mean that you will never see Atkinson cycle for this kind of two-mode hybrids. .............................

Dennis
[/b]

Here's a graphical example of the verbiage above:


[attachmentid=3761]

[john1701a]

<div class='quotetop'>QUOTE(usbseawolf2000 @ Jun 10 2006, 02:36 PM) [snapback]269256[/snapback]</div>

The benefit of always having the electric path is torque-on-demand. That's why Prius is so responsive passing on the highway. For GM's two-mode hybrid, to prepare for high speed passing, it will need to go from mode-two -> 4th gear -> 3rd gear -> mode 2 -> 2nd gear. GM claims that the transition will be smooth but how about the power delivery delay??? Since each gear has fixed ratio, as you gain speed, more shifts need to occur. For example, 2nd gear -> mode 2 -> 3rd gear. ICE RPM will also rev up and down therefore it can't stay at the optimal RPM like HSD. Their two-mode hybrid transaxle is basically a souped up 4-speed automatic transmission.[/b]

It's the "That's why Prius is so responsive passing on the highway." that I find fascinating.

I used that very same comment yesterday for an entirely different reason.

In your case, it was in reference to new a "full" hybrid design that will operate in a way we are quite unfamiliar with. Having an internal component inventory that's obviously different, expecting the same response is really pushing it. The change from extremely-efficient-cruising-mode to give-me-a-bunch-of-power doesn't appear to be a quick process. There's a clutch involved. The shifting is an extra step that doesn't exist in the "full" hybrid system Prius has. Using it will require additional time, an undeniable delay... hence not as responsive.

In my case, it was in reference to hydrogen fuel-cells. The person I was arguing with was so focused on the technology itself that he had completely overlooked how it could actually be used in a practical application: a car. On paper, you can demonstrate the complete energy cycle of hydrogen to make a convincing enough claim to warrant continued discussion. But the second you ask how a fuel-cell, which is a steady-state electricity provider, will react when passing on the highway the discussion comes to a screeching halt... because being able to respond rapidly is shortcoming for fuel-cells. A secondary source of electricity is needed to fulfill that need, which means a fuel-cell alone can't do the job. It has to be a hybrid. Adding a battery-pack would do the trick, and it would provide a place for electricity created by regenerative braking to be stored.

Lots of people still don't understand hybrids well, because they make decisions based on limited information. It's the very reason why Toyota decided to include a Multi-Display from the beginning. Seeing how much the efficiency equation involves is a very eye-opening opportunity.

[Mardikes]

<div class='quotetop'>QUOTE(Denny_A @ Jun 10 2006, 07:11 PM) [snapback]269367[/snapback]</div>

Here's a graphical example of the verbiage above:
[attachmentid=3761]
[/b]

Where did you find the graph? Any idea how old it is? Is the reference to current Prius the 2004-2006 version?

Thanks.

George

[Mardikes]

<div class='quotetop'>QUOTE(john1701a @ Jun 11 2006, 07:19 AM) [snapback]269482[/snapback]</div>

In my case, it was in reference to hydrogen fuel-cells. The person I was arguing with was so focused on the technology itself that he had completely overlooked how it could actually be used in a practical application: a car. On paper, you can demonstrate the complete energy cycle of hydrogen to make a convincing enough claim to warrant continued discussion. But the second you ask how a fuel-cell, which is a steady-state electricity provider, will react when passing on the highway the discussion comes to a screeching halt... because being able to respond rapidly is shortcoming for fuel-cells. A secondary source of electricity is needed to fulfill that need, which means a fuel-cell alone can't do the job. It has to be a hybrid. Adding a battery-pack would do the trick, and it would provide a place for electricity created by regenerative braking to be stored.

[/b]

Which is why GM is doomed and Toyota is going to be hard to beat. GM, in taking the position that hybrids are not the short term answer, are losing out on 10+ years of incremental refinement of the CVT, batteries and electrical systems. The more efficient the hybrid system, the smaller the fuel cell needed.

My guess is fuel cells never make it for application in cars. Once the plug-ins take off, then the race to get efficient and fast charging batteries takes off. The race has actually already started.

If capacitors work,

http://www.sciencentral.com/articles/view....le_id=218392803

then charging stations, similar to toll booths, where you just drive in and charge up, will become the model and the gasoline station as we know it will cease to carry the fuel, just big gulps and maybe a charging station.

I could see a process where my Prius is charged as I drive out of the parking lot from work as I stop and wave my access card, to raise the gate, the car's battery is automatically charged.

And to think GM had the EV-1 and walked away from it.

George

[usbseawolf2000]

<div class='quotetop'>QUOTE(Mardikes @ Jun 15 2006, 05:58 PM) [snapback]271858[/snapback]</div>

Where did you find the graph? Any idea how old it is? Is the reference to current Prius the 2004-2006 version?

Thanks.

George
[/b]

You can read the whole thing online or get the PDF version.

I think it came out in 4th Q of 2003. THS II is 04-Current HSD Prius. Current Prius is the classic 01-03' Prius.

Dennis