Volkswagen TwinDrive - The People's Plug-in

[PistonsFan]

Originally Posted by K-1

PistonsFan (I think) posted a visual of the 6(?) operating conditions a while back. The range-extender does charge the batteries to maintain a minimum SOC.

[john1701a]

#2 will look like #4 at times. Due to the RPM optimization, you'll get some trickle charging of the battery-pack.

[baloo]

Originally Posted by MechEng

I don't see how Volkswagen's mechanical process is going to be more efficient. As a the gearing and clutches + having to tune the engine to run at all rpm ranges will have higher losses than the Volt's specific tuned ICE to generator to electric motor to differential to wheels setup.

Well, a mechanical drive system is obviously more efficient than the Volt's electric system because you can avoid the conversion losses. The disadvantage is that the ICE needs to cover a wider load/rpm range, as you already said. However, that's not a huge problem for diesels because they typically have a wider efficiency 'sweet spot' than gas engines.

[igotzzoom]

Originally Posted by baloo

Well, a mechanical drive system is obviously more efficient than the Volt's electric system because you can avoid the conversion losses. The disadvantage is that the ICE needs to cover a wider load/rpm range, as you already said. However, that's not a huge problem for diesels because they typically have a wider efficiency 'sweet spot' than gas engines.

Wider "sweet spot"? Most diesels I've seen are only in their sweet spot between about 1,600-3,500 rpm. Gasoline engines generally have a larger dynamic operating range, but I guess the "sweet spot" might be debatable, but it's certainly usually more than a 2,000 rpm range.

[PistonsFan]

Originally Posted by baloo

Well, a mechanical drive system is obviously more efficient than the Volt's electric system because you can avoid the conversion losses. The disadvantage is that the ICE needs to cover a wider load/rpm range, as you already said. However, that's not a huge problem for diesels because they typically have a wider efficiency 'sweet spot' than gas engines.

A diesel engine for a Gen-set is an obvious choice from an efficiency point-of-view. But, if sold in the US, the emissions issues would seriously drive up the cost. The Volt's gasoline engine was picked because it was cheap, available, and it would get the job done.

A diesel engine in the Volt would have had a very significant impact on the cost (which is already very high).

More exotic (non-conventional ICE), purpose-built range-extenders are currently being researched all over the world.

This VW system is NOT something that nobody else has done or drawn up. It's just something that was bypassed by most because it's an excessively expensive technology.

Originally Posted by igotzzome
Wider "sweet spot"? Most diesels I've seen are only in their sweet spot between about 1,600-3,500 rpm. Gasoline engines generally have a larger dynamic operating range, but I guess the "sweet spot" might be debatable, but it's certainly usually more than a 2,000 rpm range.

A diesel gets very high efficiency at most mid-to-high loads. A throttled SI gasoline engine will have a small island of high efficiency on the operating map. I'll see if I can provide a visual...

[goblue]

The VW system is essentially the 2 mode plug in, except that the 2 mode is more powerful (2 55kW motors) and the 4 mode is on the way. They've limited the speeds and acceleration just like every parallel plug in hybrid. This gets annoying to the operator as the powertrain has different performance parameters depending on the mode its in. Avoiding this was a key parameter of VOLTEC.

Now, in a narrow set of operation parameters, you can get slightly higher efficiency with that mechanical connection - but that's balanced against a whole host of inefficiencies, first being to allow all speed / all acceleration EV operation you have some element of redundancy. Also, in most speed ranges, series EV operation will win out. Look at the stats. VOLTEC is using a 75hp genset and 160hp worth of electric motors. Twin Drive is using a 120hp generator and only a 110hp worth of EMs. The drive system on the VOLT allows more hp avail to the EMs with a smaller ICE.

VOLTEC is compatible with diesel, or even a fuel cell (not advocating that), which TwinDrive is not.

Plug in parallel hybrids are not the most efficient for pure EV operation and range extending capability. It is a good design for a truck with towing capability (for now).

[rsbaker]

Originally Posted by Bravada

The problem here is that both companies took approaches quite uncharacteristic of their usual conduit. GM went for expensive technology with the assumption they are making an unusual, special product that will pay for it. Volkswagen went for the smallest common denominator trying to utilize as much of the olden tech they've already got to keep the unit cost low and streamline the route to the market.

The more I think about it, the more I believe VW is actually right here. You can't out-Prius the Prius, this was a one-off Toyota managed to pull and others have to just get over it. The very next opportunity is making the hybrid your USUAL car, not a SPECIAL car. If VW indeed succeeds, the Volt will be instantly obsolete, rather than market-leading.

I disagree! The Volt is literally months away from Pre-production. This decidedly experimental Golf is not even on the Horizon for production yet. VW may well decide to go into production with this Twin Drive system but it won't be next year Like Volt. Volt will probably be readying it's 2nd generation version by the time this Twin-Drive is in the showroom. The article also implied that the VW will also be expensive at first given it complexity. I will Say though that Volt and VW twin drive are both going to be better than the current Prius, in other words, maximum electric only distances with minimal gasoline assist is the way too go. Will Toyota meet the Challenge? Time will tell but Toyota was one of the loudest skeptics of plug-in Extended range electric vehicles a year or so ago and now they are scrambling to modify the Prius for plugin electric only capability. It's been reported that it would get in the neighborhood of 10 electric only miles. Volt and VW twin drive will be superior no doubt.

[megeebee]

Originally Posted by MechEng

Here is my understanding of how the Volt range extending mode will work.

The engine will have differing set points. Lets say at 1500rpm it produces 18KW's of energy and at 2000rpm it produces 25KW's of energy.

Now lets say the car is cruising at 70MPH consuming ~20KW of energy. The battery hits the magic state of charge of about 30% and the generator comes on. Initially the Volt will turn on the generator at 2000rpm producing 25KW's of energy. 20KW's will go direct to the electric motor and the remander will charge the battery. When the battery hits about 35% SOC the generator will then turn down to 1500rpm producing 18KW's of energy. All 18KW's will go to the electric motor while the remaining 2KW's will come from the battery. When the battery hits 30% the process repeats.

The numbers used are just made up for demonstration purposes but this is my understanding of how the process works.

This way the engine can be tuned with variable intake and exaust manifolds and variable valve timing to be efficient at a series of specific rpm's.

I don't see how Volkswagen's mechanical process is going to be more efficient. As a the gearing and clutches + having to tune the engine to run at all rpm ranges will have higher losses than the Volt's specific tuned ICE to generator to electric motor to differential to wheels setup.

Thank you.

[baloo]

Originally Posted by igotzzoom

Wider "sweet spot"? Most diesels I've seen are only in their sweet spot between about 1,600-3,500 rpm. Gasoline engines generally have a larger dynamic operating range, but I guess the "sweet spot" might be debatable, but it's certainly usually more than a 2,000 rpm range.

I'm not talking about the power band, I'm talking about efficiency, or how much fuel it needs per horsepower it's producing.

Say you have a gas engine that produces 60 hp and consumes 60 grams of fuel per minute. Now what happens when you open the throttle and ask it to produce 90 hp? One could think it will consume 50% more fuel (= 90 grams/min) because it's producing 50% more power, but because gas engines are less efficient at wide open throttle, it will actually be something like 120 grams/min. The same is true for low loads. If you ask it to produce 10 hp, it will probably consume 25 g/min rather than 10 g/min.

This problem is less pronounced with diesel engines, meaning they don't start guzzling fuel as soon as you move away from the 60 hp sweet spot.

[PocoToro]

Originally Posted by baloo

Well, a mechanical drive system is obviously more efficient than the Volt's electric system because you can avoid the conversion losses. The disadvantage is that the ICE needs to cover a wider load/rpm range, as you already said. However, that's not a huge problem for diesels because they typically have a wider efficiency 'sweet spot' than gas engines.

If your statement, about mechanical drive, is even remotely true, then all of the diesel-electric rail locomotives are inefficient. And with the amount of fuel it takes to run a rail road, a one percent improvement in fuel mileage is a lot of money at the end of a year. So in your mind a rail locomotive should be mechanical drive and there not.
As for short term yes VW may be right just like the Prius, but long term the volt will win, it has the most versatile fueled dive train in the industry. It can eventual built to run on any fuel out there.

[baloo]

Originally Posted by PocoToro

If your statement, about mechanical drive, is even remotely true, then all of the diesel-electric rail locomotives are inefficient. And with the amount of fuel it takes to run a rail road, a one percent improvement in fuel mileage is a lot of money at the end of a year. So in your mind a rail locomotive should be mechanical drive and there not.

From a transmission standpoint nothing is more efficient than direct drive. However, other factors can play a role. Trains for example have a very low power-to-weight ratio, so you'd need one hell of a clutch and an impractical number of gears to get it moving, so it makes sense to use a diesel-electric drive system.

[Bravada]

VW may well decide to go into production with this Twin Drive system but it won't be next year Like Volt.

Why?

The article also implied that the VW will also be expensive at first given it complexity.

I understand the reason VW went for that over the more "ideal" setup in the Volt is that they can build it more easily and cheaply using already available components, while keeping costs down and development time short. This vehicle is there available to be driven by journalists. Is the Volt much ahead of that?

[Masospaghetti]

Originally Posted by Bravada

The more I think about it, the more I believe VW is actually right here. You can't out-Prius the Prius, this was a one-off Toyota managed to pull and others have to just get over it. The very next opportunity is making the hybrid your USUAL car, not a SPECIAL car. If VW indeed succeeds, the Volt will be instantly obsolete, rather than market-leading.

How is that? The additional hardware and complexity of having the engine directly driving the wheels seems likely to outweigh the benefits. For one, the mechanical link from the motor to the wheels will have some losses; the single-stage reduction and axles will add to the car's weight; and the motor itself has to be able to run at a wider range of RPM's, which is not good for efficiency.

The Volt gets it correct IMO. The losses from the generator through the battery pack, while significant, saves a lot of hardware needed for a direct-drive mechanical link. This will also help with ongoing maintenance costs with fewer components to wear out.

Peak alternator efficiency is around 88% (1), assuming a 20 HP (14 KW) alternator, which depends mostly on the rotational speed. Since the VOLT's engine can operate at a steady state, it can easily be designed to operate at this peak value. The battery charge and discharge efficiency ranges from 88% to 98% depending on load (2), so lets assume a 95% efficiency for the battery. The motor efficiency is 92% (1), assuming a 125 HP motor, thus leading to an overall efficiency of (0.88)*(0.95)*(0.92) = ~77%.

Now, in VW's mechanical system, the gear reduction probably has an efficiency of 90%-93% (assuming a 2-stage reduction) and a 97% from axle joint losses. If VW is using a torque converter as the coupling, then figure a 8-90% efficiency there. It is also unknown how much losses the engine will experience from operating at a wide range of speeds as opposed to the Volt's steady-state operation.

So as an estimate, VW's system has an overall efficiency between (0.93)*(.97) = ~90% if there is a lossless clutch used and the motor is 100% efficient at all speeds, and (.90)*(.97)*(.8)*(.8) = ~56% if we account for a lossy torque converter and non-perfect ICE engine.

So what? VW's system has a lot of unknowns but is likely to be between 56%-90% efficient. The Volt's system is approximately 77% efficient with less complexity. It seems to me that the Volt system is a better tradeoff.


(1) http://www.engineeringtoolbox.com/el...cy-d_655.html#
(2) http://www.bdbatteries.com/peukert.php

[baloo]

Originally Posted by Masospaghetti

So what? VW's system has a lot of unknowns but is likely to be between 56%-90% efficient. The Volt's system is approximately 77% efficient with less complexity. It seems to me that the Volt system is a better tradeoff.

There's one big problem with your calculation: The VW can use either system. They can leave the clutch open and use the generator/motor just like the Volt, or they can close it and use a mechanical link whenever that is more efficient.

[Masospaghetti]

Originally Posted by baloo

There's one big problem with your calculation: The VW can use either system. They can leave the clutch open and use the generator/motor just like the Volt, or they can close it and use a mechanical link whenever that is more efficient.

My point was that, when VW's system directly connects the engine and wheels together, its likely to be only marginally more efficient (if at all) with the added complexity of having a gear reducer, clutch or torque converter, and axles.