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All About E-Flex - a universal propulsion system


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GM E-Flex Press Release


:forum:Chevrolet Volt Image Gallery
:forum:Chevrolet Volt Press Release

GM’s E-FLEX System moves the automobile toward a new electric age


DETROIT – General Motors Corp. will introduce multiple propulsion systems that fit into a common chassis, using electric drive to help the world diversify energy sources and establish electricity from the grid as one of those sources.

GM refers to this family of propulsion systems as the “E-flex System.”

“The DNA of the automobile has not changed in more than 100 years,” said Larry Burns, GM vice president of research and development and strategic planning. “Vehicles still operate in pretty much the same fashion as when Karl Benz introduced the ‘horseless carriage’ in 1886.

“While mechanical propulsion will be with us for many decades to come, GM sees a market for various forms of electric vehicles, including fuel cells and electric vehicles using gas and diesel engines to extend the range. With our new E-flex concept, we can produce electricity from gasoline, ethanol, bio-diesel or hydrogen.

“We can tailor the propulsion to meet the specific needs and infrastructure of a given market. For example, somebody in Brazil might use 100-percent ethanol to power an engine generator and battery. A customer in Shanghai might get hydrogen from the sun and create electricity in a fuel cell. Meanwhile, a customer in Sweden might use wood to create bio-diesel.”

The Chevrolet Volt, introduced at the North American International Auto Show in Detroit , is just the first variant of the E-flex System. The Volt uses a large battery and a small, 1L turbo gasoline engine to produce enough electricity to go up to 640 miles and provide triple-digit fuel economy. GM will show other variations of the propulsion systems at future auto shows.

GM is building a fuel cell variant that mirrors the propulsion system in the Chevrolet Sequel (fuel cell vehicle),” Burns said. “Instead of a big battery and a small engine generator used in the Volt, we would use a fuel cell propulsion system with a small battery to capture energy when the vehicle brakes. Because the Volt is so small and lightweight, we would need only about half of the hydrogen storage as the Sequel to get 300 miles of range.”

Future concepts might incorporate diesel generators, bio-diesel and pure ethanol (E-100).

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Personally, I believe that the ideal future vehicle configuration is one built around a gas-turbine generator and electric drive.

I say this for the following reasons:-

(1) A 100 kW gas turbine (~134hp) is roughly the size of a 100 disc stack of CDs. Even with the generator attached it is no larger than a waste paper basket. You cannot match the space and mass efficiency of a turbine with ANY kind of piston engine.

(2) An advanced centrifugal gas turbine is about 30% thermally efficient at its optimum speed. This is better than gasoline piston engines in general. If higher efficiency is desired, a combined cycle arrangement where the gas turbine exhaust is used to create steam to drive an auxiliary steam turbine will boost efficiency past 40% (better than direction injection turbodiesels). The entire setup is still smaller and lighter than a 1.6 liter piston engine.

(3) A gas turbine requires no coolant, no radiator, no coolant pumps, etc. A simple cycle turbine itself has only ONE moving part and zero inherent vibrations. This again reduces complexity, space requirements and maintenance hassles.

(4) The main disadvantages of a turbine -- horrible throttle lag, inertial overruns on throttle release, poor efficiency at all but ideal speed range, etc are ALL irrelevant when the turbine is completely decoupled from the drive train and use only at optimum speed and only to generate electricity for the battery bank.

(5) A gas turbine is inherently multi-fuel. You can feed it Diesel, Gasoline, Kerosene, Ethanol, natural gas or even hydrogen without making any physical modification to the turbine itself.

A gas-turbine electric vehicle with a 20 mile electric reserve can be used as an electric car for short commutes and will be able to generate its own power with greater efficiency than a piston engine based hybrid for longer trips. The basic architecture is extensible to an Ethanol, Gasoline, NG, Hydrogen or electric future. The vehicle will also be extremely refined in terms of NVH as the engine as effectively no vibrations and no exhaust pulses.

I also believe that -- like computer components such as memory and harddrives which have become "non-serviceable" items -- automobiles will go down this path very soon. Instead of having a mechanic diagnose and fix increasingly advanced systems, I believe that at some point it'll make sense to simply move on to an automobile with no serviceable parts. Instead, the drive train will be broken into half a dozen "modules" such as the powerplant, the electric motor, the battery, the inverter, etc. Each is not serviceable and completely sealed from the factory. If there is a failure or an impending one, the "modules" can be removed and swapped with a functional one in 10 minutes. All servicing and refurbishment will be done at the factory in an automated, large scale process for greater efficiency and quality consistency. Labor is reduced to the minimum and one will sign maintenance contract for a particular period which covers all component swaps needed instead of paying for repairs as needed.

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Would this be like a turbo-electric drive on ships?

Yes, pretty much. Like the Integrated Power System on the DDG-1000. And, if a gas turbine as well as a secondary closed cycle steam turbine is used in a COGAS-E arrangement, like the the arrangements used in some of the latest cruise ships like Celebrity's Millenium class and a few others.

The system has all of the advantages of a gas turbine with none of its disadvantages. Simple powerplant with minimum moving parts, no reciprocating assembly and no coolant requirements. Ultra compact powerplant with power densities several times higher than a piston engine. Basically no vibrations. No turbine lag and intertial overdrive because the turbine is not directly coupled to the drive train. No inefficiencies at part load and non-optimum turbine speeds because the turbine ALWAYS operate at ideal speed. With a steam recuperative auxiliary turbine, thermal efficiency can equal or beat the finest diesel piston engines. The engine can run on multiple fuels or even a combination of different kinds of fuels without physical modifications -- although emissions may prove a problem because catalysts usually are not that adaptive. Energy conversion is also very efficient -- a good generator is over 95% efficient and good Permanent Magnet motor is also over 95% efficient. The presence of a battery -- even a modest one good for 10~20 miles of driving is makes it at least partly a plug in chargeable car for short hops and allows for energy recovery through regenerative braking. Most importantly, unlike traditional hybrids the small size and miniscue weight of the turbine power unit offsets the added mass and space requirements of the electric drive drive components and the battery.

No matter how you look at it, the ability to throw the radiator out and make the engine 1/4~1/6th the size is very significant. In fact, it changes the design paradigm of a car significantly because the engine is small enough that it no long has to occupy a big compartment in the front of the car! This may mean the end to the tradtional 2-box/3-box design paradigms. Imagine a 100hp engine which fits in the glove box and you get the idea. I am not saying you'll put it there, but you can tuck it in a whole host of places.

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Excellent idea-using a turbine-but how would it hold up in an accident? It might come down to a safety thing. And to make it safe you might need lots of shielding, which would add weight, which would require bigger electric motors...

I like the idea, and I'm not trying to shoot it down. I'm just thinking it through...

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  • 3 weeks later...

Excellent idea-using a turbine-but how would it hold up in an accident? It might come down to a safety thing. And to make it safe you might need lots of shielding, which would add weight, which would require bigger electric motors...

I like the idea, and I'm not trying to shoot it down. I'm just thinking it through...

Shielding? I mean we are not talking about a nuclear reactor here!

The worst thing that can happen during a crash is a broken fuel line dumping fuel onto hot metal and starting a fire. A piston engine is has exactly the same problem with regard to the hot exhaust manifold. This is why it is common to have a fuel cut off switch which respond to the an impact force and kills the fuel pump or put it on reverse polarity (sucking fuel backwards).

If you are worried about the turbine grenading and sending fragments out, that is a non issue. The chances of a centrifugal turbine distintegrating with enough centrifugal force to send the impeller's fragment through the aluminum or Inconel housing is non-existent because operating speeds are relatively low. Even if an axial turbine is used it is also no-existent given the thickness of the turbine's housing and the relatively small diameter of the turbine used in a car. In aviation, they put a Kevlar liner on the inside of the engine pod housing, but that is because the engine walls have to be made as thin and as light as possible for a flight weight powerplant. This is not done for instance in a marine derivative turbine like the LM2500 or the MT30. And it is never done on a centrifugal turbines like the AGT1500 or LV100. Let's put it this way, if you don't have to do it with a turbocharger, you don't have to do it with a turbine.

A turbine doesn't turn at significantly (if at all) higher speeds than a turbocharger (80,000~250,000 rpm). A typical turbo that makes about 14.5psi has a pressure ratio of 2 (by definition). The even when lag is not an issue, the limit a single stage compressor can efficiently achieve is a pressure ratio of between 3 to 4 (~ 43.5 to 58 psi absolute pressure). Beyond that you typically fall of the compressor map of even the most advanced and aerodynamically optimized compressors. A General Electric GE90 engine has a high pressure compressor with a 23:1 pressure ratio. A gas turbine with that kind of performance will have a thermal efficiency of about 44% (which is fantastically good). But that is done with 9 stages. We do not and cannot expect that kind of performance in an automotive turbine costing a few thousand dollars. But even with a fraction of that a thermal efficiency of about 25~27% can be achieved using a simple cycle turbine (this is as good as a modern gasoline piston engine). Using a combined cycle (with auxiliary steam turbine) About 35~40% can be expected which beats most of the direct injection turbo diesels.

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