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Monday, October 11, 2010

The Chevrolet Volt is not a true Series Hybrid

While studying some of the cut away technical renderings (click on the picture above to enlarge) released by General Motors as they announced the official launch of the Chevrolet Volt yesterday we couldn't quite understand the use of a planetary gear cluster inside the electric motors rotor.

General Motors have launched the Volt calling it an electric vehicle with extended range as opposed to just another hybrid. They have emphasized during the marketing campaign over the past 3 years that the Internal Combustion Engine (ICE) in the Volt never directly drives the wheels.

Well, it turns out that's not strictly true. The planetary gear cluster inside the electric motors rotor is directly connected to the electrical generator and the ICEs flywheel. When above 70 mph or in battery depleted mode the ICE and the generator can 'assist' the electric motor in driving the wheels. The percentage of 'assistance' is unknown.

General Motors claim they haven't released details about the transmission until now because the patent was still pending up until September 21.

Some reports are saying that the ICE assistance is required due to limitations of the electric motor spinning "too fast to be efficient" above 70 Mph. If that were the case then GM have gone to a lot of effort to work around an inefficient electric motor design. By comparison, the AC Induction motor in the Tesla Roadster is "efficient" at up to 14,000 rpm. The GM Volt BLDC electric motor is said to become inefficient at around 6,500 rpm.

Electric Motors are approx 90% energy efficient while an ICE is around 25%. It's hard to imagine any situation where the electric motors efficiency drops to a point where it becomes more 'energy efficient' to assist it with the ICE.

A more likely explanation for GM choosing to use ICE assistance above 70 mph is that aero load increases at the square of speed. When the road speed of a vehicle doubles the aerodynamic drag increases four times. Rolling resistance and aero drag are the two biggest energy consumers of any vehicle but particularly electric vehicles.

On an electrically driven vehicle such as the Volt with a usable battery capacity of only 8 kWh, GM engineers/management look to have decided that extended high speed highway driving in EV only mode would deplete the battery too quickly to be acceptably to American consumers.

There is also the issue of extended highway running in battery depletion mode. While driving in an urban area, brake regeneration will recharges the battery and make up some of the short-fall between the 63 kw 1.4 lt ICE and the 111 Kw EV traction motor.

However during extended highway driving where opportunities for regeneration are rare, the power deficiency between the two power units may become more of a problem. It will be interesting to test drive a Volt to see how GM engineers have handled these many and varied driving scenarios.

Quite a few on-line news sources have gone over the top saying GM lied while others are lampooning GMs claims of 230 MPG seemingly without any understanding of how or why that MPG number was calculated.

More than likely this negative press can be credited as a hang-over from the documovie "Who Killed the Electric Car" in which GM were painted as the villain. While it is disappointing that the production version of the Volt is closer to being a Plug-In series-parallel hybrid, much like the Toyota Prius, than it is to being a true series hybrid, I'm sure that once the car makes it's way into the hands of consumers the Volt's press will turn around.

After all, The Chevy Volt is still a plug-in hybrid that can drive for up to 50 miles on nothing but electricity.


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Anonymous said...

I am disappointed. =(

Malcolm Faed said...

I think you mean "the square of speed" instead of "square root of speed"

Sarah Angelina DeLagostti said...

EV designers know that hybrids aren't any good since one system ultimately fights the other system. Gas and Electric don't mix. So I'm not surprised at all. Disappointed that they could've done better ? Perhaps. Yes, only since they have a lot of people that work for them....

Speed matching / assistance - is an issue ( factor ) while the batteries run down at a variable loss operational performance curve. Thus. The non avoidable - system to system fight.

In the end. We have predict ( as EV designers ) that systems that are designed to this manner of functionality shall have only but a short EV - Hybrid life-span at best.

Sadly I'm sure they ( the EV designers ) must have known all this as well ?

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