Question about Electric Cars: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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July 9, 2009 7:17 a.m. ZOOMiata Dork

Another magazine profiles future electric cars in their most recent issue. It seems that products such as theChevrolet Volt and the Fisker Karma use a gasoline engine exclusively to power a generator to increase the range of the vehicle, and to keep the battery pack at an optimal charge.

I have a question. Does a generator produce more "charge" by spinning faster? I was curious, because most stationary generators I am aware of use a diesel engine spinning at a consistent, low rpm. But the Volt and the Karma use gasoline engines that rev quite high.

It would seem if more rpms equals more charge from a generator, then a rotary engine could be useful due to its compact dimensions (although I suspect fuel economy would be a compromise one would need to make).

Any insights (no pun intended).

Rob
July 9, 2009 7:29 a.m. walterj Dork

I have a theory that these vehicles would be more fun if we removed the electric motor and just used a more powerful gasoline engine to drive the wheels directly. If on-board electrics are necessary a small battery and alternator circuit can be used which wil greatly decrease the weight of the vehicle. Has anyone tried this?
July 9, 2009 8:25 a.m. alfadriver HalfDork

IIRC in my electronics class, you can set a generator up to run efficiently at various RPM's.

Bear in mind, unlike the stationary geneator, both the Fisker and the Volt have to have some flexibility to keep the battery charged. While they all claim that it's not driving anything, the need to go up a hill with a low battery means that the motor-generator set has to be capable of more energy production than just keeping the batteries charged for a short drive. Say, if you started up some of the longer passes, and have a goal of x% still available after starting at 30%, you'll need some decent power to generate.

While the intention is to be more "plug in electric" than "hybrid", in the end, it still has to be designed to run up a pass at posted speed limits starting with a 30% charge. So it IS a hybrid.

As for the rotary- being that they have FE problems as well as some serious emissions problems, they are not a great candidate for this kind of application. A direct injection, UUBER high compression Sterling cycle would be better (the high mechanical compression ratio is offset by the real compression ratio with very late intake valve closing).

Still not a huge fan, but I see where the direction of these kinds of cars are going....

Eric
July 9, 2009 9:03 a.m. nderwater Reader

In reply to walterj:

lol. wait, i think the answer is miata. or e30.
July 9, 2009 10:03 a.m. Raze Reader

Don't forget P71...
July 9, 2009 11:57 a.m. jikelly New Reader

Sterling cycle?

Is that like the sterling engine?
July 9, 2009 12:11 p.m. alfadriver HalfDork

jikelly wrote:

Sterling cycle?

Is that like the sterling engine?

Somewhat- but for gas/Otto engines, the way to get a larger expansion stroke than compression is to keep the intake valve open during the compression stroke, blowing some of it back into the intake.

Bad for power, good for thermal efficiency. Perfectly fine if you generate torque via a electric motor. Pain to fuel, since some is pushed back- why I would suggest DI.

Eric
July 9, 2009 7:18 p.m. Toyman01 HalfDork

House power generators turn 1800 rpm using a 4 pole armature to give 60 cycles. Portable generators turn 3600 rpm using a 2 pole generator to give the same 60 cycles. 8 poles would let you turn 900 rpm. Frequency is as important as voltage when using AC power.

With DC power frequency doesn't matter, only voltage, so you can turn the generator as fast or slow as you want. As long as the voltage is regulated you're good to go. Most alternators produce more amperage as you turn them faster. That is probably what they are doing on the Volt and Karma. They need lots of amps to keep up with the motors.
July 9, 2009 7:34 p.m. Trans_Maro Reader

alfadriver wrote: Somewhat- but for gas/Otto engines, the way to get a larger expansion stroke than compression is to keep the intake valve open during the compression stroke, blowing some of it back into the intake.

Bad for power, good for thermal efficiency. Perfectly fine if you generate torque via a electric motor. Pain to fuel, since some is pushed back- why I would suggest DI.

Eric

I think you guys are talking about an Atkinson cycle engine:

http://en.wikipedia.org/wiki/Atkinson_cycle
July 9, 2009 10:44 p.m. erohslc Reader

There are very few true DC generators. Automotive generators and alternators are both AC devices. The difference is that the generator uses a mechanical arrangement for converting the AC to DC (the brushes and commutators), while an alternator uses rectifier diodes. In both cases, the output voltage is regulated by controlling the current through the 'field' winding, which is outside and stationary on a generator, and which is inside and rotates on an alternator. A true DC generator typically consists of a thin copper disk (sometimes with radial slots to control eddy currents) with thicker sections at the hub and rim. An axial magnetic field is created by field windings or magnets. As the disk is driven to rotate in the magnetic field, DC current flows between the hub and rim (Right Hand rule for magentic fields, electric current, and motion) and brushes at the hub and rim conduct the generated current. Voltage from a single disk is rather low, although current can be high, so multiple disks (each with it's own set of brushes) would be required to achieve even 12 Volts. Motors of similar construction were used in mine cars around the turn of the previous century (1900's). The high currents limited efficiency and brush maintenance must have been a nightmare.

Carter