[cbermudez8]

I know that the gas powered engine in the Karma only powers a generator which in turn recharges the batteries. However, from an environmental perspective, how is that different than having a gas powered engine directly powering the drivetrain? Are emissions somehow different when the gas engine is powering a generator rather than a drivetrain? Thanks.

 

[AnOutsider]

I think, in theory, it's supposed to be more efficient as it hits an ideal rpm and stays there.

 

[doug]

You'd hope that would be the case, but that would be a rather high RPM and the practicalities of NVH engineering mean you can't really do that.

To first order thermodynamically, it is less efficient since it adds a couple energy conversion steps.

 

[Fabulist]

To first order thermodynamically, it is less efficient since it adds a couple energy conversion steps.

The clutch and transmission of a conventional drivetrain also add conversion steps and opportunity for power to be converted into heat instead of being transmitted to the rear wheels, and without comparing the relative efficiency of the two approaches, it is difficult to say which one is more efficient. As I understand it, the most energy efficient locomotives for example are rigged the same way as a Fisker: ICE connected to a generator with electric motors driving the wheels, the same is true with modern ships using Azimuth thrusters instead of a conventional propeller shaft and propeller. So the relative efficiency should be pretty well understood.


-- Fab.

 

[mattjs33]

I know that the gas powered engine in the Karma only powers a generator which in turn recharges the batteries. However, from an environmental perspective, how is that different than having a gas powered engine directly powering the drivetrain? Are emissions somehow different when the gas engine is powering a generator rather than a drivetrain? Thanks.

The EV-ER concept isn't really about reduced emissions, but rather reduced fuel usage. Although the times when the Karma is in EV mode the emissions are of course zero. So I guess if you get enough EV time, the emissions would in fact be less than that of a similar directly gas powered car.

I think, in theory, it's supposed to be more efficient as it hits an ideal rpm and stays there.

You'd hope that would be the case, but that would be a rather high RPM and the practicalities of NVH engineering mean you can't really do that.

As far as I understand it, cars with CVT transmissions do in fact run this way; the engine gets to an ideal rpm and stays there, while the CVT adjusts ratios accordingly. I've never driven one but read reviews of such. They said it's a wierd sensation, the engine revs not directly corresponding to forward thrust.

As I understand it, the most energy efficient locomotives for example are rigged the same way as a Fisker: ICE connected to a generator with electric motors driving the wheels...

I think the main reason that locomotives use this type of arrangement is that there isn't a clutch or transmission in the world that could withstand the slippage necessary to get that much weight moving with the torque it would require.

 

[Fabulist]

As far as I understand it, cars with CVT transmissions do in fact run this way; the engine gets to an ideal rpm and stays there, while the CVT adjusts ratios accordingly. I've never driven one but read reviews of such. They said it's a wierd sensation, the engine revs not directly corresponding to forward thrust.

I currently drive a standard (parallel) hybrid with a CVT transmission and in my experience, when the ICE is running, it behaves exactly as you would expect in response to the throttle position regardless of the speed or engine load. In my experience, the advantage of the CVT is that it can convert torque from two very different power sources (ICE and Electric Motor) very smoothly so that the transition from one to the other is completely seamless and under high acceleration, power from the two sources can be blended together smoothly. It definitely does not keep the ICE at a fixed RPM in differing load conditions since the RPM in my car changes immediately when I step on the accelerator and falls just as quickly when I take my foot off.

-- Fab.

 

[cbermudez8]

I know that the gas powered engine in the Karma only powers a generator which in turn recharges the batteries. However, from an environmental perspective, how is that different than having a gas powered engine directly powering the drivetrain? Are emissions somehow different when the gas engine is powering a generator rather than a drivetrain? Thanks.

The EV-ER concept isn't really about reduced emissions, but rather reduced fuel usage. Although the times when the Karma is in EV mode the emissions are of course zero. So I guess if you get enough EV time, the emissions would in fact be less than that of a similar directly gas powered car.

I think, in theory, it's supposed to be more efficient as it hits an ideal rpm and stays there.

You'd hope that would be the case, but that would be a rather high RPM and the practicalities of NVH engineering mean you can't really do that.

As far as I understand it, cars with CVT transmissions do in fact run this way; the engine gets to an ideal rpm and stays there, while the CVT adjusts ratios accordingly. I've never driven one but read reviews of such. They said it's a wierd sensation, the engine revs not directly corresponding to forward thrust.

As I understand it, the most energy efficient locomotives for example are rigged the same way as a Fisker: ICE connected to a generator with electric motors driving the wheels...

I think the main reason that locomotives use this type of arrangement is that there isn't a clutch or transmission in the world that could withstand the slippage necessary to get that much weight moving with the torque it would require.

I get that the overall emissions are lower in the Karma because the emissions are essentially 0 when the Karma is in electric mode. However, if there is little environmental advantage to having a gas powered engine recharge an electric battery versus power a conventional drivetrain, how is the extended range EV any better than an electric/gas hybrid?

 

[doug]

I currently drive a standard (parallel) hybrid with a CVT transmission and in my experience, when the ICE is running, it behaves exactly as you would expect in response to the throttle position regardless of the speed or engine load.

At least part of that behavior is the NVH engineering. Efficiency is sacrificed to an extent so that the engine noise acts more inline with driver expectations.

 

[Fabulist]

At least part of that behavior is the NVH engineering. Efficiency is sacrificed to an extent so that the engine noise acts more inline with driver expectations.

That's possible. Another factor is that the output of the electrical motor and the storage capacity of the battery is so much smaller than that of the ICE, it tends to get maxed out very quickly, and from that point on the ICE has to modulate the power reserve to respond to the power demand.
-- Fab.
[hr]

I get that the overall emissions are lower in the Karma because the emissions are essentially 0 when the Karma is in electric mode. However, if there is little environmental advantage to having a gas powered engine recharge an electric battery versus power a conventional drivetrain, how is the extended range EV any better than an electric/gas hybrid?

The whole point of the ICE-Generator is to allow you to have a single car for both short and long distance driving so that you could get the EV benefit every day and pay the pollution price only when you absolutely have to. By comparison, a pure EV vehicle never emits any tailpipe emissions but is range-restricted so that you would have to have another car, most likely equipped with an ICE for trips that you know would exceed the EV's maximum range. So depending on your driving patterns, you can either get two cars (or rent an ICE-equipped car when you need it) or get a car such as the Fisker that can fulfill both requirements, but not as efficiently as either a pure EV or a pure ICE car.

Contrary to Fisker propaganda, there is a significant compromise at the heart of the design and that is the dual power source. When you are driving a Fisker in pure EV mode, you are dragging around a 400 LB. motor-generator that serves absolutely no function except dead weight. Likewise, when you have to use the ICE, the 600 LB. battery serves almost no useful purpose other than as a sort of flywheel to smooth out the power changes.

If you made a Karma with just one or the other power source, you would improve the efficiency of operating that power source by itself but then you lose the inherent flexibility of having access to both power sources. So there are no perfect solutions. The Fisker is an attempt to reduce your pollution as much as possible without giving up the option of driving long distances without having to stop for an electrical charge.

-- Fab.

 

[cbermudez8]

At least part of that behavior is the NVH engineering. Efficiency is sacrificed to an extent so that the engine noise acts more inline with driver expectations.

That's possible. Another factor is that the output of the electrical motor and the storage capacity of the battery is so much smaller than that of the ICE, it tends to get maxed out very quickly, and from that point on the ICE has to modulate the power reserve to respond to the power demand.
-- Fab.
[hr]

I get that the overall emissions are lower in the Karma because the emissions are essentially 0 when the Karma is in electric mode. However, if there is little environmental advantage to having a gas powered engine recharge an electric battery versus power a conventional drivetrain, how is the extended range EV any better than an electric/gas hybrid?

The whole point of the ICE-Generator is to allow you to have a single car for both short and long distance driving so that you could get the EV benefit every day and pay the pollution price only when you absolutely have to. By comparison, a pure EV vehicle never emits any tailpipe emissions but is range-restricted so that you would have to have another car, most likely equipped with an ICE for trips that you know would exceed the EV's maximum range. So depending on your driving patterns, you can either get two cars (or rent an ICE-equipped car when you need it) or get a car such as the Fisker that can fulfill both requirements, but not as efficiently as either a pure EV or a pure ICE car.

Contrary to Fisker propaganda, there is a significant compromise at the heart of the design and that is the dual power source. When you are driving a Fisker in pure EV mode, you are dragging around a 400 LB. motor-generator that serves absolutely no function except dead weight. Likewise, when you have to use the ICE, the 600 LB. battery serves almost no useful purpose other than as a sort of flywheel to smooth out the power changes.

If you made a Karma with just one or the other power source, you would improve the efficiency of operating that power source by itself but then you lose the inherent flexibility of having access to both power sources. So there are no perfect solutions. The Fisker is an attempt to reduce your pollution as much as possible without giving up the option of driving long distances without having to stop for an electrical charge.

-- Fab.

Fab - Thanks for the response but my question is how the Karma dual system compares to a gas/electric hybrid. Is there an advantage to the Fisker configuration over a car that can run on both electric and gas power directly. Thanks.

 

[doug]

I guess the OP is asking about the efficiency differences when comparing parallel versus series hybrids.

It's really all in the specific details. To a first order approximation parallel can be more efficient for the reasons I gave. But then serial could be more efficient for the reasons AnOutsider (and Fab) gave.

In practice parallel hybrid has so far turned out to much more efficient when running on gas.

 

[ct-fiskerbuzz]

Fab - Thanks for the response but my question is how the Karma dual system compares to a gas/electric hybrid. Is there an advantage to the Fisker configuration over a car that can run on both electric and gas power directly. Thanks.

Here's where terminology is helpful:

"Serial hybrid": gasoline engine powers generator, generator powers batteries, batteries power electric motor, electric motor turns wheels.

"Parallel hybrid": gasoline engine powers generator and turns wheels; generator powers batteries, batteries power electric motor, electric motor turns wheels.

The question, then, is: "which is better, serial or parallel hybrid"?

The answer, as unsatisfying as possible , is: "it depends".

GM went with parallel on the Volt because in their design, it was slightly better for their purposes.

Honda went with serial on the Insight because in their design, it was slightly better for their purposes.

Toyota went with parallel on the Prius because in their design, it was slightly better for their purposes.

Fisker went with serial on the Karma because in their design, it was slightly better for their purposes.

Four cars, two of each design; each one is the best for the designer's purposes. If the purpose changes, or niggling details of the design change, the one that is the "best" may change as well. Goals can include CO2-per-distance; electric mileage, gasoline mileage, total (mixed mode) mileage; maximum power and/or maximum torque; curb weight; space (physical size); and pretty much anything else. There are tradeoffs between all of these.

 

[doug]

GM went with parallel on the Volt because in their design, it was slightly better for their purposes.

Honda went with serial on the Insight because in their design, it was slightly better for their purposes.

Toyota went with parallel on the Prius because in their design, it was slightly better for their purposes.

Fisker went with serial on the Karma because in their design, it was slightly better for their purposes.

Some corrections here:

Honda Insight is a parallel hybrid (IMA).

GM's Volt actually operates as series hybrid most of the time, but can be parallel in certain situations (above 70mph or under hard acceleration) in order to increase efficiency.

Fisker went with a strictly series hybrid probably because it's the simplest to engineer.

 

[kabalah70]

Fisker went with the configuration they have because that is what their partner Quantum Technologies had developed, plain and simple.

 

[ct-fiskerbuzz]

Fisker went with the configuration they have because that is what their partner Quantum Technologies had developed, plain and simple.

Well, yes, but that counts as "for their purposes".

Seriously, if you want to keep CO2-per-km low by running off electricity even on the Autobahn at 90 mph, you want a serial hybrid (Karma), not a parallel hybrid (Volt), even if the latter is more petrol-efficient. So goals really do matter.

Of course, the Q-Drive's original goals were military. But Fisker and Quantum together have plenty of engineering ability to rework the drive system if they care to do so. (And if they put in a transmission, that will mean that they do care to do so, at least in some ways. Note that transmissions give tradeoffs as well. Tesla were going to use one but could not build one strong enough to stand up to the torque in the Roadster.)

 

[BillyO]

[/quote]
Is there an advantage to the Fisker configuration over a car that can run on both electric and gas power directly. Thanks.

[/quote]

Yes. In my case I don't expect to use the ICE more than 5% of the time I'm driving. The gas electric automatically uses gas whenever it is called for by the operating software. You don't have a choice. I could choose to use no gas if I wanted to.

Billy O

 

[mattjs33]

I currently drive a standard (parallel) hybrid with a CVT transmission and in my experience, when the ICE is running, it behaves exactly as you would expect in response to the throttle position regardless of the speed or engine load. In my experience, the advantage of the CVT is that it can convert torque from two very different power sources (ICE and Electric Motor) very smoothly so that the transition from one to the other is completely seamless and under high acceleration, power from the two sources can be blended together smoothly. It definitely does not keep the ICE at a fixed RPM in differing load conditions since the RPM in my car changes immediately when I step on the accelerator and falls just as quickly when I take my foot off.

-- Fab.

I guess I was remebering reviews I had read of early, non-hybrid, CVT equipped cars, where the engine did rev up like I said. It's a concept similar to a snowmobile drive. I haven't any firsthand CVT experience, hybrid or otherwise, so forgive my ignorance!

 

[Fabulist]

I guess I was remebering reviews I had read of early, non-hybrid, CVT equipped cars, where the engine did rev up like I said. It's a concept similar to a snowmobile drive.

@Mattjs33: I did not mean to overstate the case, just sharing my personal experience which is limited to one hybrid car with CVT transmission, so what you have read may be true for pure ICE cars.

-- Fab.