[SoCalGuy]

So I've been wondering about how Fisker might optimize MPG in Sport mode. It was always my understanding that ICE's have an efficiency curve, where, depending on the engine, it may operate at its peak fuel efficiency at say, 2500 RPMs. If that is the case, wouldn't it make sense for an EV with range extender to operate the ICE exactly at that RPM level to maximize fuel economy and minimize emissions?

Let's say you pop the car into Sport mode when you get on the highway. We all know it takes more power to accelerate than to maintain. So maybe when you go from 0 to 70mph, you're using all of the available ICE power at 2500 RPMs. Then once you reach your cruising speed, the ICE still operates at 2500 RPMs and stores the excess power in the battery. An intelligent Sport mode would then balance out when it would need to run the ICE and when it can borrow the energy from the battery. This seems a lot more efficient than starting/stopping the ICE and running it at various (less efficient) RPMs depending on system reqs (eq acceleration, maintaining speed, etc).

Am I missing something here?

 

[Ross a klein]

That would be great cruise and charge.

 

[Fabulist]

I found chart on a GM hobbyist blog and it is supposed to show the HP and torque curve of the gas engine in the Karma:

If I am reading this correctly, at 3000 RPM, the engine is producing 150 HP which translates to about 110KW. I have noticed that cruising at 60 to 70 MPH requires about 25 - 35 KW but when you accelerate to pass another car or go up a hill, the usage climbs very rapidly up to 100KW and beyond. I guess if you are on a steady, 65 MPH cruise on a flat road, 3000 RPM would be enough to cruise+charge but at higher speeds or climbing, you would have to crank up the engine.

 

[SoCalGuy]

I found chart on a GM hobbyist blog and it is supposed to show the HP and torque curve of the gas engine in the Karma:

Which curve is which... also, the chart doesn't show efficiency (unless I'm missing something?). The idea would be that in instances that require a whole lot more power than is available at the 'efficient' RPM, it would borrow that from the battery (e.g. passing, climbing, etc) and pay it back when cruising. This would minimize the noise intrusion, emissions, while maximizing gas efficiency as well. Obviously, if you floor the Karma or climb up a steep grade for an extended period of time, the car may have to switch to a higher RPM level, which it should do only if battery reserves prove too low.

 

[Fabulist]

I found chart on a GM hobbyist blog and it is supposed to show the HP and torque curve of the gas engine in the Karma:

Which curve is which... also, the chart doesn't show efficiency (unless I'm missing something?). The idea would be that in instances that require a whole lot more power than is available at the 'efficient' RPM, it would borrow that from the battery (e.g. passing, climbing, etc) and pay it back when cruising. This would minimize the noise intrusion, emissions, while maximizing gas efficiency as well. Obviously, if you floor the Karma or climb up a steep grade for an extended period of time, the car may have to switch to a higher RPM level, which it should do only if battery reserves prove too low.

The curve on the left is the torque curve and the one on the right is the Horse Power curve. This chart does not show fuel consumption. My point was that just to move the car, you need to run the engine hard enough to produce around 100KW. If you have any extra power at that point, then you can bank it with the battery.

Not sure where the most efficient part of the curve is. I have heard 2400 RPM mentioned as a possible number but nothing definitive from Fisker.

 

[Nimisys]

The number your looking for is the Brake Specific Fuel Consumption (or BSFC) for the engine. Basicly its how much power does it make for a given fuel flow, and i have not been able to find it for the LNF in either stock or factory upgraded form. i have seen some thrown around by enthusiasts, but they were for use in sizing injectors and not very specific or all that accurate.

now engine theory does say for engines of equal displacment and technology, more cylinders will give more power but at more fuel usage, fewer cylinders will be less power but also less fuel usage.

the LNF also is a torque controlled motor, so it tunes the A/F ratio along with the boost pressure to obtain the desired torque request, which is why it uses a quick spooling albeit smallish twin scroll turbo and can produce a very flat torque curve across the rev range.

 

[Dutch]

I'd prefer not to use the ICE at all (I'm not fond of the characteristics because of the contrast with Stealth Mode, and it defies the purpose of the car which is extreme fuel efficiency). Unfortunately I am forced to use the ICE twice a day because my commute - highway at 70 mph - is 2 x 42 miles and that's too far. So I really, really would like to be able to buy an upgraded battery pack, which can go at least 50% further. I know Fisker plans on offering such an option, a very well-informed source told me. But I hope we don't have to wait too long.

 

[flying dutchman]

Dutch,
I am sure that will bring the cO2 output below 50 grams.
I think all Fiskers in Holland will go for that because of the tax advantage.
FD

 

[Dutch]

Dutch,
I am sure that will bring the cO2 output below 50 grams.
I think all Fiskers in Holland will go for that because of the tax advantage.
FD

That's an extra and very important consideration. I am currently not driving my Karma privately, in order to avoid a yearly tax of € 6400 ($ 8300). The Karma just missed the target for 0% tax by 2 grams of C02 (51 per km instead of the required 49). That's a pity. And for many people in our country a reason not to buy the Karma. A battery with increased range would surely cause hundreds of extra sales. This should be very interesting for Fisker.

(Sorry for taking it a bit off-topic)

 

[SoCalGuy]

The number your looking for is the Brake Specific Fuel Consumption (or BSFC) for the engine. Basicly its how much power does it make for a given fuel flow, and i have not been able to find it for the LNF in either stock or factory upgraded form. i have seen some thrown around by enthusiasts, but they were for use in sizing injectors and not very specific or all that accurate.

now engine theory does say for engines of equal displacment and technology, more cylinders will give more power but at more fuel usage, fewer cylinders will be less power but also less fuel usage.

the LNF also is a torque controlled motor, so it tunes the A/F ratio along with the boost pressure to obtain the desired torque request, which is why it uses a quick spooling albeit smallish twin scroll turbo and can produce a very flat torque curve across the rev range.

So pardon the elementary question - but isn't torque the more relevant number for things like acceleration etc? If so, since the torque curve is fairly flat at relatively low RPMs, whats the point for having the car rev higher for more HP to turn a generator?

@Dutch - I recognize your point about the relatively stark contrast between Sport and Stealth from a driver's experience perspective. However, 40-70 miles of range is simply not practical for a car in this price range. Thus, the Sport mode becomes important -and we should endeavor to find a way to maximize its efficiency. Since we have the tools like the battery at our disposal, my post is aimed at suggesting there is a more efficient way to leverage the system as a whole such that even in Sport or Range Extend mode, we can get better fuel economy than we currently get. I suspect the current paradigm we're using (i.e. graduated use of the ICE vs. using it at specific RPM levels) was the product of the noise intrusion during the Roadshow cars primarily. My recollection from the two roadshow drives I did (the Irvine at the beginning, Muffler 1.0 and the NYC one with Muffler 2.0) was that they had specific RPM levels at which they operated and they did not 'rev up' or 'rev down' like our production cars do. My sense is that those were the optimal ways to run the ICE from an efficiency perspective, but not necessarily from a driver comfort/noise perspective, thus the current system we have. If we could operate the ICE only at its most efficient level, we could use the battery to both augment the ICE power in high power moments like acceleration and pay back that borrowed power/energy when we're running the ICE while having low power reqs (e.g. while cruising on the freeway). Thus, we wouldn't sacrifice MPGs for the sake of managing noise PLUS we'd be able to recharge the battery on the fly, giving us maybe a fully charged battery after an hour or two of highway driving. Anyone disagree?

 

[Dutch]

I agree with you that it would be pleasant to have the ICE run at a constant speed, making it less protrusive. At the moment the ICE reacts to every pedal input by revving up and down, while in theory the battery could absorb those fluctuations in the required output (the ICE can then keep on running constantly, at a slightly higher level than required, so that the battery can even gain some range).

 

[Nimisys]

the engine revving in responce to throttle input was from feedback during the roadshow, where the ICE holding one RPM was not well recieved.

 

[JCMorrill]

Ok. Nobody's hit this posting for a while, so I'm a new owner, I have a new spin on this question.

My version of the question is ... In highway cruise, does the ICE rev at different speeds to correspond to the power pull that the electric motors want? Or does it just spin at a fairly consistent speed to spin the generator to provide max electrical power? Because, if it's the latter ... wouldn't that mean that you'd get better mileage in sport mode by going faster? Same engine load, but for a shorter time?

Or, am I just being hopelessly ignorant!

 

[ct-fiskerbuzz]

Ok. Nobody's hit this posting for a while, so I'm a new owner, I have a new spin on this question.

My version of the question is ... In highway cruise, does the ICE rev at different speeds to correspond to the power pull that the electric motors want? Or does it just spin at a fairly consistent speed to spin the generator to provide max electrical power? Because, if it's the latter ... wouldn't that mean that you'd get better mileage in sport mode by going faster? Same engine load, but for a shorter time?

Or, am I just being hopelessly ignorant!

From experience (driving in Sport on I-80 through UT/NV/CA), the engine will rev higher when going uphill at a constant 70 or 75 or 80 mph, than when going downhill. This means there's some degree of "demand" control based on inverter/battery draw.

Overall you get (much) better mileage at 55 mph than at 70 to 80 mph: upwards of 25 mpg at 55 on long straight flat sections, somewhere around 21 or 22 mpg at 75. This is quite clearly due to air drag.

 

[Fabulist]

Ok. Nobody's hit this posting for a while, so I'm a new owner, I have a new spin on this question.

My version of the question is ... In highway cruise, does the ICE rev at different speeds to correspond to the power pull that the electric motors want? Or does it just spin at a fairly consistent speed to spin the generator to provide max electrical power? Because, if it's the latter ... wouldn't that mean that you'd get better mileage in sport mode by going faster? Same engine load, but for a shorter time?

Or, am I just being hopelessly ignorant!

I used to have a very long and boring commute and could only listen to so much NPR or Economist Audio. So I got a Bluetooth OBDII scanner and used an app to monitor things like engine RPM during my long drives.

What I observed was that the ICE stayed very close to 3 specific RPMs. It would move around to give the impression of throttle response, but for the most part it was either at 1200, 2500, or 3600 RPM. For the vast majority of the time, the ICE stayed close to 2500 RPM at highway speeds. If I planted my right foot down hard, it would jump up to 3600 RPM and stay there until I backed off the throttle, and it would step down to 2500 RPM again. For smaller variations, the engine RPM would rise and fall slightly just to give proper feedback, but it would stay very close to the 2500 RPM as much as possible.

My understanding is that the engine RPM is directly controlled by one of the engine management computers and throttle is simply an input to the computer that determines how many RPMs to demand from the ICE. The particular motor in the Karma (GM Ecotec LNF) can actually rev much higher and provides its maximum HP (260) at 5300 RPM, but in order to protect other components, such as the interface to the generator, and to maximize efficiency, the computer tries to keep the motor running around 2600 RPM that is the most efficient RPM for this particular engine.

It would probably be more efficient to run the engine at a single speed (2600 RPM) all the time and use the battery to either provide additional power when needed, or to store extra power when it is not needed, but the experience will be very different and I guess the Karma engineers decided that a more familiar throttle response would be better received.

This is all based on my personal observations and experimentation during my commutes. I am confident about my observations, but my reverse engineering may be way off, and we have a lot of actual experts that can jump in and clarify things if I got it wrong.

 

[JCMorrill]

If Fab is correct, it would seem to indicate to me that going as fast as you can and still keeping the engine revs at 2500 would give you the best overall mileage as you're leaving the engine revving at that speed for a shorter time period. Or, am I missing something?

 

[Fabulist]

If Fab is correct, it would seem to indicate to me that going as fast as you can and still keeping the engine revs at 2500 would give you the best overall mileage as you're leaving the engine revving at that speed for a shorter time period. Or, am I missing something?

From my observation, that's what the car tries to do. Most of the time when I was driving at highway speed, the ICE RPM was around 2500. It only went to 3600 RPM when I needed more power to pass. Since the HV battery never drains all the way to zero, the car can use it to provide an instantaneous power boost, and ICE can then replace the amount used.

The whole Q-Drive system is deceptively simple in concept and mind-bogglingly complex in execution. This is one of the reasons I like this car so much. A pure BEV is much simpler. Serial hybrid is a lot more complicated.

 

[AlJOHN]

Not sure of all the dynamics of the ICE. However, I seem to recall in discussion at a dinner meeting here with Hendrick that they changed the ICE timing based on CO2 emissions. Originally the Karma was advertised to charge while driving. They then changed the software to "hold" the battery at what ever charge level it had when it was placed in SPORT. Then in the last software release, It was limited to holding charge at 26 miles of battery and below. I seem to recall that was done to make the CO2 comply with the Netherlands Super Low Emission tax exemption.

I may be incorrect, but based on those discussion it was always possible to operate the ICE and charge the battery.

 

[ct-fiskerbuzz]

I may be incorrect, but based on those discussion it was always possible to operate the ICE and charge the battery.

"Always" is too strong; it's "usually". The amount of power (measured in kW or horsepower) needed to move the car varies from moment to moment. The amount of energy (kWh) needed is the integral of power with respect to time. For simple cases you can simply multiply: 8 kW, times 2 hours, equals 16 kWh, for instance.

If you're accelerating from 0, or going uphill, though, it can take more kW (hp) than the engine actually provides: the Karma has 300 kW (403 hp) of electric drive. Even accounting for the de-rating, you could easily use over 200 kW for a short period. The engine/generator produces a maximum of 175 kW, so in order to use 300, or 250, or even just 200 kW, you must get some kW from the battery.

But you can't accelerate from 0 continuously, nor go uphill forever. Eventually you hit cruising speed and flat or downhill areas. Then it's possible for the engine to recharge the battery, since the available 175 kW is way more than the needed 5, 10, or even 70 or 80 kW you might need to sustain speed (depending on actual speed).

The current software won't do it, though. To bring the battery level up, you must drive up a mountain in Sport (holding it at 26-or-fewer miles), then go down the mountain using regeneration to push the level higher, take the car out of Sport briefly, and now you can hold the new level provided it's no higher than 26. (I have done this going down the mountain from Alta/Snowbird, and down the Sierras from Tahoe towards Sacramento or Reno.)

 

[FiskerPhilly]

All of you are correct in most accounts here. According to the software in the HCU (hybrid control unit) which is the brains of this operarion, if the car is in maintain status (sport mode at 26 mile range or below) the HCU WILL recoup consumed voltage in the battery so if you drive up a hill and consume more than the GEN can produce (as CT explained) when you level off, the gen will recharge back to the range you had when you switched to sport. Ive seen it several times, put car in sport at say 16 miles, drive it hard or up a hill it drops to 15 or even 14 then when you come to a stop GEN runs at 25kw or so to recharge and range gradually comes back up to where you started at 16.
As far as the reasoning for this, AI you are correct that it was changed from original software, and may have had something to do with Netherlands, but from what I understand and makes sense as well is that it was changed to meet the US EPA standards for overall fuel economy. If car is driven with a 40 mile range in sport mode and gen is forced on to recharge battery up to 45 while driving (albeit very cool feature) according to epa you are wasting fuel by generating and loading engine and not using your battery first. If it were up to them sport mode would use battery down to zero and only use engine to boost power when demand requires. The compromise when Fisker argued the point was 26.
Id still like to find the old software files and install it just for fun. But problem was that software was loaded with driveability issues and check engine light would be a daily part of life...