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efficiency + range <> efficiency
By drycrust3 on 10/28/2011 1:03:37 PM , Rating: 3

When I look at the 52 mpg-equivalent I just can't put my finger on why it looks wrong. To me, the 32 miles of running on the electric charge is the range of the vehicle on the electric charge (which is related to the size of the battery and the efficiency of running under electric power) and isn't an indication of the efficiency of the vehicle in converting energy into motion, which is what the mpg rating is all about. By the logic shown in this rating, if Fisher had increased the size of the battery the mpg-equivalent rating would have gone up, even though the extra weight would have made the car even less efficient than it currently is.
According to Wikipedia the electrical efficiency of this vehicle is 65kWh / 100 miles or 1462 kJ / km, which presumably equals 2353 kJ /mile (1462/0.6214) or 2.353 MJ/mile. The total range of the vehicle is 230 miles, meaning that with an electric range of 32 miles, the electrical power is 14% of the total range.
Since the petrol engine rating is 20 mpg, that equates to 0.05 US gallons of petrol per mile. 1 litre of fuel contains 32 MJ of energy, which presumably means 1 US gallon = (32/0.2642)= 121 MJ of energy per gallon, thus the car uses 6 MJ of petrol energy to travel one mile.
While there are lots of ways one could apportion the efficiency of these two means of motive power, the fairest way is to use their proportion of the total range of the car.
Since the electrical energy amounts to 14% of the total range of the vehicle and 86% of the range is from petrol, thus the car uses 520 MJ to travel the 86 miles and 33.6 MJ of electrical energy to travel the remaining 14 miles, thus using 554.4MJ to travel 100 miles, which equates to 5.544 MJ per mile, which is (5.544/121) 0.0458 gallons of fuel (real + equivalent) to travel one mile, or 21.8 miles per US gallon of fuel (real + electrical equivalent).

RE: efficiency + range <> efficiency
By Keeir on 10/28/2011 4:26:45 PM , Rating: 2
Hello... you obviously need to do a bit more thinking

The Fisker Karma is rated as

52 MPGe or stated as 65 kWh/100 mile for upto 32 miles on Electric Power. This is using the Combined EPA cycle calculation that is a combination of 5 EPA cycles.

After 32 miles on electric power, it becomes 20 MPG on the same combined cycle.

Putting an artifical label like 100 miles at a time is dangerous. Everyone drives different distances.

The Karma is capable of 32 miles of Electric Range between charges. The overall efficieny will be significant different at 25, 50, 75, or 100 miles between charges. The EPA is not attempting to project this number.

At one point in time the EPA was considering doing such a thing. By using data gathered in 2001, SAE created a curve that projected the average electric utilization rate of a car given its AER. The SAE then recommended to report the gasoline usage and electric usage over 100 miles of range using this curve as the "combined" cycle numbers. Unfortunely, this is just a little too complicated statistically for a population who do not really even have a grasp on how many miles they drive each day.

For the Karma, the numbers would have been
over 100 miles the average US driver should expect to use 39 kWh of electricity and 2 gallons of gas... or 31.7 MPGe. A pretty good number considering that a Porsche Hybrid would get 25 MPGe.

GM jumped the gun on this method and went straight to "X" MPG + Y Electricity. That was a big big mistake in my mind. People are not able to think that way.

I've always favor more of a chart approach

Always Electric: 52 MPGe (65 kWh/100)

Average US Driver: 31.7 MPGe ( 39 kWh + 2 Gallons /100 miles)

Always Gasoline: 20 MPGe (4 gallons/100 miles)

This type of chart gives a better idea to a perspective buyer than the current one and does a better job of being able to compare these different Hybrid.

For instance, the Volt and Prius Plug-in would seem natural compeditors.

The Volt gets 93 MPGe electric and 37 MPGe Gasoline versus 87 MPGe electric and 50 MPGe Gaslone for the Prius. At first glance, the Prius seems significant better. However for the Average US driver the Volt will be 67 MPGe (1 gallon/100 miles) and the Prius would be 63 MPGe (1.3 gallon/100 miles).

The Average US driver will use more Gasoline with the Prius Plug-In Hybrid than the Volt Plug-in Hybrid despite the Prius's 35% higher MPG number. If you drive more than the average US drive, then the Volt's number falls faster than the Prius number.... and the chart would give you that concept but not the point at which it becomes better.... since this does depend on how far you drive each time you get in the car, not even on average.

RE: efficiency + range <> efficiency
By drycrust3 on 10/28/2011 8:57:20 PM , Rating: 2
Hello ... I hate to disappoint you, but I do think, which is why I think that figure is nonsense! The text clearly states the 52 mpge rating is obtained by adding the 32 miles range of the batteries with the 20 mpg fuel efficiency rating. Thus, there is no indication of the efficiency of rectifiers, the batteries, and the electric motor in this rating. You could have used thermionic diode rectifiers and lead acid batteries and still got the same result, but the cost in your monthly power account would have been very different. There is no indication to the end user as to how much energy is used when driving, so the user has no idea what it will cost to travel. While I accept there is a need to present fuel efficiency figures in terms we can all understand, the current rating system is just playing with numbers to please hybrid car sales people.
I calculated the MPGe would be about 21.8 MPG over a distance of 230 miles, which is the maximum range of the vehicle using petrol and electricity. 14% of that distance was considered to be powered by electricity and the other 86% by petrol.
To me, the only fair way to take into account the different fuel injection techniques, the different battery sizes, and the other 101 other differences between one make and model and another make and model of hybrid is to base the efficiency rating on the maximum distance it can travel, then relate the electrical efficiency to the distance to it contributed, and the petrol efficiency to the distance it contributed.
The current system has an "efficiency" rating based upon an indeterminate distance and varies from driver to driver, thus it cannot be used to compare one car with another.

RE: efficiency + range <> efficiency
By Keeir on 10/30/2011 12:29:51 AM , Rating: 2
You certainly don't sound like you thought things through. I recommend for one thing not trusting Kaiser and reading at least one of the other 3 articles on the Karma here or go straight to the EPA label.

In your method of calculation, all the Karma would need to do to raise its efficiency rating is to reduce the size of the gas tank... or increase the size of the battery. Maximum Range has very little to do with efficiency.

We can both agree that Efficiency should be measured as X Consumed/ Y Miles or Y Miles/ X Consumed

But what should Y miles be? Clearly it should be consistent from car to car. It should also be a number relevant to the consumer. For example, the average car trip in the US is ~18 miles long from start to finish. Maybe that's a good point to start? No... in that case the consumer would have many unanswered questions as several plug-ins would get huge scores.

SAE developed using 2001 US Driver Data a profile of the Average US driver that tableted over a year the number of 10 mile, 20 mile, 30 mile, etc, etc driving days. They then created a curve to relate Electric Range and Electric Utilization for the Average US Driver in 2001. IE, for the Karma the Average US Driver in 2001 would use electricity 60% of the time! Even better this system accounted for mixed mode driving by allowing any two sets of propulsion to be mixed... even if there are not discrete.

In this SAE Method, efficiency is shown as a sample of 100 miles of the average efficiency over the entire year.

I still think this is a good place to start. For the average US driver of 2001, the Karma has an overall efficiency of ~31 MPGe.

Your method means next to nothing. Hopefully the average consumer would not wait till their fuel ran out to charge the battery. Most people do not drive 230 miles a day after all.

RE: efficiency + range <> efficiency
By drycrust3 on 10/31/2011 6:59:28 AM , Rating: 2
Maximum Range has very little to do with efficiency.

Well, if maximum range has little to do with efficiency then why is the MPGe rating obtained by adding the electrical range to an actual efficiency rating (MPG)?

RE: efficiency + range <> efficiency
By Keeir on 10/31/2011 2:29:24 PM , Rating: 2
Read the EPA sticker.

Go to

Search Fisker Karma EPA Sticker

Carefully look at the sticker, at let me know what you think the sticker says... its pretty clear to me.

The Karma gets 52 MPGe in Electric Mode and 20 MPG in Gasoline Mode

