Where can I find information about electric cars?
I want to know the efficiency of an average electric car (How much electricity goes in, and how far I can go in one charge) but I can't find much info about this. I also want to know how a recharged battery compares to a gallon of gas, but I can't figure out where to find this either. I mean, how many kilowatthours can I get out of a generator and a gallon of gas? Anyone know much about this?
- 6 years agoFavorite Answer
You have asked several marginally related questions. It might be better to ask them all separately.
Efficiency is a term we commonly use and sometimes misuse. From a physics perspective we are concerned about the energy that goes into an engine and how much useful work is produced. But some people use the word to simply mean something that is "better." And then there is the term "fuel efficiency" which might more appropriately be called "fuel consumption." This last seems to be what you are asking about by using the term "efficiency."
Electric vehicles can be made in many different ways. A "fuel cell" vehicle is an electric vehicle that makes electricity on the vehicle. A subway car is an electric vehicle that has electricity transmitted to the vehicle. A battery electric vehicle stores an electric charge in a battery. In today's commercially available battery electric cars the size of the battery is not consistent. The Tesla has the largest batteries at about 85 kWh. The Leaf battery is 24 kWh while the mitsubishi I has a battery capacity of only 16 kWh. A smaller battery will not allow the vehicle to travel as far but it will charge quicker.
Vehicles are rated by the US EPA and you can look up those values on http://www.fueleconomy.gov/feg/Find.do?action=sbs&... The EPA sticker lists a combined mileage for electric vehicles in MPGe or miles per gallon equivalent. This is obtained by comparing the heat (energy) value of gasoline and electricity. 34.56 kWh of electricity has about as much energy as one gallon of gasoline http://alternativefuels.about.com/od/resources/a/g... So, even the biggest battery in the Tesla model S contains less energy than 3 gallons of gasoline but due to the car's efficiency it will take the vehicle more than 250 miles.
A generator contains a petrol engine. Such an engine running at its most efficient RPM will be closer to the thermodynamic theoretical efficiency for this otto cycle engine of about 27% efficiency. But it also drives a generator where it loses some efficiency and so the final rating might be between 15% and 20% efficiency.
And so if a gallon of gasoline contains about 114,000 BTU of energy and the engine produces electricity at 20% efficiency the result will be (114,000 x .20) about 22800 BTU of electrical energy. The conversion from BTU to electricity is 3400 BTU / kWh. So we might produce 6.7 kWh of electricity from one gallon of gasoline using a generator. Different generators will have a different efficiency.
If we had this generator in the trunk of a Nissan Leaf which the EPA says uses 30 kWh / 100 miles (or .3 kWh / mile) 6.7 kWh, produced from gasoline through a generator, could take the car an average of about 22 miles. The same electrical energy as in the initial gasoline if instead stored in the battery could take the vehicle the EPA rating of 114 MPGe. The problem is the horribly inefficient petrol engine.