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Gas-electric hybrids, is gas cheaper or electricity to drive a given distance?

  1. Apr 29, 2007 #1
    IT is possible to modify a gas-electric hybrid so that you recharge the battery from the outlet, and run the car on battery. This costs electricity which costs money.

    Of course you can use gas. which also costs money.

    Which is cheaper to drive 20 miles, gas or electricity?

    Personally I would think such mods would be very dangerous. For one thing you would need to hack directly into the car's electrical grid.

    Would it be a good idea to equip gas-electric hybrids with a standard outlet or an ac/dc converter outlet to help customers save money by fueling up on electricity rather than gas? (or improve mgp by shifting power away from gas and toward electricity?)
  2. jcsd
  3. Apr 29, 2007 #2

    Ivan Seeking

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    If we assume 125000 BTUs of energy per gallon at a typical efficiency of 30%, then a gasoline engine can produce about 37,500 BTUs of work for every gallon of gasoline. At $3.00 per gallon, this is 12,500 BTUs of work per dollar

    – or 13.2 MJ [Mega Joules] of work per dollar

    For electricity, if we assume typical values of 90% efficiency of the electric motors, and 80% efficiency for the batteries, we get about 72% efficiency overall [before the drive train]. Electricity sells for about $0.10 to $0.15 per KW-Hr [depending on where you live], and a KW-Hr is 3.6 MJ, so at 72% efficiency we get about 2.6 MJ of work per KW-Hr of electricity , or 2.6 MJ / ($0.10 to $0.15)...

    = 26MJ to 17 MJ of work per dollar.

    So it would appear to be as much as twice the expense to burn gas than to run an electric motor - if you only pay 10 cents per KW-Hr for electricity.. If you are paying 3$ for gas and 15 cents per KW-Hr of electric energy, there is only about a 30% price increase going from electric to gasoline - from 17MJ/$ for electric, to 13MJ/$ for gas.

    But then you have to factor in the purchase price, lifespan, and the maintenance costs of the vehicle. For example, right now batteries are expensive and they don’t last long. A standard deep cycle lead-acid battery discharged to 50% capacity with each cycle is only expected to last for about 1000 charge cycles. Also, electric cars and hybrids are quite a bit more expensive than gasoline powered cars. So your question is not as simple as it might appear...not to mention the fact that some cars get 40mpg, and some get 10 mpg. In principle the work done per gallon should be about the same for all gas engines, but obviously some cars are implicity more efficient due to their low profile and higher aerodynamic efficiency, more efficient transmissions, etc.

    Biodiesel has 118300 BTUs per gallon with a typical efficiency of 40% for diesel engines. Also selling for about $3.00 per gallon, you would get about

    - 16.6 MJ of work energy per dollar from your diesel running carbon neutral biodiesel
    - about 23% better than gas, but no batteries. And diesel engines are well known for their long lifespan as compared to gas engines.

    That was for 100% pure biodiesel. If you are running B20, which will be the most popular blend of biodiesel with regular diesel, you now have 127,259 BTUs per gallon which leads to

    - 17.9 MJ of work energy per dollar of fuel, or about 35% more energy than gasoline for the price. So for those paying 15 cents per KW-Hr or more for electricity, it would be cheaper to drive a B20 powered diesel car than to drive an electric car.

    Note: 1055 joules = 1 BTU.

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    Last edited: Apr 29, 2007
  4. Apr 30, 2007 #3
    I read/heard that some Prius owners have come up with a couple of mods to their cars. One is a conversion (actually an add on) to make it a plug in electric hybrid. The other mod I've heard of is to somehow change (reprogram?)the controller (computer) to get the car to run longer (at higher speeds) on it's electric motor before the gas engine kicks in. I don't know how well these mods have worked, wether they've gone beyond the "shade tree mechanic" stage to an aftermarket product, but apparently it's being done.
  5. Apr 30, 2007 #4

    Ivan Seeking

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    Running the batteries lower than intended will only decrease the life of the batteries. And this could end up costing far more than any potential savings.
    http://www.usbattery.com/pdfFiles/Avg Life Cycles.pdf

    It is best to leave the engineering to the engineers who designed these cars.
  6. May 11, 2007 #5

    Ivan Seeking

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    Regarding the engineering choices made for charging and discharging procedures, this happened to come up today.

  7. May 12, 2007 #6
    What about directly recharging the battery as described?
  8. May 13, 2007 #7

    Ivan Seeking

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    My only point was this all has to be carefully engineered to maximize the benefits. As is true with all cars, it is probably inadvisable to modify hybrids unless you really know what you're doing. There are many considerations that go into the design choices made, and in many cases these involve variables that may not be obvious. And when it comes to batteries, an engineering error can lead to a fire, the venting or accumulation of deadly gasses, or even an explosion.
  9. May 15, 2007 #8
    Only too true, as you pointed out Ivan at the least an engineering error in the battery/power pack design (also improper utilization) can cause inefficiencies which lead to a shortened lifespan of the batteries. Which then of course offsets any hoped for fuel savings.

    BTW thank for the charts. THey're always helpful.
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