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AskPablo: Well to Wheel Efficiency Part III

| Monday April 30th, 2007 | 1 Comment

In Part I we learned about the energy required to overcome rolling resistance and aerodynamic drag. In Part II we learned about the energy consumed in acceleration. Now it’s time to bring it all together. We know how much energy it takes to get a vehicle up to a certain speed and to keep it there. We also know that the fuel we put into the tank contains more energy than we get back out. My car’s efficiency came out to be 19.9% (see Part I), but where does the other 80.1% go? And is there anything else to consider?


On the EPA’s Fuel Economy site there is a lot of interesting information about vehicle efficiency. On average a vehicle’s internal combustion (IC) engine is 37.6% efficient. Most of the energy is lost as waste heat, removed by the radiator or expelled through the exhaust system. Diesel engines are around 30% more efficient. A further 17.2% of fuel energy is spent on standby or idling, leaving 18.2% of the input fuel energy.
vehicleenergy.gif
Of the remaining 18.2%, 2.2% is lost to accessories such as AC, lights, and radio. Finally, 5.6% is lost in the vehicle’s transmission and other drive train components. This means that only 12.6% of the input fuel energy is turned into useful work to propel the vehicle. This number is lower than my 19.9% because my vehicle is several mpg over the national average and because my efficiency calculation did not take into account any idling or increased fuel consumption due to acceleration.
According to the EPA figures, 2.6% of the remaining 12.6% is lost in overcoming drag and 4.2% is lost to overcome rolling resistance. This leaves 5.8% for accelerating up to speed, all of which is lost to braking (except in hybrid vehicles which recapture about 10% of this energy). Now, if you recall from Part I, around 15% of the embodied energy of gasoline is incurred in extracting, transporting, refining, and transporting again. So, of the energy that comes out of the ground in the form of crude oil only about 10.7% actually move your vehicle from Point A to Point B.
All this talk of how inefficient our personal transportation is is pretty discouraging. And knowing that the average vehicle in the US gets just above 20 mpg does not help either. There is another way to look at vehicle efficiency. In the book Natural Capitalism Lovins et al suggest that most of the energy used by vehicles is used to move the vehicle itself, not the occupants. If you have read all three sections of this column you now have the technical knowledge to understand the forces and energy involved. If you think about it though, the goal is not to transport a 3,000 lb steel box everywhere you go, the goal is mobility. It is getting us, our families, and our groceries from one place to another. By looking at the energy used per person-km (number of passengers x distance traveled) we get a much better understanding of the inefficiency of a single-occupant Hummer vs. a fully utilized vanpool.
This weekend’s collapse of the 80/580/880 interchange in the San Francisco Bay Area only serves to remind us of how dependent we are on personal automobiles for transportation and how vulnerable our infrastructure is. According to one CNN.com report around 280,000 commuters, including myself, will be affected. I am grateful that I have marginal access to public transportation and I look forward to the day when we live in a society that is interconnected by an efficient and equitable system that provides personal mobility.
In the meantime, what can we do?

  • Keep the pedal off the metal. Accelerating less will get you there with less fuel.

  • Slow it down. Every twofold increase in speed quadruples the drag force. Driving at the posted speed limit will help you save fuel and money.
  • Lighten the load. Weight adds to the force required to accelerate, climb hills, and overcome rolling resistance. Avoid unnecessarily carrying heavy loads and leave your fat uncle Larry at home (sorry Larry, you really should walk…). When shopping for a new vehicle, look for something a little bit lighter (leave the Hummers for Baghdad).
  • Carpool or take public transit!
  • Keep your tires properly inflated. Not only is this safer but it will reduce rolling resistance and tire wear.
  • If you have a roof rack use a fairing to deflect oncoming air (If you have an extra Thule fairing I would love to accept your donation). Also, don’t be “that guy” driving down the freeway with the levitating mattress about to tear off of your roof. And finally…
  • If you are going to crash your tanker truck and 8,600 gallons of gasoline, please don’t do it on my commuting route and have the decency to offset the carbon emissions from all the fuel that was burned (8,600 gallons of gasoline = 23,993 kg. 24,000 kg of fuel x 3.087 kg CO2/kg fuel = 74 metric tons). That will be $555 here please.

Pablo Päster, MBA
Sustainability Engineer
www.AskPablo.org


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  • Tormod

    Hello Pablo
    I see I am arriving a bit late to the party here, but again, thank you for an excellent article.
    One thing I found a bit surprising is this:
    On average a vehicle’s internal combustion (IC) engine is 37.6% efficient
    I thought this was a bit optimistic. This figure is probably what you can get under the most favourable circumstances, with optimal load being drawn from the engine. Can it really be applied as an average? I believe it will be different if say 100 hp is used vs. if only 20 hp is needed like in slow traffic. I see you have accounted for a portion of standby/idle, but still..
    Calculating these figures are OK, but I would like to see real measured values (torque meter on the driveshaft etc.) for a “standard” commuter trip, including cold start, idling, slow traffic and everything. I would expect to see even more discouraging figures then.
    OK, now we have seen that the Common Car is a truly wasteful and inefficient machine.
    What would these figures look like on an EV or HEV…or PHEV? Or a scooter?
    I guess a scooter would give some interesting numbers, because I don’t think the well-to-wheel is all that impressive, but it wins in the end because the payload is a much larger percentage of the gross weight.
    Since in the end when you get back home, all the energy has been lost in some way, I guess a more relevant figure would be to use liter per passenger-km as we say here in Europe. Then we could also compare trains, planes and boats on the same scale. I’d love to see the figures for a cruise-ship… Would any of these be interesting as a topic for your next article?
    Again, thank you for this and many other interesting articles.
    Regards Tormod