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Automobile fuel efficiency

Discussion in 'off topic' started by Wilson, May 15, 2019.

  1. Darth Vader

    Darth Vader From the Dark Side

    There is no simple answer to the OP Q. However from Chemical thermodynamics (and vastly over simplifying) we have E=G+H where E is the total energy available from the chemical reaction (here it is burning the hydrocarbon petrol), G is the Gibbs free energy that is available to do useful work like propelling the car and H is enthalpy or wasted heat.

    So you see that we can get useful energy and energy wasted as heat. Now it comes to pass that the faster you take the energy from a chemical reaction you'll get more H and less G. Alternatively the slower you take the energy you get more G and less H. So if you took the energy infinitely slowly you'd get it all as G i.e. useful work likewise if you took the energy infinitely fast you'd get it all H i.e. wasted heat.

    Therefore you can only rate the efficiency of a car engine with associated transmission losses under specific conditions. Having said that and assuming that the car is driven at maximum efficiency it will burn more fuel as the speed increases if for no other reason than drag mainly through wind resistance will increase and extra power will be needed to overcome this.

    Whether this will be noticeable or not at 20 and 30 mph is another matter.


    Wilson likes this.
  2. sean99

    sean99 pfm Member

    My brother in law commutes 20 miles each way every day. Takes an hour each way. The difference between a prius (50mpg) and a V8 (<20mpg) is > 5 gallons a week, or 250-300 gallons a year - even at US prices ($3 per US gallon) that's a chunk of money. Agreed on the speed limits though.
  3. Tony Lockhart

    Tony Lockhart pfm Member

    Looks like the ideal situation for an EV, if it fits into your BiL's life in other ways. Sweet chocolate monkeys... an hour to do twenty miles, twice per day. In fact, I'd seriously consider an electric bike!
    Snufkin and sean99 like this.
  4. Minio

    Minio pfm Member

    It depends a lot on what mode of driving you are involved with.

    A city delivery driver will these days be doing short and speed limited mileages that don't add up to much.

    Whereas an engineer with a nationwide clientele will do hundreds.

    I've been in both scenarios. In the first, a fill up is only required once a month , if that.
  5. sean99

    sean99 pfm Member

    It's mostly on the freeway !!!! (Hence no e-bike). I agree an EV would be a good option. Used Leafs are cheap here - $12k for a 3 yr old leaf.

    It would drive me mad, but I suppose one has to eat (and have health insurance).
    Tony Lockhart likes this.
  6. Mike Reed

    Mike Reed pfm Member

    Think the U.S. gallon is smaller than the U.K.'s but not sure by how much. Our gallon is around 4.545 litres with diesel at £1.33 upwards and petrol about 5p lower than that (regions vary).
  7. Tony Lockhart

    Tony Lockhart pfm Member

    US gallon is a puny 3.78 litres.
  8. Seeker_UK

    Seeker_UK Waiting for the streetcar..

    So about 62p a litre, I reckon.
  9. stevec67

    stevec67 pfm Member

    Tonys figures look right to me. The way to remember it is that a US pint is 16 oz against 20 oz in the UK. The rest is the same.
    Mike Reed likes this.
  10. MichaelC

    MichaelC pfm Member

    I would rather drive a V8 any day than a Prius.
    Tony Lockhart likes this.
  11. Rockmeister

    Rockmeister pfm Member

    well quite, but not everyone has 1500 quid spare p,a.for a smile.
  12. stevec67

    stevec67 pfm Member

    The last thing I would want to crawl through traffic every day is a V8. What a waste of a great car. I wouldn't wear a designer silk shirt and cashmere suit to cook in either, or italian shoes to cut the grass.
    Minio likes this.
  13. Rana

    Rana pfm Member

    I attempted to do this in my post (# 3). Bottom line if it's a modern auto, there won't be a big difference between 20 and 30mph. For a manual, it's about getting into that target engine speed range, but not causing transient fuelling enrichment is also a key factor.

    Beyond the E=G+H model, alongside the useful energy released by the fuel combustion, we need to consider volumetric efficiency, pumping losses and total powertrain frictional losses (which can be significant if the valvetrain design used is operating in the inefficient part of the Stribek curve at lower engine speeds). In addition, exhaust heat can now be recovered in a few different ways. All this helps define specific fuel consumption map for the engine that further defines the optimum gearing and gear change points for the vehicle road loads.
    Snufkin, martin clark and Wilson like this.
  14. -alan-

    -alan- pfm Member

    I'd gladly pay the 20 bucks a week just to avoid the Pious :)
    MichaelC and Tony Lockhart like this.

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