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How much petrol does a car consume?

  1. Jan 25, 2016 #1
    So, I wanted to figure out how much energy is needed to drive my car .......

    A car weighing 1000 kg, moving at 50km/h travels 10 miles. If the energy content of petrol is about 30 MJ/L, how much petrol do I need?

    What's the answer?
  2. jcsd
  3. Jan 26, 2016 #2

    Simon Bridge

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    Welcome to PF;
    That's a pretty good question.

    There are a few ways to go about the calculation - but the best way is probably to weigh the fuel before and after the trip.
    You could take a shorter trip or use a smaller fixed amount of fuel.

    A theoretical calculation would find the work to move that distance (so you need the force) and adjust for the efficiency of the engine.
    You can estimate by looking up your car's fuel efficiency in miles per gallon and dividing the miles by the efficiency.

    Ultimately it depends on what you need the calculation for ...
  4. Jan 26, 2016 #3


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    my Toyota Yaris ( 1300cc) on open road steady cruising at 100 km/hr is rated at 5.8 litres / 100km
  5. Jan 26, 2016 #4


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    This is a sort of circular excercise which could be very informative and it is best suited to a practical (measurement) approach rather than a theoretical one - you never know the best place to start.
    I assume you already have an idea of the fuel economy of your vehicle.
    On a level road and at constant speed, the only energy needed is to overcome losses. The ideal answer, without losses, is Zero if you return to the same altitude you started from. (No overall work done, except against the brakes when you slow down at the end.)
    You could get an idea of the forces (hence the work) involved, you could try towing the car (in neutral) behind another vehicle, with a strong Force meter. That would give a good idea of the energy the engine is supplying. You would then need to relate this to the fuel needed by assuming an arbitrary 'efficiency' factor for the engine and transmission. Alternatively, you could find the efficiency by actually measuring the fuel used.
    Alternatively, you can fit drawing pins to the accelerator and brake pedals and drive with bare feet. That could give you extremely good fuel economy. (The added advantage would be that you could annoy a large number of fellow road users.)
  6. Jan 26, 2016 #5


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    Welcome to PF!

    The best answers you will get are either from a practical test (presumably the car has an odometer and you've filled the tank with gas a few times..) or from the rating provided by the manufacturer.

    And FYI, the fuel economy doesn't very directly depend on weight except for "city" driving. Aerodynamics is a bigger issue as is engine power capacity.
  7. Jan 26, 2016 #6


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    It's impossible to answer this question with the given information. It depends on things like aerodynamic drag, rolling resistance in the tires and transmission, and the efficiency of the engine itself. All of these vary from one model of car to another, and even from one individual car to another depending on its condition and state of repair.
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