What makes a car fuel efficient?

Lspiffycashew on 19/06/2015 - 09:26

I've always been confused as to what exactly makes a car fuel efficient. I'm driving a '89 corolla now which gets me about 10L/100km, which in my opinion isn't fantastic for a small car with its underpowered engine (granted, it's old and without the technology of newer cars)

But my confusion is that don't cars with smaller engines like mine need to work harder to keep the car at pace with other cars on the road? I need to rev mine pretty hard to keep up sometimes, and this can't be good for efficiency, so what factors make a car efficient?

I mean, how does driving a Hyundai getz like a hoon yield better efficiency than driving a commodore like one? Assuming they're both keeping up pace with each other

Automotive

Comments

  • retiredfeline on 19/06/2015 - 09:29
    +3

    Engine capacity, weight, air resistance, and technology are some factors. On the driver's side is the driving technique.

    Remember that smaller cars are also lighter and carry less (because they don't have the room).

    Old cars will also be poorer because of wear and tear, and older engine technology.

    • airzone on 19/06/2015 - 10:37

      As well as auto slushboxes vs something like a cvt, they have efi and the computer calculates fuel input based multiple factors vs a probably poorly tuned carbie, etc.

  • ESEMCE on 19/06/2015 - 10:08
    +2

    Assuming they're both keeping up pace with each other

    That's a massive assumption! I'd anticipate that (at least driving in a straight line that) driving a Commodore just above normal would keep pace with a Hyundai Getz being thrashed.
    More cylinders = more fuel required to keep the cylinders pumping.
    But above a certain speed the wind resistance puts more load on the engine so you need more power.. More cylinders = more power strokes = more power at a certain rev = less revs required to get to/maintain a speed.

    So efficiency depends on driving style and circumstances..
    A Commodore on the highway will rapidly close the efficiency gap to be close to the efficiency of a Getz on the Highway.

  • Evil-Elmo on 19/06/2015 - 10:11
    +1

    You're not wrong and Top Gear once did a (clearly biased) demonstration to prove this. They took a Toyota Prius and drove it around the test track as fast as it could go. They followed it with a BMW M3 keeping up but not pushing any harder. The M3 used less fuel. This is an unsurprising result and pretty much validates what you're talking about. However, it is not a fair comparison of realistic driving (where the M3 would lose - although I'd still rather have the M3!).

    How a car is driven certainly has a significant effect. But your puny little engine can only use so much fuel no matter how hard you push it. The larger engines can use significantly more, which uses more fuel overall (on average). Also, they require more fuel to tick over. In addition to this, the larger engines are heavier, which requires more power (fuel) to accelerate to the same speed etc. Usually they have bigger tyres to help transfer the extra power, but pushing those bigger tyres uses more fuel too.

    A lot of fuel consumption is to do with how a car is driven. But also, smaller, lighter cars with small displacement engines are inherently more fuel efficient. Also, now days a lot of effort is put into fuel efficiency, and that's why you see cars with tiny little engines being coupled with turbos and superchargers (think VWs, Alfas) to get more power out of a smaller displacement which is, at the end of the day, more efficient.

    • Diji1 on 19/06/2015 - 10:47
      +1

      I think the problem is we're aren't defining what efficiency means ;) … seems to me the cars are better than each other in terms of efficiency depending on whether they're driven on a racetrack, a highway or stop starting round the city.

  • RockyRaccoon on 19/06/2015 - 10:17
    +1

    Also GEARing Many new cars have Variable transmissions or 6+ gears, so engine doent have to work as hard. Just like pedalling a bike with no gears vs 21 etc

    But really OP typing in your post headline into a search engine gives far better information. ⁉️

    https://www.google.com.au/search?client=safari&rls=en&q=what…

  • highon2str on 19/06/2015 - 11:09

    Your right foot.

  • Davo1111 on 19/06/2015 - 14:02

    HUGE number of factors including

    • aerodynamics
    • Vehicle weight and tyre size
    • Engine size/displacement
    • fuel type
    • Tolerances in engine manufacturing
    • gearing

    Also, vehicles aren't necessarily fuel efficient at the speed you want. I've found our fuel efficiency sweet spot is around 80km/hr. I think it's written in the vehicle book.

  • billy_bob on 19/06/2015 - 14:35

    Driving style and driving conditions are HUGE factors on fuel economy. If you do a lot of stop/start driving and short trips where the engine hasn't had time to warm up, your fuel economy will be worse than average. Car engines can take 10-20 minutes to reach optimum temperature, so if your trip is only 15 minutes, the engine is running cold and will use more fuel. On a long trip, where the engine is warmed up and your are doing highway driving at a constant speed, you will get your best fuel economy. As an example, our family car is a Falcon 6 cylinder, and it will average around 12.5 l/100km in city driving, but on a long trip (highway) it will do about 7.5 l/100km.

  • Euphemistic on 20/06/2015 - 09:56

    I had a 91 Corolla (manual) and it didn't use above about 8l/100 and less on the highway. I didn't nurse it around town and it kept up well with traffic without fogging it to death. I wonder if there was a model change, or something is wrong with yours.

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