Why a smaller engine doesn't mean better fuel economy every time

Quite a common misconception amongst everybody

Max K posted in All Rants

2w ago

10.7K

There must have been times when you thought you have chosen the ideal car for your future (let's say a Range Rover Velar with the V6 the sake of this story), then you show it to some of your friends and family to see what they think about it, only for in return they say on the lines of: "Gosh, an 6 cylinder, good luck spending your time at Shell" or "pick a smaller engine. It won't hurt you instead benefit you". You get the idea.

Taking the RR Velar is a good idea. With a very diverse engine choice from a 2.0L 4 cylinder to a supercharged V8, it has it all.

Generally when you think about big engines, probably one of the first things you think about it is that its going to sip fuel like a thirsty dog, and a small engine would be more gentle on the liquid as it has got less cylinders to sweep through less engine volume.

Or is it?

With a bigger engine, a lesser proportion of its power output is required for it to do what its required to do then, such as accelerating to motorway (highway/freeway) speeds from standing still at a stop light, meaning the driver only needs little throttle input to do that in a conventional time period, allowing less fuel to be burnt as RPMs are kept low.

With a smaller engine in the same car however, using the same example used before, to get the car up to speed of the road from a standstill in the same time as how the bigger engine one does it in, a higher proportion of its power output is required as power to weight ratio is worse, meaning more throttle input is required from the driver to build up RPM and speed, resulting in more combustion of fuel.

So theoretically, although the bigger engine is the more fuel inefficient one, with the needs of driving these days that constantly require that little gain of speed in a short span of time, whether its getting the car to move from a standstill, overtaking someone or accelerating to a higher speed limit, more strain is put on the smaller engine to get the power needed for the car to do the same task as the one with a bigger engine which is naturally has a better power:weight.

However, this case does not apply to cars with a well-balanced P:W ratio, such as small cars with small engines like a Subaru BRZ where the power is suited to the weight of the car.

So enthusiasts, y'all can keep your big engines without having the thought at the back of your head consistently telling you to switch to a smaller engined car to save costs. Sometimes.

Thanks for reading.

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Comments (11)

  • I agree. If I was to put a tiny 3 cylinder petrol in a Range rover the engine constantly working hard to keep it moving, however with a V6 or V8 the engine would not have to work as hard to propel the car.

      19 days ago
    • I mean a 3 cylinder is possible within the next 3-4 years if a 4 cylinder without a hybrid is already available now, just don't see the point in it except for cheaper tax

        19 days ago
  • This is like the people who buy the ecoboost mustang but then drive extremely hard. If they had the real mustang they wouldn't need to drive as hard and would save fuel. Also they wouldn't be the person that bought a ecoboost mustang.

      18 days ago
  • If only it was a simple as that.

    An ICE has a power band, and somewhere in the rev range it will produce the best power for the amount of energy put in. As that power point will vary with throttle position and engine load, an engine needs to be suited to the type of normal driving the person does.

    Then comes the mass of the vehicle. To accelerate any given mass at a constant rate will take the same amount of energy. This is just simple physics and is independent of the number of cylinders the power source has, see the earlier point.

    Then there is internal resistance within the engine. Generally the less cylinders an engine has, the less friction there is. So less parasitic losses.

      18 days ago
  • Just not true, sorry. Any ICE that isn’t running with an open throttle at the peak of its power band is inefficient. A 200bhp engine at 25% output is less efficient than a 50bhp engine at 100%.

      12 days ago
  • There has always been a optimal power/ weight ratio, I cannot understand why manufacturers don’t work to this and produce the sports std as a special

      14 days ago
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