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I know that one difference between these two types of small aircraft are that in a twin engine, one engine can continue working if the other suddenly fails.

Wouldn’t a single engine plane weigh less and thus can fly longer on a specific amount of fuel comparative to a twin engine plane?

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    $\begingroup$ Possible duplicate of What are the pros and cons of single-engine vs. twin-engine? $\endgroup$
    – Dan Pichelman
    Commented Sep 26, 2017 at 15:27
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    $\begingroup$ @DanPichelman - I was about to VTC for same reason, but other post does not address main point, I edited the question to reflect the main objective. $\endgroup$
    – user14897
    Commented Sep 26, 2017 at 15:31
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    $\begingroup$ Answers to the proposed duplicate don't mention range as pro or con. Not a duplicate. $\endgroup$
    – mins
    Commented Sep 26, 2017 at 16:50
  • $\begingroup$ If you ask it this way, it will be hard to answer because there is hardly an existing pair of instances with everything else being equal. However, you can refactor the question to like "if we design an airplane to be twin engine instead of single engine with all the same performance, would fuel economy be worse?" Or "is there any gain in fuel economy to minimize number of engines" that will be more abstract and easier to answer. $\endgroup$
    – user3528438
    Commented Sep 26, 2017 at 16:59

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enter image description here
(Cessna 185 Skywagon and Cessna 340)

See here for the major points: What are the pros and cons of single-engine vs. twin-engine? Regarding your specific question:

Wouldn’t a single engine plane weigh less and thus can fly longer on a specific amount of fuel comparative to a twin engine plane?

Both airplanes above are powered by the same 300 hp engine, so yes the twin will be heavier. But it can also lift more weight (i.e. more fuel), and cruise at a higher speed for the same power setting, or cruise at the same speed for lower power setting.

Either the higher speed or lower power, combined with the more fuel, gives the twin a longer range.

In fact, double. 2,603 km compared to 1,333 km.

If you wanted to compare the twin to a single with a 600 hp engine, then there is no weight advantage. Only the bigger engine is harder to maintain.

On a technical note, weight is not a big issue in cruise. The power to overcome the induced drag from the lift (which counters the weight), decreases with speed, i.e., it takes less power to create lift in cruise, than at slow speeds. (See graph below for the induced drag.)

Same fuel

Twins are designed to fly on one engine, which means half the power potential is enough, which is helped by the bigger span wing. So the same fuel load will give the same or more range for the twin. Unless, you want 600 hp vs 2x300 hp, in that case the single will win because of its smaller form.

Some numbers:

For the two planes above, here are the gallons-per-hour and cruise speeds:

             gph             cruise speed kts
Twin         26.0-34.0       202-229
Single       11.7-15.5       122-140

As you can see, the higher fuel consumption is offset nicely by the speed, and note that the twin will reach the cruise altitude in less time as well.

Sources: Cessna 185, Cessna 340

If we compare a twin-piston to a single-turbine then it won't be an apples-to-apples comparison, but the single-turbine plane will be faster, and will have longer range, but that's mainly due to the different engine types (combined with different cruise speeds). For the Cessna Caravan, the turboprop engine (minus the accessories) weighs as half as a single 300 hp piston engine, which can allow for 200-400 kg more fuel, but the turboprop engines are very, very expensive. So depending on the operation, a fuel saving here might be overdone by the initial cost.

enter image description here
(wikimedia.org)

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    $\begingroup$ Perhaps the best way to compare is to compare a single and twin with same total HP output. (i.e. 300 HP single vs a 300 HP twin (such as original Apache). $\endgroup$
    – Devil07
    Commented Sep 26, 2017 at 17:42
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    $\begingroup$ @Devil07 - I don't think this is what OP wanted since he stressed on weight in his example, but feel free to submit such an answer, it will be interesting :) $\endgroup$
    – user14897
    Commented Sep 26, 2017 at 17:45
  • $\begingroup$ I thought such an answer would be interesting also, I was hoping someone else would do it. :) On second read, I think you adequately address it. $\endgroup$
    – Devil07
    Commented Sep 26, 2017 at 18:40
  • $\begingroup$ The main form of drag for most powered aircraft is form (parasitic) drag, which increases with the square of air speed. For the same air frame, same level of attack in level flight, the air speed increases with the square root of the fractional increase in weight (increased weight / original weight). The combined effect is form drag increases in proportion to increase in weight. $\endgroup$
    – rcgldr
    Commented Sep 26, 2017 at 21:39

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