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Old 16th September 2013, 22:34   #1
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Default Engine size inversely proportional to Fuel efficiency?

I often hear from my friends and read on the net that bigger engines give less mileage. For example a 2 litre engine would give less mileage than an 1.5L engine. Is this absolutely true without considering any other parameters?

I would like to start a discussion on this topic to learn more about the actual technicals that affects or dictates FE.

Even if engine size is inversely proportional to mileage (FE), could we roughly arrive at the percentage?

Looking forward to some learning.
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Old 16th September 2013, 23:35   #2
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Default re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by kaizanfan View Post
I often hear from my friends and read on the net that bigger engines give less mileage. For example a 2 litre engine would give less mileage than an 1.5L engine. Is this absolutely true without considering any other parameters?

I would like to start a discussion on this topic to learn more about the actual technicals that affects or dictates FE.

Even if engine size is inversely proportional to mileage (FE), could we roughly arrive at the percentage?

Looking forward to some learning.

This would heavy Depend of a lot of factors and situations.

The driving conditions would also matter.

In b2b heavy traffic with AC on all the time a 1.2L 1.8L and 2.0L all give the same millage of 8-10KM and nothing more than that.
This is my personal experience.

But on the open road a smaller engine will give better F.E.

More importantly the way the engine is designed and tuning also matters.

A 1.5L Honda city will give you much better FE than a 1.2L Ford Figo.
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Old 16th September 2013, 23:36   #3
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Default re: Engine size inversely proportional to Fuel efficiency?

Typically, yes: larger engines take more air, hence need more fuel (by volume, not as a percentage) compared to smaller engines; if all other factors are the same (e.g. tuning, gearing, etc.)

So, if you have a large engine and a small engine both idling, then the larger one will consume more fuel. Similar behaviour is observed under light loads where the smaller engine is not being taxed and is in its "comfort zone".

There are specific cases where the larger engine might be more efficient - these are typically usage cases where the larger engine is in its "comfort zone", such as accelerating up from a standstill to a particular speed where the larger engine puts out more torque. In these cases, the smaller engine might have to be worked harder (e.g. a higher RPM) to achieve the same result, but at the cost of more fuel.

However, in most cases, with all else being same, the larger engine takes more fuel.

This is the reason why manufacturers try to make their engines behave like both a large engine, and a small engine at the same time - this is done by variable displacement technologies:
  • Some manufacturers use forced induction (e.g. turbocharging) such that a physically smaller engine can perform like a larger one by forcing in more air (hence more fuel). At lower speeds, the turbo doesn't force in more air, so the engine behaves like a smaller displacement engine.
  • Other manufacturers shut down a subset of the cylinders, so that only a portion of the engine capacity is fed with fuel (e.g. a V6 engine might have 3 cylinders switched off like this, simulating a smaller capacity engine)
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Old 16th September 2013, 23:48   #4
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Default re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by kaizanfan View Post
I often hear from my friends and read on the net that bigger engines give less mileage. For example a 2 litre engine would give less mileage than an 1.5L engine. Is this absolutely true without considering any other parameters?
This is not necessarily true - I'll present explanations of both sides of the argument.

A larger engine can give a higher mileage too, if you don't consider the other parameters. For instance, the Dodge Charger has a V8 engine but it can shut off half of the cylinders to increase mileage. Also, a hybrid can have a larger engine and shut off the engine and run on electric power for a part of the time. Okay, so perhaps this answer isn't what you're expecting because it involves shutting off all or part of the engine.

You can have a smaller engine running at a higher rpm to provide a similar output to a larger engine and in this case, it is possible that the larger engine would be more efficient (though not always true). Also, perhaps you could get a super charged or turbo charged smaller engine to match the lower fuel efficiency of a larger engine (I'm not too sure about this one).

The Wankel rotary engine was popular for producing a lot of power with a compact engine size. However, the rotary engines were discontinued because of their low fuel efficiency.

If you were to choose an indirect fuel injection engine that is smaller, it may still not be able to match the fuel efficiency of a larger direct fuel injection engine because of the heat losses that occur in indirect fuel injection engines.

Smaller engines can also be more fuel efficient. When idling, a smaller engine has to be filled with lesser fuel to keep it running when compared to the greater volume of a larger engine. The engine wouldn't be operating at peak capacity all the time and in cities you would be spending quite a bit of time idling, so a smaller engine would be more efficient.

A smaller engine also means lower weight. Having to carry around lesser weight is another way to increase fuel efficiency. In city driving, we have stop-and-go traffic, and the force needed to move a smaller mass is lower.
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Old 17th September 2013, 00:18   #5
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Default re: Engine size inversely proportional to Fuel efficiency?

What if told you my 2l jetta gave me close 30kmpl when i decided to drive cruising at 65kmph. This i tested for a 200km drive and i can say the fe was close to 28-30.

@ 85kmph it dropped to 25-28

@similar speed my swift would be belting around 20kmpl or max 22.

Last edited by VW2010 : 17th September 2013 at 00:19.
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Old 17th September 2013, 06:07   #6
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Default

Quote:
Originally Posted by kaizanfan View Post
I often hear from my friends and read on the net that bigger engines give less mileage. For example a 2 litre engine would give less mileage than an 1.5L engine. Is this absolutely true without considering any other parameters?

I would like to start a discussion on this topic to learn more about the actual technicals that affects or dictates FE.

Even if engine size is inversely proportional to mileage (FE), could we roughly arrive at the percentage?
If I'm not wrong then a higher capacity engine would revv less than a smaller capacity engine on highways, Right?!

The speed:RPM would be lower for a higher capacity engine. Or is it dependant on tuning?

Anurag.
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Old 17th September 2013, 08:57   #7
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Default re: Engine size inversely proportional to Fuel efficiency?

From what basic formula I had learnt a few years back, fuel consumption is theoretically based on how much power you need to extract from the engine. Also volume of the engine does matter, but not entirely given that todays engines can vary fuelling by a good extent. Hence, a small engine driven hard can give you lesser mileage than a bigger engine driven with a light foot. Thats because the smaller engine would have been working at its limits while the bigger engine may be at less than half its potential. Hence, with the help of modern technology, mileage greatly depends upon driving style. However, when we compare two engines of different size in the same place and same driving style, the smaller engine would have a FE that is a notch higher, as it is a smaller engine with lesser volume afterall.
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Old 17th September 2013, 10:14   #8
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Default re: Engine size inversely proportional to Fuel efficiency?

Mileage is not inversely proportional to engine SIZE, but it is inversely proportional to engine SPEED. And the engine speed mostly depends on the load it has to lug around.

Swift's 1.3 litre multijet engine will give a good mileage in a light weight car like Swift, Ritz, Vista or Punto. But if you plonk the same engine onto a Tata Safari, its mileage will suffer because its engine will have to work harder (higher engine speed) to get the beast moving.

If you somehow manage to fit the Safari's engine onto a Swift, it should theoretically give you more mileage per litre of diesel than the 1.3 litre engine, on a stretch of say 100 km - because you can get the job done with the engine spinning at much lower RPM.
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Old 17th September 2013, 11:46   #9
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Default re: Engine size inversely proportional to Fuel efficiency?

On a broader level, definitely yes. It is inversely proportional to the engine size. Eg: A 1 liter engine will definitely give better FE than a 6 liter engine.

However, for small differences in displacement, a lot of other factors come into account, and the bigger needn't necessary be the one that guzzles more fuel.
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Old 17th September 2013, 12:39   #10
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Default re: Engine size inversely proportional to Fuel efficiency?

As someone said earlier, at idling, yes the size is inversely proportional to mileage. Also, during operation, you have engine load and rpm. These two factors decide how much fuel is being drawn to get the work done. Also, the rpms at which a 6L engine will achieve a set speed will be different from that of a smaller displacement engine and therefore, fuel draw.
In short, it is a generic statement and holds good only under certain scenarios.
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Old 17th September 2013, 12:51   #11
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Default re: Engine size inversely proportional to Fuel efficiency?

In most cases the larger would be less efficient than a smaller engine but as mostly everyone has said here - its a generic statement. To quote a real word example , maybe not the best but nevertheless - my T-Jet which is a 1.4 Turbo engine is less fuel efficient than my Fabia which is a 1.6.

The Fabia dishes out mileage relatively easily in the 11.5-13 KMPLS range in the city in mostly all conditions , in the same conditions the T-Jet would give me closer to 8-10.
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Old 17th September 2013, 12:57   #12
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Default re: Engine size inversely proportional to Fuel efficiency?

Well, famous quote is "There is no replacement for displacement"
Smaller engines develop max power and torque at higher rpm. And bigger engines develop it at very low rpm. Smaller engines if operated for maximum power are less fuel efficient. So it all depends on requirement, if you need very less power and idling most of the times then smaller are good. But you require power consistently then bigger engines are better. That is why Generators has bigger engines because power requirement is at 1500 rpm and are more efficient.

If we have continuous power requirement then Bigger engines are more efficient. If we have two cars weighing same with engines. One is with small displacement engine and other with bigger engine. Load is same, and we have to use max power of engine. Bigger engines are more efficient any day.

So solutions are designed based on application.

Last edited by sushantr5 : 17th September 2013 at 13:03.
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Old 17th September 2013, 14:55   #13
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Default re: Engine size inversely proportional to Fuel efficiency?

Taking the Ford Ecosport 1L engine as an example, would it be running at higher RPMs compared to an i10 with 1.2L engine?

I would also like to bring in power and torque parameters into this discussion. Are power and torque proportional to the size of the engine?
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Old 18th September 2013, 02:02   #14
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Default Re: Engine size inversely proportional to Fuel efficiency?

Power to Weight Ratio is also one important factor for fuel efficiency. The point (RPM) at which the Power and Torque figures are produced help in extracting fuel efficiency.

If the Power and Torque comes live at higher RPM then in a daily usage point that RPM will not be touched so the usability will be low thus straining the engine. Lower the RPM at which torque is produced better is the drive ability and lesser gear changes with better fuel efficiency.

The engine must be lugged at the same time else efficiency drops.

Anurag.
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Old 18th September 2013, 10:56   #15
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by a4anurag View Post

The engine must be lugged at the same time else efficiency drops.
I think you meant something else, but ended up writing this . AFAIK, engine lugging is quite detrimental to the engine's health; you will not just end increasing your car's fuel consumption but could also damage the gears due to the constant slipping.
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