[ahmad.007]

For me it simply comes down to power output and the inherent efficiency of engine to produce that power. Engines from 20 years ago were not very fuel efficient due to the old tech.
I say if a car has 100 bhp power and the driver often visits the redline, it will definitely be lesser efficient than an engine (of similar grade) producing 80 bhp and visiting redline often. And secondly, it comes down to the weight of the vehicle.
When you visit high rpms you get access to all the power, and that power is obviously produced by burning fuel. You may say more the power, more the speed/acceleration and so this should compensate the extra fuel burned, but it is not the case. Correct me if I am wrong but laws of physics somewhere state that a 200 bhp car will not have twice the acceleration (or mayve topspeed) of a 100 bhp car, at max it may have around 1.5 times of it.

On the other hand if you drive within rpm limits and only access the odd 30-40 bhp produced by the engine at early revs, there is no reason you should not get good fuel efficiency.

I would like to summarise it with the line, FE is inversely proportional to the power produced by the engine. (providing similar weight of car).

Any corrections are most welcome, I am no expert.

[sgiitk]

@slamnos; You left out compression. Looking at the same generation of engines in the same car, by and large the bigger engines do tend to be more thirsty, but not necessarily. As has been correctly pointed out, a smaller engine under stress will be less efficient that its larger brother lazing along. As for the ARAI figure, take it as a guide, and it will show a >20% optimistic figure, in general.

[Aroy]

As Spike said, it all boils down to specific fuel consumption for a given power, which in turn boils down to the torque produced at a given RPM.

As torque is proportionate to the engine displacement, a bigger engine will produce more torque at a given RPM compared to a smaller displacement engine (for the same technology). In B2B traffic you are normally using lower torque, and the larger engines produce it at lower RPM, there fore they normally use less fuel. Both my Alto K10 and the Honda City return the same FE - 10ish in B2B traffic. After 100 it is a different story.

[delix]

My understanding with regards to turbo kicking in a vehicle at different points of time ( or rather at different rpm) may also affect the FE. In vehicles which generally run more in stop and go traffic, it should be a boon for those vehicles where turbo kicks in at higher rpms because as soon as turbo kicks in more air is fed in the engine to get more power. I would assume that the vehicle is not revved high while being driven in this kind of traffic.
Any thoughts on this?

[slamnos]

Originally Posted by sgiitk

@slamnos; You left out compression.

Yes thank you for adding the factor.

[LOL_FWD]

Originally Posted by audioholic

If the Ecoboost is utilised to its full potential or even close to full potential.

I just found this online! Lords of cubic capacity have spoken

http://www.youtube.com/watch?v=tmZMwk14r4c#t=41

[k_nitin_r]

Originally Posted by ahmad.007

Correct me if I am wrong but laws of physics somewhere state that a 200 bhp car will not have twice the acceleration (or mayve topspeed) of a 100 bhp car, at max it may have around 1.5 times of it.

Increased friction along moving parts would add to the inefficiency. Wind resistance plays a role in preventing twice the power from creating twice the acceleration and top speed too. If you are inching forward in stop-and-go traffic, there's virtually no wind resistance at all unless it is a particularly windy day. However, if you are traveling on the highway at 120 km/hr, the same vehicle would be subjected to significant wind resistance.

Oh, and that's another factor that hasn't been mentioned on the thread yet - aerodynamics. Compare the Maruti Omni or Tata Sumo with a Maruti Swift and the difference in wind resistance would seem apparent.

[ahmad.007]

Originally Posted by k_nitin_r

Increased friction along moving parts would add to the inefficiency. Wind resistance plays a role in preventing twice the power from creating twice the acceleration and top speed too. If you are inching forward in stop-and-go traffic, there's virtually no wind resistance at all unless it is a particularly windy day. However, if you are traveling on the highway at 120 km/hr, the same vehicle would be subjected to significant wind resistance.

Oh, and that's another factor that hasn't been mentioned on the thread yet - aerodynamics. Compare the Maruti Omni or Tata Sumo with a Maruti Swift and the difference in wind resistance would seem apparent.

Apart from the wind resistance, I was pointing towards the Kinetic Energy equation*
KE=1/2 mv^2
This states that for a two fold increase in speed, the object will require four fold increase in energy.
So, this equation keeps on working at all speeds. And this also governs acceleration I guess somewhat.

[katchkamalesh]

Originally Posted by delix

My understanding with regards to turbo kicking in a vehicle at different points of time ( or rather at different rpm) may also affect the FE. In vehicles which generally run more in stop and go traffic, it should be a boon for those vehicles where turbo kicks in at higher rpms because as soon as turbo kicks in more air is fed in the engine to get more power. I would assume that the vehicle is not revved high while being driven in this kind of traffic.
Any thoughts on this?

As per my understanding, the use of turbo is to force more air i.e increase the pressure of the air, thereby leading to a finer atomization of fuel. finer the atomization more the power(more efficient the engine). Turbos are set to kick in at different RPM based on the design/requirement. Then there are variable geometry turbos, in these, the are multiple paths for the pressurised air to flow. Various path translates to various pressures which in turn means change in atomization.

Aerodynamics plays a key role only after a certain speed, but it is critical for highway runs, less aerodynamic the car the is (read: Omni) more power the car need to do say 200 kmph where as a car like swift/Alto would require considerably lower power when compared to Omni.

The size of the engine though important, it does not necessarily relate to the fuel efficiency, the usage of the vehicle is dictates the FE. For eg, one does not need a car with V8 engine for city runs and 800cc Nano for highway runs.

If you want to cruise at 140 kmph with easy get a car with a bigger engine and if you want to drive at 50 Kmph get a car with a smaller engine.

P.S I am not saying that Nano is not suitable for highway runs, I am just saying that it is not good enough to cruise on the highway say at 100-120 Kmph.

Link to Top gear fuel economy run:

http://www.youtube.com/watch?v=JmxUsGiGp3w

Check from 04:19 onwards

[ahmad.007]

Originally Posted by katchkamalesh

Check from 04:19 onwards

Well said, but one correction I would like to make, and that is, when a turbo spools up, it forces more air inside the chamber, but with that air, it also adds more fuel to maintain the correct air:fuel mixture.

[bsdbsd]

Here are some real life facts, notwithstanding all technicalities:

1. Colleagues 1.2L (or 1.1?) i10 AT -Mileage: 7-8 Kms.
2. My Santrao 1.1L AT - Mileage: 9-10 Kms. 15 in long drives.
3. My Honda 1.5 AT - Mileage: 9 Kms. Long drives not tested.
4. Colleagues 1.6 Verna AT - Mileage: 8.5 Kms.
5. Colleagues 2.4 (or 2.6 or ??) New Honda Accord AT - Mileage: 8.5 Kms

I see that a wide range of engine sizes from 1.1 to 2.6 liters ALL ARE returning approximately between 8-9 kms per liter! Thus, I decuce that engine size does not drastically alter mileage in a real life scenario (at least under Bangalore driving conditions).

[katchkamalesh]

Originally Posted by ahmad.007

Well said, but one correction I would like to make, and that is, when a turbo spools up, it forces more air inside the chamber, but with that air, it also adds more fuel to maintain the correct air:fuel mixture.

Agreed Ahmad.

More Air and more fuel is fed into the system to increase the power but there is also different type of turbocharger which is used to increase the fuel efficiency (rarely used AFAIK, correct me if wrong).

[ahmad.007]

Originally Posted by katchkamalesh

Agreed Ahmad.

More Air and more fuel is fed into the system to increase the power but there is also different type of turbocharger which is used to increase the fuel efficiency (rarely used AFAIK, correct me if wrong).

Im sorry, I am not aware of any other type of turbocharger that may increase fuel efficiency in particular. But, generally turbochargers are more fuel efficient than a naturally aspirated engine because burning fuel with high pressurized air results in better thermal power extraction and better torque production I guess (I am not too sure though), and because the same amount of power/torque produced by a naturally aspirated engine would be done at normal pressures.

[rangakishen]

Hello,
First of all lets clear things up a little.

Efficiency - is to do with how much output energy the engine is able to deliver when compared to the input energy.
This is clearly not the focus of this discussion. But in any case, efficiency has no direct correlation to the size of the engine. In fact IIRC, the world's largest diesel (although 2 stroke) is also the world's most efficient Internal combustion engine.

Now for the second part or the focus of this discussion.
Fuel Economy (km/lt) - This is given basically how many kilometers the vehicle can run on a litre of fuel (on an average).

Now it been stated above that larger engine would be less economical than a smaller one at idling speeds (Neutral Gear). At anything greater than this, the economy will heavily depend on gearing of the vehicle.
Smaller engines burn less fuel for a given RPM but also do less work because have shorter gearing.
Larger engines burn more fuel for the same RPM but do more work because of longer gearing.
If this ratio and the loads are assumed to be constant, there would be no difference in the economy. Ultimately the efficiency comes down to the target of the manufacturer. If the car has been designed to cruise at triple digit speeds all day long, it'll give good mileage on the long run thanks to it long gearing (lower running RPM for given speed). If the car has been designed for city running, It'll suffer a bit on the highways as it has to work pretty at higher RPM to maintain the same speed.

Simplest example is the Punto 90 hp. IIRC it runs at a relatively high RPM for even 100kph. Don't remember exact numbers. This would mean that even for essentially the same engine (Fiat MJD), a car with longer gearing would me more economical than Punto.
Same principles apply when different sizes are compared.

My 2 cents!

[PGA]

Engine size inversely proportional to the fuel efficiency! Ceteris paribus, YES, but one can draw very wrong conclusions if one sticks to this simple answer and it cannot be said that it will stand good for majority of conditions, like:-

A larger diesel engine is likely to be more efficient than comparatively smaller petrol engine.

Things in the realm of manufacturers like design, manufacturing precision, engine control system and drive train will dictate its efficiency.

Drag on the engine which constitutes aerodynamic drag, load of the vehicle and its occupants, type of road or lack of it on which the vehicle is running, will dictate its efficiency, it is normally indirectly proportional to the above said factors.

Things in our control that is condition of the engine which includes its tuning state, cooling state, internal friction state and external friction state (our driving state, I mean drinking and driving will reduce fuel efficiency.)

But yes size does matter like for everything else in this world.