[madhav]

Guys,
I've had this question unanswered for a long time... hoping that some of you can throw some technical light on this one!

Consider the following...
A Tata Indica Turbo diesel vehicle 1405cc, weighing 1050 (kg) and producing 68 ps gives approx 15Kmpl of diesel.
A Tata Safari 2956cc, weighing 2650(kg) and producing 115ps gives approx 10Kmpl of diesel
Whereas a Volvo BR7 (bus available in India) with 6L engine, weighing almost 16200 kg and producing 260 ps returns almost 3.5-4Kmpl of diesel (Source: KSRTC volvo bus driver)

Now comparing the three the Volvo seems to be the most fuel efficient. Shouldn't the Safari which which weighs 1/6th the Volvo, has almost 1/2 the power and engine capacity be more fuel efficient?

[condor]

Madhav, the calculations dont work in such simple equations. Also, you have considered kerb weight for the Indica and GVW for the Safari and the Volvo.

[bhogalrajnish]

What about Sakoda it has a efficiancy better than all of these ....

[naveendhyani]

i m sure FE is only determined by the kerb weight & engine capacity. it will have lots of other parameters to it.

[Psycho]

The key part for fuel efficiency can be broadly laid out to the following:

1) Torque : Higher the torque lower the operating RPM lower the fuel consumption and higher gearing can be used
2) VE: Volumetric Efficiency of the engine to convert chemical energy into mechanical energy... most passenger car only have about 75 - 85% VE, however cars with turbo's have a higher VE

Therefore high revving engines with shorter strokes give great BHP whereas low revving engines with long strokes give better torque. Therefore Higher torque means better pulling power, hence heavier vehicles focus more on torque rather than just BHP which leads to better FE.

[sumitkalindi]

Fuel efficiency depends on many different parameters.

It is affected by shape of the vehicle, the weight of the vehicle, and as well as the matching of the engine to the intended vehicle.

A 800 cc engine is very efficient for maruti, but will drink like a fish if installed in a Indica. So, though the Indica gives 15 kmpl, actually the volvo engine may be more fuel efficient.

We usually cunfuse between an efficent engine and a efficient car.

It may happen that due to some technical reason, a really fuel efficient engine may result in a gas guzzling car.

There you go. Better leave all the compleceted calculations and enjoy the drive.

[ac 427]

FE depends on all the factors mentioned like the weight of the car, the shape of the vehicle, the TYRES which you put on the same, the DRIVING STYLE of the individual drivers but the most imp factor i think is the ENGINEERS. i.e how well they can balance the amout of FUEL being pumped in the engine compartment, how efficiently they can BURN the fuel and in WHICH WAY THEY CAN MAXIMISE the power from the same.
once that equation is set, the other parameters come in picture.

Skoda seems to have cracked it.

experts kindly correct me if m wrong.

[amu1983]

Larger engines will clearly have more FE (in terms of power/distance covered, etc). Looks like the principles of "economy of scale" apply to the engines too :-). Otherwise.. we would have had trucks made of 100s of small Luna engines!!

Just to compare... a Luna engine has about 60 cc, hardly 3-4 BHP, and gave 75-80 kmpl. Imagine a car with 10 Luna engines producing 30-40 BHP (eqv. to Maruti 800) and giving mere 7-8 kmpl. The larger the engine, more efficient it gets.

[appuchan]

Do you know that Airbus A380 is more fuel efficient than an Indica or even a Prius, that too while flying at 30k ft? Ofcourse, this is cost per person...

[abhilashvk]

I think psycho has given the best explanation. Its the technology that goes into the car that determine the torque and hence the efficiency. Like a 5.5 Litre Beamer or a Bentley churns out 550 bhp and torque close to 50 KGM where as 1.4 litre indica churns out only 72 bhp....

[AkMar]

@ sumitkalindi & psycho:

Really well explained. thanks for the posts