Fuel Efficiency vs Fuel Consumption of a Car

Preamble
Last time we discussed the pro’s and con’s of switching over to the new generation/detergent pre-mixed petrols in the Mpfi vehicles, or for that matter, the equivalent/’turbo-diesel’ for diesel vehicles. Let’s have a look now as to what really is meant by ‘Fuel-efficiency’ and ‘Fuel-consumption’ of a Vehicle – suffice to say that they are NOT the same…

The real life ‘Fuel Consumption’ of any vehicle is one of the most controversial issues and at the outset, I’d like to drive home a point that no where in the world can these figs be guaranteed. At best these can be taken as ‘indicative’ only.

For starters, ‘traffic-movement-wise’, can any one define what’s a ‘City’? If you have been to some of the Metros like Delhi, Mumbai and Kolkatta, you’ll know. Over and above these, traffic jams can hit you anywhere out of the blue. Therefore, same Car with the same driver on ‘up’ route will show one level of FC and another on the way down ‘down’!!

Btw, all Cars today are more or less equally ‘Fuel-Efficient’. Their actual ‘Fuel-Consumption’ depends on the ‘Bhp’ you demand out of them - by way of acceleration, cruising speed, AC etc - which in turn depends on their ‘Kerb-Weight’ and the ‘CC’ of the Engines they have.

Waiting at Traffic Lights:

It may come as a surprise to know that it’s wasteful not only of fuel but also from wear and tear considerations on one’s Battery AND Starter Motor - to switch off the engine if the wait is going to be less than 2-mins - which generally it is.

Pick-up or ‘Mileage’!

According to reliable road test data, these for the two popular ones are as below:

Santro: 0-60/100 kph = 5.55/14.66 secs @ Highway kpl of 16.2. (3-Valves/Cyl; 1086cc)
W-R: 0-60/100 kph = 6.31/16.60 secs @ Highway kpl of 16.2. (4-Valves/Cyl; 1061cc)

From these fig’s, it can be seen that the first engine having lower # of valves AND higher ‘cc’ accelerates faster than the other one having higher # of valves/lower ‘cc’, to deliver the same/highway fuel ‘consumption’. The point I’m making here is that in these days of ‘MPFi’ tech., it’s a child’s play to get acceleration and FC figs as one likes - within overall limits of course.

Speed vs ‘mileage’:

Let’s take the example of an MPFi Zen/since phased out. It could easily do 21 kpl @ 60-70 kph and 18+kpl at speeds of 100+ kph on a highway.

This is quite normal, for it’s a well known fact that wind resistance goes up with speed in a complex manner - starting with square of it and going upto the power of 4 - depending on its magnitude.

In other words, say at a cruising speed of 50 kph, 40% of the engine power goes towards over coming the vehicle’s ‘rolling resistance’ and the remaining 60% to overcome its ‘wind resistance’. Assume these as 5 and 7.5 = 12.5 Bhp.

Further, assuming that the rolling resistance remains more or less constant beyond 50 kph, at 100 kph the engine will have to produce 5 + (7.5 x 4) = 35 Bhp. So as far as the engine is concerned, for 2x the speed, it has to produce 3x the power.

Therefore, in terms of ‘Specific Fuel Consumption’ which is defined as ‘grams per Bhp per hour’ - that’s how the engine sees it - its FC stands to go up by 3x but then, at 2x the speed, it also has to run for half the time to cover the same distance - or effectively the FC stands to go up by 1.5x to cover the same distance. In other words, if it was doing say 25 kpl @ 50 kph, at 100 kph it stands to do ~ 17 kpl.

And mind you, the ‘SFC’ that we just talked about above also varies with the absolute Bhp demanded from it coz an Internal Combustion Engine’s ‘fuel efficiency’ drops above and below its ‘peak torque’ rpm.

Further, since an ICE’s max rpm AND Bhp is limited, 59 Bhp in this case, it follows that at 100 kph, it’s got only 59 - 35 = 24 Bhp left to propel it any faster and going by the above relationship, this will get used up by another 40 kph or so - leading to its top speed ‘capability’ of ~ 140 kph.

To sum-up, to get the best ‘FE’ (as opposed to absolute Fuel Consumption/kpl) out of your Car, cruise as close to its peak torque rpm as practicable.