More on fuel economy

MOTORING MATTERS (in last week's `Small is truly beautiful') looked at the main problems with the use of private motor vehicles on Indian roads and suggested some measures to ameliorate them. To recap, these problems are: rapidly increasing congestion and pollution as well as the increasing use of imported petroleum. It was suggested that encouraging the use of smaller vehicles would help tackle all these problems. Japan has successfully used a similar strategy.

An alternative strategy is to classify vehicles according to their fuel efficiency (or fuel consumption) and use that as a basis for taxing them. This is because fuel consumption and size (or weight) have a fairly close correlation, barring some exceptions. One advantage of such a scheme is that it is easy to administer and less open to abuse.

We could use the Indian National Driving Cycle that is already used to test vehicles for their pollution levels as a basis for measuring fuel consumption under typical Indian conditions. These measurements could be carried out by ARAI, Pune or VDRE, Ahmednagar which already have the necessary facilities.

A possible duty structure for petrol and diesel driven three and four wheeled vehicles is shown below. The duty structure for diesel vehicles is slightly "stiffer" because, under the administered price mechanism, diesel is artificially cheaper than petrol. On the other hand, the duties on vehicles, petrol or diesel, that meet Euro III, or equivalent, emission norms could be reduced by 5 per cent in each category. Similarly, the duties on vehicles factory-fitted with CNG fuel systems could be reduced by 5 per cent because of their clean running.

On differentials

Numerous readers of this column have asked me what a differential is. The last straw, as far as I am concerned, was when a nephew of mine who has an engineering degree asked the same question!! So here goes...

Historically, the invention of the differential is usually attributed to the Frenchman Onesiphore Pecqueur, who in 1828 patented a steam vehicle that included a differential gear on the driving axle to help it easily negotiate corners. Why? When a vehicle with two or more wheels on an axle goes around a curve, the outer wheels travel a greater distance than the inner ones by rotating faster. Common sense shows that if they 'free wheel', there is no problem. If they are driven, however, they will tend to skid because the outer wheel is prevented from rotating faster than the inner one. Fitting a 'differential' divides the torque (turning force) equally between the two driven wheels, regardless of their relative speeds of rotation and stops the skidding.

The ordinary differential, on the other hand, is at a disadvantage when equal traction is not available to both driven wheels, typically when one is on a loose or slippery surface. That is when one needs a limited slip differential of the kind fitted to a Tata Safari.

Four wheel drive vehicles are capable off-road and under other difficult conditions albeit at the expense of added cost, complication and fuel consumption. More often than not, however, only two wheels need to be driven when it is obviously desirable that the others free wheel saving power and fuel and reducing wear and tear. The early Willys Jeeps and their Mahindra successors did not come with free wheeling front hubs unlike the Maruti Gypsies which came with them as standard, but the change to a wider track in the mid 1990s saw their unfortunate withdrawal.

Mr. Sunil Subbiah, a Bangalore based automobile enthusiast and industrialist, designs and manufactures selectable free wheeling hubs for Gypsies and Mahindra jeeps that can be retrofitted by any easily competent mechanic. Fitting these hubs should improve the power and fuel consumption of most four wheel drive vehicles by about 10 per cent in typical usage.

Manmohan Reddy

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