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All about fuel efficiency

Good news for consumers with the environment on their conscience is that modern diesel engines not only consume less fuel than same model petrol cars but are also no more polluting than petrol engines. Fuel efficient cars, in the very near future, are slated to be more economical in their operational costs, as they will give much higher mileage, over 30% of the current models. This would save 204 liters per year for those consumers who drive their cars about 600 km a month.

Why do we need fuel efficient cars?

India is now probably world’s 7th largest auto market. It produced 2.3 million passenger vehicles, 0.56 million LCVs, 0.25 million heavy trucks and buses, 0.6 million three wheelers and 10.5 million two wheelers in 2009-10 (Source SIAM). This consistent growth of 12% in automobile sector is not complemented by fuel efficiency measures, thus putting more & more pressure on the non-renewable energy resources. Certain fuel efficiency measures are urgently required to be adopted to conserve precious fuel resources, strengthen national energy security and address increasing levels of CO2 in the atmosphere.

Auto companies are also taking fuel efficiency (FE) and pollution control very seriously as automobiles are the largest consumer of liquid fuels and consequently among the largest contributors to global pollution and global warming. New technologies are progressing rapidly with introduction of clean burning engines that not only consume less fuel but are also less polluting. Unfortunately they cost more to make and automakers need the enactment of legislation to ensure that consistent efficiency improvements take place within defined time lines.

Alternate fuels for automobiles

CNG (compressed natural gas) and LPG (liquid petroleum gas) are emerging as popular automotive fuels, because they are less polluting than conventional fuels. Even though the government prices them much lower than petrol or diesel but there is an initial cost of the conversion kit which can only be fitted to petrol engines. Therefore these alternate fuels are economical only for vehicles like taxis that have to cover long distances. Users also suffer a small loss of power but the engines are quieter. CNG, being a pure gas, cannot be compressed so it has to be piped in at high pressure from the refineries and thus is only available in Mumbai and Delhi areas. LPG is however a liquid that can be stored at low pressure and transported, like other fuels, by tankers to any town.

Electric vehicles, that produce zero pollution, are slowly gaining popularity despite being expensive. They also suffer from a limited driving range of roughly 80 Kms. They are however fun to drive as they accelerate quickly and make almost no noise. Like other cars they are heavily taxed so the central and state governments must give them a complete tax holiday until they become popular.

What do you save by being fuel efficient?

Cars having new technologies are expensive but most fuel efficient cars are going to be more economical in their operational cost as they will give much higher mileage (over 30% of the current/old ones), therefore about 30% saving is achievable. This would save you 935 (17 litres) per month or 11200 (204 litres) per year if you are using your car for about 600 km a month. Thus cars incorporated with new technologies are most fuel efficient in the long run. The above details indicate billions of gallons of fuel and foreign exchange can be saved every year by using fuel efficient car.


All domestic Indian vehicles have to pass a process of homologation or conformity before they are allowed to be sold. The government test centers like ARAI (Automotive Research Association of India) not only check to confirm their specifications, engine power etc., but also the fuel consumption under standard test conditions. These results are for ideal conditions and do not take into account factors like traffic, excessive heat, aggressive driving, bad roads, etc. In real traffic situations, the fuel consumption would be roughly 20 to 25% higher than the test results.

Difference between Fuel Efficiency & Fuel Consumption:

It is important to under-stand that there is a difference between fuel efficiency and fuel consumption. Most modern engines may be almost equally fuel efficient but their fuel consumption may be different as they are affected by many factors. The engine of a small Tata Nano may be no more fuel efficient than a big Toyota Corolla but will consume less fuel as it is lighter in weight. Also the exhaust emissions tend to correspond with the fuel consumption though some engines are a little cleaner than others. The companies concentrate on better burning of fuel with MPFI (Multi Point Fuel Injection), multi valves, variable valve timing, etc., as well as better scrubbing of the exhaust gases with catalytic converters etc.

Presently, the excise tax on most passenger cars is between 20-24% depending upon category. But if the FE labeling programme is to be made successful, govt. is expected to consider further reduction in the excise duty as fiscal incentive to the buyers to encourage sale of fuel efficient cars, particularly in entry level small family cars that still continue to dominate the market. This would largely help mitigate climate change by reduction of CO2 emissions. The graph below lists typical vehicles on the Indian roads showing fuel consumption and other parameters of some selected (petrol engine) cars under ARAI ideal test conditions:

The Close Correlation Between Vehicle Weight And HP With Fuel Consumption And Emissions Is Very Clear From This Graph. Graphs Showing Relative Difference Between HP, Mileage And CO2 Emission: Diesel Engines

Are latest diesel engines most fuel efficient & least polluting?

Since the advent of high pressure common rail fuel injection, diesel engines are comparable to petrol engines in terms of power and exhaust emissions though superior in fuel efficiency. A characteristic of diesel engines is that they deliver power at lower engine speeds enabling the vehicle to drive at lower engine speeds leading to better fuel consumption. Modern diesel vehicles are more than double as fuel efficient and half as polluting as compared to ten-year-old diesel vehicles. But on the other hand, diesel is more polluting in terms of NOX and particulate matter, till the time EURO VI is made available. Even top luxury brands like Audi, BMW and Mercedes are making diesel vehicles that are as quiet as their petrol models.

The graph below profiles a set of Indian cars for which data is available. These show that modern diesel engines consume less fuel and are no more polluting than petrol engines.

fuel efficiency graph

Upcoming innovation to help consistent improvement in efficiency

Improved engine technologies are the main element in improving fuel consumption but there are many other factors as well. The impact of some recent innovations like variable valve timing, petrol direct injection, high pressure diesel injection, digital valve activation; 5/6 speed gear box and other various factors would further enhance the fuel efficiency in the coming years.

Measures that improve Fuel Efficiency

FE labeling of cars is yet to become a reality in India, but there are certain measures that can tangibly improve the efficiency of the vehicles. This is above and beyond the labeling of the cars as the fuel efficiency measures would have to be followed to get higher mileage and conserve fuel.

Listed below are some of the main fuel consumption killers that drivers can largely control themselves:

  • Cold engines: If your car does a 5-minute trip to the market and then goes on a series of short trips to the bank, school, friends, etc., your engine never gets warm enough to achieve thermal efficiency and your fuel consumption may go by up to 50%. 
  • Driving at high engine speeds: If you like to rev up your engine and drive at high engine speeds in each gear, you have to pay for it. Conversely if you keep the engine at minimum speeds your fuel consumption will be better but driving in too low speeds will also cause knocking and shorten engine life. So drive at a steady speed to get higher mileage.
  • Too many stops and starts: Driving in traffic with long periods of idling at red signals or when stuck at traffic is a fuel efficiency killer. If you spend 10 minutes idling on a 30-minute road trip, your fuel consumption may double, so switch-off your engines at red signals. If not, switch-off your AC to reduce fuel consumption. 
  • Bad Roads: These can combine the impact of too many stops with the need to use high engine speeds in low gears to keep your vehicle moving so it can play havoc with your fuel consumption.
  • The weight of the car: Most modern cars must meet current crash safety requirements. They are heavier than earlier models and therefore eat more fuel. Consider that over 108,000 people were killed on Indian roads last year so a bit more fuel is a small price for lesser deaths or injuries. If not necessity, never go for bigger cars as they are heavier and less fuel efficient.
  • The load that you carry: This weight factor is naturally increased if your car is overloaded with six people and loads of luggage. If you carry baggage on a roof rack than you will also have the impact air resistance. Thus adjust your baggage, smallest in the front & largest in the rear, tightly tie & cover luggage to minimise air resistance.
  • Engine tuning and timing: Most modern engines with MPFI and other electronic engine management systems need very little tuning unlike the old carburetors but the nozzles and injectors do need to be cleaned. So do not neglect your regular servicing to get better efficiency from your cars.
  • Air, oil and fuel filters: Choked filters can play havoc with fuel consumption & also efficiency so ensure that these are regularly serviced and changed when they are dirty or clogged.
  • Tyre pressure: Maintain correct pressure recommended by the car maker to make your car to be more fuel efficient.
  • Evaporation losses: All vehicles suffer from a little evaporation loss especially in the hot summer months. This loss will be higher in many cars that stand idle for any length of time so the fuel consumption may seem high in any vehicle that is unused for some time.
  • Hybrid cars: Hybrid cars with electric motors working in tandem with petrol engines, are being promoted by many auto makers to demonstrate their concern for the environment. Their performance is now very acceptable but they are still so expensive that they will not be popular for a long time. Many car makers are also making Hydrogen fueled cars as well as cars fueled by fuel cells but these too are for the future.

Two wheelers, the largest selling Indian vehicles:

These factors apply not only to cars but to motorcycles, and all two wheelers. Under the pressure of legislation, the noisy and polluting 2 stroke engines have now been replaced by 4 stroke engines while severe competition has made them much more consumer and environment friendly. A few models now have twin spark plugs for better combustion and fuel injection will be available soon.

Fuel consumption of vehicles:

The fuel consumption of all vehicles depends on their laden weight and the number of Kms they drive. Typically most personal cars drive about 600 Kms a month so a medium sized car at 10 Km per liter may consume 60 liters per month. By contrast a heavy truck or bus that has a big 100 to 400 HP engine may consume 4 KMPL on 5000 Kms of driving every month so it would consume about 2,000 liters… or 33 times as much fuel as a car.

This article has been made possible by the support of the Climate Works Foundation & I.C.C.T of the US on climate change mitigation The study is also supported by Shakti Sustainable Energy Foundation of India. The contents are the responsibility of Consumer VOICE and do not necessarily reflect the views of any of the other organisations.

References: GPF, SIAM, PCRA, ICCT, ARAI, MoEF, ET&TOI etc.

Consumer VOICE

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