Updated: December 18, 2015 11:44:37 am
For all cars and utility vehicles with big engines, diesel is the default choice of fuel — the Honda CRV (2.4L AWD) and hybrids such as the new Toyota Camry being possibly the only exceptions. The Supreme Court order banning the registration of diesel vehicles with engines larger than 2,000 cc capacity in Delhi until March 31, 2016, will thus hit a wide range of vehicles.
So, why is diesel the fuel of choice for bigger vehicles? Apart from the fact that it is cheaper than petrol in India, the diesel engine is more efficient. The diesel combustion cycle yields a leaner fuel-air mixture to operate at optimal efficiency as compared to petrol engines. Measured by volume, diesel is more energy-dense than petrol. The combustion cycle itself works best at leaner mixtures, and diesels deliver a torque curve that works better for bigger cars and trucks as compared to petrol.
Plus, most diesels are equipped with a turbocharger, which offers a sharp surge in power delivery after a certain RPM, a feature that is popular with customers of bigger cars and SUVs. Modern diesels also emit lesser carbon dioxide than petrols, something that has been instrumental in the spread of diesels in markets such as Europe.
On the flip side, diesel engines also emit higher levels of nitrogen oxides, and over seven times more particulates as compared to petrol engines — pollutants that cause respiratory ailments.
In 1998, California identified diesel exhaust particulate matter as a toxic air contaminant based on its potential to cause cancer, premature death, and other health problems. But Europe has been a proponent of diesel until very recently, a position influenced by the powerful diesel car lobby in mainland Europe, most of whom are leaders in the diesel technology — led by Renault and PSA Peugeot Citroen in France, Volkswagen in Germany, and Fiat in Italy. But the Volkswagen emissions scandal has now cast a shadow over all diesels, prompting the EU to reassess it testing process and announce plans to introduce new real driving emissions tests (which are considered far more accurate than the current lab tests) from September 1, 2017.
The big reason for the higher efficiency of diesel engines boils down to the engineering design. While both petrol and diesel engines work by internal combustion, they do so in slightly different ways. In a petrol engine, fuel and air is injected into small metal cylinders, and then a piston compresses the mixture, making it explosive. A small electric spark from a sparking plug sets fire to it, which makes the mixture explode, generating thrust. This then pushes the piston down the cylinder, and through the crankshaft, turns the wheels.
In pure design terms, diesel engines are simpler. First, air is let into the cylinder and the piston compresses it, but in this case, much more than in a petrol engine. While in a petrol engine, the fuel-air mixture is compressed to about a tenth of its original volume, in a diesel engine, the air is compressed by anything between 15 and 25 times.
Compressing a gas generates heat, and once the air is compressed, a mist of fuel is sprayed into the cylinder by an electronic fuel-injection system, which works a bit like an aerosol spray. The air is so hot that the fuel instantly ignites and explodes — without the need for a spark plug. This controlled explosion pushes the piston back out of the cylinder, producing the power that drives the vehicle.
Diesel engines tend to be up to twice as efficient as petrol engines. The reasons for this include the lack of a spark-plug ignition system. As a result, the fuel is compressed much more, which makes it burn more completely in combination with the air in the cylinder, thereby releasing more power. Also, in a petrol engine that is working at less than full power, more fuel (less air) needs to be suppled to the cylinder to keep it running, while diesel engines actually consume less fuel when they are working at lower power. This lowers fuel usage while idling.
Plus, measured by volume, diesel fuel is more energy-dense than petrol, and thereby offers more energy per litre than petrol. Diesel — which is a lower-grade, less-refined product of petroleum made from heavier hydrocarbons — is also a better lubricant than petrol, with the result being that a diesel engine runs with less friction, thereby generating better efficiency.
A higher compression ratio (a result of the squeezing of the air in the chamber) means the parts of a diesel engine have to withstand far greater stresses than those in a petrol engine, with the result that they need to be sturdier and, therefore, heavier. Diesels are also noisy, and they produce a lot of unburnt soot particles and nitrogen oxides. However, since diesel engines are more efficient, they typically use less fuel and thereby produce lower carbon dioxide emissions. Due to its better build quality (needed because of the higher stresses on the engine), diesel engines also tend to cost more initially than petrol engines, though their lower running costs and longer operating life offsets that costs over a period of time. In India, where diesel is priced lower than petrol, this price tradeoff is reduced, resulting in the rush for diesel cars.
Home-grown car manufacturers such as Mahindra & Mahindra and Tata Motors, alongside Toyota India, have most of their SUV/MUV offerings in the 2000 cc or higher engine capacity category. Luxury carmakers such as Mercedes Benz, BMW and Volkswagen-owned Audi too have most of their offerings in the over-2000 cc engine capacity range, with a majority of the models made by these manufacturers using diesel as fuel — a pure reflection of customer demand for diesel as a fuel for bigger engine category vehicles.
Diesel exhaust contains particulate matter and gases including benzene and nitrogen dioxide, which are agents of serious disease.
* Lung cancer, even for non-smokers; damage to lungs, possibly brain damage
* Exacerbated allergies, asthma; shortness of breath; eye irritation; nausea
* Stress responses in brain; cellular damage linked to Alzheimer’s, Parkinson’s
A 2013 study by the International Council on Clean Transportation titled Impact of Standards on Premature Mortality and Emissions: India in Global Context, found that approximately
* 10% of all years of life lost from exposure to vehicle emissions around the world occur in India
* 4 times is the factor by which the number of early deaths in urban areas will multiply without new limits on vehicle emissions
* 84% is the factor by which the number of early deaths in India can drop by 2030 if new emission limits are set
* 6.2 mn is the cumulative years that can be gained by 2030 with such limits
Brazil: Diesel cars not allowed due to a policy to keep taxes low on diesel
China: No differentiation in taxation between diesel and petrol. Beijing has not allowed diesel cars since 2003
Sri Lanka: Imposes a heavy duty on diesel cars in comparison to petrol cars in order to discourage the sale of diesel cars
Denmark, Germany: High taxes on diesel cars, including a higher annual tax
France: Diesel cars not allowed in Paris on high-smog days. Mayor has said she wants Paris freed of diesel cars by 2020.
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