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Tribune News Service
If I replace my old diesel car with a less polluting new petrol one, how long will it take for the reduced pollution of the new car to outweigh the increased pollution caused by the manufacture of the new car and the disposal of the old, assuming average usage?
**This assumes that the new petrol car will be"less polluting" than the older diesel. However, by several measures, it will be the other way around.
The major pollutants created during manufacture are the carbon emissions associated with smelting the steel and aluminium, and generating electricity. Virtually all the old polluting European smelters and generating facilities have been scrapped, but if the vehicle or its parts came from a South Asian country, the production of nitrogen oxides (NOx), sulphur oxides, ozone and carbon may be appreciable. Moreover, many vehicles manufactured in Europe were designed to be dismantled and recycled at the ends of their life, minimising emissions.
For emissions associated with running the vehicle, the best petrol engines cannot match, let alone beat, the fuel efficiency of even an elderly diesel. In general, the thermal efficiency of a petrol engine is between 25 and 30 percent. This means up to 75 percent of the fuel's energy is wasted as heat. For diesels built more than 20 years ago (with indirect injection into a pre-combustion chamber), thermal efficiency will be between 35 and 40 percent. In diesels made within the past 20 years, this can reach as much as 45 percent, giving them a big advantage in carbon emissions.
In practice, the diesel advantage is even larger. Typically, engines deliver their best efficiency around the point of peak torque. In a small diesel car, peak torque occurs between 1700 and 2500 revolutions per minute, which is where the engine operates in normal use. On the other hand, the maximum torque of non-turbo petrol engines is developed above 4000 rpm, an engine speed rarely used in normal driving.
Even if the question referred to emissions of carbon monoxide, NOx, unburned hydrocarbons and soot, the advantage is not clearly on the side of the modern petrol engine. In general, diesels have not been good at controlling NOx and carbon particles and need add-on equipment to meet today's regulations. However, diesel exhaust contains virtually no carbon monoxide and very little unburned hydrocarbon.
Petrol engines cannot match the torque and fuel efficiency of diesel engines used for heavy vehicles
In contrast, petrol engines have always needed add-on equipment to control carbon monoxide and unburned hydrocarbons. The catalytic converter, which performs this role, does not work until it reaches at least 200'b0C, so there is no emissions control for the first few kilometres of a vehicle's journey.
Some modern petrol engines have adopted diesel techniques to boost power, reduce the engine speed at which torque peaks and lower unburned hydrocarbon emissions. Even so, they cannot match the combination of lower peak torque and higher fuel efficiency that makes diesel the power choice for heavy vehicles and construction equipment.
So, the answer to the question of when the petrol engine will save enough emissions to balance those released during its manufacture could be never.
**If by"pollution" your correspondent means carbon dioxide, then the answer is probably never. Diesel cars are extremely fuel-efficient and a new petrol car is likely to produce more carbon dioxide than a five or 10-year-old diesel. Only a hybrid, regularly recharged with low-carbon electricity, is likely to do significantly better. But even then, it will probably take longer than the lifetime of the car to recover the energy cost of its manufacture.
However, if your correspondent means the toxic pollutants NOx in particular has been in the news a lot recently then the answer is: very quickly.
Toxic emissions from car manufacture are much more difficult to quantify than the carbon dioxide produced, but since the manufacture mostly happens well away from city centres, the impact on local air pollution is small.
**The carbon cost of a new car is estimated at 720 kilograms per A£1000 spent, so for an average hatchback this might amount to as much as 14 tonnes of carbon dioxide. This car might produce around 125 grams of carbon dioxide per kilometre, compared with 165 grams per kilometre for a car 10 years older. This means you would need to drive the new car 350,000 kilometres before you were making a carbon saving compared with the old one.
If we assume that the older car had become less efficient over time and generated 175 grams per kilometre of carbon dioxide, it would still take 280,000 kilometres of driving to balance out. At average UK mileage of about 12,700 km per year, it would take over 22 years before the new car became more efficient.
If you were to buy a more fuel-efficient car that produced only 80 grams of carbon dioxide per kilometre, you would still need to drive for 11 years before the carbon cost balanced out. An alternative, perhaps, is to buy a newer second-hand car rather than a brand new model, so that you don't need to consider the carbon footprint of new-car production.
