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Petrol and diesel
Conventional petrol and diesel cars can be green (or greener) if they are highly fuel-efficient – in other words, if they have high fuel economy, 'miles-per-gallon' or 'mpg'. Cars with good fuel economy need not necessarily be small or under-powered – within all vehicle classes, 'mpg' can vary by up to 30%.
How do petrol and diesel engines work?Return to top
Spark-ignition (petrol) engines utilise the four-stroke cycle (see animation). During the induction stroke a small amount of fuel and air are drawn into the cylinder. The petrol-air mixture is then compressed into a small volume and then ignited by an electrical spark from the spark plug. The explosion causes the gases to expand (power stroke) forcing the piston and turning the crankshaft. The burned gases are then expelled from the cylinder (exhaust stroke) via the exhaust valve.
Whereas older designs used a carburettor to mix the fuel and air before combustion, newer engines employ electronic fuel injectors to provide the correct amount of petrol. In order to comply with EU legislation, three-way catalysts are fitted new petrol car exhausts – these use precious metals to catalytically reduce the amount of carbon monoxide, nitrogen oxides and unburned hydrocarbons from the exhaust.
Although diesel engines are also four-stroke, only air is compressed in the cylinder instead of an air-fuel mixture, and at the end of the compression stroke the fuel is directly injected into the combustion chamber. Once the diesel is injected into the cylinder it immediately vaporises and spontaneously ignites, due to the high air temperature caused by compression. This combustion process produces a mixture of hot gases that then drive the piston.
The latest diesel technologies include common rail injection and advanced diesel turbo-chargers. In common rail systems, the injection pressure is independent from engine speed and load. This enables the injection parameters to be freely controlled leading to reductions in engine noise and NOx emissions. Diesel after-treatment systems have also been developed to comply with new Euro standards –these include diesel particulate filters (DPFs), which are now fitted to the vast majority of new diesel cars.
Animation: UtzOnBike. For more info, see: wiki/Petrol_engine and wiki/Diesel_engine
.What petrol and diesel cars have the best 'mpg'? Return to top
Most motorists assume that conventional petrol and diesel engines with small cylinder capacity are more fuel-efficient, with better 'mpg'. However, for each engine size fuel economy can vary widely, and is dependent on how well the engine is designed, built and 'tuned'. It is more accurate to think in terms of vehicle mass (or weight) – generally speaking, the smaller the mass, the better the fuel economy. In other words, the correlation between vehicle mass and fuel-efficiency is stronger than that between engine capacity and fuel-efficiency.
In recent years, fuel economy in European vehicles has gradually improved, first due to an agreement between the European Commission and the ACEA (the association of European car manufacturers) to reduce carbon dioxide emissions, and more recently, the improvement has accelerated following the fuel price peaks in 2008 and the 'credit crunch' – these have creating a renewed interest in smaller 'city' cars and superminis with better 'mpg'.
Using data from the Society of Motor Manufacturers and Traders, the average fuel economy of new petrol cars in the UK is approximately 42 mpg (6.8 litres/100km) and for new diesel cars is 48 mpg (5.9 litres/100km). These averages hide the large variations in fuel economy across vehicle classes that range from 86 mpg (3.3 litres/100km) for the most fuel-efficient small diesel citycar, to 18 mpg (16.0 litres/100km) for one of the most polluting petrol Sports Utility Vehicles (SUVs).
However, one of the most important car myths is that you need to trade off vehicle size and/or safety to get a car with good fuel economy. The SMMT has calculated that "if the lowest CO2 emitting vehicles in each segment were used then average CO2 emissions would fall by 30 per cent". Therefore, whatever type of car you are looking to buy or use, it is worth checking the fuel economy label, and/or searching the WhatGreenCar database for information about the car's fuel economy (mpg), emissions and fuel costs.
Engine technology is also an important consideration regarding fuel economy. In order to comply with future emissions standards, new petrol engine technologies have been developed and are beginning to appear in production cars. One of the most important is Gasoline Direct Injection (GDI) (also known as Fuel Stratified Injection FSI) which reduces fuel use and vehicle CO2 emissions by up to 20%. Hybrid electric cars also offer significant improvements in fuel economy (of around 25%), and in a real sense, are considered as the next technological evolution of 'conventional' cars.
What are the environmental impacts of petrol and diesel cars? Return to top
Focusing on greenhouse gas emissions (such as CO2), higher 'mpg' implies less fuel use, which in turn means fewer emissions of carbon and less environmental impact. For example, a small diesel city car capable of 86 mpg (3.3 litres/100km) emits only 88 gCO2/km, whereas a petrol 'gas-guzzler' with 18 mpg capability (16.0 litres/100km) will emit 376 gCO2/km.
Considering regulated emissions, the link between fuel economy and vehicle emissions is far weaker – in other words, some small cars with good fuel economy may emit as many particulates or NOx as larger cars – a general rule is hard to identify.
Engine type also has an impact on the level and type of vehicle emissions. The fuel efficiency of diesel engines is higher than for petrol units due to their higher combustion temperature. Diesel also has slightly higher energy content than petrol per unit volume. As a result of the differences in fuel composition and engine conditions, petrol and diesel cars differ in their relative emissions performance – petrol vehicles emit fewer NOx and particulates, and diesel vehicles produce 15%-20% fewer CO2 emissions. To some extent, there is a trade-off between reductions in local and global emissions.
The WhatGreenCar ratings on this site provide an accurate comparison between specific models and fuel types. Indeed, one of the reasons that WhatGreenCar developed the WGC rating system was precisely to address the complex scientific issues outlined above.
What are the costs of owning a petrol or diesel car? Return to top
Higher 'miles-per-gallon' implies less fuel use, which in turn means lower fuel costs. For an average petrol or diesel car, and assuming petrol and diesel pump prices at around 100p per litre, for each 5 mpg improvement in fuel economy (equivalent to around 1 litre/100km improvement), the fuel savings are between 1.2p and 1.5p per mile (depending on whether petrol or diesel is used).
The important fact to remember here is that you need not trade off vehicle size, performance and/or safety to get a car with good fuel economy. SMMT's findings (see above) suggest that fuel cost can be cut by at least 30% without having to choose a smaller car, but instead by switching to a more fuel-efficient model.
Currently, all petrol and diesel vehicles have to pay the daily London Congestion Charge. However, it is possible that this will be graduated due to carbon emissions, meaning that petrol or diesel models under a certain emissions threshold may be exempt.
STOP PRESS: London Mayor, Boris Johnson, has proposed to replace the Congestion Charge's 'Alternative Fuel Discount' with a 'Greener Vehicle Discount'. If accepted, the Greener Vehicle Discount (GFD) would provide a 100% discount for all cars (hybrid or otherwise) that emit 100 gCO2/km or less and meet the Euro V standard for air quality. See news article for details.
Where can I buy a fuel-efficient petrol or diesel car? Return to top
For the reasons discussed above, fuel-efficient petrol and diesel models can be found in all vehicle classes, and are available from most vehicle manufactures.
All that is required is for you to check the fuel economy information – the fuel economy label, which is provided in the showroom for every new car, and is also now being provided at an increasing number of used car dealerships.
The WhatGreenCar database also provides a good up to date comparison of the most fuel-efficient petrol or diesel cars available in the UK.