Conventional 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 and performance classes, 'mpg' can vary by up to 30%.
Q. How do modern petrol and diesel engines work?Return to top
Spark-ignition (petrol) engines utilise the four-stroke cycle. 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 electronically controlled fuel injectors to provide the correct amount of petrol. In order to comply with EU legislation, three-way catalysts are fitted most new petrol vehicles. These use precious metals (within the exhaust pipe) to catalytically reduce the amount of carbon monoxide, nitrogen oxides and unburned hydrocarbons from the exhaust. As the metals are poisoned by lead and sulphur, the introduction of catalysts has been dependent on the availability of lead-free and low sulphur fuels.
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 by a fuel injection pump. Typical compression ratios of 20:1 are used, which is sufficient to raise the air temperature to over 400oC. Once the diesel fuel is injected into the cylinder it immediately vaporises and spontaneously ignites. This combustion process produces a mixture of hot gases that then drive the piston. Diesel combustion is more explosive than petrol combustion. This explains why diesel engines are generally noisier and vibrate more than their petrol counterparts.
Modern diesels use direct injection fuel delivery and computerised engine management systems. These replace indirect injection engines in which the fuel is injected into a pre-chamber before entering the cylinder. New 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 are also being developed in order to comply with new Euro standards. These include the use of diesel particulate filters (DPFs), which are already fitted to a number of production cars.
Q. What petrol and diesel cars are most fuel-efficient? Return to top
As a general rule, conventional petrol and diesel engines with small cylinder capacity ('cc' or 'litres') are more fuel-efficient (ie have better 'mpg'). However, there are exceptions and large range variations in fuel economy for each engine size (eg 1 litre, 1.5 litre, etc). Therefore, it is more accurate (and useful) to think in terms of vehicle mass (or weight in everyday language) - 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 'cc' and fuel-efficiency.
Other considerations are the increasing number of energy-hungry features such as power steering and air conditioning now considered as standard on many cars. The increasing use of these features, together with tighter emission and safety standards that have tended to increase vehicle mass, has offset improvements in engine efficiency. So much so, that, despite significant improvements in engine design, between the mid 1980s and the late 1990s the fuel economy of cars in the UK improved by less than 5%. However, since 1998 the fuel economy of new cars sold in Europe has improved by almost 10% due to a voluntary agreement between the European Commission and ACEA (the association of European car manufacturers) to reduce vehicle carbon dioxide emissions. The fuel economy of conventional cars is predicted to improve by at least a further 10% by 2008 under the ACEA agreement.
Using data from the Society of Motor Manufacturers and Traders (SMMT), the average fuel economy of new petrol cars is approximately 39 mpg (7.2 litres/100km) and for new diesel cars is 45 mpg (6.2 litres/100km). These averages hide the large variation in fuel economy across vehicle classes that range from 69 mpg (4.1 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, performance 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 energy label (see below) 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%. In a GDI engine petrol is sprayed directly into the combustion cylinder. This results in a cleaner burn and an increase in power. Hybrid electric cars also offer significant improvements in fuel economy (of around 25%), and in a real sense, can be considered as the next technological evolution of 'conventional' cars - indeed, companies such as Toyota are considering hybridising their entire passenger car range.
Q. Are fuel-efficient cars better for the environment? Return to top
The short answer is 'yes’. Focusing on greenhouse gas emissions (such as CO2), higher 'mpg' implies less fuel use, which in turn means fewer emission of carbon. For example, a small diesel city car with an 'mpg' of 69 mpg (4.1 litres/100km) emits 109 gCO2/km, whereas a petrol 'gas-guzzler' with an 'mpg' of 18 mpg (16.0 litres/100km) will emit 376 gCO2/km. However, considering regulated emissions, the correlation 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.
The long answer requires some discussion of the relative benefits of petrol and diesel cars to appreciate the effect of vehicle type on emissions and the environment. The fuel efficiency of diesel engines is higher than for spark-ignition units due to their higher combustion temperature and lower rate of heat loss. Diesel also has a 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 (per mile), and diesel vehicles produce 15%-20% fewer CO2 emissions. To some extent, there is a trade-off between reductions in local and global emissions - diesel's relatively high NOx and low CO2 emissions are both the result of the engine's high combustion temperature.
Q. Are fuel-efficient cars cheaper to own and run? Return to top
If the question is about fuel costs (one of the most significant running costs), the answer is 'yes'. Higher 'mpg' 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 between 95p and 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). Changing from a conventional to a hybrid car results is even bigger savings of around 25% (equivalent to around 2.8p/mile for a mid range car).
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 finding (see above) that using the lowest emitting vehicles in each segment would cut carbon emissions by 30% implies that motorists could also cut their fuel costs by a similar amount without having to buy a smaller car - but instead by switching to a more fuel-efficient model.
If the above question is about purchasing and depreciation costs (the two most significant ownership costs), then the answer is slightly more complex. If you decide to buy a smaller car (which is likely to be more fuel-efficient due its reduced mass), then the purchase price is likely to be lower. Keeping within the same vehicle class (size), finding a definitive rule about costs is almost impossible due to the wide variation in brands, styling, performance etc. That said, although more fuel-efficient conventional petrol and diesel cars may not cost more, switching to a hybrid will mean paying more 'up-front' when buying the car - perhaps by as much as £2000.
Q. Where can I buy a fuel-efficient petrol or diesel car? Return to top
For the reasons discussed above, fuel-efficient petrol and diesel cars 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 - either on the new car energy label (which should be provided for every new car), or on the Vehicle Certification Agency website that provides fuel economy and emissions data for all cars available in the UK.
However, if you want to use some general 'rules-of-thumb' to help you choose a fuel-efficient car then remember that: the smaller the better (cars with less mass tend to have higher fuel economy); and engine technologies that are improving the fuel economy of conventional cars include Gasoline Direct Injection (GDI) (also known as Fuel Stratified Injection FSI) and hybrid-electrics.