Petrol or Diesel

Your conventional petrol or diesel cars can be green should they be very fuel-efficient, that is to say, should they have high fuel economy or 'miles-per-gallon', otherwise known as 'mpg'. Cars that have good fuel economy do not have to be small or low-power. ‘MPG’ differs by 30% for all vehicles and performance categories.

Q: How do modern petrol and diesel engines work?

Spark-ignition (petrol) engines use a four-stroke cycle. This means that during the induction stroke a little bit of fuel and air are captured in the cylinder. The petrol-air mixture is compressed into a tiny volume, and then ignited by an electrical spark from the spark plug. This explosion triggers the expansion of the gases, placing pressure on the piston and turning the crankshaft. The burned gases are released from the cylinder (exhaust stroke) through the exhaust valve.

More antiquated designs utilise a carburettor to mix the fuel and air prior to combustion. The newer engines use electronically controlled fuel injectors to give the right amount of petrol. For compliance with EU legislation, three-way catalysts are fitted on the majority of petrol vehicles, which employ precious metals (in the exhaust pipe) to catalytically lower the amount of carbon monoxide, unburned hydrocarbons and nitrogen oxides from the exhaust. Lead and sulphur poison the metals. Consequently, hence, the use of catalysts has depended on lead-free and low sulphur fuels.

Even though diesel engines are also four-stroke, just air is compressed in the cylinder, not an air-fuel mixture. The fuel is injected into the combustion chamber by a fuel injection pump at the end of the compression stroke.

Usual compression ratios of 20:1 are employed; they are enough to raise air temperature to 400ºC plus. Upon the diesel being injected into the cylinder it instantly vaporises and ignites spontaneously. The combustion process results in a combination of hot gases that subsequently drive the piston. Petrol combustion is less explosive than diesel combustion. Hence, diesel engines are on the whole more noisy and vibrate more than petrol ones.

Modern diesels employ direct injection fuel delivery as well as computerised management systems for the engines. They replace indirect injection engines wherein fuel is injected into a pre-chamber prior to entering the cylinder. Current diesel technologies include advanced diesel turbo-chargers and common rail injection. Common rail systems are where the injection pressure is independent from engine load and speed. This ensures that the injection parameters are freely controlled, enabling reductions in engine noise as well as NOx emissions. Diesel after-treatment systems are presently being developed for compliance with new Euro standards, they include use of diesel particulate filters (DPFs) which are now to be found fitted to some cars.

Q. Which petrol and diesel cars are most fuel-efficient?

Both normal petrol and diesel engines have a small cylinder capacity ('cc' or 'litres') which makes them more fuel-efficient (better 'mpg'). There are exceptions and great range variations in fuel economy, for each engine size (e.g. 1 litre, 1.5 litre, and so on).

Hence, it is more precise and helpful to think in terms of vehicle mass (or weight), The smaller the mass, the more enhanced the fuel economy. The interrelationship between vehicle mass and fuel-efficiency is stronger than that between 'cc' and fuel-efficiency.

Other energy-hungry features include power steering and air conditioning which are now standard in most cars. The rising use of these features, as well as stricter emission and safety standards that led to increased vehicle mass; this had been off set by improvements in engine efficiency.

In spite of great improvements in engine design, from the mid 1980s to late 1990s the fuel economy of cars in the UK improved by only 5%. Since, 1998 the fuel economy of new cars sold in Europe has been enhanced by nearly 10% as a result of a voluntary agreement between the European Commission and ACEA (the association of European car manufacturers) to cut vehicle carbon dioxide emissions. The fuel economy of normal cars is forecast to improve by more than 10% by 2010.

The Society of Motor Manufacturers and Traders (SMMT) data cites that the average fuel economy of new petrol cars is around 39 mpg (7.2 litres/100km), with new diesel cars being 45 mpg (6.2 litres/100km). These averages cloak the huge variation in fuel economy across vehicle classes which range from 69 mpg (4.1 litres/100km) for the top fuel-efficient small diesel city car to 18 mpg (16.0 litres/100km) for a really polluting petrol Sports Utility Vehicles (SUVs).

A great car myth is that you need to trade off vehicle size, performance and safety to purchase a car with good fuel economy. It is said that should you buy the least CO2 emitting vehicles in each category then average CO2 emissions would decline by 30%. So whichever car you are considering buying, it is worthwhile scrutinising the energy label for information regarding the car’s fuel economy ('mpg'), fuel costs and emissions.

Engine technology is another important factor associated with fuel economy. For compliance with future emissions standards, new petrol engine technologies have been developed and are starting to appear in production cars. One of the most important is Gasoline Direct Injection (GDI) (also known as Fuel Stratified Injection FSI) which cuts fuel use and vehicle CO2 emissions by around 20%.

Hybrid electric cars offer huge improvements in fuel economy (c. 25%), and can be said to be the next technological evolution of 'conventional' cars. In fact, companies like Toyota are considering hybridising their whole passenger car range.

Q. Are fuel-efficient cars better for the environment?

In short, yes!

With regard to greenhouse gas emissions (such as CO2), a higher 'mpg' indicates less fuel use, which means lower emission of carbon.

The long answer necessitates some discussion of the comparative benefits of petrol and diesel cars to understand the effect of vehicle type on emissions and the environment. The fuel efficiency of diesel engines is greater than for spark-ignition units as a result of the higher combustion temperature and the lower rate of heat loss.

In addition, diesel has minimally higher energy content than petrol per unit volume. Hence, due to the differences in fuel composition and engine conditions, petrol and diesel cars differ in their comparative emissions performance; petrol vehicles emit fewer NOx and particulates (per mile), and diesel vehicles produce 15%-20% fewer CO2 emissions.

There is a trade-off between reductions in local and global emissions.

Q. Are fuel-efficient cars cheaper to own and run?

Fuel efficient cars cost less in terms of fuel. Higher 'mpg' means less fuel use thus lower fuel costs.

Opting for a hybrid car gives you about 25% savings (i.e., c.2.8p/mile for a mid range car).

With regard to purchasing and depreciation costs the answer becomes more complicated. Should you opt to purchase a smaller car (one which is more fuel-efficient due its reduced mass), the purchase price is most likely to be lower too.

More fuel-efficient normal petrol and diesel cars may not be more expensive. However, changing to a hybrid means paying more 'up-front' when buying the car, by as much as £2000.

Q. Where can I buy a fuel-efficient petrol or diesel car?

Fuel-efficient petrol and diesel cars are present in all vehicle classes, and they are available from the majority of vehicle manufactures. You can check out the fuel economy information, with regard to the new car energy label or on the Vehicle Certification Agency website which gives you fuel economy and emissions data for all cars available in the UK.

The key general 'rule-of-thumb' to assist you in your selection of a fuel-efficient car is: the smaller the better. Engine technologies which are enhancing the fuel economy of normal cars include Gasoline Direct Injection (GDI) (or Fuel Stratified Injection FSI) and hybrid-electrics.