How do modern gasoline and diesel engines work?Return to top

Spark-ignition (gasoline) engines utilise the four-stroke cycle. During the induction stroke a small amount of fuel and air are drawn into the cylinder. The gasoline-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 gasoline. In order to comply with EU legislation, three-way catalysts are fitted most new gasoline 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 400^oC. 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 gasoline combustion. This explains why diesel engines are generally noisier and vibrate more than their gasoline 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.

Animation: UtzOnBike. For more info, see: wiki/gasoline_engine  and  wiki/Diesel_engine

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What gasoline and diesel cars have the best 'mpg'? Return to top

As a general rule, conventional gasoline and diesel engines with small cylinder capacity ('cc' or 'litres') are more fuel-efficient (ie have better 'miles-per-gallon'). However, there are exceptions and large range variations in fuel economy for each engine size (eg 2 litre, 2.5 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 standard on most models. Air conditioning alone can worsen fuel economy by up to 40% (when in use). 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, over the last decade, average fuel economy of cars in the US improved by less than 0.5% per year.

Using data from the Department for Transport (DoT), the average fuel economy of new cars in the US is approximately 22.5 mpg (10.5 litres/100km). This average hides the large variation in fuel economy across vehicle classes – for example the fuel economy for current midsize cars ranges from 46 mpg (5.1 litres/100km) to 11 mpg (21.4 litres/100km). Therefore, whatever type of car you are looking to buy or use, it is worth checking the fuel economy label for information about the car's fuel economy (city and highway) and annual fuel costs. By choosing carefully (using the published 'mpg' data), you do not necessarily need to trade off vehicle size, performance and/or safety to get a car with good fuel economy.

Engine technology is also an important consideration regarding fuel economy. New gasoline 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%. Gasoline-hybrid 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 in the process of hybridising their entire passenger car range.

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 midsize gasoline car with an 'mpg' of 50 mpg (4.7 litres/100km) emits only around 175 gCO2/mile, whereas a 'gas-guzzler' with an 'mpg' of 10 mpg (23.5 litres/100km) will emit just over 870 gCO2/mile. However, considering emissions other than carbon dioxide, 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.

A longer answer requires some discussion of the relative benefits of different fuel types (such as gasoline and diesel) to appreciate the effect of different fuels on emissions and the environment. The fuel efficiency of diesel engines (more popular in Europe) 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 gasoline per unit volume. As a result of the differences in fuel composition and engine conditions, gasoline and diesel cars differ in their relative emissions performance - gasoline 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.

The quick answer is to use the WhatGreenCar ratings on this site. Indeed, one of the reasons that WhatGreenCar developed the WGC rating system was precisely to address the complex scientific issues outlined above. The ratings can also take account of all fuel types including, ethanol (E85), natural gas, LPG and hydrogen.

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 'miles-per-gallon' implies less fuel use, which in turn means lower fuel costs. For an average gasoline or diesel car, and assuming gasoline and diesel pump prices at around $2.50 per gallon, improving from fuel economy from 30mpg to 25mpg leads to fuel savings of around 1.7c per mile (for a gasoline car). Changing from a conventional gasoline to a gasoline-hybrid car results is even bigger savings of around 25% (equivalent to around 2.5c/mile for a midsize model).

The important fact to remember here is that you do not necessarily need to trade-off vehicle size, performance and/or safety to get a car with good fuel economy. If using the lowest emitting vehicles in each segment can cut carbon emissions by up to 40% (see above) then this implies that motorists can also cut 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 gasoline and diesel cars may not cost more, switching to a gasoline-hybrid will mean paying more 'up-front' when buying the car - perhaps by as much as $2,500.

Where can I buy a fuel-efficient gasoline or diesel car? Return to top

For the reasons discussed above, fuel-efficient gasoline and diesel cars can be found in all vehicle classes, and are available from most automakers. All that is required is for you to check the fuel economy information - either on the EPA Fuel Economy Label, or search the WhatGreenCar database for the most fuel-efficient gasoline or fuel-efficient diesel cars available in the US.

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 gasoline-hybrids.