Which fuels for which vehicles?
Combustion engines can use three broad types of fuels:
- Liquid fuel made from petroleum: gasoline for spark-ignition engines, diesel fuel for diesel engines, as well as fuel for homogeneous combustion engines that is still being finalized (CAI, CCS, HCCI).
- Gas, including liquefied petroleum gas - LPG - made of a mix of butane and propane, and natural gas for vehicles - NGV - composed primarily of methane. The use of hydrogen in combustion engines is still in the research and demonstration stage.
- Biomass fuels or biofuels, including ethanol and its derivatives used in spark-ignition engines, and biodiesel made mostly from vegetable oil for diesel engines. These biofuels can be used as is or mixed with each of the conventional fuels in variable proportions depending on the country and engine adaptability.
Conventional fuels: Gasoline and diesel fuel
Gasoline and diesel fuel provide more than 96 % of the world's energy for road transportation, roughly 1.5 billion tons per year.On average, worldwide gasoline consumption is higher than that of diesel fuel: 850 million tons of gasoline compared to 650 million tons of diesel fuel, or a ratio of 1.3.
In the USA, the ratio is 2.5. In the 27 countries of the European Union, the ratio is 0.5 and in France it is 0.27.
Why are these fuels so popular?
The existence of enormous oil reserves, high energy density (between 8.9 kWh/l for gasoline and 9.9 kWh/l for diesel fuel), a liquid form that is convenient for distribution and filling gas tanks, relatively low drilling costs, and a very low risk of explosion in adapted tanks are all reasons which have undoubtedly contributed to the development of these two fuels.
From the beginning, these two fuels have evolved alongside engines to optimize both fuel consumption and engine performance.
But what are the constraints today?
The use of petroleum faces two types of constraints: local and global environmental restrictions related to the two types of combustion engine emissions, as well as constraints tied to the location of fossil fuels and their limited supply.
Planning for the post-oil era by beginning to move away from oil dependency now is one way to ensure the future of sustainable mobility.
Petroleum was formed between ten million to a hundred million years ago through the slow accumulation of the remains of organic matter transformed by the pressure and temperature beneath the earth's surface. It is a mix of hundreds of hydrocarbons and small amounts of elements such as nitrogen and sulfur. The refining process breaks this matter down into products of varying complexity that differ according to their evaporation temperature.
Properties of various types of gasoline
As seen in the section on improving combustion engine efficiency, gasoline or spark-ignition engines work by propagating a flame lit by the spark plug. Since the air-gasoline vapor blend is subject to increasing temperature and pressure, autoignition must be prevented. In fact, if autoignition doesn't occur at the optimal moment in relation to the movement of the pistons, engine knocking occurs, which leads to a drastic reduction in efficiency and may even destroy the engine.
The main property of gasoline is therefore its resistance to auto-ignition, as indicated by its octane rating. The higher the rating, the more resistant the fuel is to auto-ignition. The octane rating of gasoline has continually improved throughout the 20th century, particularly in response to the request of automobile manufacturers looking to improve engine performance by increasing the compression ratio. But increasing the compression ratio without provoking auto-ignition required simultaneously increasing the octane rating of gasoline.
The octane rating thus went from 50 in the early 20th century to 80 in the 1950s, thanks in large part to the use of leaded additives. Those additives were then eliminated to allow for the use of catalytic converters. (Leaded substitutes are now available in gas stations to protect the valve seat inserts of old engines, especially collector cars.)
Finding the optimum octane rating for specific engine fuels was made easier thanks to the development of electronically controlled ignition in the 1990s.
Now most fuel in Europe is unleaded with an octane rating of 95 or 98. Since the 1990s, all engines have run on SP 95 gasoline, with very few exceptions. Thanks to electronic combustion control, using SP 98 can increase fuel efficiency to a point that offsets its higher cost. However this is not a given, as an engine that is designed for a specific octane rating can run on a fuel with a higher rating without necessarily improving its efficiency. On the other hand, using fuel with a lower rating than recommended by the manufacturer can have severe consequences.
Octane is the main component of gasoline. It's a saturated hydrocarbon in the alkane family with the following formula: C8H18. The simplified equation for combustion can be written as:
2C8H18 + 25 O2 → 16 CO2 + 18 H2O
Properties of diesel fuels
Since the end of the 19th century, diesel fuels have developed alongside gasoline. The combustion quality of diesel fuel is mainly defined by its cetane number. A high number ensures good control over diesel combustion and therefore efficiency, noise, and polluting emissions. The redevelopment of diesel fuels led to an almost total elimination of sulfur: in Europe and the US, the level of sulfur went from 3,000 to 10 ppm between 1990 and 2010! The presence of aromatic compounds was divided by 3.5, while the cetane number stayed above 50.
These advances in diesel fuel composition, associated with the revolutionary engine innovations in the 2000s (like direct common-rail injection under very high, electronically-controlled pressure) led to a drastic reduction in emissions of nitrogen oxide (NOx) and particulate matter.
Another characteristic of diesel fuel is that it freezes in cold weather. Countries with very cold weather have adopted standards establishing the cold filter plugging point of diesel fuel. For example -30 °C in the coldest European regions.
Cetane is a saturated hydrocarbon in the alkane family with the formula C16H34.
Comparison between gasoline and diesel fuel
Ratio of CO2 density
/ fuel density