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Sustainable MobilityTechnical and environmental challenges for the automotive sector

Week 2 Session 2 Fuels (Part1)

Ludivine Pidol

IFPEN / IFP School 2014

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We will see together the main properties of fuels: both gasoline and Diesel fuels.

Fuel specifications

Around the world, fuels are subjected to certain specifications. This means that fuels must comply toconstraints in terms of properties and performances.

First, specifications guarantee a minimum quality for the user.

Specifications also allow harmonization of fuel properties within a large territory. For example, in

Europe, more than thirty countries comply with the European specifications. For gasoline, the name

of the specification is EN228 and for Diesel fuel, it is EN590.

In China and in India, there are some specifications for the whole country, their names are indicated

on the map for gasoline and Diesel fuel respectively. Moreover, in the biggest cities such as Beijing,

Shanghai, Guangdong, Delhi, Mumbai, Kolkata, etc, there are some specific requirements for fuels

due to the high level of pollutants in these towns.

Indeed, fuel specifications help to control pollutant emissions. By improving fuel properties, it is

possible to reduce pollutant emissions.

The goal of a fuel specification is to ensure the quality of the fuel throughout its chain of use, from

the fuel production, until the vehicle emissions. This diagram presents the main steps of fuel use.

First, we have the storage of the fuel in a fuel station or in the vehicles tank.

Then, the fuel is injected into the combustion chamber, and during the vaporization process, it is

mixed with the air.

Next, there is the combustion step and finally the emission of pollutants.

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In order to ensure the fuel quality all along this chain, specifications define minimum requirements

regarding some properties. For example, in the European Diesel fuel specification EN590, 21 criteria

are specified.

In this lesson, we will focus on 7 main properties which are defined for gasoline or Diesel fuels:

the density: this property affects all the steps of the fuel use;

the volatility and especially the distillation;

the cold flow properties. That means the behavior of the fuel at low temperatures;

the Octane number and the cetane number. These properties strongly affect combustion.

Octane is for gasoline whereas cetane is specific to Diesel fuel;

the sulfur content, which affects emissions;

the energy content: this property is not in the gasoline or Diesel fuel specifications but it is

very relevant information as it is directly linked to fuel consumption.

Density

The density is the weight per volume; the unit is kg per cubic meter.

A certain density range is necessary for satisfactory vehicle operation. Automobile manufacturers

take this into account when designing fuel injection systems and when choosing the appropriate flow

rates for the various components such as the injectors.

Concerning fuel consumption, when density increases, it means that the fuel contains more energy

per liter. So, the driver can do more kilometers. Higher density leads to lower fuel consumption.

In terms of emissions, it is well-known that heavy fuels contain some chemical species, called

aromatics, which are precursors of soot. Indeed, when these heavy chemical compounds are burnt in

the combustion chamber, they can lead to the formation of particles.

For all these reasons, we need to define a density range in the fuel specification.

The Density Ranges for gasoline and Diesel fuels are presented here for Europe.

It is obvious that these 2 fuels are really different in terms of density.

Diesel fuel is heavier than gasoline. That is one reason why Diesel engines usually produce more

particles than Spark Ignition engines.

The specifications for gasoline density are fairly homogeneous all over the world. It is not the case

for Diesel fuels because the density range mainly depends on the climate of the country. Indeed,

Diesel fuels in cold countries are usually lighter than ones in warm countries.

Volatility

Volatility is described by the distillation curve for gasoline and Diesel fuel and also by vapor pressure

for gasoline.

The distillation curve is built using the laboratory test ASTM D86.



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It consists in warming the fuel and at each temperature step, on the Y axis of the curve, the volume

of fuel which is evaporated is measured. This evaporated volume is on the X axis of the curve. And

this gives us the distillation curve.

For example, the initial boiling point is the temperature where the liquid fuel starts changing into

vapor.

The final boiling point is the temperature where all the fuel is evaporated.

The distillation residue is the fuel which cannot be evaporated.

The first part of the distillation curve represents the light fractions of the fuel. For gasoline: the

volatility directly impacts the start-up of the vehicle and its drivability.

Volatility requirements also control the emissions by evaporation. For instance, the light fraction of

the gasoline can lead to emissions by evaporation from the tank or from the pump for example.

The last part of the distillation curve represents the heavy fractions of the fuel. This heavy fraction

can affect the atomization of the fuel into fine droplets and consequently, can degrade the injection.

Finally, when the heavy fraction decreases, the combustion quality can be improved and the exhaust

emissions decrease.



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These curves give a comparison between distillation ranges of gasoline and Diesel fuels.

For gasoline, the distillation range is between approximately eighty and two hundred degrees

Celsius; whereas for Diesel fuel, it is between one hundred and eighty degrees and three hundred

and sixty degrees Celsius. So, the gasoline is lighter than the Diesel fuel, with lower boiling

temperatures.

Cold flow properties

The third important property concerns the cold flow behavior of Diesel fuels. To ensure cold

operation and start of the Diesel vehicle at low temperatures, it is crucial to master the behavior of

Diesel fuels at low temperatures.

Some Diesel fuel compounds, called waxes, may crystallize at low temperatures and consequently,

clogg the filter as presented on this picture. This picture was obtained with a Diesel fuel, especially

made to produce the worst case in terms of cold flow properties.

The diesel injection can be delayed and the fuel flow can be irregular, leading to power loss.



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In the case of complete filter clogging, the engine stops and you can have strong damages to

sensitive fuel equipment such as the injector.

Cold flow properties of Diesel fuel are described by the cloud point and the Cold filter plugging pointCFPP.

For the Cloud point, a Diesel fuel sample is slowly cooled down and its visual aspect is observed.

When a sort of cloud is noticed, this is the cloud point. For example, in France, the limit is -5C in

winter and +5C in summer.

The Cold filter plugging point, CFPP, is the temperature at which it is too slow to filter a Diesel fuel

sample.



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Cloud point and Cold filter plugging are specified only for Diesel fuels. Their limits depend on the

countries, and their climate. They are not the same all over Europe for example. These limits also

change with the seasons. The cold flow properties depend on the Diesel fuel composition. In

particular, waxes are not good and they increase the risk of filter plugging.



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