Particulate Matter (PM)
The term particulate matter refers to the very complex set of solid particles and liquid droplets dispersed in the air, with a diameter between a few nanometres and a few tens of micrometres; it is traditionally divided into three fractions:
- Solid fraction (SOL)
- Soluble Organic fraction (SOF)
- Particulate matter from sulphates (SO4)
The solid fraction of the particulate matter is composed of carbon particles with a diameter of less than 1 mm, known as soot, which originate in those areas of the combustion chamber where there is more fuel and then they are not oxidized. Another component of the solid fraction of the PM is the metallic ash, which contains sulphates, nitrates, metal oxides and particles removed from metal parts of the engine due to wear.
On the solid and insoluble carbon particles, a layer of heavy hydrocarbons in the liquid state which constitute the soluble organic part of each particle (SOF) is deposited by condensation and absorption. These hydrocarbons essentially come from fuel and also from lubricating oil. On the graphitic nucleus, together with the liquid hydrocarbons, sulphates are also deposited with the addition of water. About 20% by mass of the particulate matter emitted by an engine powered by an HFO containing 3% by mass of sulphur is made up of soot; the remaining part essentially consists of sulphates and sulphates compounds with water, depending on the quality of the fuel. The ashes also contribute significantly to increasing the emitted particulate so that it is possible to distinguish a rate originated by the composition of the fuel and made of sulphates, water linked to them and ashes and a rate originating from the combustion process in the engine made of soot and hydrocarbons (HC).
Among the engine variables that have a significant effect on diesel engine emissions, and for which the optimal combination must be found, there are:
- the air / fuel ratio
- the injection time and the combustion phase
- the air temperature in the combustion chamber.
When the C/O ratio is unitary, there is enough oxygen for the carbon to be converted to CO and so there is no particulate matter formation. If there is more oxygen, C/O<1, the excess oxygen will be used in the conversion from CO to CO2. If oxygen is less, the PM begins to form, therefore from the stoichiometric point of view, particulate formation is expected when the C/O ratio>1. In reality the amount of oxygen actually available is less than the theoretical one, because in the combustion processes both CO and CO2 are formed; in fact, experimental measurements revealed the formation of particulates also for C / O values close to 0.5.
In diesel engines, the fuel, in the form of droplets, and the air must be mixed in a very short time, therefore a considerable excess of air is necessary for the combustion to take place completely. At an excessive value of the air/fuel ratio, the temperature in the cylinder after combustion is low to burn the residual liquid hydrocarbons, and then the soluble organic fraction of the particulates is high. At a low value of the air/fuel ratio, less oxygen is instead available for the oxidation of the soot, whose presence therefore grows.
The temperature of the air in the combustion chamber also directly affects particulate matter emissions. If the air temperature increases, its density decreases, so a smaller mass of air enters the combustion chamber; this decreases the ratio between air and fuel and therefore, as already mentioned, the soot of emissions increases. On the other hand, if the temperature decreases, the air-fuel ratio increases and therefore the hydrocarbons emissions.
Particulate matter particles have irregular shapes, therefore they are described with reference to the equivalent aerodynamic diameter, defined as the diameter of a spherical particle having unit density and aerodynamic behaviour (in particular the sedimentation rate) equal to that of the particle considered under the same conditions of temperature, pressure and relative humidity.
The concept of aerodynamic equivalent diameter is useful for the classification of the particulate matter in categories. In this sense, U.S. EPA (Environmental Protection Agency) defines:
- PM10: particles with an aerodynamic diameter less than 10 µm (coarse particulate);
- 5: particles with an aerodynamic diameter less than 2.5 µm (fine particulate).