**9. Contaminants**

The most common contaminants for automotive fuels are water and particulate matter which can be picked up in the distribution system.

Water can accumulate in the fuel due to leaks or condensation in pipelines. Since liquids are held together by intermolecular forces the hydrocarbon solvent molecules would have to overcome these forces in the water solute in order to find their way between the solute molecules. This process is most efficient when the intermolecular forces of solvent and solute are similar. As water is a polar molecule (while hydrocarbons are not polar) its solubility in hydrocarbons is limited. In fact, oil and automotive fuels do not mix well because the water molecules are strongly attracted to each other and will not allow the weakly attracted oil molecules between them.

Although the degree of water solubility depends on composition, the bigger the density variance the lower the solubility is expected. For example, water solubility in unoxygenated gasoline fuel is expected to be lower than gas oil fuel with


#### **Figure 17.**

*Main paths of reactions for water detection in fuels.*

no biofuel as the hydrocarbon molecules are smaller and thus have less intermolecular forces when compared to the polar water. Water is thus not a contaminant which greatly effects gasoline, but it must be monitored for gas oil fuels as it may cause injector and piston groove deposits and corrode engine components. This is achieved by Karl Fisher technique where the reaction below is used to measure water. As water and iodine are consumed in a 1:1 ratio in the below reaction, when all of the water present is consumed, the presence of excess iodine is detected voltametrically by the titrator's indicator electrode. That signals the end-point of the titration (**Figure 17**).

Another source of contamination is particulate matter. This encompasses any solid material which finds its way to the automotive fuel during transportation. The most common would be rust particles which could plug filters and injectors and thus starves the engine of the required combustion energy for optimal operation. This parameter is tested in gas oil fuels gravimetrically by EN12662 or IP440 [4] where the fuel is filtered through a glass fiber filter of 0.7 μm porosity. Thus, particles larger than 0.7 μm are trapped and weighed in percentage of the fuel filtered.

Appearance by ASTM D4176 is quick indicator of water and particulate matter. A rough indication is that 200 ppm of water at 20°C is expected to be the threshold for water retention in gas oil fuel not containing biofuels. The test method also specifies to observe and report any particulates in the sample.

It is worth noticing that when oxygenates are added to hydrocarbon based mineral fuels the water retention is greatly increased. This is due to the increased polarity of the oxygenates such as esters (biodiesel) in gas oil which are better suited to interact with the water molecules. This increased interaction leads to a much higher water retention threshold for biodiesel to about 1500 ppm at 20°C. When the biodiesel is then mixed with mineral gas oil the overall blend could become hazy as the water retained in the biodiesel is not soluble in the mixture.

### **10. Safety**

During storage and transportation one of the main concerns is the volatility of the fuels. This is directly linked with the intermolecular forces acting on the various classes of hydrocarbons. As discussed in the Energy density section, aromatics have the strongest forces. However, gasolines are very volatile when compared to gas oil fuels due to their shorter chains.

At ambient temperature gasoline fuels are considered as volatile products. Thus, gasoline fuels are stored and transported in an inert atmosphere by removing oxygen and preventing combustion. The test which controls the volatility of gasoline fuels is the RVP which is normally tested by ASTM D5191. ASTM D5191 determines the total vapor pressure exerted in a vacuum by air-containing chilled, air-saturated, volatile, liquid petroleum products. This test method is performed at 37.8°C (100°F) at a 4:1 vapor-to-liquid ratio (internal volume that is five times that of the total test specimen introduced into the chamber). The sum of the partial pressure of the sample and the partial pressure of the dissolved air is obtained. The dry vapor pressure equivalent (DVPE) is calculated by removing the partial pressure exerted by the water vapor in the air.

#### *Quality and Trends of Automotive Fuels DOI: http://dx.doi.org/10.5772/intechopen.94167*

Alternative methods are available such as ASTM D323, D4953, D5190, D5482 and D6378. D323 is a manual procedure where a chamber is filled with air saturated sample and immersed in a bath at 37.8°C (100°F) until a constant pressure is observed. D4953 is a modification of D323 where the interior surfaces of the liquid and vapor chambers be free of water. Hence, this method is often referred to as the Dry Reid method. D5190 and D5482 are very similar to D5191, with the exception that the test chamber is not evacuated at the start of the test. D5482 employs a small volume test chamber. D6378 is also similar to D5191 with the exception is the fact that air-saturation and chilling of the sample is not required. The relative bias correlating D6378 to D5191 is also known.

RVP also affects starting gasoline fuels performance, warm-up, and tendency to vapor lock with high temperature or high altitudes. The RVP results can be converted to the true vapor pressure at ambient conditions by API MPMS Ch 19.4 or AP-42 by EPA formulae which is a prerequisite to determine if a storage tank or vessel tank are adequate for storing a volatile product.

As gas oil fuels are less volatile, the temperature at which they can ignite when exposed to a flame is tested. This is the definition of the flash point which is normally tested by closed cup Penske Martens by ASTM D93. Note that flash point has no relationship with the combustion characteristics and is used only as a safety measure. In general the temperature of the product is kept at least 10˙C below the flash point during storage and transportation in order to ensure that the product cannot combust.

Other concerns are also addressed by limiting the total aromatics in gasoline. Benzene is the simplest aromatic compound but others common in gasoline include toluene and xylene. As aromatics, particularly benzene, are known to be carcinogenic. They are limited in gasoline fuel and not in gas oil since aromatics are resistant to auto-ignition and cetane number specifications would not be met for highly aromatic gas oils. For the same reason they are desirable for gasoline fuels and thus they are limited as this is the most practical way of limiting human exposure to these substances from evaporative losses and in exhaust emissions.

Alternative high octane products are used in substitution of aromatics such as oxygenates and olefins. Both come with limitations though as olefins induce instability and thus adversely affect oxidation stability test while oxygenates can impact RVP. In fact mixing ethanol and mineral gasoline may increase evaporative emissions. The relationship between vapor pressure and ethanol content of a blend is non-linear as the hydrogen bonding previously holding the ethanol molecules together are greatly reduced when the ethanol is blended as mineral oil gasoline is devoid of oxygen. Thus, the mixture may have a higher vapor pressure than either product alone.
