**12. Composition**

In a refinery process the atmospheric and vacuum distillation are the first steps for separation of crude oil into various fractions or "cuts" which are composed of large numbers of hydrocarbons. These cuts have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residuum. Modern refineries are much more complex and are able to extract a variety of streams from cracking processes. These are carefully distilled to fit into the required cut boiling points making this parameter as a primary selection criteria for automotive fuels.

Both the volatile and heavy components in automotive fuels are needed and thus a distillation curve is tested by ASTM D86 to ensure that all the requirements are met. The distillation curve is the temperature vs. percentage volume recovered. It is related with the volatility and flash point of the fuel initially and with density at the back end as the high boiling point components tend to be long chain and/or aromatic which increases the density.

The distillation curve can be split in three for a better understanding [9]. The front end is typically the first 25% recovered, the next 60% is the mid-range while the final 15% is the back end which are all regulated (**Figure 19**).

If the front end is too volatile it would incur is evaporative losses and flash point issues for gas oil fuel and high RVP for gasoline fuel. If gasoline fuels are too volatile they could also vaporize in the fuel lines when the engine is hot impeding fuel flow (known as vapor lock). If less volatile combustion on a cold day would be problematic as the fuel would not vaporize easily.

The bulk of the curve is the mid-range. If this section is too volatile the majority of the chains would be composed of shorter chain molecules and thus would tend to have a poorer cold properties for gas oil fuels and increased icing tendency for gasoline fuels. If it is not volatile enough it would release less chemical energy and it would incur in poor warm-up time, rough acceleration and poor short trip economy.

The last section deals with the heaviest components. If these are too volatile poor long trip economy would result as these give the highest combustion chemical energy release. If this is not volatile enough these heavy components would result in increased incomplete combustion that would give soot or smoke in the exhaust emissions. These heavier components have more potential for incomplete vaporization and combustion and limiting the high-end temperatures reduces their proportion giving cleaner burning. The products left after incomplete combustion can cause oil dilution and increased cylinder wear and may lead to combustion chamber and inlet system deposits and spark plug fouling in spark ignition engines and injector deposits in compression

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

**Figure 19.** *Chart illustrating the effects of having more or less volatile components in the gas oil pool.*

ignition engines. Note that the end point in a gas oil fuel is related to the polyaromatic hydrocarbon content which typically have the highest boiling points.

## **13. Environmental aspects**

Environmental specifications include RVP in gasoline to control evaporative emissions. The specifications recognize that the ambient temperatures varies in different regions and thus allows for various ranges.

Another environmental aspect which has been regulated is the sulfur content. Sulfur in automotive fuels is converted to sulfur dioxide and small amount of sulfur trioxide. If released to the atmosphere, they will form acid rain. Sulfur trioxide and dissolve in water to form sulfuric acid that will cause engine corrosion. However, the main contributor to sulfur reduction in automotive fuels is environmental as only 1 to 3% of sulfur dioxide is oxidized to sulfur trioxide in the combustion chamber. Sulfur occurs naturally in crude oils and must be removed to an acceptable level during the refining process by hydrodesulphurization (HDS) as previously discussed for effects on lubricity. The current specification for both automotive fuels is set at 10 ppm.

Sulfur in automotive fuel reduces the efficiency of catalytic converters as sulfur dioxide inhibits most gaseous heterogeneous catalytic reactions by strongly competing with the exhaust pollutants for space on the active catalyst surface [10].

Developing gasoline engine technologies such as gasoline direct injection (GDI) and lean burn will require advanced catalyst technology in order to control of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) exhaust emissions [11]. Sulfur inhibition varies in degree according to the gasoline sulfur level, the catalyst formulation, catalytic function, combustion products from various air/fuel mixtures, and exhaust temperature range.

In compression ignition engines the diesel oxidation catalyst (DOC) is a fundamental device of exhaust after-treatment. Reduction of the NOx emission is achieved mainly by a significant delay of the fuel injection and using the high rates of the exhaust gas recirculation. However, these cause a significant increase in the emission of products of the non-complete combustion process i.e. carbon monoxide, hydrocarbons and particulate matter. In order to keep the emissions low it is necessary to eliminate such compounds by using the catalyst DOC which is able to oxidize CO and HC and the soluble organic fraction (SOF) of the particulate matter.

Thus, apart from its direct environmental detrimental effects, sulfur also contributes to formation of particulate matter (PM) in engine exhaust and affects the performance of vehicle emissions control equipment. It therefore has an indirect effect on emissions of carbon monoxide, hydrocarbons and NOx.

In gas oil fuels another environmentally regulated parameter is the polyaromatic hydrocarbons. These aromatics contain multiple benzene rings and their boiling points are in the gas oil range. PAHs contribute to particulate emissions while some PAHs such as benzo(a)pyrene are known to be carcinogenic. These tend to be the heaviest components in gas oil and are the slowest to burn due to their extended delocalization over several benzene ring systems. Thus, they contribute the most to incomplete combustion and thus to hydrocarbon and particulate matter emissions. The current specification has reduced the limit for PAH from 11mass% to 8mass% maximum which are tested using an HPLC technique. An additional indirect way to control PAH in gas oils is to limit the end point of the distillation curve. In fact, as PAH are the heaviest components in gas oil fuel, limiting the end point will also reduce the PAH content.

#### **14. Renewable components**

Mineral oil gas oil and gasoline fuels are derived from crude oil. This is a fossil fuel because it was formed from the remains of tiny sea plants and animals that died millions of years ago and thus cannot be regenerated. On the other hand, renewable components are any type of fuel with can be replenished and thus will not be depleted within the foreseeable future. They are part of a closed cycle and do not create a net surplus of carbon dioxide greenhouse gases when used. The most common are biodiesel and ethanol. Biodiesel is blended with gas oil fuel while ethanol and MTBE (as well as other oxygenates) are blended with gasoline fuel.

All renewable fuels contain oxygen. Thus, their combustion characteristics are significantly different from mineral oil derived fuels.

#### **Ethanol**

Ethanol is one of the main octane boosters used due to its high knock resistance. The presence of a hydroxyl group on the chain affects otherwise paraffinic chemistry scheme. The main combustion pathways are (**Figure 20**):

Hydroperoxyalkyl (RO2) radicals can now, besides isomerization to QOOH and subsequent low temperature branching reactions as for paraffins, also react via three other routes that compete with the chain branching reactions (**Figures 21**–**23**).

#### **Figure 20.**

*H atom abstraction favors C*α*-H, forming* α*-hydroxypropyl sites radicals.*

#### **Figure 21.**

*The first involves HO2 concerted elimination of* α*-hydroperoxyalkyl radicals, including one via a 5-membered transition ring, forming enol and HO2.*

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

#### **Figure 22.**

*The second reaction concerns the formation of* α*-hydroperoxyalkyl directly form carbonyl and HO2.*

#### **Figure 23.**

*The third and final route occurs via Waddington type chemistry for* β*-hydroperoxyalkyl and involves H atom transfers from C-OH to C*β*-OO radical sites, thereby producing two aldehydes and an OH radical.*

Thus, low temperature reactivity of alcohols is greatly reduced by the presence of the hydroxyl group as radicals first have to form the enol or aldehyde/ketone intermediates. As for olefins this slows down the overall reaction rate. Branched alcohols with less paraffinic chain will have longer ignition delays and thus be the preferred octane booster.

When blending ethanol in gasoline particular attention must be given to RVP. Ethanol has a relatively low RMM but it is a liquid at ambient temperature due to a type of intermolecular force that is possible due to the hydroxyl radical. This is the hydrogen bonding where a hydrogen on one molecule is attracted to the lone pairs of an oxygen on another molecule. This is clearly seen when comparing the RVP of ethanol (having a molar mass of 46.07 and an RVP of about 2 psi) to a paraffin such as butane (having a molar mass of 58.12 and RVP of 51.7 psi). However, when blending ethanol in gasoline below 10%vol, the ethanol molecules are now separated by hydrocarbons and are thus unable to interact via the hydrogen bonding. Thus, ethanol molecules become very volatile and the RVP of the blend is higher than the highest component. When the ethanol percentage increases above 10%vol the ethanol molecules start encountering each other more often and hydrogen bonding starts to be restored thus lower the RVP again.

In order to accommodate increased ethanol use as an octane component both EPA and EN228 introduced ethanol waivers. These give RVP allowances on the upper limit in view of this occurrence. As RVP is related to the distillation, ethanol will also decrease the distillation points mostly up to the back-end section. Because of its effect on distillation it affects the driveability index (intended to control cold start and warm-up driveability) and the vapor lock index (intended to protect against excessive volatility in the lines, pumps and carburettors impeding flow).

Also, ethanol has an affinity for water as both molecules interact with hydrogen bonding. Thus care must be taken not have excessive water in the distribution system and storage [12]. If a gasoline/ethanol blend encounters excessive water, it can pull the ethanol out of the blend resulting in tank bottoms comprised of water, ethanol, and some hydrocarbon content. When the water is drained out this will result in a volume loss as well as an octane loss.

Finally ethanol in gasoline blends may cause the elastomers (namely Neoprenerubber, Nitrilerubber, hydrogenatedNitrilebutadienerubber (HNBR), and Polyvinylchloride/Nitrilebutadienerubber blend (PVC/NBR)) and two types of plastic materials (namely Nylon-66 and Polyoxymethylene) in vehicle fuel systems to swell and lose strength [13]. This would lead to failures of critical components such as fuel pumps, engine seals, gaskets, fuel system seals and hoses and promote risk of fire.
