**3. Gas oil automotive engine**

The gas oil automotive engine injects only air at the start of the cycle. This is compressed and consequently the temperature also rises. Ignition is induced by timing of the fuel injection in the hot air already in the combustion chamber. The fuel can be heated by a glow plug before injection to enhance efficient combustion. As the air temperature and pressure inside the combustion chamber are designed to be above the fuel's ignition point, spontaneous ignition of the most combustible components commences. This in turn induces combustion in the balance of the components. This is called compression ignition engine and relies on the spontaneous combustion as given temperatures and pressures.

Note that while spontaneous ignition is not desirable in spark ignition engine, it is the basis of compression ignition engines. The ideal diesel cycle is shown as follows together with the relative P-V diagrams:

Intake stroke is performed by an isobaric (same pressure) expansion of air represented by point 1 to point 2 (**Figure 7**).

The air is compressed adiabatically (no heat loss) causing a rise in pressure and temperature from point 2 to point 3 (**Figure 8**).

The fuel is injected and point 3 to point 4 represents a constant pressure heating following injection. This is caused by the initial combustion of the fuel which is a slow process compared to gasoline in the Otto cycle. Thus, an isobaric process is observed as opposed to isochoric in Otto cycle (**Figure 9**).

When the remainder of the fuel ignites the products force the volume in the combustion chamber to increase leading to the power stroke. This is shown in point 4 to point 5 (**Figure 10**).

The exhaust stroke is characterized by isochoric (constant volume) cooling compression processes as per point 5 to point 6 (**Figure 11**).

**Figure 7.** *Drop of piston pulls in air.*

**Figure 8.** *Return of piston compresses air adiabatically.*

**Figure 9.** *Constant pressure heating.*

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

#### **Figure 10.**

*The power stroke: isentropic expansion.*

#### **Figure 11.**

*The exhaust valve opens as the piston reaches the bottom of its travel, dropping the pressure to atmospheric pressure.*

#### **Figure 12.**

*Rise of piston drives out burned gases. Exhaust valve closes at 1 and intake valve opens.*

And the cycle is completed by an isobaric compression as represented from point 6 to point 1 (**Figure 12**).

## **4. Combustion characteristics**

As seen in the previous section, the main fuel characteristic which dominates both spark ignition engines and compression ignition engines is auto-ignition. Note that other engine design factors also come into play for auto-ignition such as combustion chamber pressure, temperature and compression ratio but we will focus on the fuel factors.

Auto-ignition is responsible for knocking in spark ignition engines. Two types of auto-ignition are known. Knocking is defined when the fuel spontaneously starts to combust in another area of the combustion chamber and the flame front clashes with the spark plug induced advancing flame front while pre-ignition is caused by hot surfaces in the combustion chamber rather than the spark plug. While knocking is the more common of the two auto-ignition types they both lead to uncontrolled combustion which, if severe, can cause major engine damage in a spark ignition engine utilizing gasoline fuel.

On the other hand, auto-ignition is a desired quality for compression engine using gas oil fuels. As the fuel is injected the faster the fuel starts combustion the more time it will have to completely burn. The time needed from when the fuel is injected in the combustion chamber to when it starts burning is called the ignition delay. The shorter the ignition delay the easier it is to start the engine, the lower the combustion generated noise and the lower the exhaust emissions of a compression ignition engine using gas oil fuel.

As fuels are composed of a mixture of hydrocarbons, the auto-ignition characteristics of each class will determine if they are more adequate to be used in a spark ignition engine or a compression ignition engine. The classes of hydrocarbons which mostly make up gasoline and gas oil fuels are paraffins (including n-paraffins, iso-paraffins and cycloparaffins), olefins and aromatics.

Based on the reaction mechanisms presented by Curran, Gaffuri, Pitz and Westbrook [1], the main reaction pathways for long chained paraffins can be drawn schematically as shown below (**Figure 13**):

The main pathways are as follows (**Figure 14**):


**Figure 13.** *Reaction mechanisms presented by Gurran, Gaffuri, Pitz and Westbrook.*

**Figure 14.** *Main paths of reaction mechanisms.*

is called the negative temperature coefficient where the rate of reaction slowly increases in this temperature range.


Note that point 3. of this pathway requires a transition state of 5 to 7 membered structure. Thus iso-paraffins and cycloparaffins are less likely to form these transition states leading to longer ignition delays compared to n-paraffins (**Figure 15**).

Olefins tend to have a slower combustion reaction rates than corresponding paraffins. In fact at low temperatures the double bond induces an addition reaction first which slows down the overall rate. At higher temperatures hydrogen abstraction tends to occur in an allyl site which leads to stable allyl radicals which also slows down the reaction. The rate decrease with respect to n-paraffins depends on the chain length and position of the double bond.

Aromatics' combustion rates are even slower than olefins. This is due to the delocalization stability of the π bond ring which makes addition, substitution and extraction of a hydrogen (as per point 1) tough. Thus low temperature combustion reactions are not observed for aromatics. At sufficiently high temperatures, electrophilic substitution and H abstraction reactions create a phenyl radical. This reacts with HO2 radical, O2 or O atom, to produce phenoxy radicals. This is a stable radical due to resonance but it is an intermediate for ring opening reactions at high temperatures. Note that low temperature combustion reactions increases for aromatics with side chains as paraffinic reactions occur in the side chain.

Thus the auto-ignition characteristics for the most common classes of corresponding hydrocarbons can be summarized as follows starting from the most prone to least prone:

Paraffins iso paraffins cycloparaffins Olefi >− > > > ns Aromatics.

Thus paraffins are the most adequate for combustion ignition engines due to their propensity for auto-ignition. In fact gas oil fuel tend to have a significant proportion of this class of hydrocarbons. Aromatics are the most adequate for spark

**Figure 15.** *Main paths or reaction mechanisms – Carbonyl and -OH radicals.* ignition due to the lack of susceptibility of auto-ignition which make it ideal for gasoline fuel.

The cetane number test as per ASTM D613 is used for gas oils to measure the auto-ignition tendency. In fact a high cetane number indicates a short ignition delay from the start if injection to start of combustion. The higher the cetane number the longer the time available for complete combustion. Thus particulate matter and carbon monoxide emissions decrease.

Various derived cetane number equipments are available today (ASTM D6890, D7170, D7668 ad D8183) which mostly exhibit improved precision limits. The cetane index is an estimation of the cetane number by calculation from distillation and density. Note that cetane number additives are not detected by cetane index calculations as the distillation and density are not altered.

Cetane number additives are thus promoting auto-ignition. They are organic nitrates (mostly 2-ethylhexyl nitrate and alkyl nitrate) or peroxides which start to auto-ignite early and thus induces the hydrocarbons to follow suit [2]. This reduces the overall ignition delay and thus increases the cetane number.

On the other hand, the RON by ASTM D2699 and MON by ASTM D2700 are used for gasolines in order to measure the resistance for auto-ignition. RON is the anti-knock performance at lower engine speed and typical acceleration conditions while MON reflects the anti-knock performance of a fuel under high engine speed and higher load conditions.

RON and MON additives are intended to inhibit auto-ignition which cause knocking. These additives decompose in the combustion chamber into a metal, metal oxides and hydrocarbon radicals which have a very limited lifetime. The metal and metal oxides role are to scavenge any radical intermediates which stops the auto-ignition in its tracks. Most common additives are based on lead, manganese and iron. Nowadays however the use of these additives is being limited on health grounds and are replaced by higher octane oxygenated components in the gasoline pool.

### **5. Energy density**

Energy density is the amount of energy stored per unit volume when the fuel is burned. The most influential factor for the determination of net calorific value for both gas oil and gasoline fuels is the density as this determines how much mass it available per unit volume. In fact, the fuel's density has a direct effect on engine maximum power output and volumetric fuel consumption. If density is reduced, heating value per volume decreases and engines need higher fuel volume in order to provide the same energy output.

Densities of different classes of hydrocarbons are influenced by:


Intermolecular attractions in n-paraffins are the relatively weak Van der Walls forces. These increase with chain length as there is more surface area per molecule and thus the density increases with carbon number.

Isoparaffins also have Van der Walls forces. These tend to be weaker than in n-paraffins as for an equivalent carbon number they have less surface area. Thus the densities of iso-paraffins are lower than for n-paraffins due to their three dimensional structure.

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


**Table 1.**

*Densities for comparison for the hydrocarbon classes discussed.*

Cycloparaffins also have similar forces to n-paraffins but they exhibit higher densities. This is due to stronger Van der Waals forces caused by their ring structure which allows for a larger area of contact. Their locked conformations also give an increased plane of intermolecular contact.

Olefins are characterized by the double bond. Since the relative π bond is more polarizable than the δ bonds, the Van der Walls attractions in olefins are augmented by polarizability. Thus, olefin densities are higher than n-paraffins but still lower than cycloparaffins which are enhanced by their structure.

Aromatics have a delocalized π bond system which further increases the polarizability and thus intermolecular attractions. Also, benzene rings must be flat in order to allow the p orbital overlap for all six carbons. This is a more compact structure than other hydrocarbons and contributes to increased density (**Table 1**).

Even though gasoline has got more aromatics (the highest density hydrocarbon) than gas oil the densities are lower due to the shorter chains. In fact, gasoline cuts typically range between C5 and C12 while gas oils range from C10 to C25. The requirement to narrow the density range of automotive fuels is driven largely by engine manufacturers to improve fuel economy and combustion through fuel management systems by regulating the fuel to air mixture to values at least 20% lean of stoichiometric. Lower density would imply a lower energy density and thus poor fuel economy and rough engine idle. A higher density would result in increased emissions due to incomplete combustion. Reductions in the upper limit have generally been for the purposes of limiting heavier aromatic components, thereby reducing emissions, principally particulates. This effect is also achieved to some extent by control of the high end of the distillation curve. The higher content in aromatics in gasolines lead to higher octane numbers (better combustion) and in contrast the higher content in paraffins leads to higher cetane numbers (better combustion and less harmful emissions in general).
