**11. Wear**

Automotive fuels protect some moving parts such as fuel pumps and injectors from wear. This is achieved by forming a layer between the moving parts thus not allowing the metal to metal contact which would otherwise result, and it is measured by the lubricity test. As gasoline fuel is composed of shorter chains this ability to form a layer between moving surfaces is less efficient. However, spark ignition engines inject the gasoline fuel and air mixture upstream of the combustion chamber and thus operates at low pressures. However, gas oil fuel is injected directly in the combustion chamber containing compressed air. Thus, compression ignition engines require a much higher degree of lubricating property. In fact lubricity is only tested for gas oil fuels with ASTM D6079 using a high frequency reciprocating

rig (HFRR) technique. This uses a laboratory rig to measure the effective wear than can be expected by determining a wear scar diameter in microns (μm). The specification is a maximum wear scar diameter of 460 μm (microns) at 60°C.

The ability of automotive fuels to form a protective layer is given by their propensity to stick to the metal surfaces. Therefor it is only natural to expect polar compounds to have this property as non-polar hydrocarbons would not have any means to interact with the metal surface. In gas oils the most polar compounds are oxygen and nitrogen containing compounds. Lubricity does not tend to be an issue for high sulfur gas oil fuels derived from atmospheric crude oil distillation as these compounds were providing adequate protection.

However lubricity issues were first noted when ULSD was first used in 1991 by sulfur removal via hydrodesulphurization (HDS) in Sweden. It was discovered that while HDS removed sulfur (to the benefit of the environment) it also attacked these oxygen and nitrogen containing compounds which imparted lubrication resulting in pump and injector failures. This required suitable additives to be added to the gas oil to restore the lubrication. It is to be noted that these polar additives must be injected at moderate doses to reach the specification limit as if their concentration is too high they can have adverse effects such as fuel injector deposits, water separation problems, or premature filter plugging.

Gasoline fuel is a blended product from a variety of sources such as isomerate, reformate, alkylate, dimate or polygasoline and catalytic naphtha which do not contain polar molecules. Thus, such blends are bound to have a lubricity higher than the gas oil specification mentioned. Even though lubrication is less important for spark ignition engines it could still cause long term wear. However, antiknock properties is conveyed by metal containing additives or by oxygenates. Both of these also impart the minimum lubricity needed. This parameter was tested in gasoline fuel and found that anti-knock additives like the ones used in LRP gasolines in 2000 enhanced the lubricating ability of these gasoline fuels (lower wear characteristics that bulk gasoline samples). Also, MTBE and ETBE lower the wear with optimum percentage in commercial gasolines around 8 to 9% per volume. Gasoline lubricity is a complex phenomenon, involving many complicated and interrelated factors, such as the presence of water, oxygenates diolefins, diaromatics, the effect of viscosity and the synergistic effect of different wear mechanisms. The lubricity mechanism of gasoline is quite different from that of diesel fuels that leads to severe adhesive wear. With low-sulfur fuels, adhesive wear is observed instead of corrosive and mild oxidative wear, and deposits build up on top land.

Metallurgy and mechanical properties of test specimens have important effects on the lubricating mechanisms of fuels. When the hardness of the lower specimen in an HFRR test is not enough to support the generated oxide films formed by the reaction between surfaces and dissolved oxygen and the adsorption films formed on top of the oxide films by gasoline polar impurities, severe adhesion and metal transfer occur.

Lubricating properties are also influenced by the relationship of viscosity to applied pressure termed as 'α'. The α-values of toluene and normal alkanes in gasoline boiling range are significantly lower than those of corresponding diesel fuel components. The α-values of these gasoline components are almost temperature independent in moderate temperature. In contrast, the α-values of long chain hydrocarbons (diesel fuel components) decreases significantly with increasing temperature. In general, the α-values of gasoline and diesel fuel obtained using film thickness results are about 20 percent lower than corresponding average α-values of their main components.

Although the variation of film does not seem to be related to the value of the corrected wear scar diameter in the case of the mixtures of gasoline refinery streams, some of the categories of the refinery streams separately present the value of the corrected wear scar diameter (CWSD1.4) to decrease exponentially with increasing

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

of the lubricating film, such as the refinery streams from the isomerisation, reformation and dimerization unit. Generally there are two areas of film variation with the corrected wear scar diameter, where in the first region is observed a relative reduction in film increasing the corrected wear scar diameter to 900 μm, while for higher values of the corrected wear scar diameter is observed a relative increase of film, but there is not a sufficiently high correlation observed. It was carried out numerical analysis for finding the optimum rate of addition for the 7 different refinery streams according to the maximum and minimum addition, as it was obtained from the data of 36 gasoline blends. The optimum addition rate is following the mixing rules that prevail in modern refineries and specifically an optimum rate of 35% for the stream of catalytic cracking FCC unit and 26% for the stream of catalytic reformer unit were observed. The methodology used to measure gasoline lubricity is the ASTM G133 standard test method for linearly reciprocating ball-onflat sliding wear [6] (**Figure 18**).

The lambda ratio (λ) is the ratio of minimum lubricant film thickness (h) to composite surface roughness (σ) which correlates with surface contact fatigue. The interface between mixed and boundary lubrication is far more questionable. In 1990, Schipper demonstrated that some micro-EHD (elastohydrodynamic) occurs in lubricated concentrated contacts even down to a lambda ratio value of 0.03. More recently ultra-thin film interferometry in conjunction with friction measurements have demonstrated that the transition from mixed to boundary lubrication can occur at even lower lambda ratio value, probable approaching to 0.01.

For the diesel fuels, friction/film thickness shows the classical Stribeck behavior and illustrates that full, speed-independent friction is reached with in the film thickness range studied. (The further drop in friction at high speed is generally considered to result from heating of the EHD film in the contact). For gasolines, the friction/film thickness plots show no sign of leveling at high film thickness.

The composite surface roughness in the MTM test was approximately 30 nm, so the maximum film thickness reached of 20 nm corresponding to lambda ratio of about 0.7. This indicates that the gasolines are probably not reaching full boundary lubrication even at the highest speed -they are still in the mixed regime [6, 7].

From 1990's fuel quality improvement has become the most important subject for automobile industry, because it is the key factor for the energy saving and the reduction of CO2, one of the most harmful greenhouse effect gas. In order to meet this requirement, energetic research activities started to establish the direct injection gasoline engine technologies that can be applied to the practical engines in the real world. It has been a common target of research activities to develop a direct

**Figure 18.** *Variation of friction coefficient with film parameter (*λ*).*

injection gasoline engine realizing greater fuel economy compared with a diesel engine at partial loads and to realize better performance than the conventional MPI (Multi Point Injection) engines at high loads. In order to realize its fuel economy potential, the direct injection gasoline engine should be operated unthrottled in an extremely lean condition by distinctively stratifying the charge. In order to achieve its higher performance potential at high loads, the direct injection gasoline engine should be operated under stoichiometric or slightly rich conditions. When the charge is stratified, soot is generated in the rich zone. Sufficient excess air should be provided around the combustion zone containing soot, in order to burn-up the generated soot. Therefore, when the average mixture strength is stoichiometric or slightly rich, that is when the equivalence ratio is larger than unity, the mixture should be homogenous to suppress the soot formation. Additional pressure through pump and injection might generate issues on gasoline lubricity in the future [8].
