Abstract
In a diesel engine, sulfuric acid derived from fuel sulfur becomes a water solution at the cylinder wall. This solution is emulsified into the engine oil and the sulfuric acid is neutralized by base contents of overbased detergents in the oil. In this paper, neutralization of overbased detergents with sulfuric acid is discussed in contrast with the neutralization in JIS HCL Method (ASTM D 664) reaction system (reported in 1st paper).
The neutralization behaviours of overbased detergents with sulfuric acid were estimated by a Neutralization Rate Estimation Device. Fig. 1 is the comparison of fresh diesel engine oil behaviour to that of used diesel engine oil. The used oil shows higher neutralization rate. This is completely different from JIS HCL Method behaviour. From the micrograph of reacted oil (Fig. 2), a reaction model of base particles with sulfuric acid emulsion is proposed as shown in Fig. 3. From the model, following items are presumed to be the factors which influence reaction rate: (1) emulsion particle size (2) intensity of adsorption film (3) base particle size.
Fig. 4 is the relation between reactivity and emulsion surface area per unit volume, which depends on emulsion particle size. As expected, smaller particle emulsion shows higher reactivity. Table 2 shows the influence of ashless dispersants added to a detergent oil. Fig. 5 is the micrograph of sulfuric acid emulsion in ashless dispersants added oils. These investigations infer that ashless dispersants have influence on reactivity through affecting emulsion particle size and adsorption film intensity. Fig. 6 shows the effect of base particle size on reactivity. Larger particles show higher reactivity. This behaviour could be elucidated by the contribution of particle size to the energy potential of interaction between two particle [Eq. (1), Fig. 7] and also to the kinematic energy.
In the reaction of base contents with sulfuric acid, it was found that non-carbonate base contents preferentially reacted with sulfuric acid as shown in Figs. 8 and 9. Difference in particle size was pointed out as one of the reasons.
