Lubricant Additives

198 Lubricant Additives: Chemistry and Applications

Basically, two options to reduce friction and improve fuel effi ciency come forward [7,8].

1. Use of low-viscosity engine oils (SAE 0W/5W-20/30) when fl uid lubrication (EHL) is the
   governing factor [9–11]. Fluid lubrication is especially prevalent in the bearings. The grad-
   ual reduction of engine oil viscosity over the years has already brought signifi cant fuel
   savings (see Figure 7.3).
   In the preceding case, oil selection is crucial. In terms of frictional characteristics,
   one must emphasize low kinematic viscosity, high viscosity index, low “high-temperature,
   high-shear” (HTHS) viscosity, and a low-pressure/viscosity coeffi cient [12,13]. However, it
   has to be realized that other base fl uid properties, such as volatility and thermal/oxidation
   stability, must not be ignored.


2. Addition of friction-reducing agents when BL and ML are the governing factors [14].
   These are prevalent in the valve train and the piston group.
   In the preceding case, additive system design is the crucial factor. One must empha-
   size selecting proper FMs and controlling additive–additive and additive–base fl uid
   interactions.

To assess possible fuel economy improvements in the engine sequences prescribed, an overview of
the lubrication regimes existing in various test engines is provided. The data those used for current
and previous ILSAC specifi cations are given in Table 7.1.

FIGURE 7.3 Relationship between SAE viscosity grades and fuel savings based on fl eet car trials.

10W-40

15W-50

5W-30

15W-40

20W-50

8

6

4

2

0

6 81012 14 16

High shear viscosity at 100°C, 4. 105 s-1 (mm/s)

Relative fuel savings (%)

TABLE 7.1

Lubrication Regimes in API Sequences VI and VIA

API Sequence VI (%) API Sequence VIA (%)

Boundary lubrication 37 24

Mixed lubrication 15 4

Elasto-hydrodynamic

lubrication

48 72