From these results it is clear that the acid dew point temperatures are now calculated to be significantly higher. Thus, for a fuel with 3.5%m sulphur and a pressure of 180 bar at the specified set of conditions, the acid dew point may be as high as 280℃, i. e. nearly 100℃ higher than previously thought. These new values would be able to explain that in the 4RTX54 tests corrosive wear was reported at temperatures up to 270℃. Interestingly, theses dew points are close to those calculated with an empirical formula from Mueller that is based upon dew points of water and a correction factor for the presence of sulphuric acid [13].
It should be emphasised, however, that these calculated dew points represent temperatures at which condensation of acid is due to start in fully equilibrated systems only. The conditions in an engine are, of course, extremely dynamic and far from equilibrium, and this is a cause of much uncertainty. For instance, since the gas temperature in the cylinder is much higher than the relatively cold liner wall on which basis the acid dew point was calculated, SO3 in the hot gas is probably still mainly present as such rather than as gaseous sulphuric acid that participates in the phase equilibrium at the liner wall. This lower acid concentration, of course, will affect the dew point. Furthermore, at the dew point temperature the amount of liquid is still equal to zero, and no information is available on the actual mount of acid condensed close to the dew point temperatures, nor on the kinetics of this condensation. Another cause for uncertainty is the largely varying corrosivity of different sulphuric acid concentrations. It may well be that just moving the concentration of the condensate into a less harmful region is already sufficient to control corrosive wear, rather than to attempt to avoid condensation of the acid altogether [14].
In any case, despite its many limitations the new set of acid dew point curves is considered a valuable tool to assess the corrosion risk with each set of engine operating parameters.
5. SCUFFING WEAR
Although in the 4RTX54 wear study, virtually no adhesive wear was observed, there is a growing number of reports that today's highly rated large bore engines installed in container ships and in power stations are indeed susceptible to show scuffing. One reason for this discrepancy may be the relatively short duration of the 4RTA54 tests, which is of course very different from running an engine in actual operating circumstances, at sea or in a power station for many thousands of hours. During such long time a variety of conditions may occur that amongst others could develop into scuffing.
The consequence of high temperatures in an engine is a low oil film thickness from reduced oil viscosity which may then be incapable of providing a sufficiently thick oil film between ring and liner at the increased pressures. Under such conditions the surface asperities on the liner and ring surfaces make direct metal-to-metal contact resulting in local welding and metal deformation. When this contact takes place at local level at low rate, for instance in the running-in phase of new hardware components, it only results in increased wear rates. Conditions can also be such, however, that this adhesive wear gets out of control through collapse of the oil film in a larger surface area and this results in unstable and high wear which is usually referred to as scuffing. As a result liner surfaces can get severely damaged and piston rings can get micro seizures and scratches to various degrees (Fig. 7).
Figure 7. Scuffed Liner (8488 total run hours) and Piston Rings (3394 run hours since overhaul)
