The quality of fuels provided for ships today has deteriorated, so combustion process have to be regulated and monitored with ever increasing attention. As fuel qualities have deteriorated it is to be expected that they will include more and more elements that are either non-combustible or so difficult to ignite and burn that they form no useful part of the combustion process. In fact, they frequently deposit out as harmful substances where fuel pump and cylinder liner wear is concerned.
The more aggressive of these are the compounds of vanadium, sulphur, and other chemicals natural to the oil. There are other contaminants that may be picked up by the oil from the refinery storage tanks or indeed the ship’s tanks. These include water (salt or fresh), and other products, such as scale, from tank and pipe walls. In some cases the contaminants can be removed during purification of the oil on the vessel; water, solids and even a proportion of sodium can be removed. Unfortunately, the more oil-soluble ones, like vanadium and sulphur, are not removed through purification even if some degree of water washing is applied. It is worth nothing that the aggressive nature of the vanadium products of combustion are further magnified when burned in the presence of sodium, particularly when the ratio of vanadium to sodium is greater than 3:1. The removal of sodium at the purification stage can thus be seen as a great benefit even though sodium may not be an original contaminant of the oil, it may be picked up by the oil from sea water or in lesser amounts from the salt laden atmosphere (possibly via the turbocharger),
The most important stage of combustion is the original ignition. Any fuel starts to burn (in the absence of some spark or flame) only when it is raised to a temperature greater than its self-ignition temperature (SIT), and, once ignited, the volatility of the fuel will then dictate the speed of combustion throughout the fuel. Temperature, in a compression ignition engine like the diesel, is transmitted to the fuel from the air in the combustion chamber, the air itself having being raised in temperature during the compression stroke. It is worth nothing that the large bore engines have relatively low compression ratios, probably in the region of 11:1, when compared to medium speed engines where compression ratios of 16:1 or higher are not unusual. Higher compression ratios still are designed into high speed engines. The reason for such a low compression ratio in the bigger engines is the limit to which the piston crown and other components forming the combustion chamber can be loaded. That is, cyclic high loadings on such large areas as the piston crowns will cause them to fail through fatigue, so the peak pressures have to be limited to more acceptable levels. This is achieved, whilst still developing a reasonable mean effective pressure, by limiting the compression pressure to as low a value as practicable. Any further reduction in compression ratio, and hence compression pressure, would not raise the temperature of the air sufficiently by the end of compression must be high enough to ensure certain and rapid heat transfer to the injected fuel.
To this end large engines should be circulated with heated jacket water prior to their being started, to ensure that during the first part of compression the air in the cylinder will absorb heat from the warmed cylinder liner, piston crown and cylinder cover. As compression continues there will be a natural increase in temperature due to the compression itself, and the terminal temperature of the compression will be much higher than that which could be achieved by simply compressing the air from scavenge temperature. Thus the ‘warming’ through of an engine is not purely to reduce the effects of thermal shocking which occur once the engine has started, but also, and probably more importantly, to ensure crisp ignition of the injected fuel. Once the engine is running the problem of ignition will be reduced, because the running temperatures of all the components will rise.
High quality fuels with their earlier ignition cause high peak pressures. The use of a peak pressure can therefore be used to monitor the quality of a fuel, assuming that the original timing of the fuel pumps was correct.