Marine Oily Handling Devices and Pollution Prevention

Marine Oily Handling Devices and Pollution Prevention Chapter 2 Lesson 4 Fuel Impact on Diesel Engines

2.4 Fuel impact on diesel engines • The main objective of this section is to make you acquainted with the relevance of fuel parameters on: • Abnormal ignition and combustion. Wear, piston ring collapse and breakage. • Gas leakage. Ignition, combustion and exhaust gas emission. • Engine damages and operational problems. Prevention of engine damages with off spec. fuels and to recognize their impact on the operation of marine diesel engines. • Abnormal ignition and combustion

2.4 Fuel impact on diesel engines • Density: • Density of a fuel oil decreases with rise in temperature. • The density has no direct importance for the engine operating condition, but a high density indicates that a fuel contains heavy or complex hydrocarbons. • High density can lead to slow combustion and an increased degree of late combustion. • Slow combustion and late combustion will have the following effects on the engine: • Increased metal temperature (increased thermal loads). Reduced efficiency and thereby, increased specific fuel consumption, increased exhaust temperature, increased tendency for fouling, increased wear due to thermal overload of the oil film on the liners.

2.4 Fuel impact on diesel engines • Viscometers, regulators: • Viscosity is a measure of resistance to flow and is not an indication of the fuel quality. • Automatic viscosity controllers are recommended in order to obtain constant viscosity regardless of engine fuel demands. • It is of great importance to know the viscosity in order to select the right temperature, to ensure that the fuel oil viscosity is correct for atomization. • The viscosity meter is installed in the fuel supply line, to ensure the right viscosity regardless of the consumption rate.

2.4 Fuel impact on diesel engines • Viscosity depends on the fuel oil temperature, (decreases with increasing temperature), but will not change by fuel treatment. • Due to the cracking process, heavy fuel oil viscosity can not be used to indicate oil quality. • Fuel oil viscosity is important for determining the preheat temperature in order to reach the correct viscosity at the injection pumps. • Typical values at injection is in the range 10 to 20 cst.

Table 2.4 Viscosity/temperature relations

2.4 Fuel impact on diesel engines • Correct viscosity at the fuel injection pumps gives optimal injection and combustion. • Too high or too low preheating temperature causes problems to fuel injection equipment and mechanical loads. • It also affects the combustion process. With high density fuel it is advisable to use a high viscosity fuel. (Lower CCAI) • Inadequate preheating will influence combustion, cause increased cylinder wear and may result in too high injection pressure, leading to excessive stress in the fuel oil system.

2.4 Fuel impact on diesel engines • Ignition properties: • Poor ignition causes increased ignition delay. • Fuel is injected into the cylinder and begins to vaporize and mix with the surrounding air. • After a short delay, the heat of compression causes spontaneous ignition to occur, and accumulated vapour formed during the internal injection phase is vigorously burned. • This delay between the commencement of injection of the fuel droplets and the moment of spontaneous ignition of the fuel vapour is known as the ignition delay period, and occurs in all diesel engines.

2.4 Fuel impact on diesel engines • Following ignition is a period of controlled combustion, which maintains pressure on the piston, and is characterized initially by the steady and even combustion for the fuel after injection has terminated. • During the ignition phase, the pressure in the engine cylinder rises rapidly and considerable stresses are imposed on the piston. • It is desirable to keep the rate of pressure rise as low as possible, and this is achieved by ensuring that the minimum quantity of fuel is present in the cylinder prior to the ignition.

2.4 Fuel impact on diesel engines • With long ignition delay, a relatively large amount of fuel droplets will have been injected, and thus vaporized in the cylinder by the time ignition occurs. • On igniting, this large amount of accumulated vapour will combust almost explosively, leading to sudden and abnormal high rate of pressure rise and high cylinder pressure, beyond that for which engine was designed or can perhaps tolerate. • Long ignition delay leads to a sudden pressure rise in the cylinder.

2.4 Fuel impact on diesel engines • Damages: • Typical damage that can occur when using fuels with poor ignition properties are: • Deterioration in piston ring operation and broken rings. • Leakage of gas past the rings (blow by). • Deterioration in cylinder lubricant oil film and extreme wear and potential damage to the cylinder liner. • Cracks and erosion damage to pistons. • It is also likely that increased mechanical load over long time can cause damage to bearings.

2.4 Fuel impact on diesel engines • Water: • Water is undesirable in fuel when it is injected into the cylinders. • In practice it is impossible to be entirely certain that the fuel is completely free of water. • Sea water can lead to problems, because the sodium will get into the cylinder. • Fresh water should not lead to serious problems if it is evenly distributed in the fuel oil.

2.4 Fuel impact on diesel engines • Problems due to high water content can be: • Steam formation, cavitations and erosion of the components in the fuel injection system. • Unstable and incomplete combustion. • Salt water will lead to fouling and high temperature corrosion in the cylinder units and on the turbocharger .

2.4 Fuel impact on diesel engines • Ash: • Ash content indicates the presence of solid particles. • These can include: • Solid impurities in particle form (sand, rust, remains of catalyst from cracking process etc.) • Metal salts (vanadium, sodium etc.) • Other inorganic impurities • Ash content above 0.1% is considered high in fuel as bunkered. • With high ash content it is important to be careful with pre-treatment.

2.4 Fuel impact on diesel engines • Wear due to particles • can cause the following types of damage: • Wear of piston and liner in the high pressure fuel pumps, low rate of pressure increase in the pumps • Scoring of pump plungers and barrels • Wear of injector needles and guides and poor performance of injector nozzles • Increased wear of cylinder liners, piston rings and grooves • Loss of piston ring performance and possible ring breakage and leakage(blow by)

2.4 Fuel impact on diesel engines • Liner wear: • Abrasive particles cause most wear in the upper part of the cylinder liner. • High wear of injection pumps can make it impossible to run on distillates due to the fact that pressure built up will be too low for such low viscosity fuel oil. This can result in the needle not lifting from the nozzle seat, and no injection occurring. • Aluminum and silicon content indicate the presence of catalytically cracked heavy cycle oil. • This fraction of oil will considerably reduce the ignition quality.

2.4 Fuel impact on diesel engines • Damage: • The combined result of catalytic impurities and poor ignition quality can result in serious damage such as: Piston ring collapse. • Extensive gas leakage. Extreme wear and crack formation. • Reduced piston ring function can also occur, due to extensive fouling as mentioned before.

2.4 Fuel impact on diesel engines • High temperature corrosion: • Vanadium and sodium in the fuel during combustion can cause high temperature corrosion if they are deposited or sticking on/to metal surfaces. • In particular, the exhaust valves, piston crowns and turbo-chargers are exposed to this type of corrosion for trunk engines. • Exhaust gas temperature of 530 is especially critical. • A ratio of sodium to vanadium of 1:3 is particularly undesirable.

2.4 Fuel impact on diesel engines • Low temperature corrosion: • Sulphur in the fuel oil will not normally lead to operating problems. • During normal operation the sulphuric acid produced by the combustion is neutralized by the use of alkaline lubricating oils, both for cylinder lubrication and system oils. • To avoid sulphur corrosion it is important to use cylinder oil with the right TBN value. • For the most common heavy fuels a TBN value of 70 will be suitable. • With very low sulphur content it might be necessary to reconsider this. • Low temperature corrosion can occur on the nozzle tips of fuel valves or valve stems.

2.4 Fuel impact on diesel engines • Fouling: • Operating diesel engines on heavy fuel will always produce fouling in the cylinder. Impurities in the fuel which produce ash can lead to fouling. • Low ash content will not normally lead to sufficient fouling to cause a problem. • Operating on heavy fuels at low load over prolonged periods will lead to increased fouling.

2.4 Fuel impact on diesel engines • Fuel impact on diesel engines • Prevention of enginedamages or minimizing risk of same when running on off spec. fuel. • For operation on fuels with poor ignition quality, increase in pre-injection angle has little effect on ignition time. • This is because the ability of this type of fuel to ignite is strongly dependent on pressure and temperature of the charge in the cylinder. • For an earlier injection, the cylinder charge will have a lower pressure and temperature, which can result in a considerable increase in the length of ignition delay.

2.4 Fuel impact on diesel engines • Increase of process temperature • The most effective method for counteracting a long ignition delay is to increase the temperature of the charge air. • This can be done by increasing the cooling water/oil temperature and increasing the scavenge air temperature in the receiver. • The best method to reduce a long delay is to increase the scavenge air temperature. • An increase in engine temperature at part load can lead to reduced fouling sand less low temperature corrosion.

2.4 Fuel impact on diesel engines • “Rough running” • Increased ignition delay will often be observed by the engineer as rough running. • This is due to the increased pressure variations in the cylinder caused by the increased ignition delay. • It leads to higher mechanical load on piston rings and other components in the system. • It is possible to reduce the mechanical stresses by increasing the engine load. • The cylinder pressure and temperature will increase and the result will be less ignition delay. • The engine will run more smoothly.

2.4 Fuel impact on diesel engines • “Rough running” breakdowns • Some breakdowns, particularly those connected with the cylinder unit indicate that it is specially important to keep the piston rings and the piston grooves in the best condition. • This, together with not allowing too much liner wear, will reduce the likelihood of piston ring failure, when using fuel oils with poor ignition quality.

2.4 Fuel impact on diesel engines • Exhaust gas emission: • IMO’s Air Pollution Annex to MARPOL aims to regulate exhaust gas emission limits for oxides of nitrogen (NOX) and sulphur (SOX). • A step-by-step solution is planned for the introduction of NOX limits, the permissible values being reduced further at a later stage. • The world merchant fleet annually burns over 100 million tones of fuel. • Propulsion diesel engines usually operate on heavy fuel oils whose composition with respect to emission-related constituents (sulphur, for example) vary widely.

2.4 Fuel impact on diesel engines • Designing engines with optimized exhaust gas emission behavior is therefore difficult. • This is one of the reasons why shipping is less environment friendly than other transport modes in terms of sulphur, soot and particle discharge per tonne-kilometre. • Tightening controls are expected to make the emissions performance of an engine even more important in the sales decision, than its fuel economy, by the end of the century.

2.4 Fuel impact on diesel engines • New projects focuses on propulsion plants and aims to reduce drastically the visible emission of soot and particles in the exhaust gas ,under all engine operating conditions, including non-steady state running. • A further goal is to decrease the level of NOX emitted by large engines by 50% through internal measures and by 95% through after treatment by catalytic reactors. These results are sought without increasing fuel consumption, an ambitious target since high thermal efficiency (and hence fuel economy) is not naturally compatible with clean exhaust emission.

2.4 Fuel impact on diesel engines • A number of routes are available which can be applied singly of in combination to reduce noxious emissions: 1. Fuel quality: using fuel with a low sulphur content, diesel oil with low exhaust gas emissions (ash, metals, and aromatic hydrocarbons), fuel/water emulsions or alternative fuels. 2. Internal engine measures: charge air cooling, water injection, delaying the start of fuel injection, optimizing the configuration of the combustion chamber or injection system, exhaust gas recirculation and non-catalytic internal NOX reduction.

2.4 Fuel impact on diesel engines • 3. Exhaust gas treatment, particle filter, soot separator, and exhaust gas scrubber, reduction-type catalytic reactor, and oxidation-type catalytic reactor. Improved thermal efficiency through high efficiency combustion and a so-called escharotics system for automatic controlling fuel injection and valve timing is also in progress. Although the question of emission is not most important here.

2.4 Fuel impact on diesel engines • Fuel Parameters: • Density • Density alone will have no impact on the engine. • Viscosity • Viscosity alone will have no impact on the engine • Carbon residue micro • Carbon residue micro may lead to piston groove fouling, exhaust turbine erosion and deposits. • Ash • Ash may lead to cylinder wear, piston wear, piston groove fouling, exhaust turbine erosion and deposits.

2.4 Fuel impact on diesel engines • Water • Water may lead to ignition delay, combustion problems. • Sulphur • Sulphur may lead to corrosive wear, cylinder wear, piston ring wear, piston groove wear. • Vanadium • Vanadium may lead to exhaust valve corrosion, exhaust turbine fouling. • Sodium • Sodium may lead to deposits and wear of injectors, piston groove wear and fouling, exhaust valve corrosion, exhaust turbine fouling.

2.4 Fuel impact on diesel engines • Aluminum/silicon • Aluminum/silicon may lead to injection pump wear, fuel valve wear, cylinder wear, piston ring wear, piston ring wear, exhaust turbine erosion and deposits. • High CCAI • High CCAI indicate ignition delay and high pressure variations in the combustion chamber. • Incompatibility • Incompatibility may lead to wear and deposits in the injector and the fuel system in general. • Asphaltenes • Asphaltenes may lead to problems in the combustion process.

End of chapter 2

Marine Oily Handling Devices and Pollution Prevention