武汉理工大学：《船舶辅机》课程教学资源（PPT课件，英文版）2.4 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 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 – 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

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

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

able 2. 4 Viscosity/temperature relations viscosity /temperature relations: Pumping viscosity, Kinematic viscosity, Centistokes ==|380 usually about 1.000 cSt 1000 500 100 Boiler atomisation usually between 15 and 60 cst 15 DIesel injection Viscosity usually between 8 and 27 cSt. 515 30405060708090100110120130140150160170 Temperature- degrees celcius

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 CCAD 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 • 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 – 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 e 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 • 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 • 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 • 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