Introduction to Diesel.pdf

ATec 3042 Diesel Mechanics (conventional) Chapter 1: Introduction I) Comparison between Gasoline and Diesel Engines Israel.T...

ATec 3042 Diesel Mechanics (conventional) Chapter 1: Introduction I) Comparison between Gasoline and Diesel Engines Israel.T 1 2  Gasoline Engine  A petrol engine is an internal combustion engine with spark- ignition, designed to run on petrol or gasoline and similar volatile fuels.  It was invented in 1876 in Germany by German inventor Nikolaus August Otto.  The process differs from a Diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. 3  Diesel Engine  Invented by Rudolf Diesel  The Diesel engine (also known as a compression-ignition engine) is an internal combustion engine generates power by burning of fuel that has been injected into the combustion chamber by injector.  Possess highest thermal efficiency due to its very high compression ratio. 4  Bosch diesel fuel injection  Bosch’s emergence onto the diesel-technology stage In 1886; Robert Bosch (1861– 1942) opened a “workshop for light and electrical engineering” in Stuttgart.  At the beginning, his field of work lay in installing and repairing telephones, telegraphs, lightning conductors, and other light-engineering jobs.  The low-voltage magneto-ignition system developed by Bosch had provided reliable ignition in gasoline engines since 1897.  The high-voltage magneto ignition system w ith spark plug followed in 1902.  In 1922, Robert Bosch turned his attention to the diesel engine. He believed that certain accessory parts for these engines could similarly make suitable objects for Bosch high volume precision production like magnetos and spark plugs. The accessory parts in question for diesel engines were fuel-injection pumps and nozzles. 5 A comparison between SI and CI engine per stroke cycle 6  Gasoline Engine Diesel Engine Both air and fuel mixture is drawn into the cylinder Only air is drawn into the cylinder Compression ratio is 6:1 – 10:1 Compression ratio is 16:1 – 25:1 Spark plug ignites air-gasoline mixture Fuel injector injects atomized diesel over compressed air Less power is developed More power is developed Light in weight Heavy in weight Light to medium duty vehicles/application Medium to Heavy duty vehicles/application Ignition system develops secondary voltage surge High pressure surge makes burning High maintenance cost Low maintenance cost Less production of hydrocarbon High production of hydrocarbon Less noise produce Produces high noise Gives maximum 25% efficiency Gives maximum 35-40% efficiency 7  ATec 3042 Diesel Mechanics (Conventional) Chapter 1: Introduction II) Diesel Fuel Israel.T 8  Oil taken directly from the ground Used to make gasoline, diesel fuel, liquefied petroleum gas, and other petroleum products 9  Crude Oil Although the composition of crude oil varies, it typically is: 84% carbon 14% hydrogen 1% to 3% sulfur, in the form of hydrogen sulfide, sulfides, disulfides, and elemental sulfur Less than 1% nitrogen Less than 1% oxygen Less than 1% metals, normally nickel, iron, vanadium, copper, and arsenic Less than 1% salts, in the form of sodium chloride, magnesium chloride, and calcium chloride The high concentration of carbon and hydrogen is why products produced from crude oil are called hydrocarbon fuels or compounds. 10 Characteristics of Diesel Fuel Diesel fuel is made from petroleum (crude oil), as is gasoline. When petroleum is refined, it is separated into three components:  gasoline,  middle distillates, and  all remaining substances. Diesel comes from the middle distillates. 11 Diesel engines burn diesel fuel oil. It is a light oil made from crude oil by the same refining process that produces gasoline. The oil must have the proper viscosity, volatility, and cetane number for use as a diesel fuel. Diesel fuel contains more heat energy than gasoline by volume  Diesel fuel is a thicker fraction of crude oil  Diesel fuel will not vaporize as easily as gasoline 12  Diesel Fuel Grades  No. 1  thinnest grade  may not provide adequate lubrication for injection system components on some engines  may be used as a winter fuel  No. 2  most commonly used in automotive diesels  provide adequate lubrication for injection system components  used with additives for winter fuel 13 The following will identify the characteristics and highlight important areas to know about diesel fuel:  Heat Energy  Specific Gravity  Wax appearance Point (WAP)  Pour Point  Viscosity  Volatility  Cetane Number  Carbon Residue  Sulfur Content  Water Content  Bacteria Content  Flash Point 14 Heat Energy:  The combustion process burns fuel and releases heat energy.  The amount of heat energy released is referred to as calories (British Thermal Units [BTU]).  The calorie is derived from determining the heat energy required to raise the temperature of one gram of water 1°C (one pound of water 1°F).  One calorie will raise one gram of water 1°C. This heat energy is converted into power by the diesel engine.  Diesel fuel has a higher calorie content than gasoline.  More calories result in more power; This explains the fuel efficiency of diesel engines. 15 Specific Gravity:  The specific gravity of a liquid is a measurement of the weight of the liquid compared to water. The specific gravity of water is one.  Diesel fuel is lighter than water, but heavier than gasoline. If it is mixed with other liquids, the specific gravity will change.  A hydrometer is used to measure specific gravity.  Specific gravity relates to the combustion process. Diesel fuel must be heavy enough to fill the entire combustion chamber before burning.  If the specific gravity is too low, the fuel starts to burn before the chamber is filled. This causes poor performance, increases engine noise, and may damage components.  If the specific gravity is too high, fuel consumption may increase and engine power may decrease. Specific gravity is for diesel fuel in vehicle use is b/n 0.82 to 0.86 16 Wax Appearance Point (WAP):  Climate and temperature affect diesel fuel more than gasoline.  Diesel fuels contain paraffin, a wax material in middle distillate fuels.  Paraffin acts the same as candle wax. After a candle is blown out, the temperature around the wick begins to cool.  As the temperature drops, the candle wax begins to solidify.  The paraffin acts the same in diesel fuel. The point when paraffin begins to solidify is the WAP, sometimes called the cloud point.  Solidified paraffin collects and clogs fuel filters or lines.  The WAP for diesel fuel is approximately 20°F (-7°C).  Refineries add flow improvers to lower this temperature. This is why you hear of summer and winter fuels. 17 Pour Point:  The pour point is the point at which the fuel solidifies.  This differs from the WAP in that the WAP points out when the wax solidifies, not the fuel.  If the pour point is reached, the fuel stops flowing.  The pour point for diesel fuel is approximately 5°F (-15°C). 18 Viscosity:  Viscosity is the measurement of how resistant a liquid is to flowing; The viscosity of diesel fuel affects the spray from the injector.  High viscosity equals more resistance and low viscosity equals less resistance; Diesel fuel needs a low viscosity so that a fine spray comes from the injector. However, if the viscosity is too low, the fuel does not provide enough lubrication.  Temperature also affects viscosity. Colder temperatures result in higher viscosity. This is another reason for blending fuel. 19 Volatility:  Volatility is the measurement of how easily a liquid changes into a vapor or how easily a liquid evaporates.  Diesel engines need a fuel with a fairly high volatility.  Gasoline vaporizes easily. It has a high volatility.  Diesel fuel has low volatility. It boils at a temperature of about 700ºF (371ºC)  The higher volatility makes combustion easier. 20 Cetane vs Octane:  The cetane number is a rating of diesel fuel ignition quality or ability to spontaneously self-ignite.  Cetane is actually a laboratory liquid with excellent ignition qualities.  The cetane rating for diesel is determined by mixing cetane that has a rating of 100 with methylnaphthalene that has a rating of zero.  Methylnaphthalene does not ignite. The cetane and methylnaphthalene are mixed to imitate the performance of the fuel being tested.  The percentage of cetane in the mixture is the cetane rating.  Automakers recommend a cetane rating of about 45.  As the cetane rating increases, the faster the fuel self-ignites. This is the opposite of octane. As the octane rating increases, the fuel resists self- ignition. Premium gasoline has a higher octane (CN) rating than regular, allowing it to be used in higher compression engines. 21 Cont... Cetane Rating 22 Carbon Residue:  Because diesel fuel contains HC, carbon residue can be produced under certain operating conditions.  The amount of carbon residue depends on the fuel quality and operating conditions.  Engines that are idled for long periods tend to produce more carbon residue due to the lack of combustion efficiency at these engine speeds.  Carbon residue that is allowed to build up can cause engine damage.  The use of high quality fuel can reduce these buildups. 23 Sulfur Content:  Sulfur is a chemical in diesel fuel and the actual quantity of sulfur depends on the quality of the fuel.  The sulfur is converted to Sulfur Dioxide (SO2) during combustion.  SO2 is undesirable because it has acidic qualities that are harmful to the environment.  Allowable sulfur levels in diesel fuels have been lowered in recent years. 24 Water Content:  Water can be in diesel fuel that has been stored or distributed improperly. If the diesel fuel appears cloudy, water is most likely in the fuel.  Water in diesel fuel will corrode the fuel system. Rust particles from the corroding fuel system components get trapped in the fuel filter and clog the system.  When temperatures fall, the water in the fuel freezes and causes damage to fuel system components.  As previously mentioned, diesel fuel is the only lubricant for the fuel injection pump and fuel injectors; Water in the fuel reduces the lubrication quality and may damage these components. Water also invites bacteria to grow and poses a threat to the components.  Most diesel injection systems have water separators to prevent water damage 25 Bacteria Content:  Diesel fuel can be inviting to bacteria, particularly in warmer climates.  The bacteria ingests the diesel fuel and excretes a corrosive substance; The substance also can clog the fuel system.  Because bacteria may be living in diesel fuel, always wash your hands thoroughly and clean up your work area after handling the fuel. 26 Flash Point:  The flash point of diesel fuel is the lowest temperature at which it can produce a flammable vapor; This temperature has little effect on the performance of the vehicle, but is important to fuel storage.  If diesel fuel is stored at a temperature warmer than the flash point, fumes develop, and the fuel could be ignited easily. Warning! Diesel engines return warm fuel to the fuel tank. This fuel is often above the flash point and can be explosive. 27  ATec 3042 Diesel Mechanics (Conventional) Chapter 1: Introduction III) Diesel Engine Combustion Process Israel.T 28 Combustion process as viewed from P-V Diagram 4-strokes in SI-Engine 4-strokes in CI-Engine Constant-P Constant-V 29  Diesel Engine Combustion Process 1. Ignition delay (A - B). 2. Flame propagation (B-C) 3. Direct combustion (C-D) 4. After burning (D-E) 30 1. Ignition Delay Period ⦿ The ignition delay period is also called the preparatory phase during which some fuel has already been admitted but has not yet ignited. ⦿ This period is counted from the start of injection to the point where the pressure- time curve separates from the motoring curve indicated as start of combustion in Fig. ⦿ The fuel does not ignite immediately upon injection into the combustion chamber. There is a definite period of inactivity between the time when the first droplet of fuel hits the hot air in the combustion chamber and the time it starts through the actual burning phase. This period is known as the ignition delay period. 31 2. Period of Flame Propagation ⦿ The period of flame propagation also called rapid combustion or uncontrolled combustion, is that phase in which the pressure rise is rapid. ⦿ During the delay period, the droplets have had time to spread over a wide area and fresh air is always available around the droplets. ⦿ Most of the fuel admitted would have evaporated and formed a combustible mixture with air. ⦿ By this time, the preflame reactions would have also been completed. 32  Cont... ⦿ The period of rapid combustion is counted from end of delay period or the beginning of the combustion to the point of maximum pressure on the indicator diagram. The rate of heat-release is maximum during this period. ⦿ It may be noted that the pressure reached during the period of rapid combustion will depend on the duration of the delay period (the longer the delay the more rapid and higher is the pressure rise since more fuel would have accumulated in the cylinder during the delay period). 33 3. Period of Direct Combustion ⦿ The rapid combustion period is followed by the third stage, the direct or controlled combustion. ⦿ The temperature and pressure in the second stage is already quite high. ⦿ Hence the fuel droplets injected during the second stage burn faster with reduced ignition delay as soon as they find the necessary oxygen and any further pressure rise is controlled by the injection rate. ⦿ The period of controlled combustion is assumed to end at maximum cycle temperature. 34 4. Period of After-Burning ⦿ Combustion does not cease with the completion of the injection process. ⦿ The unburnt and partially burnt fuel particles left in the combustion chamber start burning as soon as they come into contact with the oxygen. This process continues for a certain duration called the after-burning period. ⦿ Usually this period starts from the point of maximum cycle temperature and continues over a part of the expansion stroke. ⦿ Rate of after-burning depends on the velocity of diffusion and turbulent mixing of unburnt and partially burnt fuel with the air. ⦿ The duration of the after-burning phase may correspond to 70-80 degrees of crank travel from TDC. 35  Diesel Knock  Occurs when too much fuel ignites at one time, producing a loud knock  Excessive diesel knock can reduce power, fuel economy, and engine life 36 Normal Diesel Combustion 37 Diesel Knock 38 Causes of Diesel Knock Diesel knock has several causes: - cold engine operation - low-cetane fuel - improper fuel injector spray pattern - incorrect injection timing 39 END OF CHAPTER-1 40