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Diesel Lecture 1 - Working Principle 4-stroke 1 Induction...

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Unformatted text preview: Working Principle: 4-stroke 1. Induction Pure air is drawn into the cylinder 2. Compression The air is compressed to high temperature and pressure 3. Power Stroke Around TDC fuel is injected, which self-ignites and burns, further increasing pressure and therby forcing the piston down. 4. Exhaust The hot gas is released to the surrounding Niklas Nordin Working Principle: 4-str0ke Pow er/me Co ressijn - lntalw mp ' Expansmn Compressed air Fuel injected near TDC IV Close EV Open — scANIA Niklas Nordin Air only Diesel Spray Combustion 100 bar : T=1000K, p= Condi ions I“ Niklas Nordin CI combustion cycle P-crank angle and P-volume diagram EWHGICN —_._._._._—~ (fmeS‘f‘uM PRESSURE PRESSURE ! T130 300 TDC BDC TDC CRANK ANGLE ID = lgnmon Delay Diesel vs. Otto Engine The main difference between the Diesel and Otto engine is: The burning of the fuel. In a Gasoline engine the air/fuel mixture enters the cylinder and creates a stoichiometric homogeneous mixture, which is ignited and the flame travels from the spark and outwards to the liner. 0 In the Diesel air enters the cylinder, fuel is injected, self-ignites and burns with a diffusion type of combustion — SGANIA Niklas Nordin Diesel vs. Otto Engine 11 Otto Engine drawbacks *9 Since the mixture always must be stoichiometric it is necessary to reduce the amount of air that is drawn into the cylinder if we reduce the fuel amount. 9 Throttling is thus necessary at low loads, which takes power from the engine. Due to the fuel/air mixture it is necessary to lower the compression ratio to avoid it from self-igniting (knock). _ SGANIA Niklas Nordin Diesel vs. Otto Engine 111 The Diesel has: higher compression ratio higher thermodynamic efficiency, no knock no throttle no pumping losses, power ~ fuel combustion is always lean lower heat losses, higher efficiency more flexibility about choice of fuel — SGANIA Niklas Nordin Diesel vs. Otto Engine IV 1.0 Spatk ignition engine 0-3 -— — - Indirect injection diesel engine. ftom figure 5.9 —’ -— Direct injection diesel engine. from figuto 5.9 stc (kg/kWh) .0 a: .o a 0.2 bmep (bar) Specific Fuel Consumption as function of load for Diesel and Otto. Otto engines have higher pump-losses at low loads, which leads to higher sfc. — SGANIA Niklas Nordin Combustion in CI Engine The combustion process proceeds by the following stages: Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the compression stroke. The liquid fuel atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure air. Premixed combustion phase (bc) — combustion of the fuel which has mixed with the air to within the flammability limits (air at high-temperature and high- pressure) during the ignition delay period occurs rapidly in a few crank angles. Mixing controlled combustion phase (Cd) — after premixed gas consumed, the burning rate is controlled by the rate at which mixture becomes available for burning. The rate of burning is controlled in this phase primarily by the fuel-air mixing process. Late combustion phase (de) — heat release may proceed at a lower rate well into the expansion stroke (no additional fuel injected during this phase). nAML-IA‘:AI" A: Ana—Io OIQA‘A-u‘u‘AI-l ':lu-¢:IJ ‘IIAI vii-AA AAA‘ :4. £A-fi Combustion steps in DI diesel engine Physmal Dela y ‘ Breaking Fuel Thermal Complete Spray Decomposition Combustion F/A Mam-fig Fue: Vaponzmg F/A Mixture Incomplete Oxidation Combusflon FN ang MMW‘? "* ~: um: Combustion Products Formation Ignition Delay Ignition delay is defined as the time (or crank angle interval) from when the fuel injection starts to the onset of combustion. Both physical and chemical processes must take place before a significant fraction of the chemical energy of the injected liquid is released. Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air. Good atomization requires high fuel-injection pressure, small injector hole diam., optimum fuel viscosity, high cylinder pressure (large divergence angle). Rate of vaporization of the fuel droplets depends on droplet diameter, velocity fuel volatility, pressure and temperature of the air. Chemical processes similar to that described for autoignition phenomenon in premixed fuel—air, only more complex since heterogeneous reactions (reactions occurring on the liquid fuel drop surface) also occur. Fuel Ignition Quality The ignition characteristics of the fuel affect the ignition delay. The ignition quality of a fuel is defined by its cetane number CN. For low cetane fuels the ignition delay is long and most of the fuel is injected before autoignition and rapidly burns, under extreme cases this produces an audible knocking sound referred to as “diesel knock”. For high cetane fuels the ignition delay is short and very little fuel is injected before autoignition, the heat release rate is controlled by the rate of fuel injection and fuel-air mixing — smoother engine operation. Cetane Number The method used to determine the ignition quality in terms of CN is analogous to that used for determining the antiknock quality using the ON. The cetane number scale is defined by blends of two pure hydrocarbon reference fuels. By definition, isocetane (heptamethylnonane, HMN) has a cetane number of 15 and cetane (n-hexadecane, C16H34) has a value of 100. In the original procedures a—methylnaphtalene (C11H10) with a cetane number of zero represented the bottom of the scale. This has since been replaced by HMN which is a more stable compound. The higher the ON the better the ignition quality, i.e., shorter ignition delay. The cetane number is given by: CN = (% hexadecane) + 0.15 (% HMN) Diesel Combustion Process Diesel Ignition Flame Front Propagation ' - Spontaneous combustion (auto ignition) due to temperature increase of reactants. - Ignition triggered by compression heating of fuel-air mixture. - Ignition initiated at random point in combustion chamber - Fast combustion process - Less complete combustion process Cetane Number Measurement The method developed to measure CN uses a standardized single-cylinder engine with variable compression ratio The operating condition is: Inlet temperature (°C) 65.6 Speed (rpm) 900 Spark advance (°BTC) 13 Coolant temperature (°C) 100 Injection pressure (MPa) 10.3 With the engine running at these conditions on the test fuel, the compression ratio is varied until combustion starts at TC, ignition delay period of 13°. The above procedure is repeated using blends of cetane and HMN. The blenc that gives a 13° ignition delay with the same compression ratio is used to calculate the test fuel cetane number. Factors Affecting Ignition Delay Iniection timing — At normal engine conditions the minimum delay occurs with the start of injection at about 10-15 BTC. The increase in the delay time with earlier or later injection timing occurs because of the air temperature and pressure during the delay period. Ini'ection guantity— For a Cl engine the air is not throttled so the load is varied by changing the amount of fuel injected. Increasing the load (bmep) increases the residual gas and wall temperature which results in a higher charge temperature at injection which translates to a decrease in the ignition delay. Intake air temperature and pressure — an increase in ether will result in a decrease in the ignition delay, an increase in the compression ratio has the same effect. Cetane vs Octane Number The octane number and cetane number of a fuel are inversely correlated. @ Octane-heptane ""' o Benzene-naphtha n Somme-gasoline n Alcohol-gasoline ' :3 Cetane-me. naphm. _ . """ " a Gas ollvteoh. me. naphmfw?“”"'"‘" ‘ + fiat-man fuels 9 6.0. and.) fuels Cetane motor method octane number Ignition delay. m 3.0 2.5 3° 0 1" u 1.0 0 1(1) 200 300 4005(1) 600 bmcp.kPa Factors Affecting Ignition Delay DI and IDI Combustion chamber designs Modern DI Cl Engine Types Two basic categories of Cl engines: i) Direct-injection — have a single open combustion chamber into which fuel is injected directly ii) Indirect-injection — chamber is divided into two regions and the fuel is injected into the “prechamber” which is connected to the main chamber via a nozzle, or one or more orifices. o For very-large engines (stationary power generation) which operate at low engine speeds the time available for mixing is long so a direct injection quiescent chamber type is used (open or shallow bowl in piston). - As engine size decreases and engine speed increases, increasing amount of swirl are used to achieve fuel-air mixing (deep bowl in piston) - For small high-speed engines used in automobiles chamber swirl is not sufficient, indirect injection is used where high swirl or turbulence is generated in the pre-chamber during compression and products/fuel blowdown and mix Rate of heat release Four Stages of Combustion in CI Engines Ignition Premixed combustion phase delay period Mixing-controlled combustion phase End of injccction Start of injection Crank angle. deg Combustion Characteristic Combustion occurs throughout the chamber over a range of equivalence ratios dictated by the fuel-air mixing before and during the combustion phase. In general most of the combustion occurs under very rich conditions within the head of the jet, this produces a considerable amount of solid carbon (soot). g0” ...
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