Engine Testing & Instrumentation: Work, Heat, and Thermodynamics in IC Engines, Summaries of Thermodynamics

Download Engine Testing & Instrumentation: Work, Heat, and Thermodynamics in IC Engines and more Summaries Thermodynamics in PDF only on Docsity! Engine Testing and Instrumentation 1 Engine Thermodynamics Engine Testing and Instrumentation 2 Thermodynamics – study of heat related to matter in motion. Engineering thermodynamics mainly concerned with work producing or utilising machines such as engines, turbines and compressors together with the working substances used in the machines. Working substance – fluids: capable of deformation, energy transfer, common: air and steam Engine Testing and Instrumentation Phase Nature of substance. Matter can exists in three phases: solid, liquid and gas Cycle If a substance undergoes a series of processes and return to its original state, then it is said to have been taken through a cycle. Engine Testing and Instrumentation 6 Process A substance is undergone a process if the state is changed by operation having been carried out on it Constant temperature process Isothermal process Constant pressure process Isobaric process Constant volume process Isometric process or isochoric process Adiabatic Process An adiabatic process is a process in which no heat is transferred. This is the case if the process happens so quickly that there is no time to transfer heat, or the system is very well insulated from its surroundings. Polytropic process A process which occurs with an interchange of both heat and work between the system and its surroundings. Nonadiabatic expansion or compression of a fluid is an example of a polytropic process. Engine Testing and Instrumentation 7 Energy Capacity of doing work Work A force is moved through a distance e.g. In a piston, ( )12 VVPALPLPAdonework −=×=×= Unit of work: Nm = J (joule) Power Rate of doing work J/s = Watt P L Engine Testing and Instrumentation 10 ( ) 1 1 2211 1 1 1 2 2 1 2 1 − − = − +− = = = +−+− −∫ ∫ n VPVP VV n C dVVC dVP donework nn V V n V V Work done in polytropic process Engine Testing and Instrumentation 11 Heat Temperature t (Celsius) = T-273.15 (Kelvin) Q – heat energy joules/kg Specific heat capacity: heat transfer per unit temperature: dt dQc = Unit: joules/kg K (joules per kg per K) Calorific value: the heat liberated by burning unit mass or volume of a fuel. e.g. petrol: 43MJ/kg Engine Testing and Instrumentation 12 Enthalpy H = U + P V U – internal energy P – pressure v – volume Specific Enthalpy h =U/m= u + P v Engine Testing and Instrumentation 15 Mechanical Power TakenTime doneWork Power = Unit: J/s= watt Electrical Power W = I V Unit: J/s=Watt Engine Testing and Instrumentation 16 The Conservation of Energy For a system Initial Energy + Energy Entering = Final Energy +Energy Leaving Potential energy = gZ 2 2 1 mCenergyKinetic = Engine Testing and Instrumentation 17 Laws of Thermodynamics The zeroth Law If body A and B are in thermal equilibrium, and A and C are in thermal equilibrium, then, B and C must in thermal equilibrium. The first law W=Q Means if some work W is converted to heat Q or some heat Q is converted to work W, W=Q. It does not mean all work can convert to heat in a particular process. Engine Testing and Instrumentation 20 Combined gas law )constant(a T PV = or 2 22 1 11 T VP T VP = Let mR=a, where m is the mass, R - specific gas constant (air: R=287 J/kgK) mR T PV = - characteristic equation of gas Engine Testing and Instrumentation 21 Joule’s law Internal energy of gas is the function of temperature only and independent of changes in volume and pressure. The specific heat capacity at constant volume vc ( )1212 TTmcUU v −=− (change in internal energy) The specific heat capacity at constant pressure pc ( )121212 )( TTmcVVPUU p −=−+− Engine Testing and Instrumentation 22 Polytropic Process CVP n = From nnn nn P P V Vand V V V V P P VPVP /1 2 1 2 1 2 1 1 2 2 1 2211 −−       =      =      = = Engine Testing and Instrumentation 25 Entropy for gas Polytropic process CPV n = Heat transferred Pdv c ncdQ 1− − = v p c c c = dv v R c ncdv T P c nc T dQds 11 − − = − − == (combined characteristic eq.) where s – specific entropy v is a unit of V. v dvR c ncds v v s s ∫∫ − − = 2 1 2 1 1 1 2 12 ln 1 v vR c ncss − − =− Engine Testing and Instrumentation 26 1 2 12 ln 1 v vR c ncss − − =− 2 1 12 ln 1 T T n nccss v − − =− 2 1 12 ln P P n nccss v − =− Engine Testing and Instrumentation 27 1 2 1 2 12 1 2 1 1 2 2 1 2 1 2 1 2 1 2 1 2 1 12 lnln . )(lnln ln 1 )(ln ln 1 )1( ln 1 ln 1 ln 1 P PR T Tcss ei Rcc T TR T Tc T T n ncc T Tc T T n ncncnc T T n ncc T T n n c c c T T n nccss p vp n n p vpp vpp vpv p vv −=− =−      −= − −+−= − −+−− = − − = − − = − − =− − Engine Testing and Instrumentation 30 Example: The air at T=288K and standard atmosphere is compress to P=5 times of standard atmosphere pressure with a temperature 456.1K. Calculate the entropy values. For air Cp=1005 J/kgK, R=287 J/kgK. Before compression 53 101.0 101.0ln287 15.273 288ln10052 =−=s After compression 53 101.0 101.0*5ln287 15.273 1.456ln10052 =−=s No change in entropy. Engine Testing and Instrumentation 31 Isentropic Process An isentropic process is one during which the entropy of working fluid remains constant. T s Engine Testing and Instrumentation 32 Petrol Engine The cycle in a four-stroke cycle modern petrol engine is called the Otto cycle after German engineer Nikolaus Otto, who introduced it in 1876. It improved on earlier engine cycles by compressing the fuel mixture before it was ignited. Rudolf Christian Karl Diesel, inventor of the Diesel engine. Born in Paris, France, March 18, 1858; died in the English Channel, September 29, 1913. Mr Diesel’s first engine a4 = compressed ait exhaust valve fuel-air mixture ignites bined gases power stroke exhaust stroke 24 Engine Testing and Instrumentation 37 Nicolaus August Otto Four Stroke Engine 1876 S Ideal Otto Cycle Research p-V diagram Center V= Volume p=pressure Combustion Process — Compression Stroke -7] constant volume process adiabatic process / Power Stroke Heat Rejection a Intake Stroke Exhaust Stroke AN Engine Testing and Instrumentation 41 Otto cycle Otto Thermodynamic Cycle is used in all internal combustion engines. Stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum. Between Stage 1 and Stage 2 the piston is pulled out of the cylinder with the intake valve open. The pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve. Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back into the cylinder, the gas volume decreases, and the pressure increases because work is done on the gas by the piston. Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly and the volume remains constant. Heat is released during combustion which increases both the temperature and the pressure, according to the equation of state. Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston is driven towards the crankshaft, the volume in increased, and the pressure falls as work is done by the gas on the piston. Stage 5: the exhaust valve is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions. Stage 6 begins the exhaust stroke of the engine during which the piston moves back into the cylinder, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself. Engine Testing and Instrumentation 42 Work Done During the cycle, work is done on the gas by the piston between stages 2 and 3. Work is done by the gas on the piston between stages 4 and 5. The difference between the work done by the gas and the work done on the gas is the area enclosed by the cycle curve and is the work produced by the cycle. The work times the rate of the cycle (cycles per second) is equal to the power produced by the engine. Ideal Otto cycle: no heat entering (or leaving) the gas during the compression and power strokes, no friction losses, and instantaneous burning occurring at constant volume. Engine Testing and Instrumentation 45 2-3 Compression stroke 1 3 2 23 3 2 23 −       =       = γ γ V V TT V Vpp Engine Testing and Instrumentation 46 3-4 Combustion Process 3 4 34 34 34 T T pp vv c QTT v = = += It is a constant volume process. Engine Testing and Instrumentation 47 4-5 Power Stroke 1 5 4 45 5 4 45 −       =       = γ γ V VTT V Vpp Engine Testing and Instrumentation 50 Work Done ∫∫ −− −= 1 2 2 1 6532 V V V V dVPdVP Engine Testing and Instrumentation 51 A Comparison between a working cycle of a piston engine and a turbo-jet engine Four Stroke Diesel Cycle Fig. 1: 4-stroke diesel engine 1 Induction stroke, 2 Compression stroke, 3 Working stroke, 4 Exhaust stroke. 1 59 Engine Testing and Instrumentation 55 Compression At bottom dead centre (BDC) the cylinder is at its maximum volume and the intake is closed. Now the piston moves round to Top dead centre (TDC) and compresses the air/fuel mixtures. The pressure is increased and the volume decreased. The necessary work for the compression increases the internal energy of the mixtures – the temperature is increased. Because of the fast compression only a small part of the energy is transferred to the environment Engine Testing and Instrumentation 56 Ignition Near the end of the compression stroke, the ignition starts the combustion and the mixture burns very rapidly. The expanding gas creates a high pressure against the top of the piston. The resulting force drives the piston downward. The rapid increase in pressure gives rise to the noise levels associated with combustion. Engine Testing and Instrumentation 57 Power stroke The force drives the piston downward to the crankshaft (the valves are closed). The volume is increased and the pressure is decreased. No more energy is added and because of this the internal energy of the gas is decreased, as is the temperature. Engine Testing and Instrumentation 60 Introduction to diesel fuels • Diesel fuels consist of a number of hydrocarbons, which have boiling points between 180° and 360 ° C • As there is no external ignition system ( yet!), diesel fuel must ignite when introduced into heated compressed air with the minimum possible delay.Ignition property of the fuel is defined as the property of the fuel, which serves to initiate auto ignition. • This ignition quality is expressed by the cetane number (CN). The higher the CN the easier it is for the fuel to ignite. • The numbers allocated are 100 for very good, down to methylnapthalene that has poor ignition qualities and a cetane of 0 • The minimum CN for diesel fuel is 45, a CN of 50 is the optimum for current engines Engine Testing and Instrumentation 61 Cold behaviour of diesel fuel • At low temperatures, paraffin crystals can cause filters to clog and block off the fuel delivery. • For untreated fuel, this crystallisation starts at 0 ° C • With this in view, cold weather additives are added at the refinery and standard UK pump fuels are good to -22 °C Engine Testing and Instrumentation 62 Density of diesel fuel • The diesel fuel’s calorific value is approximately related to its density. • It increases with increasing density • If fuels with a differing density are used with the same fuel injection equipment at the same settings, there will be fluctuations in the calorific values which in turn will lead to increased smoke and sot emissions from higher density fuels. • Reference fuel barrel storage methods are critical to ensure minimal loss of light ends. Engine Testing and Instrumentation 65 Diesel 4 stroke cycle Engine Testing and Instrumentation 66 Engine Movie (Ford)…