MIT OpenCourseWare http ocw mit edu 2 61 Internal Combustion Engines Spring 2008 For information about citing these materials or our Terms of Use visit http ocw mit edu terms Diesel Engine Combustion 1 Characteristics of diesel combustion 2 Different diesel combustion systems 3 Phenomenological model of diesel combustion process 4 Movie of combustion in diesel systems 5 Combustion pictures and planar laser sheet imaging DIESEL COMBUSTION PROCESS PROCESS Liquid fuel injected into compressed charge Fuel evaporates and mixes with the hot air Auto ignition with the rapid burning of the fuelair that is premixed during the ignition delay period Premixed burning is fuel rich As more fuel is injected the combustion is controlled by the rate of diffusion of air into the flame DIESEL COMBUSTION PROCESS NATURE OF DIESEL COMBUSTION Heterogeneous liquid vapor and air spatially non uniform turbulent diffusion flame The Diesel Engine Intake air not throttled Load controlled by the amount of fuel injected A F ratio idle 80 Full load 19 less than overall stoichiometric No end gas avoid the knock problem High compression ratio better efficiency Combustion Turbulent diffusion flame Overall lean Diesel as the Most Efficient Power Plant Theoretically for the same CR SI engine has higher f but diesel is not limited by knock therefore it can operate at higher CR and achieves higher f Not throttled small pumping loss Overall lean higher value of higher thermodynamic efficiency Can operate at low rpm applicable to very large engines slow speed plenty of time for combustion small surface to volume ratio lower percentage of parasitic losses heat transfer and friction Opted for turbo charging Large Diesels f 55 98 ideal efficiency Disadvantages of Diesel Engines Cold start difficulty Noisy sharp pressure rise cracking noise Inherently slower combustion Lower power to weight ratio Expensive components NOx and particulate matters emissions Diesel Engine Characteristics compared to SI engines Better fuel economy Overall lean thermodynamically efficient Large displacement low speed lower FMEP Higher CR CR limited by peak pressure NOx emissions combustion and heat transfer loss Turbo charging not limited by knock higher BMEP over domain of operation lower relative losses friction and heat transfer Lower Power density Overall lean would lead to smaller BMEP Turbocharged would lead to higher BMEP not knock limited but NOx limited BMEP higher than SI engine Lower speed overall power density P VD not as high as SI engines Emissions more problematic than SI engine NOx needs development of efficient catalyst PM regenerative and continuous traps Applications Small 7 5 to 10 cm bore previously mainly IDI new ones are high speed DI passenger cars Medium 10 to 20 cm bore DI trucks trains Large 30 to 50 cm bore DI trains ships Very Large 100 cm bore stationary power plants ships Common Direct Injection Compression Ignition Engines Fig 10 1 of text Image removed due to copyright restrictions Please see Fig 10 1 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 a b c Quiescent chamber with multihole nozzle typical of larger engines Bowl in piston chamber with swirl and multihole nozzle medium to small size engines Bowl in piston chamber with swirl and single hole nozzle medium to small size engines Common types of small Indirect injection diesel engines Fig 10 2 of text Image removed due to copyright restrictions Please see Fig 10 2 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 a Swirl prechamber b Turbulent prechamber Common Diesel Combustion Systems Table 10 1 Image removed due to copyright restrictions Please see Table 10 1 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 bar Typical Large Diesel Engine Performance Diagram BMEP 14 3 bar Configurations 4 cyl 11 8 MW 16000 bhp 5 cyl 14 7 MW 20000 bhp 6 cyl 17 7 MW 24000 bhp 7 cyl 20 6 MW 28000 bhp 8 cyl 23 5 MW 32000 bhp 9 cyl 26 5 MW 36000 bhp 10 cyl 29 4 MW 40000 bhp 12 cyl 35 3 MW 48000 bhp oC Piston speed 6 46 m s Max Pressure Compression Pressure Scavenge Air Pressure gauge Exh Temp Turbine Inlet and Outlet kg kWh Rating Speed 102 Rev min g kWh 1 9 m stroke 0 9 m bore bar Sulzer RLB 90 MCR 1 Turbo charged 2 stroke Diesel 140 120 100 80 60 40 20 2 5 2 0 1 5 1 0 0 5 0 500 450 400 350 300 250 200 13 12 11 10 9 8 7 210 205 200 195 190 185 180 Specific air quantity Specific fuel consumption 4 6 10 12 BMEP bar 8 14 16 Diesel combustion process direct injection 1 2 3 4 Ignition delay no significant heat release Premixed rapid combustion Mixing controlled phase of combustion Late combustion phase Note 2 is too fast 4 is too slow Rate of Heat Release in Diesel Combustion Fig 10 8 of Text Image removed due to copyright restrictions Please see Fig 10 9 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 A Simple Diesel Combustion Concept Fig 10 8 Image removed due to copyright restrictions Please see Fig 10 8 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988