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.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 imagingDIESEL 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 fuel-air 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 flameDIESEL COMBUSTION PROCESS NATURE OF DIESEL COMBUSTION • Heterogeneous – liquid, vapor and air – spatially non-uniform • turbulent • diffusion flameThe 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 leanDiesel as the Most Efficient Power Plant • Theoretically, for the same CR, SI engine has higher ηf; but diesel is not limited by knock, therefore it can operate at higher CR and achieves higher ηf • Not throttled - small pumping loss • Overall lean - higher value of γ - 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: ηf~ 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 emissionsDiesel 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 trapsApplications • 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, shipsCommon 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) Quiescent chamber with multihole nozzle typical of larger engines (b) Bowl-in-piston chamber with swirl and multihole nozzle; medium to small size engines (c) Bowl-in-piston chamber with swirl and single-hole nozzle; medium to small size enginesCommon 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 prechamberCommon Diesel Combustion Systems (Table 10.1) Image removed due to copyright restrictions. Please see Table 10-1 in Heywood, John B. Internal CombustionEngine Fundamentals. New York, NY: McGraw-Hill, 1988.Typical Large Diesel Engine Performance Diagram 140 Max Pressure Scavenge Air Pressure (gauge) Exh. Temp, Turbine Inlet and Outlet Specific air quantity Specific fuel consumption Compression Pressure 120 100 80 60 40 20 2.5 2.0 1.5 1.0 0.5 (g/kWh) (kg/kWh) ( oC) (bar) (bar) Sulzer RLB 90 - MCR 1 Turbo-charged 2-stroke Diesel – 1.9 m stroke; 0.9 m bore Rating: 0• Speed: 102 Rev/ min 500 – Piston speed 6.46 m/s • 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) 450 400 350 300 250 200 13 12 11 10 9 – 7 cyl: 20.6 MW (28000 bhp) 8 7 – 8 cyl: 23.5 MW (32000 bhp) 210 – 9 cyl: 26.5 MW (36000 bhp) – 10 cyl: 29.4 MW (40000 bhp) 205 200 195 190 – 12 cyl: 35.3 MW (48000 bhp) 185 180 4 6 10 148 12 BMEP (bar) 16Diesel combustion process ― direct injection 1) Ignition delay ― no significant heat release 2) “Premixed” rapid combustion 3) “Mixing controlled” phase of combustion 4) “Late” combustion phase Note: (2) is too fast; (4) is too slowRate 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,