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 injection ignition and fuel air mixing 1 Fuel spray phenomena 2 Spontaneous ignition 3 Effects of fuel jet and charge motion on mixingcontrolled combustion 4 Fuel injection hardware 5 Challenges for diesel combustion DIESEL FUEL INJECTION The fuel spray serves multiple purposes Atomization Fuel distribution Fuel air mixing Typical Diesel fuel injector Injection pressure 1000 to 2200 bar 5 to 20 holes at 0 15 0 2 mm diameter Drop size 0 1 to 10 m For best torque injection starts at about 20o BTDC Injection strategies for NOx control Late injection inj starts at around TDC Other control strategies Pilot and multiple injections rate shaping water emulsion Diesel Fuel Injection System A Major cost of the diesel engine Performs fuel metering Provides high injection pressure Distributes fuel effectively Spray patterns atomization etc Provides fluid kinetic energy for charge mixing Typical systems Pump and distribution system 100 to 1500 bar Common rail system 1000 to 1700 bar Hydraulic pressure amplification Unit injectors 1000 to 2500 bar Piezoelectric injectors to 1800 bar Electronically controlled EXAMPLE OF DIESEL INJECTION Hino K13C 6 cylinder 12 9 L turbo charged diesel engine rated at 294KW 2000 rpm Injection pressure 1400 bar duration 40oCA BSFC 200 g KW hr Fuel delivered per cylinder per injection at rated condition 0 163 gm 0 21 cc 210 mm3 Averaged fuel flow rate during injection 64 mm3 ms 8 nozzle holes at 0 2 mm diameter Average exit velocity at nozzle 253 m s Fuel Atomization Process Liquid break up governed by balance between aerodynamic force and surface tension Webber Number Wb gasu 2 d Critical Webber number Wb critical 30 diesel fuel surface tension 2 5x10 2 N m Typical Wb at nozzle outlet Wb critical fuel shattered into droplets within one nozzle diameter Droplet size distribution in spray depends on further droplet breakup coalescence and evaporation Droplet size distribution f D Size distribution f D dD probability of finding particle with diameter in the range of D D dD 1 f D dD 0 D Average diameter Volume distribution 1 dV V dD D f D D dD 0 f D D3 3 f D D dD 0 Sauter Mean Diameter SMD D 32 3 f D D dD 0 2 f D D dD 0 Droplet Size Distribution Image removed due to copyright restrictions Please see Fig 10 28 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Fig 10 28 Droplet size distribution measured well downstream numbers on the curves are radial distances from jet axis Nozzle opening pressure at 10 MPa injection into air at 11 bar Droplet Behavior in Spray Small drops micron size follow gas stream large ones do not Relaxation time d2 Evaporation time d2 Evaporation time small once charge is ignited Spray angle depends on nozzle geometry and gas density tan 2 gas liquid Spray penetration depends on injection momentum mixing with charge air and droplet evaporation Spray Penetration vapor and liquid Fig 10 20 Shadowgraph image showing both liquid and vapor penetration Image removed due to copyright restrictions Please see Fig 10 20 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Back lit image showing liquidcontaining core Auto ignition Process PHYSICAL PROCESSES Physical Delay Drop atomization Evaporation Fuel vapor air mixing CHEMICAL PROCESSES Chemical Delay Chain initiation Chain propagation Branching reactions CETANE IMPROVERS Alkyl Nitrates 0 5 by volume increases CN by 10 Ignition Mechanism similar to SI engine knock CHAIN BRANCHING EXPLOSION Chemical reactions lead to increasing number of radicals which leads to rapidly increasing reaction rates Formation of Branching Agents Chain Initiation RH O R HO 2 2 RH ROOH R RO 2 R CHO R O RO 2 Chain Propagation Degenerate Branching etc R O2 RO 2 O H ROOH RO O HO R CHO O R C 2 2 Cetane Rating Procedure is similar to Octane Rating for SI Engine for details see10 6 2 of text Primary Reference Fuels Normal cetane C16H34 CN 100 Hepta Methyl Nonane HMN C16H34 CN 15 2 2 4 4 6 8 8 Heptamethylnonane Rating Operate CFR engine at 900 rpm with fuel Injection at 13o BTC Adjust compression ratio until ignition at TDC Replace fuel by reference fuel blend and change blend proportion to get same ignition point CN n cetane 0 15 x HMN Ignition Delay Image removed due to copyright restrictions Please see Fig 10 36 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Ignit i on delays measured in a small four stroke cycle DI diesel engine with rc 16 5 as a function of load at 1980 rpm at various cetane number Fig 10 36 Fuel effects on Cetane Number Fig 10 40 Image removed due to copyright restrictions Please see Fig 10 40 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Ignition Delay Calculations Difficulty do not know local conditions species concentration and temperature to apply kinetics information Two practical approaches Use an instantaneous delay expression T P P nexp EA T and solve ignition delay id from 1 dt t si T t P t Use empirical correlation of id based on T P at an appropriate charge condition e g Eq 10 37 of text 1 t si id 1 1 21 2 0 63 id CA 0 36 0 22Sp m s exp E A 17190 P bar 12 4 R T K EA Joules per mole 618 840 CN 25 Diesel Engine Combustion Air Fuel Mixing Process Importance of air utilization Smoke limit A F 20 Fuel jet momentum wall interaction has a larger influence on the early part of the combustion process Charge motion impacts the later part of the combustion process after end of injection CHARGE MOTION CONTROL Intake created motion swirl etc Not effective for low speed large engine Piston created motion squish Interaction of fuel jet and the chamber wall Image removed due to copyright restrictions Please see Fig 10 21 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Sketches of outer vapor boundary of diesel fuel spray from 12 successive frames 0 14 ms apart of high speed shadowgraph movie Injection pressure at 60 MPa Fig 10 21 Interaction of fuel jet with air swirl Image removed due to copyright restrictions Please see Fig 10 22 in Heywood John B Internal Combustion Engine Fundamentals New York NY McGraw Hill 1988 Schematic of fuel jet air swirl interaction is the fuel equivalence ratio distribution Fig 10 22 Rate of Heat Release in Diesel