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 mixing-controlled combustion 4. Fuel injection hardware 5. Challenges for diesel combustionDIESEL 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 μm • For best torque, injection starts at about 20o BTDC Injection strategies for NOx control • Late injection (inj. starts at around TDC) • Other control strategies: ¾ Pilot and multiple injections, rate shaping, water emulsionDiesel 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 controlledEXAMPLE 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/sFuel Atomization Process • Liquid break up governed by balance between aerodynamic force and surface tension ρgasu2d Webber Number (Wb ) = σ • 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 evaporationDroplet 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 D 0 Average diameter Volume distribution ∞ 1 dV f(D) D3 D =∫ f(D)D dD V dD =∞ 0 ∫ f(D)D3dD 0 Sauter Mean Diameter (SMD) ∞ ∫ f(D) D3dD D32 =∞ 0 ∫ f(D) D 2dD 0Droplet 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 τ∝d2 • Evaporation time ∝ d2 – Evaporation time small once charge is ignited • Spray angle depends on nozzle geometry and gas density : tan(θ/2) ∝√(ρgas/ρliquid) • Spray penetration depends on injection momentum, mixing with charge air, and droplet evaporationSpray 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 liquid-containing coreAuto-ignition Process PHYSICAL PROCESSES (Physical Delay) ¾ Drop atomization ¾ Evaporation ¾ Fuel vapor/air mixing CHEMICAL PROCESSES (Chemical Delay) ¾ Chain initiation ¾ Chain propagation ¾ Branching reactions CETANE IMPROVERS ¾ Alkyl Nitrates – 0.5% by volume increases CN by ~10Ignition 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 ChainInitiation RO 2 + RH ⇒ ROOH + R RH + O2 ⇒ R + HO 2 RO 2 ⇒ R′CHO + R′′O ChainPropagation DegenerateBranching R + O2 ⇒ RO 2,etc. ROOH ⇒ RO + O H R′CHO + O2 ⇒ R′C O + HO 2Cetane Rating (Procedure is similar to Octane Rating for SI Engine; for details, see10.6.2 of text) Primary Reference Fuels: ¾ Normal cetane (C16H34): CN = 100 ¾ Hepta-Methyl-Nonane (HMN; C16H34): CN = 15 (2-2-4-4-6-8-8 Heptamethylnonane) Rating: ¾ Operate CFR engine at 900 rpm with fuel ¾ Injection at 13o BTC ¾ Adjust compression ratio until ignition at TDC ¾ Replace fuel by reference fuel blend and change blend proportion to get same ignition point ¾ CN = % n-cetane + 0.15 x % HMNIgnition Delay Igniti on delays measured in a small four-stroke cycle DI diesel engine with rc=16.5, as aImage removed due to copyright restrictions. Please see Fig. 10-36 in Heywood, John B. Internal Combustion Engine Fundamentals. New York, NY: McGraw-Hill, 1988. 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 τ(T,P) = P-nexp(-EA/ T) and solve ignition delay (τid) from 1= ∫tsi+τid 1dttsi τ(T(t),P(t)) • Use empirical correlation of τid based on T, P at an appropriate charge condition; e.g. Eq. (10.37 of text) ⎡ 1 1 21.2 0.63 ⎤ τid(CA) = (0.36 + 0.22Sp(m/ s))exp ⎢⎢EA(R~T(K) − 17190)) + (P(bar) − 12.4) ⎥⎥ ⎣ ⎦ 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