High Efficiency Engines through Dilution Opportunities and Challenges Dr. Terry Alger Southwest Research Institute
Efficiency Drivers from the Marketplace and Regulators Oil price volatility CO 2 and CAFE regulations Emissions Standards mg/km NMOG CO Nox PM T2B5 56 26 43 6 T2B2 6 1305 12 6 T2B1 0 0 0 0 E5-petrol 68 00 60 5 E5-diesel 50 500 180 5 E6-diesel 90 500 80 5 2 6//2011
What Limits the Efficiency of an SI Engine? 22 20 BSFC [g/kw h] Behavior in this region sets the CR of the engine BMEP [bar] 18 16 14 12 8 6 4 2 280 260 Knock 230 240 280 Combustion Phasing Losses Pumping Work Friction 350 260 260 1500 2000 2500 0 3500 4000 4500 5000 Engine Speed [rpm] 3 6//2011
Utility of EGR Cooled and/or Uncooled 4 6//2011
Improved Cycle Efficiencies Through Knock Reduction 80 70 Cylinder Pressu ure [bar] 80 60 40 20 8 6 4 2 0% EGR % EGR 15% EGR 20% EGR 25% EGR Cylinder Pressure [bar] 60 50 40 30 20 0-30 -15 0 15 30 45 60 Crank Angle [deg atdcf] Modern GDI engine 1500 rpm / 60% load 1 0.04 0.06 0.08 0.1 0.2 0.4 0.6 Cylinder Volume [L] 5 6//2011
Improved Cycle Efficiencies Through Charge Properties and Pumping Work Cooler combustion improves the ratio of specific heats and reduces HT losses Lean burn results in a slightly larger improvement 1.302 Cycle-Averaged γ [-] 1. 1.298 1.296 1.294 1.292 1.290 0 5 15 20 EGR [%] ~ +1 % BTE Use of either internal EGR (through cam phaser) or cooled EGR decreases throttling losses Also not quite as good as lean operation 6 6//2011
Emissions Reduction With EGR Cool combustion reduces PM, 25 PN, CO and NOx Lower postflame 20 temperatures 15 lead to small HC increase Stoichiometric exhaust composition yields low-cost, low-emissions x [g/kwh] x ] BSCO, BSHC, BSNOx BSPM [mg/kwh x BSCO BSHC BSNOx BSPM BSPN 5 BSFC [g/kwh] 240 235 230 225 0 5 15 20 25 EGR [%] Data from modern GDI application @ 0 rpm / 75% load 0 solution 0 5 15 20 25 8 6 4 2 0 BSPN [#/kwh/1e e12] EGR [%]
Exhaust Temperature Reduction With EGR Pre-Turbin ne Temperature [ o C] 900 880 860 840 820 800 780 760 740 720 EGR cools exhaust via: Higher thermal mass Slower reaction rates 4000 RPM 0% EGR (Engine φ shown as label) 15% EGR (Engine φ = 1 at all conditions) 0.94 0.94 0.97 1 1.11 More advanced combustion phasing at high load 1.17 1.18 4 6 8 12 14 16 18 BMEP [bar]
EGR Challenges $ 9 6//2011
Misfire and Stability Issues Dilution reduces laminar burning velocities Decreases volumetric HR Slows reaction rates Reduces flame temperatures In an engine, slower burn rates can lead to: Unstable operation Full Misfire Primary affect felt on 0-2% MFB Solutions come in two categories Igniter and igniter hardware Fuel and charge enhancement Degrees Crank Angle 35 30 25 20 15 0-2% MFB duration [deg] 50-90% MFB duration [deg] 2-50% MFB duration [deg] 0 5 15 20 25 30 EGR [%] 6//2011
Ignition System Solutions for Dilute Engines Alternative system approach Long duration, continuous discharge OR Volumetric ignition Continuous discharge Overcomes HT losses to plug and gasses Couples with the flow field Forces flame kernel growth Volumetric ignition Large initial flame area Very fast burn rates 90 0-50% MFB [deg] 80 70 60 50 40 Stock System Multi-strike system DCO System CoV IMEP [%] High EGR tolerance potential 0 5 15 20 25 EGR [%] 8 6 4 2 0 Increasing EGR tolerance of volumetric igniters
Fuel or Charge Enhancement Solutions Fuel-based solutions Pilot ignition using diesel or diesel-like fuel Charge Enhancement Chemical supplementation 220 H 2 enhancement 2 2 Engine Design Based High turbulence High CR operation High S/B ratio gisfc (g/kwh) 200 190 180 Electrical ignition 15 25 35 45 55 % EGR Pilot ignition
Boosting Challenges for Dilute Engines High EGR rates High MAP and low pre-turbine temperatures H = U + PV & = m corr m& P T 01 01 High expansion ratio required for turbine power High pre-turbine pressure Low Dilution Low corrected flow rate Low U/C (blade speed ratio) High Dilution Low Turbine Efficiency Turbine is small relative to the compressor Shaft speed will be too low for best efficiency From SAE Paper 2011-01-0358
Boosting Solutions Two stage boost systems can achieve very good torque curves More expensive than non- EGR systems Cheaper than a light-duty diesel with aftertreatment Other options Pursue diesel torque curve with high EGR gasoline application Similar efficiency / Lower cost Limit EGR flow at certain portions of the engine map Torqu ue 1.6 L Engine Torque Curves < 235 g/kwh BSFC 2-stage boost (20% EGR) 2-stage boost (25% EGR) Production Gasoline Production Diesel (with overboost) 00 2000 0 4000 5000 6000 Engine Speed [rpm]
EGR Control Challenges Transient control of EGR is vital At high loads and during tip-ins Too little EGR can lead to knock and high pre-turbine temperatures Too much EGR can lead to misfire At low loads and during tip-outs EGR required to improve BSFC Too much EGR can lead to misfire Hardware design and advanced air handling control are vital to EGR control success
Materials and Design Requirements Peak Pressure Peak pressure is the key limiting parameter for torque above 2500 rpm in 2-stage TC applications Spark retard to limit PCP leads to instability At least 130 bar will be 40 required in the future Powertrain Architecture EGR system design will be key to control and packaging Heat rejection requirements in LPL EGR mode will need to be accounted for Downspeeding potential offers increased S/B ratio Cylinder Pr ressure [bar] 80 70 60 50 30 20 0 Increasing EGR 0% 25% -60-30 0 30 60 Crank Angle [CAD]
After addressing the challenges, the result is a high-efficiency, ultra-low emissions engine Single-stage boost system BSFC [g/kwh] 22 20 BSFC [g/kwh] 2-stage boost system 18 16 240 14 230 220 BMEP [bar] 12 8 6 260 4 2 1500 2000 2500 0 3500 4000 4500 5000 Engine Speed [rpm] Both engines operate at ~25% EGR 17 6//2011
High EGR engines have become a realistic option for Tier 4 (Final) applications Single-stage boost system BSFC [g/kwh] Similar torque as an offroad diesel engine of similar displacement Low-cost, high-power, low-co2 engine for Tier IV Main Fuel: HEEE Gasoline, Ignition: Diesel and/or SI 20 18 200 16 14 18 Diesel short-block converted to run as pilotignited gasoline engine TWC compliant for ultralow emissions BMEP (bar) 12 8 6 4 2 260 230 240 220 280 2 00 1200 1400 1600 1800 2000 2200 2400 Engine Speed (rpm) 6//2011
The future of high EGR engines 12 14 16 18 BS FC (g/kwh ) BMEP - Brake Mean Effective Pressure (bar) 6//2011 19 1200 1 1400 1500 1600 1700 1800 1900 2 4 6 8 BMEP - Brake Mean Effective Pressure (bar) E ng ine S pe ed (RP M) Dual-fuel 13 L on-highway engine (predicted BSFC)
Summary High EGR engines allow operation at high CR with low emissions Significant FE improvement potential Challenges remain Ignition Boosting Control Addressing these challenges, and others, will determine the ultimate application potential of the high EGR engine Latest results indicate that the high EGR engine can challenge the LD diesel at much lower cost and tailpipe emissions levels
What is next in High Efficiency? The industry needs to continue to address key issues: PM emissions and their effect on operating strategy for high efficiency Lean burn is it or isn t it a realistic option? Limits of downsizing LSPI as a limiting factor New drive cycles Friction improvements Increased requirement for cylinder pressure Knock reduction in SI engines will be a key item for the future
Reducing Knock in SI Engines through Oil Properties Altering the lubricant s chemical or physical properties can make it less reactive Lower CN lubricants allow more spark advance or higher boost levels Potentially significant enabler for downsizing / downspeeding 0 90 15W40 High CN Mid CN Low CN 89 ISFC [g g/kwh] 244 242 240 238 236 234 15W40 Oil Fully-Formulated IQT CN: 62, 91, 78 High CN Oil PAO base-stock IQT CN: 74, 8, 89 Mid CN Oil AN base-stock IQT CN: 49, 77, 64 Low CN Oil Ester base-stock IQT CN: 26, 45, 38 IQT derived Cetane Number 80 70 60 50 40 30 78 15W40 fully-formulated PAO base-stock R&D formulation PAO base-stock 64 R&D formulation AN base-stock 38 R&D formulation Ester base-stock 232 230 228 22 20 18 Spark Timing [deg btdc] 16 ~ 3.0% 14