Improving the Spark-Ignition Engine

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Albert Barnett
7 years ago
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1 M I T Improving the Spark-Ignition Engine John B. Heywood Sun Jae Professor of Mechanical Engineering Director, Sloan Automotive Laboratory M.I.T. Engine Research Center Symposium University of Madison, Wisconsin June 8-9, 2005

2 Theme Why can t a spark-ignition engine be more like a diesel? 2

3 Key Diesel Characteristics 1. High compression ratio 2. Operates lean 3. Operates unthrottled 4. Usually turbocharged 3

4 Contributors This presentation includes results from several of my graduate students: Ferran Ayala, Mike Gerty, Josh Goldwitz, Ziga Ivanic, Bridget Revier, Dan Sandoval, Jenny Topinka 4

5 Comparison: Spark-Ignition and Diesel Engine 5

6 Agenda: Improving the Spark-Ignition Engine 1. Lean operation vs. stoichiometric (EGR): Efficiency/emissions trade-off 2. Benefits of higher compression ratio 3. Reducing throttling losses 4. Turbocharging and downsizing 5. Dealing with knock constraint 6

7 Definitions 7

8 Definitions (Continued) 8

9 SI Engine Friction FMEP (kpa) Friction Model Ford 2.0L engine Speed (rpm) 9 Mechanical plus accessory friction mep vs. speed (Sandoval, Ivanic)

10 Comparisons: What Hold Constant? 1. Vehicle performance Maximum torque/brake power Vary engine displaced volume bmep = 4! x torque/displaced volume Then, imep = bmep + fmep Simpler: Cylinder volume Net imep constant Brake performance: subtract friction 06/08/05

11 U.S. Emissions Requirements 1. Gasoline spark-ignition engine Operate stoichiometric Use three-way catalyst: 98% effective Probably meet PZEV emissions 2. High-speed direct-injection diesel Particulate and NO x exhaust treatment technology? Active control system required Reducing agents for NO x : 5 % fuel penalty Are PZEV levels feasible? 11

12 Change Dilution:Air and/or EGR 12 06/08/05

13 10-90% Burn Duration (CAD) Air Lean, EGR: Effects on Burn Rate EGR 3.5 bar NIMEP Rc= Thermal Dilution Parameter (TDP) Source: Ayala 06/08/05

14 Lean, EGR: Effects on Efficiency, COV IMEP Engine net indicated efficiency Efficiency - Air Efficiency - EGR COV - Air COV - EGR 3.5 bar NIMEP Rc= COV of NIMEP (%) Thermal Dilution Parameter (TDP) Source: Ayala 0 06/08/05

15 Explanation: Dilution Efficiency Limit 37% 35% Gamma + Heat Transfer + Pumping Gamma + Heat Transfer Efficiency 33% 31% 29% Data All Effects Baseline Efficiency 27% Lengthening Burn Duration 25% Relative Air/Fuel Ratio Source: Ayala

16 Compression Ratio Effects Net Indicated Efficiency NIMEP = 2.0 bar, 1500 rpm NIMEP = 2.0 bar, 2500 rpm NIMEP = 4.0 bar, 1500 rpm NIMEP = 4.0 bar, 2500 rpm NIMEP = 8.0 bar, 1500 rpm NIMEP = 8.0 bar, 2500 rpm R c 16 Change in net efficiency with compression ratio for a range of loads;! = 1.0 (Gerty). 06/08/05

17 Compression Ratio, Boost, and Downsizing Change in Brake Efficiency (%) Without Downsizing With Downsizing Change in Efficiency (%) Net Efficiency Brake Efficiency R c NIMEP Boost Level (%) 17 Increases in net and brake efficiency with r c and boost, with Engine downsizing for constant max. torque. 2.6 bar bmep, 1500 rpm,! = 1.0 (Gerty). 06/08/05

18 Knock Constraint 1. Knock is the process that produces an audible (sharp, clanging) sound outside the engine. 2. Caused by rapid autoignition of the unburned end-gas in a fraction of the engine s cycles. 3. Onset first occurs at time of peak pressure. 4. Key variables are end-gas temperature, pressure and composition; time/speed; fuel octane rating. 18

19 Knock: Sensitivity to Air and Fuel Flow 13 IPW [ms] (measure of fuel flow) Boxed Numbers = ONR Constant Output Constant Fuel Constant MAP Constant Lambda Intake Manifold Pressure [bar] Octane requirement map: primary reference fuels (Topinka) 19

20 Benefits of Turbocharging 1. Significantly increases WOT bmep 2. At constant vehicle performance allows engine downsizing 3. At typical part load (25% of max. torque), bmep is then higher 4. Part-load brake efficiency increases due to reduced pumping work AND increased mechanical efficiency 20

21 Benefits of Turbocharging (Continued) Bmep, bar 21 Performance maps of naturally aspirated and turbocharged engines. (Gerty).

22 Dealing with Knock 1. Higher octane (e.g. premium) gasoline 2. With VVT, delay intake valve closing 3. Homogeneous gasoline direct injection 4. Variable compression ratio engine 5. Increase mixture octane with H 2 plus CO from on-board reformer Direct injection of ethanol

23 Gains from Engine Boosting and Downsizing Reduction Fuel economy, in V d Test efficiency, gain Study % cycle % Ivanic (MIT) 30 U/H FTP Gerty (MIT 30 light load 16 FEV (Lang, et al) 30 NEDC Ricardo (Web) engine data 6-17 IFP (Lecointe) 40 DI engine 25 Average

24 Diesel More Efficient Than SI Engine Typical driving, diesel mpg some 50 percent higher than SI engine mpg. Higher fuel energy/gallon 8% Lean operation (above EGR) 5% Compression ratio higher 5% High levels of boost 20% Combustion efficiency 6% Total ~ 50% 24

25 Summary 1. The biggest opportunity for improving the sparkignition engine is boosting and downsizing. 2. Stoichiometric operation enables very low air pollutant emissions. 3. Many other design variables could contribute: e.g. increase compression ratio, variable valve control, lower friction 4. The major challenge is controlling knock % higher part-load efficiency plausible 25

Engine Heat Transfer. Engine Heat Transfer

Engine Heat Transfer. Engine Heat Transfer Engine Heat Transfer 1. Impact of heat transfer on engine operation 2. Heat transfer environment 3. Energy flow in an engine 4. Engine heat transfer Fundamentals Spark-ignition engine heat transfer Diesel