Background NOx and PM Standards have driven diesel engine design for two decades Test methods have evolved over that time Manufacturers have adopted efficiency initiatives where customer return on investment demands would be satisfied Climate change and energy concerns have now initiated vehicle efficiency standards The emissions and efficiency requirements are not fully aligned
Progression of Complexity in US Diesel Engine Controls (NOx & PM Reduction) Mechanical Injection Electronic Injection (Injection Timing Authority) Boost Management (Wastegate, Electronic Wastegate) Cooled Exhaust Gas Recirculation Multiple Injections Oxidation Catalysts (some buses) Diesel Particulate Filtration Urea Selective Catalytic Reduction Increasingly Sophisticated Control Progression differed in Europe parallel SCR & non- SCR (EGR) tracks for low NOx From Clark 2011 Fall ASME ICE Keynote
History of NOx Efficiency Tradeoff Unregulated on-road mechanically injected diesel engines typically produced 10-15 g/kw hour NOx Optimal engine efficiency Manageable component temperatures Simple injection systems Modest NOx reductions were possible simply by retarding injection timing Loss of efficiency Approach used to reach about 5 g/ kw hour NOx NOx (g/s) Advancing timing on mechanical Mack Engine CO 2 (g/bhphr) -- -15% -12% -7% 11% NOx (g/bhphr) -- 61% 131% 183% 270% Data Source: WVU chassis dynamometer data 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 NOx = 0.0165 CO2-0.0558 R² = 0.7448 0 20 40 60 CO2 (g/s)
NOx-CO 2 Relationship: Electronic Injection and EGR Chassis dynamometer testing of OTR tractor Early EGR management NTE not yet enforced High scatter of NOx relative to CO 2 : linear relationship is lost Data Source: CRC E-55/59 Program NOx (g/s) 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 NOx = 0.0046 CO2 + 0.0082 R² = 0.6578 0 20 40 60 80 CO2 (g/s) 5
US FTP versus On-Road Operation 140.00 120.00 100.00 80.00 60.00 Torque 40.00 20.00 0.00-40.00-20.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00-20.00 FTP emphasizes operation near rated speed and torque -40.00-60.00-80.00 Speed 140.00 120.00 100.00 80.00 Source: Thesis research Radermacher, WVU Torque 60.00 40.00 20.00 0.00-40.00-20.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00-20.00-40.00-60.00-80.00 Speed
Off-Cycle Data 1.4 1.2 Regain efficiency and protect engines Steady-State Post Holes y = 0.0043x + 0.1906 R 2 = 0.7104 NOx Emissions (g/s) 1.0 0.8 0.6 0.4 Cruise Transient 0.2 y = 0.0012x + 0.1049 R 2 = 0.3387 0.0 0 50 100 150 200 250 300 350 Dispersed and Time Aligned Axle Horsepower (ahp) Source: WVU data Clark SAE Keynote
US Measures to Insure Broader NOx Reduction Source: Clark SAE Keynote
NOx Fuel Interactions Cases where both NOx and CO 2 are reduced together Reduced friction / Better lubricants Reduced load (e.g. lighter vehicle) Cases where both NOx and CO 2 increase together DPF Regeneration Exhaust back pressure Cases where NOx and CO 2 trade off Retarded timing Exhaust gas recirculation Indicated efficiency Pumping work to 2002) Complex cases (e.g. enable reduction but demand power) EGR cooling demand Multiple injections & rate shaping Krishnamurthy et al. (Atmos. Environ 2007) show ~10% fuel use increase for NOx reduction from 5 to 2.5 g/bhp-hr standard (US 1995 High pressure injection Downspeeding / managing powertrains / hybrid technology Upstream implications of urea Driving to find urea! Choice of units / engine & vehicle / engine sizing
Causes of Measurement Variability Sensitivity of EGR & timing strategy to transient operation Effects of changing exhaust backpressure with DPF DPF regeneration fuel use Cold start strategies SCR thermal effects & control effects Increasing difficulty in quantifying very low levels Hybrid operation brings additional complexities See SAE J2711 Powertrain controls to a lesser degree Vehicle-based efficiency measurements and modeling results cannot characterize small efficiency differences accurately
DPF Regeneration raising Chassis dynamometer test data from 2007 Cummins ISL 320 over OCTA driving schedule REGENERATION DOES NOT INFLUENCE NOx- CO 2 relationship substantially, but both have highest values during regeneration NOx and CO 2 NOx (g/s) 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 Normal Regen NOx = 0.0028 CO 2 + 0.0028 R² = 0.6317 (regen) NOx = 0.0027 CO 2-0.0017 R² = 0.6549 (normal) Data show that relationships between NOx and power and CO 2 and shaft power are affected. 0.04 0.02 0 0 10 20 30 40 50 CO 2 (g/s) Source: WVU chassis data CRC Presentation
OCTA Driving Schedule NOx over Three Repeat Runs per Bus (Data from CAFEE Database, DOE, DOT & LYNX data) NOX (g/mile) 45 40 35 30 25 20 15 10 5 0
NOX Measurment Coefficient Variance OCTA Driving Schedule NOx Variability over Three Repeat Runs per Bus (Data from CAFEE Database, DOE, DOT & LYNX data) 18% 16% 14% 12% 10% 8% 6% 4% 2% 0%
EGR and SCR To meet 2002-2010 2.5 g/bhp-hr and 1.2 g/bhp-hr NOx heavy-duty on-road levels, the US manufacturers opted for in-cylinder and injection controls, and for cooled EGR. Euro IV levels were met by using either SCR or EGR in Europe. [Erkkila & Nylund report shows SCR offers better efficiency in most cases]. Post-2010 US and Euro VI employ SCR, which may be used with or without EGR. SCR accommodates higher engine-out emissions, usually offering an efficiency gain, but SCR must be active to reduce NOx. [US studies of school bus efficiency favor SCR].
Test-to-test Variability: Urea SCR 2012 OTR Tractor 2011 Diesel engine (Mack MP8) Urea-SCR exhaust aftertreatment Integrated NOx (grams) 16 14 12 10 8 6 4 2 Cold Warm Hot #1 Hot #2 Hot #3 Hot #4 Post SCR Exhaust Temp (C) 300 250 200 150 100 50 Cold Warm Hot #1 Hot #2 Hot #3 Hot #4 0 0 200 400 Time 600 (s) 800 1000 1200 0 0 200 400 600 800 1000 1200 Time (s) WVU Data CRC Presentation 15
Hot- and Cold-start NOx with SCR 2010 30 foot Transit Bus Paris Cycle Hot Start Emissions Cold Start Emissions Source: SAE 2011-01-2469 Clark, McKain, Wayne, Carder & Gautam, WVU
NOx below 0.2 g/bhp-hr in the US California has funded a study aiming at 0.02 g/bhp-hr NOx from heavy-duty engines (additional 90% reduction) Diesel Natural Gas Different possible pathways for diesel More intensive cooled EGR with SCR (reduced engine-out) Increasingly sophisticated SCR (sensors and models) Aggressive injection of reductant with cleanup Ammonia concerns Roberts (2011) and Johnson (2012) have discussed engine-out vs. aftertreatment tradeoffs
Summary & Observations NOx reduction through retarded timing and cooled EGR reduces engine efficiency. Some EGR cooling burdens are not measured in a test cell SCR has offered a pathway to recovering efficiency, but urea is now required. Urea has an upstream footprint and cost Emissions will be high if the catalyst is inactive Present pathways suggest that further NOx reduction will imply engine efficiency loss and/or higher urea usage. Regulatory tools are not fully aligned with on-road use and are about to face an information and control onslaught. Low emissions levels are hard to measure. Small changes in efficiency are hard to measure.