In a US Department of Energy (DOE)-sponsored project, engineers at Cummins are developing a Tier 2 Bin 2 emissions compliant diesel for application in a light duty pickup (ATLAS, Advanced Technology Light Automotive Systems, earlier post). Tier 2 Bin 2 requirements are only slightly less stringent than the CARB LEVIII-SULEV20 requirements. (Earlier post.) Fuel economy targets for the vehicle are 22.4 mpg US (10.5 l/100km) city and 34.3 mpg US (6.9 l/100 km) highway.
At the recent 2103 SAE World Congress, Cummins discussed key engine technology enablers—including air-handling, fuel system, and base engine design— and development of the combustion system that will help in achieving the target emission levels and fuel economy.
|US Light Duty Diesel Emission Standards|
(full useful life: 150,000 miles)
|Tailpipe emissions [g/mile]||US EPA Tier 2 Bin 2||CARB LEVIII-SULEV20|
|NOx + NMOG||0.03||0.02|
The baseline engine for the work is a Euro IV-compliant inline 4-cylinder 2.8L diesel equipped witha single-stage wastegate turbocharger and generating 160 hp (119 kW) of power and 265 lb-ft (359 N·m) of torque, with a compression ratio of 16.9:1. Emission control is via high pressure cooled EGR with a diesel oxidation catalyst.
The test vehicle is a MY2010 Nissan Titan crew cab 2x4; the truck’s 5.6L V8 was replaced with the 2.8L. The upfitted Titan was tested on a chassis dyno to establish baseline emissions, performance and fuel economy for FTP-75, LA-4 and HFET drive cycles.
Based on the baseline emissions profile, the Cummins team determined that they had to reduce NOx by 99%, PM by 90%, and NMHCs by 67% to meet the design target.
The team started by upgrading the 1600 bar injector system to a 2000 bar system, using 8-hole injectors rather the original 7-hole injectors. The concept architecture calls for upgrading the original single-stage turbocharger to a more efficient variable geometry turbine and a larger compressor wheel/housing to meet requirements for power density and lowered fuel consumption.
To achieve the emissions targets, higher levels of EGR are needed. (Increased EGR rates results in decreased oxygen concentration in the intake manifold, resulting in reduced engine-out NOx.)
The EGR circuit has both high pressure (HP) and low pressure (LP) loops. (LP EGR has an advantage compared to HP in lowering intake oxygen concentration.) The HP loop recirculate exhaust from exhaust manifold to intake manifold via an HP EGR valve; there is no physical EGR cooler in the HP loop. The LP loop runs from downstream of an integrated DOC-SCRF aftertreatment system. AN exhaust throttle and LP EGR valve work together to control mass flow rate. An EGR cooler in the LP loop helps reduce gas temperature before mixing with fresh air upstream of the turbo.
The team found that by optimizing LP EGR, they could reduce fuel consumption by approximately 16% compared to the optimal HP EGR solution. Their work also showed the need for dual loop EGR in certain areas of the drive cycle to help improve fuel economy.
...the dynamics of dual loop EGR varies over the engine operating map driven by duty cycle and emission requirements. There is no single strategy that will work consistently across the engine map and hence several iterations of optimization will be required both analytically and experimentally to arrive at the best possible solution. Several factors like air-handling thermodynamics, turbocharger selection, emission requirements during drive cycle and cold start have a significant effect on calibration development and optimization.—Suresh et al.
To select the variable geometry turbine, they ran through multiple hardware combinations and duty cycle optimizations.
Other studies have shown that lowering compression ratio can help to reduce engine-out smoke levels along with enabling premixed combustion modes favoring low NOxformation. To confirm this experimentally, the Cummins team made two combustion bowls with 16.5 CR and 15.3 CR. These were installed on two different engines with 8-hole injectors.
With higher EGR, they found a reduction in oxygen concentration for the 15.3 CR down to 15.1%, resulting in a significant reduction in NOxemissions. The additional piston bowl volume also helps to achieve better in-cylinder charge-fuel mixing resulting in lower smoke emissions when compared to the 16.5 CR bowl.
The improved turbine match with the 15.3 CR engine further helped in reducing fuel consumption by 3% when compared to 16.5 CR and this can be attributed towards reduced pumping losses resulting in an improvement in open cycle efficiency.—Suresh et al.
The Cummins team found a significant reduction in smoke emissions via the combination of lower CR, 8-hole nozzles and high in-cylinder swirl. The team is also exploring the concept of generating variable swirl through the use of variable valve timing as a possible future design feature.
Running a version of the hardware (ATLAS 1.7) in a testbed, they reported finding:
0.40 g/mile engine-out NOx based on a modal roll-up LA-4 cycle. The reduction in NOxwas more than a factor of 4 when compared to the baseline engine.
PM emissions were <0.04 g/mile—“stellar”, due to advancements in air-handling and combustion systems.
Calibration optimization helped bring down the engine-out unburned HC to 0.27 g/mile. Improved injector nozzle technology and optimization around combustion bowl-nozzle matching may result in further engine-out HC reductions.
Fuel economy for a modal rollup LA-4 cycle was 25.4 mpg (9.3 l/100 km), exceeding the city target.
Fuel economy for a modal rollup based on HFET was 32.6 mpg (7.2 l/100 km)—falling short of the target. The engineers said that optimization involving engine hardware, calibration, controls and aftertreatment integration will help in bridging the gap.
Mule testing showed a multi-fold reduction in NOxemissions by 36%, 55% and 28% for FTP_75, LA-4 and HFET cycle respectively, compared to the baseline. Equipped with an 8-speed automatic, the mule delivered fuel economy of 24.8, 26.7 and 34.4 mpg US (9.5, 8.8 and 9.1 l/100 km) for FTP-75, LA-4 and HFET cycles, respectively.
The ultimate goal would be to integrated ATLAS 1.7 technologies and 8-speed automatic transmission in the test vehicle with appropriate refinements around design, packaging ,performance, power management and reliability for meeting Tier 2 Bine 2 emissions. Vehicle noise and vibration will be an area of concern for 4-cylinder light duty diesels. Advanced technologies related to engine design as well as challenges around combustion noise improvements need to be addressed in the future.—Suresh et al.
Suresh, A., Langenderfer, D., Arnett, C., and Ruth, M. (2013) “Thermodynamic Systems for Tier 2 Bin 2 Diesel Engines,” SAE Int. J. Engines 6 (1): 167-183 doi: 10.4271/2013-01-0282