Increasing the high load limit of effective premixed charge compression ignition via intake valve closure modulation and late injection
Diesel engines have superior efficiency and power density. However, traditional diesel engines can produce large amount of nitrogen oxide (NOx) and particulate matter (PM) emissions. In addition, the EPA and NHTSA have recently established fuel economy and green house gas emission standards which require the diesel engine to be more efficient than ever before. Premixed charge compression ignition (PCCI) is an advanced combustion mode that can reduce PM and NOx emissions and maintain, or even improve, the engine efficiency. However, there are several factors, including mixing timing, pressure rise rate and gas exchange that limit the PCCI operating range. In order to extend the effective PCCI operating range, this study was performed as a continuation of work conducted by the author's colleagues. Specifically, this effort focuses on numerically identifying, and experimentally validating, the high load limit of effective PCCI. This thesis first presents results for the calibration of a GT-Power engine model. This model was then utilized to run several design of experiments (DOEs) with four different input variables, including the VGT position, rail pressure, intake valve closing timing (IVC), and start of injection (SOI). Regression models were created based on the results from the DOEs. These models were then used to optimally determine the required inputs for effective PCCI. Strict engine-out emissions were set as constraints for effective PCCI. Specifically, bsNOx, bsPM, bsUHC, and bsCO must be less than 0.2, 0.1, 7, and 775 g/hp-hr, respectively. Air-fuel ratio and the overlap between start of combustion and end of injection (SOC-EOI) in the simulation were considered as proxies for these emission constraints. Pressure rise rate (PRR) was used as a proxy for noise. The optimization targeted maximizing torque at a given speed and fueling amount without violating any the following constraints: AFR≥18, SOC-EOI≥-2°CA, PRR<100 bar>/ms. To be classified as effective PCCI, the optimal result must also maintain, or improve, brake thermal efficiency (BTE) compared to conventional operation at the same speed and fueling point. The final results from this study indicated that BTE limited effective PCCI operation at 800 rpm, which was also illustrated by the author's colleagues' prior experimental work. However, this study, both numerically and experimentally, demonstrated an increase in the maximum achievable effective PCCI load from 5.2 to 7.3 bar BMEP at 1200 rpm by utilizing late SOI, high rail pressure, fully opened EGR valve, and early IVC. This finding is profound, as 1200 rpm 7.3 bar BMEP corresponds to the highway cruise condition for modern class 8 trucks. This result demonstrates that the combination of late SOI (after top dead center) and IVC modulation enables highway cruise operation with no engine fuel penalty and EPA-compliant engine out NOx emissions, suggesting that urea consumption during cruise can be eliminated. At 1600 rpm, the numerical work indicated that, using a similar strategy, the maximum effective PCCI load can be increased from 4.4 to 5.5 bar BMEP.
Shaver, Purdue University.
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