Thermal efficiency and emission analysis of advanced thermodynamic strategies in a multi-cylinder diesel engine utilizing valve-train flexibility

Chuan Ding, Purdue University

Abstract

Stringent emission regulations and a growing demand for fossil fuel drive the development of new technologies for internal combustion engines. Diesel engines are thermally efficient but require complex aftertreatment systems to reduce tailpipe emissions of unburned hydrocarbons (UHC), particulate matter (PM), and nitrogen oxides (NOx). These challenges require research into advanced thermodynamic strategies to improve thermal efficiency, control emission formation and manage exhaust temperature for downstream aftertreatment. The optimal performance for different on-road conditions is analyzed using a fully flexible valve-train on a modern diesel engine. The experimental investigation focuses on thermal management during idling and high-way cruise conditions. In addition, simulation are used to explore the fuel efficiency of Miller cycling at elevated geometric compression ratios. Thermal management of diesel engine aftertreatment is a significant challenge, particularly during cold start and extended idle operation. For instance, to be effective, NOx-mitigating selective catalytic reduction (SCR) systems require bed and gas inlet temperatures of at least 200°C, and diesel oxidation catalysts coupled with upstream fuel injection require inlet temperatures of at least 300°C in order to raise diesel particulate filter inlet temperatures to at least 500°C for active regeneration. However, during peak engine efficiency idle operation, the exhaust temperatures only reach 120 and 200°C for unloaded (800 rpm/ 0.26 bar BMEP) and loaded (800 rpm/ 2.5 bar BMEP) idle, respectively, for a typical modern-day diesel engine. For this and other engines like it, late injections or throttling (for instance via an over-closed variable geometry turbocharger) can be used to increase exhaust temperatures above 200°C (unloaded idle) and 300°C (loaded idle), but result in fuel consumption increases in excess of 100% and 67%, respectively. Fortunately, and as this thesis describes, cylinder deactivation can be used to increase exhaust temperatures above 300°C at the loaded idle condition without increasing fuel consumption. Further, at the unloaded idle condition, the combination of cylinder deactivation and flexible valve actuation on the activated cylinders allows 200°C exhaust temperatures without a fuel consumption penalty. At both operating conditions the primary benefits are realized by reducing the air flow through the engine, directly resulting in higher exhaust temperatures; and as good, or better, open cycle efficiencies compared with conventional 6 cylinder operation. In all cases, comparisons are made with strict limits on engine out NOx, unburned hydrocarbons, and particulate matter emissions. Internal exhaust gas recirculation (iEGR), late intake valve closure (LIVC) and cylinder deactivation (CDA) were experimentally investigated as methods for fuel economy and thermal management at 1200 RPM and 7.58 bar brake mean effective pressure (BMEP), which corresponds to the highway cruise condition for over the road trucks. These strategies were compared with conventional operation on the basis of optimized fuel consumption, exhaust temperature, and exhaust power at three NOx targets. Physical constraints and emission limits were set to ensure realistic engine operation and emission regulations. The results show that conventional valve profiles lead to the best fuel economy, but iEGR, LIVC and CDA increase achievable exhaust temperature by 57-216 °C. iEGR increases exhaust temperatures by eliminating the heat rejection that occurs when using external EGR. Both LIVC and CDA increase combustion temperature by reducing the air to fuel ratio. Advanced thermodynamic strategies such as the Miller cycle and Atkinson cycles have been realized on production spark ignition engine through variable valve timing. However, fewer efforts have been directed to compression ignition engines. Increases in geometric compression ratio typically lead to increased thermal efficiency, but the application is constrained by physical limits including peak cylinder pressure and turbine inlet temperature. An experimentally validated model was used to obtain the trade-off; between fuel economy and NOx emissions in order to thoroughly investigate Miller cycling at elevated geometric compression ratio. The results demonstrate the expected improvement in thermal efficiency, however, as expected, the maximum in-cylinder pressure and temperature violate the physical constraints at elevated power conditions. These challenges can be addressed through the use of Miller cycling via a reduced effective compression ratio through the modulation of intake valve closure. Miller cycling enables the engine operation with elevated geometric compression ratio at maximum power condition and further improves fuel economy by advancing combustion. The results present a 5% fuel economy improvement at operating conditions without EGR and equivalent fuel consumption when EGR is incorporated. Brake thermal efficiency (BTE) is improved by 0.1%-2% using Miller cycle at elevated GCR. Although EGR was able to achieve very low NOx emissions, fuel economy was sacrificed at medium load condition. Moreover peak cylinder pressure (PCP) and turbine inlet temperature (TIT) exceeded the upper limits at maximum power condition using EGR with elevated geometric compression ratio.

Degree

Ph.D.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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