Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine

While operating at light loads, diesel pilot-ignited natural gas engines with lean pre-mixed natural gas suffer from poor combustion efficiency and high methane emissions. This work investigates the limits of low-load operation for a micro-pilot diesel natural gas engine that uses a stoichiometric mixture to enable methane and nitrogen oxide emission control. By optimizing engine hardware, operating conditions, and injection strategies, this study focused on defining the lowest achievable load while maintaining a stoichiometric equivalence ratio and with acceptable combustion stability. A multi-cylinder diesel 6.7 L engine was converted to run natural gas premix with a maximum diesel micro-pilot contribution of 10%. With a base diesel compression ratio of 17.3:1, the intake manifold pressure limit was 80 kPa (absolute). At a reduced compression ratio of 15:1, this limit increased to 85 kPa, raising the minimum stable load. Retarding the combustion phasing, typically used in spark-ignition engines to achieve lower loads, was also tested but found to be limited by degraded diesel ignition at later timings. Reducing the pilot injection pressure improved combustion stability, as did increasing pilot quantity at the cost of lower substitution ratios. The lean operation further reduced load but increased NOx and hydrocarbon emissions. At loads below the practical dual-fuel limit, a transition to lean diesel operation will likely be required with corresponding implications for the aftertreatment system

Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine

While operating at light loads, diesel pilot-ignited natural gas engines with lean premixed natural gas suffer from poor combustion efficiency and high methane emissions. This work investigates the limits of low-load operation for a micro-pilot diesel natural gas engine that uses a stoichiometric mixture to enable methane and nitrogen oxide emission control. By optimizing engine hardware, operating conditions, and injection strategies, this study focused on defining the lowest achievable load while maintaining a stoichiometric equivalence ratio and with acceptable combustion stability. A multi-cylinder diesel 6.7 L engine was converted to run natural gas premix with a maximum diesel micro-pilot contribution of 10%. With a base diesel compression ratio of 17.3:1, the intake manifold pressure limit was 80 kPa(absolute). At a reduced compression ratio of 15:1, this limit increased to 85 kPa, raising the minimum stable load. Retarding the combustion phasing, typically used in spark-ignition engines to achieve lower loads, was also tested but found to be limited by degraded diesel ignition at later timings. Reducing the pilot injection pressure improved combustion stability, as did increasing pilot quantity at the cost of lower substitution ratios. The lean operation further reduced load but increased NOx and hydrocarbon emissions. At loads below the practical dual-fuel limit, a transition to lean diesel operation will likely be required with corresponding implications for the aftertreatment system

COMBUSTION DEVELOPMENT OF A HIGH EFFICIENCY DIESEL MICRO PILOT NATURAL GAS ENGINE

Dual fuel engine operation with premixed natural gas as the main fuel and diesel pilot ignition has been gaining interest among research and industry as natural gas is among the most promising existing alternative fuels. The dual fuel engine performance has been shown to equal and sometimes, depending on operating conditions, better the performance and efficiency of the diesel engine. Along with its advantages on the combustion high efficiency, diesel-like performance, and emissions of NOx and particulate matter reduction, some disadvantages are brought by the application of such operation. At light load conditions, there is an increase in CO and HC emissions, low fuel efficiency and combustion stability. While operating at higher loads, the dual fuel engine performance showed to be limited by combustion knock. This effectively reduces the maximum break mean effective pressure (BMEP) the engine can output when compared to a diesel engine. Although combustion knock is well defined in SI and diesel engines, dual fuel knock characterization still needs more investigation. This project centers on developing a fuel system for a diesel engine conversion to dual fuel to deliver high load and high efficiency. The selected engine has been converted to the dual fuel operation and dual fuel combustion has been demonstrated. After achieving the project goal of a high load and high efficiency dual fuel engine, the combustion knock in dual fuel operation will be characterized and a method for detection and intensity calculation will be modeled. The characterization will also be compared to spark ignition (SI) and reactivity-controlled compression ignition (RCCI) operating engines

COMBUSTION DEVELOPMENT OF A HIGH LOAD HIGH-EFFICIENCY MICRO-PILOT DIESEL NATURAL GAS ENGINE

The conventional internal combustion engine will continue to exist for a long time. Likewise, demand for higher output efficiencies, higher specific power output, increased reliability, and lower emissions will continue to grow. There is also a growing requirement to run on various gaseous fuels and natural gas, whether for environmental, economic, or resource conservation reasons. This dissertation investigates a 6.7L diesel engine converted to run stoichiometric diesel micro-pilot / natural gas premix combustion with a maximum diesel contribution target of 5% of the total fuel energy with a three-way catalyst aftertreatment. The research centers on investigating the dominant factors and their impact on the critical barriers of this technology, including the positive and negative impact on combustion stability at low loads, the most influential factors and their impact on maximizing thermal efficiency at medium loads, the controlling parameters at preventing combustion knock at high-loads, and the ability of the three-way catalyst to minimize emissions. A diesel-like efficiency of 41% brake thermal efficiency was achieved with a high load output of 23 bar brake mean effective pressure when operating in the micro-pilot mode. This operating condition reduced up to 25% brake-specific CO2 emissions compared to diesel-only. Low loads can be achieved by delaying combustion phasing, reducing the injection pressure, adding exhaust gas to the intake, and increasing the total diesel pilot quantity. Maintaining stable ignition of the diesel pilot becomes a challenge at low loads, as the intake pressure is reduced; the chamber pressure at diesel injection decreases, and the presence of a near-stoichiometric mixture of NG will act to inhibit the diesel ignition. As such, maintaining the stoichiometric combustion resulted in a minimum load output of 5 bar BMEP. The pilot injection pressure reduction improved combustion stability at lower loads. While lean operation enabled further load reduction, it precludes using a three-way catalyst to control NOx emissions. At medium loads, a design of experiments investigation revealed that, when the equivalence ratio is constrained at stoichiometric, exhaust gas recirculation and pilot injection timing are the most influential factors in controlling combustion and performance metrics. In contrast, intake air temperature and pilot injection pressure showed the least sensitivity. While it was possible to achieve 25 bar BMEP for high loads, such operation was limited by pre-ignition. Exhaust gas recirculation and pilot injection timing can mitigate abnormal combustion effectively. At a steady-state, near stoichiometric condition, it was observed that the catalyst operates efficiently, consistent with a three-way catalyst operation with very low NOx and unburned methane emissions. Overall, this dissertation demonstrates that diesel-like performance can be achieved with the stoichiometric micro-pilot concept and provides an understanding of the primary controlling factors and their limitations

Model predictive control of a dual fuel engine integrated with waste heat recovery used for electric power in buildings

Waste heat recovery (WHR) system uses the thermal energy from the exhaust gases of an internal combustion engine (ICE) to assist in the electricity generated by the ICE generator in buildings. This paper presents a model predictive control (MPC) framework to minimize the fuel consumption of an ICE by integrating it with a WHR system. To this end, a control oriented model of a WHR system is developed and then integrated to a control oriented model of a turbocharged dual fuel diesel-natural gas ICE. The ICE model is derived based on experimental data collected from a 6.7 L Cummins ISB engine modified for dual fuel operation. The designed MPC framework optimizes the ICE combustion, turbocharger, and organic Rankine cycle (ORC) system in the WHR to minimize fuel consumption of the ICE. The designed control framework also allows to meet time-varying exhaust gas temperature requirements of the ICE to meet exhaust emission constraints. The results show that the optimal operation of the WHR and the ICE reduces the fuel consumption of the ICE by 6.7%

Development of a medium-duty stoichiometric diesel micro-pilot natural gas engine

Fueling a compression-ignition engine with premixed natural gas offers the potential to combine a clean-burning, low-carbon fuel with a high compression ratio, high-efficiency engine. This work describes the development of a multi-cylinder 6.7 L diesel engine converted to run stoichiometric diesel micro-pilot/ natural gas premix combustion with a maximum diesel contribution target of 5% of the total fuel energy and a three-way catalyst aftertreatment system. Results are given by comparing the stoichiometric combustion to the diesel baseline operation, showing combustion characteristics differences, including the rapid two stage heat release. A high load output of 23 bar brake mean effective pressure was obtained with diesel-like brake thermal efficiency of 41%. This operating condition enabled a brake specific CO2 emissions reduction of up to 25% when compared to diesel. It was observed that the low load output is limited by combustion stability when operated at stoichiometric condition. The three-way catalyst is observed to run at peak efficiency with an equivalence ratio of 1.01. Injector fouling was observed through the inspection of the nozzle and its internal parts, indicating carbon build-up similar to that seen in injector coking mechanisms. A comparison of the developed engine to other engine technologies is given, showing that the diesel micro-pilot natural gas engine performance is in good standing among other diesel and gas engines in the market

Multi-Variable Sensitivity Analysis and Ranking of Control Factors Impact in a Stoichiometric Micro-Pilot Natural Gas Engine at Medium Loads

A diesel piloted natural gas engine\u27s performance varies depending on operating conditions and has performed best under medium to high loads. It can often equal or better the fuel conversion efficiency of a diesel-only engine in this operating range. This paper presents a study performed on a multi-cylinder Cummins ISB 6.7L diesel engine converted to run stoichiometric natural gas/diesel micro-pilot combustion with a maximum diesel contribution of 10#x00025;. This study systematically quantifies and ranks the sensitivity of control factors on combustion and performance while operating at medium loads. The effects of combustion control parameters, including the pilot start of injection, pilot injection pressure, pilot injection quantity, exhaust gas recirculation, and global equivalence ratio, were tested using a design of experiments orthogonal matrix approach. Specific outcomes from this research lead to fundamental and essential new knowledge in identifying the dominant factors for optimizing engine performance (i.e., thermal efficiency, combustion stability, combustion duration). The results provide a path forward for developing a high-efficiency engine and an optimized fuel and air handling system by ranking different controlling parameters for each performance metric studied. It was observed that exhaust gas recirculation and diesel pilot start of injection are the most influential parameters controlling medium load performance. In contrast, intake air temperature and pilot injection pressure have the least impact on the condition studied

Investigation of high load operation of spark-ignited over-expanded Atkinson cycle engine

A boosted spark-ignited over-expanded engine was investigated through 1-D engine simulation. A conventional 4-stroke turbocharged spark-ignited engine with 10.5:1 compression ratio (CR) was selected as the baseline engine. The Atkinson cycle engine model was developed and calibrated based on a multi-link mechanism. The compression ratio (CR) and over expansion ratio (OER) of the Atkinson cycle engine are 10.5 and 1.5, respectively. Two speed and load conditions of 1500 rpm, 13 bar net indicated mean effective pressure (IMEPnet) and 3500 rpm, 20 bar IMEPnet with valve timing optimization were investigated. Results depict that the increase in indicated efficiency of Atkinson cycle engine was from both portions of over-expansion and non-over-expansion portion of the cycle. Atkinson cycle engine benefits from lower knock propensity and lower exhaust temperature. At 1500 rpm, 13 bar IMEPnet, the simulation results indicated that energy loss due to combustion phasing was 2.1% and 0.4% for baseline and Atkinson cycle engine. Net indicated efficiency of Atkinson cycle engine was increased by 16%. At 3500 rpm, 20 bar IMEPnet, baseline engine was operated at knock limited spark timing and fuel enrichment to reduce the turbine-inlet temperature. Net indicated efficiency of optimized Atkinson cycle engine at 3500 rpm 20 bar IMEPnet was higher by 27% in comparison to the optimized baseline engine. The combustion phasing loss was 1.2% and 0.6% for baseline and Atkinson cycle engine, respectively. The energy loss due to fuel enrichment was 6.0% and 1.6% for baseline engine and Atkinson cycle engine, respectively, indicating that the Atkinson cycle engine was beneficial to maximize its efficiency

Process for study of micro-pilot diesel-NG dual fuel combustion in a constant volume combustion vessel utilizing the premixed pre-burn procedure

A constant volume spray and combustion vessel utilizing the pre-burn mixture procedure to generate pressure, temperature, and composition characteristic of near top dead center (TDC) conditions in compression ignition (CI) engines was modified with post pre-burn gas induction to incorporate premixed methane gas prior to diesel injection to simulate processes in dual fuel engines. Two variants of the methane induction system were developed and studied. The first used a high-flow modified direct injection injector and the second utilized auxiliary ports in the vessel that are used for normal intake and exhaust events. Flow, mixing, and limitations of the induction systems were studied. As a result of this study, the high-flow modified direct injection injector was selected because of its controlled actuation and rapid closure. Further studies of the induction system post pre-burn were conducted to determine the temperature limit of the methane auto-ignition. It was found that for sufficient induction and mixing time determined from experimental observations and CFD modeling studies, a maximum core temperature of 750 K at the time of micro-pilot diesel injection can be achieved. Although lower than TDC temperatures in diesel CI engines, this temperature is sufficient for studying dual fuel injection and auto-ignition with high cetane fuels. Results from this work confirm the feasibility of dual fuel combustion using the proposed CV process and provide constraints for further micro-pilot diesel-NG dual fuel combustion studies