Carrie Hall

Closed-loop combustion control of biodiesel–diesel blends in premixed operating conditions enabled via high exhaust gas recirculation rates

Stringent emissions and fuel economy regulations motivate the investigation of advanced combustion strategies such as premixed charge compression ignition. However, controlling premixed charge compression ignition is challenging as there is no direct trigger for the combustion event. In addition, fuel-flexible applications must account for the impact of variable fuel properties on the premixed combustion proces. This paper identifies the control challenges in fuel-flexible premixed charge compression ignition and presents a control strategy for mitigating the differences in combustion phasing, the increases in nitrogen oxide emissions, and the decreases in power output typically encountered when operating with biodiesel in premixed operating conditions. When biodiesel is used in premixed operating conditions with the engine operating with the stock calibration for diesel fuel, the timing of the start of combustion and the peak heat release can shift by over 2° crank angle and 4° crank angle respectively, the nitrogen oxide emissions can increase by over 100%, and the torque output can drop by over 30% with respect to the engine performance with diesel. The control techniques presented in this paper use a biodiesel estimation method in conjunction with a physics-based accommodation strategy based on accounting for the differences between the fuel energy densities and the oxygen contents of diesel and biodiesel. This control method dictates fueling quantities on an energy basis instead of the traditional mass basis and maintains a nearly constant in-cylinder oxygen fraction for both diesel and biodiesel. At the premixed operating points considered in this study, this strategy is demonstrated to eliminate increases in the biodiesel nitrogen oxide emissions, decreases in the torque, and shifts in the combustion timing. The results demonstrate, first, that combustion timing control and nitrogen oxide emissions control can also be achieved through control of the in-cylinder oxygen fraction alone and, second, that energy-based fueling provides consistent in-cylinder oxygen fractions for diesel and biodiesel while also matching the torque outputs for the two fuels when considered at the same premixed operating point (the same speed, intake manifold temperature, charge flow, effective compression ratio, and combustion timing).

Combustion phasing modeling and control for compression ignition engines with high dilution andboost levels

Because fuel efficiency is significantly affected by the timing of combustion in internal combustion engines, accurate control of combustion phasing is critical. In this paper, a nonlinear combustion phasing model is introduced and calibrated, and both a feedforward model–based control strategy and an adaptive model–based control strategy are investigated for combustion phasing control. The combustion phasing model combines a knock integral model, burn duration model, and a Wiebe function to predict the combustion phasing of a diesel engine. This model is simplified to be more suitable for combustion phasing control and is calibrated and validated using simulations and experimental data that include conditions with high exhaust gas recirculation fractions and high boost levels. Based on this model, an adaptive nonlinear model–based controller is designed for closed-loop control, and a feedforward model–based controller is designed for open-loop control. These two control approaches were tested in simulations. The simulation results show that during transient changes, the CA50 (the crank angle at which 50% of the mass of fuel has burned) can reach steady state in no more than five cycles and the steady-state errors are less than ±0.1 crank angle degree for adaptive control and less than ±0.5 crank angle degree for feedforward model–based control.

Modeling and control of fuel distribution in a dual-fuel internal combustion engine leveraging late intake valve closings

In internal combustion engines, cycle-to-cycle and cylinder-to-cylinder variations of the combustion process have been shown to negatively impact the fuel efficiency of the engine and lead to higher exhaust emissions. The combustion variations are generally tied to differences in the composition and condition of the trapped mass throughout each cycle and across individual cylinders. Thus, advanced engines featuring exhaust gas recirculation, flexible valve actuation systems, advanced fueling strategies, and turbocharging systems are prone to exhibit higher variations in the combustion process. In this study, the cylinder-to-cylinder variations of the combustion process in a dual-fuel internal combustion engine leveraging late intake valve closing are investigated and a model to predict and address one of the root causes for these variations across cylinders is developed. The study is conducted on an inline six-cylinder heavy-duty dual-fuel engine equipped with exhaust gas recirculation, a variable geometry turbocharger, and a fully flexible variable intake valve actuation system. The engine is operated with late intake valve closure timings in a dual-fuel combustion mode in which a high reactivity fuel is directly injected into the cylinders and a low reactivity fuel is port injected into the cylinders. The cylinder-to-cylinder variations observed in the study have been associated with the maldistribution of the port-injected fuel, which is exacerbated at late intake valve timings. The resulting difference in indicated mean effective pressure between the cylinders ranges from 9% at an intake valve closing of 570° after top dead center to 38% at an intake valve closing of 620° after top dead center and indicates an increasingly uneven fuel distribution. The study leverages both experimental and simulation studies to investigate the distribution of the port-injected fuel and its impact on cylinder-to-cylinder variation. The effects of intake valve closing as well as the impact of intake runner length on fuel distribution were quantitatively analyzed, and a model was developed that can be used to accurately predict the fuel distribution of the port-injected fuel at different operating conditions with an average estimation error of 1.5% in cylinder-specific fuel flow. A model-based control strategy is implemented to adjust the fueling at each port and shown to significantly reduce the cylinder-to-cylinder variations in fuel distribution.

Cylinder-specific model-based control of combustion phasing for multiple-cylinder diesel engines operating with high dilution and boost levels

Accurate control of combustion phasing is indispensable for diesel engines due to the strong impact of combustion timing on efficiency. In this work, a non-linear combustion phasing model is developed and integrated with a cylinder-specific model of intake gas. The combustion phasing model uses a knock integral model, a burn duration model, and a Wiebe function to predict CA50 (the crank angle at which 50% of the mass of fuel has burned). Meanwhile, the intake gas property model predicts the exhaust gas recirculation fraction and the in-cylinder pressure and temperature at intake valve closing for different cylinders. As such, cylinder-to-cylinder variation of the pressure and temperature at intake valves closing is also considered in this model. This combined model is simplified for controller design and validated. Based on these models, two combustion phasing control strategies are explored. The first is an adaptive controller that is designed for closed-loop control and the second is a feedforward model–based control strategy for open-loop control. These two control approaches were tested in simulations for all six cylinders, and the results demonstrate that the CA50 can reach steady-state conditions within 10 cycles. In addition, the steady-state errors are less than ±0.1 crank angle degree with the adaptive control approach and less than ±1.3 crank angle degree with feedforward model–based control. The impact of errors on the control algorithms is also discussed in the article.

Instantaneous and cycle optimization of fuel usage on a dual fuel vehicle leveraging gasoline and natural gas

Recent increases in natural gas supply have led to a desire to leverage this fuel in the transportation sector. Dual fuel engines provide a platform on which to use natural gas efficiently; these engines, however, require new hardware and new control strategies to properly utilize two fuels simultaneously. This paper explores the impact of implementing dual fuel capabilities on a sedan and demonstrates that a dual fuel E10 and compressed natural gas engine is able to improve the average engine efficiency by up to 6.5% compared to a single fuel engine on standard drive cycles. An optimal control technique is also developed, and the proposed approach allows factors including fuel cost and fuel availability to be taken into account. Optimization at each time instant is investigated and contrasted with optimization over the entire cycle. Cycle optimization is shown to have particular value for cases in which the level in one fuel tank is low.

Model-based control of integrated diesel engine and selective catalytic reduction systems

Selective catalytic reduction (SCR) is one of the most promising solutions to meet the future nitrogen oxides (NOx) emissions regulations for heavy-duty diesel vehicles. However, during transient operations, SCR inlet temperatures can drop below the optimal range and poor NOx conversion can result. To address the low NOx conversion problems encountered particularly in low load conditions, a new integrated engine and aftertreatment control strategy was investigated. This integrated approach improves the SCR system efficiency by using available feedback and modulating the upstream air/fuel ratio to provide more favorable SCR inlet conditions. In order to integrate the engine and aftertreatment system, a model of the SCR dynamics was created and validated and a simple model of the relationship between the engines air/fuel ratio and resulting exhaust temperature and composition was leveraged. The new model-based control strategy is proven to be effective in improving the NOx conversion efficiency of the SCR system from 35% to 85% at low load operating conditions.

Nonlinear model-based control of combustion timing in premixed charge compression ignition

One of the major challenges in the control of advanced combustion modes, such as premixed charge compression ignition, is controlling the timing of the combustion event. A nonlinear model-based controller is outlined and experimentally shown to be capable of controlling the engine combustion timing during diesel premixed charge compression ignition operation on a modern diesel engine with variable valve actuation by targeting the desired values of the in-cylinder oxygen mass fraction and the start of injection. Specifically, the experimental results show that the strategy is capable of controlling the start of combustion and the intake oxygen mass fraction to within 1° crank angle and 1% respectively. A stability analysis also demonstrates that this control strategy ensures asymptotically stable error dynamics.

Flatness-Based Control of Mode Transitions Between Conventional and Premixed Charge Compression Ignition on a Modern Diesel Engine With Variable Valve Actuation

Energy needs in the transportation sector and strict emissions regulations have caused a growing focus on increasing engine efficiency while simultaneously minimizing engine out emissions. One method for accomplishing this is to leverage advanced combustion strategies which are efficient yet very clean. One such combustion mode is premixed charge compression ignition (PCCI). PCCI can lead to drastically lower emissions than conventional diesel combustion while still maintaining engine efficiencies; however, the engine operation region over which it can be utilized is limited. In order to take advantage of this advanced combustion mode, engines must be designed to move between conventional diesel combustion and PCCI. To achieve transitions between different combustion modes, a control strategy was developed which utilizes a extensively validated gas exchange model and flatness-based methods for trajectory planning and trajectory tracking to enable smooth transitions between different combustion modes on a modern diesel engine with variable valve actuation. Since the engine considered here has the ability to alter valve timings, the control method exploits both capabilities to control the gas exchange process as well as the effective compression ratio of the engine. Simulation results indicate that this flatness-based approach is effective in enabling mode transitions.Copyright