Sage L. Kokjohn

Fuel reactivity controlled compression ignition (RCCI): a pathway to controlled high-efficiency clean combustion

A fuel reactivity controlled compression ignition (RCCI) concept is demonstrated as a promising method to achieve high efficiency – clean combustion. Engine experiments were performed in a heavy-duty test engine over a range of loads. Additionally, RCCI engine experiments were compared to conventional diesel engine experiments. Detailed computational fluid dynamics modelling was then used to explain the experimentally observed trends. Specifically, it was found that RCCI combustion is capable of operating over a wide range of engine loads with near zero levels of NO x and soot, acceptable pressure rise rate and ringing intensity, and very high indicated efficiency. For example, a peak gross indicated efficiency of 56 per cent was observed at 9.3 bar indicated mean effective pressure and 1300 rev/min. The comparison between RCCI and conventional diesel showed a reduction in NO x by three orders of magnitude, a reduction in soot by a factor of six, and an increase in gross indicated efficiency of 16.4 per cent (i.e. 7.9 per cent more of the fuel energy was converted to useful work). The simulation results showed that the improvement in fuel conversion efficiency was due both to reductions in heat transfer losses and improved control over the start- and end-of-combustion.

Reactivity controlled compression ignition and conventional diesel combustion: A comparison of methods to meet light-duty NOx and fuel economy targets:

This study compares conventional diesel combustion and reactivity controlled compression ignition combustion in a light-duty engine at NOx levels equivalent to US Tier 2 Bin 5 and proposes a simple method to account for the added fluid consumption required to meet NOx constraints using aftertreatment. Reactivity controlled compression ignition and conventional diesel combustion are compared assuming that the conventional diesel combustion mode uses selective catalytic reduction to meet NOx constraints. The results show that reactivity controlled compression ignition is capable of meeting cycle-averaged NOx targets (equivalent to Tier 2 Bin 5) without NOx aftertreatment. In addition, efficiency comparisons show that reactivity controlled compression ignition offers a 4% improvement in fuel consumption and a 7.3% improvement in total fluid consumption (fuel + diesel exhaust fluid) over conventional diesel combustion with selective catalytic reduction. The fuel consumption improvement is due primarily to lower heat transfer losses. Additionally, it was found that the efficiency of reactivity controlled compression ignition can be further improved by careful selection of operating conditions and the combustion chamber configuration. The modeling shows that over 52% gross indicated efficiency can be achieved in the light-duty engine while meeting NOx targets in-cylinder.

Investigation of the Roles of Flame Propagation, Turbulent Mixing, and Volumetric Heat Release in Conventional and Low Temperature Diesel Combustion

In this work, a multi-mode combustion model, that combines a comprehensive kinetics scheme for volumetric heat release and a level-set-based model for turbulent flame propagation, is applied over the range of engine combustion regimes from non-premixed to premixed conditions. Model predictions of the ignition processes and flame structures are compared to measurements from the literature of naturally occurring luminous emission and OH planar laser induced fluorescence (PLIF). Comparisons are performed over a range of conditions from conventional diesel operation (i.e., short ignition delay, high oxygen concentration) to a low temperature combustion mode (i.e., long ignition delay, low oxygen concentration). The multi-mode combustion model shows excellent prediction of the bulk thermodynamic properties (e.g., rate of heat release), as well as local phenomena (i.e., ignition location, fuel and combustion intermediate species distributions, and flame structure). The results of this study show that even in the limit of mixing controlled combustion, the flame structure is captured extremely well without considering sub-grid scale turbulence-chemistry interactions. The combustion process is dominated by volumetric heat release in a thin zone around the periphery of the jet. The rate of combustion is controlled by transport of reactive mixture to the reaction zone and the dominant mixing processes are well described by the large scale mixing and diffusion. As the ignition delay is increased past the end of injection (i.e., positive ignition dwell), both the simulations and optical diagnostics show that the reaction zone spans the entire jet cross-section. In this combustion mode the combustion rate is no longer limited by transport to the reaction zone, but rather by kinetic timescales. Although comparisons of results with and without consideration of flame propagation show very similar flame structures and combustion characteristics, the addition of the flame propagation model reveals details of the edge or triple-flame structure in the region surrounding the diffusion flame at the lift off location. These details are not captured by the purely kinetics based combustion model, but are well represented by the present multi-mode model.Copyright

A Computational Investigation of Two-Stage Combustion in a Light-Duty Engine

The objective of this investigation is to optimize light-duty diesel engine operating parameters using Adaptive Injection Strategies (AIS) for optimal fuel preparation. A multi-dimensional Computational Fluid Dynamics (CFD) code with detailed chemistry, the KIVA-CHEMKIN code, is employed and a Multi-Objective Genetic Algorithm (MOGA) is used to study a Two-Stage Combustion (TSC) concept. The combustion process is considered at a light load operating condition (nominal IMEP of 5.5 bar and high speed (2000 rev/min)), and two combustion modes are combined in this concept. The first stage is ideally Homogeneous Charge Compression Ignition (HCCI) combustion and the second stage is diffusion combustion under high temperature and low oxygen concentration conditions. Available experimental data on a 1.9L single-cylinder research engine is used for model validation. The results show that the computations are able to adequately predict the emissions trends and quantities over an injection timing sweep for the Partially Premixed Compression Ignition (PCCI) cases investigated. A preliminary investigation was performed to gain an understanding of two-stage combustion in the light duty engine. At this condition it was found that pure HCCI combustion could yield very low engine out emissions, but extreme pressure rise rates would lead to excessive combustion noise. A multi-dimensional optimization code, NSGAII, was used for optimization of six objectives (NOx, soot, CO, HC, ISFC, and peak PRR) by adjusting four parameters (boost pressure, EGR rate, fraction of premixed fuel, and start of late injection timing). The optimization has shown that two-stage combustion is a feasible concept for noise reduction while maintaining reasonable emissions and fuel consumption. A Pareto solution yielding a peak pressure rise rate of 4.3 bar/deg was found using a high EGR rate (54%), relatively low IVC pressure (1.74 bar), premixing 36% of the total fuel, and injecting the remainder of the fuel at 2.9 degrees after TDC. Introduction Since its introduction in the late 1800’s, the diesel engine has been utilized in almost every aspect of modern life, from transportation to energy generation to food production. Several emissions are of prime concern for air pollution: nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (HC), and particulates (soot). These pollutants are damaging to the environment and human health. Reduction of these harmful pollutants while maintaining fuel economy has been a primary driving factor for internal combustion engine research in recent years. Homogeneous Charge Compression Ignition (HCCI) and Premixed Charge Compression Ignition (PCCI) concepts have been shown as promising techniques for simultaneous NOx and soot reduction [1-3]. However, a major concern for light duty engines is noise generated during the combustion process. HCCI combustion tends to produce high rates of pressure rise and therefore can result in higher combustion noise than conventional diesel operation. Sun [4, 5] has shown the possibility of emissions reduction using a Two-Stage combustion concept in a heavy-duty diesel engine. The first stage is HCCI combustion and the second stage is diffusion combustion under high temperature and low oxygen concentration conditions. Because only a fraction of the total fuel is burnt in pure HCCI combustion it may be possible to use the TSC concept for noise reduction. This study aims to apply the TSC concept to a light-duty diesel engine in order to minimize pollutant emissions and to improve fuel economy while maintaining low engine noise. A multi-dimensional CFD code with detailed chemistry, the KIVA-CHEMKIN code, was employed in this investigation. Model validation was performed using experimental data of Opat et al. [3]. After model validation, a preliminary investigation was performed to gain a basic understanding of the two-stage combustion concept in a light-duty diesel engine. With a basic understanding of two-stage combustion, a multi-objective genetic algorithm (MOGA) was used to optimize engine parameters to minimize pollutants, engine noise, and fuel consumption.

Investigation of charge preparation strategies for controlled premixed charge compression ignition combustion using a variable pressure injection system

Abstract This paper uses a multi-dimensional computational fluid dynamics (CFD) code coupled with detailed chemistry, the KIVA-CHEMKIN code, to provide guidelines for solving problems with premixed combustion strategies, namely, lack of combustion phasing control, excessive pressure rise rate, and spray wall impingement due to early injections. A multiple injection concept is used to control combustion phasing and reduce the rate of peak pressure rise. To address spray—wall impingement, an adaptive injection strategy (AIS) using two-injection pulses at different injection pressures is employed. The combustion process considered is at a mid-load operating condition for the light-duty engine of the present study (nominal indicated mean effective pressure (IMEP) of 5.5 bar and high speed, 2000 r/min) and the effects of first and second pulse injection pressure and timing, swirl ratio, and spray targeting are explored. The investigation showed that an optimized low-pressure early cycle injection combined with a high-pressure near top dead centre (TDC) injection allows combustion phasing to be well controlled while achieving premixed compression ignition (PCI)-like emissions levels. An improved solution was found with near-zero nitric oxides (NO x ) and soot, a net indicated specific fuel consumption (ISFC) of only 175 g/kW h, and a peak pressure rise rate of ∼8 bar/deg.

Investigation of the sources of combustion instability in low-temperature combustion engines using response surface models

In the search for more efficient and cleaner engines, low-temperature combustion engines have emerged with the potential to simultaneously increase engine efficiency and reduce nitrogen oxides and soot emissions. Although promising, low-temperature combustion engines have not been widely implemented due to difficulties controlling combustion phasing, combustion duration, and cycle-to-cycle variation. This article develops a methodology using detailed computational fluid dynamics simulations to investigate cycle-to-cycle instability of homogeneous charge compression ignition and reactivity-controlled compression ignition engines. Using computational fluid dynamics modeling, a large design of experiment is performed with small perturbations to the intake and fueling conditions. A response surface model is then fit to the design of experiment results to predict the combustion characteristics and exercised to determine the main sources of cycle-to-cycle variation. As expected, the results show that reactivity-controlled compression ignition and homogeneous charge compression ignition have significantly more variation than conventional diesel combustion. Reactivity-controlled compression ignition combustion phasing (CA50—crank angle when 50% of fuel is burned) is most sensitive to variations in diesel fuel mass, level of exhaust gas recirculation, and charge gas temperature. The peak pressure rise rate of reactivity-controlled compression ignition combustion is most sensitive to variations in gasoline fuel mass. The sources of variation for homogeneous charge compression ignition are similar to those of reactivity-controlled compression ignition combustion; however, trapped gas pressure and cylinder liner temperature become significant factors. Because of the late combustion phasing required for homogeneous charge compression ignition to maintain acceptable pressure rise rate, its cycle-to-cycle variation was found to be higher than that of reactivity-controlled compression ignition combustion for the same input variations.

Blending the benefits of reactivity controlled compression ignition and gasoline compression ignition combustion using an adaptive fuel injection system

Computational optimizations of dual-fuel reactivity controlled compression ignition combustion and gasoline compression ignition combustion were performed using a novel adaptive dual-fuel injector capable of direct injecting both gasoline and diesel fuel in a single cycle. Optimization used the Engine Research Center KIVA code coupled with a multiobjective genetic algorithm. Model validation was performed by comparing simulation results to conventional diesel, reactivity controlled compression ignition, and gasoline compression ignition combustion, and the validated model was used to develop an optimum reactivity controlled compression ignition–gasoline compression ignition combustion strategy. The reactivity controlled compression ignition optimization results showed that by direct injecting gasoline and diesel fuel, the gasoline quantity can be held at a high percentage across the range of loads considered. In this study, the mode weighted gasoline percentage was 91%. At the lightest load point, direct injecting the gasoline gave optimum results, whereas for the other load points, premixing the gasoline yielded the optimum results. The optimized results were compared with conventional diesel combustion, and it was seen that reactivity controlled compression ignition combustion gives a cycle-averaged improvement of 33% in gross indicated efficiency over conventional diesel combustion. The cycle-averaged NOx and soot emissions were reduced by 95% and 75%, respectively. To demonstrate operation over the entire operating map, an optimization was performed at a high-speed–high-load (16 bar, 2500 r/min) condition. Optimization results showed that a gross indicated efficiency of 46.4% with near zero NOx and soot emissions could be achieved using gasoline compression ignition at this load point.

Investigation of the Effect of Injection and Control Strategies on Combustion Instability in Reactivity-Controlled Compression Ignition Engines

This paper uses detailed computational fluid dynamics (CFD) modeling with the kiva-chemkin code to investigate the influence of injection timing, combustion phasing, and operating conditions on combustion instability. Using detailed CFD simulations, a large design of experiments (DOE) is performed with small perturbations in the intake and fueling conditions. A response surface model (RSM) is then fit to the DOE results to predict cycle-to-cycle combustion instability. Injection timing had significant tradeoffs between engine efficiency, emissions, and combustion instability. Near top dead center (TDC) injection timing can significantly reduce combustion instability, but the emissions and efficiency drop close to conventional diesel combustion levels. The fuel split between the two direct injection (DI) injections has very little effect on combustion instability. Increasing exhaust gas recirculation (EGR) rate, while making adjustments to maintain combustion phasing, can significantly reduce peak pressure rise rate (PPRR) variation until the engine is on the verge of misfiring. Combustion phasing has a very large impact on combustion instability. More advanced phasing is much more stable, but produces high PPRRs, higher NOx levels, and can be less efficient due to increased heat transfer losses. The results of this study identify operating parameters that can significantly improve the combustion stability of dual-fuel reactivity-controlled compression ignition (RCCI) engines.

Experimental and computational investigation of soot production from a premixed compression ignition engine using a load extension injection

Low temperature, highly premixed compression ignition strategies have proven to produce high efficiency and low soot emissions, but struggle to reach high loads within normal operating constraints. Recent research has suggested that a mixed mode combustion strategy using a premixed main heat release followed by a mixing controlled load extension injection can retain the part-load thermal efficiency and emissions reduction potential of premixed compression ignition strategies, while enabling high load operation. However, soot emissions under this type of mixed mode combustion strategy have been shown to be problematic. This work investigates soot formation and mitigation methods using a combination of detailed engine experiments and computational fluid dynamics modeling. A premixed compression ignition combustion event was achieved using a premixed charge of gasoline and n-heptane to control combustion phasing, and a load extension injection of gasoline was added near top dead center. The experiments showed negligible engine out soot under the premixed compression ignition operating conditions (i.e. without the load extension injection). When the load extension injection was added, soot increased by several orders of magnitude. Detailed experiments were used to isolate the effects of injection timing, injection pressure, charge conditions (e.g. air–fuel ratio), and fuel type. Computational fluid dynamics modeling considering polycyclic aromatic hydrocarbon chemistry up to pyrene was then used to explain the experimentally observed soot trends. As expected, the soot emission results showed a strong impact of oxygen concentration and injection pressure for injection timings near top dead center; however, as the load extension injection event was delayed beyond the end of the premixed compression ignition heat release, the soot formation decreased and became independent of oxygen concentration. At these conditions, the computational fluid dynamics modeling showed that soot formation is dependent solely on temperature. The results identify a pathway to enable premixed compression ignition load extension, while minimizing soot emissions.

Examination of Initialization and Geometric Details on the Results of CFD Simulations of Diesel Engines

Computational fluid dynamic simulations using the AVL FIRE and KIVA 3V codes were performed to examine commonly accepted techniques and assumptions used when simulating direct injection diesel engines. Simulations of a steady-state impulse swirl meter validated the commonly used practice of evaluating the swirl ratio of diesel engines by integrating the valve flow and torque history over discrete valve lift values. The results indicate the simulations capture the complex interactions occurring in the ports, cylinder, and honeycomb cell impulse swirl meter. Geometric details of engines due to valve recesses in the cylinder head and piston cannot be reproduced axisymmetrically. The commonly adopted axisymmetric assumption for an engine with a centrally located injector was tested by comparing the swirl and emissions history for a noncombusting and a double injection low temperature combustion case with varying geometric fidelity. Consideration of the detailed engine geometry including valve recesses in the piston altered the swirl history such that the peak swirl ratio at TDC decreased by approximately 10% compared with the simplified no-recess geometry. An analog to the detailed geometry of the full 3D geometry was included in the axisymmetric geometry by including a groove in the cylinder head of the mesh. The corresponding emissions predictions of the combusting cases showed greater sensitivity to the altered swirl history as the air-fuel ratio was decreased.