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Journal Article

Diesel Engine Size Scaling at Medium Load without EGR

Several diffusion combustion scaling models were experimentally tested in two geometrically similar single-cylinder diesel engines with a bore diameter ratio of 1.7. Assuming that the engines have the same in-cylinder thermodynamic conditions and equivalence ratio, the combustion models primarily change the fuel injection pressure and engine speed in order to attain similar performance and emissions. The models tested include an extended scaling model, which scales diffusion flame lift-off length and jet spray penetration; a simple scaling model, which only scales spray penetration at equal mean piston speed; and a same speed scaling model, which holds crankshaft rotational velocity constant while also scaling spray penetration. Successfully scaling diffusion combustion proved difficult to accomplish because of apparent differences that remained in the fuel-air mixing and heat transfer processes.
Journal Article

Fuel Reactivity Controlled Compression Ignition (RCCI) Combustion in Light- and Heavy-Duty Engines

Single-cylinder engine experiments were used to investigate a fuel reactivity controlled compression ignition (RCCI) concept in both light- and heavy-duty engines and comparisons were made between the two engine classes. It was found that with only small changes in the injection parameters, the combustion characteristics of the heavy-duty engine could be adequately reproduced in the light-duty engine. Comparisons of the emissions and performance showed that both engines can simultaneously achieve NOx below 0.05 g/kW-hr, soot below 0.01 g/kW-hr, ringing intensity below 4 MW/m2, and gross indicated efficiencies above 50 per cent. However, it was found that the peak gross indicated efficiency of the baseline light-duty engine was approximately 7 per cent lower than the heavy-duty engine. The energy balances of the two engines were compared and it was found that the largest factor contributing to the lower efficiency of the light-duty engine was increased heat transfer losses.
Technical Paper

A Computational Investigation of the Effects of Swirl Ratio and Injection Pressure on Mixture Preparation and Wall Heat Transfer in a Light-Duty Diesel Engine

In a recent study, quantitative measurements were presented of in-cylinder spatial distributions of mixture equivalence ratio in a single-cylinder light-duty optical diesel engine, operated with a non-reactive mixture at conditions similar to an early injection low-temperature combustion mode. In the experiments a planar laser-induced fluorescence (PLIF) methodology was used to obtain local mixture equivalence ratio values based on a diesel fuel surrogate (75% n-heptane, 25% iso-octane), with a small fraction of toluene as fluorescing tracer (0.5% by mass). Significant changes in the mixture's structure and composition at the walls were observed due to increased charge motion at high swirl and injection pressure levels. This suggested a non-negligible impact on wall heat transfer and, ultimately, on efficiency and engine-out emissions.
Journal Article

Experimental Investigation of Piston Heat Transfer in a Light Duty Engine Under Conventional Diesel, Homogeneous Charge Compression Ignition, and Reactivity Controlled Compression Ignition Combustion Regimes

An experimental study has been conducted to provide insight into heat transfer to the piston of a light-duty single-cylinder research engine under Conventional Diesel (CDC), Homogeneous Charge Compression Ignition (HCCI), and Reactivity Controlled Compression Ignition (RCCI) combustion regimes. Two fast-response surface thermocouples embedded in the piston top measured transient temperature. A commercial wireless telemetry system was used to transmit thermocouple signals from the moving piston. A detailed comparison was made between the different combustion regimes at a range of engine speed and load conditions. The closed-cycle integrated and peak heat transfer rates were found to be lower for HCCI and RCCI when compared to CDC. Under HCCI operation, the peak heat transfer rate showed sensitivity to the 50% burn location.
Technical Paper

A Computational Investigation of Stepped-Bowl Piston Geometry for a Light Duty Engine Operating at Low Load

The objective of this investigation is to optimize a light-duty diesel engine in order to minimize soot, NOx, carbon monoxide (CO), unburned hydrocarbon (UHC) emissions and peak pressure rise rate (PPRR) while improving fuel economy in a low oxygen environment. Variables considered are the injection timings, fractional amount of fuel per injection, half included spray angle, swirl, and stepped-bowl piston geometry. The KIVA-CHEMKIN code, a multi-dimensional computational fluid dynamics (CFD) program with detailed chemistry is used and is coupled to a multi-objective genetic algorithm (MOGA) along with an automated grid generator. The stepped-piston bowl allows more options for spray targeting and improved charge preparation. Results show that optimal combinations of the above variables exist to simultaneously reduce emissions and fuel consumption. Details of the spray targeting were found to have a major impact on the combustion process.
Journal Article

Effect of Piston Bowl Geometry on Dual Fuel Reactivity Controlled Compression Ignition (RCCI) in a Light-Duty Engine Operated with Gasoline/Diesel and Methanol/Diesel

A single-cylinder light-duty diesel engine was used to investigate dual fuel reactivity controlled compression ignition (RCCI) operated with two different fuel combinations: gasoline/diesel fuel and methanol/diesel fuel. The engine was operated over a range of conditions, from 1500 to 2300 rpm and 3.5 to 17 bar gross IMEP. Using the stock re-entrant piston bowl geometry, both fuel combinations were able to achieve low NOx and PM emissions with a peak gross indicated efficiency of 48%. However, at light load conditions both gasoline and methanol yielded poorer combustion efficiencies. Previous studies have shown that the high-levels of piston induced mixing that are created by the stock piston are not required, and in fact are detrimental due to increased heat transfer losses, for premixed combustion. Thus a modified piston featuring a shallow, flat piston bowl with nearly no squish land was also investigated.
Technical Paper

Soot Structure in a Conventional Non-Premixed Diesel Flame

An analysis of the soot formation and oxidation process in a conventional direct-injection (DI) diesel flame was conducted using numerical simulations. An improved multi-step phenomenological soot model that includes particle inception, particle coagulation, surface growth and oxidation was used to describe the soot formation and oxidation process. The soot model has been implemented into the KIVA-3V code. Other model Improvements include a piston-ring crevice model, a KH/RT spray breakup model, a droplet wall impingement model, a wall-temperature heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process. Experimental data from a heavy-duty, Cummins N14, research DI diesel engine operated with conventional injection under low-load conditions were selected as a benchmark.
Technical Paper

Modeling the Effects of EGR and Injection Pressure on Soot Formation in a High-Speed Direct-Injection (HSDI) Diesel Engine Using a Multi-Step Phenomenological Soot Model

Low-temperature combustion concepts that utilize cooled EGR, early/retarded injection, high swirl ratios, and modest compression ratios have recently received considerable attention. To understand the combustion and, in particular, the soot formation process under these operating conditions, a modeling study was carried out using the KIVA-3V code with an improved phenomenological soot model. This multi-step soot model includes particle inception, surface growth, surface oxidation, and particle coagulation. Additional models include a piston-ring crevice model, the KH/RT spray breakup model, a droplet wall impingement model, a wall heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process.
Technical Paper

Modeling the Effects of Geometry Generated Turbulence on HCCI Engine Combustion

The present study uses a numerical model to investigate the effects of flow turbulence on premixed iso-octane HCCI engine combustion. Different levels of in-cylinder turbulence are generated by using different piston geometries, namely a disc-shape versus a square-shape bowl. The numerical model is based on the KIVA code which is modified to use CHEMKIN as the chemistry solver. A detailed reaction mechanism is used to simulate the fuel chemistry. It is found that turbulence has significant effects on HCCI combustion. In the current engine setup, the main effect of turbulence is to affect the wall heat transfer, and hence to change the mixture temperature which, in turn, influences the ignition timing and combustion duration. The model also predicts that the combustion duration in the square bowl case is longer than that in the disc piston case which agrees with the measurements.
Technical Paper

Reduction of Emissions and Fuel Consumption in a 2-Stroke Direct Injection Engine with Multidimensional Modeling and an Evolutionary Search Technique

An optimization study combining multidimensional CFD modeling and a global, evolutionary search technique known as the Genetic Algorithm has been carried out. The subject of this study was a 2-stroke, spark-ignited, direct-injection, single-cylinder research engine (SCRE). The goal of the study was to optimize the part load operating parameters of the engine in order to achieve the lowest possible emissions, improved fuel economy, and reduced wall heat transfer. Parameters subject to permutation in this study were the start-of-injection (SOI) timing, injection duration, spark timing, fuel injection angle, dwell between injections, and the percentage of fuel mass in the first injection pulse. The study was comprised of three cases. All simulations were for a part load, intermediate-speed condition representing a transition operating regime between stratified charge and homogeneous charge operation.
Technical Paper

Multidimensional Modeling of the Effects of Radiation and Soot Deposition in Heavy-duty Diesel Engines

A radiation model based on the Discrete Ordinates Method (DOM) was incorporated into the KIVA3v multidimensional code to study the effects of soot and radiation on diesel engine performance at high load. A thermophoretic soot deposition model was implemented to predict soot concentrations in the near-wall region, which was found to affect radiative heat flux levels. Realistic, non-uniform combustion chamber wall surface temperature distributions were predicted using a finite-element-based heat conduction model for the engine metal components that was coupled with KIVA3v in an iterative scheme. The more accurate combustion chamber wall temperatures enhanced the accuracy of both the radiation and soot deposition models as well as the convective heat transfer model. For a basline case, (1500 rev/min, 100% load) it was found that radiation can account for as much as 30% of the total wall heat loss and that soot deposition in each cycle is less than 3% of the total in-cylinder soot.
Technical Paper

Development of an Ignition and Combustion Model for Spark-Ignition Engines

A new ignition and combustion model has been developed and tested for use in premixed spark-ignition engines. The ignition model is referred to as the Discrete Particle Ignition Kernel (DPIK) model, and it uses Lagrangian markers to track the flame-front growth. The model includes the effects of electrode heat transfer on the early flame kernel growth process, and it is used in conjunction with a characteristic-time-scale combustion model once the ignition kernel has grown to a size where the effects of turbulence on the flame must be considered. A new term which accounts for the effect of air-fuel ratio, was added to the combustion model for modeling combustion in very lean and very rich mixtures. The flame kernel size predicted by the DPIK model was compared with measurements of Maly and Vogel. Furthermore, predictions of the electrode heat transfer were compared with data of Kravchik and Heywood. In both comparisons the model predictions were in good agreement with the experiments.
Technical Paper

Assessment of Wall Heat Transfer Models for Premixed-Charge Engine Combustion Computations

Two-dimensional computations of premixed-charge engine combustion were made using the KIVA-II code. The purpose of the study was to assess the influence of heat transfer and turbulence model boundary conditions on engine combustion predictions. Combustion was modeled using a laminar- and turbulent-characteristic-time model. Flow through the piston-cylinder-ring crevice was accounted for using a phenomenological crevice-flow model. The predictions were compared to existing cylinder pressure and wall heat transfer experimental data under motoring and fired conditions, at two engine speeds. Two different wall heat transfer model formulations were considered. The first is the standard wall function method. The second is based on solutions to the one-dimensional unsteady energy equation, formulated such that the standard wall function method is recovered in the quasi-steady limit. Turbulence was modeled using the standard k-ε turbulence model equations.
Technical Paper

Progress in Diesel Engine Intake Flow and Combustion Modeling

The three-dimensional computer code, KIVA, is being modified to include state-of-the-art submodels for diesel engine flow and combustion. Improved and/or new submodels which have already been implemented are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops. Progress on the implementation of improved spray drop drag and drop breakup models, the formulation and testing of a multistep kinetics ignition model and preliminary soot modeling results are described. In addition, the use of a block structured version of KIVA to model the intake flow process is described. A grid generation scheme has been developed for modeling realistic (complex) engine geometries, and initial computations have been made of intake flow in the manifold and combustion chamber of a two-intake-valve engine.
Technical Paper

A Study of Diesel Cold Starting using both Cycle Analysis and Multidimensional Calculations

The physical in-cylinder processes and ignition during cold starting have been studied using computational models, with particular attention to the influences of blowby, heat transfer during the compression stroke, spray development, vaporization and fuel/air mixture formation and ignition. Two different modeling approaches were used. A thermodynamic zero dimensional cycle analysis program in which the fuel injection effects were not modeled, was used to determine overall and gas exchange effects. The three-dimensional KIVA-II code was used to determine details of the closed cycle events, with modified atomization, blowby and spray/wall impingement models, and a simplified model for ignition. The calculations were used to obtain an understanding of the cold starting process and to identify practical methods for improving cold starting of direct injection diesel engines.
Technical Paper

Modeling Combustion in Compression Ignition Homogeneous Charge Engines

The combustion mechanism in a Compression Ignition Homogeneous Charge (CIHC) engine was studied. Previous experiments done on a four-stroke CIHC engine were modeled using the KIVA-II code with modifications to the combustion, heat transfer, and crevice flow submodels. A laminar and turbulence characteristic time combustion model that has been used for spark-ignited engine studies was extended to allow predictions of ignition. The rate of conversion from one chemical species to another is modeled using a characteristic time which is the sum of a laminar (high temperature) chemistry time, an ignition (low temperature) chemistry time, and a turbulence mixing time. The ignition characteristic time was modeled using data from elementary initiation reactions and has the Arrhenius form. It was found to be possible to match all engine test cases reasonably well with one set of combustion model constants.
Technical Paper

3-D Modeling of Diesel Engine Intake Flow, Combustion and Emissions

Manufacturers of heavy-duty diesel engines are facing increasingly stringent, emission standards. These standards have motivated new research efforts towards improving the performance of diesel engines. The objective of the present program is to develop a comprehensive analytical model of the diesel combustion process that can be used to explore the influence of design changes. This will enable industry to predict the effect of these changes on engine performance and emissions. A major benefit of the successful implementation of such models is that engine development time and costs would be reduced through their use. The computer model is based on the three-dimensional KIVA-II code, with state-of-the-art submodels for spray atomization, drop breakup / coalescence, multi-component fuel vaporization, spray/wall interaction, ignition and combustion, wall heat transfer, unburned HC and NOx formation, and soot and radiation.
Technical Paper

Improvements in 3-D Modeling of Diesel Engine Intake Flow and Combustion

A three-dimensional computer code (KIVA) is being modified to include state-of-the-art submodels for diesel engine flow and combustion: spray atomization, drop breakup/coalescence, multi-component fuel vaporization, spray/wall interaction, ignition and combustion, wall heat transfer, unburned HC and NOx formation, soot and radiation and the intake flow process. Improved and/or new submodels which have been completed are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops.
Technical Paper

Visualization and Modeling of Pilot Injection and Combustion in Diesel Engines

An endoscope-based image acquisition-and-processing camera system was used for diagnostics of pilot injection combustion in a single-cylinder heavy duty diesel engine. A study of the pilot injection or light load is of interest because the spray breakup, mixing and vaporization processes are less influenced by heat feedback from the flame than in full injection cases. This allows the spray process to be decoupled from the combustion process. The experimental cases were modeled using a version of the KIVA-II code that includes improvements in the turbulence, wall heat transfer, spray, ignition and combustion models. Pilot injections of three different amounts (10, 15 and 20% of the fuel injected at 75% load and 1600 RPM) at different start-of-injection timings were studied. The imaging system included an endoscope, an intensified CID camera, a frame grabber and the control circuitry.
Technical Paper

Multidimensional Modeling of Engine Combustion Chamber Surface Temperatures

A two-dimensional transient Heat Conduction in Components code (HCC) was successfully set up and extensively used to calculate the temperature field existing in real engine combustion chambers. The Saul'yev method, an explicit, unconditionally stable finite difference method, was used in the code. Consideration of the gasket between the cylinder wall and head, and the air gap between the piston and liner were included in the code. The realistic piston bowl shape was modeled with a grid transformation and piston movement was considered. The HCC code was used to calculate the wall temperature of an Isuzu ceramic engine and a Caterpillar heavy-duty diesel engine. The code was combined with the KIVA-II code in an iterative loop, in which the KIVA-II code provided the instantaneous local heat flux on the combustion chamber surfaces, and the HCC code computed the time-averaged wall temperature distribution on the surfaces.