Martti Larmi

Hydrotreated Vegetable Oil (HVO) as a Renewable Diesel Fuel: Trade-off between NOx, Particulate Emission, and Fuel Consumption of a Heavy Duty Engine

Hydrotreating of vegetable oils or animal fats is an alternative process to esterification for producing biobased diesel fuels. Hydrotreated products are also called renewable diesel fuels. Hydrotreated vegetable oils (HVO) do not have the detrimental effects of ester-type biodiesel fuels, like increased NO x emission, deposit formation, storage stability problems, more rapid aging of engine oil or poor cold properties. HVOs are straight chain paraffinic hydrocarbons that are free of aromatics, oxygen and sulfur and have high cetane numbers. In this paper, NO x ‐ particulate emission trade-off and NO x ‐ fuel consumption trade-off are studied using different fuel injection timings in a turbocharged charge air cooled common rail heavy duty diesel engine. Tested fuels were sulfur free diesel fuel, neat HVO, and a 30% HVO + 70% diesel fuel blend. The study shows that there is potential for optimizing engine settings together with enhanced fuel composition. HVO could be used in optimized low emission diesel power trains in captive fleet applications like city buses, indoor fork-lift trucks, or mine vehicles.

Reductions in Particulate and NOx Emissions by Diesel Engine Parameter Adjustments with HVO Fuel

Hydrotreated vegetable oil (HVO) diesel fuel is a promising biofuel candidate that can complement or substitute traditional diesel fuel in engines. It has been already reported that by changing the fuel from conventional EN590 diesel to HVO decreases exhaust emissions. However, as the fuels have certain chemical and physical differences, it is clear that the full advantage of HVO cannot be realized unless the engine is optimized for the new fuel. In this article, we studied how much exhaust emissions can be reduced by adjusting engine parameters for HVO. The results indicate that, with all the studied loads (50%, 75%, and 100%), particulate mass and NO(x) can both be reduced over 25% by engine parameter adjustments. Further, the emission reduction was even higher when the target for adjusting engine parameters was to exclusively reduce either particulates or NO(x). In addition to particulate mass, different indicators of particulate emissions were also compared. These indicators included filter smoke number (FSN), total particle number, total particle surface area, and geometric mean diameter of the emitted particle size distribution. As a result of this comparison, a linear correlation between FSN and total particulate surface area at low FSN region was found.

Valve Train Design for a New Gas Exchange Process

The design and testing of the valve train for a new two-stroke diesel engine concept [1,2] is presented. The gas exchange of this process requires extremely fast-acting inlet valves, which constituted a very demanding designing task. A simulation model of the prototype valve train was constructed with commercially available software. The simulation program served as the main tool for optimizing the dynamic behavior of the valve train. The prototype valve train was built according to the simulations and valve acceleration measurements were performed in order to validate the simulation results. The simulations and measurements are presented in detail in this paper.

Imbalance wall functions with density and material property variation effects applied to engine heat transfer computational fluid dynamics simulations

Heat transfer is a significant factor affecting internal combustion engine efficiency, emissions and performance. This study concentrates on model development for convective heat transfer and near-wall turbulent flow. The solution of complete fluid dynamics equations within the very thin-wall boundary layers is, and will be in the near future, very impractical in engineering scale flows with any present day method. An advanced numerical near-wall treatment method within the Reynolds averaged Navier–Stokes framework has been developed. The method solves simplified boundary layer equations for enthalpy, momentum, turbulent kinetic energy and dissipation in wall adjacent cells on cellwise high-resolution subgrids, adaptive to local conditions. The boundary layer equations include temperature gradient–induced density/multicomponent material property variations and complete imbalance contributions, for example, convection, transients, pressure gradient and external sources, in compact forms. The resulting numerical wall functions are valid with near-wall grid resolution ranging from the viscous sublayer to the fully turbulent region, thus avoiding the conflicting near wall resolution requirements of common low–Reynolds and high–Reynolds number turbulence models. The advanced near wall treatment method, comprising the numerical imbalance wall functions and accordingly modified low–Reynolds number turbulence model, is implemented in STAR-CD 4.12, extensively utilized in engine simulations. The near wall treatment method is validated against available measurements and direct numerical simulation data of strongly heated pipe flow. Performance of the near-wall treatment method in engine conjugate heat transfer simulations is also demonstrated. Local and average effects of variable properties and imbalance contributions on piston surface heat transfer, friction and turbulent sources are elaborated and contrasted to the standard high–Reynolds number near-wall treatment.

Study of Miller timing on exhaust emissions of a hydrotreated vegetable oil (HVO)-fueled diesel engine

The effect of intake valve closure (IVC) timing by utilizing Miller cycle and start of injection (SOI) on particulate matter (PM), particle number, and nitrogen oxide (NOx) emissions was studied with a hydrotreated vegetable oil (HVO)-fueled nonroad diesel engine. HVO-fueled engine emissions, including aldehyde and polyaromatic hydrocarbon (PAH) emissions, were also compared with those emitted with fossil EN590 diesel fuel. At the engine standard settings, particle number and NOx emissions decreased at all the studied load points (50%, 75%, and 100%) when the fuel was changed from EN590 to HVO. Adjusting IVC timing enabled a substantial decrease in NOx emission and combined with SOI timing adjustment somewhat smaller decrease in both NOx and particle emissions at IVC −50 and −70 °CA points. The HVO fuel decreased PAH emissions mainly due to the absence of aromatics. Aldehyde emissions were lower with the HVO fuel with medium (50%) load. At higher loads (75% and 100%), aldehyde emissions were slightly higher with the HVO fuel. However, the aldehyde emission levels were quite low, so no clear conclusions on the effect of fuel can be made. Overall, the study indicates that paraffinic HVO fuels are suitable for emission reduction with valve and injection timing adjustment and thus provide possibilities for engine manufacturers to meet the strictening emission limits. Implications: NOx and particle emissions are dominant emissions of diesel engines and vehicles. New, biobased paraffinic fuels and modern engine technologies have been reported to lower both of these emissions. In this study, even further reductions were achieved with engine valve adjustment combined with novel hydrotreated vegetable oil (HVO) diesel fuel. This study shows that new paraffinic fuels offer further possibilities to reduce engine exhaust emissions to meet the future emission limits. Supplementary Materials: Supplementary materials are available for this paper. Go to the publishers online edition of the Journal of the Air & Waste Management Association for a complete list of analysed PAH compounds.

Large-Eddy Simulation of Subsonic Jets

The present study deals with development and validation of a fully explicit, compressible Runge-Kutta-4 (RK4) Navier-Stokes solver in the opensource CFD programming environment OpenFOAM. The background motivation is to shift towards explicit density based solution strategy and thereby avoid using the pressure based algorithms which are currently proposed in the standard OpenFOAM release for Large-Eddy Simulation (LES). This shift is considered necessary in strongly compressible flows when Ma > 0.5. Our application of interest is related to the pre-mixing stage in direct injection gas engines where high injection pressures are typically utilized. First, the developed flow solver is discussed and validated. Then, the implementation of subsonic inflow conditions using a forcing region in combination with a simplified nozzle geometry is discussed and validated. After this, LES of mixing in compressible, round jets at Ma = 0.3, 0.5 and 0.65 are carried out. Respectively, the Reynolds numbers of the jets correspond to Re = 6000, 10000 and 13000. Results for two meshes are presented. The results imply that the present solver produces turbulent structures, resolves a range of turbulent eddy frequencies and gives also mesh independent results within satisfactory limits for mean flow and turbulence statistics.

Conjugate Heat Transfer in CI Engine CFD Simulations

The development of new high power diesel engines is continually going for increased mean effective pressures and consequently increased thermal loads on combustion chamber walls close to the limits of endurance. Therefore accurate CFD simulation of conjugate heat transfer on the walls becomes a very important part of the development. In this study the heat transfer and temperature on piston surface was studied using conjugate heat transfer model along with a variety of near wall treatments for turbulence. New wall functions that account for variable density were implemented and tested against standard wall functions and against the hybrid near wall treatment readily available in a CFD software Star-CD. INTRODUCTION Accurate prediction of the piston surface temperature and heat flux is difficult mainly because of the very large temperature and velocity gradients of highly turbulent flow near the wall and also because of the transient interaction between the gas phase and solid wall temperatures. Instead of resolving the temperature and velocity profiles down to the wall, wall functions based on some simplifying assumptions of the near wall turbulence are used. Standard wall functions and hybrid near wall treatment in the form they are usually implemented in commercial CFD codes assume incompressible flow. Therefore straightforward adoption of them into engine simulations might not be appropriate, since the gas in the cylinder is highly compressible indeed and large density variations are likely to appear near the walls. To tackle this problem momentum and energy equations near the wall are integrated in their compressible form to new modified wall functions that are sensitive to density variation. Also variable turbulent Prandtl number near the walls is included in the modified wall functions. Heat transfer in combustion engines has bee widely studied, e.g. by Schubert, Wimmer and Chamela [1], who developed quasi-dimensional heat transfer models for combustion ignition engines, by Han and Reitz [2], who studied the effect of density variations on convective heat transfer, by Urip, Liew, Yang and Arici [3], who studied the effect of unsteady thermal boundary conditions, by Urip, Yang and Arici [4], who studied conjugate heat transfer in actual internal combustion engine CFD simulations, by Tiainen, Kallio, Leino and Turunen [5], who studied heat transfer in diesel engines with density dependent wall functions in CFD and combining the obtained average heat transfer coefficients to FEM calculations of the heat transfer in solid piston material and also by Huuhilo [6], who studied and developed a FEM code for transient heat transfer in the piston surface. A good motivation for this work is figure 1 from Huuhilo’s Master’s Thesis that shows the effect of the piston surface material on the transient maximum piston surface temperature. Fig. 1. The simulated maximum temperature of the piston surface layer fabricated from steel, aluminium or zirconium oxide [6]. VARIABLE DENSITY WALL FUNCTIONS In deriving the wall functions following assumptions are made: 1) wall normal derivatives are much larger than tangential ones, so the tangential derivatives are neglected. 2) near wall fluid flow is tangential to wall. 3) near wall pressure gradient can be neglected. 4) near wall heat flux consists only of laminar and turbulent conduction. 5) gas obeys ideal gas law 6) near wall specific heat capacity is constant 7) near wall mass fractions of mixture components are constant. 8) near wall shear stress and heat flux are constant. Now the simplified near wall momentum and heat equations (1) and (2) can be written as

Analyzing Local Combustion Environment with a Flamelet Model and Detailed Chemistry

Measurements have been done in order to obtain information concerning the effect of EGR for the smoke and NOx emissions of a heavy-duty diesel engine. Measured smoke number and NOx emissions are explained using detailed chemical kinetic calculations and CFD simulations. The local conditions in the research engine are analyzed by creating equivalence ratio temperature (Phi-T) maps and analyzing the CFD results within these maps. The study uses different amount of EGR and the standard EN590 diesel fuel. The detailed chemical kinetic calculations take into account the different EGR rates. The CFD calculations are made with a flamelet based combustion model together with detailed chemistry. The results are compared to a previous study where a hybrid local flame area evolution model combined with an eddy breakup type model was used in the CFD simulations. It was observed that NOx emission trends can be well captured with the Phi-T maps but the situation is more difficult with the engine soot. Hence, the conclusion is the same as in the previous study with the hybrid combustion model. However, the local reaction zone is qualitatively very different with the flamelet model as compared to the hybrid model. Phi-T maps were also constructed for the total fixed nitrogen, using a detailed description of the nitrogen chemistry. EGR is typically used as an NOx abatement technique, having the purpose to lower the temperature and thus the formation of thermal-NO. However, these maps revealed a new functionality of EGR as a NOx abatement method.

Experimental Investigation of Characteristics of Transient Low Pressure Wall-impinging Gas Jet

This paper describes an investigation of the jet structure and mixture formation process of wall-impinging gas jet injected by a low pressure gas injector in a constant volume chamber at room conditions. The tracer-based planar laser-induced fluorescence (PLIF) technique is applied to qualitatively evaluate the mixture formation process. The macroscopic structure and concentration distribution of wall-impinging jet were studied based on a series of time evolution high-definition images. In particular, the effects of injection pressure on characteristics of turbulence were investigated. Experimental results show that vortex structure with large scale is one of important characteristics for wall-impinging jet, and the interaction among jet flow, impingement wall, and surrounding air plays a dominant role in the mixture formation. The comparative study about the effect of injection pressure on wall-impinging jet reveals higher injection leads to higher mixing efficiency and better mixture formation.

Influence of the in-cylinder gas density and fuel injection pressure on the combustion characteristics in a large-bore diesel engine:

Engine research is going toward higher specific power and downsizing. This topic is of increasing interest in compression-ignition engines, due to high output demand and more restrictive legislation. This study focuses on the influence of in-cylinder gas density and fuel injection pressure on the combustion and performance in a large-bore medium speed research engine. From a baseline setup close to a commercial engine, the specific power density was augmented by higher in-cylinder pressure ranging from 200 to 300 bar. This corresponds to gas density between 65 and 90 kg/m3 at top dead center. In addition, fuel injection pressure was varied between 1500 and 2400 bar. Based on the literature, this is the first study to report engine operation with 300 bar cylinder pressure. The results show that ignition delay is significantly reduced by increasing the injection pressure, while gas density has only a moderate effect. In addition, specific nitrogen oxides increase strongly with higher injection pressure but only moderately with higher gas density. Running with high gas density permits to improve the engine fuel economy. The test outcomes also show that the combustion duration can be decreased by increasing the in-cylinder density.