Rolf D. Reitz

Turbulence Modeling of Internal Combustion Engines Using RNG κ-ε Models

Abstract The RNG κ-e turbulence model derived by Yakhot and Orszag (1986) based on the Renormalization Group theory has been modified and applied to variable-density engine flows in the present study. The original RNG-based turbulence transport approximations were developed formally for an incompressible flow. In order to account for flow compressibility the RNG e-equation is modified and closed through an isotropic rapid distortion analysis. Computations were made of engine compressing/expanding flows and the results were compared with available experimental observations in a production diesel engine geometry. The modified RNG κ-e model was also applied to diesel spray combustion computations. It is shown that the use of the RNG model is warranted for spray combustion modeling since the ratio of the turbulent to mean-strain time scales is appreciable due to spray-generated mean flow gradients, and the model introduces a term to account for these effects. Large scale flow structures are predicted which ar...

Mechanism of atomization of a liquid jet

In the atomization regime of a round liquid jet, a diverging spray is observed immediately at the nozzle exit. The mechanism that controls atomization has not yet been determined even though several have been proposed. Experiments are reported with constant liquid pressures from 500 psia (33 atm) to 2500 psia (166 atm) with five different mixtures of water and glycerol into nitrogen, helium, and xenon with gas pressures up to 600 psia (40 atm) at room temperature. Fourteen nozzles were used with length‐to‐diameter ratios ranging from 85 to 0.5 with sharp and rounded inlets, each with an exit diameter of about 340 μm. An evaluation of proposed jet atomization theories shows that aerodynamic effects, liquid turbulence, jet velocity profile rearrangement effects, and liquid supply pressure oscillations each cannot alone explain the experimental results. However, a mechanism that combines liquid–gas aerodynamic interaction with nozzle geometry effects would be compatible with our measurements but the specific...

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.

A temperature wall function formulation for variable-density turbulent flows with application to engine convective heat transfer modeling

A temperature wall function was derived for variable-density turbulent flows that are commonly found in internal combustion engines. Thermodynamic variations of gas density and the increase of the turbulent Prandtl number in the boundary layer are included in the formulation. Multidimensional computations were made of a pancake-chamber gasoline engine and a heavy-duty diesel engine under firing conditions. Satisfactory agreement between the predicted and measured heat fluxes was obtained. It was found that gas compressibility affected engine heat transfer prediction significantly while the effects of unsteadiness and heat release due to combustion were insignificant for the cases considered.

Mechanism of Soot and NOx Emission Reduction Using Multiple-injection in a Diesel Engine

Engine experiments have shown that with highpressure multiple injections (two or more injection pulses per power cycle), the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with the conventional single-pulse injections, reducing both soot and NOx emissions significantly. In order to understand the mechanism of emissions reduction, multidimensional computations were carried out for a heavy-duty diesel engine with multiple injections. Different injection schemes were considered, and the predicted cylinder pressure, heat release rate and soot and NOx emissions were compared with measured data. Excellent agreements between predictions and measurements were achieved after improvements in the models were made. The improvements include using a RNG k-e turbulence model, adopting a new wall heat transfer model and introducing the nozzle discharge coefficient to account for the contraction of fuel jet at the nozzle exit. The present computations confirm that split injection allows significant soot reduction with out a NOx penalty. Based on the computations, it is found that multiple injections have a similar NOx reduction mechanism as single injections with retarded injection timings. Regarding soot reduction, it is shown that reduced soot formation is due to the fact that the soot producing rich regions at the spray tip are not replenished when the injection is terminated and then restarted. With split injections, the subsequently injected fuel burns rapidly and does not contribute significantly to soot production. The present work also demonstrates the usefulness of multidimensional modeling of diesel combustion to reveal combustion mechanisms and to provide design insights for low emission engines. EXTENSIVE RESEARCH is in progress to reduce both nitrogen oxides (NOx) and particulate (soot) emissions from diesel engines due to environmental concerns. One of the emission-control strategies is in-cylinder reduction of pollutant production. It is well known that it is very difficult to reduce both NOx and soot production simultaneously during the combustion process. Many emission-reduction technologies developed so far tend to increase soot emission while reducing NOx emission, and vice versa. For example, retarding fuel injection timing can be effective to reduce NO formation. However, this usually results in an increase of soot production. On the other hand, although increasing fuel injection pressure can decrease soot emissions, it can also cause higher NOx emissions at the same time [1]*. Recently, it has been shown experimentally that with high-pressure multiple injections, the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with single-pulse injections, reducing both soot and NOx emissions significantly [2-4]. Nehmer and Reitz experimentally investigated the effect of double-pulse split injection on soot and NOx emissions using a single-cylinder Caterpillar heavy-duty diesel engine [2]. They varied the amount of fuel injected in the first injection pulse from 10 percent to 75 percent of the total amount of fuel and found that split injection affected the sootNOx trade-off. In general, their split-injection schemes reduced NOx with only a minimal increase in soot emissions and did not extend the combustion duration. Tow et al. [3] continued the study of Nehmer and Reitz [2] using the same engine, and included different dwells between injection pulses and triple injection schemes in their investigation. They found that at high engine load (75%), particulate could be reduced by a factor of three with no increase in NOx and only a 2.5% increase in BSFC compared to a single injection, using a double injection with a relatively long dwell between injections. They also found that triple injection could reduce NOx and soot emissions at both light and high loads. Another important conclusion of Tow et al. [3] is that the dwell between injection pulses is very important to control soot production and there exits an optimum dwell at a particular engine operating condition. The optimum dwell of a double-injection was found to be about 10 degree crank angles at 75% load and 1600 rev/min for their engine conditions. * Numbers in brackets designate References at the end of the paper. Pierpont et al. [4] confirmed that the amount of fuel injected in the first pulse affects the particulate (smoke) level in experiments where the NOx emission level was held constant. However, the best double injections were found to also depend on the spray nozzle included angle. For a production injector with a 125 included angle, which results in significant wall impingement on the piston bowl, the best double injections were found to be those with 50% to 60% of the fuel injected in the first pulse. They also found that with a combination of EGR and multiple injections, particulate and NOx were simultaneously reduced to as low as 0.07 and 2.2 g/bhp-hr, respectively, at 75% load and 1600 rev/min. Other multiple injection studies can be also found in the open literature [5, 6]. The published experimental works indicate that multiple-injection is an effective mean to control NO and particulate production during the diesel combustion process. In general, multiple injections allow the injection timing to be retarded to reduce NOx emission while holding the particulate at low levels. Both the amount of fuel injected in the first pulse and the dwell between pulses are important for an optimum injection scheme. With the application of multiple injection technology, the goal of improved injection scheme design and better control of engine combustion is made difficult by the fact that design variables are added with flexible injectors. It is thus helpful to simulate the engine processes with the use of computational models, which can provide detailed temporal and spatial information of precisely parameter-controlled injection and combustion processes. Patterson et al. [7] performed multidimensional computations of multiple injections using an improved KIVA code. They tried to reproduce the experimental results of Nehmer and Reitz [2] and achieved a fair success. However, the accuracy of their model prediction deteriorated for double-pulse injections as the amount of fuel injected in the second pulse increased. Kong, Han and Reitz [8] modified the code by including a modified RNG k-e turbulence model and turbulence boundary conditions [9]. Predictions of combustion and emissions of single-injections were shown to be improved significantly [8]. These successes motivated the application of the code to multiple injections in the present study. It is clear that a good model is necessary in order to predict engine combustion and emissions accurately. Accordingly, the submodels used by Kong, Han and Reitz [8] were implemented together with improved heat transfer and injection models. The models were first applied to the experimental results of the double injections of Nehmer and Reitz [2]. For better understanding of the formation of NO and soot during multiple-injection combustion processes, a set of designed singleand double-injection schemes were computed. Based on the computational results, a mechanism of emission reduction using multiple-injection is suggested.

Comparisons of computed and measured premixed charge engine combustion

Abstract Comparisons are presented of computed and measured cylinder pressure in a reciprocating engine with a pancake combustion chamber and premixed propane/air charges. Engine operating conditions range over volumetric efficiency of 30–60%; equivalence ratio of 0.87–1.1; and rpm of 1000–1500. The computations start from the actual spark times and simulate the growth of the flame kernel into a fully developed turbulent flame by taking into account the increasing influence of turbulent eddies on the growing flame kernel. A k-ϵ submodel is used for turbulence. The species conversion submodel assumes that the species (C3H8, O2, H2O, CO2, CO, H2, and N2) concentrations approach their local thermodynamic equilibrium values with a characteristic conversion time that is a combination of a turbulent mixing time and a chemical conversion time in laminar propaneair flames. In all cases computed and measured cylinder pressure agree well in trends and magnitudes during the entire duration of combustion. The difference in magnitudes generally is much less than 8%. The main conclusion is that laminar flame processes must be explicitly accounted for in order to reproduce certain elements of premixed charge engine combustion.

Effect of drop breakup on fuel sprays

Recently developed computer models are being applied to calculate complex interactions between sprays and gas motions. The three-dimensional KIVA code was modified to address drop breakup and was used to study fuel sprays. The results show that drop breakup influences spray penetration, vaporization and mixing in high pressure sprays. The spray drop size is the outcome of a competition between drop breakup and drop coalescence phenomena, and the atomization details at the injector are lost during these size rearrangements. Drop breakup dominates in hollow-cone sprays because coalescence is minimized by the expanding spray geometry. The results imply that it may be possible to use a simple injector and still control spray drop size and vaporization if the flow details are modified so as to enhance drop breakup and coalescence.

An experimental study on the effects of oxygenated fuel blends and multiple injection strategies on DI diesel engine emissions

Experimental studies on effects of oxygenated fuels in conjunction with single and split fuel injections were conducted at high and low loads on a Caterpillar SCOTE DI diesel engine. At high loads, a significant beneficial effect of oxygenated fuels was seen to reduce soot emissions with little or no penalty on NOx emissions. Also, at high loads, split injection had an additional favorable effect on soot emissions as compared to single injections, but the soot reducing influence of the oxygenates was not as marked as that seen with the single injection cases. This result indicates that the soot reduction due to the addition of oxygenate to the fuel is most effective in rich combustion as split injections are known to be effective at leaning-out the charge. In fact, at low engine loads when the overall mixture is further leaned-out, the oxygenated fuels had only a slight effect on particulate emissions. Split injections were effective in reducing particulate emissions at low loads particularly at advanced fuel injection timings when overall temperatures would be expected to be higher.

Modeling high-speed viscous liquid sheet atomization

Abstract A linear stability analysis is presented for a liquid sheet that includes the effects of the surrounding gas, surface tension and the liquid viscosity on the wave growth process. An inviscid dispersion relation is used to identify the transition from a long wavelength regime to a short wavelength regime, analogous to the first and second wind induced breakup regimes of cylindrical liquid jets. This transition, which is found to occur at a gas Weber number of 27/16, is used to simplify the viscous dispersion relation for use in multi-dimensional simulations of sheet breakup. The resulting dispersion relation is used to predict the maximum unstable growth rate and wave length, the sheet breakup length and the resulting drop size for pressure-swirl atomizers. The predicted drop size is used as a boundary condition in a multi-dimensional spray model. The results show that the model is able to accurately predict liquid spray penetration, local Sauter mean diameter and overall spray shape.

Modeling engine spray/wall impingement

A computer model was used to study the impingement of sprays on walls. The spray model accounts for the effects of drop breakup, drop collision and coalescence, and the effect of drops on the gas turbulence. A new submodel was developed to describe the spray/wall interaction process. Predictions of the effect of engine swirl, ambient gas pressure (density), wall inclination angle and the distance from the nozzle to the wall, were in good qualitative agreement with the experiments