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The Effect of Tumble Flow on Efficiency for a Direct Injected Turbocharged Downsized Gasoline Engine
Abstract
Direct gasoline injection combined with turbo charging and down sizing is a cost effective concept to meet future requirements for emission reduction as well as increased efficiency for passenger cars. It is well known that turbulence induced by in-cylinder air motion can influence efficiency. In this study, the intake-generated flow field was varied for a direct injected turbo charged concept, with the intent to evaluate if further increase in tumble potentially could lead to higher efficiency compared to the baseline. A single cylinder head with flow separating walls in the intake ports and different restriction plates was used to allow different levels of tumble to be experimentally evaluated in a single cylinder engine. The different levels of tumble were quantified by flow rig experiments. Two series of experiments were performed, one aiming to evaluate tumble in the region of low to medium load and engine speed, mainly focusing on efficiency, and one for the high load region to evaluate any negative consequences of increased tumble. The results indicate that tumble positively can influence the efficiency and emissions, however, the shape of the incoming flow dictate the level of impact significantly. Even for a relatively small change in tumble great differences in heat loss were seen. The efficiency increase seen originated mainly from lower heat loss through the exhaust gases. Additional gain came from lower in cylinder heat loss for the more favorable shape of the flow, where CFD indicates that the incoming air initially follows the cylinder head to a greater extent. Negative consequences are also associated with increased tumble. For instance, excessive pressure rise rates which can result in noise issues at higher loads. However, from a combustion perspective, the turbulence induced by the tumble positively effects the main parameters with reduced combustion duration, increased stability and increased exhaust gas recycling tolerance as well as increased combustion efficiency, features that are beneficial especially for a direct injected down sized turbo charged concept.
... As identified in several papers [1,123,134], and frequently referenced during the newer engine developments, downsizing works well when high levels of tumble are used, allowing improved combustion stability, increased EGR / dilution tolerance and reducing knock sensitivity [192,193]. Bandel et al. [136] suggest that charge motion of around twice thencurrent levels would provide sufficiently fast and stable combustion to allow increases in compression ratio and/or reduction in enrichment for component protection. The rationale is that use of a suitably-matched turbocharger enables boost pressure to be raised to overcome any loss in volumetric efficiency inherent with ports designed for charge motion in preference to high flow coefficients, making high-charge-motion port design synergistic with turbocharging and downsizing. ...
... It was hypothesised that a high level of in-cylinder turbulence associated with high levels of tumble would cause an increase in heat loss [192]. However, this was found to be only partly true: heat loss could be significantly increased by directing the tumble motion towards the cylinder head and cylinder wall surfaces; otherwise the improved combustion resulting from higher tumble allowed an overall reduction in heat loss. ...
... Berntsson et al. [192] commented that while higher tumble ratios produced faster combustion, by enabling combustion closer to MBT they also resulted in higher rates of pressure rise for a given level of mean effective pressure. Therefore, it could be expected that at some point, even if heat transfer to the cylinder does not become limiting, refinement may become a concern with very rapid increases in cylinder pressure. ...
... Among the various attributes, turbulence plays a critical role in determining the engine performance, especially in spark-ignition engines where the combustion takes place based on flame propagation. This is mainly because a higher turbulence level can increase the turbulent flame speed, which may result in improved thermal efficiency (by bringing it close to the constant-volume combustion) [1] as well as mitigation of knocking phenomena (in case the flame propagates ahead of the auto-ignition of the end gas) [2,3]. Moreover, it has been demonstrated in many researches that turbulence enhancement can also increase the EGR tolerance [3,4], which is favorable for engines to meet the emission regulations. ...
... This is mainly because a higher turbulence level can increase the turbulent flame speed, which may result in improved thermal efficiency (by bringing it close to the constant-volume combustion) [1] as well as mitigation of knocking phenomena (in case the flame propagates ahead of the auto-ignition of the end gas) [2,3]. Moreover, it has been demonstrated in many researches that turbulence enhancement can also increase the EGR tolerance [3,4], which is favorable for engines to meet the emission regulations. For all these reasons, turbulence is a primary aspect that the 0D model should correctly predict. ...
Turbulence is one of the most important aspects in spark-ignition engines as it can significantly affect burn rates, heat transfer rates, and combustion stability, and thus the performance. Turbulence originates from a large-scale mean motion that occurs during the induction process, which mainly consists of tumble motion in modern spark-ignition engines with a pentroof cylinder head. Despite its significance, most 0D turbulence models rely on calibration factors when calculating the evolution of tumble motion and its conversion into turbulence. In this study, the 0D tumble model has been improved based on the physical phenomena, as an attempt to develop a comprehensive model that predicts flow dynamics inside the cylinder. The generation and decay rates of tumble motion are expressed with regards of the flow structure in a realistic combustion chamber geometry, while the effects of port geometry on both charging efficiency and tumble generation rate are reflected by supplementary steady CFD. The developed tumble model was integrated with the standard k-ε model, and the new turbulence model has been validated with engine experimental data for various changes in operating conditions including engine speed, load, valve timing, and engine geometry. The calculated results showed a reasonable correlation with the measured combustion duration, verifying this physics-based model can properly predict turbulence characteristics without any additional calibration process. This model can suggest greater insights on engine operation and is expected to assist the optimization process of engine design and operating strategies.
... The combustion duration can be decreased by increasing turbulence. For example, an increase in tumble level has been shown to be beneficial for the overall combustion duration and engine stability [24,25,26,27]. ...
... This increase in combustion rate can be explained by the piston speed at the moment the intake valve was opened. The piston reached its maximum speed when the intake valve was fully opened, which may result in higher tumble motion and turbulence, and consequently increased turbulent flame speed [24,25,26,27]. ...
This study investigated how the amount of dilution applied can be extended while maintaining normal engine operation in a GDI engine. Adding exhaust gases or air to a stoichiometric air/fuel mixture yields several advantages regarding fuel consumption and engine out emissions. The aim of this paper is to reduce fuel consumption by means of diluted combustion, an advanced ignition system and adjusted valve timing. Tests were performed on a Volvo four-cylinder engine equipped with a dual coil ignition system. This system made it possible to extend the ignition duration and current. Furthermore, a sweep was performed in valve timing and type of dilution, i.e., air or exhaust gases. While maintaining a CoV in IMEP < 5%, the DCI system was able to extend the maximum lambda value by 0.1 - 0.15. Minimizing valve overlap increased lambda by an additional 0.1. For dilution by exhaust gases, an increase of 9% was noted for the ignition system and a further increase of 9% was obtained by minimizing the valve overlap at 1500 rpm/5.00 bar BMEP. This exchange of internal combustion residuals for external dilution resulted in a further decrease of fuel consumption but increased engine out NOx. Using air as a diluter resulted in a 6% fuel consumption benefit over exhaust gas dilution, mainly due to the enhanced combustion efficiency arising from higher oxygen concentrations. However, the lower oxygen concentration when using exhaust gases as a diluter led to 50% lower engine out NOx levels at the dilution limit.
... The literature on tumble motion is quite wide: here only some references are reported. In particular the methodological approach on the tumble motion analysis could be based on the CFD simulations [6][7][8][9][10][11][12][13][14][15] or on the experimental results [16,17]. As far as the squish motion is concerned, the squish flow control is a key technology for improving the knock limit in the spark ignition engines [18,17]. ...
... In particular an estimation of the variation of the squish velocity at TDC is performed. The squish velocity at TDC is calculated by Eq. (14) under stationary condition hypothesis. In Fig. 8 the ratio between the radius variation ∆r and the cylinder area versus the C/D ratio for a percentage squish area of 25% (K S =0.25) and 30% (Ks=0.30) ...
Both in the automotive and in the motorcycle fields the requirement of step-by-step improvements for optimizing the engine cycle is still present. In particular the focus of the optimization process is to reduce the raw emissions and at the same time to not penalize the engine performance. In this research field the engine modeling is of great importance because the application field of the experimental measurements is very narrow, time-consuming and expensive. Hence the modeling technique is a wide used and a wide recognized instrument for helping in the design process. Another important function of the modeling is to provide the engine designers with the most important guidelines. The main focus is to fast provide designers with some fundamentals during the first designing stage which, if not the conclusive, is close to the final project.
... Compare with the battery electric vehicle (BEV) and fuel cell vehicle (FCV), the main obstacles of ICE are relatively low tank-to-wheel (TTW) efficiency and in-use emissions [1][2]. While the brake thermal efficiency (BTE) of modern commercial petrol ICE has improved significantly by implementation of advanced engine optimization technologies including lean/diluent combustion [3], Atkinson/Miller cycle [4], intensive in-cylinder tumble flow [5], variable compression ratio [6], controllable auto-ignition [7], etc., the overall TTW efficiency remains under 45% [8]. The requirement for novel combustion concept which is capable in realizing extreme high thermal efficiency while achieving zero emission is becoming more and more urgent. ...
Direct water injection provides feasible solution for combustion optimization and efficiency enhancement within internal combustion Rankine cycle engine, while the feedback signal of close-loop direct water injection control is still absent. Ion current detection monitors in-cylinder electron variation which shows potential in revealing direct water injection process. For better understanding of unprecedented augment of ion current signal under direct water injection within internal combustion Rankine cycle engine, a chemical kinetic model is established to calculate the effect of intake oxygen fraction, fuel quantity, initial temperature and residual water vapor on in-cylinder electron formation based on GRI Mech 3.0 and ion current skeleton mechanism. The simulation results indicate direct water injection process show significant impact on in-cylinder electron formation through chemical interactions between H2O and other intermedia species including HO2, O2, CH3 and H, these reactions provides additional OH radical for propane oxidation facilitation, which result in large portion of CH radical formation and therefore, lead to higher in-cylinder electron generation. The initial temperature plays a vital role in determining whether residual water vapor show positive or negative effect by in-cylinder temperature coordination of direct water injection. Results of this work can be used to explain phenomenon related to direct water injection and ion current signal variation under both internal combustion Rankine cycle or traditional petrol engine.
... In this work, a pent-roof gasoline engine was chosen as an experimental test case where strong tumble motion of the in-cylinder gases is known to significantly affect heat losses [90,91]. On one hand, tumble is known to increase the burning rate and thus the thermal efficiency of an engine. ...
A non-intrusive laser diagnostics technique has been developed for simultaneous measurements of velocity and gas temperature in optically accessible internal combustion en-gines. The technique, thermographic PIV (T-PIV) combines phosphor thermometry and particle image velocimetry (PIV) and offers the possibility of simultaneous measurement of gas temperature and velocity.Suitable phosphor materials were selected by testing three commercially available phosphors: BAM:Eu2+, ZnO and ZnO:Zn. The lumines-cence emission and the spectral response to var-ious parameters including temperature were measured yielding a temperature-dependent cal-ibration curve to be used for signal interpreta-tion in engine experiments. The ZnO:Zn phos-phor shows the highest sensitivity to tempera-ture allowing higher temperature precision. Therefore, ZnO:Zn phosphor was chosen as the suitable candidate for engine measurements.Measurements were performed in an internal combustion engine at a speed of 1200 rpm with a sampling rate of 10 Hz between 180 and 540°CA under motored conditions. The temper-ature and velocity fields were measured success-fully at various times throughout the compres-sion and the exhaust stroke. The obtained tem-perature fields are compared with simulated bulk-gas temperatures from a 0D model-based simulation showing a temperature deviation of around 1% (200°CA) to 14% (480°CA) from the model. The measurement accuracy was found to be 55 K (18%) at 300 K and 2 K (0.3%) at 614 K for the 200-cycles average.The potential of the diagnostics was tested also in in cylinder post-combustion gases. In this case, the diagnostics was failing probably due to the characteristics of the phosphor used, which does not seem to resist to high combustion tem-peratures degrading its luminescence properties. The potential of T-PIV in post-combustion gases remains under the conditions of finding more resistant phosphor particles.
... Berntsson et al. [73] studied the effect of tumble number using modified intake ports. Tumble numbers were varied between a baseline of 1.12 up to 1.5 and were established using flow rig measurements. ...
... To realize the designed in-cylinder tumble motion, various kinds of charge motion control valves have been employed near the intake valves [17,18] or far away from the intake ports [19][20][21][22][23]. Fischer et al. [19] investigated the CCV in an optical stratified-charge gasoline direct injection (GDI) engine with variable tumble systems using PIV, spark emission spectroscopy, and high-speed flame visualization. ...
Cycle-to-cycle variations (CCVs) limit the extension of the operating range by inducing load variations and even misfire and/or knock for direct injection spark ignition (DISI) engines and hence need to be controlled. One of the effective and flexible ways to reduce CCV is to employ a charge motion control valve. This study is aimed to analyze the flow characteristics and CCV using large eddy simulation (LES) and fast Fourier transform (FFT) in a non-reacting, DISI engine equipped with a tumble flap (i.e., a specific type of charge motion control valve) inside the intake port. The in-cylinder flow characteristics are analyzed in detail, and the possible effects of multi-scale structures of the fluid field on the subsequent ignition and combustion processes are also discussed. Computational results indicate that closing the tumble flap helps enhance the intensity of the coherent structures and increase the total integral length scale (ILS) while decreasing the Kolmogorov scale and stabilizing the flow field by suppressing the CCV of tumble ratio and tumble center. Furthermore, based on a newly developed FFT triple decomposition, each instantaneous flow field is decomposed into three subfields, termed ensemble mean part and low- and high-spatial frequency parts, respectively. It is found that switching the tumble flap position greatly affects the first two subfields, but it has negligible effect on the last part. With the closed tumble flap, the energy portion of the mean part increases, the rate of energy decay reduces, and the CCV of the low- and high-spatial frequency parts decreases.
Downsizing, decrease of engine size, shows up as a significant method of enhancing fuel utilization of engines while keeping up bit of leeway of low emisions these days in market. The downsizing methodology with turbocharged applications is presently to an ever-increasing
extent situated towards fuel economy, because of specific arrangements. The utilization of gasoline direct injection combined with turbochargers gives a few methods of improving aversion to knock particularly at heavy load and lower engine speeds where current PFI motors
with forced induction are as yet restricted. In this study we are discussing about the performance,
test conditions and emissions with respect to standard naturally aspirated engines. We have also
compared various methodologies used for downsizing rather than turbocharging and how they
help in overcoming the issues with turbocharging like low boost and turbo lag. Reducing the weight of moving parts like piston and crankshaft, thus reducing the overall weight of engine which increases its efficiency, by 3D printing methods is also studied. Shortcomings and
conclusions drawn from the use of various Alternate Fuels and comparisons of various methodologies like Exhaust Gas Recirculation, Water Injection, coupling of a small compressor etc. are recorded and discussed
div class="section abstract"> Homogeneous lean combustion (HLC) can be utilized to substantially improve spark ignited (SI) internal combustion engine efficiency. Higher efficiency is vital to enable clean, efficient and affordable propulsion for the next generation light duty vehicles. More research is needed to ensure robustness, fuel efficiency/NO<sub>x</sub> trade-off and utilization of HLC. Utilization can be improved by expanding the HLC operating window to higher engine torque domains which increases impact on real driving. The authors have earlier assessed boosted HLC operation in a downsized two-litre engine, but it was found that HLC operation could not be achieved above 15 bar NMEP due to instability and knocking combustion. The observation led to the conclusion that there exists a lean load limit. Therefore, further experiments have been conducted in a single cylinder research DISI engine to increase understanding of high load lean operation. HLC is known to suppress end-gas autoignition (knock) by decreasing reactivity and temperatures, but during the experiments knock was observed to be prominent and increasing in severity when engine load was increased despite operating ultra-lean close to lambda 2. Knock is normally mitigated by spark retardation which decreases peak cylinder pressure. However, to maintain stable combustion at lambda = 2 the combustion phasing had to be kept close to TC which resulted in high peak cylinder pressures. Therefore, the combustion event had to be balanced in a window where early combustion promoted knock and late resulted in instability and partial burns. A tumble flap was introduced to increase in-cylinder tumble which reduced knock and improved combustion stability. It could be observed that for most load-points assessed; increased tumble could suppress knock and increase the air-dilution limit which proved beneficial in decreasing the NO<sub>x</sub> emissions. The highest engine load that could be achieved with highly diluted combustion was 16 bar NMEP.
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The present paper is focused on the development of a high-performance, monofuel, spark ignition engine running on natural gas, featuring a high volumetric compression ratio and a variable valve actuation system. More specifically, the cylinder head geometry effect has been analyzed and the compression ratio has been optimized by means of steady-state and transient simulation activity, as well as of an extensive experimental campaign. The compression ratio effect was mainly investigated by means of experimental tests but a few 3D simulations were also run in order to quantify its impact on the in-cylinder tumble and turbulence. The main novelty of the paper are, first, the adoption of very high engine compression ratio values, second, the combined optimization of the cylinder head design and compression ratio. The main results can be summarized as follows. The engine configuration with mask showed a decrease in the average discharge coefficient by 20–30% and an increase in the tumble ratio by around 200% at partial load. Moreover, the simulation of the engine cycle indicated that the presence of the piston modifies the tumble structure with respect to the steady-state simulation case. An increase in the tumble number and turbulence intensity by around 90% and 10%, respectively, are obtained for the case with mask at 2000 rpm and 4 bar. With reference to the combustion duration, on an average, the presence of the masking surface led to a reduction of the combustion duration (from 1% to 50% of mass fraction burned) between 2 and 6 degrees. As far as the engine compression ratio is concerned, the value of 13 was finally selected as the best compromise between combustion variability, engine performance at full load and fuel consumption at partial load.
Most of us are not familiar with the concept that an internal combustion (IC) engine is working on a four-stroke six-event principle. The six events are suction, compression, combustion, expansion, blow-down and exhaust. However, the expansion and blow-down happen in the power stroke and they can be clubbed together. Therefore, we can say that an engine, whether diesel or gasoline, is working on four-stroke five-event principle. The purpose of this chapter is to make the reader to familiarize with the complexities involved in the working of a four-stroke engine. The five events which are completed in four strokes are: suction, compression, combustion, expansion and exhaust. Application of computational fluid dynamics (CFD) principles for each process mentioned above is a challenging job. The difficulty in understanding the working of an IC engine is due to the fact that we cannot see what is happening inside the cylinder piston arrangement. All that is described in textbooks is based on our knowledge gained over a period of time by conducting experiments. There is no doubt in saying that “seeing is believing”. As it is next to impossible to have a complete experimental flow visualization, nowadays CFD comes in handy to have theoretical flow visualization. Well-developed software is available for the simulation in 3D geometries. In this chapter, we explain step-by-step the details required for the CFD simulation of various processes in an IC engine. Extensive results obtained over a period of 20 years of research by the application of CFD in analysing the flow in engines are comprehensively presented and discussed. CFD can be very well applied for analysing any particular process. It can also be used for the modification of the existing engine design or can also be employed for a new design of an engine. It is hoped that readers may be benefitted in understanding the application of CFD for fluid flow analysis and engine design by reading this chapter. Therefore, the main aim of this chapter is to make the reader appreciate how exactly CFD can be applied for design of an engine. As it is application oriented, we are not going deep into the equations, modelling, etc. A number of case studies are presented and discussed.
This paper describes the experimental study of a tumble-flap mounted in the intake port of a single-cylinder spark-ignited gasoline engine. The research question addressed was whether an optimal tumble level could be found for the combustion system under investigation. Indicated fuel consumption was measured for a number of part-load operating points with the tumble-flap either open or closed. The experimental results were subjected to an energy balance analysis to understand which portion of the fuel energy was converted to work and how much was lost by incomplete combustion, heat losses to walls and to the exhaust gases, as well as to pumping losses. Closing the tumble-flap resulted in reduced fuel consumption only in a small area of the operating map: only at low-speed, low-load operation, a benefit could be obtained. One way of achieving this was by means of de-throttling due to increased valve overlap, since the combustion system could tolerate a higher level of internal exhaust gas recirculation (EGR) when the tumble-flap was closed. Improved combustion efficiency was another way of achieving lower fuel consumption. For higher engine speeds and loads, fuel consumption actually deteriorated for closed tumble-flap operation, indicating that an optimal tumble level exists and that it was reached in this study. The reason for the worse fuel consumption was that the intake ports in the baseline combustion system already set up such a strong charge motion that a further increase of turbulence (by closing the tumble-flap) no longer contributed to improved conversion of fuel into products or a higher tolerance to internal EGR. On the contrary, it was found that restricting the intake port resulted in increased pumping losses. A tendency of increased wall heat losses for the highest tumble levels could also be observed. Combustion duration generally became shorter for higher levels of in-cylinder turbulence, but for some operating points with extreme tumble level combustion speed actually decreased.
Volvo Car Corporation has recently launched the first three members of a completely new engine family called Drive-E. It consists of four gasoline and four diesel engines that are characterized by high specific performance, low friction and low fuel consumption. The number of cylinders is limited to four, which implies that these down-sized, boosted engines will replace present five and six cylinder engines. This paper focuses on the development of the Drive-E gasoline combustion system, characterized by a centrally-mounted direct-injection fuel injector in combination with high-tumbling intake ports. The break-down of the large-scale tumble motion into a high level of turbulence leads to fast and stable combustion as well as excellent air/fuel mixing and limited wall wetting. The four gasoline engines span a wide range of power levels. This means that the engine at the entry power level needs to breathe less air than the 2.0 L engine at the highest power level (225 kW, 306 HP). Consequently, intake port design was an important part of the combustion system development. Different intake port geometries are applied at the different power levels, and this paper describes the trade-off between a design that sets up a strong tumble motion (leading to fast combustion and thus reduced fuel consumption), and a design that optimizes engine breathing (enabling class-leading specific power, while keeping fuel-efficiency at a good level).
In scooter/motorbike engines coherent and stable tumble motion generation is still considered an effective mean in order to both reduce engine emissions and promote higher levels of combustion efficiency. The scientific research also assessed that squish motion is an effective mean for speeding up the combustion in a combustion process already fast. In a previous technical paper the authors demonstrated that for an engine having a high C/D ratio the squish motion is not only not necessary but also detrimental for the stability of the tumble motion itself, because there is a strong interaction between these two motions with the consequent formation of secondary vortices, which in turn penalizes the tumble breakdown and the turbulent kinetic energy production. The present paper starts with the development of a theoretical model which will be applied to different engine classes demonstrating how the relative importance of the tumble and squish varies changing the typical design parameters. The final achievements will be to show that the squish flow importance in promoting the turbulence reduces moving to small bore square engines. CFD simulation of the intake flow will be then performed with AVL Fire v. 2010 for investigating in more detail the stroke influence on the in-cylinder turbulence generation.
In PFI and GDI engines the tumble motion is the most important charge motion for enhancing the in-cylinder turbulence level at ignition time close to the spark plug position. In the open literature different studies were reported on the tumble motion, experimental and not. In the present paper the research activity on the tumble generation at partial load and very partial load conditions was presented. The added value of the analysis was the study of the effect of the throttle valve rotational direction on the tumble motion and the final level of turbulence at the ignition time close to the spark plug location. The focus was to determine if the throttle rotational direction was crucial for the tumble ratio and the turbulence level. The analyzed engine was a PFI 4-valves motorcycle engine. The engine geometry was formed by the intake duct and the cylinder. The CFD code was FIRE AVL code 2013.1. The intake and the compression phases till TDC were simulated: inlet boundary conditions from 1D simulations were imposed. The modelled fluid was only air because the next step, now in progress, is the analysis of the mixture formation at the same partial load conditions modelling the injection. Once assessed the influence of the throttle valve on both the tumble motion and the turbulence, the authors' focus is the analysis of the mixture formation process and the evaluation of the final mixture index value.
During the last years the deep re-examination of the engine design for lowering engine emissions involved two-wheel vehicles too. The IC-engine overall efficiency plays a fundamental role in determining final raw emissions. From this point of view, the optimization of the in-cylinder flow organization is mandatory. In detail, in SI-engines the generation of a coherent tumble vortex having dimensions comparable to the engine stroke could be of primary importance to extend the engines' ignition limits toward the field of the dilute/lean mixtures. For motorbike and motor scooter applications, the optimization of the tumble generation is considered an effective way to improve the combustion system efficiency and to lower emissions, considering also that the two-wheels layout represents an obstacle in adopting the advanced post-treatment concepts designed for automotive applications. The aim of the paper is to use the 3D-CFD simulation tool to assess the intake duct geometry influence on the tumble motion generation during both the intake and the compression strokes. All the CFD simulations presented in the paper were performed on a SI 4 valve engine characterized by an unit displacement of 250 cm3. The tumble structure was changed during the analysis by changing the angle set defining the intake port shape. The stroke-to-bore engine ratio was kept constant to 0.7. The effects of the tumble variations were evaluated on the in-cylinder vortex characteristic length and vorticity, and on the cylinder mean flow structure at IVC. 3D-CFD simulations were performed by AVL-FIRE v.2010 CFD code.
Faster combustion and lower cycle-to-cycle variability are mandatory tasks for naturally aspirated engines to reduce emission levels and to increase engine efficiency. The promotion of a stable and coherent tumble structure is considered as one of the best way to promote the in-cylinder turbulence and therefore the combustion velocity. During the compression stroke the tumble vortex is deformed, accelerated and its breakdown in smaller eddies leads to the turbulence enhancement process. The prediction of the final level of turbulence for a particular engine operating point is crucial during the engine design process because it represents a practical comparative means for different engine solutions. The tumble ratio parameter value represents a first step toward the evaluation of the turbulence level at ignition time, but it has an intrinsic limit. The tumble ratio parameter represents the value of the angular velocity of a single macro vortex, while the flow-field is often characterized by multiple vortexes, sometimes some rotating and some counter-rotating. The idea at the basis of the paper is: To develop a quasi-predictive 0D model for defining the final mean level of the turbulence at the ignition time. The model is fed by the curtain intake valve mass flow rate and the intake valve lift trend. In order to validate the 0D model the results were compared versus 3D CFD results.To extract from a 3D CFD flow field at IVC the type and the number of the vortexes. The 3D CFD RANS simulations were performed by AVL Fire code v. 2010.To demonstrate through some 3D CFD results that the flow field structure was a function only of the engine type and the load condition. Finally the flow field structure could be used in the 0D model on varying the engine speeds as a means of improvement of the model prediction capability.
Engine downsizing and downspeeding using GTDI technology improves fuel economy while maintaining vehicle performance. The downsizing potential of an engine application is limited by engine knock at low engine speeds as well as turbocharger inlet and catalyst temperatures at high speeds, requiring high spark retard and fuel enrichment, respectively. Both spark retard and fuel enrichment reduce the overall real world fuel economy benefit. Cooled exhaust gas recirculation (EGR) has been investigated as a way of reducing knock and lowering exhaust gas temperatures. This paper discusses the use of low-pressure route cooled EGR for knock mitigation at low engine speeds in order to improve full load performance and fuel consumption and increase the potential for engine downsizing. In particular, this study focuses on increasing the EGR tolerance of the combustion system through the use of very high tumble ratios exceeding that of current production systems, enhanced ignition and increased compression ratio. An externally boosted single cylinder research engine is used to investigate the effects of these various concepts on the EGR tolerance and the subsequent effects on low speed, full-load engine performance and fuel consumption. Computational fluid dynamics (CFD) is used to understand the mixture preparation and turbulence differences expected when using high tumble intake ports. A high tumble combustion system was found to provide high EGR dilution tolerance, improved thermal efficiency and higher load capability through advancement of the knock limited combustion phasing. However, extremely high tumble ratios were found to increase the knocking tendency of the engine.
Today turbo-Diesel powertrains offering low fuel consumption and good low-end torque comprise a significant fraction of the light-duty vehicle market in Europe. Global CO 2 regulation and customer fuel prices are expected to continue providing pressure for powertrain fuel efficiency. However, regulated emissions for NOx and particulate matter have the potential to further expand the incremental cost of diesel powertrain applications. Vehicle segments with the most cost sensitivity like compacts under 1400 kg weight look for alternatives to meet the CO 2 challenge but maintain an attractive customer offering. In this paper the concepts of downsizing and downspeeding gasoline engines are explored while meeting performance needs through increased BMEP to maintain good driveability and vehicle launch dynamics. A critical enabler for the solution is adoption of gasoline direct injection (GDi) fuel systems. GDi provides the ability to utilize increased scavenging without sacrificing hydrocarbon emissions because fueling and air controls can be separated. In-cylinder injection with GDi also provides charge cooling benefit yielding the knock reduction necessary for turbocharged applications. Several options within GDi are explored including multi-hole and single pintle spray generators, as well as side-mount versus central-mount applications. Both 3-cylinder and 4-cylinder base engine configurations are explored for turbocharged engines downsized to 1.2 L. Improvements in hydrocarbon emissions, heat losses and scavenging favor fewer cylinders. This is re-enforced by packaging and cost considerations. The next system aspect considered is emissions aftertreatment. Stoichiometric turbocharged GDi provides the lowest cost using the established 3-way catalyst. Stratified GDi is expected to require a lean NOx trap (LNT) and Diesel Euro 6 systems require addition of a diesel particulate filter (DPF) and possibly NOx aftertreatment. Although the aftertreatment system to meet Euro 6 NOx requirement is not known today, addition of a DPF, DPF with LNT, or DPF with urea selective catalytic reduction (SCR) represent significant cost and packaging challenges. The analysis concludes by comparing the 3-cylinder turbocharged GDi to other offerings in the context of value tradeoff of CO 2 reduction to system on-cost. Stratified 3-cylinder turbocharged GDi systems offer up to 22% CO 2 reduction compared to a baseline port fuel injected 4-cylinder engine. CO 2 reduction of 18% is possible for a stoichiometric GDi mechanization that employs variable valve actuation (VVA) and a conventional 3-way catalytic converter. At Euro 6 emission levels, stoichiometric 3-cylinder turbocharged GDi powertrains offer excellent value. The 3-cylinder turbo-Diesel offers similar value if it is capable of meeting the NOx emissions standard without lean aftertreatment. Powertrains with higher engine-out NOx levels that require lean aftertreatment are significantly disadvantaged. Based on the price sensitivity of the compact car segment, the value analysis predicts the 3-cylinder turbocharged GDi engine as the powertrain of choice.