EGR Dilution Research Articles

Collaborative effects of fuel properties and EGR on the efficiency improvement and load boundary extension of a medium-duty engine

EGR dilution combustion has problems such as weakened anti-knock capability at high load, slow combustion speed and poor combustion stability, which results in limitations in the thermal efficiency improvement and load boundary extension of medium-duty highly-downsized engines. It is necessary to combine EGR dilution and other measures to collaboratively control the in-cylinder thermodynamic state and combustion process. The experimental investigations in this study isolate the effect of the ethanol blending ratio in ethanol gasoline on the anti-knock performance, combustion performance and thermal efficiency, and verifies the potential of collaborative optimization of fuel properties and EGR in improving the thermal efficiency and extending the load boundary for a medium-duty highly-downsized engine. The results show that as the load increases, the improvement effect of increasing the blending ratio of ethanol in the anti-knock performance, combustion speed, and the turbine inlet temperature reduction will become more obvious. At high load, using E20 fuel can improve the EGR tolerance, advance the spark timing and CA50, and thus increase the BTE. As the speed decreases, the thermal efficiency improvement effect of E20 fuel gradually increases, and the improved load range extends. The collaborative optimization of E20 fuel and EGR can further extend the high thermal efficiency area of the engine. And the Max. achievable load is 0.11 MPa higher than that of E10, which effectively extends the upper load limit during the stoichiometric combustion.

A novel concept of active pre-chamber engine with a single injector – The passive main chamber approach

The pre-chamber spark ignited combustion is recognized as one of the most promising concepts utilizing a lean burn combustion approach towards the increase of efficiency and reduction of NOx emissions in spark ignited engines. Numerous investigations indicate the possibility to reduce the engine-out NOx emissions under the regulation limits by using active pre-chamber at very high dilution levels. Regardless of the dilution approach, one of the main challenges is to achieve the proper mixture formation and scavenging of the pre-chamber from residual gases to ensure stable engine operation without misfires. Recent investigations imply that the auxiliary air injection in the pre-chamber might be necessary to ensure stable operation at such highly diluted mixtures, especially if an EGR dilution approach is utilized, further increasing the complexity of such a concept. In this work, a new approach is investigated which can be described as a passive main chamber approach and relies on a single fuel injector supplying both the pre-chamber and the main chamber. Contrary to the passive pre-chamber systems, the injector in this approach is mounted in the pre-chamber, while the main chamber is supplied indirectly through the pre-chamber nozzles. To verify the feasibility and the potential benefits of such an approach, a numerical framework consisting of 3D-CFD and 1D/0D models is applied. The performed study included different pre-chamber variants, previously investigated on an experimental gasoline-fuelled engine, to verify the applicability of such a concept regardless of the pre-chamber geometry. The performed study showed that the proposed concept is a feasible option and, despite the inability to directly control the excess air ratio in the pre-chamber, results with the desired values of λPC ≈ 0.8 at the MBT optimized spark timings. At very lean mixtures (λglobal > 2.0), the average residual gas concentration in the pre-chamber and around the spark plug at the ignition timing is reduced from 10 % and 13 % respectively to 2.5 %, along with a 1.4 percent point increase of indicated efficiency when compared to normal operation. When compared to the reference gasoline fuelled cases, a highest efficiency of 37.7 % is achieved at moderately lean mixture (λglobal ≈ 1.6) and MBT optimized spark timing with NOx levels of 1.41 g/kWh, which represents a 0.7 percent point increase in indicated efficiency but 75 % increase of NOx emissions. At NOx optimized spark timing however (NOx < 0.4 g/kWh), an identical indicated efficiency of 34.7 % is obtained. Overall, the results indicate that with the proposed concept, it is possible to achieve the same scavenging effect as with auxiliary air injection while eliminating the need for both the main chamber injection system and the pre-chamber auxiliary air supply system and without sacrificing the overall performance.

Flame morphology of hydrous ethanol combustion under EGR dilution for port fuel injection and direct injection in a spark ignition optical engine

The increasing popularity of plug-in hybrid vehicles has prompted investigation of options such as range extender units, which may include small engines powered by biofuels. To minimize energy consumption, the best technologies must be chosen, including the use of exhaust gas recirculation (EGR, as a charge dilution technique) and the best alternative for fuel injection systems. Therefore, a combustion evaluation was conducted on an optically accessible engine fueled with hydrous ethanol to determine the effects of different injection modes, such as direct injection (DI) and port fuel injection (PFI), combined with EGR. The study employed high-speed camera imaging to analyze flame morphological characteristics and understand their impact on in-cylinder thermodynamics and engine emissions. The DI mode presented more stability than PFI, although the dilution limit was lower for DI.

Numerical assessment of water injection for improved thermal efficiency and emissions control in a medium-duty hydrogen engine for transportation applications

The existing political environment and the growing worries about climate change have resulted in the imposition of stringent regulations on traditional propulsion systems reliant on fossil fuels. While new technologies are being investigated to substitute internal combustion engines, their current technological advancement necessitates further development and refinement in order to emerge as a viable alternative to the conventional internal combustion engine. The utilization of hydrogen-powered internal combustion engines has showcased their capacity to rapidly achieve complete decarbonization in the transportation industry. However, these engines continue to encounter limitations regarding their operational range that need to be addressed. The utilization of EGR for controlling combustion instabilities and mitigating NOx formation still has significant limitations in operating at high engine loads, mainly affecting the boost requirements for reaching high power densities. The present work evaluates the potential of substituting the EGR dilution with a water injection system for reaching full load operation on a conventional diesel engine, assuming the minimum modifications required for it to work under H2 combustion. The study is carried out assuming a multi-cylinder engine representing the medium- to high-duty transport sector. The evaluation includes a comparison of the engine performance and emissions obtained when using EGR and water injection at full load, comparing the requirements of the boost system for both strategies to reach power densities of 24 bar of BMEP equivalent to modern diesel applications. The results show that WI allows for significantly reduced boost pressure requirements lowering intake pressure by up to 1 bar, while achieving very similar engine efficiency and a reduction of NOx emissions around 80%, resulting a similar effectiveness compared with EGR. Additionally, WI provides additional prevention against undesired autoignition.

The interaction of charge motion and EGR on anti-knock performance and efficiency of a medium-duty gasoline engine: An experimental study

Due to the common operating characteristics of medium duty (MD) gasoline engines, achieving higher load at the expense of fuel efficiency is not an acceptable tradeoff. EGR dilution-stoichiometric combustion can suppress the knocking and improve the thermal efficiency, but has problems such as slow combustion rate and poor combustion stability, which have limitations in optimizing combustion for MD gasoline engines with strong swirl. The experimental investigations in this study focus on the interaction of charge motion and EGR on anti-knock performance and thermal efficiency under different load conditions, and verifies the potential to improve the thermal efficiency of inclined swirl combustion system. The results show that at low-medium load, the tumble-dominated inclined swirl scheme can enhance the effectiveness of EGR in knock suppression, thus reducing the introduction of EGR to avoid its adverse effects on the combustion duration. At medium-high load, although the above benefit wears off, it can enhance the anti-knock performance and thus advance the spark timing by increasing the EGR tolerance. The EGR ratio corresponding to the highest thermal efficiency increases with higher load. The difference in the in-cylinder flow field will affect the combustion rate in a high EGR dilution and high load condition. The scheme adopting the compound intake ports and inverse cuneiform-shaped combustion chamber can improve both EGR tolerance and initial combustion rate under high EGR dilution, with the most significant improvement in thermal efficiency, and the improvement range will increase along with the increase of load.

Numerical assessment of fuel physical properties on high-dilution diesel advanced compression ignition combustion

Using traditional lean air-fuel mixture aftertreatment technologies to reduce NOx at low exhaust temperatures is limited below 175°C. Recent low engine load NOx emission regulations encouraged researchers to explore the potential of fuel physical properties to enable low-temperature, low-NOx combustion under challenging conditions in which the selective catalytic reduction device might be unable to treat engine-out NOx. Three diesel range fuels were developed to explore how the distillation profile affects and may enable low-load diesel advanced-compression ignition. The least volatile fuel is a conventional, commercially available diesel #2 fuel with a cetane number of 45.3. The second-most volatile fuel is diesel #1 fuel, which can also be found commercially. The last fuel, which is the most volatile, is a new formulation designed to target a final boiling point approximately 50 K below diesel #1 fuel and was termed diesel #0 fuel. A computational fluid dynamics model was developed and validated against experimental data to investigate the underlying effects of each of the developed fuels in the combustion process. Finally, each fuel physical property (e.g., density, viscosity, etc.) was changed independently to assess its effect under the same operating conditions and identify potential directions for fuel formulation that may enable improved NOx emissions and reduce the reliance on aftertreatment systems for NOx control. The results showed that although changes in the fuel distillation curve and other physical properties affect the air-fuel mixtures substantially, changes were not sufficient to result in a large impact to low-load NOx formation. The maximum predicted NOx reduction was 11% uncorrected by combustion phasing. The results suggest that further research efforts should focus on changing the chemical properties of the fuel, like the cetane number, to allow higher EGR dilution and stable combustion.

Effect of pre-chamber scavenging strategy on EGR tolerance and thermal efficiency of pre-chamber turbulent jet ignition systems

Dual Mode, Turbulent Jet Ignition (DM-TJI) is an engine combustion technology that incorporates an auxiliary air supply apart from the auxiliary fuel injection inside the pre-chamber of a divided chamber ignition concept. Compared to other active (auxiliary fueled) and passive pre-chamber ignition technologies, the DM-TJI system has the distinct capability to operate with very high level (up to ∼50%) of recirculated exhaust gas (EGR). Thus, unlike typical lean (excess air dilution) operated pre-chamber ignition technologies, the DM-TJI system enables the use of widely utilized low cost three-way-catalyst (TWC) while still running at high level of dilution (with EGR). The supplementary air supply to the pre-chamber enables effective purging and ignitable mixture formation inside the pre-chamber even with very high external EGR rate. The current work presents the results of experimental investigation conducted on a Prototype III Dual Mode, Turbulent Jet Ignition (DM-TJI) (or Jetfire ignition) single cylinder metal engine. Different pre-chamber scavenging/fueling strategies (active vs passive) were investigated in order to compare the EGR dilution tolerances between different scavenging configurations under identical pre-chamber design parameters (pre-chamber volume and nozzle configuration). Tests were conducted at two regularly encountered operating conditions (6 and 10 bar IMEPg at 1500 rpm) in typical drive cycles. Results are also compared with the conventional SI (spark ignition) configuration on the same engine. The results indicate that to maintain very high EGR diluted (up to ∼50%) operation the auxiliary air supply to the pre-chamber is of paramount importance. The analysis found that DM-TJI/Jetfire ignition system is more effective in terms of thermal efficiency at high load knock limited situation due to its considerably higher external EGR dilution tolerance. Higher EGR rate offered better combustion phasing and improved thermal efficiency considerably. It was found that with the elevated 13.3:1 compression ratio and 10 bar load, SI could not maintain knock free stable operation and DM-TJI/Jetfire delivered 7%–9% improvement in thermal efficiency compared to TJI mode of operation with no air delivery to the pre-chamber.

A study of the influence of orifice diameter on a Dual Mode, Turbulent Jet Ignition engine through variable engine speed

The increasingly strict worldwide emission regulations for internal combustion engines force car manufacturers to exploit innovative combustion technologies. One expects a change from pure thermal engines to more advanced internal combustion engine. A high engine compression ratio with high EGR dilution has an impact on increasing engine efficiency. A compelling concept to burn diluted combustion can be a DM-TJI combustion system, which consists of a pre-chamber and main chamber connected with one or more small orifices. This technology effectively achieves diluted combustion by improving engine dilution tolerance and knock performance. This paper focused on the impact of engine speed with variation in pre-chamber nozzle orifice diameter. Experiments were conducted with three different pre-chamber nozzle orifice diameters of 1.25, 1.5, and 1.75 mm with an engine speed range from 1500 to 2300 rpm. A numerical analysis was conducted to investigate the nozzle orifice diameter effect on the engine speed beyond 2500 rpm. Engine speed variation affects the performance of each orifice’s diameters. Small orifice diameter performs better for lower engine speed, and large orifice diameter performs better for higher engine speed. The 1.25 mm orifice diameter provided fast combustion. However, when the engine speed increases, the combustion stability suffers. Orifice diameter of 1.5 mm has better combustion stability in all tested speed ranges and higher efficiency in the speed range of 1500–2000 rpm, with a maximum indicated efficiency of 44.18%. The 1.75 mm orifice diameter performs better in the high engine speed ranges between 2100 and 2300 rpm, with a maximum indicated efficiency of 44.56% at an engine speed of 2300 rpm. This study has demonstrated the capability of the DM-TJI system to work with a high level of EGR dilution (up to 45%) and delivered a better indicated efficiency than the conventional spark ignition engines.

Investigations into EGR dilution tolerance in a pre-chamber ignited GDI engine

EGR is a useful means to improve fuel economy in spark-ignited gasoline engines, especially under part-load conditions by means of reducing throttling losses. There is also a significant reduction in NOx emissions due to the reduced peak cylinder temperatures. However, the ability of conventional spark ignition systems to reliably ignite dilute mixtures limits the dilution tolerance due to the onset of combustion instability. In this study, an active pre-chamber system was evaluated at part load condition with various EGR rates using a regular grade full boiling range gasoline. A premixed air-fuel mixture was supplied to the pre-chamber, which assists in scavenging the residual mass fractions to enable reliable ignition of EGR dilute mixtures in the main combustion chamber. A range of pre-chamber injection, air-fuel mixture formulation, and ignition timing strategies were evaluated during the EGR sweeps. In addition, 0-D and 1-D simulations were conducted in GT-Power to assess the thermodynamic state and composition in the pre-chamber, as well as estimate the jet momentum at the various dilution rates. The active pre-chamber extended the EGR dilution limit from 20% for conventional SI to above 30%, with a higher pre-chamber flow rate (more scavenging) resulting in increased combustion stability. A tradeoff between the jet momentum and spark timing was observed when analyzing the GT-Power simulations and the pre-chamber jet momentum was found to decrease with increasing EGR rate. At unstable high EGR operation, no relation could be found between the pre-chamber indicated data and main chamber combustion stability.

Economy and emission characteristics of the optimal dilution strategy in lean combustion based on GDI gasoline engine equipped with prechamber

EGR and excess-air dilution have been investigated in a 1.5 L four cylinders gasoline direct injection (GDI) turbocharged engine equipped with prechamber. The influences of the two different dilution technologies on the engine performance are explored. The results show that at 2400 rpm and 12 bar, EGR dilution can adopt more aggressive ignition advanced angle to achieve optimal combustion phasing. However, excess-air dilution has greater fuel economy than that of EGR dilution owing to larger in-cylinder polytropic exponent. As for prechamber, when dilution ratio is greater than 37.1%, the combustion phase is advanced, resulting in fuel economy improving. Meanwhile, only when the dilution ratio is under 36.2%, the HC emissions of excess-air dilution are lower than the original engine. With the increase of dilution ratio, the CO emissions decrease continuously. The NOX emissions of both dilution technologies are 11% of those of the original engine. Excess-air dilution has better fuel economy and very low CO emissions. EGR dilution can effectively reduce NOX emissions, but increase HC emissions. Compared with spark plug ignition, the pre chamber ignition has lower HC, CO emissions, and higher NO emissions. At part load, the pre-chamber ignition reduces NOX emissions to 49 ppm.

Performance Comparison of LPG and Gasoline in an Engine Configured for EGR-Loop Catalytic Reforming

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;In prior work, the EGR loop catalytic reforming strategy developed by ORNL has been shown to provide a relative brake engine efficiency increase of more than 6% by minimizing the thermodynamic expense of the reforming processes, and in some cases achieving thermochemical recuperation (TCR), a form of waste heat recovery where waste heat is converted to usable chemical energy. In doing so, the EGR dilution limit was extended beyond 35% under stoichiometric conditions. In this investigation, a Microlith®-based metal-supported reforming catalyst (developed by Precision Combustion, Inc. (PCI)) was used to reform the parent fuel in a thermodynamically efficient manner into products rich in H&lt;sub&gt;2&lt;/sub&gt; and CO. We were able to expand the speed and load ranges relative to previous investigations: from 1,500 to 2,500 rpm, and from 2 to 14 bar break mean effective pressure (BMEP). Experiments were conducted to determine the effects of the H/C ratio of the fuel on H&lt;sub&gt;2&lt;/sub&gt; production and on the engine efficiency in order to compare E10 gasoline (H/C = 1.95) and liquified petroleum gas (LPG), comprised primarily of propane (H/C = 2.67). Additionally, the compression ratio of the engine was increased to ascertain whether further efficiency improvements could be realized based on a reduced knock propensity of the dilute EGR mixture with the reformed fuel. Both the gasoline and propane reforming strategies provided efficiency gains up to 1.85 percentage points and further efficiency improvements with the increased compression ratio were realized. The fuel specific effects of gasoline vs. LPG, the effect of engine operating condition on reforming, and knock limits of the reformed mixture are discussed in detail.&lt;/div&gt;&lt;/div&gt;

Comparison of Excess Air (Lean) vs EGR Diluted Operation in a Pre-Chamber Air/Fuel Scavenged Dual Mode, Turbulent Jet Ignition Engine at High Dilution Rate (~40%)

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Charge dilution is widely considered as one of the leading strategies to realize further improvement in thermal efficiency from current generation spark ignition engines. While dilution with excess air (lean burn operation) provides substantial thermal efficiency benefits, drastically diminished NOx conversion efficiency of the widely used three-way-catalyst (TWC) during off-stoichiometric/lean burn operation makes the lean combustion rather impractical, especially for automotive applications. A more viable alternative to lean operation is the dilution with EGR. The problem with EGR dilution has been the substantially lower dilution tolerance limit with EGR and a consequent drop in thermal efficiency compared to excess air/lean operation. This is particularly applicable to the pre-chamber jet ignition technologies with considerably higher lean burn capabilities but much lower EGR tolerance due to the presence of a high fraction of residuals inside the pre-chamber. Dual Mode, Turbulent Jet Ignition (DM-TJI) technology with its unique ability to work with high external EGR dilution (up to 40% wet/mass basis) due to its additional air delivery to the prechamber offers a viable alternative to the lean burn strategy. DM-TJI could be the technology pathway to realize high EGR diluted combustion with comparable dilution limits to those of the lean burn strategy while still enabling effective use of TWC technology. Present study compares the excess air versus EGR dilution strategy under identical level of dilution (up to 40 %) in a DM-TJI equipped single cylinder engine operating on a high (13.3: 1) compression ratio. The results show that compared to the lean burn operation, EGR dilution provides marginally lower but still comparable thermal efficiency benefits with a marked improvement in NOx reduction, especially in a high compression, knock limited situation. This study showcases that high EGR dilution rates comparable to lean burn operation can be maintained with the DM-TJI system to achieve high thermal efficiency while still operating at stoichiometric air-fuel ratio.&lt;/div&gt;&lt;/div&gt;

Streamers Variability Investigation of a Radio-Frequency Corona Discharge in an Optical Access Engine at Different Speeds and Loads

Engine research community interest in the Radio-Frequency corona-based ignition systems is currently gaining in importance mainly due to their capability to ensure robust combustion at challenging operating conditions such as very lean mixture and/or high EGR dilution. The benefits of Corona low-temperature plasma foster the early flame development thanks to combustion precursors production and to a more energetic and volumetric discharge, resulting in a larger amount of involved mixture. The corona discharge generates ionizing waves, named streamers, whose temporal and spatial variability in orientation, length and branching can affect the combustion onset and, therefore, the engine cycle-to-cycle variability. In this work, the discharge natural luminosity of a RF corona igniter, characterized by four tips electrodes, was recorded in an optically accessible engine via high-speed camera detection. A preliminary statistical analysis of the spatial and temporal streamer variability was performed by operating in motored conditions. Four different engine speeds and two different loads were explored in order to deeply investigate the streamer behaviour at diverse engine operating conditions. A comparison between a motored and a lean operating condition is also proposed to analyse, at a specific engine speed, the mixture influence on the streamers propagation before the start of the combustion.

The mitigation of mode-switching challenges of spark-assisted compression ignition engines through powertrain electrification

Spark-assisted compression ignition (SACI) is a novel combustion strategy that combines the fuel efficiency benefits of high EGR dilution, high compression ratio, and lean operation. Though the efficiency benefit is significant, SACI has several control and calibration challenges. To resolve this limitation, switching combustion modes between SACI and SI-only combustion is implemented. This work demonstrates the synergistic benefit of mild hybridisation on improving torque tracking compared to a non-hybrid SACI vehicle. A tip-in performed on a one-dimensional engine model shows a significantly reduced mode-switching challenge as the electric machine can adapt torque during mode changes, offsetting transport delays and actuator constraints. Furthermore, a vehicle model demonstrates that an electric machine can reduce the number of required mode transitions over four drive cycles from 15-40%. In addition to control benefits, the SACI powertrain also demonstrated 4-5% higher fuel economy compared to an SI-only powertrain in each vehicle configuration.

Experimental study of combustion strategy for jet ignition on a natural gas engine

To explore a suitable combustion strategy for natural gas engines using jet ignition, lean burn with air dilution, stoichiometric burn with EGR dilution and lean burn with EGR dilution were investigated in a single-cylinder natural gas engine, and the performances of two kinds of jet ignition technology, passive jet ignition (PJI) and active jet ignition (AJI), were compared. In the study of lean burn with air dilution strategy, the results showed that AJI could extend the lean limit of excess air ratio (λ) to 2.1, which was significantly higher than PJI’s 1.6. In addition, the highest indicated thermal efficiency (ITE) of AJI was shown 2% (in absolute value) more than that of PJI. Although a decrease of NOx emission was observed with increasing λ in the air dilution strategy, THC and CO emissions increased. Stoichiometric burn with EGR was proved to be less effective, which can only be applied in a limited operation range and had less flexibility. However, in contrast to the strategy of stoichiometric burn with EGR, the strategy of lean burn with EGR showed a much better applicability, and the highest ITE could achieve 45%, which was even higher than that of lean burn with air dilution. Compared with the most efficient points of lean burn with pure air dilution, the lean burn with EGR dilution could reduce 78% THC under IMEP = 1.2 MPa and 12% CO under IMEP = 0.4 MPa. From an overall view of the combustion and emission performances under both low and high loads, the optimum λ would be from 1.4 to 1.6 for the strategy of lean burn with EGR dilution.

Investigation on the EGR effect to further improve fuel economy and emissions effect of Miller cycle turbocharged engine

Miller cycle and EGR have been proved to be the effective ways to improve the engine performance. In order to realize Miller cycle with high compression ratio, the piston and intake cam profile were redesigned for a turbocharged GDI engine. In-cylinder pressure was taken to clarify the difference of working cycles between Miller and Otto cycle. At full load, Miller engine with high compression ratio leads to the knock at a low speed. But at a high speed, engine can operate nearer stoichiometric conditions and get better economy. At partial load, engine can adopt higher compression ratio and intake pressure. The reduction of pumping losses and exhaust gas energy have become the key factors to improve fuel economy. On the basis, LP cooled EGR was introduced to study the further effect on Miller cycle. In-cylinder temperature is reduced due to the EGR dilution and heat capacity effect. Thus, engine can adopt larger ignition advance angle to obtain better combustion phase. For emission analysis, lower temperature can effectively reduce NOx emissions. However, because of the extended combustion duration and complex piston shape, insufficient combustion will result in the increase of THC. The constant excess-air coefficient makes the CO emissions almost unchanged.

Identification of factors affecting exergy destruction and engine efficiency of various classes of fuel

The fuels of internal combustion (IC) engines are gradually becoming diverse. However, differences in physicochemical properties among fuels result in different efficiency potentials. In this work, the factors affecting exergy destruction and engine efficiency of various classes of fuel were identified based on the first- and second-law of thermodynamics. The results demonstrate that exergy destruction fraction (fEx_D) shows strongly positive correlation to the ratio of fuel chemical exergy (LExV) to lower heating value (LHV) (ε) and dimensionless entropy change (αS), and the first-law efficiency has strongly positive correlation to the product ratio of specific-heat during combustion (γc). Specifically, small molecule fuels have lower first-law efficiency than large ones, while the second-law efficiency of hydrocarbons are higher than that of oxygenates due to their lower exergy destruction losses. However, the lower exergy destruction does not always lead to improved second-law efficiency, but increased exhaust exergy. With increasing air or EGR dilution, both the first- and second-law efficiency increase, but the increased exergy destruction results in the reduced exhaust exergy. As a result, it is more important and reasonable to design or reform fuel for increasing the thermal efficiency, rather than reducing the exergy destruction of IC engines.

CFD modelling of natural gas combustion in IC engines under different EGR dilution and H2-doping conditions

The present paper provides a contribution to the CFD modelling of reacting flows in IC engines fueled with natural gas. Despite the fact that natural gas has been widely investigated into in the last decades, the literature still lacks reliable models and correlations to be exploited so as to efficiently support the design of internal combustion engines. The paper deals with the development of an accurate CFD model, capable of capturing the effects of the engine working conditions and mixture compositions on the combustion process. The CFD model is based on the Extended Coherent Flame Model combustion model coupled to a laminar flame speed one through a user subroutine, which replaces the commonly adopted empirical correlations. The flame speed values have been derived from the application of a reaction mechanism for natural gas-air-residual gases mixtures.In the second part of the paper, the model is validated and applied to the investigation of the dependence of the combustion quality on the fuel doping with hydrogen as well as on the mixture dilution with EGR. As a matter of fact, the attractiveness of the mixture dilution with EGR relies on the potential in containing engine-out NOx emissions as well as in reducing the pumping losses, thus further abating fuel consumption at part loads. Finally, the effects of fuel blending with H2 on the EGR tolerance is discussed in the paper.

Experimental Study of EGR Dilution and O2 Enrichment Effects on Turbulent Non-Premixed Swirling Flames

ABSTRACT This paper presents experimental results of the effects of CO2 and H2O dilution on CH4-air flames enriched by oxygen. The study focuses on pollutant emissions, flame stability through the determination of liftoff heights and flow fields by LDV measurements. Different parameters of the burner are studied such as the swirl number, the global equivalence ratio, and the fractions of O2, H2O, and CO2 in the mixture. The fraction of diluents varies from 0% to 20%, O2 enrichment from 21% to 30% (in vol.), and the swirl number from 0.8 to 1.4. The experiments are carried out on a coaxial swirled burner placed in a vertical combustion chamber. The flame is visualized by the chemiluminescence technique on OH* in order to locate the flame front. Measurements of combustion products such as NOx, CO, and CO2 are done using a HORIBA PG250 multi-gas analyzer. Flow fields are measured by LDV measurement for the longitudinal velocity component and its fluctuations. Results show that the CO2 and H2O dilution influences significantly the flame characteristics. With dilution the liftoff height increases but the flame remains stabilized. With O2 enrichment the liftoff height decreases and flame stability is enhanced. If we therefore want to dilute more, we should enrich with oxygen. The increase of dilution rate induces a decrease in NOx emission and exhaust gas temperature and an increase in CO emissions. LDV measurements showed the longitudinal velocity distribution of flow and its fluctuations in reactive and non-reactive conditions. Note that the presence of the flame induces an increase in the longitudinal velocity downstream of the flow due to the expansion of gases.

The Effect of EGR Dilution on the Heat Release Rates in Boosted Spark-Assisted Compression Ignition (SACI) Engines

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;This paper presents an experimental investigation of the impact of EGR dilution on the tradeoff between flame and end-gas autoignition heat release in a Spark-Assisted Compression Ignition (SACI) combustion engine. The mixture was maintained stoichiometric and fuel-to-charge equivalence ratio (&lt;i&gt;ϕ&lt;/i&gt;&lt;sup&gt;′&lt;/sup&gt;) was controlled by varying the EGR dilution level at constant engine speed. Under all conditions investigated, end-gas autoignition timing was maintained constant by modulating the mixture temperature and spark timing. Experiments at constant intake pressure and constant spark timing showed that as &lt;i&gt;ϕ&lt;/i&gt;&lt;sup&gt;′&lt;/sup&gt; is increased, lower mixture temperatures are required to match end-gas autoignition timing. Higher &lt;i&gt;ϕ&lt;/i&gt;&lt;sup&gt;′&lt;/sup&gt; mixtures exhibited faster initial flame burn rates, which were attributed to the higher laminar flame speeds immediately after spark timing and their effect on the overall turbulent burning velocity. The increasing trends in peak heat release rate and peak autoignition rate as a function of &lt;i&gt;ϕ&lt;/i&gt;&lt;sup&gt;′&lt;/sup&gt; were consistent for all intake pressures ranging from 80 kPa to 150 kPa. For a constant spark timing, the mass fraction burned at the onset of autoignition correlated well with &lt;i&gt;ϕ&lt;/i&gt;&lt;sup&gt;′&lt;/sup&gt;, regardless of intake pressure. Reducing the unburned mass fraction at the onset of autoignition was effective at limiting the peak heat release rates. This behavior was attributed to the reduced energy content of the end-gas and also to the reduced end-gas burn rates. Possible explanations for the latter observation were the greater buffer gas effect due to a larger burned gas mass, and the higher reactivity stratification of the end-gas being closer to the walls.&lt;/div&gt;&lt;/div&gt;