Exhaust Gas Recirculation Levels Research Articles

A CFD ignition model to predict average-cycle combustion in SI engines with extreme EGR levels

Control of the combustion process in Spark-Ignition (SI) engines operated with extreme dilution from exhaust gas re-circulation (EGR) represents one of the major limitations in the industrial design of such technology. Numerical approaches able to describe in detail the formation of the early flame kernel become essential to face such an ambitious task. This work presents a RANS-based multi-dimensional model of the combustion process, including an advanced description of the ignition stage to consider its stochastic re-ignitions within the average cycle prediction. The spark-channel is described as a column of Lagrangian parcels that represent early flame kernels, whose growth is controlled by the laminar flame speed and energy input from the electrical circuit. The spatial evolution of each parcel is computed according to a scaled value of the average-flow speed, to mimic the smooth but short elongation of the mean-cycle channel produced by stochastic restrikes affecting the single-cycle arcs. To clarify this phenomenon and assess the proposed CFD method, a series of experiments are performed in a single cylinder SI engine with optical access, running at a low-load cruise-speed operating condition. Increasing EGR levels are tested up to the onset of misfire, with measurements of the secondary-circuit features and of the flame evolution through high-speed imaging. Satisfactory results are achieved in terms of numerical-experimental comparison of the cycle-averaged in-cylinder pressure, discharge parameters, and spatial flame distribution, demonstrating the reliability of the proposed numerical approach.

Impact of engine control variables on low load combustion efficiency and exhaust emissions of a methane-diesel dual fuel engine

A conventional diesel engine when operated in the reactivity-controlled compression ignition (RCCI) combustion strategy faces challenges of high total hydrocarbon (THC) and carbon monoxide (CO) emissions leading to poor combustion efficiency at low engine loads. The oxides of nitrogen (NOx) versus smoke trade-off, encountered in conventional diesel combustion, is replaced by the NOx-THC trade-off in the RCCI operation. This work focuses on addressing the NOx-THC trade-off issue by systematically investigating the effects of engine control variables, such as, fuel injection timing, exhaust gas recirculation (EGR) and intake throttling, on a light duty compression ignition engine running in a methane-diesel dual fuel mode. The engine is operated at a load of 3 bar gross indicated mean effective pressure and at a speed of 1500 rev/min. Based on relationships identified between engine control variables, combustion parameters, emissions and engine performance, a bottom-up approach is used to combine the control variables synergistically to improve the NOx-THC trade-off. A combination of advanced start of injection timing of diesel (−35 degree crank angle (°CA) after top dead centre), 50% premix ratio and 55% EGR levels along with the end of port fuel injection of methane in the middle of the intake stroke (−270°CA), has resulted in a ∼34 percentage points (from 56% to 90%) improvement in combustion efficiency and a ∼9.5 percentage points improvement in thermal efficiency compared to the baseline low load dual fuel operation while maintaining good combustion stability. THC emission is reduced from 105 to ∼25 g/kWh whilst maintaining low levels of NOx (<0.3 g/kWh).

Experimental evaluation of spark behavior under diluted conditions in an optically accessible engine

An optically accessible single-cylinder spark-ignition engine operated under homogeneous, part-load conditions is experimentally investigated using optical and spark diagnostics to evaluate the relationship between the spark, flow, and flame with increasing dilution using several levels of exhaust gas recirculation (EGR). Voltage and current measurements of the secondary spark circuit are compared with simultaneous high-speed spark plasma imaging, particle image velocimetry measurements of the flow field, and burned gas images. Specifically, characteristic restrike cycles and normal cycles are examined under the 0 % EGR and 12.9 % EGR conditions to reveal a relationship between the magnitude and direction of the velocity near the spark plug and the spark’s behavior coupled with that of the subsequent flame propagation. Through the use of conditional statistics and correlation analysis of data sets of all non-restrike and all restrike cycles, the horizontal velocity across the spark gap was identified as a critical quantity in facilitating more stable and faster combustion under diluted mixture conditions.

Analyzing the effects of cooled EGR on the knock of hydrogen-fueled Wankel rotary engine

Hydrogen-fuel Wankel rotary engine (HWRE) has higher power density than current marketed gasoline-fueled engines, however, it suffers from the knock. Hence, the purpose of this work is to investigate the impacts of cooled exhaust gas recirculation (EGR) on the knock in a HWRE by analyzing the relationship between some knock-relevant parameters and EGR level, which is conducted at 1500 r/min, wide-open throttle, stoichiometric ratio, and respectively constant ignition timing and CA50. The results show that cooled EGR can be used as an effective means of eliminating the knock in HWRE. As the EGR level is increased, the size and distribution of knock-relevant parameters, such as knock intensity, knock duration, crank angle of peak knock pressure and so on, show their own change characteristics. Besides, the role of the EGR level on the knock-relevant parameters is also influenced by the ignition timing and CA50. Overall, cooled EGR has a positive effect on the suppression of knock.

The Influence of Flow on Cycle-to-Cycle Variations in a Spark-Ignition Engine: A Parametric Investigation of Increasing Exhaust Gas Recirculation Levels

Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex.

An experimental study on biodiesel production and impact of EGR in a CRDI diesel engine propelled with leather industry waste fat biodiesel

The purpose of the predominant endeavor is the way of through an experiment aspect for the reticular effect of exhaust gas recirculation (EGR) on combustion, performance, emissions besides energy characteristic of common rail direct injection (CRDI) engine fuelled with leather industry waste fat biodiesel and diesel blends. Biodiesel was converted by the transesterification process from the raw fat oil extracted with leather waste fat feedstock. This work takes advantage of blends of both like B10, B20, B30 along with engines that were run with various EGR rates (5%, 10%, and 15%) to evaluate engine outcome parameters. The maximum in-cylinder pressure was examined with the blend B20 with EGR 5% rate about 6.65% higher than pure diesel fuel. The high heat release rate (HRR) was ascertained with the blend B30 with EGR 10% rate about 23.81% higher than neat diesel fuel. The brake thermal efficiency (BTE) for the blend B10 with EGR 5% rate is higher than other test conditions but 4.2% let down associated with simple diesel fuel. The nitrogen oxides (NOx) were lesser than the clean diesel fuel at all test fuels, the highest NOx emission reduction was achieved in blend B30 with EGR 15% rate of about 87.59% than neat diesel fuel operation. The smoke opacity increased by raising the EGR level, the combination B30 with EGR 5% rate displayed low smoke opacity about 64.5% lower than neat diesel fuel. The hydrocarbon (HC) emission was lower than neat diesel at a low EGR rate, the blend B10 with EGR 5% rate displayed the lowest HC emission about 85.71% decreased then neat diesel operation. The carbon monoxide (CO) emission displayed lower than neat diesel at low EGR rate, the blend B10 with EGR 5% rate displayed the lowest CO emission about 50% than neat diesel. As of energy analysis, it was noted that the heat equivalent to brake power and heat lost to engine cooling water is higher in diesel fuel than biodiesel blends and the heat lost to exhaust gases and heat unaccounted loss were lower in diesel than biodiesel blend. Finally, the TOPSIS optimization study was carried out for finding the optimum input conditions which could result in improved combustion, performance, and reduced emissions. The B30% blend with EGR 15% at full load achieved 1st rank with 0.849 relative closeness value. So, this operating condition was projected as the best prospect for the application of a CRDI diesel engine.

Effects of pre-injection on ignition, combustion and emissions of spray under engine-like conditions

Large-eddy simulation (LES) of multiple-injection spray combustion under engine-like conditions is performed using a coupled model of an Eulerian stochastic field transported probability density function (ESF T-PDF) and a flamelet generated manifold (FGM). This coupled model allows the use of the T-PDF methods in modeling the interaction of turbulence and chemistry at affordable computational costs for engine applications. Simulation results are compared with the available experimental data for spray flames with multiple-injection and at a high level of exhaust gas recirculation (EGR) under conditions relevant to internal combustion engines. It is shown that the T-PDF/FGM LES model is capable of reproducing not only global combustion characteristics, such as the pressure rise and ignition delay time but also replicating the evolution of liftoff length and the spray flame structure.The effects of pre-injection strategies are then investigated, by systematically varying the pre-injection duration and the dwell time between two injections, while keeping the total injected fuel mass constant. The LES results reveal different mechanisms by which a pre-injection can change the ignition delay time, the combustion mode and the emissions in spray flames, depending on the injection timing. It is shown that even an extremely short non-igniting pre-injection can substantially change the ignition and emissions characteristics of the main-injection. It is shown that the combustion mode of a single-injection can be altered by splitting the injection to pre- and main-injections. The current study also demonstrates that decreasing the dwell time, within the range that is examined here, at a given pre-injection, will potentially increase the soot oxidation rates while it does not significantly change the rate of soot formation.

A study on effect of engine operating parameters on NOx emissions and exhaust temperatures of a heavy-duty diesel engine during idling

Though NOx emissions from on-road heavy duty vehicles (HDVs) have reduced drastically over the past three decades, upcoming North American regulations may call for further reductions. Lowering NOx emissions below current levels is challenging, especially at lower operating loads such as idling and/or colder climates when the exhaust gas temperature is insufficient for high NOx conversion in the HDV’s exhaust after treatment (EAT) system. This paper investigates the impact of various engine operating parameters on the three-way trade-off between NOx emissions, exhaust temperature, and fuel consumption using a single-cylinder, heavy-duty diesel engine outfitted with a conventional EAT system and diesel exhaust fluid (DEF) injection. The engine parameters investigated include exhaust gas recirculation (EGR) ratio, intake pressure, intake temperature, injection pressure, multiple injections, and engine fluid temperatures. In order to simulate cold climate operation, the baseline intake air, coolant, and lubricating oil temperatures are maintained at 10°C, 40°C, and 50°C, respectively. Results show that using a moderate level of EGR has the dual benefit of reducing the engine-out NOx and increasing the intake charge temperature. Increasing the intake or injection pressure does not result in significant benefit in improving the three-way trade-off. However, using double injection with the second injection at the start of the expansion stroke can help reduce NOx emissions and increase exhaust temperature with marginal impact on fuel efficiency. Based on the parametric study, three particular engine operating conditions are chosen for further investigation with the EAT online. At an identical DEF dosing rate, the rates of NOx reduction in the SCR are similar for the three conditions (∼96%), and the lowest tail-pipe NOx observed for this study is 0.032 g/kWh.

Experimental investigation on combustion and particulate emissions of the high compressed natural gas reactivity controlled compression ignition over wide ranges of intake conditions in a multi-cylinder engine using a two-stage intake boost system

Compressed natural gas (CNG)/diesel reactivity-controlled compression ignition was investigated to understand the combustion and emission characteristics over wide ranges of intake pressure and exhaust gas recirculation (EGR) rate at high load condition. Reactivity-controlled compression ignition is a dual fuel engine combustion strategy that employs in-cylinder fuel blending with two different reactivity fuels, and multiple injections for the in-cylinder fuel reactivity control to optimize the combustion phasing, duration, and magnitude as well as NOx and soot reduction. High CNG substitution was of interest since it could lead to greater CO2 and particulate matter reductions. The present 80% CNG substitution required substantial intake boosting due to the charge air displaced by CNG. The experimental engine is commissioned for the reactivity-controlled compression ignition regime using natural gas and diesel fuel with a two-stage turbocharging system. The intake boosting system achieved the intake pressure and EGR rate of 210 kPa and 25%, respectively, when the EGR valve was fully open. The intake pressure and EGR levels varied in the ranges of 150 to 210 kPa and 0 to 25%, respectively. The corresponding equivalence ratio ranged from 0.85 to 0.5. The intake system achieved the highest turbocharger efficiency of 56% at the maximum boosting operation. The highest CO2 reduction was 23.4% at the maximum boosting condition. The improvements under the enhanced intake condition were significant from the standpoints of both efficiency and emissions, including in terms of the particle concentrations.

Cycle-to-cycle variability in spark-assisted compression ignition engines near optimal mean combustion phasing

Stoichiometric spark-assisted compression ignition (SACI) combustion with exhaust gas recirculation (EGR) dilution has demonstrated higher part-load thermal efficiencies compared to spark-ignited engines, while maintaining ultra-low tailpipe emissions. However, SACI is often characterized by high cyclic variability in heat release or torque output, which poses a challenge to its implementation in light-duty vehicles. This paper presents an experimental investigation of cyclic variability in stoichiometric SACI combustion under EGR-dilute conditions, while maintaining mean combustion phasing (θ50) near optimal timing for thermal efficiency. The present work focuses on SACI conditions where the flame-based heat release fraction is between approximately 10% and 40% of the overall heat release, and therefore contributes significantly to the combustion process. For the SACI conditions examined, the variability in θ50was driven by variability in autoignition timing, which in turn correlated with the start of measurable heat release (θ02). High variability in θ50caused unstable work output due to very late combustion with poor or no end-gas autoignition. The use of a high ignition energy dual-coil offset ignition system had negligible impact in reducing θ50variability. Analysis of experimental data from close to 1000 operating conditions showed that the magnitude of θ50variability correlates with the flame-based heat release fraction ([Formula: see text]), for a large range of intake pressures, spark timings and exhaust gas recirculation levels. For the SACI conditions examined, combustion phasing variability was largely determined by flame-based combustion and particularly the initial flame formation (θIGN-02), and minimally by the end-gas autoignition heat release. The analysis also demonstrated that θ50variability is amplified as the contribution of the flame to the overall heat release increases.

Assessment of air-management strategies to improve the transient performance of a gasoline engine under high EGR conditions during load-decrease operation

Advanced gasoline engines in hybrid powertrains are currently regarded as the most widespread solution for passenger cars in the upcoming years, due to the strong commitment of many countries to reduce greenhouse gas emissions and the limitations of pure electric vehicles at present. At the same time, it is widely proven that exhaust gas recirculation (EGR) through the low pressure (LP) configuration allows reducing fuel consumption and, consequently, CO2 emissions in gasoline engines. However, it is challenging to extract the full EGR potential, since it means operating close to the EGR tolerance, and this can compromise the engine transient response because of the long transport delays associated with LP EGR systems. Hence the paper presents an analysis on the capability of three strategies to accelerate the transient response of a gasoline engine operating with high LP EGR levels during tip-out maneuvers, while guaranteeing the combustion stability. Such strategies are the optimization of valve synchronization (based on applying different delays between the throttle actuation and pedal demand), the integration of a secondary air-path which bypasses the compressor, and the use of a pressurized air tank directly connected to the intake manifold. Both experimental and modeling data were utilized to assess the advantages and drawbacks of these strategies: engine tests for the validation of 1D model transient performance and the evaluation of combustion stability and torque evolution, while the fluid-dynamics and transport phenomena were analyzed by simulation.

Tailored Air-Handling System Development for Gasoline Compression Ignition in a Heavy-Duty Diesel Engine

The simultaneous application of new low-NOx emissions standards and greenhouse gas (GHG) rules has placed great pressure on the commercial vehicle industry and has driven demand for innovative solutions. One potential solution, gasoline compression ignition (GCI), utilizes gasoline’s lower reactivity to promote partially premixed combustion and extract efficiency while reducing the PM-NOx trade-off curve. Gasoline’s volatility allows for the use of higher levels of exhaust gas recirculation (EGR), a key enabler of GCI combustion. In order to deliver higher levels of EGR while maintaining sufficient boost pressure, a tailored and efficient air-handling system is critical. This work presents the analysis-led development of a low-NOx GCI air-handling system including both turbocharger matching and EGR configuration for a prototype heavy-duty GCI engine based on a model year 2013 Cummins ISX diesel engine using low octane gasoline (RON80). In the analysis-driven development process, a 1D engine system-level analysis was closely coupled with closed-cycle 3D CFD GCI combustion development. Three different boost systems were investigated using a validated 1D engine model: 1) the production turbocharger; 2) an off-the-shelf single-stage waste-gate turbocharger; 3) a prototype single-stage variable geometry turbocharger. For each boost system, three EGR configurations were evaluated: 1) a high-pressure EGR route; 2) a low-pressure EGR route; 3) a dual-loop EGR route. The air-handling system performance was first investigated over five steady-state engine operating conditions extracted from the ramped modal cycle supplemental emissions test. Then, through cosimulation using a Simulink-based engine controls model, the best performing candidates under transient operation through the Heavy-Duty Federal Test Procedure certification cycle were identified. The production turbocharger, designed for 4–6 g/kWh engine-out NOx, suffered from low combined turbocharger efficiency under the low-NOx GCI thermal boundary conditions. The prototype 1-Stage variable geometry turbocharger, when used with a high-pressure EGR configuration, demonstrated higher combined efficiencies, while the waste-gate turbocharger showed the best results when used with a dual-loop EGR system. All low-pressure only EGR configurations were found to incur additional pumping penalties due to the need for a back pressure valve to drive sufficient EGR levels. In the transient test cycle analysis, the single-stage high-pressure EGR system was capable of delivering the target boost and EGR, while the off-the-shelf waste-gate turbocharger, with its higher mass inertia, showed slower turbine response and a resulting lag in boost response. Unsurprisingly, the dual-loop EGR system also suffered from delays in EGR delivery during engine acceleration. In summary, the prototype single-stage variable geometry turbocharger with a high-pressure EGR system produced the best performance over both the steady-state and transient engine cycles and was identified as the best candidate for the prototype low-NOx heavy-duty GCI engine.

Next-Cycle Optimal Fuel Control for Cycle-to-Cycle Variability Reduction in EGR-Diluted Combustion

Dilute combustion using exhaust gas recirculation (EGR) is a cost-effective method for increasing engine efficiency. At high EGR levels, however, its efficiency benefits diminish as cycle-to-cycle variability (CCV) intensifies. In this simulation study, cycle-to-cycle fuel control was used to reduce CCV by injecting additional fuel in operating conditions with sporadic misfires and partial burns. An optimal control policy was proposed that utilizes 1) a physics-based model that tracks in-cylinder gas composition and 2) a one-step-ahead prediction of the combustion efficiency based on a kernel density estimator. The optimal solution, however, presents a tradeoff between the reduction in combustion CCV and the increase in fuel injection quantity required to stabilize the charge. Such a tradeoff can be adjusted by a single parameter embedded in the cost function. Simulation results indicated that combustion CCV can be reduced by as much as 65% by using at most 1% additional fuel. Although the control design presented here does not include fuel trim to maintain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda = 1$ </tex-math></inline-formula> for three-way catalyst compatibility, it is envisioned that this approach would be implemented alongside such an external controller, and the theoretical contribution presented here provides a first insight into the feasibility of CCV control using fuel injection.

Experimental and Simulation Analysis of Natural Gas-Diesel Combustion in Dual-Fuel Engines

The recent strict emission regulations and the necessity to reduce the global CO2 levels have promoted research studies over alternative fuel combustion technologies. Dual fuel technology is a fascinating approach as it could combine the benefits of two fuels to achieve high thermal efficiency with lower harmful emissions. Natural gas burns cleaner than other fossil fuels and its recent low price makes it an attractive fuel as a near-term solution. The utilization of natural gas with diesel in a compression ignition engine can result in high engine performance with lower CO2 emissions. However, at low engine loads, natural gas-diesel operation suffers from low combustion efficiency with high unburned hydrocarbon and carbon monoxide emissions. The present paper collates the work of experimental and simulation studies on natural gas-diesel dual fuel operation over three different engine set-ups. Two single-cylinder, with and without optical access, and a multi-cylinder engine were used to analyze the effects of varying diesel injection and air-path strategies on the combustion performance, with a focus on the low load operation zone. The results showed that the injection timing, as well as intake charge dilution, can severely promote clean combustion at the low load zone. An early diesel injection can enhance the diesel-natural gas homogeneity within the cylinder and improve the combustion efficiency of the engine. On the other hand, high levels of exhaust gas recirculation (EGR) are mandatory to suppress NOx and further enhance the brake thermal efficiency of the engine.

Ability of Particulate Matter Index to describe sooting tendency of various gasoline formulations in a stratified-charge spark-ignition engine

This study investigates the ability of Particulate Matter Index (PMI) to describe the sooting behavior of various gasoline formulations in a stratified-charge (SC) spark-ignition engine. The engine was operated at 2000 rpm with an intake pressure of 130 kPa where soot formation is known to primarily occur in the bulk gases. Exhaust soot emissions were measured for nine test fuels at various exhaust gas recirculation levels. A comparison between measured soot levels and PMI shows that PMI is a relatively poor predictor of the sooting tendency of the tested fuels under lean SC combustion. Among the fuels, three fuels, namely the di-isobutylene blend, High Olefin, and E30 fuels exhibit measured soot behavior opposite of that predicted by PMI. Optical diagnostics were utilized to further investigate the in-cylinder phenomena for these three fuels. Analysis of natural luminosity and diffused back-illumination extinction imaging suggests that fuel-induced differences in the amount of soot formed are responsible for a majority of the discrepancy in measured versus predicted sooting tendency. Fuel-induced differences in soot oxidation and spray development seem to play minor roles. Because the combustion and air-fuel mixing processes for lean SC combustion are different from conventional stoichiometric operation it is hypothesized that the PMI correlation needs to be modified to account for differences in stoichiometric air-fuel ratio and level of oxygenation between fuels. Furthermore, the role of fuel volatility in PMI possibly needs to be de-emphasized for SC operation with fuel injection into compression-heated gases.

Experimental investigation on Jatropha oil Methyl Ester fuelled CI engine using high EGR

The present experiment is carried out to study the effect excess cooled exhaust gas recirculation (EGR) on four-stroke, single-cylinder, air-cooled direct injection (DI) diesel engine. The biodiesel used is 100% JOME (Jatropha Oil Methyl Ester). Here jatropha oil methyl ester fuel is transesterified to reduce the viscosity as per ASTM standard. The experiment is conducted by different BMEP with a constant speed of 1500 rpm. An Extensive level of EGR (up to 75%) was used to achieve lower emissions. Experimental results were showed a reduction in NOx higher rate compares to another emission. Exhaust gas Emissions of hydrocarbons, Nitrous oxide, carbon monoxide was measured using gas analyzer. Thermocouple mounted on tailpipe is used to observed the exhaust gas temperature. The calculated thermal efficiency and exhaust emissions are compared with diesel. Reduction in NOx was 62.5% at 75% EGR at 50% load was observed but hydrocarbon, carbon monoxide emissions are increased with rising of EGR percentage.

Investigating the influence of oxygenated fuel on particulate size distribution and NOX control in a common-rail diesel engine at rated EGR levels

The effect of butanol-diesel blend (B20) and different rates of exhaust gas recirculation (EGR) on particulate matter (PM) characterisation and NOX emissions were investigated in this experimental work. The different rates of EGR (0%, 15% and 30%) and oxygenated fuel (B20) were used in this study. The brake thermal efficiency (BTE) of the engine increased under without used EGR technique compared to the presence of EGR during combustion process for B20 and diesel fuel. Further, higher BTE produced during the combustion of B20 than that produced from combustion of diesel for different rates of EGR. The BSFC slightly increased with B20 combustion compared to the diesel combustion. The cylinder pressure and ROHR improved with combustion of B20 under without using EFR. The results showed that PM reduced from the combustion of B20 more than to the diesel fuel for different rates of EGR. Besides, the average of particle diameter reduced from B20 combustion by 20, 22 and 25 nm compared to the diesel by 31, 37 and 43 nm for 0%, 15% and 30% of EGR, respectively. The soot particles generated from B20 combustion are lower than to the diesel fuel combustion for different rates of EGR. Lower level of NOX emissions can be achieved with using EGR (15% and 30%) than to the without EGR technique, this effect is more clearly with oxygenated fuel. It is indicated that the EGR dramatically increase the THC and CO emissions when exceeds the 30% with no obvious influence on NOX emissions.

The impact of intake pressure on high exhaust gas recirculation low-temperature compression ignition engine combustion using borescopic imaging

In diesel engines, high levels of exhaust gas recirculation can be used to achieve low-temperature combustion, resulting in low emission levels of both nitrogen oxides (NO x) and particulate matter. This work studied the effects of varying the intake manifold pressure on in-cylinder combustion processes and engine-out emissions from a light-duty single cylinder diesel engine under conventional and high exhaust gas recirculation low-temperature combustion regimes. The work was conducted at a part-load cruise condition of 1500 r/min and at an indicated mean effective pressure of approximately 600 kPa. Exhaust gas recirculation rates were varied between 0% and 65% at absolute intake pressures of 100–150 kPa. Very low NO x emissions were achieved (&lt;10 ppm, ∼0.05 g/kW h) for intake oxygen mass fractions below about 11%, independent of boost pressure. Smoke emission levels were lower than for non–exhaust gas recirculation combustion at oxygen mass fractions below ∼9%, depending on the boost pressure. High intake pressures reduced fuel consumption by 15% and combustion by-product emissions by 50%–60% compared to low boost. For the low intake boost case, little visible flame was apparent through borescope imaging. At higher boost pressures, intense flame luminosity was observed within the piston bowl early in the expansion stroke. Spatially averaged soot luminosity based on photomultiplier tube data showed that peak soot luminosity was five times greater and occurred 8 °CA earlier for high boost. This work demonstrates how the combination of appropriate boost pressures and exhaust gas recirculation rates can be used to mitigate the emissions and thermal efficiency penalties of high-dilution low-temperature combustion to achieve near-zero NO x operation.

Learning reference governor for cycle-to-cycle combustion control with misfire avoidance in spark-ignition engines at high exhaust gas recirculation–diluted conditions

Cycle-to-cycle feedback control is employed to achieve optimal combustion phasing while maintaining high levels of exhaust gas recirculation by adjusting the spark advance and the exhaust gas recirculation valve position. The control development is based on a control-oriented model that captures the effects of throttle position, exhaust gas recirculation valve position, and spark timing on the combustion phasing. Under the assumption that in-cylinder pressure information is available, an adaptive extended Kalman filter approach is used to estimate the exhaust gas recirculation rate into the intake manifold based on combustion phasing measurements. The estimation algorithm is adaptive since the cycle-to-cycle combustion variability (output covariance) is not known a priori and changes with operating conditions. A linear quadratic regulator controller is designed to maintain optimal combustion phasing while maximizing exhaust gas recirculation levels during load transients coming from throttle tip-in and tip-out commands from the driver. During throttle tip-outs, however, a combination of a high exhaust gas recirculation rate and an overly advanced spark, product of the dynamic response of the system, generates a sequence of misfire events. In this work, an explicit reference governor is used as an add-on scheme to the closed-loop system in order to avoid the violation of the misfire limit. The reference governor is enhanced with model-free learning which enables it to avoid misfires after a learning phase. Experimental results are reported which illustrate the potential of the proposed control strategy for achieving an optimal combustion process during highly diluted conditions for improving fuel efficiency.

Effect of Renewable Fuels and Intake O2 Concentration on Diesel Engine Emission Characteristics and Reactive Oxygen Species (ROS) Formation

Renewable diesel fuels have the potential to reduce net CO2 emissions, and simultaneously decrease particulate matter (PM) emissions. This study characterized engine-out PM emissions and PM-induced reactive oxygen species (ROS) formation potential. Emissions from a modern heavy-duty diesel engine without external aftertreatment devices, and fueled with petroleum diesel, hydrotreated vegetable oil (HVO) or rapeseed methyl ester (RME) biodiesel were studied. Exhaust gas recirculation (EGR) allowed us to probe the effect of air intake O2 concentration, and thereby combustion temperature, on emissions and ROS formation potential. An increasing level of EGR (decreasing O2 concentration) resulted in a general increase of equivalent black carbon (eBC) emissions and decrease of NOx emissions. At a medium level of EGR (13% intake O2), eBC emissions were reduced for HVO and RME by 30 and 54% respectively compared to petroleum diesel. In general, substantially lower emissions of polycyclic aromatic hydrocarbons (PAHs), including nitro and oxy-PAHs, were observed for RME compared to both HVO and diesel. At low-temperature combustion (LTC, O2 &lt; 10%), CO and hydrocarbon gas emissions increased and an increased fraction of refractory organic carbon and PAHs were found in the particle phase. These altered soot properties have implications for the design of aftertreatment systems and diesel PM measurements with optical techniques. The ROS formation potential per mass of particles increased with increasing engine O2 concentration intake. We hypothesize that this is because soot surface properties evolve with the combustion temperature and become more active as the soot matures into refractory BC, and secondly as the soot surface becomes altered by surface oxidation. At 13% intake O2, the ROS-producing ability was high and of similar magnitude per mass for all fuels. When normalizing by energy output, the lowered emissions for the renewable fuels led to a reduced ROS formation potential.