High Exhaust Gas Recirculation Rates Research Articles

Evaluation of the residual gas tolerance of homogeneous combustion processes with high exhaust-gas recirculation rates

The development of concepts with low emissions and fuel consumption for gasoline engines requires an early knowledge of the combustion process’ residual gas tolerance. At the Institute for Internal Combustion Engines and Automotive Engineering at Vienna University of Technology a method was developed that allows the dimensioning of this significant parameter based on CFD simulation. For that reason two engines were investigated experimentally at the engine test bench as well as with CFD simulation methods to create an increased charge motion.

Hydrogen-Enhanced Gasoline Stratified Combustion in SI-DI Engines

Experimental investigations were carried out to assess the use of hydrogen in a gasoline direct injection (GDI) engine. Injection of small amounts of hydrogen (up to 27% on energy basis) in the intake port creates a reactive homogeneous background for the direct injection of gasoline in the cylinder. In this way, it is possible to operate the engine with high exhaust gas recirculation (EGR) rates and, in certain conditions, to delay the ignition timing as compared to standard GDI operation, in order to reduce NOx and HC emissions to very low levels and possibly soot emissions. The results confirmed that high EGR rates can be achieved and NOx and HC emissions reduced, showed significant advantage in terms of combustion efficiency and gave unexpected results relative to the delaying of ignition, which only partly confirmed the expected behavior. A realistic application would make use of hydrogen-containing reformer gas produced on board the vehicle, but safety restrictions did not allow using carbon monoxide in the test facility. Thus, pure hydrogen was used for a best-case investigation. The expected difference in the use of the two gases is briefly discussed.

Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine

Abstract Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NOx production and is used on most modern high-speed direct injection (HSDI) diesel engines. However EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease of peak heat release, decrease in O2 concentration (and thus of global air/fuel ratio (AFR)) and flame temperature, increase of lift-off length, etc.), and thus the influence of EGR on NOx and particulate matter (PM) emissions is not perfectly understood, especially under high EGR rates. An experimental study has been conducted on a 2.0 l HSDI automotive diesel engine under low-load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NOx/PM emissions. The increase of inlet temperature with EGR has contrary effects on combustion and emissions, thus sometimes giving opposite tendencies as traditionally observed, as, for example, the reduction of NOx emissions with increased inlet temperature. For a purely diffusion combustion the ROHR is unchanged when the AFR is maintained when changing in-cylinder ambient gas properties (temperature or EGR rate). At low-load conditions, use of high EGR rates at constant boost pressure is a way to drastically reduce NOx and PM emissions but with an increase of brake-specific fuel consumption (BSFC) and other emissions (CO and hydrocarbon), whereas EGR at constant AFR may drastically reduce NOx emissions without important penalty on BSFC and soot emissions but is limited by the turbocharging system.

Effect of exhaust gas recirculation (EGR) temperature for various EGR rates on heavy duty DI diesel engine performance and emissions

DI diesel engines are well established today as the main powertrain solution for trucks and other relevant heavy duty vehicles. At the same time emission legislation (mainly for NOx and particulate matter) becomes stricter, reducing their limit to extremely low values. One efficient method to control NOx in order to achieve future emissions limits is the use of rather high exhaust gas recirculation (EGR) rates accompanied by increased boost pressure to avoid the negative impact on soot emissions. The method is based on the reduction of gas temperature level and O2 availability inside the combustion chamber, but unfortunately it has usually an adverse effect on soot emissions and brake specific fuel consumption (bsfc). The use of high EGR rates creates the need for EGR gas cooling in order to minimize its negative impact on soot emissions especially at high engine load were the EGR flow rate and exhaust temperature are high. For this reason in the present paper it is examined, using a multi-zone combustion model, the effect of cooled EGR gas temperature level for various EGR percentages on performance and emissions of a turbocharged DI heavy duty diesel engine operating at full load. Results reveal that the decrease of EGR gas temperature has a positive effect on bsfc, soot (lower values) while it has only a small positive effect on NO. As revealed, the effect of low EGR temperature is stronger at high EGR rates.

ESTIMATE MEASUREMENT OF SOOT DIAMETER AND VOLUME FRACTION INSIDE THE BOWL OF A DIRECT-INJECTION-COMPRESSION-IGNITION ENGINE: EFFECT OF THE EXHAUST GAS RECIRCULATION

An original application of the Laser-Induced-Incandescence (LII) technique was set up to quantify soot particles inside the combustion chamber of an optically accessible Direct-Injection Diesel engine. Planar soot concentration and local particle diameter were measured for several Exhaust Gas Recirculation (EGR) rates. The impact of the injection timing on the soot evolution for the highest EGR rate was also studied. Based on the analysis of LII images it is shown that the planar distribution of soot becomes more and more uniform across the combustion chamber and globally the soot maximum more important with the EGR rates increase. High EGR rates, combined with a retarded start of injection may lead to lower soot production inside the combustion chamber. Comparison between exhaust and in-cylinder soot concentration highlights the effect of post-combustion oxidation on the particle-emissions amount.

Increase of passenger car engine efficiency with low engine-out emissions using hydrogen–natural gas mixtures: A thermodynamic analysis

In this study a state of the art passenger car natural gas engine was optimised for hydrogen natural gas mixtures and high exhaust gas recirculation (EGR) rates in the part load domain. With optimal combinations of spark timing (ST) and EGR rate the achievements are significant efficiency increase with substantially lower engine-out NO x while total unburned hydrocarbons or CO-engine-out emissions are not affected. Comprehensive investigations of the parameter space using design of experiments (DoE) algorithms provided a complete picture of the potential of such applications. Combustion analysis on the other hand allowed to identify improvements on the basis of accelerated combustion caused by the hydrogen as well as the reduced gas exchange losses due to EGR and associated less required throttling for a given engine output. The best combinations of EGR rate, hydrogen-fraction in the fuel and ST exhibited optimal in-cylinder pressure characteristics accompanied by moderate combustion peak temperatures and low expansion cylinder temperatures.

Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fuelled with primary reference fuels

The effects of exhaust gas recirculation (EGR) of primary reference fuel on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine are investigated. The results show that EGR delays the ignition timing, slows down the combustion reaction rate, reduces the pressure in cylinder, and extends the HCCI operation region. The maximum indicated thermal efficiency of fuels with different octane numbers appears in the region of high EGR rate and large fuel: air equivalence ratio. In the region of high EGR rate, hydrocarbon emissions rise up sharply as the EGR rate increases. Carbon monoxide emissions increase with the increase in EGR rate. Nitrogen oxides emissions increase abruptly as knocking combustion occurs.

Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol

The controlling strategies of homogeneous charge compression ignition (HCCI) fueled by dimethyl ether (DME) and methanol were investigated. The experimental work was carried out on a modified single-cylinder diesel engine, which was fitted with port injection of DME and methanol dual fuel. The results show that exhaust gas recirculation (EGR) rate and DME percentage are two important parameters to control the HCCI combustion process. The ignition timing and combustion duration can be regulated in a suitable range with high indicated thermal efficiency and low emissions by adjusting the DME percentage and EGR rate. EGR cannot extend the maximum indicate mean effective pressure (IMEP) of HCCI operation range with dual fuel, but can enlarge the DME percentage range in normal combustion. The combustion efficiency largely depends on DME percentage, and EGR can improve combustion efficiency. The results also show that HC emissions strongly depend upon DME percentage, and CO emissions have good coherence to the peak mean temperature in cylinder. In normal combustion, adopting large DME percentage and high EGR rate can attain an optimal HCCI combustion.

High Frequency Individual Cylinder Estimation for Control of Diesel Engines

Increasingly stringent pollution regulations have spurred a broad interest in the reduction of engine emissions. Two strategies can be considered: after-treatment or engine emissions reduction. For Diesel engines, after-treatment has a high cost induced by the required technology and implementation issues. In this context, the Highly Premixed Combustion combustion mode (HPC) has emerged as a topic of major interest in recent years. It represents one of the most promising ways for new generation of CI engines using high Exhaust Gas Recirculation (EGR) rates. Experimentation on the testbench brought the proof of significant emission reduction. However, actual vehicle implementation implies frequent transient which fall far beyond steady state experimentation on the testbench. Several key challenges have to be addressed. With this combustion technology, accurate torque balancing at all engine modes is a requirement. Even slight unbalance between the cylinders can have dramatic consequences and induce important noise, possible stall and higher emissions. To achieve this goal, we develop high frequency individual cylinder estimators. These serve as tools for real time control strategies.

Super clean diesel by usage of wide range, high boosted and cooled EGR system in single cylinder engine Japanese national project: super clean diesel engine

The heavy duty diesel engines have achieved status as prime movers for CO2 emissions. However, in the future heavy duty diesel engines will be required to have lower NOx and particulate matter (PM) emission levels than today. In this study high boost and lean diesel combustion has been attempted by a single cylinder engine in order to obtain a good engine performance and clean exhaust emission. The experiment has been done with intake air quantity up to five times of a naturally aspirated (NA) engine and 200MPa injection pressure. The adopted pressure booster is an external supercharger; and this experimental system can control the intake air temperature and also produce a high exhaust gas recirculation (EGR) rate in a wide speed and high load range.

Effect of Compression Ratio and Spray Injection Angle on HCCI Combustion in a Small DI Diesel Engine

An experimental investigation was performed on a small direct injection (DI) diesel engine equipped with a common-rail injection system to find the optimal operating conditions of a homogeneous charge compression ignition (HCCI) engine. It is generally agreed that NOx is formed within the stoichiometric diffusive flame and may continue to form in the hot combustion products and that particulate matter is formed under a rich fuel/air mixture condition. Therefore, if an adequately diluted fuel/air mixture is formed before the start of ignition by prolonging the ignition delay, a homogeneous lean mixture is burned at a low temperature. In this way, simultaneous reduction of NOx and soot can be achieved. To realize this fundamental concept and find the optimal operating conditions, injection timing was varied from top dead center (TDC) to 80° before TDC and up to 45% of exhaust gas recirculation (EGR) was tested. The features of the base engine were modified as follows. First, the geometric compression ratio was reduced from 17.8 of the base diesel engine to 15 for expanding ignition delay by modification of the piston shape. Second, the injection angle was reduced from 156° of the base engine to 60° to reduce fuel deposition on the wall of the combustion chamber when using early injection timing. Experimental results showed that clean operating conditions existed either at an early start of injection timing with/without EGR or at a very late start of injection timing near TDC with high rates of EGR.

Combustion optimization in the low-temperature diesel combustion regime

A microgenetic algorithm (μGA) code was applied to optimize experimentally an HSDI single-cylinder diesel engine equipped with a common rail fuel-injection system in order to reduce NOx, soot, and b.s.f.c. simultaneously. Four control factors were used, namely, start-of-injection (SOI) timing, intake boost pressure level, cooled exhaust gas recirculation (EGR) rate, and fuel-injection pressure. The search space was designed to be within the experimental capabilities of the engine and control system. The engine testing was done at 1550 r/min, and 25 per cent load. The optimum results showed significant improvements for the NOx and soot emissions. Through analysis of the combustion characteristics, the mechanisms of emission reduction were revealed. The optimum featured a long ignition delay due to retarded SOI timing, and low combustion temperatures as a result of high EGR rates. The resulting long time for mixing and low temperatures helps suppress soot formation. To explore further the effect of mixing on emissions in the low-temperature combustion regime, factors that enhance turbulent mixing rates, including the use of high injection pressures and post injections were examined. The results show that optimal post injections are useful further to reduce emissions when they feature a short injection pulse with an optimal dwell time between injections.

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 2. Effect of operating conditions and EGR on HCCI combustion

In Part 1, the effects of octane number of primary reference fuels and equivalence ration on combustion characteristics of a single-cylinder HCCI engine were studied. In this part, the influence of exhaust gas recirculation (EGR) rate, intake charge temperature, coolant temperature, and engine speed on the HCCI combustion characteristics and its emissions were evaluated. The experimental results indicate that the ignition timing of the first-stage combustion and second-stage combustion retard, and the combustion duration prolongs with the introduction of cooled EGR. At the same time, the HCCI combustion using high cetane number fuels can tolerate with a higher EGR rate, but only 45% EGR rate for RON75 at 1800 rpm. Furthermore, there is a moderate effect of EGR rate on CO and UHC emissions for HCCI combustion engines fueled with n-heptane and RON25, but a distinct effect on emissions for higher octane number fuels. Moreover, the combustion phase advances, and the combustion duration shorten with the increase of intake charge temperature and the coolant out temperature, and the decrease of the engine speed. At last, it can be found that the intake charge temperature gives the most sensitive influence on the HCCI combustion characteristics.

Spray and combustion characteristics of dimethyl ether fuel

Dimethyl ether (DME) has been recognized as an alternative fuel for better combustion and less pollutant formation than diesel, even though they have a cetane number. In this study, characteristics of DME spray were observed experimentally under various ambient conditions in a constant-volume chamber with a common-rail injection system. To understand the combustion characteristics of DME, radicals from the flame and exhaust gases were also investigated. Radicals from DME combustion were measured by a high-speed camera with stereoscope. Flame spectra were taken near 310 and 400-530 nm emitting from OH and CH or C2 radicals respectively. Flames of diesel were found to be much stronger than those of DME because of the smoke emitted from CH and C22 radicals. Moreover, NOx emission is substantially decreased without increasing in the smoke emission with a high exhaust gas recirculation (EGR) rate in DME combustion. It was confirmed from these results that DME has the potential for a low emission fuel alternative to diesel.

An experimental study of the effects of combustion systems and fuel properties on the performance of a diesel engine

This paper investigates the effects of combustion system and fuel properties on spray, mixture formation, combustion process, and emissions, using a constant volume chamber and single- cylinder engine. In a combustion system, high-pressure injection, exhaust gas recirculation (EGR), and impingement spray are applied while visualizing and analysing exhaust gas. Also, light diesel fuel, which has approximately the same cetane value as the diesel but has low values of PAHs, boiling point, density, viscosity, and surface tension, were used. Spray and combustion images were taken using a high-speed video camera and analysed by their penetration and evaporation characteristics in comparison with current diesel fuel. The results showed that the mixture formation of the high- pressure injection, impingement spray, and light diesel fuel is faster than that of diesel fuel, depending on physical properties. In the engine test, a considerable reduction of smoke emission was observed using the high-pressure injection with supercharging, due to the enhanced fuel atomization and air entrainment. In addition, a high reduction of NOx emission can be achieved with a high EGR rate, and with supercharging can achieve a reduction of NOx emission without deteriorating BSFC.

Extended Spark Duration Improves Engine Operation at High Exhaust Gas Recirculation Rates

Abstract Abstract-The amount of exhaust gas recirculation (EGR) which a single-cylinder engine tolerated without misfire increased when the duration of the spark was increased. Also, when minimum spark advance for best torque was maintained, engine efficiency improved as EGR increased. In addition to misfire, two other engine combustion problems were identified: partial combustion, and excessively slow combustion. Low flame speeds associated with high EGR probably caused these other problems, which could be prevented with advanced spark timing. Misfire, partial combustion, and excessively slow combustion were detected by monitoring with an oscilloscope the output of a bare wire thermocouple located in the exhaust port of the engine. This simple method could be suitable for quantifying combustion problems in multi-cylinder engines.