Exhaust Gas Recirculation Gas Research Articles

Evaluation of energy-exergy performance and sustainability index of a DI engine integrated with designed electromechanical EGR cooling system

Exhaust Gas Recirculation (EGR) system is a commonly used pretreatment technique to improve pollutant emissions from modern diesel engines. Recently, the EGR cooling strategy focuses on improving combustion stability, NOx emission and fuel consumption values by controlling oxygen dilution, in-cylinder and exhaust gas temperatures. However, there are not enough studies focusing on energy and exergy analysis to improve the deterioration caused by hot EGR gas in engine performance and energy distributions. Therefore, in this study, classical and electronically controlled external EGR cooling systems are designed. The present study investigates the effect of the various EGR rates and EGRout gas temperatures on classical/electromechanics EGR cooling systems in a DI diesel engine and reveals the effects of energy-exergy distributions on each other and the sustainability index (SI) perspective. The tests are performed with various EGR rates (5%, 10%, 15%) and EGRout gas temperatures (75 °C, 90 °C, 110 °C) at 1700 and 2000 rpm. At 1700 rpm test conditions, the highest thermal-exergetic efficiencies for low and high load are recorded at 15% EGR rate-75 °C EGRout gas temperature and 5% EGR rate-75 °C EGRout gas temperature, respectively. At 2000 rpm test conditions, the highest thermal-exergetic efficiencies for low and high load are achieved at 15% EGR rate-110 °C EGRout gas temperature and 5% EGR rate-90 °C EGRout gas temperature, respectively. The main findings show that the EGR rate rather than the EGRout gas temperature has a more dominant effect on the in-cylinder combustion performance and thermal-exergetic efficiency. While the increase in the EGR rate from 5% to 15% contributes positively to the thermal-exergetic efficiency and SI performance at low engine load, the opposite is observed at high load. The study recommends that the EGR rate and EGRout gas temperature should be chosen at the optimum level for the engine performance and energy efficiency.

Effects of CO2 on the laminar burning velocities of toluene reference fuel (TRF) with increasing initial temperatures and pressures

Spherically expanding flames of TRF/air mixtures diluted with different levels of CO2 content at elevated temperatures and pressures were experimentally studied in this work to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO2 in EGR gases on the laminar flame characteristics of gasoline fuel. The experiments have been performed using a constant volume combustion bomb with schlieren method. The laminar burning velocities (LBV) and markstein lengths of the TRF flame with different dilution level of CO2 (0–20%) at the elevated temperature of 373 K, 443 K and pressures of 1 bar, 3 bar and 5 bar have been studied. Numerical decoupling simulations were then applied to investigate the individual dilution, thermal and chemical effects of CO2, respectively. The CO2 addition has a suppressing effect on the LBV of the TRF fuel at all the tested conditions. The LBV of the TRF mixture diluted with 20% CO2 reduced to about 60% of the non-diluted flame at 373 K and atmospheric pressure. The LBV were promoted by the elevation of initial temperature, while inhibited by the promotion of initial pressure. The increasing of initial temperature extended both the lean and rich limits, while the elevation of initial pressure would facilitate the combustion on the leaner flame. The Markstein length had a slight increase, indicating a small increase of flame instability, with the increase of the of CO2 dilution ratio for the rich flames and decreased for the lean flames. The crossover equivalence ratio, φ*, was witnessed to shift slightly towards the leaner side, while the critical equivalence ratio, φc, moved to the richer side. Among the three factors, the dilution effect played the dominant role and the chemical effect came up the next. The thermal effect took up the least portion, yet non-negligible. In addition, the chemical effect showed a more sensitive respond to the temperature variation.

Effect of nitric oxide and exhaust gases on gasoline surrogate autoignition: iso-octane experiments and modeling

Exhaust gas recirculation (EGR) is widely used in advanced internal combustion engines to reduce engine emissions as well as control combustion phasing. Among various species present in EGR gases, CO2 and H2O are two major components that can thermally and chemically affect fuel autoignition. It is of fundamental interest to isolate the thermal and chemical effects of CO2 and H2O on fuel autoignition, especially as such an effort has not been reported in the literature. Moreover, nitric oxide (NO) is known to exhibit strong chemical effects on fuel autoignition, which in turn affects engine combustion phasing. The effects of ultra-low NO addition (< 100 ppm) on fuel autoignition at low temperatures are also not well understood. Recognizing these problems, autoignition experiments of iso-octane (a major gasoline surrogate component) in air are performed in this study using a rapid compression machine at varying compressed pressures, equivalence ratios, dilution levels with an EGR gas analogue (consisting of CO2, H2O, O2, and N2) and N2 only, and varying amounts of NO addition. The thermal and chemical effects of the EGR gas analogue are isolated and evaluated by comparing the ignition delay time datasets of EGR and N2-only diluted cases. It is found that the dilution/thermal effect on autoignition dominates at all test conditions, and the combined chemical effect of CO2 and H2O is insignificant. The NO doping experiments show that the promoting and inhibiting effects of NO addition on iso-octane ignition delay time are temperature dependent. Furthermore, a chemical kinetic model for gasoline surrogate components and NOx is developed by validating it against the experimental data obtained in this study and those available in the literature for various gasoline surrogate components. The validation of the model across different experimental datasets and for several fuels demonstrates its reliability and wide applicability in several operating conditions. Comparisons with previously developed models shed light on the gaps still present in understanding the effect of nitrogen species on the reactivity of gasoline surrogate molecules. Chemical kinetic analyses have also been conducted to identify the important reaction pathways controlling iso-octane autoignition at various conditions, and to elucidate the underlying mechanism leading to different reactivity trends at different NO doping levels and temperatures.

A comparative study into the effects of pre and post catalyst exhaust gas recirculation on the onset of knock

Exhaust gas recirculation (EGR) is proven as a valuable technology for controlling knock whilst maintaining lambda one operation, and is also capable of providing efficiency gains at low load. Despite this few studies in the literature address the question of EGR composition effects, namely whether the EGR gas is sourced from before or after the catalyst, and this remains an area which is often overlooked whilst investigating EGR performance. This paper demonstrates a novel method combining experiment air-path emulation and in-depth data processes to compare the effect of EGR catalysis on the angle of knock onset in a 1L GDI engine. Since initial temperature and pressure have a significant impact on knocking behaviour, an artificial boosting rig replaced the turbomachinery. This enabled fine control over the engine boundary conditions to ensure parity between the catalysed and un-catalysed cases. To overcome the difficulty of comparing stochastic phenomena in an inherently variable dataset, a pairing method was combined with Shahlari and Ghandhi’s angle of knock onset determination method to assess the effects of EGR composition on knock onset for EGR rates ranging from 9% to 18%. The air path emulation system stabilised the engine combustion to provide a suitably rich dataset for analysing knock using the pairing method. Catalysed EGR improved the mean knock onset angle by 0.55 CAD, but due to the inherent variability in cylinder pressure data this only equated to a 58.3% chance of a later knock onset angle for catalysed EGR in any given pair of comparative cycles.

On the Role of Nitric Oxide for the Knock-Mitigation Effectiveness of EGR in a DISI Engine Operated with Various Gasoline Fuels

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The knock-suppression effectiveness of exhaust-gas recirculation (EGR) can vary between implementations that take EGR gases after the three-way catalyst and those that use pre-catalyst EGR gases. A main difference between pre-and post-catalyst EGR gases is the level of trace species like NO, UHC, CO and H&lt;sub&gt;2&lt;/sub&gt;. To quantify the role of NO, this experiment-based study employs NO-seeding in the intake tract for select combinations of fuel types and compression ratios, using simulated post-catalyst EGR gases as the diluent. The four investigated gasoline fuels share a common RON of 98, but vary in octane sensitivity and composition. To enable probing effects of near-zero NO levels, a skip-firing operating strategy is developed whereby the residual gases, which contain trace species like NO, are purged from the combustion chamber.&lt;/div&gt;&lt;div class="htmlview paragraph"&gt;Overall, the effects of NO-seeding on knock are consistent with the differences in knock limits for preand post-catalyst EGR gases. This suggests that for a majority of the conditions studied here, variations in NO concentration dominate the autoignition-influencing role of trace species that are present in pre-and post-catalyst EGR gases. The specific response of the knock limits to NO vary greatly between fuels, and the response also varies with compression ratio for a given fuel. Generally, the greatest sensitivity to NO exists in the 0 - 50 ppm range for all fuels, with increasing [NO] strongly promoting end-gas autoignition. For higher levels of NO, the autoignition-enhancing effect of NO saturates. This is particularly true for the Alkylate fuel with low octane sensitivity, for which addition of NO beyond 100 ppm reverses the trends, with further NO addition actually suppressing knock, although not beyond the level achieved without NO present.&lt;/div&gt;&lt;/div&gt;

Study on Diesel Engine Characteristics under Large Transient EGR

Modern diesel engines tend to employ up to 50-70% exhaust gas recirculation (EGR) together with high intake pressure and injection strategies to enable low temperature combustion (LTC) cycles for reducing NOx and soot emissions simultaneously. Obviously, the combustion conditions and exhaust emissions are sensitive at such high EGR rate. And any slight fluctuation in the EGR quantity will bring unintended deviations from the desired engine performance, thus LTC mode is only limited at partial engine operation points. So the engine has to switch combustion mode frequently between compression ignition (CI) and LTC region within a few engine cycles in real application, which may result in combustion cyclic variations and even misfire, especially during transient operation. In order to investigate effect of heavy EGR transient process on engine combustion cycles, the experimental work was carried out on a four-cylinder VM common-rail turbocharged diesel engine. The results show that the oxygen concentration in the intake charge almost maintains at steady level at EGR steady conditions, while the exhaust oxygen concentration is affected by exhaust values opening/exhaust values closing (EVO/EVC), and result in intra-cycle fluctuation, which will approximately bring 2% calculation error bandwidth for EGR ratio. From 37% to 55% EGR ratio, the EGR gas is mainly driven by the pressure ratio of intake and exhaust duct, and it will experience a long accumulating process to reach a new equilibrium. And the inordinate delayed injection timing will promote in-cylinder cycle-to-cycle variation and even misfire, especially during transition from CI to LTC region.

Study on the effects of intake conditions on the exergy destruction of low temperature combustion engine for a toluene reference fuel

Low temperature combustion (LTC) is a new mode for efficient and clean combustion in engines, and the process is mainly controlled by chemical reaction kinetics of fuel. In this paper, the numerical model of exergy analysis for LTC engine was established. The temperature, pressure and species concentration were obtained by using the homogeneous charge compression ignition (HCCI) engine model of Chemkin-Pro software with the simplified mechanism of toluene reference fuel (TRF). The exergy items were further calculated to explore the effect of different intake conditions on exergy destruction in LTC process. It was found that the irreversibility related to the combustion proceeding mainly happened in low temperature and high temperature reaction zones. With the increase of exhaust gas recirculation (EGR) rate, the proportion of working exergy increases but the irreversibility destruction of combustion decreases. Besides, N2 component concentration in EGR gas has the greatest influence on the distribution of exergy.

Parametric Study on EGR Cooler Fouling Mechanism Using Model Gas and Light-Duty Diesel Engine Exhaust Gas

Exhaust gas recirculation (EGR) and high-pressure fuel injection are key technologies for reducing diesel engine emissions in the face of reinforced regulations. With the increasing need for advanced EGR technologies to achieve low-temperature combustion and low emission, the adverse etableffects of EGR must be addressed. One of the main problems is fouling of the EGR cooler, which involves the deposition of particulate matter (PM) due to the thermophoretic force between the cooler wall and flow field. A large amount of deposited PM can reduce the effectiveness of the heat exchanger in the EGR cooler and the de-NOx efficiency. In the present study, the effects of the variables that affect EGR cooler fouling are investigated by a comparison of laboratory-based and engine-based experiments. In the laboratory experiment, a soot generator that could readily provide separate control of the variables was used to generate the model EGR gas. Through control of the soot generator, it was possible to perform a parametric study by varying the particle size, the EGR gas flow rate, and the coolant temperature as the dominant parameters. A decrease in these factors caused an increase in the mass of the deposit and a drop in the effectiveness of the heat exchanger, related to fouling of the EGR cooler. In the engine-based experiment, engine-like conditions were provided to analyze real exhaust gas without a soot generator. Different variables were found to induce fouling of the EGR cooler, and the results of the engine-based experiment differed from those of the laboratory experiment. For example, in the engine-based experiment, a decrease in the EGR gas flow rate did not lead to a more pronounced drop in the effectiveness of the heat exchanger because of the increase in the concentration of PM in the EGR gas. This result shows that the conditions of the engine exhaust gas are different from those of the soot generator.

Adaptation Strategy for Exhaust Gas Recirculation and Common Rail Pressure to Improve Transient Torque Response in Diesel Engines

Fuel injection limitation algorithms are widely used to reduce particulate matter (PM) emissions under transient states in diesel engines. However, the limited injection quantity leads to a decrease in the engine torque response under transient states. To overcome this issue, this study proposes an adaptation strategy for exhaust gas recirculation (EGR) and common rail pressure combined with a fuel injection limitation algorithm. The proposed control algorithm consists of three parts: fuel injection limitation, EGR adaptation, and rail pressure adaptation. The fuel injection quantity is limited by adjusting the exhaust burned gas rate, which is predicted based on various intake air states like air mass flow and EGR mass flow. The control algorithm for EGR and rail pressure was designed to manipulate the set-points of the EGR and rail pressure when the fuel injection limitation is activated. The EGR controller decreases the EGR gas flow rate to rapidly supply fresh air under transient states. The rail pressure controller increases the rail pressure set-point to generate a well-mixed air-fuel mixture, resulting in an enhancement in engine torque under transient states. The proposed adaptation strategy was validated through engine experiments. These experiments showed that PM emissions were reduced by up to 11.2 %, and the engine torque was enhanced by 5.4 % under transient states compared to the injection limitation strategy without adaptation.

EGR control on operation of a tar tolerant HCCI engine with simulated syngas from biomass

In combined heat and power plants operated with biomass syngas, the removal of condensible tars is a necessary but expensive step (up to one third of the installation and maintenance costs). This step is required because the syngas has to be cooled down to avoid knocking in the spark ignition engines traditionally used in such plants. To remove the tar condensation problem, we developed an alternative system based on an Homogeneous Charge Compression Ignition (HCCI) engine operated at intake temperatures above the tar dew point. To address the challenge of power derating of such engine setups, the current paper focuses on the application of Exhaust Gas Recirculation (EGR) as a control parameter that would indirectly allow the improvement of the engine performance. Based on a conservative estimate of tar dew points, HCCI combustion was studied at an intake temperature of 250°C using synthetic biomass syngas and synthetic EGR on a mono-cylinder HCCI engine operated at 1000 RPM. The effects of charge dilution, thermal and kinetic damping due to the EGR gases were also analysed to understand their main effects. The use of EGR successfully increased the maximum achievable Indicated Mean Effective Pressure from 2.5bar at EGR=0% up to 3.3bar at EGR=25%, through damping the maximum pressure rise rate and allowing higher equivalence ratios.

Influence of diluents on combustion and emission characteristics of a GDI engine

Exhaust gas recirculation (EGR) and lean burn are two common examples of dilution combustion that have been widely applied to improve the performance of gasoline direct injection (GDI) engines. This paper studied influences of air and EGR gas components (CO2, N2, and Ar) on combustion and emission characteristics of a GDI engine, in order to understand their independent effect and provide bases for optimal control of diluents. The results indicated that CO2 dilution showed the strongest influence on combustion process, while Ar showed the weakest. As increasing CO2 addition ratio, the flame development and combustion duration were extended, the combustion center was delayed and the cycle-by-cycle combustion variations was deteriorated significantly. Under the same addition ratio, Ar dilution gave the best fuel economy, while CO2 dilution could more effectively suppress NOx, HC and particulate matter emissions. Moreover, Ar dilution offered the best relationship between fuel economy and NOx emission, followed by N2 and then CO2, and it got the lowest brake specific fuel consumption at the same NOx level. However, CO2 dilution is the most effective method in suppressing particles at the same level of fuel economy.

An experimental study on removal of exhaust gas recirculation cooler fouling using washer fluid steam

Lowering the temperature of recirculated gas in an exhaust gas recirculation (EGR) system reduces the emission of nitrogen oxide (NOx). However, the EGR cooler can become polluted by PM in recirculated EGR gas, which reduces the cooling efficiency and the gas flow rate. A steam EGR cooler fouling remover (SECFR) system was designed using evaporated washer fluid to remove fouling on the cooler surface. However, the injection pressure is only 5bar, which is not sufficient to atomize the washer fluid. Therefore, in this study, an impinging plate was used to promote the atomization of sprayed washer fluid, and the droplet area ratio (DAR) and droplet uniformity index (DUI) were measured through quantitative analysis of high-speed images. Finally, the performance. of the SECFR system was evaluated in a real diesel engine.

Metal foams as gas coolers for exhaust gas recirculation systems subjected to particulate fouling

This paper presents experimental results indicating the benefits and challenges associated with the use of metal foams as Exhaust Gas Recirculation (EGR) coolers. Fouling of such heat exchangers is a critical issue and, as such, special attention has been paid to address this very issue in the present study where a soot generator has been employed to simulate the engine running condition. Effects of aluminium foam PPI and height as well as gas velocity are investigated. It has been noted that proper design of the foam can lead to significantly higher heat transfer rate and reasonable pressure drop compared to no-foam cases. More interestingly, it is demonstrated that the foams can be cleaned easily without relying on expensive cleaning techniques. Using simple brush-cleaning, the foams can be reused as EGR gas coolers with a performance penalty of only 17% (compared to a new or clean foam).

G0700201 マイクロフローリアクタ-による排気再循環を伴う予混合気の自己着火特性の把握

Effects of large-amount dilutions of exhaust gas recirculation (EGR) and dedicated-EGR (DEGR) on the ignition characteristics of stoichiometric methane-air premixed air were experimentally studied by using a micro flow reactor. The flames were formed for various EGR/DEGR ratios and equivalence ratios Φ_A in the DEGR cylinder Stable flames and oscillating flames were observed; the combustion characteristics varied because the mixtures were highly diluted with EGR/DEGR gases. Their ignition temperatures were defined by the positions of the flames referred to a measured temperature profile in the quartz tube. Since EGR gas is composed of inert species such as N_2 and CO_2, the ignition temperatures of the premixed gas with EGR gas were higher than that without EGR gas; the EGR gas dilution interferes the ignition timing in reciprocating engines. On the other hand, the ignition temperature of the premixed gas with the dilution of DEGR gas whose equivalence ratios correspond to Φ_A = 1.6 and 1.8 were lower than that with EGR gas. This is because highly-reactive intermediate species such as H_2 and CO. Particularly, the ignition temperatures of the premixed gases highly diluted (40%) with DEGR gas of Φ_A = 1.6 and 1.8 were lower than that without EGR gas or DEGR gas. Therefore, it was found that gases with DEGR have potential for shorting the ignition delay.

Experimental investigation of the thermal and diluent effects of EGR components on combustion and NOx emissions of a turbocharged natural gas SI engine

Exhaust gas recirculation (EGR) is one of effective measures used in natural gas (NG) engines to reduce nitrogen oxides (NOx) emissions. Each component of EGR gases can exert different effects on NG combustion and NOx formation rates, such as thermal effect, diluent effect, and chemical effect. In this study, the thermal and diluent effects of the main components of EGR gases, including carbon dioxide (CO2) and nitrogen (N2), were experimentally investigated. The experiments were arranged based on an electronically controlled heavy-duty natural gas spark-ignition (SI) engine with multi-point injection and 6-cylinder. In order to define the diluent effect of EGR components, argon (Ar) was introduced to the test, for its low and unchangeable specific heat capacity under different temperatures. The results showed that the contribution of the diluent effect on NOx reductions was 50–60% and 41–53% for N2 and CO2 respectively, and the relevant contribution of the thermal effect was 40–50% and 47–59% respectively. CO2 had greater effects on NG combustion and NOx formation rates than N2 at the same dilution ratio. Increasing Ar improved the thermal efficiency due to the higher specific heat ratio provided. Meanwhile, NOx emissions were found to be decreased with increasing DR as a result of the diluent effect of Ar on combustion and NOx formations.

Effects of CO2 Dilution on Methane Ignition in Moderate or Intense Low-oxygen Dilution (MILD) Combustion: A Numerical Study

Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution (MILD) combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation (EGR), especially CO2 in EGR gases, on ignition characteristics. Specifically, effects of CO2 addition on autoignition delay time were emphasized at temperature between 1200 K and 1600 K for a wide range of the lean-to-rich equivalence ratio (0.2∼2). The results showed that the ignition delay time increased with equivalence ratio or CO2 dilution ratio. Furthermore, ignition delay time was seen to be exponentially related with the reciprocal of initial temperature. Special concern was given to the chemical effects of CO2 on the ignition delay time. The enhancement of ignition delay time with CO2 addition can be mainly ascribed to the decrease of H, O and OH radicals. The predictions of temperature profiles and mole fractions of CO and CO2 were strongly related to the chemical effects of CO2. A single ignition time correlation was obtained in form of Arrhenius-type for the entire range of conditions as a function of temperature, CH4 mole fraction and O2 mole fraction. This correlation could successfully capture the complex behaviors of ignition of CH4/air/CO2 mixture. The results can be applied to MILD combustion as “reference time”, for example, to predict ignition delay time in turbulent reacting flow.

Potential of in-cylinder exhaust gas recirculation stratification for combustion and emissions in diesel engines

It is difficult to decrease the emissions of nitrogen oxides (NO x) and soot simultaneously in conventional diesel engines. Low-temperature combustion concepts have been studied in an effort to overcome this problem. Low-temperature combustion has the potential to reduce NO x and soot emissions, but it has many limitations, including narrow operating ranges, high carbon monoxide and hydrocarbon emissions, and difficulties with ignition control. Exhaust gas recirculation (EGR) stratification is another combustion concept used to reduce NO x and soot emissions simultaneously using the local non-uniformity of EGR gas instead of increasing the overall EGR rate. In this study, the EGR stratification concept was improved using computational fluid dynamics. First, a two-step piston was developed to maximize the stratified EGR effects by obtaining a favourable EGR distribution pattern and injecting fuel into the high-EGR region. Then, the possibility of combustion and emission control using stratified EGR was estimated. The ideally distributed EGR in the cylinder results showed that the region of locally high EGR effectively influences the combustion characteristics and, thus, horizontally and centrally stratified EGR has the potential to reduce the nitric oxide (NO x) and soot emissions at the same time. Engine simulation results also showed simultaneous reductions in the NO x and the soot emissions.

Parametric study on particle size and SOF effects on EGR cooler fouling

Abstract Recent diesel engine technologies, developed for enhanced regulation of exhaust emissions, are characterized by high exhaust gas recirculation (EGR) rates and high-pressure fuel injection. The use of high EGR rates, by which high temperatures can be avoided in a cylinder, is an effective method for reduction of nitrogen oxide (NO x ) emissions. High-pressure fuel injection leads to smaller soot particles, which decreases mass-based soot emission. These technologies, however, also have effects on particulate matter (PM) characteristics that are closely related to EGR cooler fouling. High-pressure fuel injection, which makes smaller soot particles, increases the concentration of soot particles, and a high EGR rate, which lowers the temperature in the cylinder, increases the soluble organic fraction (SOF). In this study, we evaluated the effects of these changes in PM characteristics on EGR cooler fouling. Instead of an engine-based experiment, in which a parametric study is nearly impossible, a laboratory experiment was performed to separate variables. A soot generator was used to make model exhaust gas because the variables could be controlled separately (e.g., mean particle size and concentration of soot particles) and this improved the reproducibility of the experiments. Additionally, n-dodecane, a model compound representing diesel fuel, was vaporized and injected into the exhaust gas to test the effects of SOF on cooler fouling. For particle sizes ranging from 41 to 190 nm in diameter, the deposition fraction was inversely proportional to particle size. Thus, smaller soot particles in the exhaust gas were more likely to cause formation of thermophoretic deposits on the wall of the EGR cooler. At an EGR gas temperature of 350 °C, the deposition fraction was greatest (84%) for the smallest particle size of 41 nm, whereas the deposition fraction was least (7%) for the largest particle size, 190 nm. The performance degradation of the EGR cooler showed a similar trend to the measured deposit mass. As coolant temperature decreased, the effect of n-dodecane injection on the growth of PM deposits increased, which significantly reduced the overall thermal conductivity of the EGR cooler. This result showed that ‘wet soot’ caused greater contamination of the EGR cooler.

저압 EGR을 적용한 디젤엔진의 희석비에 따른 연소 특성 비교

Exhaust gas recirculation (EGR) is more effective than selective catalytic reduction (SCR) or lean NOx trap (LNT) for the reduction of NOx emissions in diesel engines. A large amount of EGR gas is necessary to satisfy the stringent regulations on NOx emissions. Low pressure loop (LPL) EGR is almost independent of the variable geometry turbocharger (VGT) at a specific boost pressure, so LPL EGR is better than conventional high pressure loop (HPL) EGR in terms of EGR supply. We compare the influence of HPL EGR and LPL EGR on the combustion characteristics at a constant boost pressure in a diesel engine. The dilution ratio was employed as an independent parameter to analyze the effect of the dilution of the intake charge for each EGR loop. At the same level of NOx emissions, the fuel consumption and smoke opacity were slightly lower for LPL EGR than for HPL EGR.

Systems Approach to the Improvement of Engine Warm-Up Behaviour

Modifications to the coolant and oil circuits of a modern production 2.4 l diesel engine have been made in an attempt to promote oil warm-up to reduce fuel consumption. The new system used oil to cool exhaust gas recirculation (EGR) gases and incorporates a number of coolant flow control valves to reduce heat loss during warm-up. The engine was run over cold-start New European Drive Cycles with various flow strategies as a screening exercise to understand the behaviour of the system. Fuel consumption benefits of up to 4 per cent were observed, but these were accompanied by 3 per cent increases in nitrogen oxide (NO x) emissions. Detailed analysis of the coolant flows and temperatures showed that, when throttling the flow, the mass of coolant in the degas bottle and radiator could be isolated from the system during warm-up, essentially reducing the thermal inertia. Heat transfer directly to the oil from the EGR gases rather than via the coolant allowed more heat to be put into the oil, with engine oil supply temperatures up to 6 °C hotter; however, it was not possible to verify that the oil was hotter at the bearings, valve train, and cylinder liner. The engine strategy was seen to react to the faster warm-up and to retard injection timing, reducing NO x but also compromising overall fuel consumption benefits. Further tests were conducted with various injection timings to establish a NO x—fuel consumption trade-off to demonstrate further benefits when the engine strategy is included in the operation of novel thermal management systems.