Cooled Exhaust Gas Recirculation Research Articles

Mapping of hydrocarbon condensation onset temperature and its sensitivity analysis for Exhaust Gas Recirculation (EGR) cooler

Exhaust Gas Recirculation (EGR) cooling technology is one of the key technologies of the engine to meet the Euro 7 emission standard. Nevertheless, the hydrocarbon (HC) condensation in the cooler leads to uncontrolled thermal-hydraulic performance, and the onset temperature of HC condensation is still debated, which is one of the obstacles to implementing the Euro 7 standard. Here, we first established a database of key parameters (carbon number, concentration, and pressure); then theoretically calculated the onset temperatures of various condensation types (wall condensation, heterogeneous nucleation, and homogeneous nucleation), plotted the entire map and verified its veracity; and finally, elucidated the contribution of each parameter through sensitivity analysis. The results show that (i) HC species are C12 to C25, from 25 to 1717 ppm concentrations, and pressures are from 1 to 4 bar. (ii) The map diagrams for the onset temperature of HC condensation have broad applicability and strong practicality. (iii) The temperature ranges of the onset of wall condensation, heterogeneous nucleation, and homogeneous nucleation are from 4 to 218 °C, from 1 to 209 °C, and from −6 to 195 °C, respectively. (iv) Carbon number has the most significant contribution, followed by HC concentration and pressure. The results can provide a valuable reference for more accurate EGR cooler design and management.

Realisation of Low Temperature Combustion in an Unmodified Diesel Engine

Heavy-duty diesel engines are an essential part of road transportation. Since viable alternatives are not expected in the short and medium term, the problematic emission characteristics of compression ignition engines have to be addressed. Low temperature combustion (LTC) is an alternative combustion method for compression ignition engines that allows low particulate matter and nitrogen oxide emissions while improving efficiency. To overcome the difficulties of market introduction, the realization of such alternative combustion methods should come with marginal engine modifications. Thus, this work investigates the possible realization of LTC in an unmodified diesel engine. LTC methods with and without injection strategy modifications were also studied to provide sufficient recommendations for other researchers. It was concluded that techniques requiring early direct injection, such as homogeneous charge compression ignition (HCCI) require a narrow cone angle injector to reduce wall impingement. It was also concluded that modulated kinetics (MK) type LTC can be easily achieved by applying a conventional injection strategy and high amounts of cooled exhaust gas recirculation. The realized MK combustion resulted in an enhanced NOx-PM trade-off and a lower peak pressure rise rate compared to normal operation.

An original nondestructive sampling method to study the effect of gravity on the deposition of micron-sized large particles in exhaust gas recirculation (EGR) cooler fouling

Fouling is one of the barriers to developing more efficient and near-zero emission internal combustion engines. The micron-sized particulate matter is one of the roots of this fouling phenomenon in exhaust gas recirculation (EGR) coolers. This fouling is inadequately evaluated quantitatively, and its deposition mechanism is unknown. To investigate the effect of gravity on the deposition of micron-sized particles, an original nondestructive sampling fouling method and experiment apparatus have been developed to effectively obtain the upper and lower bottom fouling in the cooler in the direction of airflow, and the area proportion, count, and diameter of large particles in the fouling using image processing software. It was found that (i) the area proportion of large particles in the lower bottom fouling was almost 2.5 times higher than the upper bottom fouling; (ii) the count of large particles in the lower bottom fouling was higher, and the maximum diameter was larger, up to 639 μm; (iii) the mass of the lower bottom fouling was 1.5 times higher than the upper bottom fouling; (iv) gravity can significantly promote the deposition of micron-sized particles and should be considered in the design and arrangement of the EGR cooler to prevent fouling.

An Experimental Examination into the Efficiency of a Shell-and-Tube Heat Exchanger for an EGR Cooler

The increasing concerns over environmental pollution and stringent regulations on emission have led to the widespread adoption of exhaust gas recirculation (EGR) systems in interna combustion engines. EGR systems help reduce nitrogen oxide (NOx) emissions by recirculating a portion of exhaust gaes back into the combustion chamber. Howerver, the elevated temperature of exhaust gases poses challenges related to thermal management, necessitating the use of exhaust gas recirculation cooler. The experimental setup allows for the controlled circulation of exhaust gases and cooling fluids, thereby facilitating the measurement of heat transfer rates, pressure drops and overall thermal efficiency. These parameters encompass the inlet and outlet temperatures of both the exhaust gases and the cooling fluids flow rates for both fluids resulting pressure drops. By systematically altering these factors, the intricate, interplay between the various parameters and their inputs on the heat exchangers thermal performances are comprehensively explored. Addition, the research develops into critical aspects often overshadowed in EGR cooler studies on fouling. Overtime, the accumulation of deposits in the heat exchanger surface can led to reduced heat transfer efficiency. The outcomes of this study are instrumental for the advancement of EGR cooler technologies fostering more sustainable and environmentally friendly and automotive engines.

Study on Selection of Diesel Engine Exhaust After-Treatment System Materials and Emission Performance for Genset Applications

The use of diesel engines for the transportation sector is ubiquitous and covers a broad continuum, like buses, trucks, agriculture, construction equipment, generators, and industrial applications. However, because of their increased use of off-road engines, they contribute more emissions in various sectors than on-road. As a result, worldwide, regardless of ongoing emission control efforts, these off-road, stationary engines also continue to be a substantial source of air pollutant emissions in the United States, European Union and Asian markets. Accordingly, while improving emissions performance, the selection of after-treatment system materials and the use of compatible after-treatment specification selections are also critical. This is only possible by effectively combining clean diesel technologies such as Fuel Injection Equipment (FIE), Turbocharger with Intercooler (TCIC), Cooled Exhaust Gas Recirculation (C-EGR), and after-treatment systems like Diesel Oxidation Catalyst (DOC), Selective Catalytic Reduction (SCR), and Diesel Particulate Filter (DPF). Overall, in this study, the influence of after-treatment system configurations and their effect on emission reduction performance are studied for three different DOC specifications based on emission testing for catalyst and size optimization. With the selected DOC catalyst specification, suitable combinations of DPF and SCR specifications are optimized further using GT-SUIT software simulation. This study was carried out during the conversion of a 15 kW NA engine to a 27 kW TCIC engine to meet diesel engine emission norms.

Design of an Optimum Compact EGR Cooler in a Heavy-Duty Diesel Engine towards Meeting Euro 7 Emission Regulations

Exhaust gas recirculation (EGR) has been an efficient emission treatment strategy employed in internal combustion engines (ICEs) to cope with NOx emission limits since the introduction of Euro 4 regulations for heavy-duty commercial vehicles. A portion of the exhaust gas is fed back into the intake port, replacing O2 in the fresh air with inert CO2 from the exhaust gas, resulting in a reduction in the combustion temperature and, hence, a reduction in NOx emissions. Considering the high exhaust temperature, this process increases the charge mixture temperature and degrades the volumetric efficiency of the engine. EGR coolers have been introduced as vital parts of EGR exhaust treatment systems with the aim of reducing the intake port temperature to increase volumetric efficiency and further reduce combustion temperatures. EGR coolers are heat exchangers (HXs) that generally employ engine coolant to reduce the EGR temperature with effectiveness values around 0.7~0.85 and downgrade with engine usage owing to soot deposition. Increasing the effectiveness of the EGR cooler has a positive effect on engine volumetric efficiency and reduces NOx, particulate matter (PM), and fuel consumption. The current study involved the design of a microchannel HX for a 500 PS heavy-duty Euro 6 diesel engine EGR cooler. The mechanical and thermal-hydraulic design calculations of the proposed HX were performed using Mathematica software. The optimum HX dimensions for the required boundary conditions were determined, and the performance of the EGR cooler was analyzed for the current and proposed options. Furthermore, Diesel-RK software was used to model the engine performance with NOx, PM, CO2 emissions, and fuel consumption predictions. The results show that the newly proposed microchannel HX design improves NOx, PM, and specific fuel consumption by 6.75%, 11.30%, and 0.65%, respectively.

Three condensation paths of exhaust and its five effects on exhaust gas recirculation (EGR) cooler fouling and thermal performance: A review

Fouling has limited the evolution of compact heat exchangers (represented by EGR cooler) to smaller and more efficient sizes, an important reason is the unclear mechanism of the effect of exhaust gas condensation on fouling and thermal performance. This paper reviews the effects of hydrocarbon vapor, water vapor and acid vapor condensation on EGR cooler fouling and thermal performance from 1998 to 2023. Three paths of vapors condensation can occurr: interaction with particles, homogeneous nucleation and interaction with fouling. Beneficial effect (3) and harmful effects (2) to EGR cooler thermal performance have identified: filling, removal and chemical are beneficial effects whereas adhesion and nucleation are harmful effects. Three pathways and five effects play a key role on the composition, the mass, the thickness, density, the morphology and thermal resistance of fouling layer. The involvement level of sub-paths and contribution degree of sub-effects of vapor condensation under different boundary conditions, standard database, holistic model and the effect of renewable synthetic fuels on fouling and thermal performance can be further explored. The results will help researchers to understand the logical relationship between exhaust vapor condensation and fouling and thermal performance from the new perspective of condensation.

Effect of soot particle deposition on porous fouling formation and thermal characteristics of an exhaust gas recirculation cooler

Exhaust gas recirculation (EGR) systems have been successfully employed to reduce the NOx emissions in diesel engines. However, the fouling problem in EGR coolers challenges their capability to comply with stringent environmental regulations. A few numerical simulations have considered the fouling growth in EGR coolers. Those studies modeled the evolving fouling layer to be a solid medium, therefore, fluid flow and convection heat transfer within the fouling layer, which is well-documented to be a porous medium permeable to gas flow, have not been considered yet. As such, the present study investigates the simultaneous effects of the formation of the evolving porous fouling layer (EPFL) at the walls of an EGR cooler and fluid flow and convection heat transfer simulation within this EPFL to determine its coupled effects on the thermal performance of the EGR cooler. This study also investigates the possibility of formation of a steady fouling layer (SFL) because of the opposing effects of the fouling layer growth and deposition rate. The effects of two pertinent dimensionless parameters, namely Darcy number (10-4≤Da≤5×10-3) and Reynolds number (100≤Re≤400) on the time history of the fouling layer growth, deterioration of the thermal performance of the duct, and average Nusselt number ratio (Nuav/Nuavt=0) are studied. The results show that the thermal performance of the duct decreases as the EPFL grows, which agrees well with the available experimental data. It is shown that the steady fouling layer is obtained due to a decrease in thermophoretic force and deposition rate, as a result of the EPFL formation. Finally, a correlation is proposed in terms of Reynolds and Darcy numbers for the time at which the SFL occurs.

Elucidation of exhaust gas recirculation deposit formation mechanism based on chemical analysis of components

To comply with strict emission regulations, vehicles are equipped with exhaust gas recirculation (EGR) systems that control nitrogen oxide emissions by returning part of the exhaust gas to the intake air. However, since deposits form in the EGR piping and valves, leading to pipe clogging, valve sticking, and in the worst case, failure of these components, there is a need to clarify the mechanism of deposit formation to predict and reduce deposit accumulation. While most of the previous studies have focused on the powdery deposits that clog the EGR cooler, this study aims to elucidate the formation mechanism of both the lacquer-like hard deposits and wet soft deposits that cause EGR valve sticking by quantitative and molecular-level analyses. In these experiments, a portion of the exhaust gas from an actual diesel engine without soot removed by diesel particulate filter (DPF) was used. Deposits were continuously generated on a simulated EGR line with a downstream temperature gradient from the engine outlet, while measuring the inner wall temperature. The collected deposits were subjected to comprehensive chemical analysis. The analysis showed that 75 % of the solvent-soluble deposit components were polycyclic aromatic hydrocarbons (PAHs), which were the main component of the hard and soft deposits. Most of the PAHs in the hard deposits that formed at a high inner wall temperature were C17 or higher. The hard deposit with an inner wall temperature of 137 °C contained 0.016 mg/cm2 of 6-ring benzo[ghi]perylene and 0.26 × 10-3 mg/cm2 of 4-ring pyrene. On the other hand, the soft deposit is formed at an inner wall temperature of 80 °C, contained 0.022 mg/cm2 of pyrene, which is significantly more than in the hard deposit. This shows that PAHs in the exhaust gas condense when the gas temperature reaches their dew points, and that the deposits formed at higher inner wall temperatures contain more PAHs with higher boiling points. High-molecular weight aromatic compounds with oxygen and nitrogen functional groups were detected in the solvent-insoluble deposit components. These compounds were found to be derived from the polymerization of oxygenated and nitrogenated PAHs through dehydration condensation and cross-linking reactions.

Effects of Cooled Exhaust Gas Recirculation Combined With Prechamber Jet Ignition on the Combustion Characteristics in a Kerosene-Fueled Spark Ignition Engine

Abstract For security and logistical convenience, the single fuel forward policy hopes to use a single kerosene fuel to replace a variety of military fuels, especially the unsafe gasoline. However, the performance of kerosene-fueled spark ignition (SI) engines is severely restricted by knock, slow combustion rate, and poor combustion stability. This work innovatively applies cooled exhaust gas recirculation (EGR) combined with prechamber jet ignition (PJI) to a kerosene-fueled engine to suppress knock and improve performance. The results show that applying cooled EGR at a fixed throttle opening can suppress knock and improve fuel economy. However, the power decreased due to the decreased intake of energy. While applying EGR with constant fresh air mass flow makes the knock more prominent because of the microboosting effect caused by the increased throttle opening. This indicates that cooled EGR alone cannot improve the power output. Moreover, combustion instability was also caused due to the slower combustion rate. Therefore, PJI was introduced to compensate for the negative impact of EGR. As a result, a synergistic effect of cooled EGR and PJI was discovered, improving the IMEP by 5% and reducing the ISFC by 4.9% compared to the baseline. The PJI can shorten the combustion duration and improve the combustion stability because of the high kinetic energy jet and high turbulence flame. In addition, a novel two-stage heat release process which includes the unusual first-stage low-temperature heat release was discovered. Overall, EGR dominates the knock suppression, and PJI contributes to the combustion rate improvement.

Effect of hydrocarbon condensation on fouling and heat exchange efficiency in EGR cooler

Based on the self-established experiment and test platform and evaluation system, we studied the effects of hydrocarbon condensation on mass, hydrophilicity, color, and micromorphology of fouling and heat transfer efficiency in exhaust gas recirculation (EGR) coolers. The results show that hydrocarbon condensation converts low mass, hydrophobic, solid black, and fluffy scales into larger mass, hydrophilic, white and yellow, and compact scales of fouling. When Total Hydrocarbons (THC) are 590 ppmC, 411 ppmC, and 359 ppmC, the temperatures to the demarcation points of the rapid condensation enhancement and stability stage are 100 °C, 120 °C, and 135 °C, respectively. However, when the temperature exceeds 160 °C, hydrocarbons' condensation is substantially suppressed, and the heat exchange efficiency is the worst. Hydrocarbons condensation will increase the fouling density, improving the heat exchange efficiency; the highest heat exchange efficiency will be achieved when the condensation is stable. Hydrocarbon and particulate matter (PM) together determine the composition and structure of the fouling, which in turn determines the heat transfer efficiency.

Technical research on improving engine thermal efficiency

To aid in addressing global climate change and reduce automobile carbon emissions, the thermal efficiency of engines must be improved. By altering the structural design of an engine body (including the cylinder blocks and cylinder heads), a rapid combustion model with a compression ratio of 16 and a maximum tumble ratio of 3.94 was developed that improved the combustion speed. Through synergistic matching of cooled exhaust gas recirculation with lean burn technology, when the exhaust gas recirculation rate was 20% and the air-fuel ratio was 20, the effect of reducing NOx emission and knocking reached the optimum level. When an electronic water pump and dual thermostat technology were used, the temperature change at the combustion chamber wall of each cylinder was <3%, and when the engine ran at speeds of up to 2400 r/min and torque of up to 60 Nm, the coolant temperature could reach 105°C, thereby reducing both heat dissipation and friction losses. The maximum thermal efficiency of the new DHE16GDI engine developed based on the DAM16N engine model increased by approximately 4.4%, and the contents of the major pollutants NOx and CO and the total hydrocarbon content were reduced by approximately 43.1%, 34.8% and 8.3%, respectively.

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.

Numerical and experimental investigation of heavy-duty EGR cooler thermal performance

Exhaust Gas Recirculation (EGR) cooler is a crucial automotive heat exchanger that decreases the temperature of the recirculated exhaust gas fed to engine cylinders to reduce the NOx emissions. The high temperature gradients in the heat exchanger and its varying hot and cold operation makes it necessary to accurately determine the temperature distribution of the EGR cooler to evaluate vulnerability to various thermal failures. This study demonstrates a thermal assessment of EGR cooler using detailed CFD simulation methods developed and validated with comprehensive experimental design and measurements. The experimental data including detailed boiling detection is collected on a gas burner test bench for eight different working conditions of the EGR cooler with conditioned cooling with an average of 3% repeatability error. The detailed and highly resolved CFD model which includes the heat transfer enhancing features of the heat exchanger is validated at these working conditions and resulted in an average of 1% difference in temperature and maximum 5% difference in pressure drop values. The developed detailed CFD methodology reduces the amount of EGR performance and boiling tests significantly and enables assessment of the EGR cooler at engine assembly conditions which affects the hot gas distribution and resulting temperature gradients. Furthermore, the CFD methodology and its correlation with the experiment proved that a coupled CFD methodology can be used instead of the development tests for analytical sign-off purposes as long as the detailed heat exchanger design is included in the CFD model. • A detailed CFD model with all geometrical features was built as essential alternative to EGR cooler tests. • The detailed CFD model does not require any additional numerical approach for pressure drop or heat transfer. • Acoustic Emission sensor was utilized for the more precise assessment of boiling level. • CFD predictions were validated with experimental data by using acoustic emission sensor measurements. • Predicted temperature and pressure change were validated by experimental results with high accuracy.

Effect of Hydrocarbon Concentration on Particulate Deposition and Microstructure of the Deposit in Exhaust Gas Recirculation Cooler

Effect of Hydrocarbon Concentration on Particulate Deposition and Microstructure of the Deposit in Exhaust Gas Recirculation Cooler

48V EGR Pump System Development and Fuel Benefit Evaluation

Upcoming global emission regulations include considerable reduction in emission of nitrogen oxides (NOx) and greenhouse gases (GHGs) for vehicles with heavy-duty (HD) diesel application. These regulations will phase between 2024 and 2030 in the United States and the European Union. CARB Regulations include up to 90% reduction in NOx levels along with ∼25% reduction in CO2. One of the primary technologies used to reduce engine out NOx emission is the use of cooled exhaust gas recirculation (EGR). Research studies carried out across multiple domains by engine/vehicle original equipment manufacturers (OEMs) and others have identified air handling as one of the technologies to help meet next-generation regulations (Joshi, 2020; Dreisbach et al. 2021). This includes more efficient turbomachinery which helps improve engine efficiency and thus reduce GHGs. This has an adverse effect on driving EGR which affects engine out NOx. In this study, the development and performance impact of the EGR pump is investigated, which allows improved engine fuel efficiency without the corresponding penalty to engine out NOx. Computational fluid dynamics (CFD) is used to optimize the EGR pump design, which leads to reduction in fluid-borne noise of the pump, which is then evaluated for fuel benefits using a calibrated GT-POWER engine model.

Improvement of an exhaust gas recirculation cooler using discrete ribbed and perforated louvered strip vortex generator

This CFD study represents the improvement of cooling performance of Exhaust Gas Recirculation (EGR) by adding newly designed vortex generators inside the engine space of an automobile. The simulation of EGR cooler model was validated comparing with the available data and a reasonable agreement has been observed. A total of twelve different shapes of proposed wing-type vortex generators were numerically investigated to compare the cooling effect of a single VG in the duct. For all the studies, a forward inclination angle of 135° was considered as it is the best inclination angle to get the highest vorticity. Among all, it was found that kite type wing VG provides the best cooling effect. Later, the VGs were arranged in several array configurations with 5 different pitch distances to find the best pitch distance. With the 3mm pitch distance in an array, among twelve different VGs, Gothic type wing VG showed the best performance in terms of cooling performance and 11% cooling improvement rather than a single placed VG was determined. Vorticity and cooling effect were also compared by changing the inlet velocity of the exhaust gas. A comparative study was carried out to highlight that Gothic type wing VG performed best in lowering the outlet temperature. Gothic type VG is 12% and 4% more effective than the discrete ribbed and perforated louvered strip VGs respectively.

Influence of the fin to baffle distance on temperature, stress distribution and fatigue life of a cooled Exhaust Gas Recirculation system

Environmental concerns and the decline of Earth's natural resources are among the most prominent issues in today's society, leading to stringent policies regarding commercial vehicle emission products, such as nitrogen oxides (NOx) and carbon dioxide (CO2) emission levels. Aiming to reduce their environmental footprint, car manufacturers have chosen to integrate the Exhaust Gas recirculation (EGR) cooler technology into their vehicles decades ago, as this is a resourceful and effective technique for reducing NOx emissions. This technique is still applied in hybrid cars and is foreseen to be used in future moved vehicles, proving its contemporaneity, adequacy, and relevancy. Nevertheless, this technology requires that different factors be considered during the design phase, such as high gas temperatures, mechanical vibrations, and other loads, as these may affect overall performance and durability. Therefore, this study aims to assess the impact of varying the inner fińs length on the applied loads and how it may affect the EGR Cooler efficiency. To this end, Computer-Aided Engineering (CAE) tools using numerical methods, such as Finite Element Method (FEM) and Finite Volume Method (FVM), were applied. This investigation revealed that varying the inner fi length affects each of the applied loads differently, and increasing the length was shown to improve the heat transfer process of the EGR; however, it also increases the thermally induced stresses at the hottest region, diminishing the fatigue resistance of the component. The advantages and disadvantages of several configurations understudy were addressed in the work carried out.

Experimental analysis of the EGR rate and temperature impact on combustion and emissions characteristics in a heavy-duty NG engine

Cooled exhaust gas recirculation (EGR) is one of the most effective methods in spark ignition (SI) natural gas (NG) engines to improve the fuel economy and reduce the nitrogen oxides (NOx) emissions. The objective of this study is to anatomize the EGR rate and temperature impact on the combustion and emissions characteristics by the experiments in a heavy duty (HD) NG engine under various spark timings. The results showed that the increasing EGR rate reduced the burning rate and extended the combustion duration. Spark timing should be optimized to copy with the retarded combustion phase. Under high EGR rate, the combustion performance could be optimized with the increase of EGR temperature. There exists a proper EGR rate in real application for the fuel consumption of the engine. Higher EGR temperature plays positive effects on fuel economy, owing to the higher combustion efficiency and lower throttle losses, and the decreasing amplitude of BSFC is 1.25% between the EGR temperatures of 333 K and 373 K. With the increase of EGR rate from 5% to 25%, NOx emissions are reduced by up to 76%, the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions are increased by 34.6% and 12.9% respectively. Higher EGR temperature could alleviate the increase of HC and CO emissions, but sacrificing the NOx emissions, and the increase extent of NOx emissions is 35.3%. The results provide an insight into the quantitative comparisons of EGR rate and temperature on the NG engine performance and emissions, which promotes the utilizing EGR strategy in SI NG engines.

A methodology to study the interaction between variable valve actuation and exhaust gas recirculation systems for spark-ignition engines from combustion perspective

Several technologies are being put in place to improve the efficiency of spark-ignition engines. Two of these technologies are variable valve actuation and exhaust gas recirculation, both of which can have an impact on combustion according to the literature. However, this impact is due to variations in both the in-cylinder temperature and composition, linked to the different characteristics of internal and external residuals, which are difficult to decouple. In this paper, a methodology aimed at improving the understanding of the temperature and composition effects on combustion parameters is developed. Internal residuals are controlled through a variable valve timing actuation, and evaluated thanks to a previously validated one-dimensional engine model, while the externally-circulated exhaust are extracted after the catalyst through a low-pressure system. Combustion is analyzed based on in-cylinder pressure acquisition and emissions measurement at the engine-out, before the aftertreatment module. Different combinations of internal and external residuals are tested with different strategies from the point of view of the spark actuation: first, a constant spark timing for each value of total residuals; then, a spark timing adapted to maintain the same temperature inside the cylinder at the combustion start, and finally an optimized calibration from the point of view of the engine efficiency. Results show a general trend to enhance combustion timings as higher share of internal residuals are introduced for the same amount of total residuals. It can be seen how this trend comes mostly from the temperature effect at low total residuals fraction, while composition effects are more significant as the amount of total residuals increase. Empirical correlations developed in the paper confirm the need to decouple internal and external residuals to have a proper estimation of the combustion timings. Despite the positive impact of internal residuals on the combustion duration, a small fuel consumption benefit is reached with cooled exhaust gas recirculation thanks to lower heat transfer losses. In terms of emissions, unburned hydrocarbon emissions tend to be reduced with increasing the share of internal residuals thanks to the higher combustion speed, while other gaseous emissions are mostly affected by the total residuals amount. Therefore, internal and external residuals could be combined to achieve the best trade-off between fuel consumption and hydrocarbon emissions.