Impact of Heat Exchanger Effectiveness and EGR on Energy and Emission Performance of a CI Engine

Abstract

This study explores the impact of intake air cooling intensity, defined by heat exchanger effectiveness (HEE) and exhaust gas recirculation (EGR), on the energy and environmental performance of a turbocharged compression ignition (CI) engine. Experimental investigations were conducted on a 1.9-litre CI engine operating at 2000 rpm under three brake mean effective pressure (BMEP) conditions (0.2, 0.4, and 0.6 MPa), which correspond to part-load engine operation. HEE was varied at 0%, 50%, and 100%, in both EGR-on and EGR-off modes. Additional numerical simulations were carried out using AVL BOOST software to analyze combustion dynamics, including engine operating cycle modeling to validate the accuracy of the combustion analysis. The results demonstrate that increasing HEE significantly improves cylinder filling and excess air ratio, leading to enhanced combustion efficiency and lower in-cylinder temperatures. This, in turn, reduces specific NO_x emissions by approximately 40% with EGR and approximately 60% without EGR; however, under EGR-on conditions, the reduced combustion intensity leads to increased smoke and unburned hydrocarbon emissions—particularly at high cooling intensities. This effect is primarily associated with the engine control unit’s (ECU) limitations on intake air mass flow to maintain the target EGR ratio. Integrated control of HEE and EGR systems improves engine performance and reduces emissions across varying conditions, while highlighting trade-offs that inform the refinement of air management strategies.

1. Introduction

The transport sector plays a key role in global energy consumption and environmental pollution [1,2]. According to the International Energy Agency (IEA), the transport sector accounts for approximately 30% of total final energy consumption and is responsible for nearly 25% of global carbon dioxide (CO₂) emissions from fuel combustion [3]. This sector’s continued reliance on internal combustion engines (ICE), particularly diesel engines, makes it critical to improve energy efficiency and reduce emissions [4,5,6,7,8].

In the context of growing urbanization and logistics demands, the energy intensity of transportation is expected to remain high. Therefore, improving engine performance and reducing pollutant output are not only technical challenges but also essential components of climate change mitigation policies [9,10].

CI engines—primarily diesel engines—remain dominant in heavy-duty transportation due to their superior thermal efficiency, favorable torque characteristics, and fuel economy compared to spark-ignition (SI) engines [11]. Diesel engines operate at higher compression ratios, which enhances combustion efficiency but also leads to increased nitrogen oxides (NO_x) and particulate matter emissions—key pollutants regulated under Euro VI and EPA standards [12,13,14].

Despite growing interest in electrification and alternative fuels, diesel engines are expected to maintain a significant presence in commercial vehicles, agricultural machinery, and marine applications over the coming decades [15,16,17]. Consequently, incremental improvements in CI engine technologies are highly relevant [18,19,20].

The dual challenge of maintaining or improving efficiency while reducing emissions has led to numerous innovations in engine design, fuel injection systems, aftertreatment technologies, and air management strategies [21,22,23]. Among these, intake air intercooling plays a crucial role in optimizing combustion quality, enhancing power output, and lowering NO_x emissions by reducing peak combustion temperatures [24,25]. Previous studies have shown that effective intercooling can increase cylinder filling by up to 10–15%, while simultaneously reducing exhaust gas temperature and the load on the engine’s cooling system [26].

While intake air intercooling via a heat exchanger (intercooler) plays a critical role in enhancing engine performance, it also involves engineering trade-offs—particularly between HEE and pressure losses, as more intensive cooling typically requires a more complex design, which increases airflow resistance and pressure drop between the turbocharger and the intake manifold. [27,28].

More efficient heat exchangers typically incorporate a larger heat transfer surface area, an extended airflow path, or increased internal turbulence to enhance heat exchange performance; however, these improvements are often accompanied by increased pressure losses across the system [29,30,31,32].

Such structural enhancements can lower intake air temperature by 20–40 °C, but they may induce pressure drops in the range of 10–20 kPa, depending on engine load and speed [26]. This pressure loss negatively affects volumetric efficiency, which in turn reduces combustion performance and instantaneous power output. Furthermore, the increased resistance can elevate turbocharger workload and contribute to response delays (commonly known as turbo lag) [33,34].

To address these challenges, engineers aim to find an optimal balance between heat transfer efficiency and flow resistance. Contemporary development practices increasingly rely on computational fluid dynamics (CFD) to evaluate how various heat exchanger geometries influence both heat exchange and pressure losses [35,36,37,38].

Another emerging trend is the application of water-cooled heat exchangers, which enable more effective heat dissipation in compact spaces [31] but require additional components such as water pumps, heat exchangers, and control systems. These are particularly relevant in high-power-density applications like motorsports or hybrid powertrains [39,40].

However, the trade-offs involved in thermal management strategies—such as increased cooling load, turbocharger efficiency, and fuel-air mixing dynamics—necessitate further investigation into how varying HEE impacts overall engine behavior under realistic operational conditions, both from an energetic and ecological standpoint.

The objective of this study is to evaluate the combined effect of intake air heat exchanger effectiveness and EGR operation on the combustion process and performance parameters of a CI engine under part-load conditions. The analysis integrates combustion modeling carried out using AVL BOOST software (Version: v2021.2.0.0.0) simulation with detailed experimental data. The aim is to identify potential heat exchanger control strategies that simultaneously enhance combustion efficiency and reduce NO_x and soot emissions. The novelty of this study lies in the combined assessment of heat exchanger effectiveness and EGR performance under part-load conditions using a comprehensive experimental—simulation approach, which enables a more holistic improvement in energy efficiency and emission reduction.

2. Materials and Methods

2.1. Methodology of Experimental Research

The experimental investigation was carried out using a four-cylinder turbocharged direct injection CI engine 1Z with a displacement of 1896 cm³ (Volkswagen AG, Wolfsburg, Germany). A schematic diagram of the experimental test bench is shown in Figure 1. The engine was equipped with a BOSCH VP37 (Robert Bosch Ltd., Gerlingen, Germany) rotary distributor-type high-pressure fuel injection pump. Fuel injection timing (start of injection) is electronically controlled by the engine’s Electronic Control Unit (ECU) through management of the high-pressure pump. Fuel is delivered via a single-injection process using a two-stage injector fitted with a dual-spring needle assembly: the primary spring enables a low-pressure pilot injection, while the secondary spring allows full needle lift for the main injection at higher pressure. This staged injection strategy improves combustion efficiency and reduces emissions. The VW 1.9 CI engine employs open-loop ECU control for both the EGR and turbocharger systems. EGR flow is regulated based on predefined maps and intake air mass estimation, without closed-loop feedback. Boost control relies on a wastegate actuator, which limits the maximum turbocharger pressure according to ECU commands, also without real-time correction. Detailed technical engine specifications are provided in

Table 1. Engine technical specifications.

Parameter	Value	Units
Number of cylinders	4	-
Bore	79.5	mm
Stroke	95.5	mm
Displacement	1896	cm3
Compression ratio	19.5	-
Rated power	66	kW
Rated speed	4000	rpm
Peak torque	180	Nm
Peak torque speed	2000–2500	rpm

. The ICE was cooled using a common coolant circulation system. Experimental investigations were conducted using a KI-5543 electric engine test bench (Gosniti Rosselhozakademii, Moscow, Russia), equipped with a lever-arm dynamometer for measuring engine torque, with measurement uncertainty ±1.23 Nm.

Fuel consumption was measured using SK-5000 electronic scales (A&D Company, Limited, Tokyo, Japan) and a stopwatch, with a relative measurement error of 0.5%. The intake air mass flow rate (kg/h) was determined using a BOSCH HFM 5 air mass meter (Robert Bosch Ltd., Abstatt, Germany) with an accuracy of 2%. The boost pressure generated by the turbocharger was measured using a Delta OHM HD 2304.0 manometer (Senseca Italy Srl, Caselle, Italy), with a measurement uncertainty of ±0.002 bar. In-cylinder pressure was also recorded during each test using an AVL GH13P pressure sensor (AVL DiTEST GmbH, Graz, Austria) installed in place of the glow plug.

The exhaust gas components were measured using the AVL DiCom 4000 gas analyzer (AVL DiTEST GmbH, Graz, Austria), the technical specifications of which are presented in

Table 2. Technical specifications of the AVL DiCom 4000 ¹ exhaust gas analyzer.

Parameter	Measurement Limits	Resolution	Units
CO	0–10	0.01	% Vol.
CO2	0–20	0.1	% Vol.
O2	0–25	0.01	% Vol.
HC	0–20,000	1	ppm Vol.
NOx	0–5000	1	ppm Vol.
Lambda, λ	0–9.999	0.001	-
Absorption (K-Value)	0–99.99	0.01	m−1
Opacity	0–100	0.1	%

¹ The device complies with the following standards and approvals: OIML R99 Class I, ISO 3930 [41], ECE R24 [42], and IEC 801-1/2/3/4.

.

A liquid-cooled heat exchanger from a Subaru Impreza 2.0T vehicle was installed in the engine air intake system (Figure 2). The heat exchanger is equipped with air inlet (1) and outlet (2) ports, as well as coolant inlet and outlet ports (3). A screw cap (4) is used for filling the coolant and for removing air locks from the heat exchanger’s cooling circuit. During the experimental investigation, the HEE of the heat exchanger was varied by adjusting the flow rate of coolant (water) through the heat exchanger. Type-K thermocouples were used to measure the coolant temperature before the heat exchanger (T₁, °C) and after the heat exchanger (T₂, °C), as well as the air temperature entering the heat exchanger from the turbocharger (T₃, °C) and the air temperature exiting the heat exchanger toward the engine (T₄, °C) (see Figure 1). The temperature measurement accuracy was ±1.5 °C.

The experiments were carried out at an engine speed of 2000 rpm, which allows the broadest usable torque range to be achieved. The engine was subjected to loads of 30 Nm, 60 Nm, and 90 Nm, corresponding to BMEP of 0.2 MPa, 0.4 MPa, and 0.6 MPa, respectively. These values represent low to medium engine load conditions (6.3 kW, 12.6 kW, and 18.9 kW), which are characteristic of typical urban transient driving conditions.

Test No. 1—HEE 0%. Prior to the test, the coolant flow through the heat exchanger was completely shut off by closing the fluid valves in order to prevent the cooling of the compressed and heated air flowing from the turbocharger into the engine. Type-K thermocouples, installed before and after the heat exchanger, recorded the temperature of the incoming and outgoing compressed air. Data were collected with the exhaust gas recirculation system both activated and deactivated. The EGR ratio was approximately estimated based on the observed reduction in intake air mass. It was determined that during EGR operation, the recirculated exhaust gas fraction decreased with increasing engine load, reaching approximate values of 0.28, 0.22, and 0.19, respectively. The EGR system was deactivated by disconnecting the electronic control connector from the vacuum solenoid controlling the EGR valve. Under these conditions, the ECU did not register any diagnostic fault codes (DTCs) related to the engine control system.

During the experimental investigation, the ROSS-TECH VCDS diagnostic tool (Ross-Tech, LLC, Montgomery County, PA, USA) was used to retrieve real-time operational data from the engine’s ECU via the OBDII interface, including the start of injection (SOI) timing. Deactivation of the EGR system resulted in a minor shift in SOI timing (up to 0.5° crank angle), which had no significant impact on the engine’s energy efficiency or exhaust emission parameters.

Test No. 2—HEE 50%. Before the test, the coolant valves were opened to 50%, allowing partial cooling of the airflow through the heat exchanger. Type-K thermocouples measured the temperature of the coolant entering and exiting the heat exchanger, as well as the air temperature before and after the heat exchanger. Measurements were conducted under the same engine operating conditions as in Test No. 1.

Test No. 3—HEE 100%. Before the test, the coolant flow through the heat exchanger was fully opened (100%) to enable maximum cooling of the air entering the engine. The engine was tested under the same conditions as in Tests No. 1 and No. 2.

Each test was repeated three times, and the results presented are the arithmetic mean values. Measurements of the 1Z engine parameters were carried out in a forced-ventilation laboratory environment, ensuring a constant intake air temperature. All tests were performed on the same day, and atmospheric pressure fluctuations were minimal and did not have a significant effect on the experimental results.

2.2. Methodology for Numerical Modeling of the Combustion Process

To perform a more detailed analysis of changes in the engine’s energy and environmental performance with varying HEE levels, numerical modeling of the combustion process is required. This modeling consists of two main components: combustion analysis and the synthesis of engine operating cycle parameters. A numerical model of the investigated engine was developed using AVL BOOST software, with its visual representation shown in Figure 3.

The combustion process analysis was conducted using the AVL BURN subprogram. Experimental data, including in-cylinder pressure throughout the cycle, fuel and air consumption, engine geometric parameters, compression ratio, fuel properties, and others, were input into the program. During the numerical analysis of the combustion process, key in-cylinder combustion parameters are determined, including the start of combustion (φ_p), combustion duration (φ_z), combustion intensity shape parameter (m), pressure rise, heat release rate, temperature rise, and temperature during the combustion process. The AVL BOOST software calculated the expected in-cylinder pressure, BMEP, and other relevant parameters. These results were subsequently used to validate the combustion analysis model.

The mathematical model of the combustion process for the investigated engine is developed using the single-zone combustion model, as it is suited to describing the combustion kinematics of a CI engine. Heat release is determined using the Vibe function [43]:

$${\displaystyle \frac{dx}{d\left(\frac{\phi }{{\phi }_z}\right)}}=6.908\left(m+1\right){\left({\displaystyle \frac{\phi }{{\phi }_z}}\right)}^m{e}^{-6.908{\left({\displaystyle \frac{\phi }{{\phi }_z}}\right)}^{m+1}}.$$

(1)

Based on the heat release rate data, the burned fuel mass fraction, the in-cylinder temperature rise, and the pressure rise were determined. These data were used to perform a more detailed analysis of the combustion process and to explain the changes in engine emission parameters (smoke opacity, CO, CO₂, HC, O₂, NO_x) and energy performance indicators resulting from variations in HEE. The results of the numerical analysis are presented in the form of diagrams showing combustion characteristics at an engine speed of 2000 rpm and a load of BMEP = 0.4 MPa, under both EGR on and off conditions.

2.3. Exhaust Gas Analysis Methodology

The air excess air ratio—lambda (λ) is an important parameter for evaluating the combustion process as well as the engine’s energy and environmental performance. Lambda is calculated based on experimental data and fuel properties [11]:

$$\lambda ={\displaystyle \frac{B_{air}}{B_{fuel}·l_0}};$$

(2)

where: B_air—engine intake air mass, kg/h; B_fuel—engine fuel consumption, kg/h; l_o—air mass required for stoichiometric combustion of 1 kg of fuel.

By evaluating the volumetric concentration of individual exhaust gas components and their molar masses, the specific emission of each pollutant was calculated:

$$s{E}_x={\displaystyle \frac{C_x·M_x·m_{ex}}{{1000·M}_{ex}·P_B}},\mathrm{g}/\mathrm{k}\mathrm{W}\mathrm{h};$$

(3)

where: C_x—pollutant component concentration, ppm; M_x—molar mass of the pollutant component, g/mol; m_ex—exhaust gas mass flow rate, kg/h; M_ex—molar mass of the exhaust gases, g/mol; P_B—engine brake power, kW.

Specific pollutant emissions allow for a more objective evaluation of the pollution generated by the engine.

3. Results and Discussion

3.1. Parameters of the Air–Exhaust Gas Mixture Supplied to the Engine Cylinder

With increasing engine load, the ECU increases the amount of fuel injected, which leads to a rise in exhaust gas flow and, consequently, an increase in the turbocharger boost pressure p_boost (Figure 4). However, when EGR is on, p_boost decreases by 6–8% compared to the EGR-off condition due to a substantial reduction in the mass of fresh air entering the cylinder. Increasing the HEE had no significant effect on p_boost under either EGR-on or EGR-off conditions. Since the turbocharger boost pressure is regulated by an open-loop ECU control strategy, p_boost is primarily determined by the energy content of the exhaust gases. While a higher HEE slightly increases the intake air mass, potentially raising exhaust energy, on the other hand, the improved combustion efficiency and faster burn rate resulting from the additional air tend to reduce it. Consequently, variations in HEE have no significant effect on exhaust energy and, therefore, on turbocharger boost pressure.

The intake air temperature drop across the heat exchanger (ΔT_cool) was significantly influenced by the intensity of HEE, engine load, and EGR activation conditions. At a load of BMEP = 0.4 MPa, ΔT_cool reached 21 °C and 19 °C under EGR-off and EGR-on conditions, respectively, with HEE at 50%. At a higher load of BMEP = 0.6 MPa, ΔT_cool reached 29 °C and 26 °C (Figure 5). When HEE was increased to 100% at BMEP = 0.6 MPa, ΔT_cool reached 52 °C and 48 °C under EGR-off and EGR-on conditions, respectively.

The intake manifold temperature (T_in), representing the temperature of the intake air and recirculated exhaust gas mixture, is approximately 40 °C to 50 °C higher under EGR-on conditions compared to EGR-off conditions (Figure 6). The temperature difference of the mixture entering the cylinder between EGR-on and EGR-off is greater at lower engine loads, as the EGR ratio is higher under low-load conditions. With a 100% increase in heat exchanger effectiveness, under EGR-off conditions and at engine loads of BMEP = 0.2 MPa, 0.4 MPa, and 0.6 MPa, T_in decreases by approximately 9 °C, 13 °C, and 11 °C, respectively. Under EGR-on conditions, T_in decreases by approximately 12 °C, 13 °C, and 12 °C, respectively. This temperature reduction increases the air density and improves the engine’s volumetric efficiency.

As the engine load increases, the intake air mass (B_air) rises (Figure 7) due to the increasing boost pressure (Figure 4). When the EGR system is activated (with EGR ratios of approximately 0.28, 0.22, and 0.19), B_air decreases by about 27%, 20%, and 18%, respectively, as part of the intake charge is replaced by recirculated exhaust gases. This substitution also raises the intake mixture temperature and reduces its density. With increasing HEE intensity, B_air shows an upward trend of up to 4.0% under EGR-off conditions, primarily due to reduced intake air temperature, which increases air density. Under EGR-on conditions, the rise in B_air is limited to approximately 1.5%, as the intake air is more strongly heated, reducing its density. The engine ECU operates with an open-loop EGR control strategy but uses mass airflow sensor data to estimate the EGR rate: an increase in measured intake airflow indicates reduced EGR flow and a corresponding reduction in EGR valve opening, which the control algorithm attempts to return to within predefined target limits.

In this study, it is relevant to analyze the measured oxygen concentration in the exhaust gases and the calculated excess air ratio (λ). As the engine load increases, a greater quantity of fuel is injected per cycle, resulting in higher oxygen consumption during combustion and a significant reduction in the oxygen concentration in the exhaust gases; the excess air ratio also decreases due to the greater amount of air required for complete fuel combustion. In the case of EGR on, as the engine load increases (BMEP = 0.2 MPa, 0.4 MPa, and 0.6 MPa), the O₂ concentration in the exhaust gases relatively decreases by approximately 17%, 18%, and 22% respectively, compared to EGR off (Figure 8a). The calculated λ also decreases due to EGR on, by approximately 29%, 24%, and 22%, respectively (Figure 8b). At lower loads, λ decreases more sharply than the observed O₂ concentration in the exhaust gases, as under lean mixture conditions, the oxygen concentration is less sensitive to EGR-induced dilution. In the case of EGR off, increasing the HEE intensity up to 100% resulted in a more pronounced effect, and at higher loads, the O₂ concentration was relatively increased by up to 3.6%, while λ rose by up to 5%. In the case of EGR on, the O₂ concentration increased by up to 1.2%, and λ increased by up to 3%. This leads to improved combustion efficiency. However, in the case of EGR off, increasing HEE intensity leads to a rise in λ across the entire load range, while under EGR on conditions, HEE intensification increases λ at lower engine loads, but the positive effect of HEE diminishes as the load increases.

As the engine load increases, the exhaust gas temperature (T_ex) rises due to the greater amount of energy released from the combustion of a larger mass of injected fuel. When the EGR valve is opened, T_ex increases (Figure 9a) as a result of the slower combustion process caused by the reduced oxygen concentration and the presence of CO₂ in the recirculated exhaust gases within the cylinder. As the HEE increases to 50% and 100%, the T_ex decreases by up to 22 °C and 20 °C, respectively, under both EGR off and EGR on conditions. A greater reduction in T_ex is observed at higher loads, as the additional air supply during combustion becomes more significant due to the increased fuel quantity. This reduction induces an increase in combustion rate and thermal efficiency, as less energy is lost through the exhaust system.

The temperature of the recirculated exhaust gases (T_EGR) correlates with T_ex but is 30 °C to 45 °C lower (Figure 9b) due to the low-intensity cooler integrated into the EGR system. Even a modest decrease in T_EGR can significantly lower the overall intake temperature and thereby improving cylinder filling. Enhanced air charging improves the combustion process, leading to reduced carbon monoxide (CO) and HC emissions, as well as lower smoke levels. Introducing a cooler mixture into the cylinder lowers the compression and combustion temperatures, resulting in reduced NO_x emissions. Improved combustion also contributes to lower fuel consumption and, consequently, reduced CO₂ emissions.

3.2. Numerical Analysis of the Combustion Process

To validate the reliability of the numerical analysis results of the combustion process, an engine operating cycle simulation was carried out using AVL BOOST software. The simulation was based on combustion analysis data (start of combustion, combustion duration, and combustion intensity shape parameter), as well as experimentally determined fuel consumption, turbocharger pressure, and other relevant parameters. Figure 10a,b presents the calculated in-cylinder pressure results, which are validated against the experimentally measured in-cylinder pressure under engine operating conditions at a load of BMEP = 0.4 MPa, with HEE = 0% and for both EGR off and on cases. Analysis of the in-cylinder pressure results shows that the variation caused by intake air cooling and EGR conditions becomes evident during the compression and combustion phases. Higher HEE under EGR-off conditions increases compression pressure due to the higher density of cooled intake air, which improves volumetric efficiency. However, when EGR is on, the compression pressure slightly decreases with increasing HEE, which is attributed to a combination of factors: (1) the engine ECU monitors the intake air mass flow and responds to its increase by raising the EGR rate accordingly and (2) the recirculation of high-temperature exhaust gases into the cylinder limits the effect of intake air cooling. According to the engine ECU data, fuel injection starts at 2° crank angle (CA) before top dead center (TDC), and ignition occurs after a certain delay—approximately 4–5° CA after TDC.

When the combustible mixture ignites under EGR-off conditions, the combustion process generates significantly higher pressure. This is driven by two main factors: higher pressure at the end of the compression stroke and a lower intake temperature, which prolongs the ignition delay and leads to increased heat release during the premixed combustion phase [11]. Based on the curves obtained during the experiment, it was determined that the maximum in-cylinder combustion pressure under EGR-off and 100% cooling conditions reached 7.5 MPa, which is ~2.7% higher compared to HEE 0% (Figure 10a), and ~10.6% higher compared to the case with EGR-on and HEE 100% (Figure 10b). The resulting elevated pressure imposes additional mechanical loads on the crank mechanism, making it essential for engine components to be resistant to withstand increased mechanical stresses.

Under EGR-off conditions and at HEE 0%, the maximum pressure rise during combustion in the cylinder reaches approximately 0.2 MPa/°CA (Figure 11a). Increasing the cooling intensity to HEE 50% and HEE 100% results in a pressure rise increase of approximately 12% and 44%, respectively. This is caused by the higher air density and the extended ignition delay due to the lower intake air temperature. When EGR is on, the maximum pressure rise decreases by approximately 20% due to slower combustion caused by the recirculation of exhaust gases into the cylinder (Figure 11b). Increasing the heat exchanger effectiveness has little impact on the maximum pressure rise, as the engine ECU ratio is prescribed by the control algorithm, thereby preventing a significant increase in intake air mass.

The results of heat release rate (HRR) were calculated using the BURN subprogram of AVL BOOST, based on in-cylinder pressure data as well as experimentally measured fuel and air consumption and other relevant parameters. It must be confirmed that, in all analyzed cases, combustion took place under consistent engine load, speed, and start of fuel injection conditions. A single-injection strategy was applied; however, the injector is equipped with a dual-spring needle design. This specific needle design contributes to the appearance of two distinct HRR peaks, corresponding to the premixed and controlled combustion phases. As shown in the graphs (Figure 12a), under EGR-off conditions, the HRR during the premixed combustion phase (at constant volume) reaches 31.5 J/°CA without heat exchanger cooling. When HEE is increased to 50% and 100%, the HRR rises by approximately 11% and 28%, respectively. The more intensively the intake air is cooled, the longer the ignition delay period becomes, allowing more unburned fuel to accumulate in the cylinder, which then burns at the onset of intense combustion together with the subsequently injected fuel. The second HRR peak is generated during the controlled combustion phase (at constant pressure) and is responsible for producing the required torque. Due to the increased HRR during the premixed combustion phase when HEE is raised, the heat release during the controlled combustion phase slightly decreases. With EGR activated, the HRR peak during the premixed combustion phase decreases by approximately 7% due to reduced combustion intensity. As the HEE increases to 100%, the HRR approaches the level observed under EGR-off conditions. The HRR peak during the controlled combustion phase also decreases by 9%, indicating a more intense later combustion phase (Figure 12b).

In the case of EGR off and HEE 0%, the temperature at the end of compression reaches 1017 K, while for HEE 50% and HEE 100% it decreases by approximately 28 K and 52 K, respectively, as cooling intensity increases (Figure 13a). This correlates with the maximum combustion temperature, which reaches 1489 K at HEE 0% and decreases by approximately 4% and 6%, respectively. When EGR on and HEE 0%, the compressed air temperature reaches approximately 933 K (Figure 13b) due to a significant increase in intake gas temperature and a corresponding decrease in its density. Increasing HEE to 100% reduces the end-of-compression temperature to 823 K as a result of lower intake air temperature.

The maximum combustion temperature under EGR on and HEE 0% conditions reaches 1349 K, which is approximately 16% lower compared to the EGR off case. With HEE increased to 100%, the maximum combustion temperature further decreases to 1229 K—representing an approximate 18% reduction compared to the EGR off and HEE 0% case. This variation in combustion temperature is primarily determined by the initial intake air temperature and the reduced combustion intensity caused by the suppressive effect of the EGR gases. Lower combustion temperatures result in reduced NO_x emissions; however, they may lead to increased smoke emissions [44].

The maximum temperature rise under EGR-off and HEE 0% conditions reaches 50 K/°CA (Figure 14a). However, when air cooling is intensified to HEE 50% and HEE 100%, the temperature rise starts later due to a lower compression temperature and a longer ignition delay, but the temperature increases more steeply—by approximately 10% and 24%, respectively.

When EGR is on and HEE is 0%, the maximum temperature rise decreases by approximately 8% compared to the EGR-off case due to slower combustion (Figure 14b). However, as HEE intensity increases, although the ignition delay becomes longer, the temperature rise remains nearly unchanged because the engine ECU maintains the predetermined EGR ratio.

3.3. Investigation of Engine Energy Indicators

The conducted studies have shown that the EGR system has a negative impact on both brake-specific fuel consumption (BSFC) and brake thermal efficiency (BTE). The use of EGR increases BSFC by 2.0–2.8% (Figure 15a), as the recirculated CO₂ and reduced excess air ratio lower the combustion temperature (Figure 13) and quality, thereby decreasing the thermal efficiency of the engine. Although combustion slows down, leading to a rise in exhaust gas temperature (Figure 15a), which in turn results in greater heat energy losses. These findings indicate that the EGR system is an effective tool for reducing combustion temperature, but it does not contribute to lowering fuel consumption [45]. The increase in fuel consumption represents a trade-off between NO_x reduction and thermal efficiency. Improved cooling of the intake air has a positive effect, reducing BSFC by up to 2.5% when the EGR system is deactivated (from 299.8 g/kWh to 292.3 g/kWh at BMEP = 0.6 MPa). A comprehensive evaluation of the combined use of EGR and HEE has shown that, when EGR is active and 100% HEE is applied, fuel consumption remains largely unchanged compared to the case with EGR off and HEE at 0%.

In the absence of EGR, increasing HEE to 50% and 100% results in a greater intake air mass, which in turn reduces both the combustion temperature (Figure 7) and exhaust gas temperature (Figure 9b). This leads to lower heat losses and an improvement in BTE of up to 2.5% (from 0.338 to 0.347 at BMEP = 0.6 MPa) (Figure 15b). This indicates that the heat exchanger has a significant impact on engine performance, although potential trade-offs between thermal efficiency and pollutant emissions should be taken into account [46,47]. Higher engine loads (0.6 MPa) result in increased BTE and simultaneously reduced BSFC, particularly under more intensive HEE operation. This suggests that higher load is associated with greater energy demand, which requires more fuel, but also increases the mechanical efficiency of the engine, thereby enabling improved BTE [46,47]. However, when EGR is applied, BTE tends to decrease, although high-intensity HEE helps to maintain BTE at a nearly unchanged level. The research results indicate that optimal engine efficiency can be achieved by balancing engine load, EGR operation, and HEE intensity; however, the engine’s environmental performance indicators must also be taken into account.

3.4. Investigation of Engine Environmental Performance Indicators

As engine load increases, smoke emissions also rise due to a decreasing excess air ratio. In the case of EGR on, smoke emissions increase significantly, as the recirculated exhaust gases—characterized by low oxygen concentration but elevated CO₂ levels—further reduce the air excess, suppress combustion, and lower the combustion temperature. Within the BMEP range of 0.2 MPa, 0.4 MPa, and 0.6 MPa, the activation of EGR results in an increase in smoke levels by approximately 4.3-fold, 4.8-fold, and 6-fold, respectively (from 0.09 m⁻¹ to 0.39 m⁻¹, from 0.11 m⁻¹ to 0.53 m⁻¹, and from 0.13 m⁻¹ to 0.81 m⁻¹, respectively) (Figure 16). This indicates that within the tested range, the increase in smoke opacity is linear under EGR-off conditions, whereas it follows an exponential trend when EGR is active. In the case of EGR off, increasing the HEE intensity to 100% reduces smoke emissions by 15–18% due to an increase in λ. However, the effect is not particularly significant, as the combustion temperature is also reduced. In the case of EGR on, increasing HEE intensity up to 100% reduces smoke emissions by up to 14% (to 0.34 m⁻¹) at a load BMEP = 0.6 MPa; however, at 0.6 MPa, smoke emissions even increase by 9% (to 0.88 m⁻¹) compared to the HEE 0% case. This is caused by the reduction in intake mixture temperature, which lowers the combustion temperature, without an increase in excess air.

NO_x formation is directly dependent on the in-cylinder temperature and the excess air ratio. High temperatures are reached when the engine operates under higher load. However, it is important to note that this study analyses specific pollutant emissions [g/kWh], which depend not only on the volumetric concentration of pollutants, but also on the total emitted pollutant mass, the molar mass of the pollutants, and the engine brake power (Equation (3)).

When comparing the cases of EGR off and EGR on, the specific NO_x emission at BMEP levels of 0.2 MPa, 0.4 MPa, and 0.6 MPa decreased from 3.57 g/kWh to 1.95 g/kWh, from 4.40 g/kWh to 3.127 g/kWh, and from 3.94 g/kWh to 2.56 g/kWh, respectively—corresponding to reductions of approximately 45%, 30%, and 35%. During testing with EGR off and HEE increased to 100%, at an engine load of 0.6 MPa, the specific NO_x emission decreased from 3.94 g/kWh to 2.30 g/kWh, representing a relative reduction of around 40% (Figure 17). Under the same conditions but with EGR on, the specific NO_x emission decreased from 2.56 g/kWh to 0.917 g/kWh, a relative reduction of approximately 60%. Compared to the case with EGR off and no intake air cooling, at BMEP = 0.6 MPa, the overall NO_x reduction achieved with EGR on and 100% HEE reached approximately 76%. This reduction is attributed to the recirculation of a portion of exhaust gases into the cylinder, which displaces part of the intake air and lowers the combustion temperature. Furthermore, the intake of cooler air contributes additionally to the temperature reduction within the cylinder.

The concentration of hydrocarbons (HC) in the exhaust gases indicates incomplete combustion. Experimental results showed that as engine load increases, specific HC emissions decrease due to improved combustion quality at higher combustion temperatures; increasing engine load also reduces the calculated specific emission. In the case of EGR on, the specific HC emission decreased by approximately 12% (from 0.077 g/kWh to 0.068 g/kWh) under low load conditions (0.4 MPa), which is attributed to the increased temperature of the air–fuel mixture and improved fuel evaporation (Figure 18). However, at medium load (0.6 MPa), with EGR active, the specific HC emission increased by around 16% (from 0.048 g/kWh to 0.056 g/kWh), primarily due to the reduced combustion temperature and lower air quantity in the cylinder, both of which negatively affect the combustion process.

Increasing HEE intensity up to 100% at engine loads of 0.2 MPa and 0.4 MPa even increased the specific HC emission (by up to 18%), as it reduced the combustion temperature, while the additional air did not improve combustion due to the already high excess air ratio (above 2.4). A slight positive effect of HEE (up to 5%) in reducing HC emissions was observed only in the case of EGR on at a load of 0.6 MPa. Since the specific HC emission is relatively low due to the excess air, HC should not be considered a primary concern when developing the HEE and EGR control strategy.

As engine load increases, the CO₂ concentration (%) rises due to the greater amount of fuel in the combustible mixture. However, as engine power increases, the specific CO₂ emission (g/kWh) decreases (Figure 19). The EGR system significantly increases the volumetric concentration of CO₂ in the exhaust gases (by 30–40%), as part of the intake air is replaced by exhaust gases, which have a high CO₂ content. Moreover, EGR slightly increases brake-specific fuel consumption (Figure 15a). On the other hand, in the case of EGR off, the intake air mass is reduced by approximately 20–28% (Figure 7), which significantly lowers the absolute mass emissions of exhaust pollutants, which are directly proportional to the specific CO₂ emissions.

When comparing EGR-off and EGR-on conditions across various engine load levels (BMEP = 0.2 MPa to 0.6 MPa), the calculated specific CO₂ emissions under EGR-on conditions increased by approximately 3.0%. Increasing HEE intensity to 100% at a load of BMEP = 0.4 MPa under EGR-on conditions resulted in a relative reduction in specific CO₂ emissions of up to 4%, thereby offsetting the emission increase caused by EGR. Under EGR-off conditions, increasing HEE intensity led to a 1.7–3.7% reduction in specific CO₂ emissions due to improved brake thermal efficiency and reduced brake specific fuel consumption (Figure 15).

4. Conclusions

The following key findings were obtained from the investigation of varying heat exchanger effectiveness (HEE) and EGR operation in a CI engine under part-load conditions:

1.

EGR and air mass effects:

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at BMEP levels of 0.2, 0.4, and 0.6 MPa, the ECU-controlled EGR rates were approximately 0.28, 0.22, and 0.19, respectively;

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corresponding intake air mass reductions reached approximately 27%, 20%, and 18%;

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with 100% HEE, intake manifold air temperature dropped by ~13 °C, while intercooler outlet temperature decreased by ~52 °C (EGR on) and ~48 °C (EGR off).

2.

Air mass and ECU regulation:

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the ECU maintains the EGR ratio by adjusting boost pressure and monitoring intake air mass.

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under EGR on, intake air mass increased only by ~1.5% at full HEE, whereas under EGR off, it rose by up to ~4%.

3.

Combustion and pressure dynamics:

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under EGR off, enhanced air cooling raised compression pressure and ignition delay, boosting heat release rate and reducing exhaust gas temperature by up to 20 °C;

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EGR on conditions, with higher CO₂ concentration, suppressed combustion intensity, lowering peak temperature by ~16%.

4.

Fuel consumption and efficiency:

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EGR increased BSFC by 2–2.8% and exhaust gas temperature by 15–17% due to reduced combustion efficiency;

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higher HEE partially mitigated these effects, improving combustion quality and reducing BSFC by up to 2.5%.

5.

Emissions:

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under EGR-off, 100% HEE reduced:

○

smoke opacity by 15–18% due to increased air–fuel ratio (λ);

○

specific NO_x emissions by 14–41% due to lower combustion temperatures;

-

under EGR-on, smoke opacity increased 4.3–6 times, with HEE reducing smoke by up to 14% only under low loads;

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NO_x emissions decreased by 30–45% due to EGR alone, and up to 60% when combined with 100% HEE;

-

HC emission reduction with HEE was limited due to the offsetting effect of lower combustion temperature.

-

CO₂ emissions:

○

decreased by up to 3.0% under EGR off with higher HEE;

○

increased by up to 4% under EGR on, partially offset by HEE at lower loads.

6.

General implications:

-

under EGR off, high HEE intensity can partially substitute EGR by reducing NO_x and enhancing combustion;

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under EGR on, HEE effectiveness is limited by ECU-imposed intake air constraints.

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combined control of EGR and HEE is essential for balancing NO_x emissions and engine efficiency across operating ranges.

Future work will focus on broader engine operation ranges and the use of renewable fuels (e.g., HVO, RME, synthetic fuels), supporting sustainable and efficient energy use.