A Review of the External and Internal Residual Exhaust Gas in the Internal Combustion Engine

: Efﬁciency and emission reduction are the primary targets of internal combustion engine research due the large number of vehicles in operation and the impact of emissions-related pollution on human and ecosystem health. Harmful components of engine exhaust gases include nitrous oxides (NO x ), carbon dioxide, hydrocarbons, and particulate matter. NO x emissions in particular are associated with signiﬁcant health threats. The recirculation of exhaust gases can reduce NO x emissions and improve engine efﬁciency when combined with other advanced techniques. On the other hand, the residual exhaust gas also effects on the quality of lubricating engine oil and therefore causes an increase in engine piston ring wear. In this review paper, the effects of external and internal exhaust gas recirculation on the performance and emission characteristics of diesel, gasoline, and alternative fuel engines are summarized and discussed in detail. Because it is difﬁcult to estimate the internal residual exhaust gas in the combustion engine by doing experiments. This review paper introduces control strategies and prediction methods for internal and external exhaust gas recirculation.

The formation of nitrous oxides (NOx) in the combustion chamber is strongly dependent on the peak firing temperature and oxygen concentration area [11,12].Reducing either may help reduce NOx emissions.Three methods are known to effectively reduce peak firing temperature and reduce NOx emissions: 1.
Using water injection [13][14][15][16]: Water is injected into the cylinder during the combustion stroke to reduce the temperature of the hottest area.This method may also help prevent engine knock [17].

2.
Using a catalyst [18][19][20]: Engine exhaust gas flows through a catalyst before it is expelled to the environment.The catalyzers NH3 and Mn-Ce/TiO2 are known to reduce NOx emissions.

3.
Exhaust gas recirculation: A portion of an engine's exhaust gas recirculates back to the engine cylinders [21][22][23][24], resulting in a reduction in peak firing temperature and oxygen concentration area in the combustion chamber, which in turn reduces NOx emissions.
In iEGR, the residual exhaust gases flow back into the combustion chamber through exhaust ports after each exhaust stroke.Premixing recirculated exhaust gases with a mixture of fresh air and fuel can affect combustion stability, charge mass, flame speed, and creation of toxic products in the next combustion stroke.This flow can be controlled by changing the intake valve timing, exhaust valve timing, and valve overlap period [30,31].
In eEGR, the gas from the exhaust pipe is routed back to the cylinder through the intake pipe.This is a useful and more effective method of reducing NOx emissions in diesel engines [32][33][34] because up to 50% of the exhaust gases can be recirculated without affecting combustion stability.However, in gasoline engines, the eEGR rate should be kept below 20% to ensure combustion stability [35].
In a review of the effects of eEGR on the performance and emission characteristics of internal combustion engines, Abd-Alla [36] found that the eEGR method is more effective and provides greater benefits in terms of reducing NOx emissions in diesel engine compared with gasoline and dual-fuel engines.In a gasoline engine, NOx emissions can be reduced by 10% to 20%, but eEGR will also affect the combustion rate and reduce combustion stability.Greater eEGR reduces brake-specific fuel consumption (BSFC).For a dual-fuel engine, engine thermal efficiency can be improved with hot eEGR because a higher intake temperature allows for reburning of unburned fuel in exhaust gases.With cooler eEGR, the improvement in thermal efficiency is lower, but the reduction in NOx emissions is greater Zheng et al. [37] reviewed the advances and novel concepts associated with eEGR in diesel engines, reporting that NOx can be reduced, but particulate matter (PM) will increase because of lower oxygen concentrations.They identified several parameters that can influence eEGR, such as the pressure of the eEGR loop system, efficiency of the eEGR cooler, engine combustion efficiency, and temperatures of the intake and exhaust pipes.From those parameters, three methods of increasing eEGR efficiency were proposed.If eEGR gases are treated before returning to the cylinder, a greater reduction in NOx emissions may be possible compared with using raw eEGR.The three methods of limiting eEGR are: -eEGR cooling: the flow is cooled to reduce exhaust gas temperatures before returning to the cylinder.-eEGR oxidation: The eEGR gases flow through a catalyst that oxidizes unburned combustibles into CO2 and H2O.This process extends the limits of eEGR.-eEGR fuel reforming: an eEGR reformer produces H2 and CO, increasing the premixed combustion rate.
In a review of the effects of eEGR on combustion ignition (CI) engine performance and emission characteristics, Harilal et al. [38] found that the eEGR method could be applied to CI engines with various fuels, including diesel oil, liquefied petroleum gas, bio-diesel, and hydrogen.At a 15% eEGR rate, fewer NOx emissions were produced without a decrease in engine efficiency or BSFC.
Thangaraja et al. [39], who reviewed the effect of eEGR on advanced diesel combustion and alternative fuels, found that, while eEGR can reduce NOx emissions, it also led to increased emissions of soot, particulate matter (PM), hydrocarbons, and carbon dioxide (CO).The increase in soot and PM can reduce the quality of engine lubrication oil, which increases engine wear.
A review of published articles confirmed that significant effects are associated with a reduction in NOx emissions from exhaust gas recirculation.Most previous reviews and research articles focused on the effects of eEGR on CI engine performance and emission characteristics.No reviews discussed the effects of iEGR on engine performance and emission characteristics, which is a significant research gap.In this review paper, the effects of both eEGR and iEGR on internal combustion engines are highlighted, and the engine parameters or methods used to control or determine eEGR and iEGR will be discussed in detail.

Effect of eEGR on Diesel Engine Performance and Emission Characteristics
The diesel engine offers relatively high fuel efficiency because it works at high compression ratios while experiencing little engine knock [40,41] compared with gasoline engines.However, these advantages come at the cost of greater NOx and PM emissions and smoke [42].Many strategies have been presented to control and reduce the harmful components of diesel emissions [43][44][45].This section discusses in detail the effect eEGR on diesel engine performance and emissions.
Figure 1 provides a brief summary of the effects of eEGR on diesel engine performance and emission characteristics.An increase in the eEGR rate can dilute the intake of air into the combustion chamber.At a high temperature, eEGR increases the heat capacity of the intake charge and the endothermic dissociation of CO2 and H2O.This explains why an increase in the eEGR rate leads to a decrease in combustion quality and peak firing temperature.A decrease in NOx emissions but lower engine power, high levels of hydrocarbons, CO, and soot are observed as a consequence.A higher soot level will negate the effect of lubricating oil additives and increase engine wear.
Torregrosa et al. [46] used a direct injection diesel engine to study the effects of eEGR on heat transfer in the combustion chamber.They found that eEGR has a large effect on heat transfer, with an increase in eEGR reducing combustion wall temperatures and cylinder heat flux.The greatest influence on heat flux was observed at the center of the cylinder head.A reduction in temperatures was associated with an eEGR rate of 18% to 30%.
Torregrosa et al. also found that the rate of decrease in percentage of NOx emissions with eEGR was 10 times larger than the increase in percentage of CO2 emissions.
An effect on carbon deposits and engine wear in an agricultural diesel engine at a constant engine speed is reportedly associated with eEGR [47].The effect of eEGR rates of 0% to 20% on engine performance and emission characteristics at different engine loads was investigated.Figure 2 shows the relationship between the eEGR rate and engine thermal efficiency and NOx emissions at different engine loads.With an increase in the eEGR rate, engine thermal efficiency tends to increase slightly at lower loads because the unburned hydrocarbons from the previous cycle will be re-burned.However, at high loads, eEGR rates of up to 20% result in little change in engine thermal efficiency because higher CO2 levels reduce the peak firing temperature and oxygen concentration, and the unburned hydrocarbons from the previous cycle are therefore less significant.The decrease in peak firing temperature and oxygen concentration can also explain the sensitivity of NOx reductions to high loads compared with low loads.Carbon deposits are more evident in engines using eEGR than those not using eEGR because of the greater soot emissions associated with engines using eEGR.
Table 1 shows a comparison between piston ring wear in engines using eEGR and those not using eEGR.In the case of piston ring number one, higher wear can be observed with engines not using eEGR because this piston ring experiences higher temperatures and pressures compared with rings in an engine using eEGR.With other piston rings, a reverse trend was evident because high soot emission in engines using eEGR reduces the quality of lubricating engine oil [48], the increase in eEGR leads to increased soot, which increases the friction between the piston ring and cylinder surface [49].This also explains a higher wear of the other piston rings with eEGR.The effects of low-and high-pressure eEGR on the performance and emission characteristics of a turbocharged marine diesel engine at a low speed have also been examined [50].Figure 3 shows the effects of high pressure eEGR on NOx emissions at various engine loads and eEGR rates: a downward trend in NOx emission at partial loads when the eEGR rate increased at both high and low pressures.With a higher eEGR rate, high-pressure eEGR was more effective in reducing NOx emissions than low-pressure eEGR, although a higher BSFC was evident.In summary, the benefits of using eEGR for a diesel engine include a reduction in NOx emissions due to low combustion temperatures and cylinder head flux.In addition, improved thermal efficiency at low loads is possible with eEGR.However, eEGR will increase hydrocarbon, CO, and soot emissions; an increase in soot emissions will lead to increased carbon deposits, which reduces the quality of lubricating oil, causing increase engine wear.

Effect of eEGR on Alternative Fuel Engine Performance and Emission Characteristics
Alternative fuels offer a promising solution to fossil fuel security problems, the threat posed by greenhouse gas emissions responsible for climate change, and the health risks associated with the harmful components of exhaust gas.In addition, alternative fuels have become common and preferred racing fuels in recent years [51][52][53][54][55][56][57].This section reviews the performance and emission characteristics of alternative fuel engines that use eEGR.
Yaopeng et al. [58] evaluated the influence of eEGR on the performance and emission characteristics of a compression ignition engine using methanol/diesel at medium loads.A three-dimensional simulation model based on the KIVA-3V code was employed to estimate the reductions in engine emissions and improvement in performance using eEGR.From the simulation results, they found that fuel efficiency improved with a higher methanol fraction and a low eEGR rate.Engine performance declined with an increasing eEGR rate, but a sharp reduction in NOx emissions was also evident.A similar investigation of the effects of eEGR on engine emissions and combustion characteristics of an engine using ethanol was conducted by Saravanan et al. [59].Their results also indicated that the peak firing temperature and exhaust gas temperature decreased due to a lack of oxygen in the combustion chamber at higher eEGR rates.The decrease in peak firing temperature and lack of oxygen was associated with a reduction in NOx emissions and an increase in CO and hydrocarbon emissions.
Chunhua et al. [60] investigated the effect of eEGR on the combustion and emission characteristics of a six-cylinder dimethyl biodiesel-fueled engine.The experiments were conducted with biodiesel blends from 0% to 20% at both low loads (brake mean effective pressure (BMEP) = 0.4 MPa) and high loads (BMEP = 0.8 MPa).They reported that the ignition delay increased with the eEGR rate because a higher eEGR rate extended the interaction time between oxygen and fuel.A higher ignition delay at high loads compared with low load conditions can be observed in Figure 4.The increase in eEGR rate from 0% to 18% was due to a decrease in the flame-kernel growth rate from 16.5 m/s to 5.5 m/s, which was a product of an increase in eEGR, which reduced the spread of the ignition spark and flame velocity [61].These results indicate that a hydrogen-fueled engine can operate without knock at a higher equivalence ratio by applying eEGR  An increase in the eEGR rate can reduce the flow of fresh air into a cylinder, leading to a reduction in oxygen and homogeneous air-fuel for the combustion process.As a result, both the combustion phase and duration tend to increase with a higher eEGR, and this effect is enhanced at high loads (Figure 5).A lack of O2 and reduced homogeneous air-fuel mixture decreases the heat release rate and pressure rise.A lagging peak heat release rate and peak pressure rise with an increase in the eEGR rate.
In these results, a significant effect from the increased eEGR rate was observed in the form of reduced NOx emissions but increased hydrocarbon and CO emissions, as in a diesel engine.
Salvi et al. [62,63] conducted an experiment and simulations using a single-cylinder, spark-ignition engine with hydrogen as a fuel to study the effect of eEGR on the flame kernel growth rate.The experiment was conducted at an engine speed of 3000 rpm; hydrogen was injected through an intake manifold by a gas injector, and the eEGR rate varied from 0% to 18% by volume.Figure 6 shows that the increase in the eEGR rate from 0% to 18% was due to a decrease in the flame kernel growth rate from 16.5 m/s to 5.5 m/s, which was a product of an increase in eEGR, which reduced the spread of the ignition spark and flame velocity [64].These results indicate that a hydrogen-fueled engine can operate without knock at a higher equivalence ratio by applying eEGR.Yasin et al. [65] studied the performance of an engine fueled by palm oil biodiesel, combining an experiment with simulations to estimate the effect of eEGR at full loads and an engine speed of 2000 rpm.They found BSFC and CO emissions increased while exhaust gas temperature and NOx emissions decreased when using eEGR compared with the same engine fueled by diesel (Figures 7 and 8).The lower heat value of biodiesel fuels means the peak firing temperature and exhaust gas temperature will be lower compared with conventional diesel fuel.As a result, lower NOx emissions and higher CO emissions were reported using biodiesel fuel with eEGR.In summary, in an engine using an alternative fuel, a downward trend in NOx emissions and an upward trend in hydrocarbon and CO emissions was associated with an increased eEGR rate.The increased eEGR led to an increase in ignition delay, combustion phase, and combustion duration because the dilution charge extended the interaction time between oxygen and the fuel.A hydrogen-fueled engine can therefore be operated without knock at a higher equivalence ratio by applying eEGR.

Effect of eEGR on Spark Ignition Engine Performance and Emission Characteristics
Among different approaches to improving engine performance, eEGR is a promising approach to suppressing knock, improving engine efficiency, and reducing NOx emissions in spark ignition engines [66][67][68].This section provides a review of the effect of eEGR on spark ignition engines.
Thomas et al. [69] studied the suppression of knock in a direct injection spark ignition engine using eEGR.The eEGR rate was increased from 0% to 13% at three load con-ditions of the indicated mean effective pressure (IMEP: 5.5 bar, 7.0 bar, and 8.5 bar).They reported that an increased eEGR helped reduce the indicated specific fuel consumption (ISFC) by 2.2%, 4.1%, and 1.0%, respectively.At 7.0 bar (the IMEP test case), the engine achieved a knock-limited maximum torque at 8 deg-CA with a 12% eEGR, which led to improved peak cylinder pressure, ISFC, and engine efficiency; the pumping work was reduced at the knock-limited maximum brake torque, and the engine achieved optimum engine load phasing.The ISFC could be further improved if the eEGR incorporated a three-way catalyst [70].The decrease in NOx emissions and increase in hydrocarbon emissions were linked to a decrease in peak firing temperature as the eEGR increased.
Dongwon et al. [66], who investigated the potential of eEGR to improve the thermal efficiency and emission characteristics of a spark ignition engine, found that engine efficiency could be improved by 16% with eEGR, lean combustion, and a fuel-air equivalence ratio of 0.7.Other research teams have reported that a similar degree of maximum thermal efficiency could be obtained with a combination of eEGR and lean burning conditions [71].
Atul et al. [72] reported the effect of exhaust eEGR on spark ignition engine performance and emission characteristics.Using a 2.0 L four-stroke engine, they increased the eEGR rate from 0% to 19%, reporting that the increase in eEGR rate was due to an increase in combustion duration because the charge dilution from incorporating eEGR decreased flame speed and propagation.The increase in eEGR reduced the amount of fresh air and oxygen in the combustion chamber, reducing the peak firing temperate and BMEP, as shown in Figure 9.A reduction of NOx and CO emissions and increase in hydrocarbons is reported.These effects can be attributed to the decrease in peak firing temperature and amount of oxygen in the air during combustion.When the eEGR rate rose to 19%, the engine achieved a 20% decrease in BMEP, and NOx and CO emissions fell from 20% to 33%.However, BSFC and hydrocarbon emissions increased by 2.5% and 50%, respectively.The ignition delay increased with the eEGR rate because a higher eEGR rate increased the charge dilution, which extended the interaction time between O2 and fuel [73].
Currently, eEGR is commonly used in spark ignition gasoline engines because of its ability to increase engine efficiency, reduce NOx emissions, and suppress knock, although the latter requires careful control techniques.

Control System and Prediction of eEGR Methods
In the eEGR method, the exhaust gas from an engine will be reburned by returning a part of exhaust gas to the intake tube and allowing it to mix with fresh air and fuel in the combustion chamber.Unburned hydrocarbons in the exhaust gas will be burned completely, and the resulting charge dilution can reduce peak firing temperature and oxygen concentrations, which reduces NOx emissions.The eEGR rate can be controlled by a valve as shown in Figure 10. Figure 10 shows that the exhaust gas in an eEGR system returns directly to the intake pipe at a high temperature due to a reduction in engine volume efficiency and a high charge dilution.As this decreases engine efficiency and promotes knock, other effective designs of an eEGR loop system are required to eliminate these drawbacks, as shown in Figures 11 and 12   Figure 11 depicts two schemas for an eEGR loop system.The eEGR flow will reduce temperatures before returning the exhaust gases to an inlet pipe, improving volume efficiency and reducing knock [74].Figure 11a describes an eEGR loop system with low-pressure eEGR flow through the turbine.Exhaust is directed to the intake pipe when P2 is higher than P1.
Figure 11b describes an eEGR loop system with high-pressure eEGR flow.The eEGR flow is directly charged to the intake pipe when P4 is higher than P3.
Figure 12 depicts an eEGR system that uses a three-way catalytic converter to reduce NOx and PM before the exhaust gases reach the intake pipe.
Previous studies have used a set of equations to calculate the eEGR rate: The high-pressure and cooled eEGR rate is defined from the mass flow rate [75]  %   .100 where meEGR is the eEGR mass, and mi is total mas intake mixture.The high-pressure and cooled eEGR rate is defined from the volume flow rate [75] where Vaf is the air flow volume rate into the engine at full load without eEGR.Vac is the air flow volume rate into the engine at part load with eEGR.
The high-pressure and uncooled eEGR rate is defined from concentration of CO2 [76] where  is the concentration of CO2 at inlet pipe. is the concentration of CO2 at exhaust pipe. is the concentration of CO2 at ambient.The low-pressure and cooled eEGR rate is defined from concentration of CO2 [77]  %    (4) where  is the concentration of CO2 at inlet pipe. is the concentration of CO2 at exhaust pipe.The simulation eEGR rate is defined from the nitrogen mass [78]  %    (5) where  is the N2 mass flow;  is the total charged air mass.The simulated eEGR rate is determined by the inherent and applied residual gas ratios for a two-stroke engine [79] where  is the applied residual gas ratio;  is the inherent residual gas ratio.Based on the experimental conditions and an eEGR loop system, a suitable eEGR-rate equation should be selected for optimal results.

Internal Exhaust Gas Recirculation (iEGR)
The eEGR flow is taken from the exhaust pipe and supplied to the cylinder through the inlet pipe, while iEGR is achieved by modifying the intake or exhaust valve profile to increase the fraction of exhaust residuals at the end of the intake stroke.This section discusses the effect of iEGR on engine performance and emission characteristics.

The Effect of iEGR on Engine Performance and Emission Characteristics
In an internal combustion engine, iEGR can reduce NOx emissions, while intake and exhaust valve timing is used to control the reverse exhaust gas flow back into the cylinder.
A comparison of the effects of eEGR and iEGR on the performance and emission characteristics of a six-cylinder, four-stroke diesel engine was conducted via experiments and simulations [80].A variable valve actuation system was used to control valve timing, which increased eEGR flow back into the cylinder.Using iEGR with the same exhaust gas recirculation rate and full load conditions resulted in lower NOx emissions, lower engine torque, and higher BSFC compared with eEGR.This can be attributed to the higher temperature and pressure of the iEGR mass in the combustion chamber, which lowered the air-fuel mixture density and decreased engine volume efficiency, reducing oxygen concentration, peak firing temperature, and engine efficiency.However, when the engine operated in a transient state, iEGR was associated with superior control of the exhaust gas recirculation rate compared with eEGR.
The combustion characteristics of a homogeneous charge compression ignition engine using biogas as a fuel can be altered using iEGR [81].An increase in the iEGR rate due to dilution of the oxygen in the air-fuel mixture causes an increase in combustion duration and timing and a lower increase in combustion duration with the iEGR band from 1.5% to 10% when compared with the iEGR band from 10% to 20% as a result.
The emission characteristics of a spark ignition engine using natural gas and ethanol as fuel can be controlled using iEGR.When the iEGR rate was increased from 9.5% to 14%, NOx emissions decreased by 67%, but CO and hydrocarbon emissions increased by nearly 75% [82] as shown in Figure 13.An increase in ignition delay and combustion duration was also evident [83].In a gasoline engine, the iEGR can be an effective method of improving efficiency and reducing NOx emissions.Such engines can achieve stable combustion rates at low loads with iEGR [84].When the iEGR rate was increased from 9.1% to 51.7%, the BMEP increased after achieving a maximum value at an iEGR rate of 22.8%, after which the iEGR continued to increase.The BSFC exhibited a contrary trend to that of the BMEP; the optimal BSFC was achieved at 202 g/kWh, with an iEGR rate of 22.8%.This makes it clear, at low load conditions, the high temperature of a suitable iEGR rate helps engines achieve superior homogeneous air-fuel mixtures and combustion processes [85].However, a high eEGR rate can cause charge dilution, increasing the incomplete combustion rate and low burning conditions and decreasing thermal efficiency [86].When an engine is idling, a high iEGR rate can effectively reduce cycle-to-cycle variation and improve combustion stability.
The close relationship between combustion duration and iEGR in small spark ignition engines fueled by gasoline has been investigated [87].In a further study, the effect of iEGR on engine performance and emission characteristics at full loads was investigated.
The iEGR rate has a significant effect on peak firing temperature and pressure rise [88].Figure 14a,b shows decreases in peak firing temperature and pressure rise with an increasing iEGR rate.This is because the higher iEGR rate was due to a diluted and less homogeneous air-fuel mixture and a lack of oxygen.This led to a decrease in the quality of combustion and the heat release rate of the power stroke.The results of the research show that the peak firing temperature decreased from 2900 K to 1250 K, and the peak pressure rise decreased from 8 bar/deg to 5.5 bar/deg, respectively.In a combustion stroke, the chemical energy trapped in the cylinder is released as heat.A certain amount of heat energy is lost through heat transfer, pumping loss, and energy that is not effectively released in the cylinder (but instead goes into the combustion products).The remaining energy is the effective release energy.A decrease in effective release energy (Figure 15a) with an increase in residual exhaust gas was observed.To increase thermal efficiency, a short burn time can lead to a faster release of heat energy.However, the increase in residual gas in the cylinder due to a longer burn time, lower peak firing pressure rise, and lower peak firing temperature reduces the effective release energy.The effective release energy decreased from 0.85 KJ to 0.53 KJ when the iEGR increased from 1% to 5%.The increase in iEGR led to a decrease in the heat release rate and effective release energy, so the decrease in engine torque is a result (Figure 15b).
Figure 16a shows that NOx emissions tend to decrease with an increasing iEGR rate for two reasons.First, an increased iEGR is associated with a lower peak firing temperature.Second, an increase in iEGR is caused by a diluted air-fuel mixture and less oxygen.NOx emissions declined to a minimum of 2.12 g/kWh at a residual gas ratio of 5%. Figure 16b and Figure 16c show that an increased residual gas ratio leads to increased HC and CO emissions.This is because the higher residual gas ratio leads to an increased diluted air-fuel mixture and an increase in the areas lacking oxygen, increasing the amounts of unburned hydrocarbons and CO emissions from 3.65 g/kwh to 18.2 g/kwh and 139 g/kwh to 450 g/kwh, respectively.In summary, iEGR and eEGR result in similar effects on engine performance and emission characteristics.Using iEGR has less of an effect on NOx because the exhaust gas recirculation rate is lower than that of eEGR.However, iEGR involves a lower initial cost, simpler packaging, a potential for retrofit, lower maintenance costs, greater reliability, and higher tolerance of high-sulfur fuels.Using iEGR also offers improved cold engine starts and shorter warm-up periods.

Control Strategies and Prediction of iEGR Methods
Engine speed, load conditions, and valve overlap can effect iEGR performance [89].To obtain a target iEGR mass at a steady state, controlling valve overlap has proven to be an effective approach.Valve overlap is the time during which both the inlet and exhaust valves are both open, which permits iEGR flow to return to the cylinder.The timing of the intake opening and exhaust closing has a strong influence on valve overlap.In an internal combustion engine, intake valve timing controls the intake flow into the cylinder while the exhaust gas flow is controlled by exhaust valve timing.Early intake valve opening may cause exhaust gases to be expelled into the intake port and back to the cylinder in the intake stroke, and late exhaust valve opening may result in the reverse exhaust flow back to the cylinder during the intake stroke [90].The iEGR rate can be controlled by changing the timing of the intake valve opening timing and the exhaust valve closing.The increase in the early opening of the intake valve or late closing of the exhaust valve can cause an increase in the iEGR rate.
The ability of a fully flexible variable valve actuation (FVVA) system without camshafts to provide a fully independent lift of intake and exhaust valves was recently tested.Such FVVA systems can be electrohydraulic [91,92], electropneumatic, or electromagnetic [93].The FVVA system allows for a secondary lift of an intake valve during the exhaust stroke (2IVO) or induces a secondary lift of an exhaust valve during the intake stroke (2EVO) [94] [95].
Benajes et al. [96] studied the effect of the 2IVO and 2EVO approaches on the iEGR rate of a heavy-duty diesel engine.The authors reported that the 2IVO strategy resulted in a higher iEGR rate than that of a 2EVO approach because the latter reduced air mass flow and disrupted swirl production.When applying the 2IVO strategy to an engine, an iEGR rate of up to 12% was achieved.Combining 2IVO and valve overlap strategies in a high-speed, direct injection engine resulted in an iEGR of up to 25% [97].An iEGR of 60% was possible using a combination of 2IVO and 2EVO strategies [98].Thompson et al. [99] investigated the effect of iEGR on the performance and emissions of a spark-ignition engine at partial loads, using FVVA to control the positive and negative valve overlaps to increase the iEGR from 5% to 35%.The engine achieved a combustion efficiency of 25% to 30%.
At each operating condition with a suitable iEGR rate, the engine will perform at optimal combustion efficiency.An accurate method to determine the iEGR rate is therefore necessary.It is difficult to determine iEGR by doing experiments, so the iEGR rate can be obtained from the inherent and applied residual gas ratio through the simulation method [79] %   .100% where  is the applied residual gas ratio;  is the inherent residual gas ratio.The total iEGR rate can be calculated based on the mass without eEGR [99][100][101]  %    .100% where  is the mass of simulated residual exhaust gas;  is the air-fuel mixture mass.
The simulated iEGR rate can be determined when the engine combines iEGR and eEGR [84]  %     .100% where  is the mass of external exhaust gas recirculation.

Conclusions
Through this review, the effect of internal and external exhaust gas recirculation was completely discussed, and the mentioned gap of internal exhaust gas recirculation reviews has been filled.Based on the review of iEGR and eEGR, the important results are summarized below:  Due to the benefits of reducing NOx emissions and improving ISFC and engine efficiency, both iEGR and eEGR are commonly used in modern internal combustion engines.Gasoline engines can improve ISFC and engine thermal efficiency at low loads, while hydrogen-fueled engines can operate without knock at a higher equivalence ratio.Exhaust gas recirculation can be used to suppress knock, improve engine efficiency, and reduce NOx emissions from a spark-ignition engine, but requires a high degree of precise technical control. Internal combustion engines using exhaust gas recirculation can reduce NOx emissions due to the resulting decrease in combustion temperatures, cylinder head flux, and oxygen concentrations.However, exhaust gas recirculation will increase hydrocarbon, CO, and soot emissions, ignition delays, and combustion duration.The increase in soot emission is due to increased carbon deposits, which also reduce the quality of lubricating engine oil and therefore causes an increase in engine wear.The increase in combustion duration is due to increased heat loss transfer.


The iEGR method reduces NOx emissions to a lesser extent than the eEGR method because the exhaust gas recirculation rate is lower with iEGR.However, iEGR involves a lower initial cost, easier packaging and the potential for retrofits, lower maintenance cost, greater reliability, and a higher tolerance of high-sulfur fuels.It offers improved cold engine starts and warm-up periods. Exhaust gas recirculation control strategies, and determination equations for eEGR and iEGR were also reported.Depending on an engine's operating condition, a

Figure 1 .
Figure 1.The effects of eEGR on diesel engine performance and emission characteristics.

Figure 2 .
Figure 2. Effect of 20% eEGR rate on: (a) engine thermal efficiency at difference engine loads; (b) NOx emission at difference engine loads.

Figure 6 .
Figure 6.Effect of eEGR rate on the flame kernel growth rate.

Figure 7 .Figure 8 .
Figure 7.A comparison of the effect of eEGR rate on BSFC and exhaust gas temperature between diesel and palm oil biodiesel.

Figure 9 .
Figure 9.The effect of eEGR on: (a) combustion at various engine speeds; (b) peak temperature at various engine speeds; (c) BMEP and BSFC at various engine speeds.
Figure10shows that the exhaust gas in an eEGR system returns directly to the intake pipe at a high temperature due to a reduction in engine volume efficiency and a high charge dilution.As this decreases engine efficiency and promotes knock, other effective designs of an eEGR loop system are required to eliminate these drawbacks, as shown in Figures11 and 12.

Figure 15 .
Figure 15.The effect of iEGR on (a) Effective release energy; (b) brake torque.

Table 1 .
Effect of eEGR on piston ring wear.