Impacts on Nox Emission Control Measures to Achieve EURO VI Limits - A Review

Impacts on Nox Emission Control Measures to Achieve EURO VI Limits - A Review

Saravanan Supramani* Chidambaram Ramesh Kumar 

Automotive Research Center, School of Mechanical Engineering, Vellore Institute of Technology, Vellore 632014, India

Corresponding Author Email: 
saravanan.s2015@vit.ac.in
Page: 
163-171
|
DOI: 
https://doi.org/10.18280/jesa.520208
Received: 
9 January 2019
|
Accepted: 
30 March 2019
|
Published: 
15 July 2019
| Citation

OPEN ACCESS

Abstract: 

The intention of this review study is to investigate the control measures of NOx emission. Even though the diesel engine sector plays a pivotal role in economic growth, it brings an unavoidable environmental deterioration. The current work reviews the effect of EGR, boost pressure variation, fuel modification, advanced engine combustion concepts, and after treatment processes for reducing NOx emission to EURO VI level. The outcomes of bio-diesel performance and other emission characteristics are too highlighted. In addition to this, specific attention is given to Porous Medium Combustion (PMC) technology adopted in diesel engines and a separate section is devoted. In order to facilitate a better understanding of PMC technology, simulation and experimental results of NOx emission are reported. Through this porous medium experimental study, it was found that NOx, HC and CO emissions were reduced significantly. Reducing the NOx and PM emissions might simultaneously increase the chance of usage of diesel and biodiesel blends fuelled CI engines in near future. The findings of this research work serve as best possible ways for achieving ultra-low NOx emission subjected to EURO VI norms level is suggested in future directions to give the reader insight and the track for the future investigation.

Keywords: 

diesel engine, low temperature, homogeneous combustion, porous medium, emission, oxides of nitrogen, smoke opacity, particulate matter

1. Introduction

Over the past few decades, there has been a tremendous increase in the vehicle population. According to the worldwide survey, over 4.2 million people died due to outdoor air pollution in the last year 2017 [1]. The only way to protect biodiversity and human being from automotive pollution is by implementing stringent emission norms. Most countries have implemented stringent regulations on engine out emissions based on European Emission Standards (EURO). The emission standard for commercial vehicles in India is proposed to attain EURO VI level. These newer emission norms have brought new diesel engine technologies in the country. So, Several developing countries have banned the operation of old diesel vehicles [2] and encourage the use of biodiesel combined with progressive engine combustion concepts. The combination of advanced engine combustion concepts and biodiesel adoption might reduce the emissions compared to a conventional diesel engine.

Biodiesel is a renewable fuel which is produced by the transesterification process of feedstock, vegetable oils, animal fats, and biowastes, etc. Jatropha, Pongamia, Palm, and Sunflower oil are some of the most commonly used raw materials for biodiesel production [3]. Biodiesels can be delivered at a lower cost when produced in large scale. It is less hazardous, has less storage risk, has a high cetane number and high flash point properties. The most commonly used biodiesel in India is ethanol. Typically sugarcane is used as a raw material to produce ethanol. Crop harvest, material softening, breaking down the sugar – hydrolysis and finally fermentation are the respective processes produce ethanol. India as 125 ethanol production plants producing around 1.25 billion liters of ethanol [3]. However, usage of fertilizers, water and agricultural land used of domestic food crop production are some of the disadvantages of producing biodiesels. Diesel engines are designed to run on diesel, not for direct biodiesels. Even though biodiesel’s calorific value is closer to diesel fuel other properties like viscosity, density has a stronger impact on mixture formation and proper auto-ignition. The overall efficiency and depreciation of the engine running on continuous biofuels are the challenging tasks to be faced when implementing them directly in diesel engines.

In order to achieve the same auto-ignition conditions of biodiesel near to diesel, researchers focused their ideas to operate it as same as diesel combustion. So, fuelling of biodiesels directly in diesel engines requires some additional changes in fuel lines, injection pressure, and injection timings. Still, these variations might reduce HC, CO and smoke emissions but increases the NOx emissions. However, HC and CO emissions can be reduced more efficiently by the three-way catalytic converter and smoke emissions might be reduced by DPF. Reducing NOx emission is the must one to achieve EURO VI emission level. Hence, the importance is given to NOx emission compared to other emissions.

The combustion of fossil fuels with nitrogen in atmospheric air under high pressure and temperature forms N2O, NO and NO2. Out of these, the biggest contributor is NO. NOx reacts with moisture in the lungs to form nitric acid, affects respiratory systems causing bronchitis, pneumonia, lung infections and visibility reduction. Many strategies are developed to reduce NOx emission and these are classified into a few major technologies as shown in Figure 1. NOx formation in CI engines is directly proportional to the combustion temperature. The following sections will briefly summarize the above-said parameters and its effects on diesel and biodiesel-diesel blended diesel engines performance and emission characteristics.

Figure 1. Strategies to control NOx emission

2. In-Cylinder Control Measures

2.1 Effects of intake charge modification

2.1.1 EGR addition

EGR, an additional heat sink replaces some part of the fresh air mainly to reduce NOx emissions. Diluents lower the combustion rate and reduce peak combustion temperature thus the NOx formation reaction diminishes. However, EGR improves combustion efficiency. In some cases, before inducting EGR is cooled close to atmospheric temperature and used up to 60 % with fresh air. Appendix A shows the combined and individual effects of EGR on emissions with other strategies like varying fuel injection pressure, injection timing, and a number of injections. Some additives like DME and CH2O can also be combined with the fuel to attain homogeneous charge. The increase in additive percentage might increase ignition quality. Yao et al studied the control strategies to attain homogeneous combustion in diesel engines. The engine selected was a single cylinder water cooled diesel engine which was modified to operate in dual fuel mode. The DME and methanol dual fuel is port injected separately at the pressure of 2.5 bar. Since the engine is mechanical governor controlled the remaining required diesel is conventionally injected through the main injection. 

From the literature, it is found that the researchers varied EGR from 0 – 80 %. Amongst most of the results approved that 30 % EGR shows better performance and emission characteristics. In the research work of Yao et al, the EGR ratio was varied from 0, 25, 40 and 65 % and DME ratio 60 to 76 % respectively. On conducting experiments, controlling the DME ratio and EGR percentage improved the homogeneous ignition with increased thermal efficiency. The results reveal that the increase in the percentage of DME reduces the HC and CO emissions. Increase in EGR ratio reduced the NOx emissions significantly [4]. Though EGR is capable to reduce NOx emission it raises HC, CO, and smoke emissions.  However, the above experiment didn’t report about smoke emissions. EGR reduces more than 30 % of the NOx emissions and on the same side, it reduces the available intake oxygen. The reduction in the oxygen level reduces the combustion speed and power output of the engine. Simultaneously PM emissions will be increased. In order to achieve the same performance and simultaneous reduction of NOx and PM emissions, the EGR technique is combined with optimized boost pressure and high fuel injection pressure. The boost pressure can be increased by means of a turbocharger or a supercharger in CRDI diesel engines. An empirical study was carried out by Usman Asad et al to find the influence of fuel injection pressure and boost pressure in combination with EGR. The EGR was cooled and used from 30 – 57 % to replace fresh air. The tests were carried out on a four-stroke, CRDI engine. The boost pressure was varied from 1.5 to 3 bar and the fuel injection pressure was varied from 600 to 1800 bar. It was reported that NOx and PM emissions were decreased by cooled EGR and increasing intake boost pressure, fuel injection pressure respectively [5].

Olsson et al tried to achieve ultra-low NOx emissions and thermal efficiency. The two different fuels namely ethanol and n-heptane were chosen on the basis that it will aid to gain homogeneity of the charge. The test engine chosen was a modified six-cylinder Scania DSC12 turbocharged diesel engine operating at 18: 1 compression ratio. The increase in inlet pressure by turbocharger allows the engine to operate at higher loads. As a result, the highest efficiency of 41.2 % is achieved with less than 0.1 g/kWh of NOx emission [6] at full load. But the CO emissions were increased significantly due to the lack of oxygen at lower loads which can be reduced by slight heating of the intake air. As a result, the dilution required to control the NOx emission necessitates the use of higher boost supply compared to the conventional mode of operation. When high boost is required, turbocharger efficiency is shown to be very important in order to reduce the pumping losses. For high load applications, two-stage turbocharging with inter-cooling was recommended. Better control of the turbocharger avoids unnecessary pumping work throughout boost. Appendix B shows the experimental outcomes of using variable geometry turbocharger.

On concluding the effect of adding EGR, it increases the heat capacity of the intake charge and displaces the fresh oxygen. So flame temperature becomes lower resulting in NOx emission reduction while HC, CO and soot emissions rose due to insufficient oxygen. Increased EGR level increases the engine wear, degradation of lubricating oil, knocking and increased fuel consumption [7]. So, to reduce the BSFC the authors tried to work with varying the compression ratio. The compression ratio of an engine has a direct impact on efficiency and at the same time ignition delay of an engine. Increasing the compression ratio produced more power output and increases the thermal efficiency of the engine. Lowering the compression ratio reduces the self-ignition spots thus increases the ignition delay. The results of varying compression ratio and fuel injection timings in an HCCI engine was investigated by Arjan Helmantel et al. [8]. Tests were performed on a Ricardo Hydra single cylinder engine. In this work, a toroidal piston bowl region is enlarged to lower the compression ratio. In addition to the bowl enlargement, the timing of the intake valve closing was altered by camshaft modifications to change the compression ratio. By conducting the experiments, NOx and PM formations drastically reduced while HC and CO emissions were increased. In the same experiment configuring injector nozzle to an angle of 60º showed excellent choice for HCCI while 140º orientation increased the PM and reduced the engine efficiency. On the overall analyzation of results are shown in Appendix A and B, lowering the compression ratio in the presence of diluted EGR with the combination of higher intake boost pressure and fuel injection pressure extends the operability limit of an HCCI engine. In addition, HC and CO emissions reduced with a minor drop in thermal efficiency. Thus the combination of EGR and compression ratio has the potential for reducing NOx and improving thermal efficiency respectively. However current emission norm EURO VI requires low NOx and PM emission level. So, researchers focused their ideas on advanced engine combustion concepts like PCI, HCCI, and PMCI via fuel modification techniques.

2.1.2 Fuel modification

The above-mentioned techniques are used to reduce the NOx emission while the forthcoming technologies experiment to a simultaneous reduction of NOx and PM emission. Attaining homogenous charge will help to reduce the PM emission. To get the air and fuel mixed well, PCI mode tends to help more in achieving homogeneous combustion. This mode of operation prepares premixed charge outside the combustion chamber. A premixed charge is obtained by an auxiliary injector placed near the intake manifold which is termed as port fuel injection [9]. Early injection, multiple injections, and late injection are the three different types of approaches that have been taken to experimentation. This mode of operation paid more attention to the formation of the homogeneous mixture before autoignition. There is no PM producing fuel rich zones since it reduces local fuel rich zones and high-temperature regions. However, these advanced engine concepts are limited to low and medium load operations. It may be extended by fuel controlled multiple split injection strategies and they are discussed below. Advancing the injection timing reduces the formation of fuel stratification [10]. This mode of combustion reduces NOx and PM emissions with a slight increase in HC and CO emissions. It is quite obvious that changes in SOI, air temperature and mass flow rate of the fuel have a great impact on particulate matter reductions [11]. Too early injection leads to wall dousing due to over penetration of the fuel and misfire and so this mode of operation is controlled by chemical kinetics and governed by fuel injection and mixing rate.

Multiple Injections technology for advanced engine concepts.

The effect of misfiring in early injection promotes researchers to work in multiple injections. Studies were carried out in an experimental setup consisting of one main injector and two auxiliary injectors at the sides by Takeshi et al. The engine chosen was single cylinder four stroke diesel engine. The fuel injection timing, quantity, and pressures are controlled separately. The main fuel injection pressure was kept constant as 2500 bar and the auxiliary injector pressure is maintained at 1000 bar. The first stage injection is set at 150º bTDC and the second stage from 2º bTDC to 30º aTDC. Looking at the results from the experiment, retarding the second injection from 2º bTDC to 30º aTDC gradually reduces 50 % of NOx and PM emissions compared to the conventional mode with improved thermal efficiency. NOx emissions were reduced due to the reduction of cylinder temperature at the end of the second stage injection [12]. To support these experiments, research conducted by Halysun et al proved the same i.e. multiple injection strategies will reduce the NOx and PM emissions [13]. After a great deal of research, Sanghoon et al evaluated the adverse effects of injection pressure, injection timings, EGR rate, swirl ratio, and several injections approach in a diesel engine to achieve clean as well as efficient combustion. The combined effect of high EGR and triple injection reduces NOx and PM emissions [14]. To investigate more about multiple injections, Lemongrass oil was tested in PCCI mode under the basis of the premixed ratio on an energy basis. An auxiliary injector is used which is placed close to the intake manifold (injection pressure - 2 bar) and controlled by an electronic unit, where the main injection is kept at 23° bTDC under 220 bar. The experiments were conducted with two different premixed ratios 5 % and 10 %. Due to the increased ignition delay period and reduced combustion duration, the NOx and PM emissions are reduced. Even though the prospective benefits are achieved in emissions, its performance is lesser compared to diesel but the experiment was mainly conducted in PCCI mode to reduce the NOx and PM emissions [15].

Yet another assessment to advancing and retarding second injection work was carried by Takayuki Fuyuto et al [16]. The results of varying second injection timing are shown in table 1. Extensive research has been done in Diesel- PCCI mode as a single post injection and double post-injection strategy by Youngsoo et al. [17]. The single post injection has longer ignition delay and reduced 33 % of NOx emissions, whereas HC and CO emissions increased compared to conventional diesel injection. In the same study, the author tried to achieve double post injections through a split injection strategy. By conducting trials at various injection timings, it is concluded that when the second injection is retarded more than 30° crank angle after the main injection, NOx levels reduced.  The experimental outcomes prove that double post injections lead to the best NOx and HC reduction compared to single post injection. The combined effect of the reduced compression ratio and retarded injection timing greatly reduced the PM and NOx emissions by the lean homogenous mixture and low flame temperature. Being interested to know the effects of combustion efficiency and other emissions of multiple injections, Naoto Horibe et al. conducted an experiment to analyze the influence of injection parameters in an 857 cc diesel engine. The engine is modified from conventional mode to Diesel-PCCI mode [18]. The progressive result of this work is presented in table 2. A retard in the fuel injection improves pressure to rise and combustion efficiency also lowers CO and HC emissions.

Table 1. Influence of second injection timing

PCCI

Injection Timing

Effect

Single injection

Advanced

↓NOx and noise; ↑ HC, CO, and SFC

Retarded

Steep ↑ HRR, NOx, and noise where ↓HC and SFC

Second Injection

Advanced

↑ PM and CO

Retarded

↑ PM, ↓ CO, and noise

 
Table 2. Influence of first and second late injection

Influence of

Engine at low speed

Engine at high speed

Quantity

Timings

Quantity

Timings

First injection

Increase

Retard

Decrease

Advance

Second injection

-

Retard

-

Advance

Effect

↓maximum pressure rise

↑ PM and CO, improved efficiency

 
 

Retarding injection timing with high compression ratio reduces the NOx and PM emissions, compared to multiple injections [19]. This trend is principally due to the longer ignition delay. The late injection is better than early injection for reducing NOx emissions, but it increases HC and CO emissions. However, HC and CO emissions can be reduced by cheap and efficient after-treatment processes. Similarly, a single retarded injection has the same performance effect compared to multiple injections but a slight increase in NOx as stated in the literature [20]. A single cylinder diesel engine was chosen to analyze the effects of changing injection pressure and timing on NOx and PM emissions. From the experimental results, it is concluded that increasing the injection pressure (from 600 to 1000 bar) alone decreases PM with little penalty in NOx while retarding injection timing (2º bTDC) with increased injection pressure simultaneously reduces the NOx and PM emissions. So advanced injection timing provides homogeneous fuel distribution inside the cylinder. This trend is principally due to the longer ignition delay [21]. The combined effect of the reduced compression ratio and retarded injection timing greatly reduced the PM and NOx emissions by the lean homogenous mixture and low flame temperature.

2.2 Effects of engine modification

2.2.1 Porous medium combustion

The performance and emission issues discussed so far in previously mentioned modes are positively influenced by the PMC. For simultaneous reduction of these aforementioned pollutants, fuel properties and combustion chamber modifications are vital. Higher burning rate, reliable lean burn ignition, ultra-low emissions, higher cycle efficiency, and equalized temperature mode are some of the potentials of PMC. Porous ceramic materials are made as a high-density wire packed grid structures. It has a large number of hotspots distributed throughout the entire structure with the properties of transparency of liquid and gas flows, capable of transferring single spray into multiple sprays and improves fuel vaporization to make the charge homogeneous. It has a higher heat accumulation, convection, and radiation capacity. The disciplined procedure associated with PMC is shown in Figure 2. The most commonly used porous materials are Silicon Carbide (SiC), Zirconia (ZrO2), Alumina (Al2O3) and Yttria-Stabilized Zirconia due to their properties of high compression strength, thermal resistance, and hardness. The combination of the large heat capacity and large specific surface area might form an excellent heat transfer characteristic which achieve quick fuel vaporization and homogeneous mixture [22].

Molecular transport and adiabatic combustion are the two mechanisms that control the propagations of combustion waves in the porous medium [23]. Using transient approach, the excess specific enthalpy achieved during porous medium combustion is given in the below relation

$\int_{T_{0}}^{T_{f}} C_{p} d T=q_{c}+q_{a}=h_{f}-h_{0}$   (1)

where, To and Tf are the initial and final temperature, qc is the heat released by chemical energy, qa is the energy added, and ho and hf are the enthalpies of initial and final states [24]. At the end of combustion, the residual enthalpy of the combusted gas gets accumulated in the porous medium. This residual enthalpy is used to increase the enthalpy of the forthcoming fuel mixture in the next cycle [25]. At a bird’s eye point of view, liquid fuel fired porous medium combustion chambers are characterized into fuel vaporizing type PM and fuel spraying type PM.

Figure 2. The porous medium ignition process

Fuel vaporizing type PM. Fuel vaporizing type concept is used to make the diesel vaporize quickly in compressed air [26]. For of all before experimenting Liu et al developed models to study the degree of sophistication attained for predicting problems.  Some of the expected issues are fuel flow, velocity, temperature, flame speed, mass fraction, combustion efficiency and emissions. The PMC analysis is carried through one dimension and two-dimensional models [27]. In that two unique models of configuring porous medium have been recommended and tried. To be specific they are named as open and closed chamber configurations. The porous medium chamber that has a perpetual contact with the cylinder content is named as an open chamber and the porous medium chamber has intervallic contact with the cylinder content named as a closed chamber. Based on these two configurations, three locations for mounting porous medium in the combustion chamber were chosen commonly and they are cylinder head, cylinder wall and in the piston head. SiC is selected as the porous medium and it is assumed to be placed in the cylinder head between the intake and exhaust valves as an open chamber system without any damage to the porous medium material. A series of tests were conducted and the results showed an obvious reduction in peak pressure compared to conventional mode of diesel operation. Also, it revealed very low CO, NOx and PM emissions with better fuel economy. One more test was carried on permanent contact PM engine with porous SiC reactor mounted in the piston head. The experimental result shows decreased NOx emission with a slight increase in CO and HC emissions. From this above literature, it is concluded that the location of PM in the chamber either moving or stationary have a direct influence on emissions. Heat accumulation in the porous medium reduces the flame temperature and thus NOx emission is reduced. Susceptibility of the valve exposed to combustion products and high thermal stress can cause damage to valve and piston tops [28]. Saravanan et al attempted simulation study to control NOx using PMC technique in a diesel engine. The software chosen was Converge 2.0 and the same conventional engine operating parameters are considered for simulation. Silicon carbide is selected as the porous medium due to its significant properties of thermal conductivity, porosity, and ppi, etc. The simulation is devoted to NOx reduction and the NOx results were found reduced. The authors observed that the NOx emission was reduced by 77 % while HC and CO emissions were increased by 80 % significantly. However, HC and CO emissions can be controlled by simple after treatment processes, the main objective of the simulation study was achieved via PMC [29].

Fuel spraying type PM. The fuel spraying type porous medium is a concept of making porous nozzles to inject diesel fuels. A new type of porous injector nozzle was reported and investigated experimentally by Reijinders et al [30]. Fuel spray interface with a porous medium is given in figure 3.  In this pioneering research, it is quite new that the nozzle with an orifice diameter has less than 50 µm was taken. The advantage of the new concept was further sophisticated by porous nozzle tip designed to achieve a spray angle nearly by 180 º. A model is created using Darcy-Forchheimer equation in COMSOL Multiphysics to predict the direction and velocity of the fuel. The simulation was analyzed by varying parameters like mixture formation, high-pressure fuel injection, different hole and sizes configuration under different loads. The results showed technical and profitable perspectives like shorter ignition delay and more homogeneous distribution compared to the conventional injector.

Figure 3. Fuel distribution of a conventional and porous injector [30]

Table 3. An average lifetime of the tested porous materials [32]

Material

Average injections

Stainless steel (316L)

58,000

Hastelloy X

1,35,535

Inconel 600

3,13,500

 

The sample of the injector with three different porous materials mounting on conventional injector tip is tested at atmospheric pressures under a pressure of 100 MPa, 5ms duration, and 15 HZ injection frequency. The results obtained from the test concluded that increasing the strength of the porous material increases lifetime (average injections) as given in table 3. High injection pressure has more additional capacity to scatter rapidly than low injection pressures [31].  Despite these potential advantages, the normal lifetime is not yet met.

Shanhangian and Ghojel et al. [33] explored the effect of injection pressure on porous media with the help of high-speed imaging technique. The combustion chamber volume was reduced with an increase in size, heat capacity, pore density and specific surface area of the porous reactor. An experimental setup was systemized in a 150 mm diameter and 135 mm depth, eight windows stainless steel constant volume chamber. The PM was centrally positioned across the entire vessel. Thermocouple and static pressure transducer were used to measure the temperature and pressure of the chamber. In addition to this a common rail diesel fuel injection system, high-speed camera, trigger circuit, and illumination circuits are connected in series. The camera used here is Shimadzu Hyper Vision HPV-1 fitted Micro Nikkor lens to capture the injection and impingement events. The high-resolution charge-coupled device camera with exposure time was adjusted to achieve the frame integrated time of 2 μs for the 125 and 63 kfps framing rates. The experiment is carried out with an Al2O3 porous sphere and repeated for a different number of layers of porous spheres. The results showed that there is a significant reduction in NOx and PM emissions with increased combustion efficiency. Looking at results from the literature, it is accomplished that the temperature of the combustion medium is reduced, which paves the way for NOx reduction. Lower oxygen concentration and uniformed hot spots may also reduce the NOx formation.

Navid Shahangian [33] et al investigated the influence of injection pressure and pore density on a porous medium. To access the technical viability high-speed imaging technique was employed and images are processed with the help of Matlab code to analyze the spray impingement.  For the experimental analysis three different SiC types of 10 ppi, 20 ppi, and 30 ppi are chosen. As the injection pressure rises, the numbers of secondary spray increases.  From the investigation, it is decided that the injection pressure of 1000 bar increases the amount of fuel penetration. Increased air entrainment and improved homogenization resulted from the experiment outcomes. Porous medium combustion technology is furthermore efficiently achieved by varying the parameters like compression ratio, swirl ratio, equivalence ratio, injector nozzle tip diameter, etc. Since HC, CO and NOx, smoke emissions have a tradeoff to rise or decrease vice versa HC and CO emissions can be reduced by a combination of porous medium combustion and by implementing after-treatment processes like Three Way Catalytic converter, Lean NOx Trap (LNT) and NOx Storage Reduction Catalysis (NSR).

3. After Treatment Control Measures

Though many techniques are employed for emission reduction in automotive industries, the use of catalytic converters is considered as the best way of reducing the emissions without any modification in the engine and its performance. The harmful pollutants discharged from engines such as carbon monoxide, unburned hydrocarbons and NOx can be changed over to safe ones utilizing a three-way catalyst through balanced oxidation-reduction responses. A three-way catalytic converter has principally three responsibilities:

Reduction of nitrogen oxides to nitrogen and oxygen

2NOx → xO2 + N2   (2)

Oxidation of carbon monoxide to carbon dioxide

2CO + O2 → 2CO2   (3)

4 HC + 5 O2 → 4 CO2 + 2 H2O   (4)

CnH2m + (n + m/2) O2 → nCO2 + mH2O   (5)

To advance this technology furthermore led to the development of multi-component LNT catalysts to abolish the shortcoming of the traditional TWCs and urea-SCR system [34]. LNT technology is the most feasible and potential technique to reduce the NOx emission from the diesel engine exhaust. These challenges led to the development of multi-component LNT catalysts to abolish the shortcoming of the traditional TWCs and urea - SCR system. LNT technology also acknowledged as NSR is the most feasible and potential technique to reduce the NOx emission from the diesel engine exhaust. But there is an unclear hypothesis on the chemistry and process mechanism of reduction of the stored NOx. In recent times, it has been suggested that ammonia is intermediate in the reduction of the stored nitrates by H2 [35]. The NSR catalytic system composed of noble metal is the most promising alternative for the removal of NOx from the exhaust of diesel engines because it does not need any additional equipment and infrastructure. Since NO2 and NO gases are acidic, various oxides of alkali and alkaline earth metals can be used for the NOx storage. The goal of this investigation was to provide analysis of the pathways operating during both lean and rich operating conditions particularly to study the behaviour of the combined NSR/SCR catalytic systems on the basis of the proposed pathways for the reduction of the stored NOx with NH3 as an intermediate species which can be used in SCR bed to further improve the total efficiency of the system [36].

4. Concluding Remarks and Future Recommendations

Internal heat recuperation, fuel injection, fine vaporization, homogenization, and self-ignition are the processes followed to attain homogeneous charge in porous medium combustion. Several simulation works have been made to focus the eye of current research workers in the progress of a porous medium combustion process. But, the experimental works carried out were found to be very few due to the complexity in the setup construction or modifying the existing one. Experimental optimization of adding porous medium inside the combustion chamber is difficult, due to the physical limitations of the porous material. Fracture, and degrade upon continual thermal cycling is one of the demerits to be assured in porous media. SiC is the best porous medium to use in a combustion chamber and mainly reported in all literature. Amongst this, a large specific surface area and heat capacity of Sintered silicon carbide may be utilized for excellent interphase heat transfer, because other porous materials are feasible, but not applicable to high-temperature processes. For a very good heat transport properties, foam structure should have a mean pore diameter of 0.1 to 5 mm.  A wide range of low calorific fuels can be used in dual fuel mode with main diesel fuel.

(1) Porous medium combustion has superior thermal efficiency and a major reduction in NOx and PM emissions

(2) It is concluded that the temperature of gas trapped in the porous medium is less dependent on the air-fuel ratio of the engine

(3) Multipoint fuel injection with electronic control unit enables to achieve efficient fuel economy

(4) Having an optimized compression ratio lowers the combustion chamber pressure

(5) Even though substantial numerical modeling research has been dedicated to porous medium combustion still there is a lack of satisfactory experimental verification owing to its intricacy in the process

(6) Investigations can be attempted on the combination of diesel fuel with onboard generated hydrogen

(7) Investigations on porous medium combustion can be done on multi-cylinder diesel engines and high-speed engines

(8) Investigations can be done on more fuel modifications

Nomenclature

aTDC

after Top Dead Centre

bTDC

before Top Dead Centre

CI

Compression Ignition

CN

Combustion Noise

CO

Carbon monoxide

CRDI

Common Rail Direct Injection

DE

Diesel Engine

DI

Direct Injection

DME

DiMethyl Ether

DPF

Diesel Particulate Filter

EGR

Exhaust Gas Recirculation

EURO

European Emission Standards

FV

Free Volume

HC

Hydro Carbon

HCCI 

Homogeneous Charge Compression Ignition

HRR

Heat Release Rate

HSDI

High-Speed Direct Injection

ID

Injection Duration

IPR

Ideal Porous Reactor

LNT

Lean NOx Trap

NOx

Oxides of Nitrogen

PCCI

Premixed Charge Compression Ignition

PM

Particulate Matter

PMC

Porous Medium Combustion

RPR

Real Porous Reactor

SCR

Selective Catalytic Reduction

SFC

Specific Fuel Consumption

SiC

Silicon Carbide

Appendix

Appendix A: Experimental results on varying EGR ratios, injection timing, and pressure

Engine type

Operating conditions

Test fuel

Injection timing

(Injection pressure)

Performance

Emission

Ref

Kirloskar -7.4 kW rated @ 1500 rpm

EGR – 8, 15 & 30 %, CR – 19.5

CNG–intake manifold inj, Diesel-Main inj

(250 bar)

↑BTE. Upto 15 % EGR @ 37 % CNG energy share

↓  NOx – 94 %, ↑PM with ↑ EGR;  CO & HC ↓ for 15 % EGR & ↑ for 30 % EGR c.t. D

[37]

6 Cylinder, 24-valve DE

CR – 16, EGR – 15 & 45 %

PFI – N-butanol, Diesel – Main inj

DI CRDI – 8° CA bTDC

At 10 % EGR- P & HRR ↑, 45 % EGR- ↓ P& HRR

↑ HC & CO for ↑ PFI, ↓ NOx & ↑PM as ↑EGR

[38]

4 Cylinder DIDE, 2 PFI

CR – 18,  EGR – 0 & 50 %

Biogas & hydrogen-PFI; Diesel- Main inj

Main inj - 10°bTDC, PFI - 60°bTDC,

CRDI (1200 bar)

↓ID

At 50% EGR, NOx ↓, ↑PM as ↑EGR

[39]

6 cyln heavy duty DE

CR – 16,  EGR – 0-45 %

40 % Butanol + D

-3° aTDC

CRDI (1400 bar)

↑P for B40 c.t. D

↑ NOx for B40, ↑ HC & CO as ↑EGR

[40]

Single cyln, DE

CR – 18, EGR – 5, 10, 15 %.

20 % soybean + D

(180 bar)

↑P & HRR with ↑ EGR with B20, ↑BSFC, ↓ BTE

↑ EGR→↓ NOx, slight ↑ smoke.↓ HC & CO c.t. D

[41]

Petter PH1W diesel engine

CR – 16.5, EGR – 5, 10, 20 %.

CNG – main inj; diesel – PFI

(200 bar)

↓P w.r.t.↑ EGR

NOx, HC and CO ↓w.r.t. ↑EGR

[37]

Single cyln, DE

CR – 17.5, EGR – 10-30 %.

1-Hexanol (blends by vol. 10, 20 & 30 %) + D

21, 23 & 25 °CA bTDC

(210 bar)

For 30% blend ↑P & HRR

↓ PM & CO but ↑ NOx & HC

[42]

Kirloskar TAF1

CR – 17.5, cooled EGR – 10, 20, 30 %

n-Pentanol + D (blends by vol.10, 20, 30 & 45 %)

23.4 °bTDC

(200 bar)

↑ID, ↓BSFC & BTE for all blends

↓ NOx @ 45% blend & 30% EGR, ↓ PM for all blends, ↑HC & CO for all trials but not significant

[43]

GW4D20 diesel engine

CR – 16.7, EGR (0-70 %)

Gasoline (37-66 %) – Diesel

SOI 1-35° aTDC, SOI2-5° aTDC

CRDI (650 bar)

↑P & HRR

↓ NOx & PM as ↑gasoline ratio

[44]

Ford “Puma”

CR – 18.2, EGR (0-60 %)

Ethanol, Diesel

CRDI(1600 bar)

↓ BTE with ↑ EGR

↓ NOx as ↑ EGR, where ↑ PM

[45]

* D-Diesel, P-Pressure, PFI-Port Fuel Injection,  inj-injection, CNG- Compressed Natural Gas
 

Appendix B: Experimental results on variable geometry turbocharger

Engine type

Operating conditions

Test fuel

Injection timing

Performance

Emission

Ref

4S DE

Turbocharged, CR-16.8

Biodiesel-D

12° bTDC – 215 bar

↑BTE & BSFC c.t.D

↑NOx & ↓CO c.t.D

[46]

XLD 418 Ford Diesel

Supercharger, CR-21.5, EGR – 27, 29, 32 %

D

-

↑P

↓NOx & PM, ↑CO and SOx

[47]

HSDI diesel passenger car engine

Steady state pre-turbo, DPF, CR-18

D

-

↑P

↑NOx,↓CO & HC, ↓Opacity for pre-turbo while ↑Opacity for post-turbo;

[48]

4-cyln diesel engine

 

Turbocharger, DOC, POC, CR-17.1,

DMDF

-

-

↓PM by 25% for D after DOC, 96% reduction after DOC & POC treatment for DMDF

[49]

6-cyln   diesel engine

inter-cooled turbocharger,CR-18.1

D(DI) + methanol (PFI)

(PFI - 4 bar)

↑ P

↓ NOx & PM; ↑ HC & CO,

[50]

* D-Diesel, P-Pressure

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