Power And Emission Control Engineering Essay

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A basic 6 cylinder engine of Volvo S6 including turbocharger was modelled and simulated in Ricardo WAVE software. The engine didn't contain emission control devices such as catalytic convertor and exhaust gas recirculation. Later on the same engine has been upgraded with catalytic convertor and exhaust gas recirculation in order to reduce emissions. Both engines were run of three different loads i.e. part load, half load and full load and graphs were obtained through WAVE post. The graphs obtained were of brake power, CO emissions, NOx emissions and HC emissions. A comparison has been made between the two engine models, the basic one and upgraded (emissions control engine) one. A brief discussion has been presented in order to justify the results obtained and conclusion has been given to justify the project form the author's point of view.

Introduction

Four stroke engines consist of four different strokes during which the crank rotates two complete revolutions; these strokes are suction stroke, compression stroke, power stroke and exhaust stroke. During suction stroke fresh fuel enters into the cylinder through inlet valve which gets compressed during compression stroke. After the ignition the combustion occurs and power stroke produces power from the engine. Exhaust stroke removes the burnt charge in from the engine. Combustion process of the engine is nothing but a reaction process from which the emissions induced as a side product. The devices shown below are several devices used to control the emissions from the engine.

Exhaust gas recirculation

Catalytic convertor

Oxygen sensor

PCV valve

Charcoal canister

Air pump

Muffler and Resonator

Oxygen sensor: Function of an oxygen sensor is to sense amount of unburnt fuel in the exhaust and accordingly sending the signals so that air fuel ratio can be controlled in such a way that amount of unburnt gases can be controlled. A figure below shows oxygen sensor installed in the exhaust manifold. The implementation of oxygen sensor led to feedback control system in modern automobiles fuelling systems.

PCV VALVE: PCV- Positive Crankcase Ventilation valve system is used to redirect the vapours produced during the normal combustion in the crank case to the intake air fuel system which will re enter into the combustion chamber along with the air fuel mixture to utilize them for better combustion. These vapours are diluted in such a way that they are in control of getting mixed with air-fuel, because when the engine runs at idle mode, the air fuel mixture will be very critical, only a part of vapours is allowed to get mixed. Also if the engine runs at high speed mode, the mixture in not much critical, since the pressure generated in the engine is higher, more vapours are allowed in to the engine creating a better combustion with emission. Thus this valve controls and monitors the complete process while running.

The valve used for this application should be the right one, because other kinds of valves may lead to clogging by creating high pressure by pushing the seals in engine and creating leakage in whole system. This ultimately reduces the performance of the engine by sucking out the entire oil out from the engine and making it to run inefficiently in idle condition.

EGR valve: EGR - Exhaust Gas Recirculation valve monitors the small amount of exhaust gas entering the induction system. The combustion chamber temperature is lowered by the recirculation of the exhaust gas which dilutes the air/fuel mixture. This enhances the excessive creation of highly pollutants like nitrogen oxides which ultimately affects the engine, if the exhaust gas is not properly re circulated. Since engine is not designed to run with its exhaust gas as an input. So in order to get rid of this problem a perfect control and monitoring is implemented for the flow of exhaust gas into the induction system. This can be made by setting up a series of electrical and vacuum switches interfaced with a computer control inside the vehicle. As the dilution of the air/fuel mixture affects the performance of the vehicle, the EGR is destined in such a way that it should not act while the engine is running in cold condition as well as in full power as burnt fuel occupies considerable amount of space in fresh charge which greatly reduces the power available from the burning of the fresh fuel as burnt charge cannot generate additional power as it can burn again.

Evaporative Controls: A property of gasoline is that it evaporates quite easily. In earlier days, the evaporative emissions get vented to the open space. About 20% of the Hydrocarbon emissions are from the gas reservoir of the vehicles. Later, due to over pollution, a legislation act was passed on the year 1970 to reduce the venting of evaporated emissions directed to the atmosphere. To act upon this rule evaporative control system was designed and developed to get rid of the pollution caused by the evaporated emissions. The role of the evaporative control system is to stop allowing the emission from escaping out from the fuel reservoir and carburettor by trapping and storing them and supply to the engine when required. A charcoal canister is used for trapping and storing the evaporated fuel. The evaporated fuel in the form of vapours will be adhered to the charcoal canister until the engine gets started to run. And the vacuum created by the engine will draw the fuel in the form of vapours from the charcoal canister. This helps in burning the vapours to be utilized for combustion along with the air/fuel mixture. A sealed gas filler cap is needed in the reservoir of the system. This filler cap is now tested and implemented in almost all state emission inspection programs. Before 1970, all the automobiles were releasing the fuel vapours to the open space by using the vented gas cap. So at present the fuel reservoir is redesigned in such a way that it uses the sealed caps which has some space to collect the vapours and diverting them towards the charcoal canister as well.

A Purge valve is used for controlling the flow of vapour into the engine. The purge valve is operated with the help of vacuum created by the engine. One main disadvantage of the purge valve is that when it gets damaged after continuous use, then the engine directly draws fuel from the reservoir to the induction system abruptly. This will enrich the fuel mixture and spoil the spark plugs and the mileage of the automobile will get dropped ultimately. In order to recover that problem, the charcoal canisters with filter should be replaced in regular intervals.

AIR INJECTION: Air injection system is an emission control system which does not provide any effect on performance of the engine. Every Internal combustion engine are designed to match the 100% efficiency, but which were not achieved so far. So the combustion of the air/fuel mixture is incomplete with increased hydrocarbon emissions. In all cases, the combustion occurs in the presence of three elements. They are Fuel, oxygen and heat. Combustion cannot occur in absence of any one of the elements. Usually there will be enough heat at exhaust manifold to support combustion. If sufficient amount of oxygen is introduced with the unburnt fuel, it will get ignited. This will ultimately reduce the hydrocarbon content in the emissions. But this combustion occurring at the exhaust manifold is uncontrolled, which is not like the combustion occurring in combustion chamber. In this case, if the fuel content is in excess in the exhaust, then explosions with popping sound occurs. During deceleration under normal conditions with excess fuel content, the injection system should be shut off in order to avoid the explosions. So for this case diverter valve is used which will divert the flow of air flowing towards the exhaust manifold instead of shutting down the pump. This can be performed only after the completion of the combustion process. It is also a kind of emission control system.

Catalytic convertor

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1d/DodgeCatCon.jpg/220px-DodgeCatCon.jpg

The main aim of the catalytic converter is to convert up to 90% of emissions like carbon monoxide, hydrocarbons, and nitrogen oxides in to less harm full gases. This basic operation done by catalytic convertor is to reduce the toxicity of emissions from an internal combustion engine. This widely introduced in 1975 by USA for the series-production automobiles, this most commonly used in trucks, busses, trains, aeroplanes, and other engine equipped machines.

However this was first introduced by a French engineer Eugene houndry in 1950 who was expert in oil refining in the US. Further it is developed by John J.Mooney and Carl D.Keith , the first catalytic converter production taken place in 1973.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/7a/Aufgeschnittener_Metall_Katalysator_f%C3%BCr_ein_Auto.jpg/220px-Aufgeschnittener_Metall_Katalysator_f%C3%BCr_ein_Auto.jpg

The catalytic converters are mainly of ceramic material this is in a honey comb shape, openings with a shape of square 1for the exhaust gases. There are 400 openings per square inch approximately (62 per cm2) 0.006 in in wall thickness. Wash coat i.e. Porous aluminium material is coated to substrate, which makes the rough surface. Then on top of the wash coat catalytic material is applied. Then a round and oval shaped casing is provided on the substrate this is of aluminium or stainless steel together.

In monolithic converters this ceramic substrate is not restrictive, but it does break when it subjected to shock or severe jolts, The manufacturing cost of monolithic convertors is very high. These can be serviced but only in a unit. To carry the gasses from the catalytic convertor to silencer or exhaust from the manifold a exhaust pipe is connected between them.

The exhaust gasses entering in to the catalytic converter from the engine contains mainly oxides of nitrogen, oxygen, carbon. These exhaust oxide gases are converted into the less harmful gases and released in to the atmosphere; this is done by the rhodium, palladium, and platinum which acts as an catalyst. The gases passing through the catalyst first separates the oxygen and nitrogen and then converted by using the fresh air into the harmless gasses like carbon dioxide and water vapour by oxidising carbon monoxide and hydrocarbons which present in the exhaust gasses.

The catalytic converter should be located near to the exhaust manifold for the efficient results because the gases from the engine need to reach catalytic converter before they cool down, If not they turn into harm full gasses.

The gasoline used in the modern automobile is a complex blend of both straight and branched Chain hydrocarbons. In simpler terms it is a mixture of different types of bunches of hydrogen and carbon. We will use the fictitious molecule C8H17 to approximate the blend of different hydrocarbon compounds found in gasoline. In simpler terms one gasoline molecule contains 8 atoms of carbon for every 17 atoms of hydrogen and nothing else.

ONE GASOLINE MOLECULE

GASOLINE IS --> C8H17

8 CARBON ATOMS + 17 HYDROGEN ATOMS BONDED TOGETHER

COMPOSITION OF AIR

1 PART OXYGEN (O2) AND 4 PARTS NITROGEN (N2)

When gasoline is mixed with air and ignited in the combustion chamber it burns, and in doing so reorganizes the hydrogen, carbon and oxygen atoms. As these atoms are reorganized they can form CO, CO2, H2O, NO (and other NOx), and of course if some of the gasoline is left unburned, C8H17 or other forms of generic HC.

Optimum combustion occurs at an A/F ratio of about 14.64:1. If all of the fuel vaporizes and takes part in combustion and no NOx is formed we would have perfect combustion. Perfect combustion would result in the formation of nothing CO2, H2O.

Perfect combustion: Air + Fuel CO2 + H2O (and nothing else). Unfortunately as more and more CO2 is formed the temperature goes up. As the temperature increases, NOx is formed. NOx formation uses up the oxygen that is needed for CO2 formation

Real World combustion: Air + Fuel CO2 + H2O + NOx + CO (and unburned HC, O2 & N2)

NOx emissions are at their highest between 14.64:1 and about 16.5:1

HC emissions increase whenever the mixture is richer or leaner than about 14.64:1. Under lean conditions, the fuel charge will sometimes fail to ignite and result in high HC emissions. This is known as a lean misfire. Under rich conditions, some of the fuel fails to burn because there is not enough oxygen.

Muffler: - Muffler is nothing but a set of perforated tubes which reflects sound waves. It is known fact that combustion in the engine produces considerable amount of noise which has to be reduced in order to have reduced noise pollution on the road. The perforated tubes in muffler reflect the sound waves which collide with each other and cancel each other.

Turbocharger

A turbocharger is a device which consists of a turbine and a radial flow compressor mounted on a common shaft. It can be referred as a supercharger powered by a turbine. The compressor is driven by the turbine which uses the energy in the exhaust gas, which sucks in outside air, recompresses it and finally is supplied to the cylinders at the pressure above atmospheric pressure. Rotor assembly, compressor housing and bearing housing are common components of turbocharger. The engine lubrication system generally supplies oil to the shaft bearing for lubrication whereas engine coolant is circulated in the housing to provide cooling. A turbocharger is generally used to increase the power and performance of the engine by increasing or giving boost to the amount of fuel which can be burned in cylinders.

Turbocharged engines: - if one wants more horsepower out of the car more amounts of fuel and air has to be supplied to burn more fuel. It can be achieved if one force feeds the engine through turbocharger. The exhaust gas comes out of the exhaust manifold. It then flows into the turbine housing in the turbocharger. This spins the turbine and then the exhaust gas exits to the exhaust system. The compressor is attached to the turbine wheel though a common shaft. Thus when the turbine wheel rotates the compressor wheel rotates. This compressor wheel is kept between the air filter and the throttle body on the intake side of the engine, which as and when rotates faster and faster, it compresses the intake air entering the engine, in turn increasing the pressure of air in the engine, so when the intake valve opens every time it allows more air to enter the cylinder of the engine and due to it more fuel is also added. Some of the exhaust gas will bypass the turbine via waste gate valve once the pressure reaches predetermined level so as to prevent the turbocharger to reach self destructive speeds.

Compressed air before entering the combustion chamber, it will go through the intercooler. Intercooler is very vital component of the turbocharger as the heat generated by the compressed air is massive. when the air compresses the temperature also increases and as one knows that hot at any given particular pressure takes up more space than the cold air at same pressure, thus it need to be cooled off before entering the cylinders so as to accommodate more air. Intercooler can be generally the radiator. Bigger the intercooler higher air content can be drawn, but bigger intercooler increases the time lag of the turbo. To minimise this time lag, dump valve are used.

Literature Review

(kajsdgbaskd ) has shown that The exhaust emission can be improved by using engine modifying approach.

1. BY USING LEANER AIR FUEL RATIO :- the carburettor in an engine can be designed and modified in such a way that during the idling and cruise operation it gives stable and lean air-fuel ratio but the idle speeds has to be increased along with the modification. Also with better manifold design, increasing the coolant temperature, heating of inlet air and use of electronic fuel injection improves the distribution of fuel.

2. IGNITION TIMIMG RETARDATION: - Time for fuel burning is increased by retarding the ignition timing. NOx emissions are reduced by retarding the temperature by decreasing the maximum temperature also the exhaust temperatures are higher in turn reducing the HC emission. on the other hand, greater cooling is required due to the retardation of ignition timing and also that some amount of fuel economy and power is lost in the process.

3. REDUCING QUENCH AREAS BY MODIFYING COMBUSTION CHAMBER :- Combustion is incomplete if due to configuration of combustion chambers flames is quenched as certain places which increases the HC emission. Thus the combustion chamber can be modified in order to avoid these flames quenching zones. This can be achieved by reducing the surface to volume ratio, reducing the squish area, and by reducing the distance of the top piston ring from the top of the piston.

4. LOWER COMPRESSION RATIO: - if the quench effect is reduced by reducing the quench area, HC emissions can be reduced. This can be achieved by having the lower compression ratios. NOx emissions can also be reduced by lowering compression ratio due to maximum temperature, again some fuel and power is lost by reducing the compression ratio. unleaded gasoline can be used.

5. REDUCED VALVE OVERLAP: - some mixture always escapes directly due to increased valve overlap which results in increased emission level thus this can be controlled by reducing the valve overlap.

Modelling In RICARDO WAVE

A simple model as shown in the figure has been modelled in Ricardo WAVE simulation software. As shown in the figure the engine has six inline cylinders, having separate fuel injectors has been designed. Various objects used in Ricardo to make the model closely related to the actual conventional engines. These objects are listed below prior to their brief explanation in order to justify their use in this particular model.

To make a comparison two different model have been modelled in Ricardo WAVE software,

Basic model (FIGURE)

This model contains no emission control devices but it has a turbocharger, muffler and silencer in it.

Emission control model

Emission control model contains emission control devices such as catalytic convertor and exhaust gas recirculation.

Figure : Emission control model developed in Ricardo WAVE 8.2

In order to understand how the engines have been modelled in Ricardo WAVE, it is important to discuss about some important features of the software which allows the simulation of the engine, therefore some of the important features as shown below have been discussed.

Ducts

Simple duct

Catalytic duct

Orifice

Y junctions

Simple Y junction

Complex Y junction

Engine block

Ducts

Simple duct

Ducts are used to connect two various parts of the engine, they allow the fluid to pass through with pressure and temperature changes along their length. It is to be noted that this simulation software is working on computational fluid dynamics where the body of any object is converted into different smaller volumes and then their solution is converted into final result. A duct has to be specified through physical properties such as its length, diameter, angle etc and friction coefficient, fluid temperature and wall temperature etc. Apart from these there are various other properties but they are not important from the current project point of view, therefore they haven't been discussed here.

Catalytic duct

Catalytic ducts are used to represent catalytic convertor; it has number of parallel ducts in it through which the fluid passes through.

Orifice

An orifice is used to connect ducts having different diameter, it one of the simplest yet important tools of Ricardo WAVE software.

Y junctions

Simple Y junction

When one duct is required to connect with more than one duct or if there is a mass less or catalytic duct in manifolds Y junctions are used. Characteristic of fluid flow highly depends upon the angles and positioning of inlet and outlet ducts of the Y junction. Also diameter of the Y junction is an important factor as it should be set according to the diameter of inlet and outlet ducts.

Complex Y junction

The only difference between simple Y junction and complex Y junction the complex Y junction is more complex and it requires many constants to be defined prior to its use. However the fluid flow through complex Y junction is more uniform and can be controlled according to the user. Apart from that some of the features can be connected with the use of complex Y junction.

Engine block

Engine block is used to model a combination of cylinders especially when there is more than one cylinder in the engine. In Ricardo two options are available

As shown in the graph there is an increment in power with increasing speed which is obvious because power is directly proportional to speed and therefore generally at initial stage the power increases due to torque and speed. When torque reaches its peak, i.e. at the time of optimum combustion the power may or may not reach its peak depending upon characteristic of engine. However it is obvious that after this peak due to decreasing torque the power will be decreasing.

An engine is modelled in Ricardo in order to obtain power versus speed graph at full load, half load and part load. It is obvious that at half load the power will be lower than it is at the full load as less amount of fuel air mixture is entering in the engine.

Discussion and Results from Ricardo

Discussion and results from Ricardo has been divided into three section as shown below,

Brake power

Brake Torque

Exhaust emissions

Brake Power

To compare the influence of EGR the engine was modelled in a simplest manner without EGR. Power and torque were obtained through wave post. The figure below shows power and torque for the simple model at full load, half load and part load. It is to be noted that while the automobile is running on the road it is not driven at constant full or half or part load. Depending upon the requirement the engine is enabled to generate power at different load. Therefore it is required to obtain power at different loading condition, in automatic or self operated machines governor is used to control the power output according to load.

As shown in the figure below air is supplied to the engine through throttle valve and its opening is manually controlled by the user (FIGURE). The throttle opening is an indication of the power requirement of the engine. At full load the engine requires full throttle opening at half load it required half throttle opening and so on. Depending upon the air flow and speed of the engine the fuel is supplied either directly in the engine or just before the engine through injectors. At lower speed and low load condition lean mixture of fuel and air is required while at higher speed and full load condition richer fuel- air mixture is required. To make the engine equivalent to the actual one the injectors have been added to the duct just before the engine.

Engine at part load

Power:

RPM

Power (KW)(Basic model)

Power (KW)(With EGR)

6999.998

78.2562

76.2852

6000

80.2581

81.2567

4999.978

76.91959

78.259

3999.983

70.20134

73.7592

3000.011

66.2869

67.2593

1999.999

53.31211

50.24

1000.003

35.00781

29.19359

Table : Tabular data presenting Power of both models at part load

Figure : RPM versus POWER graph of engine at part load

As shown in the figure above at part load the engine with EGR is generating higher amount of power compared to the basic model without EGR. Though the increment is not significant it can be seen that with EGR the power curve is smoother the more uniform than the engine without EGR. Supply of burnt exhaust gases has increased the temperature of the fresh fuel and therefore it has made the combustion of charge in more uniform way which in turns has made the power output curve uniform.

Engine at half load

RPM

Power (KW)(Basic model)

Power (KW)(With EGR)

6999.998

117.8892

122.3359

6000

129.282

128.8131

4999.978

124.4141

131.0452

3999.983

118.8355

121.14

3000.011

100.7309

107.8101

1999.999

78.55713

80.33776

1000.003

41.8782

32.10911

Table : Tabular data presenting power of both models at half load

Figure : RPM versus Power graph comparing power of both models at half load

From above figure it can be said that power at half load in model having EGR is uniform and higher compared to the basic one. The reason is the same as discussed previously in case of part load condition.

Engine at full load

RPM

Power (KW)(Basic model)

Power (KW)(With EGR)

6999.998

172.5996

177.3045

6000

194.4271

201.2561

4999.978

195.7352

200.5689

3999.983

186.85

191.5637

3000.011

153.4799

160.9297

1999.999

104.9373

102.3013

1000.003

49.57498

40.07592

Figure : Tabular data presenting power of both models at full load

Figure : RPM versus Power graph comparing power of both models at full load

As shown in the figure there is still increment in power at full load but in terms of uniformity both are almost same. Possible reason may be at high load the throttle is completely open and due to higher power requirement the engine consumes considerable amount of charge therefore EGR is not making considerable amount of change in uniformity of the curve and actually the exhaust gas recirculation device hasn't been preferred during full load as it reduces available power at full load.

It is to be noted that during the operation the engine is not running at constant load. Geology of the road, speed required are few of the factors which can affect the load requirement from the vehicle. Therefore it is important to find out performance of the engine at part load, half load and full load because generally at lower speed when the power requirement is lower the engine is running at part load, at mean speed or half speed the engine is running at half load and during high speed the engine is running at full load condition.

As per the reasons discussed above the reference line was drawn connecting all three power curves at different load to obtain power at different speeds and loads. The figure below shows power curves at different loads.

Figure : RPM versus Power curves for all loads with and without EGR including reference line

As per the reasons discussed above the reference line was drawn connecting all three power curves at different load to obtain power at different speeds and loads. The figure below shows power curves at different loads.

Emissions

Generally three types of emissions are of main concern for any automotive; NOx, COx and HC emissions.

NOx emissions: - It is known fact that fresh air has 79% of nitrogen in it therefore the fuel air mixture entering in the engine is enriched with nitrogen; however process between nitrogen and oxygen won't initiate before certain temperature which is required for nitrous oxide formation in the engine. After 11000C nitrogen reacts with oxygen and nitrous oxides are formed (Mathur, M. L. & Sharma, R. P., 2003). It is obvious that due to this reason nitrogen oxide formation is directly proportional to the break mean effective pressure of the engine as increase in pressure leads to high temperature combustion and causes formation of NOx gases. ASDSADSA has discussed three major factors affecting the NOx formation they are as shown below,

Fuel air ratio

Burnt gas fraction

Ignition timing

Fuel air ratio

If the burnt gases have fuel air ratio more than one (1.1) means if the burnt fuel is richer then lesser amount of oxygen in the fuel air mixture won't allow nitrogen oxide formation to occur. The formation of nitrous oxides required certain amount of oxygen and therefore leaner mixture but not least so that the temperature reduction leads to impossibility of the reaction process. A slight lean fuel air mixture (0.9) is the most favourable to the nitrous oxide formation as it has required amount of oxygen and temperature for the reaction.

Burnt gas fraction

If more amount of burnt gas remains in the combustion chamber after combustion process gets over, overall temperature of the burnt charge remains lower and due to that formation of NOx decreases as its formation is directly proportional to the temperature within the combustion chamber.

Ignition timing

NOx formation within the combustion chamber depends greatly on ignition timing; pressure developed after the combustion in the chamber is inversely proportional to the ignition delay, means ignition delay should be kept optimum in order to have appropriate pressure which shouldn't be more than one which initiate NOx formation.

The figure below shows results obtained from Ricardo WAVE in terms of NOx formation at full load, half load and part load.

Figure : Comparison of NOx formation at part load

It is obvious from the above figure that at high load the amount of NOx formation is higher than it is at lower loads. This is mainly due to higher mean effective pressure at high loads as ignition advance is increased in order to have proper burning of charge before it leaves the combustion chamber this causes proper combustion of fuel but on the other side amount of NOx is increased.

It can be seen that at full load after implementing emission control devices the NOx formation has been decreased but not by considerable margin as exhaust gas recirculation has not been used during full load as it reduces overall power due to exhaust gas fraction in the fresh charge.

At lower loads i.e. half load and part loads amount of NOx formation is less than it is at full load due to the reason discussed above. It can be seen in the figure that there is a considerable margin between NOx emissions of basic model and emission control model. This is because at part load and half load fraction of burnt fuel is sent back to the inlet manifold through exhaust gas recirculation which reduces resultant NOx formation as burnt gas replaces a fraction of combustible charge and this reduces pressure and temperature in the cylinder.

HC emissions: - If the charge is not burnt properly in the combustion chamber then a part of the burnt fuel enters into exhaust manifold along with the combustion products, this event cause HC emissions. It is to be noted that generally in four stroke engine amount of HC emissions are lower than two stroke engines as in two stroke engines larger amount of fresh charge goes into exhaust manifold due to valve overlap. In four stroke engine also valve overlap is introduced in order to have proper scavenging process, but a designer must compromise between scavenging and unburn fuel emissions.

According to Mathur, M. L. & Sharma, R. P(2003) design of a combustion chamber plays an important role in CO formation within the combustion chamber as a fraction of fuel remains far from the centre where combustion occurs and therefore incomplete combustion becomes unavoidable in the chamber which leads to unburned fuel emissions. Figure below shows HC emissions obtained through Ricardo WAVE.

As shown in the figure above amount of HC emissions have remained almost unchanged even after implementing emission control model.

At lower speed engine under full load is emitting more amount of HC emissions as at full load richer charge is supplied to the engine cylinder which causes more amount of unburnt charge in the cylinder.

It can be seen that implementation of emission control devices have made a little impact on HC emissions, which means unburnt hydrocarbons has remained an unsolved problem for the engine.

For improvement an engine with direct compressed air injection can be used which allows more amount of air in the cylinder which in turn causes proper combustion of fuel and therefore reducing HC emissions.

CO emissions

Co emissions occur due to incomplete combustion of fuel in the combustion chamber. It is known fact that combustion of carbon with insufficient oxygen leads to CO formation. Being one of the most poisonous gases the formation of CO must be avoided. When the fuel air mixture is rich, in other words if air fuel ratio is less than the ideal 14:1 than the fuel won't get appropriate amount of oxygen for complete combustion which leads to the scenario listed in the paragraph above. The ideal air fuel mixture should be above 16:1 in order to avoid CO formation as abundant amount of oxygen becomes available for the combustion of fuel. The most obvious situation apart from rich charge is deceleration as the throttle fails to respond sudden decrement of charge in the combustion chamber. A complete opposite condition is during acceleration when throttle is pressed and initially leaner charge is supplied to the engine due to failure of injectors to provide fuel proportional to the air supplied to the combustion chamber.

CO emissions have been obtained through Ricardo WAVE software after simulation as shown in the figure below. It can be seen in the figure that the results are obtained maintaining the engine at full load, half load and part load.

CO emissions are generally unavoidable according to Mathur M, L. (2003), however it can be reduced by considerable margin using leaner fuel, however leaner fuel means more amount of air made available which means increase in NOx emissions therefore an in between ratio should be preferred in order to have least CO emissions.

It can be seen in the figure that basic model at part load and half load is generating highest amount of CO emissions mainly due to richer fuel which means at part load when a fraction of throttle opening is available for the air, rich charge is entering into the cylinder and less amount of oxygen for the combustion of carbon is leading towards incomplete combustion and generation of CO.

At full load the engine performance is better in terms of reduced CO emissions, as the throttle is fully open and appropriate amount of air is becoming available for the fresh charge.

Emission control systems have made a significant impact mainly at part load and half load conditions due to implementation of catalytic convertor.

However at full load the convertor hasn't made a big impact mainly due to high temperatures as catalytic convertor is sensitive to the temperature between certain values.

Power and emission control

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