IC Engine Design Strategies Vs Exhaust Emissions

P M V Subbarao

Professor

Mechanical Engineering Department

An Efficient Engine Design is not Obviously Eco-friendly Design…

Concentration of any chemical is pollution but not the quantity…..

Emission sources in a gasoline fuelled car

Emission sources in a diesel engine powered bus

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Adverse Health Effects of IC Engine Generated Air Pollutants

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Pollutants Short-term health effects Long-term health effectsCarbon

monoxideHeadache, shortness of

breath, dizziness, impaired judgment, lack of motor

coordination

Effects on brain and central nervous system, nausea, vomiting, cardiac and pulmonary functional

changes, loss of consciousness and death

Nitrogen dioxide Soreness, coughing, chest discomfort, eye irritation

Development of cyanosis especially at lips, fingers

and toes, adverse changes in cell structure of lung wall

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Oxidants Difficulty in breathing, chest tightness, eye

irritation

Impaired lung function, increased susceptibility to

respiratory functionOzone Similar to those of NO2 but

at a lower concentrationDevelopment of

emphysema, pulmonary edema

Sulfates Increased asthma attacks Reduced lung function when oxidants are present

TSP/Respirable suspended particulate

Increased susceptibility to other pollutants

Many constituents especially poly-organic

matter are toxic and carcinogenic, contribute to

silicosis, brown lung

Emissions Control

• Three basic methods used to control engine emissions:

• 1)Engineering of combustion process -advances in fuel injectors, oxygen sensors, and on-board computers.

• 2) Optimizing the choice of operating parameters -two Nox control measures that have been used in automobile engines are spark retard and EGR.

• 3) After treatment devices in the exhaust system -catalytic converter.

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Three chemical reactions form the Zeldovich reaction are:

Zelodvich reaction is the most significant mechanism of NO formation in IC engines.

Forward rate constants:

Tk

Tk

Tk

f

f

f

/450exp101.7

/4680exp108.1

/38370exp108.1

10,3

7,2

11,1

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Global Reaction Rate

• Using the chemical reactions given, one can write the following expression for the rate of change of nitric oxide concentration.

• Where the brackets denote concentrations in units of molecules/m3.• Approximations to solve above equation:

• The C-O-H system is in equilibrium and is not perturbed by N2 dissociation.

• This means that the pressure, temperature, equivalence ratio and residual fraction of fluid element only are required to calculate NO concentration.

• N atoms change concentration by a quasi-steady process.• This means that one can solve for the N atom concentration by

setting the rate of change of atoms to zero.

HNOkOHNkONOkONkNNOkNOkdt

NOdbfbfbf ,3,3,22,2,12,1

0,3,3,22,2,12,1 HNOkOHNkONOkONkNNOkNOkdt

Ndbfbfff

Using two approximations together:

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1

12

1

160

RRR

R

pdt

dxNO

Where xNO is mass fraction of NO and is the ratio of instantaneous NO mass fraction to equilibrium mass fraction of NO.

eNO

NO

x

x

,

eef

eef

eef

OHNkR

ONkR

NOkR

,33

2,22

2,11

Practical Aspects of NOx Reactions

• NO formation is kinetically controlled.

• The NOx reactions are quasi steady.

• The concentrations of [O2], [O], [OH], [H] & [N2] are in equilibrium.

• Formation of N is slow and hence [N] is always in steady state.

• The practical kinetics of formation of NO shows:

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Effect of Equivalence Ratio on NO Concentration

Effect of Various Parameters on NO Concentration

Increased spark advance and intake manifold pressure both result in higher cylinder temperatures and thus higher NO concentrations in the exhaust gas

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Exhaust NO Concentration Reduction

Since the formation of NO is highly dependent on cylinder gas temperatureany measures taken to reduce the AFT are effective:

• increased residual gas fraction• exhaust gas recirculation (EGR)• moisture in the inlet air

In CI engines the cylinder gas temperature is governed by the load and injection timing

EGR for Reduction of NO

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EGR System

The EGR flow is directed by a solenoid valve that is controlled by the Power train Control Module (PCM). The PCM uses inputs such as engine speed, intake-manifold pressure, and engine temperature to determine how much exhaust flow to meter back to the air-fuel mixture.

Closing Remarks on NO Control

• Unfortunately, EGR also reduces flame speed.

• Excessive EGR rates can slow burn rate so drastically as to cause surge, misfire, and other combustion problems.

• However, EGR can increase fuel economy.

• When used optimally, EGR can reduce NO and improve fuel economy while avoiding rough engine operation.

• This optimum varies with speed, load, and engine design.

• Nox also varies with spark timing.

• More spark advance means that parts of the mixture spend more time at high temperatures, so more NO is formed.

Hydrocarbon Emission Sources

Crevices – these are narrow regions in the combustion chamber into which the flame cannot propagate because it is smaller than the quenching distance.Crevices are located around the piston, head gasket, spark plug and valve seats and represent about 1 to 2% of the clearance volume.The crevice around the piston is by far the largest, during compression the fuel air mixture is forced into the crevice and released during expansion.

Hydrocarbon Emission Sources for SI Engines

There are six primary mechanisms believed to be responsible for hydrocarbon emissions:

% fuel escaping Source normal combustion % HC emissions

Crevices 5.2 38Oil layers 1.0 16Deposits 1.0 16Liquid fuel 1.2 20Flame quench 0.5 5Exhaust valve leakage 0.1 5

Total 9.0 100

RT

pxx

Tdt

HCdOHC 2

ˆˆ18735

exp107.6 15

HC emission control:

• Three basic types of emission control systems are used in modern vehicles; pre-combustion, post-combustion and evaporating control system.

• Pre-combustion control; (Positive Crankcase Ventilation (PCV).

• Engine Modification System; (Secondary air or air injector systems, and Catalytic converters).

• The evaporative control system; (traps the fuel vapors from fuel tank, maniolds and carburetor).

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Setting of Air/fuel Ration at Idling Conditions

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Selection of Idling RPM Vs HC Emissions

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Coolant Temperature Vs HC Emissions

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Ignition Timing Vs HC Emissions

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Effect of Misfiring on HC Emissions

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Bore to Stroke Ratio Vs HC Emissions

• There will be a direct consequence of lower heat losses at high S/B ratio and much greater than combustion higher combustion temperatures,.

• These are associated with increasing the stroke and making the chamber portions more lean.

• It could be speculated that the long-stroke (small-bore) engine would be advantageous in terms of unburned hydrocarbon emissions since crevice volumes, and hence trapped unburned charge in such volumes, much smaller in the small-bore engine.

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