Shaping the Future Emissions Formation and Control

Formation and In-Cylinder Control  Prevention is better than cure ! (and its cheaper )  Consider formation and in-cylinder control for;  Carbon Monoxide (CO)  Hydrocarbons (NMOG)  Nitrogen Oxides (NOx)  Particulates

Formation and In-Cylinder Control  Prevention is better than cure ! (and its cheaper )  Consider formation and in-cylinder control for;  Carbon Monoxide (CO)  Hydrocarbons (NMOG)  Nitrogen Oxides (NOx)  Particulates

CO Formation & In-Cylinder Reduction  Produced through incomplete combustion  Affected by ;  oxygen starvation (rich mixtures)  fuel vapourisation rates;  poor injection quality  low ambient temperatures  Not a significant concern for diesel applications and other lean burn systems

Formation and In-Cylinder Control  Prevention is better than cure ! (and its cheaper )  Consider formation and in-cylinder control for;  Carbon Monoxide (CO)  Hydrocarbons (NMOG)  Nitrogen Oxides (NOx)  Particulates

HC Formation & In-Cylinder Reduction Pre Ignition HC Formation Excess air and excess fuel can result in very slow flame speeds or no ignition Large (cold) surface areas can cause flame quenching

HC Formation & In-Cylinder Reduction Post Ignition HC Formation Local pyrolysis of the fuel will prevent vapourisation Slow air-fuel mixing results in local over-rich mixtures

In-Cylinder Spatial and Oil Distribution Effects HC Formation & In-Cylinder Reduction

In-Cylinder Spatial Effects - Crevices HC Formation & In-Cylinder Reduction

In-Cylinder Spatial Effects - Crevices HC Formation & In-Cylinder Reduction

Air Fuel Ratio HC Formation & In-Cylinder Reduction High air-fuel ratio (reduced equivalence ratio) for example during transient accelerations reduces laminar (and hence turbulent) flame speeds causing unstable combustion (high COV of IMEP) and increased HC (HC Tail)

HC within wall quench layer increased by the presence of surface deposits (more traps) – less HC from a new engine Oil layer absorbs HC during compression and combustion and release during exhaust  Blow down turbulence ejects HC from port area  HC scraped off cylinder walls during exhaust stroke  Coldest gas (highest HC) ejected at the end of the stroke Time Dependence HC Formation & In-Cylinder Reduction In-Cylinder Spatial and Oil Distribution Effects

In-Cylinder HC Reduction ;  Good transient Air-Fuel Ratio control  Good combustion chamber design to improve Air-Fuel Ratio excursion tolerance  Minimising crevice volumes  Minimising surface areas (chamber & port surfaces)  Minimising oil and other deposits  High coolant and chamber wall temperatures (good warm up)  Optimum fuel preparation – no large droplets (min sac volumes) HC Formation & In-Cylinder Reduction

Formation and In-Cylinder Control  Prevention is better than cure ! (and its cheaper )  Consider formation and in-cylinder control for;  Carbon Monoxide (CO)  Hydrocarbons (NMOG)  Nitrogen Oxides (NOx)  Particulates

NOx Formation & In-Cylinder Reduction Nitrogen from the air is oxidised during the combustion process. There are six oxides of nitrogen: ImportantUnimportant nitric oxide NOnitrogen sesquioxideN 2 O 3 nitrogen dioxide NO 2 dinitrogen tetroxideN 2 O 4 nitrous oxide N 2 O dinitrogen pentoxideN 2 O 5 (s) Fortunately only NO, NO 2 and N 2 O are important in the automotive industry. Only NO and NO 2 are regulated as NO x Nitric Oxide is the predominant oxide in exhaust gas: gasoline engines NO 2 /NO <2% diesel engines NO 2 /NO 10-20%

NO concentration can be predicted with reasonable accuracy using chemical rate kinetics; Reaction rate constants for the forward (f) and reverse (r) directions are available so enabling the rate of NO formation to be given by (Zeldovich) The rate ‘constants’ are highly temperature dependent ~ NOx formation is very much dependent on combustion temperature NOx Formation & In-Cylinder Reduction

NOx is formed at peak combustion temperatures During the expansion stroke the temperatures fall and the NO reaction chemistry ‘freezes’ The higher the peak combustion temperatures the greater the NOx Hence strong AFR dependence…

HC NOx Formation & In-Cylinder Reduction  Reducing peak combustion temperatures by ;  Delaying onset of combustion  Prolonging combustion  Increasing the inert charge mass (thermal sponge) through internal or external Exhaust Gas Recirculation  Reducing the compression ratio  Running lean  Most NOx reduction methods also reduce the thermal efficiency  Often an HC – NOx trade off

High Pressure Low Pressure EGR acts as a thermal sponge and significantly reduces NOx Cooled EGR – higher density NOx Formation & In-Cylinder Reduction

High EGR causes HC, fuel economy and particulates issues NOx Formation & In-Cylinder Reduction Particulates

 Increasing the injection duration (multiple injections?) reduces NOx slightly  Similarly delaying SOI  Both adversely affect Specific Fuel Consumption and HC NOx Formation & In-Cylinder Reduction

Formation and In-Cylinder Control  Prevention is better than cure ! (and its cheaper )  Consider formation and in-cylinder control for;  Carbon Monoxide (CO)  Hydrocarbons (NMOG)  Nitrogen Oxides (NOx)  Particulates

A collection of solid largely carbon particles (primary particles or spherule) often ‘stuck’ together with semi liquid matter Ref Eastwood et al Big particles are many spherules stuck together, not bigger spherules Primary Particle Formation Transportation & Evolution Deposition Destruction Particulate Formation

Particulate Composition of Diesel Engine Exhaust

The Primary Particle (Spherule)

EGR significantly increases primary particle sizes under most engine speed & load conditions The Primary Particle (Spherule)

Particulate Size Evolution

Particulate - NOx Trade Off Effect of Start of Injection (SOI)

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