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
Equivalence Ratio
Rich (Excess Fuel)Lean (Excess Air)
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:
Important Unimportant
nitric oxide NO nitrogen sesquioxide N2O3
nitrogen dioxide NO2 dinitrogen tetroxide N2O4
nitrous oxide N2O dinitrogen pentoxide N2O5 (s)
Fortunately only NO, NO2 and N2O are important in the automotive industry. Only NO and NO2 are regulated as NOx
Nitric Oxide is the predominant oxide in exhaust gas:
gasoline engines NO2/NO <2%diesel engines NO2/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 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 Formation
Particulate Composition of Diesel Engine Exhaust
Particulate
Soluble (Volatile) Fraction
InSoluble (Non Volatile) Fraction
Sulphates OrganicsInorganics
(Ash)Carbon Nitrates
4-5 Ring Poly Aromatic HC Rings
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)
Thank You for Listening