Week 4

Question 1

How do four stroke engines work?

Answer 1

Inlet stroke receives gas
Compression stroke has piston moving up
Constant volume combustion
Expansion/power stroke moves piston down due to heat released
Constant volume exhaust - exhaust valve opens
Exhaust stroke - piston moves up and gas escapes

Question 2

How do ICEs work in petrol cars?

Answer 2

Spark-ignited
Require stoichiometric (perfect) gas fuel mix
Spark plug detonates ignition
Intake air throttled - leads to pump loss
Homogeneous fuel air mix reduces emissions

Question 3

How do ICEs work in diesel cars?

Answer 3

Compression ignition
Lean air fuel mix (more air)
Ignition occurs as fuel sprayed in hot compressed air
No throttle so higher efficiency
Inhomogeneous fuel mix - leads to incomplete combustion as higher concentrations of fuel occur in spots

Question 4

How does a catalyst work?

Answer 4

Affects rate of combustion, unaffected by reaction. Reduces activation energy for reactants to react and converts HC, CO and NOx to CO2, H2O, and N2

Question 5

What does catalyst selectivity mean?

Answer 5

Species catalyst acts on

Question 6

What is the catalyst layout?

Answer 6

A substrate with catalyst covering (Al2O3) and a nano particle coating (platinum). In grid structure with high surface area and porosity to increase reaction rate

Question 7

How does conversion rate relate to temperature?

Answer 7

Low temp - rate controlled by chemical kinetics and activation energy
Medium temp - controlled by diffusion into pores
High temp - controlled by bulk mass transfer rate of gas to pore entrance as diffusion and reactions fast

Question 8

How can catalysts be deactivated ?

Answer 8

1. Fouling by lead and sulphur
2. Thermal deactivation where platinum particles migrate and form larger particles reducing surface area
3. Thermal induced changes close pores
4. Reactions with intake particles

Question 9

How does the three way catalyst for spark ignited engines work?

Answer 9

Stoichiometric operation required
CO and HCs oxidised by NO, reducing NO to N2, and making CO2 and H2O

Question 10

When lean operation is used for three way catalyst how does this affect NOx conversion and CO and HC conversion?

Answer 10

More oxygen means NOx conversion less efficient - more O2 to make NO and NO2
But efficiency for HC and CO2 improves

Question 11

How are diesel emissions controlled?

Answer 11

Turbo intakes fresh air and sends to combustion cylinder
Exhaust recirculated to turbo with exhaust sent to intake to reduce NOx by less oxygen particles and lower temp
Exhaust gas sent through oxidation catalyst to remove CO and HC
+ additional removal processes outlined in next questions

Question 12

What is exhaust gas recirculation?

Answer 12

NOx absorbers trap NOx and reduce it to N2. This is released to reset trap

Question 13

How does Selective Catalytic Reduction work?

Answer 13

Adds ammonia to exhaust to oxidise NOx
This ammonia is added in the form of urea solution

Question 14

What is the Diesel Particulate Filter? (DPF)

Answer 14

This is a filter which captures emissions and removes particles through soot oxidation (burning)

Question 15

How can pollutant dispersion be modelled?

Answer 15

Computational fluid dynamics at street level (10m-100m)
Regional scale models from 1-1000m
Gaussian plume model for street to city level

Question 16

How does the computational fluid dynamics model work to improve street safety? Why is it limited in scale?

Answer 16

Captures pollutant trapping and determines safe side of road
Computationally expensive

Question 17

How is the regional scale model created?

Answer 17

With an input emissions inventory and meteorology data
Equations are implemented to account for photochemistry and aerosol dynamics

Question 18

How is the Gaussian plume model set?

Answer 18

It assumes wind disperses from plume with the height of the plume increasing in y and z downstream
The spread of the plume is described by a normal distribution in y and z at each point along the plume in x
Assesses concentration at x,y,z as g/m3

Question 19

What are assumptions made by the Gaussian plume model?

Answer 19

Constant emission rate
Wind speed constant in time and elevation
Pollution not lost by decay or reactions
Flat terrain

Question 20

How is the concentration assessed when there is a ground level below the source of release at z=h ?

Answer 20

Making a mirror source at z= -h and adding the concentrations from both sources

Question 21

How does the atmospheric condition affect plumes?

Answer 21

The rate temperature changes with altitude (lapse rate) affects the plume as it impacts the spread in z and y (sigma z and sigma y)

Question 22

How are atmospheric stability classes used?

Answer 22

There is a rating system of A (very unstable ) to F (stable)
Plots relate these classes and downstream distance to sigma y and sigma z
Otherwise equations can be used with constants related to the stability classes

Question 23

How are infinite line sources modelled ?

Answer 23

Q in atmospheric dispersion equation becomes QL = Q/L (g/m/s)
Plume model is integrated from y= -inf to y=inf and assessed at z=0 with the concentration multiplied by 2 (image term)
Concentration is g/m2