Emissions

Question 1

What are the main pollutants

Answer 1

unburnt hydrocarbons

carbon monoxide

NOx

particulate matter (CI)

Question 2

What does NOx encompass

Answer 2

Nitric oxide NO
- hightly reactive
- not toxic
- major precursoer of smog (NO2)

Nitrogen dioxide (NO2)
- brown poisonous gas
- NO2 < 10% of NOx
- adverse health effects

Question 3

explain NOx formation

Answer 3

thermal mechanims -> N2 is attached by O2, this occurs at high temperatures in oxygen rich environments

Prompt NO mechanism -> reaction between N2 and CH, not temperature dependant

Question 4

compare emissions to equiv ratio for an SI engine… explain the trend

Answer 4

at low equiv ratio -> low combustion temp so we have low NOx formation. Lean combustion means that all the fuel is burnt (so low UHC) and all C is converted into CO2 due to the abundance of O

at stoich conditions -> combustion temp is the highest so NOx is highest

at high equiv ratio -> temp is low again so NOx doesnt form, as the fuel is rich there is lots of UHC as combustion and CO as there is not enough air for complete combustion

Question 5

How can we reduce NO

Answer 5

premixed
- use lean combustion (high AFR and low temperature
- improve exhaust mixing

non premixed
- difficult to recude as always have fuel lean condition
- reduce hot spots in the cylinder and improve mixing