The Reduction of Pollutants from Diesel Engines

By Sean McSweeney

How A Diesel Engine Works• Air is taken in and compressed by the piston.• As the pressure of the air rises, so does the

temperature. • When the temperature is over 200 C, fuel is

injected.• The fuel is ignited by the hot air, and the energy

released pushes the piston.• Exhaust is released, containg NOx and

particulate matter.

Diesel Engines vs. Petrol Engines

• Diesel engines are more efficient than petrol engines– It will give more torque at lower speeds– It energy content is 38.6 MJ/L – Lower auto ignition temperature

However, diesel engines produce higher levels of particulate matter.

Major Pollutants

• Soluble Organic Fraction (SOF)

• Particulate Matter (PM)

• NOx

Diesel Oxidation Catalysts

• Honeycomb Monolith Structure• Oxidation of CO, HC, SOF to less hazardous

components• Coated with precious metals to help reduce posionous

emissions

Diesel Particulate Filter (DPF)

Regenerating the DPF

• Catalysed DPFsCatalysing Washcoat on filter

• Fuel-Borne Catalysts• Hydrogen Trap• Hydrocarbon Dosing

Hydrogen trap• Traps HC’s at lower temperatures• Zeolites adsorb HC’s

Fuel Borne CatalystAdvantages-• Reduces combustion temperature• Regeneration is faster and more complete • FBC’s are more tolerant to high levels of fuel sulphur • Regeneration is independent of engine NOx/PM ratios • Do not produce additional NO2 emissions

Fuel – Borne Catalysts

Hydrocarbon Dosing

Reduction of NOx

Primary MethodsDelayed Injection, Miller Cycle

Secondary MethodsExhaust Gas Recirculation (EGR)Nitrogen Enriched Air (NEA)Selective Catalytic Reuction (SCR)Lean NOx Traps (LNT)

Exhaust Gas Recirculation

• Lowers temperature of fuel ignition

• Up to 50% NOx reduction

• NOx – PM trade off

Nitrogen-Enriched Air

• Some oxygen stripped from air.

• NOx reduction of up to 50%

• Less adverse effects than EGR

Selective Catalytic Reduction

• Ammonia used to convert NOx to N2 and water

• Usually using a V2O5 / TiO2 catalyst

• Can achieve an 80-90% reduction of NOx

• Problems with unburnt fuel, NH3 slippage

Selective Catalytic Reduction

Lean NOx Trap

• NO converted to NO2 using Pt oxidation catalyst

• Alkaline metal oxide in washcoat of catalyst traps NO2

• NO2 reduced by rhodium catalyst to N2

• Can reduce 60-70% NOx

• Problem with sulphur poisoning

Comparison of fossil fuel versus biodiesel

References

• Lamas, M.I. and Rodriguez, C.G. (2012) ‘Emissions from Marine Engines and NOx Reduction Methods’, Journal of Maritime Research, IX (1), 77-82• Heck, R., Farrauto, R.J., Gulati, S. T. (2009) Catalytic Air

Pollution Control: Commercial Technology, 3rd ed., USA: Wiley

• http://dieselnet.com/tech/dpf_cat.php• Biodiesel: A realistic fuel alternative for diesel engines.

Ayhan Demirbas. 2008