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Tuesday November 21st 2017

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Emissions Control: In-Cylinder

This is the most desirable approach to reduce engine emissions, however is the most complicated.

 

SI engines

The following graph shoes the Variation of HC, CO and NO Concentration with Fuel/Air Equivalence ration in an SI Engine (Heywood 1988).

Carbon monoxide (CO) is mainly dependent on whether the engine is running lean, stoichiometric or rich. CO emissions are controlled via running the engine close to stoichiometric and fitting an exhaust catalytic converter to the exhaust system.

Hydrocarbon (HC) emission problem is related to the amount of unburned fuel inside the engine combustion chambers. The In-Cylinder Controls implemented for HC emissions are:

  • Reduce crevice volumes
  • Good control of the fuel injection process (especially when the engine is started and warming up)
  • A combustion process that produces a fast flame propagation process with low cycle-to-cycle variation.

SI engines are a significant source of NOX emission because of the very high burned gas temperatures. In-cylinder NOX control is achieved by recycling between 5 and 20% of the engine’s exhaust gases into the intake. This recycled exhaust dilutes the incoming fuel-air mixture so that after combustion, the burned gas temperatures are reduced.

Gasoline Direct Injection (GDI) spark ignition improve fuel economy and therefore reduce CO2 emissions. However, since it usually operate lean at light load, they require a new technology to reduce NOx (e.g. new catalyst technology).

Alternative fuels such as natural gas (NG) and liquid petroleum gas (LPG) produce lower emissions. However, the problems are the vehicle conversion cost, the fuel availability and the cost of these fuels.

 

CI Engines

Compression Ignition Engines emissions of HC and CO are low because combustion is almost complete and the engine always operates lean, with excess air.

NOX emissions are high because burned gas temperature is high. In-Cylinder control of NOx emissions has been achieved by:

  • Careful control of engine inlet air temperatures.
  • Substantial injection retard to delay most of the combustion process to the early part of the expansion stroke (Worsens fuel consumption).
  • Recycling exhaust gases.

Particulate emissions have been reduced:

  • Use of fuel injection equipment with very high fuel injection pressure (~2000bar)
  • Use of low-sulphur fuels has reduced the sulphur component of the particulate.
  • Increasing fuel-air mixing rates by carefully matching of the geometry of the bowl-in-piston combustion chamber, air motion and spray geometry.
  • Careful control of lubricant behaviour has reduced the high molecular weight hydrocarbon particulate component that is absorbed onto the soot.



Also interesting:

Emissions Control: After Treatment Systems

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