Question

Consider the following:

1. Catalytic convertor

2. Use of high compression ratio

3. Oxidation catalyst in the exhaust manifold

4. Use of high degree of supercharger

By which of these techniques, can the oxides of nitrogen in the exhaust be controlled?
1. 1 only
2. 2 and 4 only
3. 2, 3 and 4 only
4. 1, 2, 3 and 4

Answer 1

Correct Answer - Option 1 : 1 only

Concept:

• NO_X or oxides of nitrogen emissions control is the most difficult problem engineers and environmentalists have been grappling with since the discovery of the automobile, since most control methods tend to present additional problems in their own right.
• Most control systems will either increase fuel consumption or introduce new forms of emissions.
• However it is important to note that NOX emissions can only be obtained by carefully considering air-fuel ratios, combustion and exhaust temperatures, design features that will reduce quench zones, fuel injection timing, fuel injection rate, compression ratios, catalytic reductants and convertors, exhaust gas recirculation systems, and advanced combustion control strategies.
• No single method can handle the issue of NO_X emissions efficiently, but by combining two or more of these methods, the objective of reducing NO_X emissions can be achieved.
  • An oxidation catalyst converts CO and HC to CO₂ and H₂O. It also reduces PM by oxidizing heavy hydrocarbons adsorbed on carbon particles at low temperatures. Unfortunately, oxidation catalysts do not affect black carbon particles and NO_X. At typical engine exhaust temperatures, a catalyst would normally reduce CO & HC in the order of 80%, and particulate matter by approximately 25%.
  • Increasing the compression ratio increases the power created by the higher thermal efficiency that accrues due to the increase in the compression ratio. However, it is noticed that a continued increase in the compression ratio increases NO_X emissions by increasing the cycle temperatures.
  • A higher degree of supercharger results in higher cycle temperatures and thus higher NO_X emissions.
  • Exhaust gas recirculation: Exhaust gas recirculation is a NOx-reduction technique, it works by redirecting a portion of the engine exhaust gas back into the combustion chamber where it is mixed with the fresh fuel-air mixture. It is done by adjusting the exhaust valve timing so that some of the combusted gas is trapped in the cylinder. This residual gas acts as a diluent for the next combustion cycle, hence lowering the combustion temperature.
  • Three-way reduction catalytic convertor: The TWC is a robust technology for reducing emissions of Nox, carbon monoxide (CO) and hydrocarbons (HC) (hence the name "three-way").