Reducing Vehicle Pollution: Exhaust Gases and Control Methods
Accumulation of toxic products in the air can cause serious damage to health. Engine exhaust contains a compound atmosphere including unburned hydrocarbons and carbon monoxide.
Measures to reduce pollutant emissions aim to achieve the lowest possible gasoline consumption, high performance, and good engine behavior.
Sources of Pollution
Engine Crankcase Gases
These gases are sent through a recirculation circuit to prevent external output.
Exhaust Gases and Gasoline Vapors
These are recycled into the cylinders during operation. The composition of exhaust gases depends on several factors:
- Combustion process: Poor or rich mixtures result in incomplete burning of fuel.
- Outside temperature and pressure
- Homogeneity of mixing
- Turbulence
- Combustion chamber shape
Harmful Exhaust Gases
Carbon Monoxide (CO)
Arises from excess fuel in the mixture. It combines with hemoglobin, replacing oxygen in red blood cells, which can result in death.
Nitrogen Oxides (NOx)
Form at high temperatures when nitrogen combines with oxygen, causing significant irritation.
Hydrocarbons (HC)
Originate from two causes: lean flame propagation from a rich mixture and a lack of oxygen during combustion.
Lead (Pb)
Comes from the composition of gasoline.
Sulfur Dioxide (SO2)
Results from fuel impurities and incomplete combustion.
Anti-Pollution Modifications to the Motor
These include modifications to the combustion chamber, heating of manifolds, variable valve timing, variable intake manifold, and ignition system retarder valves.
Treatment of Exhaust Gases
1. Air Injection into the Exhaust
Introducing a volume of air into the exhaust manifold to complete the combustion of off-gases from the cylinder. This reduces HC and CO. Lean-burn engines have enough oxygen to oxidize these gases. Engines needing a rich mixture require additional air entry, which is performed on the exhaust valve to tap high temperatures.
Air Pump Systems
A vane pump, driven by the motor, takes air from the air filter and sends it to the exhaust manifold.
Valve Systems
Instead of an air pump, an oscillating valve with a steel membrane seals or releases air. These oscillations occur due to pulsations of exhaust gases.
2. Exhaust Gas Recirculation (EGR)
EGR reduces NOx by returning exhaust gases to the intake manifold to be introduced into the combustion chamber along with fresh gas. The mixture formed is impoverished, reducing temperature and pressure, which reduces NOx formation.
3. Catalytic Converters
Catalytic converters provide further treatment of exhaust gases. Precious metals such as rhodium and platinum are used within a metal casing placed like a muffler, located near the exhaust manifold to maintain high temperatures. In contact with these metals, NOx is broken down into oxygen and nitrogen, while CO and HC combine with oxygen.
Structure:
The converter consists of a stainless steel casing with a ceramic substrate inside, channeled in a honeycomb shape for better contact between gases and metals. This honeycomb is coated with a “washcoat” of aluminum oxide for better adhesion of exhaust gas. An expandable mantle prevents dilation of the ceramic substrate relative to the housing, providing thermal insulation. Effectiveness is between 400-800 ºC, located on the edge of the exhaust gas outlet pipe to isolate the catalyst collector.
Types:
Two-Way Converters:
These converters perform oxidation processes alone, transforming CO and HC but not NOx. They are used in lean-burn engines and combined with exhaust air injection systems to provide sufficient oxygen.
Three-Way Converters with Air Injection:
These open-loop systems consist of two ceramic monoliths, reducing NOx and oxidizing CO and HC.
Three-Way Converters with Lambda Probe:
These closed-loop regulation systems adjust the air/fuel mixture, maintaining the air coefficient (lambda) at the most appropriate value (lambda = 1), between 0.95 and 1.05. In lean mixtures, potassium and calcium oxides are added to combine with NOx, forming barium nitrate. A homogeneous mixture is then introduced (3 seconds) to release NOx.
Fuel Vapor Pollution
Fuel vapor absorption occurs when the temperature rises. A canister containing active carbon absorbs the vapors and returns them to the cylinders via a valve. The canister contains a capsule and membrane-type valve to allow passage when the membrane is deformed, as well as a rollover safety valve.