important Vehicle Emissions and their control remedies, turbocharger

Vehicle  Emissions and control remedies

Emission from CI engine:

•        NOx  

•        HC

•        CO

•        Soot

•        Particulates matter(PM)

Soot :

•          Soot represents the solid particles of carbons products containing upto 99% of Pure Carbon

•        Presence of soot in the spent gases is the cause of black smoke in Diesel engine

•        Greatest amount of soot is formed during the diffusion combustion in burner jet core , particularly at full load operation of diesel engine

•        It may affect lungs, and active carrier of cancer producing substances  

Particulates :

•         Diesel engine emits  30 – 80 times more particulate matter than SI Engine. This particulates includes all substances ( with the exception of unburned water ) which under normal conditions are present as solids ( ash, carbon) or liquids in Exhaust gases

•        SO₂ and H₂S ( Hydrogen Sulphide)  are formed during burning the sulphur in diesel fuel.

Emission Control Technologies in CI Engine:

•        Combustion Chamber Design

•        EGR

•        DOC

•        DPF

•        SCR

•        Low CR Engine

Combustion Chamber Design

•        The main source of hydrocarbon emissions is unburnt fuel that is in contact with the combustion chamber walls. For this reason the surface area of the walls should be kept as small as possible and with the least complicated shape.

•        A theoretical ideal is a sphere but this is far from practical.

•        Good swirl of the cylinder charge is important, as this facilitates better and more rapid burning.

•        Perhaps more important is to ensure a good swirl in the area of the spark plug. This ensures a mixture quality that is easier to ignite.

•        The spark plug is best positioned in the centre of the combustion chamber as this reduces the likelihood of combustion knock by reducing the distance the flame front has to travel.

Valve Timing

•        The effect of valve timing on exhaust emissions can be quite considerable.

•        One of the main factors is the amount of valve overlap. This is the time during which the inlet valve has opened but the exhaust valve has not yet closed. The duration of this phase determines the amount of exhaust gas left in the cylinder when the exhaust valve finally closes.

•        This has a significant effect on the reaction temperature (the more exhaust gas the lower the temperature), and hence has an effect on the emissions of NOx.

•        The main conflict is that, at higher speeds, a longer inlet open period increases the power developed.

•        The down-side is that this causes a greater valve overlap and, at idle, this can greatly increase emissions of hydrocarbons. This has led to the successful introduction of electronically controlled valve timing.

Compression Ratio on Emission

•        The higher the compression ratio, the higher, in general, the thermal efficiency of the engine and therefore the better the performance and fuel consumption.

•        The two main drawbacks to higher compression ratios are the increased emissions and the increased tendency to knock.

•        The problem with emissions is due to the high temperature, which in turn causes greater production of NOx.  The increase in temperature makes the fuel and air mixture more likely to self-ignite, causing a higher risk of combustion knock.

•        Countries which have had stringent emission regulations for some time, such as the USA and Japan, have tended to develop lower compression engines.

•        However, with the changes in combustion chamber design and the more widespread introduction of four valves per cylinder, together with greater electronic control and other methods of dealing with emissions, compression ratios have increased over the years.

Exhaust Gas Recirculation (EGR)

•        High Combustion Temp – NOx is high

•        Lowering Combustion Temperature

•        Inert gas – No combustion

•        Reduction of NOx

•        Less Compression Ratio

Exhaust gas circulation techniques are:

Turbocharger

A turbocharger uses the heat of the exhaust to power a turbine wheel and therefore does not directly reduce engine power.

In a naturally aspirated engine, as much as 50% of the heat is lost to the exhaust system. Some of this lost energy is regained by using a turbocharger that uses the normally wasted combustion heat energy to perform useful work. Another 25% is lost through radiator cooling. Only about 25% is actually converted to mechanical power.

As exhaust gas enters the turbocharger, it rotates the turbine blades. The turbine wheel and compressor wheel are on the same shaft so that they turn at the same speed. Rotation of the compressor wheel draws air in through a central inlet, compresses and sends it to a cooler.

Cooler cools the engine to required temperature and the engine sucks this air during suction stroke.

DOC (Diesel Oxidation Catalyst)

As the name suggests they are used in diesel engines for the purpose of oxidation.

This is also a catalytic converter which oxidizes the following:

·       Harmful Carbon Monoxide (CO) into less harmful Carbon Dioxide    (CO₂)

·       Hydrocarbons (HC) into Carbon Dioxide (CO₂) and Water Vapor (H₂O)

·       Harmful Nitrogen Oxide (NO) into less harmful Nitrogen Dioxide (NO₂)

DPF Regeneration

Soot particulates in the gas remain trapped on the DPF channel walls where, over time, the trapped particulate matter will begin to clog the filter.

This builds up the exhaust back pressure which increases fuel consumption, reduces power output and can potentially cause engine damage. So the soot will be burnt off in the regeneration process. Based on the pressure drop the regeneration process starts.

For the regeneration to take place the temperature should be more than around 550^o C. The oxidization process in the DOC produces some heat and the rest of the heat required can be obtained  by using vaporizer technology wherein extra fuel is injected and burnt.