Particulate matter is one of the most common and harmful constituents of the engine`s exhaust. Diesel engines produce more particulate matter than any other fuel. Since pollution norms are becoming stringent decade after decade (Remember we are skipping BS-5 and preponing BS-6) manufacturers come up with new technologies for holding a position in the market. Diesel particulate filter is a component to remove these particulate matters. DPFs are a part of exhaust system of a car and are generally mounted after the catalytic converter.
Particulate matters are produced due to improper combustion and they are nothing but very tiny particles of carbon of size in micrometres. When deployed causes air pollution and many health hazards therefore they are filtered and flushed away safely. DPF`s role comes into picture here. The components of a DPF are a filter made to filter out the particles, a differential pressure sensor to measure the back pressure to detect blockage, sometimes an electric heating system.
The working can be explained as follows.
- Filtering the particulate matter.
- Regeneration (Removing these particles from the filter).
Table of Contents
Filtering:
Filtering is done by a filter (usually has a honeycomb structure) which is made of materials like cordierite (Most common), metals, ceramics, papers, etc. These filters have very tiny holes in their structure which would let the other substances and makes these soots to stay on their surface. There is a differential pressure sensor connected between the inlet and outlet section of the
DPF. These play a major role in regeneration. They sense the amount of soot particles on the DPF just by measuring the inlet and outlet pressures respectively (Measures the degree of blockage). An indicator is placed on the instrument cluster gets input from this sensor and warns the driver if there any external action to be taken if the vehicle could not remove the particles automatically.
Regeneration:
Now let us talk about regeneration. Since DPF filters out particles, once if blocked it cannot let the exhaust gases to pass through it. therefore, they have to be cleaned either dynamically or at least periodically. The system is designed in such a way that it takes care of this process by itself. But it needs some servicing when the system cannot handle by its own. Regeneration is done in three different stages passive, active and forced where the first two needs no human intervention and the other one needs some external support.
Principle of operation:
Before getting into these three types let us see how Regeneration is done. Heat is the main requirement here. DPF needs around 600 oC to flush away these soots. At these temperatures these soot burns giving away CO2 as a product (Very less amount).
Passive:
This is the regeneration process that needs no manual input. The engine is totally responsible to provide such high temperatures and the heat is carried by the exhaust gases from the engine to DPF.
Active:
This happens when vehicle was driven for only short distances at slower speeds. The engine in such situations cannot produce any heat to burn off the soot particles. Therefore, when diagnosed the system injects some amount of fuel to the exhaust manifold in unburnt condition which then combines with appropriate catalyst and reaches the DPF at high temperatures burning these particles. The fuel can be either injected by the injector in the combustion chamber during exhaust stroke or can be injected by an injector mounted at the exhaust manifold.
In some vehicles, the indicator goes on when it senses a need of an active regeneration therefore running the vehicle at higher speeds for a prolonged time can promote passive regeneration.
Forced:
This happens only when the other two methods of regeneration failed this will make the indicator glow and immediate action should be taken. The DPF system is connected to a computer externally and the computer maintains the engine rpm until the pressure sensor detects no blockage.
If there is a problem even after this, it is very likely that the DPF cannot be serviced and needs an immediate replacement.
Manually these blockages can be detected when the performance of the engine is poor.
Need for DPF:
Regulation is the main reason why DPF is used. For instance, BS-4 norms permitted 0.05 to 0.1 grams of particles per kilometre travelled by a diesel vehicle (M and N category) whereas in BS-6 it is just 4.5 mg. which is very difficult to attain. On the other hand, this was not even regulated till 1996 norms