Automotive Technician Training – Auto365 www.atttraining.com
ŠTom Denton
1 Engines 1.1.1 Diesel particulate filters (DPFs) Introduction The main approach to the further lowering of diesel engine emissions are internal engine improvements because improved fuel combustion prevents, as far as possible, the formation of pollutants and also reduces fuel consumption. In this respect, automobile manufacturers and their component suppliers have already achieved a great deal.
Figure 1 This DPF may need replacing!
Engine management Heavy passenger cars will not meet the latest standards without treatment systems. EDC (Electronic Diesel Control) handles the management of particulate filters and nitrogen oxide storage catalytic converters. It matches injection flexibly to the requirements of the exhaust emission treatment systems, for example by altering injection timing, quantity and process. EDC also matches the amount of combustion air fed to the engine to the respective demand. This is done by controlling the exhaust gas recirculation, determining the setting of the throttle valve and the operating pressure of the exhaust gas turbocharger. Sensors convey information to the EDC about the exhaust gas temperature, backpressure and composition. Engine management can, therefore, not only determine the condition of the particulate filter and the nitrogen oxide storage catalytic converter, but also improve the quality of combustion.
Figure 1 Diesel injection and management system
A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot and under certain conditions can attain soot removal efficiencies of close to 100%. The particulate filter shown here from Bosch is made of sintered metal and lasts considerably longer than current ceramic models, since its special structure offers a high storage capacity for oil and additive combustion residues. The filter is designed in such a 1
Automotive Technician Training – Auto365 ©Tom Denton www.atttraining.com way that the filtered particulates are very evenly deposited, allowing the condition of the filter to be identified more reliably and its regeneration controlled far better than with other solutions. The diesel particulate filter is designed to last as long as the vehicle itself.
Figure 1 Diesel particulate filter (Source: Bosch Media)
Filtration systems The two main systems are those with additive and those without. To enable a vehicle to use a system without additive the particulate filter must be fitted close to the engine. Because the exhaust gases will not have travelled far from the engine they will still be hot enough to burn off the carbon soot particles. In these systems an oxidising catalytic converter will be integrated into the particulate filter. In other systems the particulate filter is fitted some distance from the engine and as the exhaust gases travel along the exhaust they cool. The temperatures required for ignition of the exhaust gas can only be achieved by the use of an additive.
Figure 1 Different fitting methods for DPF
Additive Use of an additive lowers the ignition temperature of the soot particles and the engine management ECU raises the temperature of the exhaust gas so the filter can be regenerated. Regeneration is usually necessary after between 300 and 450 miles, depending on how the vehicle is driven. The process takes about 5 – 10 minutes and the driver shouldn’t notice it is occurring, although sometimes there may be a puff of white smoke from the exhaust during regeneration. The additive is stored in a separate tank and is used at a rate of about 1 litre of additive to 3000 litres of fuel. It works by allowing the carbon particles trapped in the particulate filter to burn at a significantly lower temperature than would usually be required (500ºC rather than 600 - 650ºC).
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©Tom Denton
Figure 1 Additive (Source: Prestone)
Filter management methods On-board active filter management can use a variety of strategies: • Engine management to increase exhaust temperature through late fuel injection or injection during the exhaust stroke (the most common method) • Use of a fuel borne catalyst (the additive) to reduce soot burn-out temperature • A fuel burner after the turbo to increase the exhaust temperature • A catalytic oxidizer to increase the exhaust temperature, with after injection • Resistive heating coils to increase the exhaust temperature • Microwave energy to increase the particulate temperature Not running the regeneration cycle soon enough increases the risk of engine damage and/or uncontrolled regeneration (thermal runaway) and possible DPF failure. Diesel particulate matter burns when temperatures above 600 degrees Celsius are attained. This temperature can be reduced to somewhere in the range of 350 to 450 degrees Celsius by use of a fuel additive which is a catalyst.
Figure 1 Sectioned view of a new filter
Passive and active regeneration There are two types of regeneration – passive and active. Passive regeneration takes place, automatically, on motorway-type runs in which the exhaust temperature is high (somewhere between 350ºC and 500ºC). As the exhaust is so hot (hot enough to ignite the soot particles) the regeneration process can carry on continuously and steadily across the platinum coated catalytic converter.
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ŠTom Denton
Figure 1 Filter and its position in relation to the engine
Regeneration Once the storage capacity of the particulate filter has been exhausted, the filter has to be regenerated by passing hot exhaust gases through it which burn up the deposited particulates. In order to produce the necessary high exhaust gas temperatures, the EDC alters the amount of air fed to the engine as well as the amount of fuel injected and the timing of injection. In addition, some unburnt fuel can be fed to the oxidizing catalytic converter by arranging for extra fuel to be injected during the expansion stroke. The fuel combusts in the oxidizing catalytic converter and raises the exhaust temperature even further. Types of DPF Unlike a catalytic converter which is a flow-through device, a DPF retains bigger exhaust gas particulates by forcing the gas to flow through the filter, however does not retain small particles and maintenance free DPF break larger particles into smaller ones. There are a variety of diesel particulate filter technologies on the market. Cordierite wall flow filters The most common filter is made of cordierite (a ceramic material that is also used as catalytic converter supports (cores)). Cordierite filters provide excellent filtration efficiency, are (relatively) inexpensive, and have thermal properties that make packaging them for installation in the vehicle simple. The major drawback is that cordierite has a relatively low melting point (about 1200 °C) and cordierite substrates have been known to melt down during filter regeneration. This is mostly an issue if the filter has become loaded more heavily than usual, and is more of an issue with passive systems than with active systems, unless there is a system break down. Cordierite filter cores look like catalytic converter cores that have had alternate channels plugged - the plugs force the exhaust gas flow through the wall and the particulate collects on the inlet face.
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ŠTom Denton
Figure 1 Cordierite filter cores
Silicon carbide wall flow filters The second most popular filter material is silicon carbide, or SiC. It has a higher (2700 °C) melting point than cordierite, however it is not as stable thermally, making packaging an issue. Small SiC cores are made of single pieces, while larger cores are made in segments, which are separated by special cement so that heat expansion of the core will be taken up by the cement, and not the package. These cores are often more expensive than cordierite ones, however they are manufactured in similar sizes, and one can often be used to replace the other.
Figure 1 Silicon carbide (left) and a cordierite filter cores (Source: NGK)
Ceramic fibre filters Fibrous ceramic filters are made from several different types of ceramic fibres that are mixed together to form a porous media. This media can be formed into almost any shape and can be customized to suit various applications.
Figure 1 Typical DPF
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Automotive Technician Training – Auto365 ©Tom Denton www.atttraining.com Metal fibre flow through filters This type is becoming more popular; it has cores made from metal fibres woven into a monolith. These cores have the advantage that an electrical current can be passed through the monolith to heat the core for regeneration purposes, allowing the filter to regenerate at low exhaust temperatures and/or low exhaust flow rates. Metal fibre cores tend to be more expensive than cordierite or silicon carbide cores and not usually interchangeable with them because of the electrical requirement.
Figure 1
Vehicle usage and reasons for failure A significant number of people don’t use motorways so passive regeneration will be possible only occasionally. In the case of a filter without additive when the soot loading reaches about 45 per cent the ECU switches off the EGR and increases the fuel injection period so there is a small injection after the main injection. These measures help to raise the engine exhaust temperature to over 600ºC which is high enough to burn off the soot particles. A warning light is triggered at a 55 per cent soot loading. In such circumstances the car needs to be driven hard in a lower gear so the temperature in the particulate filter will be sufficient to burn off the soot. If the driver ignores the warning and continues to use the car as normal, the soot will continue to build until it reaches 75 per cent. Additional warnings will then be given using the malfunction indication lamp (MIL) for example. It will not now possible to clear the DPF by driving and it may need to be replaced - if loading reaches 95 per cent then the DPF will definitely need to be replaced.
Figure 1 DPF warning light
Maintenance Filters require more maintenance than catalytic converters. Ash, a byproduct of oil consumption from normal engine operation, builds up in the filter as it cannot be converted into a gas and pass through the walls of the filter. This increases the pressure before the filter. Warnings are given to the driver before filter restriction causes an issue with driveability or damage to the engine. Regular filter maintenance is a necessity and replacement is also recommended in many cases. 6
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Figure 1 DPF cleaner (Source: Wynn's)
Summary Diesel particulate filters do exactly as their names suggests and filter particulates (soot) out of the exhaust of diesel engines. A regeneration process that involved heating and burning of the soot into is controlled by the engine management system. Some systems also include a catalyst additive that allows the burning to work at a lower temperature. DPFs can be very expensive and therefore it is vital that the regeneration process takes place. However, because of particular driving cycles this may not always take place and replacement can become necessary.
Figure 1 Common rail diesel system
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