One of the prominent by-products of diesel combustion is particulate matter, which consists primarily of soot formed by unburned fuel. This soot, along with other particulate matter and combustion by-products, is emitted as a dense black smoke - typically when the engine is under heavy load.
Soot, and the other fine particles emitted from diesel engines, is harmful if ingested and can cause issues such as lung damage and cancer. In urban areas, in particular, the sheer quantity of diesel-powered traffic contributes significantly to air pollution-related illnesses. Ways to filter this particulate matter out of the exhaust stream began being investigated in the late 1960s, resulting in the development of particulate filters. The first production automotive system was offered by Mercedes-Benz, in its Californian-market 1985 300D, for example.
The systems, which later became known as 'diesel particulate filters' - DPFs - relied on trapping the exhaust soot in a filter assembly. When the filter became full, a 'regeneration' cycle would be carried out. This process would raise the temperature in the particulate filter to such high levels that the unwanted stored by-products would be burnt and oxidised - and then released as less harmful carbon dioxide.
As diesel became more popular, and congestion in cities led to increasingly unpleasant air quality issues, the use of DPFs became more widespread. By 2009, as emissions regulations continued to tighten, it became necessary for all new diesel cars to have a diesel particulate filter.
How does a particulate filter work?
Inside a diesel particulate filter is a structure called a 'filter substrate', which is the element responsible for capturing the soot particles. It is typically made out of ceramic materials and its walls are often porous. Two common forms of design are used: the wall-flow and the partial-flow filter. The first involves all the exhaust gas flowing through the porous walls of the filter, which removes particulates effectively but can result in the assembly clogging easily. Partial-flow filters, alternatively, contain channels that flow and weave - so when the exhaust gas changes direction quickly, it deposits soot on the wall of the substrate. These filters don't clog easily but are much less effective.
In either case, the basic principle is the same. Exhaust from the engine is fed into the diesel particulate filter and the gases flow through the porous material or channels within. The particles in the exhaust stream cannot pass through the walls or the channels in the filter, so they are then deposited within the DPF - while the rest of the exhaust gas makes its way out of the tailpipe.
The deposits in the DPF, left unchecked, would eventually block the filter assembly and cause significant performance issues. In order to clean the filter, without removing it from the car, a process called 'regeneration' takes place. This involves increasing the temperature inside the particulate filter to the point that the debris stored within are burnt off and oxidised into a gaseous form, typically carbon dioxide. The process, which generates emissions less harmful than simply releasing the soot to atmosphere, can occur passively or actively.
Passive regeneration occurs when the exhaust gas temperature is high enough to burn off the material within the filter; such conditions may be experienced when the car is being driven at motorway speeds - and, as a result, the process remains invisible to the driver.Active regeneration, on the other hand, involves increasing the temperature within the filter artificially. One approach is to alter the behaviour of the engine - delaying the injection process, for example, so the combustion continues in the exhaust and heats the filter. Alternatively, a heating system may or fuel may be injected directly into the exhaust, burning in the system to raise its temperature.
In some cases, such as the driver continually only doing short trips, the filter may not get a chance to regenerate. This leads to it ultimately blocking, reducing engine performance and causing problems; as a result, many technicians can force a DPF to regenerate in order to clean them out. Although particulate filters can be troublesome, they are at least efficient - with one manufacturer claiming that its wall-flow filters capture 90 per cent or more of soot particles emitted by the engine.
Do petrol cars use particulate filters?
One slight irony of the recent backlash against diesels is that many modern petrols - specifically those that rely on direct injection - are themselves not free from emissions-related issues. In particular, one three-year Swiss study revealed that several direct-injection cars emitted ten to 100 times more 'ultrafine' particulate matter than a modern diesel equipped with a particulate filter.
These finer particles are produced by the combustion processes and mixtures experienced by direct-injection engines. More problematically, they are potentially even more harmful than conventional particulate matter - as their finer size means they can penetrate into soft tissues, such as those in the lungs, more easily.
Consequently, the results have prompted some manufacturers to equip their petrol vehicles with particulate filters. Again, Mercedes-Benz was one of the leading forces in the adoption of 'gasoline particulate filters' GPFs. In 2017, some petrol versions of the W222-gen S-Class were equipped with particulate filters.
Unlike the company's diesel filters, the petrol versions use cordierite - a silicon material that contains magnesium, iron and aluminium - in their filter assembly. Cordierite filters are less expensive than the silicon carbide-based ones used in diesels, and cannot withstand as high temperatures, but the material does not expand as much when heated. Consequently, cordierite-based particulate filters are easier to package. Their filtration performance, however, is still excellent.
Other brands using GPFs include Ford, in the 2018 Ecoboost Mustang and the Fiesta ST, and Volkswagen - which uses them in the Up GTI and Tiguan.