Catalytic converters & diesel particulate filters
How to make sure your emissions systems are up to scratch.
Although internal combustion engines are compact, lightweight and reliable sources of power, one of their downsides is the pollutants that emerge from the exhaust. With more vehicles on the road than ever before, most technological advances in recent decades have been dedicated to reducing the negative impact on both the environment and human health. Modern vehicles now contain a plethora of emission control systems, including catalytic converters and diesel particulate filters (DPFS), and motorists have an obligation, both legally and morally, to ensure they are kept in optimum condition. Maintaining and repairing exhaust pipe-mounted technologies can also save you money in the long run.
Taking control
Strict engine regulation is vital to ensuring the efficiency and reliability of emissions control equipment. In the case of petrol engines, a catalytic converter’s ability to work effectively over a typical vehicle’s lifetime depends greatly on the fuel system maintaining an optimum 14.3:1 air-to-petrol mix. This optimum ratio is referred to as Lambda 1, which explains not only why Lambda/oxygen sensors are used within exhaust systems to measure the gases’ oxygen content, and promote the engine management system to adjust the fuelling accordingly, but also is why Lambda is measured as part of a non-hybrid petrol MOT emissions test. Should excess fuel be introduced into the catalyser due to a rich mixture – caused by an engine fault, poor maintenance, or unsuccessful push-starting, for example – the converter’s internal honeycomb can overheat and crack, melt or collapse. Unsurprisingly, its pollutant reduction function is rendered useless.
Ironically, with diesel engines, dosing fuel into the exhaust system is desirable to control soot emissions under certain conditions. This is not to benefit the diesel catalytic converter (which is unharmed by the process) but rather the diesel particulate filter (DPF) that is mounted further downstream. Unlike a catalyser, the filter must be emptied regularly and it does this automatically by super-heating its contents. Should natural fast-road driving not generate sufficiently high temperatures to
regenerate the DPF, the engine electronics will step in to perform an ‘active regeneration’, which introduces fuel into the exhaust system on purpose. The success of this process relies on a specific set of variables being satisfied, which depends upon the condition of other systems and even the fuel level. Therefore, be aware that, for modern diesel engines that are equipped with DPFS, a blocked particulate filter may be a symptom of a fault elsewhere and not the cause.
What goes wrong with cats
Catalytic converters are very reliable. Most issues are caused by either a rich mixture, or contamination originating from a faulty engine component, such as a failing turbo and the valve stem seals that discharge oil into it, or a head gasket that causes the converter to be ‘poisoned’ by silicon from the coolant. Impact damage may be a factor, too, especially if your suspension has been lowered, raising the risk of the cat being struck. Thermal shock, which might be caused by driving through deep floodwater, can crack the delicate internals, too. In some cases, broken ceramic material can be scoopedup by the flowing exhaust gases and pinned against the outlet; the resulting blockage causes back-pressure high enough to cause a loss of power under load.
In most cases, cat failure indicates another fault. Nothing lasts forever and, after very high mileages have elapsed, the precious metals can become detached from the honeycomb brick and get blownout. Interestingly, these fragments remain so valuable that one British university has developed a system by which precious metals from catalytic converters can be extracted from road sweepings. This does little to benefit the car owner, who cannot scoop them up and ‘glue’ them back into the exhaust system!
As the catalytic converter’s performance drops, the first an owner may know about it is when an MOT emissions test is failed, or an engine management fault code is generated. Various fuel additives have been developed that claim to either clean the catalyser’s internals of soot or even to restore some of the metals that have been lost. The success rate depends on the reasons why the catalyser has failed and if the claims made by the additive maker are true.
A diesel particulate filter relies on a catalytic converter to function, but it is a soot trap and not a catalyser. Fitted to some Euro IV emissions vehicles from around 2004, DPFS fulfil the mandatory Euro V environmental directive by capturing soot, which includes particles of unburned hydrocarbons and oil, rather than expelling them into the atmosphere.
The filter medium is housed within a canister and the core consists of numerous internal channels, each of them are blocked at one end. Passing gases are forced through the porous ceramic walls, which trap the soot deposits. In fast, open-road driving, the gases flow through the walls easily and the particles become trapped at the end of the filter. As the soot is combustible, it may burn away naturally, due to the high temperatures created at fast speeds and elevated loads – this is called 'passive regeneration'. The pressure difference is detected by either one, or two, electrical sensors, which take a measurement via one, or a pair, of hollow tubes attached to the DPF. In slow-moving traffic, the gases do not pass through the conduits as readily, meaning that trapped soot builds layers in the internal channels within the centre of the filter, resulting in a high pressure difference between the filter’s entrance and exit ports.
Once a pre-determined pressure is reached – typically around a 45% loading – the car will superheat the DPF by either injecting fuel into the exhaust system via a separate fuel injector or, more commonly, the engine fuel injectors fire extra pulses of diesel during the exhaust stroke, which passes through the manifold tract and into the catalyser and DPF. This process – 'active regeneration' – demands that the car is driven at set speeds and throttle positions for around 20 minutes, which permits the catalyser and DPF to reach around 600°C. During this time, it is common for the engine to create load by activating electrical accessories (including, possibly, the air-conditioning), triggering the radiator cooling fans, closing the EGR valve to raise the intake air temperatures and even restricting the airflow, prior to advancing the injection timing and dosing extra fuel into the exhaust.
Naturally, all of these stages rely heavily on engine management settings, complex algorithms and whether, or not, the right conditions are met for a successful regeneration. Trying to thrash the car hard in short bursts of maximum acceleration in low gears tends to increase the DPF’S soot loading. Unfortunately, during passive regeneration, the engine oil is contaminated with wasted diesel and this is why you should reset any service indicator lamp diagnostically postmaintenance, change the oil regularly and never overfill the sump.
Once the DPF has regenerated, the vacated particulates cause the pressure differential to drop and the cycle repeats itself. However, the vapourised soot leaves ash behind. This cannot be removed in subsequent regenerations and even on-car cleanings. While the car’s ECU’S algorithms estimate the ash levels, depending on mileage, conditions and the number of successful regenerations, most drivers needn’t worry about having the DPF removed to clean-out the ash until at least 150,000 miles have elapsed, provided that maintenance advice has been followed.