Reducing emissions in CI engines By Alex, Kieran, and Jade
Hope you’ve learnt something new from reading this leaflet!
Diesel particulate filters
Catalytic converters
EGR valve
Diesel particulate filter
Page 2
Diesel particulate filters are used to meet the European emission legislation It’s used to remove the soot and other small particles that are produced during the combustion cycle. When a car is fitted with a diesel particulate filter there should be no visible smoke coming from the exhaust. It is different to a catalytic converter because it is a filter. A DPF needs to operate within a certain heat range to be efficient. A DPF needs to be maintained to prevent it from becoming blocked. Some can be replaced through a service however most of them use a regeneration cycle. During the regeneration cycle the DPF should reach around 600 degrees Celsius for the best result. The DPF can become blocked because of lots of slow driving and short journeys. This is because the DPF cannot get up to the correct working temperature.
DPF regeneration will be initiated by the ECU every 300 miles or so depending on vehicle use and will take 5 to 10 minutes to complete. You shouldn't notice anything other than perhaps a puff of white smoke from the exhaust when the process is completed
There are pressure and temperature sensors mounted in the exhaust system so that the engine management system can monitor the DPF and then maintain the correct function.
You can have two types of regeneration: Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don't get this sort of use car manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process. Active regeneration is when the soot loading in the filter reaches a set limit (about 45%) the ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is a bit stop/start or you take your foot off the accelerator while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked. It should be possible to start a complete regeneration and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph. If the regeneration is unsuccessful the extra fuel injected will not burn and will drain into the sump. Oil quality will deteriorate as a result of this and the level will rise. It is important that you check that the oil level does not increase above the maximum level on the dipstick as diesel engines can run on excess engine oil – often to the point of destruction.
Well some common issues are...
The pressure differential sensor can fail.
Too many DPF regenerations have been carried out. (They have a limited lifespan).
The DPF has become overloaded and automatic regeneration is not possible. In this case you need to force the regeneration process using suitable diagnostic equipment.
The DPF can break apart internally.
The temperature sensor can fail. The additive tank can run empty. (If fitted).
Diesel particulate filter Catalytic converter
A catalytic converter is a vehicle emissions control device that converts toxic pollutants in exhaust gas to less toxic pollutants, by catalysing a redox reaction. Catalytic converters are used in internal combustion engines fuelled by either petrol or diesel. For compression ignition engines the most common catalytic converter is diesel oxidation catalyst, this type uses oxygen in the exhaust gas stream to convert carbon monoxide, to carbon dioxide and hydrocarbons to water and carbon dioxide. This type of catalytic converter works at 90% efficiency virtually eliminating harmful gases and soot. Most vehicles make most of their pollution within the first 5 minutes of starting the car, because the catalytic converter takes time to warm up to running temperature, the running temperature of a common diesel car is 100650c, in 1999 BMW introduced an electrically heated catalytic converter which they called “E-CAT” this used heating coils inside which are electrified just after the engine is running bringing it up to running temperature more quickly therefore eliminating more emissions.
DPF additives The most commonly fitted type of DPF has an integrated oxidising catalytic converter and is located very close to the engine where exhaust gases will still be hot. This heat means that passive regeneration is more likely to be successful. Some models, across a wide range of manufacturers, use a different type of DPF which relies on a fuel additive to lower the ignition temperature of the soot particles. The additive is stored in a separate tank next to the fuel tank and is automatically mixed with the fuel whenever you fill up. Only very small quantities are used so a litre of additive should treat around 2800 litres of fuel – enough to cover 25,000 miles at 40mpg. It lasts about 70000 miles and is replenished during a service – at extra cost. You will have to pay to get the additive tank refilled at some time in the car's life – expect to pay between £150 and £200 including fluid and labour Don't be tempted to ignore a warning light showing that the additive tanks need refilling. It's absolutely essential this tank is refilled as without it regeneration is unlikely to be successful and a new DPF may be needed – at significant cost. Fuel consumption can increase as a result of failed regenerations too.
EGR valve
In many countries around the world, the emissions of NOx from diesel and petrol vehicles are restricted by legislation. NOx is formed in the combustion chamber of engines, when high temperatures cause oxygen and nitrogen (both found in the air supplied for combustion) to combine. A widely adopted route to reduce NOx emissions is Exhaust Gas Recirculation (EGR). This involves recirculating a controllable proportion of the engine's exhaust back into the intake air. A valve is usually used to control the flow of gas, and the valve may be closed completely if required. The substitution of burnt gas (which takes no further part in combustion) for oxygen rich air reduces the proportion of the cylinder contents available for combustion. This causes a lower heat release and peak cylinder temperature, and reduces the formation of NOx. The presence of an inert gas in the cylinder further limits the peak temperature (more than throttling alone in a spark ignition engine). The gas to be recirculated may also be passed through an EGR cooler, which is usually of the air/water type. This reduces the temperature of the gas, which reduces the cylinder charge temperature when EGR is employed. This has two benefits- the reduction of charge temperature results in lower peak temperature, and the greater density of cooled EGR gas allows a higher proportion of EGR to be used. On a diesel engine the recirculated fraction may be as high as 50% under some operating conditions. What Is EGR? Exhaust gas recirculation (EGR) is a system that takes exhaust gas from a diesel’s exhaust manifold and reintroduces it into the intake of the engine to reduce the combustion chamber temperature. Why do modern diesels have EGR? Basically, EGR is used to reduce the formation of NOx emissions, which are created when the combustion chamber temperature exceeds 2,370 degrees. Pumping exhaust (EGR) back into the engine helps to cool the combustion chamber because EGR has almost no oxygen in it. Without oxygen, the temperature stays below the NOx-producing threshold, and the emissions coming out the exhaust are cleaner.
Common Faults EGR valves do come with their own annoying faults a key fault being the buildup of soot causing the valve to stick open If the EGR valve is stuck open, it will essentially cause a vacuum leak, leading to inefficient combustion, rough idling, hesitation, and sometimes stalling in extreme cases. This is because the car cannot combust on carbon dioxide from the tailpipe — it needs atmospheric oxygen. If the combustion chamber is flooded with exhaust from an open EGR valve, it will not function properly. If the EGR valve is stuck closed, emissions of nitrogen oxides will rise, and the car may start to knock. Spark knock happens when the fuel mixture in the combustion chamber ignites before it has been reached by the explosion in the cylinder, resulting in a disruption of the engine timing. In this instance, the engine should be warmed up and revved to see if the EGR valve will move. In both cases, simple repairs are possible. The EGR valve can be clogged, and a simple cleaning of a mechanical EGR valve may solve the problem. In the case of an electrically controlled EGR valve or a more complex mechanical problem, replacement of the valve may be needed. The EGR valve is an expensive motor vehicle component, and care should be taken to ensure that it needs to be replaced rather than repaired.