While many technicians understand why wintry short trips can block diesel particulate filters, Rob Marshall looks at how other parts of the exhaust system are affected.
It tends not to be realised, even by technicians, that burning a litre of petrol, or diesel, within a combustion engine produces almost the same quantity of water. While some of it ends up in the engine oil, most escapes as steam into the exhaust system. As the flowing gases reach the catalytic converter, many toxins present are converted into yet more water.
Clearly, this moisture cannot simply disappear. In warm weather, especially after a run, the steam that emerges from the exhaust pipe cannot be seen – yet, it is still there. In colder weather, the steam may condense within the exhaust system, causing water droplets to be expelled from the tailpipe. The water vapour may also condense as soon as it hits the open air, creating plumes of visible ‘smoke’. Some concerned owners consult ‘Doctor Google’ and conclude, incorrectly, that their cylinder-head gasket has failed, when no such drama has occurred; all that can be blamed is the weather.
While short journeys can prevent the engine from reaching its optimum operating temperature, the exhaust system can also be affected. As after-treatment systems (such as catalytic converters and DPFs) require high temperatures to work, manufacturers place them as close to the engine as possible. Not helped by a flow of cold air passing over the rest of the exhaust system as the car moves forward, the rest of the pipework runs cooler. Therefore, steam is more likely to condense inside those colder parts, especially during short runs in cold weather.
While most OEM and quality aftermarket exhausts are manufactured from aluminised steel, they are not impervious to the resultant corrosion, caused by the slightly acidic water that becomes trapped within the system. Ironically, due to their stop-start nature, hybrid petrol vehicles tend to have exhausts that run at cooler temperatures, making them more prone to ‘rot’.
This explains why exhausts can rust from the inside, out. Interestingly, diesel exhausts can last at least twice as long as those fitted to equivalent petrols. While diesel’s oily particulates have given legislators and carmakers particular headaches, because of their negative impact on air quality, they add a protective anti-corrosion coating within the exhaust system’s surfaces. For this reason, you may encounter cars over a decade-old, wearing original exhausts in good condition.
As excessively-rich fuel/air mixtures can damage the precious metals that coat the catalytic converter’s internal porous monolith, repeated engine stop and restarts over short periods risk dispensing unburnt fuel into the catalyst, especially when the engine is cold. This is not relevant to modern automatic start-stop systems, which possess algorithms that protect the converter to prevent this situation from happening.
While a catalytic converter reaches its operating temperature far quicker than coolant and engine oil, it needs to attain a far higher 350 degrees Celsius, which is unlikely to be reached, let alone maintained, on very short runs in the cold. Repeated abuse in this way coats the active surfaces with un-burnt fuel particles. These prevent the gases from interacting with the precious metals and, because the chemical reaction can now not take place fully, raised CO and HC emissions might result in an MOT failure. While some cleansing fuel additives are available, you may wish to advise a low-mileage customer to drive the car for around 20 minutes non-stop, to allow the converter to reach its operating temperature and burn-off the accumulated hydrocarbons.
Selective Catalytic Reduction appeared in passenger diesel cars approximately five years ago, as the most popular means of meeting Euro VI NOx emissions regulations. Our new car report shows how the system is developing with the Volkswagen’s Arteon’s twin dosing technology.
Aside from user error, which can range from spilling AdBlue over the operating electronics on top of the tank, to dosing it directly into the diesel, short journeys present additional complications over and above DPF clogging. The AdBlue solution, a mixture of distilled water and urea, tends to crystallise under these conditions. These can clog not only the tubing between the tank and the exhaust system but also the injector.
Inferior AdBlue quality also poses a risk. Yet, all AdBlue possesses a limited two years storage life, which reduces
if it has been exposed to strong sunlight and temperature extremes. Therefore, it may be worth advising owners of low mileage diesel vehicles to top-up their AdBlue with fresh fluid, if they have not done so for several years.
Extra value checks…
Clearly, the MOT Test will uncover excessively deteriorated exhaust systems and inefficient anti-pollution equipment. Yet, come service time, perform closer inspections of the exhaust’s integrity and advise the customer if you conclude that it will not last for at least another year of relatively infrequent and low mileage use.
We are especially grateful to Klarius for its technical expertise on which we have lent for this feature.
Catalyst replacement costs unlikely to decline…
The 600% rise of UK catalytic converter thefts within the last eighteen months, with London being the epicentre, highlights their precious metals’ high scrap values. It seems that greater quantities of these valuable elements are used on hybrid cars, judging by the theft figures. The market value of these materials continues to skyrocket, meaning that replacement catalytic converters are becoming more expensive. The value of rhodium, for example, increased threefold during 2020. According to BM Catalysts, demand is fuelled by the global drive to improve air quality by targeting tailpipe emissions. As there are few signs of this abating, market forces are making it unlikely that replacement catalytic converter prices will drop, at least in the short term.