We tend to take it for granted that cars just get steadily greener, consuming less fuel and emitting lower pollutant levels with each new generation. But engineers haven’t got magic wands. How do they keep making improvements, year after year?
In a short series of articles, I’ll look at the technical challenges associated with catalysts, stop-start systems, engine downsizing and other CO2 reduction measures.
Clean air legislation in the USA during the 1970s drove the introduction of catalysts to treat exhaust pollutants from cars. By the end of the 1980s catalysts had become the norm on petrol engines, which meant all engines had to operate with the chemically-correct air-fuel mixture (no more rich or lean running). Fuel injection and electronic engine management became universal, in order to give sufficient accuracy of control over the fuelling.
The three-way catalysts used with petrol engines clean up three types of emissions: unburnt hydrocarbons, CO and NOx. To be effective, the catalyst has to be positioned sufficiently close to the engine where the exhaust gases are hot enough to ‘light off’ the catalyst and keep it lit the whole time the engine is running.
As legislation has set progressively lower limits for toxic emissions from vehicles, engineers have had to make catalysts more effective. Emissions test results include all the pollutants produced from the moment the engine is started. Once lit, a typical catalyst is so effective that the emissions drop almost to zero, so most of the pollutants from a vehicle emerge during the first crucial seconds after starting.
To meet lower limits for emissions, engineers had to encourage the catalyst to light off more quickly by mounting it closer to the engine. This compromised the exhaust design on many engines by preventing the use of a long, branched manifold and, during the 1990s, the power output of several four cylinder ‘performance’ engines fell as a result. Since most performance derivatives nowadays are turbocharged, this is not such a problem, but the thermal mass of the turbocharger and manifold must be kept to a minimum to avoid absorbing too much heat from the exhaust.
Bigger engines, such as four or five litre V-8s and V-12s, struggled to light off the catalysts quickly enough because, when first started, the engines were not working very hard so the exhaust was relatively cool, but the large engine displacement was generating lots of emissions. The answer was to use artificial heating in the catalysts, such as electrical heating elements. Smaller engines often satisfied emissions requirements by running with the ignition retarded to increase the exhaust gas temperature quickly after start-up.
Diesel engines present a slightly different problem for emissions treatment because they run much leaner than petrol engines, with excess air present in the exhaust. Because of this, for many years diesels have used two-way catalysts that oxidise unburnt hydrocarbons and CO using this surplus air.
Greater restrictions on diesel emissions nowadays has meant controlling NOx and particulates (soot) as well, which makes the catalyst system more complicated. Vehicles fitted with a diesel particulate filter (DPF) have to be operated periodically at speeds that may be impractical in urban conditions, in order to clean the filter. Other approaches include injecting urea solution into the exhaust to give selective catalytic reduction (SCR) of the NOx, or using a lean NOx trap containing alkaline materials. The disadvantage for the vehicle owner is that some of these systems have to be regenerated frequently, by running the engine with surplus fuel, which increases fuel consumption.
In the next edition we discuss stop-start technology.