High-temperature strength of charge air coolers is crucial for clean diesel engines
Governments around the world are pressuring the makers of diesel engines and heavy-duty trucks to meet increasingly stringent emissions requirements for particulate matter (PM), oxides of nitrogen (NOx), nonmethane hydrocarbons (NMHC), and other pollutants. The U.S. EPA's standard for 2007 represents a 90- and 95-percent reduction in PM and NOx, respectively. By comparison, the 2004 standards represent only a 50-percent reduction based on today's standards. According to the EPA, these emission standards are feasible using catalyzed diesel particulate traps, NOx absorbers, and various other diesel technologies. Original-equipment manufacturers are not limited to these technologies, and they don't have to use them -- they just have to meet the emissions standards.
Most diesel-engine manufacturers agree that clean diesel engines will require more complex turbochargers and heat exchangers. More heat at higher temperatures will create higher underhood temperatures that will have to be reduced to prevent potential catastrophic damage.
One widely publicized technology, exhaust-gas recirculation (EGR), uses exhaust gas as an oxygen-depleted gas to be partially recirculated back to the intake manifold and into the combustion chamber.
The exhaust gas has to be cooled by an EGR cooler and then compressed by the turbocharger and perhaps cooled again. Meanwhile, the usual intake air is compressed by the turbocharger and cooled in the air cooler, commonly called the intercooler. At some stage, the exhaust gas and atmospheric air must be mixed.
Tube brass retains much of its tensile strength at elevated temperatures. Aluminum tubes experience a severe drop in strength and are subject to fatigue cracking above 200 fC.
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The aluminum tubes in current-generation air coolers can be unreliable above 180 fC, and maximum specification temperatures for aluminum air chargers typically are even lower. Serious problems can occur at temperatures above 200 fC, for which the strength of aluminum typically drops 40-60 percent compared to its strength at 150 fC.
The introduction of CuproBraze technology has greatly changed the design parameters for intercoolers and hence for turbochargers as well. A copper-brass cooling element can withstand inlet temperatures of 290 fC, retaining much of its strength and avoiding metal fatigue.
A useful interpretation of strength-versus-temperature data for brass versus aluminum is to calculate strength ratios at different temperatures. For example, brass tube is 1.7 times as strong as aluminum tube at room temperature, but it's four to five times as strong as aluminum at 250 fC.