Most diesel emissions catalyst systems are designed to be applied on the back end of an engine system hence the term “aftertreatment . ” But Roman Konieczny, general manager, application light-duty at emissions control specialist Emitec, explained how the application of a small oxidation catalyst in front of the turbocharger can help meet exhaust emissions requirements for Euro 5 and beyond, while reducing the overall cost of the catalyst system.
“Future emissions regulations require new diesel engine technologies, in particular advanced combustion processes, which lead to lower exhaust gas temperatures,” said Konieczny. “Low gas temperatures cause increasing emissions of hydrocarbons (HC) and carbon monoxide (CO), thus demanding more efficiency from the oxidation catalyst.”
Emitec has developed its Pre Turbo Metalit, an emissions reduction system that involves the mounting of a small oxidation catalyst in front of the exhaust gas turbine, which is designed to optimize the operation of post-turbo catalysts.
Konieczny confirmed how the positioning of a Pre Turbo Metalit in front of the turbine allows for oxidation of HC and CO before the turbocharger, so that any other catalyst can be optimally designed to perform new functions. “Splitting the oxidation catalyst volume in two parts and mounting a very small
part in front of a turbocharger on a Euro 3 or 4 engine has proved very efficient,” Konieczny said. “New experimental results with a Euro 5 development diesel engine provided by Siemens VDO — have confirmed our previous observations.”
Because temperatures at the exhaust gas turbine are up to 100°C higher than downstream, a smaller Metalit
catalyst achieves HC and CO conversion rates comparable to a considerably larger post-turbo catalyst, explained the manufacturer.
Konieczny considered how the rising cost of precious materials used in the catalytic coating makes it very significant to utilize a smaller volume but highly efficient upstream catalyst, which allows for a lower-volume downstream catalyst, with the result being a reduction in overall system cost.
One concern arising from the positioning of the catalyst right in front of a turbocharger is that any failure in the metal substrate or the coating could result in damage to the turbine and/or loss of the turbine functionality.
Konieczny illustrated how the Pre Turbo Metalit unit has been designed with the help of thermal stress simulations and finite element analysis. This has been validated on the test bench to assure a very
robust design of the metal substrate, the company said.
“Due to the limited package space before the turbocharger, the catalyst needs to be optimally integrated without
creating a high pressure drop,” Konieczny said. “That would severely affect the turbine and consequently the
engine performance.“The higher overall temperature level and temperature gradients require the substrate and catalyst coating to withstand higher thermal stress. The turbine wheel is extremely sensitive to the impact of parts onto the turbine blades. Therefore, no loss of catalyst material — substrate or coating — is tolerable.”
Experiments on coating durability have been carried out by Emitec on a component test bench combining an electrodynamic shaker, which delivers the required mechanical load, along with a power-modulated burner, which is intended to expose the catalyst to the maximum temperature and transients. In addition, tests on engine test benches have also been successfully completed, the company said.
Konieczny said that results confirmed the mechanical durability has been assured, proving the capability of the Pre Turbo Metalit to withstand the loads occurring in a pre-turbo position in modern diesel engines.
