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Exhaust Gas Recirculation (EGR) is an effective technique for reducing NOx emissions in order to achieve the ever more stringent emissions standards. This system is widely used in commercial vehicle engines in which thermal loads and durability are a critical issue. In addition, the development deadlines of the new engine generations are being considerably reduced, especially for validation test phase in which customers usually require robust parts for engine validation in the first stages of the project. Some of the most critical issues in this initial phases of program development are heavy boiling and thermal fatigue. Consequently it has been necessary to develop a procedure for designing EGR coolers that are sufficiently robust against heavy boiling and thermal fatigue in a short period of time, even when the engine calibration is not finished and the working conditions of the EGR system are not completely defined.

Exhaust Gas Recirculation (EGR) has been in use for many years to control NOx emissions in commercial vehicle applications. Emissions limits are tighter with every new regulation while durability requirements continue to increase, so EGR system manufacturers must be able to provide high performance and robust designs even with high thermal loads. The commercial vehicle market is characterized by lower production rates than passenger car programs and the same engine must cope with multiple applications that have totally different engine calibrations. In some cases it is necessary to design two or more EGR systems for an engine platform, with a consequential impact on cost and development timeline. The optimal design of an EGR system needs to take into consideration several topics related with performance and durability: efficiency and pressure drop, fouling, boiling, thermal fatigue, vibration, pressure fatigue and corrosion among others.

Recent emissions standards have become more restrictive in terms of CO2 and NOx reduction. This has been translated into higher EGR rates at higher exhaust gas temperatures with lower coolant flow rates for much longer lifetimes. In consequence, thermal load for EGR coolers has been increasing and the interaction of boiling with thermal fatigue is now a critical issue during development. It is almost impossible to avoid localized boiling inside an EGR cooler and, in fact, it would not be strictly necessary when it is below the Critical Heat Flux (CHF). However when CHF is exceeded, film boiling occurs leading to the sudden drop of the heat transfer rate and metal temperature rise. In consequence, thermal stress increases even when film boiling is reached only in a small area inside the part. It is very difficult to accurately predict under which conditions CHF is reached and to establish the margins to avoid it.

The increasingly restrictive emission standards in the automotive industry require higher thermal requirements in the EGR loop in terms of gas mass flow, gas temperature and lower coolant flow rate. Also, their performance has to be sustained over a longer period of time. Therefore, thermal load for EGR components, especially EGR coolers, has been increased and thermal fatigue durability is now a critical issue during their development. One of the most challenging issues during product validation is to define a thermal fatigue test with the same field cumulative fatigue damage in order to guarantee durability during vehicle life. A new analytical procedure has been developed in order to define the equivalent thermal fatigue test which has the same cumulative damage as the real application in the field or to estimate durability in the field on the basis of a previous thermal fatigue test result.

Recent emissions standards have become more restrictive in terms of CO2 and NOx reduction. This has been translated into higher EGR rates at higher exhaust gas temperatures with lower coolant flow rates for much longer lifetimes. In consequence, thermal load for EGR components, specially EGR coolers, has been increased and thermal fatigue durability is now a critical issue during the development. Consequently a new Thermo-Mechanical Analysis (TMA) procedure has been developed in order to calculate durability. The TMA calculation is based on a Computational Fluid Dynamics simulation (CFD) in which a boiling model is implemented for obtaining realistic temperature predictions of the metal parts exposed to possible local boiling. The FEM model has also been adjusted to capture the correct stress values by submodeling the critical areas. Life calculation is based on a Multiaxial Fatigue Model that has also been implemented in FEM software for node by node life calculation.