Integrated cascaded charge air cooling
Raising specific output in a turbocharged engine with reduced displacement is a common way to achieve low consumption and emissions values without sacrificing driving performance. However, the combustion process, the engine and its components, and the engine peripherals are subjected to significant effects. The technological effort increases while the thermal and mechanical loads are more intense.
Increasingly greater charge air pressures and the associated rising charge air temperatures, as well as more stringent emissions regulations require more efficient charge air cooling systems that allow further reductions in charge air temperature. MAHLE has recently developed and presented integrated cascaded charge air cooling in the air intake module for gasoline engines. The system cools the charge air in two stages, first by means of the high-temperature coolant circuit in the engine, and then by means of the low-temperature circuit for indirect charge air cooling. The results from engine testing have demonstrated that cascaded charge air cooling is an “enabler” for further downsizing steps and higher turbocharging levels. The disadvantages currently associated with a high level of turbocharging (such as retarded ignition, enrichment on the full-load curve) are eliminated thanks to integrated cascaded charge air cooling. The cooler charge air addresses the root cause, i.e., the combustion conditions, thus making downstream measures unnecessary. It also offers greater freedom in selecting the compression ratio. This results in consumption savings in the load ranges relevant to the cycle.
Motivated by the positive results, the next step was to investigate the potential of integrated cascaded charge air cooling in the diesel engine. Engine testing has shown that reducing charge air temperatures also has fundamental advantages for diesel engines. Under partial load, emissions were reduced, while under full load the effect can be used either to reduce fuel consumption for a constant power output, or the other way around to increase performance at a constant consumption rate. In addition, the greater mass flow increases the charger speed and thus improves engine responsiveness.
The advantages identified by MAHLE on the test bench must be classified according to their feasibility and the effort required. Assuming an initial basis of direct charge air cooling, converting to an indirect solution integrated in the air intake module provides tremendous potential due to the higher cooling capacity and the ability to regulate the cooling capacity. The pressure losses in the air path for this variant are also reduced significantly.
Cascaded charge air cooling, also integrated in the air intake module, provides even greater increases in cooling capacity, in addition to full control flexibility. This approach is necessary if the heat dissipation of the low-temperature circuit is no longer sufficient.
All of the advantages from the test bench measurements under full and partial load can also be implemented in the vehicle: the emissions benefits under partial load can be obtained using a single-stage, conventional indirect, integrated charge air cooling, while the additional consumption benefits under full load require the cascaded variant. This was confirmed by computation of vehicle circuits for all variants. In both cases, a prerequisite is a full-face low-temperature radiator at the front end. The capacity of the high-temperature radiator must be checked for cascaded charge air cooling. This power reserve was sufficient for the tested engine and compensated for the loading with cascaded charge air cooling, even under full load, without making changes to the radiator design. An additional increase in thermal load from parallel, full-load EGR requires that the high-temperature radiator be adjusted. Condensate management can be handled by simple means due to the controllability of both variants.
Due to the very good emissions results under partial-load operation, as well as fuel savings under full load, increasing use of indirect integrated charge air cooling in both gasoline and diesel engines can be forecast with high market share. In the medium term, cascaded charge air cooling with several cooling circuits connected in series for additional significant reductions in charge air temperatures will also see growing market application.