Multiple Compressors

Hannu Jääskeläinen

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Abstract: Multiple compressors can provide a number of benefits, such as full load torque curve shaping and increased boost for high EGR rates and/or for Miller valve timing. Two or more compressors can be arranged in series—providing two or more stages of compression—or in parallel. Parallel arrangements can be further broken down into simple turbocharger installations where exhaust and intake flows are equally split between two or more turbochargers or into parallel sequential arrangements where the flows to the individual turbochargers are controlled depending on the engine operating conditions.

Benefits of Multiple Compressors

There are numerous reasons to use multiple compressors to supply charge air to an internal combustion engine. For large multicylinder engines, a single turbocharger may be so large that dynamic response would be unacceptable. For high BMEP engines or engines using high levels of charge dilution such as EGR, a single stage of compression may not be able to provide both the required pressure ratio and range of air flow required to meet the engine’s performance requirements. In these cases, a manufacturer may need to resort to a charging system with multiple compressors. Such a charging system may consist of a supercharger and turbocharger or two or more turbochargers.

Two or more compressors can be arranged in series—providing two or more stages of compression—or in parallel. Parallel arrangements can be further broken down into simple turbocharger installations where exhaust and intake flows are equally split between two (or more) turbochargers or into parallel sequential arrangements where the flows to the individual turbochargers are controlled depending on the engine operating conditions. Series arrangements can also be a straight forward series arrangement or a series-sequential (or series modulated) arrangement where one (or more) of the compressors can be deactivated at some engine operating conditions. Within these general configurations—series, parallel sequential and series sequential—numerous refinements are possible through the selection of compressor/turbocharger sizes and bypass arrangements. Broadly speaking, the different multiple compressor arrangements can be classified as those with a single stage of compression or those with two (or more) stages of compression.

Benefits of multiple compressors to deliver intake air to an engine are similar to those for assisted turbocharging—namely full load torque curve shaping and the reduction of turbocharger lag to improve transient response. To this list, one can add increased boost pressure for the successful implementation of various emission control strategies such as high EGR rates and Miller valve timing.

Full Load Torque Curve Shaping. Figure 1 compares the full load torque curve for some examples of the above charging systems in a light-duty diesel engine. Sequential arrangements generally give improved low speed engine torque and reduced turbocharger lag but at best, only modest increases in boost pressure compared to a single turbocharger. Series arrangements offer the potential for significant boost pressure increases and if a series-sequential arrangement is used, low speed torque can be improved, boost pressure increased and turbocharger lag reduced [2640].

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Figure 1. Potential full load engine performance improvements possible with different turbocharger arrangements

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