Turbocharging Challenges

Hannu Jääskeläinen

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Abstract: Achieving maximum engine torque at low engine speeds can be a challenge with a turbocharged engine, especially in downsized engines that rely heavily on turbocharging. Another key challenge with the design of turbocharged engines is the turbocharger lag, which can lead to smoke emissions and to a significant reduction in the maximum torque available during vehicle acceleration. Turbocharger lag can be controlled through minimizing the moment of inertia of the turbocharger, reducing turbocharger friction, and other strategies.

Low Speed Boost

At low engine speeds, achieving maximum engine torque can be a challenge with a turbocharged engine. If a fixed geometry turbocharger without a wastegate and sized for engine rated power is used, examination of a typical turbine mass flow versus expansion ratio curve shows that at the low mass flows typical of low speed operation, the expansion ratio across the turbine would be low (expansion ratio strongly influences the power generated by the turbine) and little boost would be generated by the compressor and consequently air flow to the engine would be limited. In diesel engines, this airflow limitation places a limit on engine torque due to smoke emission considerations while in stoichiometric gasoline engines, the torque would be limited from charge flow considerations, Figure 1 [2727]. The turbine mass flow versus expansion ratio curve suggests that expansion ratio across the turbine could be increased at low engine speeds by choosing a turbine with a lower flow capacity and using a wastegate. A variable geometry turbine provides some flexibility in its operation and even higher expansion ratios are possible at low exhaust flow conditions.

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Figure 1. Effect of various charging options on low speed engine torque

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