Turbo vs naturally aspirated engine power band width around peak power

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Discussion Overview

The discussion revolves around the comparison of power band width between turbocharged and naturally aspirated gasoline engines, particularly focusing on the average power around maximum power RPMs. Participants explore the implications of engine design, tuning, and performance characteristics in relation to peak power output.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that naturally aspirated engines will always have a wider power band compared to turbocharged engines.
  • Others suggest that the width of the power band is influenced by the tuning of the inlet and exhaust, with turbochargers being more effective at high RPMs.
  • One participant proposes that if both engine types have the same peak power, a turbocharged engine may only engage at lower RPMs, potentially increasing the overall power band.
  • There is a discussion about the efficiency of turbochargers at different RPMs, with some noting that they may be less efficient at low RPMs compared to superchargers.
  • Concerns are raised about the need for a turbocharger to function efficiently across a wide RPM range, particularly at lower RPMs where it may struggle.

Areas of Agreement / Disagreement

Participants express differing views on whether naturally aspirated engines inherently possess a wider power band than turbocharged engines. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants highlight various assumptions regarding engine design, tuning, and the operational characteristics of turbochargers versus naturally aspirated engines. There are references to specific performance metrics and conditions under which each engine type may excel.

gen x
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Which engine have wider "power band", more average power around max power rpm-s(rpm range that car use for max acceleration), high revs naturally aspirated engine or low revs turbo engine, if both engines have same peak power?(gasoline engines)
That can be calculated from area under power curve at that specific rpm range, divide by rpm interval(definite integral).
 
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The naturally aspired engine will always have a wider power band.
 
Lnewqban said:
The naturally aspired engine will always have a wider power band.
Can you elaborate more, maybe put some power curves plots that support that?
 
The width of the power band will be determined by the tuning of the inlet and the exhaust. The turbo will be most important at high RPM, as it will extend the power at high RPM, when there is little time to induct air into the cylinders.

A turbo will enable more power from the same engine displacement, or the same power from a smaller displacement engine.

gen x said:
... high revs naturally aspirated engine or low revs turbo engine, ...
It is more likely that a low revs engine would have a supercharger than a turbocharger. A high revs engine will benefit from a turbocharger.
 
If both engines are identical and have the same peak power, it can be assumed that the turbo is bypassed at high rpms. Therefore, the turbo only kicks-in during low rpms, thus increasing the overall power band compared to the naturally-aspirated engine.

Baluncore said:
It is more likely that a low revs engine would have a supercharger than a turbocharger. A high revs engine will benefit from a turbocharger.
A turbocharger can work just as well on a low revs engine:

2011-6.7-cummins-horsepower-torque-curve.webp
 
gen x said:
Can you elaborate more, maybe put some power curves plots that support that?
It is all about breathing.
A naturally aspired engine that has a healthy breathing along a wide range of rpm's is said to have a flat performance curve, and can enjoy a transmission with three or four steps only.

Turbines are only efficient in certain narrow range of rpm's and delta pressures (which depends on good engine breathing), which limits the range of rpm's in which it is able to provide the best boost to its engine.

The case described in post #5 is a good example of a turbo specifically designed to make a peak curve flatter, which is opposite to the case of a turbo meant to increase the peak power of an engine (maximum breathing-rpm's combination).
 
jack action said:
A turbocharger can work just as well on a low revs engine:
Yes, but to get power across the widest RPM band, the turbo will need to function at the low end of the revs, where it is most inefficient. As I understand it, turbocharger efficiency follows an RPM4 power, while with a positive displacement supercharger, efficiency does better than RPM2, and it follows closer to the stoichiometric air volume required.

I know the vast majority of the turbo compressed air can be dumped at the top end, but that does not relieve the requirement for a higher capacity turbocharger, that will work sufficiently well at the bottom end.
 

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