Why does CI engine performance change with load and speed?

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Discussion Overview

The discussion centers on the performance characteristics of a compression ignition (CI) engine, specifically how mechanical efficiency and brake thermal efficiency change with varying load and speed. Participants explore the relationship between engine load, throttle position, and efficiency metrics under constant speed conditions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why mechanical and brake thermal efficiency initially increase with load before reaching a maximum and then decreasing at constant speed.
  • Another participant suggests that increased load leads to a wider throttle opening, proposing that an engine achieves optimal efficiency when it is un-throttled.
  • A different viewpoint is expressed, indicating that the relationship may resemble a bell curve for efficiency, with thermal efficiency potentially decreasing at an exponential rate as power increases.
  • One participant notes that volumetric efficiency tends to decline as engine speed rises, implying a potential connection to the overall efficiency discussion.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between load, throttle position, and efficiency, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Participants do not fully resolve the assumptions regarding the efficiency curves or the specific conditions under which these observations hold true, leaving some aspects of the discussion open to interpretation.

kannans
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why the mechanical efficiency and brake thermal efficiency curve increases as load increases and reaches a maximum value and then again decreases( in contand speed test)
 
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Because with more load at a constant speed you have a wider throttle opening, an engine is at best efficiency when completely un-throttled.
 
Kozy said:
Because with more load at a constant speed you have a wider throttle opening, an engine is at best efficiency when completely un-throttled.

Are you saying that across the board? It seems to me that you would be more or less looking a bell curve for this efficiency. It seems that the thermal efficiency is going to decrease at a near exponetial rate as the power increases.
 
Volumetric efficiency tends to fall as speed rises, if that is what you mean?
 

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