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Old Apr 12th 2017, 12:10 AM   #1
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Why engine efficiency drops for light loads?

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well. Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well. Thanks!
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Old Apr 12th 2017, 08:02 AM   #2
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It would help if you could define what you mean by "load." If what you mean is usable torque that an engine produces, there is some amount of inefficiency in an engine due to things like internal friction of the engine and transmission. So if the engine is idling - in other words producing no usable torque - its efficiency is zero. At the other end of the spectrum, an engine which is approaching red line has a fall off in torque due to inefficiencies in fuel combustion and the larger amount of power that is consumed getting the pistons to cycle up and down at a faster rate. In other word at higher revs it takes more work per stroke of the piston to get it to accelerate and decelerate at each end of its stroke, which means the power required to move the mass of the pistons goes up as the square of the RPM. Hence the efficiency of the engine suffers. So somewhere in between 0 RPM and redline is the sweet spot for efficiency.
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