Engine map with load curve in 5th gear:fuel consumption varies weirdly

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SUMMARY

The discussion centers on the fuel consumption behavior of an engine at different RPMs in 5th gear, specifically comparing 4000 RPM and 5500 RPM. At 4000 RPM, the engine consumes 280 g/kWh, while at 5500 RPM, it consumes only 270 g/kWh despite producing more power. This counterintuitive result is attributed to the engine's increased efficiency at higher loads and lower speeds, which is typical for optimal combustion. The analysis reveals that at 5000 RPM, the engine consumes even more fuel due to a higher power output, emphasizing the importance of load conditions on fuel efficiency.

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marellasunny
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http://imageshack.com/a/img824/2641/1cpe.png

The black curve I drew there represents the load curve in 5th gear. Why is it that at 5500 rpm(160kph), I have a lesser fuel consumption(270g/kWh) than at 4000 rpm(280g/kWh)?

Intuitively,if I produce more power at 180kph(ie 5500rpm),I should consume more fuel,right? But,this is not the case here.Why?
 
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It is consuming more fuel at 5000rpm, it's a specific fuel consumption map.

4000 rpm = approx. 20kW
5000 rpm = approx 50kW

We can set a steady run for 1 hour (to remove the h term from kWh)

So in an hour:
At 4000rpm we burn
20 kW * 280 g/kWh = 5600 g fuel

At 5000rpm we burn
50*270 = 13500 g fuel


Per unit of power, it's using less fuel because that engine combusting more efficiently. The specific reasons are not described by the map. Typically engines work most efficiently at high load low speed, it's why cruising gears are chosen to keep the engine operating in the 2000-3000rpm band at higher throttle openings (ie low specific fuel consumption)
 

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