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

AI Thread Summary
The discussion centers on an engine's fuel consumption at different RPMs in 5th gear, revealing an unexpected trend where fuel consumption decreases at higher RPMs. At 5500 RPM (160 kph), the engine consumes 270 g/kWh, which is less than the 280 g/kWh at 4000 RPM (20 kW). This counterintuitive result suggests that the engine operates more efficiently at higher RPMs due to better combustion efficiency. The analysis indicates that engines typically achieve optimal efficiency at high load and low speed, explaining the choice of cruising gears to maintain RPMs in the 2000-3000 range. Overall, the engine's performance highlights the complexities of fuel consumption relative to power output.
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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