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

1. Mar 23, 2014

### marellasunny

http://imageshack.com/a/img824/2641/1cpe.png [Broken]

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?

Last edited by a moderator: May 6, 2017
2. Mar 23, 2014

### xxChrisxx

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)