Originally Posted by sendler
What is the vertical aspect of the iso lines? It can't be torque as it extends above the maximum torque curve on this particular graph although most other graphs always stop at the torque curve.
Yes, the vertical axis for the fuel consumption iso lines is also torque. In this case Nm = Newton meters. That's fairly common, but you will also see ft lbs. Both are measured at the flywheel. But many graphs show cylinder pressure, BMEP = brake mean effective pressure, in bars or rarely psi. That is convertible to torque.
As I said, the iso lines seem to go above the maximum torque, simply to show what the fuel consumption was on the maximum torque line. It isn't as if the engine could get to that iso line. It's handy to know that at maximum torque at, say 3500 rpm, the fuel consumption is hardly better than 275. But it would be useful different information if that 275 iso line zoomed off showing that at 3500 rpm the fuel consumption was almost 250. Put it another way, showing an iso line outside the engine operating area allows you to determine the bsfc at points where the engine does operate.