Equation of non-symmetric cone for fuel consumption maps

aLEx7

Dear Brains

I am working on fuel consumption maps to be included in vehicle simulation software to calculate the fuel consumption. To be able to create the map you need 3 things : RPM, Engine Power, and consumption (kgr/h) or efficiency of the engine (Depending on the consumption). You take all the points and you make a graph like this when using rpm, engine power and consumption: http://www.google.it/imgres?um=1&hl=...8&tx=133&ty=92

which is 2 polynomials equation z(x,y) (or something like this)

If you substitute the consumption with the efficiency you will get a graph like this:http://www.google.it/imgres?um=1&hl=...r:2,s:20,i:146

I know that here the y-axis is torque but is the same idea

This i can describe it with my little knowledge of mathematics as a non-symmetric cone. Is anyone that could give a general equation that will described this graph? I know that is not only one equation it should be multiple equations. But I can not find any info around the internet or I am really not searching well

Thank you for your help