What does this mod do?

This is a mod that adjust propellent utilization on engines, as was done in real life on the J-2 rocket engine. EMR stands for Engine Mixture Ratio

How do I use it

ERMController works in two modes: "open", and "closed" loop modes. Open loop takes the Oxidizer:Fuel mixture ratio you want to use, and runs the engine at that mixture. Closed loop takes a look at your remaining fuel, and continually adjusts your mixture ratio to exhaust both fuel and oxidizer at the same time.

We'll cover how to

1. Set up an EMRController configured engine in the VAB, and
2. Use that engine in flight

VAB

In the VAB, the first step is to click the Enable EMR button. By doing so, you'll get a wide range of options to configure:

Starting EMR: This is the mixture ratio the engine will (by default) start at

Final EMR: this is the mixture ratio the engines will (ideally) end at

Note: Both Starting and Final EMR have vacuum ISP and Thrust displays visible

Percentage at Final EMR: This is the percentage of fuel you plan to use at your Final EMR (This is used to determine how much fuel we should put in our tanks)

Boiloff Reserve Percentage: If you're using a mod that simulates propellent boiloff, this slider allows us to add up to 50% more fuel or oxidizer, whichever you want to have more of. Drag right for fuel (the more common choice), or left for oxidizer.

Now when you add your fuel (assuming you're using RealFuels), the fuel tank should properly select the right mixture for you.

In Flight

When operating in Open Loop, the following options are available:

Current EMR: The currently active EMR (again, with Vac ISP and Thrust values below) Reserve: Assuming you stayed at the current EMR, you would be left with this much fuel/oxidizer Change to Closed Loop (X.X:1): clicking this will switch you to Closed Loop mode, which would change your EMR to X.X:1

Clicking on that, the UI changes. The slider for Current EMR gets replaced by text that updates, and generally, Reserve will show "None". An exception to this is when Closed Loop mode is operating at the configured EMR limits

Configuration

Each configuration for an EMRController module takes 2 MIXTURE configs (an upper and lower limit.) There are some key parameters on a MIXTURE config as follows:

configName: if using ModuleEngineConfigs, you can specify the config this
            MIXTURE should be applied to

ratio:      the oxidizer:fuel ratio that this mixture uses.  
            Note: this is by mass (and not volume, as is what KSP normally uses)

maxThrust:  The maximum thrust this engine produces at this ratio

minThrust:  The minimum thrust this engine produces at this ratio
            This is optional, and defaults to 0

atmosphericCurve:  a standard FloatCurve for the ISP at this MIXTURE

A sample MM config:

@PART[liquidEngine2]:FOR[EMRController]
{
    MODULE
    {
        name = EMRController

        MIXTURE
        {
            configName = something
            ratio = 1
            maxThrust = 200
            minThrust = 0

            atmosphereCurve
            {
                key = 0 325
                key = 1 260
                key = 6 0.001
            }
        }
        MIXTURE
        {
            configName = something
            ratio = 1.5
            maxThrust = 300
            minThrust = 0

            atmosphereCurve
            {
                key = 0 300
                key = 1 200
                key = 6 0.001
            }
        }
    }
}