The same way they control other emissions: locked-down software in the engine control system. Possible, though often illegal, to circumvent. Edit: the software

The same way they control other emissions: locked-down software in the engine control system. Possible, though often illegal, to circumvent. Edit: the software can detect if the engine is running on ethanol or gasoline (or mixed) by looking at the air/fuel ratio, which is set using the oxygen sensor in the exhaust. It's called flex-fuel. Quite simple for the software to only allow operation on pure ethanol.