Originally Posted by clett
Does the Prius calculate how much fuel to inject on the next cycle based on the data from the oxygen sensor in the exhaust?
Most cars with catalytic converters/oxygen sensors do this. It's called stochiometric operation, or complete combustion. Too little air and you get unburned hydrocarbons in your exhaust; this pollutes and gums up the catalyst. Too little fuel and you get poor combustion, less power and more nitrous oxides in the exhaust. The system tweaks the amount of fuel to maintain the ideal 14.7 air/fuel ratio.
So you can just inject extra fuel and the computer will back off the regular fuel, right? Well, not always. Most systems have expected values that should work for a given air pressure/temperature/coolant temperature etc - this is how they start, given that the oxygen sensor has to be hot to work and takes a few minutes to warm up. Even once the loop is closed, the computer may balk at having to cut fuel back to near-zero due to dumping more fuel into the line directly.
In addition, you have some other problems. If the system wants zero power from the engine, and you're feeding it fuel, it may cause trouble. The engine may continue to run with an error code, or the system may cut the ignition and cause a buildup of fuel in the intake manifold - a very bad thing. Also, changing the fuel mix significantly may cause preignition, poor combustion, or incomplete combustion.
Hence my earlier advice to try it in very small amounts. You also need some way to stop the flow when the engine is delivering below X percent of power. A fuel-flow switch might accomplish this, although you'd have to put it between the fuel manifold and the injectors. (In most modern cars, fuel is always circulating from the tank through the pump and back to the tank.) An intake manifold vacuum switch could also do it - normal pressure or high vacuum (idle) should shut off the alternative fuel, but a moderate vacuum could enable it.