I've looked into it quite a bit myself and I find some of Sharkey's conclusions to be a bit... optimistic. I like a lot of his engineering though. Like Forced Induction says, the gain is only because you are adding more fuel.

It works like this:
Propane is injected into the air stream, usually ahead of the turbo to increase the mixing.
The air and propane mix enter the combustion chamber on the intake stroke of the engine.
The intake valve closes and the piston begins the compression stroke. As the gases (atmospheric air and propane) are compressed they release heat according to Boyle's law.
At the top of the piston stroke +/- a few degrees the injection pump forces a fuel charge into the combustion chamber. The fuel is subjected to high heat from the compressed gases and begins to burn. Once the oxygen is reduced to a certain level by the burning diesel (depending on how much propane you are injecting), the hot propane gas reaches a proper air to fuel ratio and combusts, adding additional heat and pressure to the power stroke.

Diesel fuel will burn in a very lean environment. Propane won't. Unless you are fueling to the max, there is always going to be more oxygen than you need to fully combust the diesel. Propane also has a higher flash point than diesel and wouldn't combust on its own without the diesel to raise the temperature.

In the end, you might as well just add more diesel and end up with something far simpler. Contrary to what Sharkey states (again, much love) the efficiency of the combustion process is not made more efficient.

It's a little like saying your wood-burning stove is made more efficient by dousing it continuously with gasoline. Sure you burn less wood, but now you're burning gasoline too.

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1978 Ford F-150 351M
2002 Kia Spectra 1.8l 5-sp
1985 Volkswagen Scirocco 8V 1.8l 5sp
1985 MB 300D Turbo California

"Die Natur verabscheut Vakuum"

I've run a propane system on my 99 F-350 for a while, and found that it did add more power and economy, BUT, the economy was based on the cost of the propane. You are just adding more fuel and any power/economy gain from 'a complete burn, etc' is minimal at best.
The system I had on the truck used a boast valve to start/stop propane flow. Under slow cruise (under 60MPH) and idle, it used no propane. When the boast went about 3 to 5 lbs, the propane would flow. (unfortunately, at 75MPH cruise, there was enough boast pressure to continually flow the propane...)

I ran all sorts of numbers, including 1/4 mile results, and all in all, for an inexpensive hot rod solution, it worked. It is not an answer for more economy.
Also, bear in mind that my F-350 CC Dually has more protection for the propane bottle than a car.

I ended up taking the setup off of the truck when I went to bigger injectors. I ended up having enough fuel but needed a better turbo for more air.

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87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100

This quote supports my theory that propane burns farther down the piston travel than diesel fuel. I think that is why propane adds power. It's not that it just has more fuel. If that were the case couldn't you just chip the computers in the newer trucks and avoid all the bull****?