Liquid Propane Gas (LPG) is both a primary and supplemental fuel in diesel engines almost since their inception a century ago for good reason. Although LPG has less energy potential by volume, it releases its energy in a slightly different way than diesel, making it a cheap and convenient way to increase horsepower and fuel economy at the flick of a switch.
Understand a few basics before introducing propane into your engine. First, propane is a fuel (not an oxidiser like nitrous oxide) and will explode sooner in the combustion chamber than diesel will. Mixing excess amounts of propane with diesel fuel will cause your engine to work against itself and blow up.
Secondly, propane's primary benefit is that it rapidly cools the incoming air and acts as a sort of chemical intercooler. Be careful about how much you inject; around 70 horsepower you'll reach a point of diminishing returns and risk blowing the motor.
What You'll Need
It really matters little whether you're working on a Powerstroke, Cummins or Duramax diesel; installation will be almost identical for any engine. A propane injection set-up won't be foreign territory for anyone familiar with home propane set-ups, as you'll find the same basic parts on any LPG stove or water heater. You'll need a propane tank, hose, regulator and pressure gauge (all of which are available at your local RV store, welding supply shop or franchise hardware store). Use a 12-volt normally closed solenoid valve (available online) and the wiring and a toggle switch to trigger the system. The entire project should set you back less than £97, not including the cost of propane.
The first thing you'll need to mount is the propane tank, then (in this order) the pressure gauge, pressure regulator and solenoid valve. Run a line from the solenoid valve to the air filter box, connect your on/off switch to the valve and you're almost done.
Although this basic set-up will indeed introduce propane into your engine, you're far from finished. You might seriously want to consider mounting a servo to your pressure regulator, which should be connected to a cab-mounted dimmer-style switch. This will give you some much-needed real-time control over the amount of propane being injected, a setting vital to preventing engine destruction while introducing enough to make power.
Tuning and Triggering
Don't trigger the propane injection if your engine is idling, decelerating or the turbo is producing less than 5 psi of boost. In other words, flip the switch only when your engine is under load and boosting. When tuning the amount of propane injected, start with the valve just open and continue to increase the amount of pressure until you start to hear pinging, then back it off by about 5 psi.