Although still relying on vapor-withdrawal, another relatively inexpensive diesel dual fuel system to consider is the Red Rooster Diesel-Propane or Red Rooster Diesel-CNG system. Unlike the simple mechanical systems, the Red Rooster system uses electronically controlled injection to control fuel flow. With user-adjustable start pressure, maximum pressure, and flow settings, the LPG/CNG flow begins at the start pressure and tops-out at the maximum pressure with a linear (straight-line) flow increase with boost pressure. Fuel flow is determined by the regulator pressure and affects the entire range of operation.
The better diesel dual fuel systems are injection systems that deliver fuel based on a load-based (boost pressure vs RPM) fuel map. The fuel map allows fuel flow to be non-linear so that relatively more flow is delivered at low loads (even at idle) and cuts back fuel flow as loads increase to ensure that detonation never occurs. The best fuel-map systems (like the LPG EcoDiesel System and CNG EcoDiesel System) deliver fuel to the intake manifold rather than upstream of the turbocharger, which eliminates control issues from fuel transport delay. The EcoDiesel System also has a self-calibrating base fuel map feature, which conservatively maximizes LPG/CNG use and is fully laptop-programmable. The design of the EcoDiesel System also prevents any surging issues sometimes prevalent with budget systems.
As mentioned earlier, the budget systems do work and the people that use them seem to be pleased with their operation. They're all different in configuration so they'll each work differently. Make sure you understand how each system works and don't just rely on testimonials before laying out your hard-earned money. Caveat Emptor!