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For Diesel-CNG systems, many people often first think of the Natruell CNG Injection System, which was popular in 2008 but has since disappeared. A review of the Natruell CNG system that appears from the Diesel Power Magazine article (Natruell CNG Conversion Kit - CNG Injection) describes it as a system developed with Bully Dog using Prins CNG system components. The article doesn't go into details about the controller used to control CNG flow so it appears that the Natruell CNG System too uses a simple control strategy, much like the Bully Dog propane system. A search for Natruell on the internet reveals very little detailed information and many people suggest the Diesel Power Magazine article as a reference. Although the Natruell Diesel-CNG system uses Prins CNG components, it is completely different from the Prins Dieselblend system.
DeLuca Fuel Products' "The Fuel Stretcher" is also available in a CNG version. It uses the same VFF30 and Model J as the propane version but adds a high pressure CNG regulator. Because CNG is a high pressure gas rather than a liquefied gas, the converter doesn't need to be heated but hot water keeps the regulator from frosting. Heating the regulator will also help to keep the fuel density more constant for better flow control. Downstream of the converter, everything remains the same as the LPG system. As with the LPG version, CNG flow is adjusted with the choice of secondary regulator springs (blue, orange, red) and the flow control valve. A installation of The Fuel Stretcher is documented in MrTruck.Net: CNG, is it the Answer for the Fuel Crisis?
Although not mentioned on their web site, Performance Diesel Inc. makes (made?) a CNG fumigation system very much in function to the ATS Torque Pro system. Rather than using an Impco Model J converter, PDI uses a regulator similar to the Impco Model EV with a custom cover assembly. Motortopia has a review of PDI's CNG system in their Green Diesel article. PDI includes a Hobbs pressure switch to the intake manifold so the system activates when boost pressure reaches the switch's setpoint. Described as being a negative-draw system in the article, the PDI CNG system includes a venturi to generate sufficient vacuum in the intake duct upstream of the turbocharger. The Model EV can either supply fuel at -1.5" WC (Blue Spring) or at -0.5" WC (Orange Spring) and I suspect that it more likely uses the orange spring. Very little is shown of the venturi as it would be a proprietary design but I would expect that it incorporates a manually-set valve (load block) to minimize overfueling. The Green Diesel article reports that PDI claims CNG usage reduces DPF regen cycle times as well.
Even when supplied with vapor only (both LPG and CNG), if the gaseous fuel system is adjusted to supply too much fuel, the engine could still suffer from detonation and very expensive repairs. DeLuca reports on his web site that he was able to substitute 96% CNG for diesel under light load but found that CNG substitution rates over 70% had detonation under heavy loads. Generally, stationary engines in power generation applications substitute as much as 90% CNG for diesel and greater amounts of substitution (closer to 95%) have diesel injector cooling issues. Because the system draws fuel in response to intake duct vacuum, constant adjustment of the manually-set fuel control control valve would be required to maintain the optimum CNG flow under varying engine speeds and loads. Again, DeLuca Fuel Products has a new microprocessor-based system in the works that addresses the risk of detonation.
Automotive CNG system installations are governed by NFPA 52 and CSA B109 and it is extremely foolish to cut corners with a fuel stored at 3000 or 3600 psi in your vehicle.
Since public CNG stations are less common than propane motor fuel stations, there area other CNG Fuelling options available.