Over the years we've seen diesel engines fueled with everything from cooking oil to ATF, and just about every other imaginable liquid or gas in between. But nothing has impressed us as much as injecting compressed natural gas (CNG) into a diesel engine. Why? It's simple really. Injecting CNG offers fuel mileage gains, offsets the fluctuating costs of diesel fuel, and CNG is more resistant to pre-ignition than propane or methanol.
Unlike many gasoline-engine conversions that are designed to run on 100-percent CNG, a typical CNG conversion for a diesel engine still uses a small percentage of diesel to initiate the combustion. Installing a CNG injection system on modern diesel engines might seem like a daunting task at first, but Natruell has recently developed a CNG kit for common-rail diesel engines with the help of Bully Dog.
When installed on an '08 Duramax...
When installed on an '08 Duramax with the LMM engine, the Natruell CNG system uses a pressure regulator (A) and four injectors (B) to deliver the CNG to the engine as needed.
To inject CNG, Natruell's kit uses injectors and a regulator from Keihin, but the real science lies in the Bully Dog-developed electronic module that controls the injection of both CNG and diesel. The Bully Dog module hooks into the truck's computer system through the OBDII port. When CNG is injected, it backs off the injection of diesel to the engine so that diesel fuel is conserved. Natruell claims that running the engine on 80-percent CNG and 20-percent diesel offers the best mileage and performance.
So What Exactly is CNG?
Compressed natural gas is the same gas that is used to heat most homes in the United States, it's just compressed to a higher pressure. Where residential gas lines contain natural gas at 40 to 50 psi, CNG is compressed to 2,000 to 3,000 psi. Therefore, it packs much more energy and power into a smaller volume. Since the pressure is so high, the tank must be very strong to contain the gas. Most CNG tanks are built from carbon fiber-reinforced aluminum.
How Do You Store It?
The tank in the 6.4L Power Stroke we tested holds the diesel equivalent of six gallons of fuel, yet it filled the front section of the pickup's bed. One of our initial concerns over the use of CNG was the risk of transporting a tank filled with so much pressure, but after researching CNG storage devices, we found there are many tanks that are DOT-certified for automotive use. CNG is also relatively safe because if it is ever released, the gas is so light it rises quickly into the atmosphere, thereby reducing the risk of fire.
 The injectors and regulator...  The injectors and regulator are produced by Keihin (www.keihin.com), which is a global supplier of fuel injection systems and components for automotive and motorcycle applications. |  Refueling the CNG system on...  Refueling the CNG system on the Duramax test vehicle we drove was as simple as opening the factory fuel door and plugging into the high-pressure CNG connector (arrow). |  The downside to most CNG systems...  The downside to most CNG systems is that you need a lot of space to carry the gas. In a pickup truck application that means you have to sacrifice bed space. On the Ford we drove, the CNG tank was covered with this sheet of aluminum diamond-plate. |