CNG Dodge Caravan

Chrysler Corp. sold versions of the 1995 Dodge Caravan, Plymouth Voyager, and Caravan cargo minivans that run on compressed natural gas. They are certified as one of the "world's cleanest internal-combustion vehicles" by the state of California, which said the minivans meet its tough standard for ultra-low-emission vehicles (ULEVs). The vehicles store the equivalent of 8.2 gallons of gasoline in their four underside tanks. The range is 160 to 180 miles, and the extra cost is about $5,000 over the regular vehicle price.

In 1992, Dodge introduced a full-sized van that ran on compressed natural gas. Two years later, in 1994, Chrysler’s compressed natural gas (CNG) minivan was first brought out; all three short-wheelbase minivans could be purchased with a CNG 3.3 L OHV SMPI V-6 engine and 41TE electronically controlled four-speed overdrive transaxle. Operating cost with CNG was less than with gasoline, and emissions were far lower as well. The engine and fuel system were developed by Chrysler engineers and the vehicles were assembled by Chrysler. Warranty coverage on the vehicle and fuel system was the same as on gasoline engine vehicles.

Fuel economy with CNG is roughly equivalent to that provided by gasoline on a energy content basis. Power is reduced approximately 15% from gasoline-fuel levels because CNG displaces more air in the cylinders, reducing volumetric efficiency.

The CNG fuel injection system used sequential multi-point injection (SMPI), a major step forward in responsiveness, performance and emission control compared to aftermarket conversions. Injectors and fuel rails were given a much higher flow capacity than gasoline injectors because of the lower energy content in CNG; the separate electronic injector-driver module was required to operate the high flow injectors. The control system included sensors for both fuel temperature and pressure to provide a correct basis for fuel flow; an oxygen sensor fine-tuned the fuel mixture. A specially formulated three-way catalyst assured extremely low emissions. Test procedures were inadequate to accurately measure the emissions from these vehicles, but they were the lowest emission levels of any vehicles available to the public at the time.

Fuel was stored in four glass and epoxy-wrapped aluminum cylinders. When filled at 3000 psi they held the equivalent of 8.5 gallons of gasoline. Two tanks were under the floor, replacing the gasoline tank. The other tanks took the place normally occupied by the spare tire. The spare was stored inside the vehicle. Fuel was supplied to the engine by pressure in the tanks, rather than by a pump. A regulator at the tank maintained a constant operating pressure. There was no return line from the engine to the tank.

The cylinders were recharged at a CNG refueling station through a quick connect coupling. At a quick-fill station this takes about five minutes. Slow refilling at a fleet garage could be done over night. A home refueling unit was also available. A conventional fuel gauge on the instrument panel sensed the amount of fuel remaining in the tanks by measuring its pressure. Regardless of the refueling system used, no evaporation or loss of fuel occurred.

To maintain durability with CNG, different materials were required for valves, valve stem seals and valve seat inserts.

Additional safety features included both ignition-activated and manual gas shutoff valves, back flow check valves in the refueling circuit, armored stainless steel fuel lines and fuel cylinders that meet rigid Department of Transportation requirements. Valve placement and mounting was designed to avoid damage in a collision.

The CNG minivans ran for a long time, albeit in small quantities each year. The first minivans to be certified as Ultra-Low Emission Vehicles in California, their production was apparently constrained partly by problems with getting sufficient supplies of tanks and other parts. In 1997 and 1998, Chrysler dropped entirely out of CNG vehicle production.