Vehicle Fuel Systems and Onboard Fuel Storage

Onboard Fuel Storage

Fuel for natural gas vehicles is stored on the vehicle as a liquid (LNG) or as a gas (CNG). How the fuel is stored is not a factor for Cummins Westport natural gas engines. What is important is that the fuel must be supplied to the engine in gaseous form at specified pressure and temperature under rated conditions. For example, the Cummins Westport ISL G requires that fuel be supplied to the engine from 70–150 pounds per square inch (psi) (483–1,034 kPa) and from -40 °F to 200 °F (-40 °C to 93 °C).

The kind of vehicle fuel storage chosen depends on a variety of factors, including the availability of CNG or LNG, size and design of the vehicle chassis, daily mileage requirements for the vehicle application, and refueling station availability.

Fuel tanks can be mounted on the vehicle in different locations, depending on the design of the vehicle. Some examples would be: on the roof of a transit bus, behind the cab or on the vehicle frame of a truck, or on the body of a refuse vehicle.

  • A typical CNG fuel system is comprised of tank storage, a fuel fill system, engine compartment components, fuel lines, regulators and a high pressure fill system. CNG is stored at pressures up to 3,600 psi. Fuel lines are connected to a pressure regulator that reduces pressure to engine, where a second regulator controls the engine fuel requirements. CNG is infused with an odor agent so that leaks can be identified by smell.
  • An LNG fuel system is a low pressure system designed for normal operating pressure of 230 psi. Fuel lines are fully annealed high-quality stainless steel hydraulic tubing type 304 or 316 seamless stainless steel. When LNG needs to be converted for use, it's drawn from the tank, run through a vaporizer, "warmed" back into gas and supplied to the engine at the required pressure. LNG has no odor, so a methane detection system is required in the vehicle.
  • With both CNG and LNG fuel systems, an automatic shutoff valve is opened in the "on" key position, and closes when the key is turned off.

Natural gas vehicle fuel storage systems are installed in a variety of ways. In most cases, original equipment manufacturers (OEMs) will install the fuel storage system at their factory. In other cases, it is installed at a different location, such as a truck-body builder (i.e. refuse) as the vehicle is being manufactured to the customer's specifications.

Watch a video on natural gas fueling options.

CNG Fuel System

The basic function of the vehicle fuel system is to store gaseous fuel and deliver it to the engine inlet connection within Cummins specifications for quality, temperature and pressure under all operating conditions.

Fuel pressure delivered at the engine inlet may vary with engine load conditions, tank pressure conditions and fuel flow rates. However, fuel pressure at the inlet must be stable within +/- 10 psi in response to transient engine operating conditions, and it should always be within the minimum and maximum values indicated on the engine datasheet.  The CNG system must be capable of maintaining acceptable engine inlet fuel pressure at maximum power conditions as the tank pressure varies from the maximum fill pressure down to 10% of the maximum rated tank fill pressure.

Download an Information Bulletin on CNG systems here.

LNG Fuel System

The typical LNG system for a Cummins Westport natural gas engine relies entirely on fuel vapor pressure -  so that if the LNG is too cold it cannot maintain minimum pressure to the engine as the fuel is used.  The basic function of the LNG vehicle fuel system is to store liquid fuel until it is used by the engine.  The vehicle fuel system also delivers fuel to the engine in accordance with Cummins’ specifications for fuel quality, temperature and pressure under all operating conditions. Fuel temperature should be between -40 °F and 200 °F at the engine fuel connection. This temperature is controlled by a fuel vaporizer that typically uses engine coolant to warm and vaporize the fuel. 

Download an Information Bulletin on LNG systems here.

Natural Gas Fuel System Maintenance

Water, dust, sand, dirt, oils and other substances in the fuel can be detrimental to the operation of natural gas engines. Cummins Westport engines are equipped with fuel filters to capture traces of water, oil or other substances and protect the engine against damage. In some cases, OEMs will also include fuel filters as part of the fuel delivery system to the engine.

It is the operators’ responsibility to perform maintenance on the fuel filter according to the schedule outlined in the owner’s manual. Daily maintenance includes a check of the fuel filter, as outlined in the owner’s manual. This involves opening the drain valve by hand 1½ to 2 turns until draining occurs. This will drain any captured water or oils from the filter.

The fuel filter is also replaced on a regular basis as part of scheduled maintenance. Consult your owner’s manual for details.