Natural-gas Vehicle Design

Natural-gas vehicles use the same basic principles as gasoline-powered vehicles. In other words, the fuel (natural gas in this case) is mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston up and down. Although there are some differences between natural gas and gasoline in terms of flammability and ignition temperatures (see chart below), NGVs themselves operate on the same fundamental concepts as gasoline-powered vehicles.

Property
Natural Gas
Gasoline
Diesel
Flammability Limits (volume % in air)
5-15
1.4-7.6
0.6-5.5
Auto-Ignition
Temperature (°F)
842
572
446
Peak Flame
Temperature (°F)
3423
3591
3729

Still, some modifications are required to make an NGV work efficiently. These changes are primarily in the fuel storage tank, the engine and the chassis.

Fuel Storage
Most NGVs operate using compressed natural gas so the fuel takes up less space. At a fueling station, gas is compressed to 3,000-3,600 pounds per square inch before being pumped into high-pressure, tube-shaped cylinders that are attached to the rear, top or undercarriage of the vehicle. The storage tanks of early NGVs were bulky and took up much of the vehicle's cargo space, but newer, more lightweight cylinders have been developed. Called Integrated Storage Systems (ISSs), these all-composite cylinders are contained within a fiberglass shell and impact-absorbing foam to protect them in the event of a crash. The cylinders also have a smaller diameter so that three of them can be housed together in a size and shape resembling a conventional gasoline tank.


Image courtesy Johns Hopkins University Applied Physics Laboratory

Engine Modifications
When the engine in an NGV is started, natural gas flows from the storage cylinders into a fuel line. Near the engine, the natural gas enters a regulator to reduce the pressure. Then the gas feeds through a multipoint gaseous fuel-injection system, which introduces the fuel into the cylinders. Sensors and computers adjust the fuel-air mixture so that when a spark plug ignites the gas, it burns efficiently. A natural-gas engine also includes forged aluminum, high-compression pistons, hardened nickel-tungsten exhaust valve seats and a methane-specific catalytic converter.


Photo courtesy Johns Hopkins University Applied Physics Laboratory
An installed natural gas engine

Chassis Modifications
Some modifications in the suspension of a NGV may be required to create space for the fuel-storage containers. In the rear of the vehicle, a semi-trailing arm suspension sometimes replaces the lateral-link suspension that comes standard in many gasoline-powered cars. This creates more open space in the rear undercarriage, yet still provides a smooth, comfortable ride. NGVs also remove the spare tire and jack, which allows for a flat floor plan. "Run-flat" tires, such as the Extended Mobility Tires from Goodyear Tire, are installed to compensate for the fact that the spare tire and jack are missing.


Photo courtesy Johns Hopkins University Applied Physics Laboratory

Differences in Refueling
Refueling a natural-gas vehicle may also be a little different. The fueling point is typically at the front of the vehicle, although in some NGVs, such as the Honda Civic GX, the fueling point is in the rear. An NGV can be fueled at a "fast-fill" pump in about the same time it takes to fuel any gasoline- or diesel-powered vehicle. Alternatively, an NGV can be fueled in five to eight hours using a "slow-fill" method. The home refueling stations offered by Honda are of the slow-fill variety, requiring car owners to refuel their vehicles overnight.

Next we'll discuss what advantages natural-gas vehicles have over gasoline-powered vehicles, as well as the disadvantages.