A CNG vehicle usually gets less range than a conventional gasoline engine. CNG tanks are also quite bulky, so integrating them into the vehicle can be a challenge. Plus, there are challenges with rapid fill. “If you want to get more gas into [the tank] quickly, you need a big store of it at a high pressure,” says Blomerus. “It takes energy, and if you compress gas, it produces heat, so that is a challenge.”
The heat of compression can be overcome by using a time fill. “You plug it in and then you leave,” says Richard Kolodziej, president, Natural Gas Vehicles for America. “It is for fleets that have vehicles that come back to a common depot and they are not used overnight.”
LNG offers two big advantages. “One is that you can put more fuel on board than you can with CNG, with the same weight and the same space,” says Kolodziej. “If you have a weight problem, then LNG is more effective. If you cannot put enough cylinders in the back of the [heavy truck] tractor or on the side rails of the tractor, then LNG is more attractive. If you want to get 500 or 600 miles of range, you are going to have to go with LNG.”
The second big advantage is the gas is not compressed. The heat of compression makes it difficult to rapidly fill a CNG tank to capacity. “If you pull up to a pump and try to put in 100 to 120 gal. of CNG equivalent, you only get about 70% of the capacity,” says Kolodziej. “It will read full, but an hour later, the pressure will drop as the heat dissipates. You don’t have that problem with LNG.”
Natural gas is liquefied by cooling it to -260° F at atmospheric pressure. At this temperature, LNG occupies 1/600th the volume of natural gas at atmospheric temperature and pressure. It is stored in double-walled vacuum-insulated stainless steel tanks. There is -200° F liquid in a thermos on the side of the vehicle. To use this fuel, it is vaporized in a heat exchanger and gets to the engine at about 110 to 150 psi.
When the engine is not running, the LNG fuel in the tank will slowly return to gaseous form as the fuel warms, increasing pressure inside the tank. LNG inside the tank is stored at up to 230 psi. If the vehicle is used on a regular basis, pressure inside the LNG tanks will be managed by the system. However, if the vehicle is parked for an extended period of time, the pressure in the tank may increase to a point that causes the tank to vent gaseous fuel and reduce tank pressure. For this reason, LNG should not typically be used on vehicles that are parked for weeks at a time.
Engine Types Available
Three types of engine technologies are currently employed for natural gas engines used on mobile equipment: spark ignited, dual fuel and high-pressure direct injection (HPDI). The technology deployed depends on the application. “But [for] the largest and most demanding engines, where efficiency and power response are core requirements, the HPDI is more suitable,” says Blomerus.
Unlike diesel fuel, natural gas will not ignite by compression alone. It needs an ignition source to make it work. This is usually a spark plug in a spark-ignited engine or a small mixture of diesel in a compression-ignited engine.
Think of a spark-ignited engine more like a gasoline engine. “It has a more peak-shaped torque curve,” explains Blomerus. “You need to rev it up to get the power, whereas a diesel cycle engine has a flatter torque curve, which gives that nice low-end response.”
Westport uses spark ignition on light-duty (1-to-5-liter) engines and Cummins Westport currently uses it on medium-duty (5.9- to-9-liter) engines. Many light-duty engines used on passenger cars and light-duty trucks are configured for bi-fuel operation. In medium-duty vehicles, natural gas can also be cost effective with a spark-ignited engine.
An upside of the spark-ignited engine is that it is much quieter than a diesel compression-ignition engine. The downside is it typically provides 85% to 90% of the power of a diesel and 80% of the torque. Cooling requirements are also increased.
Spark-ignited engines do have emissions benefits. “On the Cummins Westport engines, you don’t have a particulate filter and you don’t need urea to control the NOx,” says Kolodziej. Instead, the engine relies on a three-way catalyst.