No matter where you go, people are discussing natural gas. While the publicity often far exceeds the current infrastructure, it’s beneficial to understand the basics, as this fuel source is poised to play a more significant role in the future.
Natural gas is an odorless, nontoxic, gaseous mixture of hydrocarbons — predominantly methane — and it possesses the lowest carbon content of any hydrocarbon fuel. Although it does produce greenhouse gases, it is much cleaner than petroleum-based fuels. It is lighter than air and disperses quickly into the atmosphere when spilled. Plus, it has a narrow flammability range, which makes it a relatively safe fuel.
Consequently, natural gas is quickly gaining favor as an affordable domestic alternative to gasoline or diesel. Any vehicle traditionally powered by gasoline or diesel can be converted to natural gas.
Capacity a Challenge
When people refer to compressed natural gas (CNG) or liquefied natural gas (LNG), they are really describing the state in which the natural gas is stored. A natural gas engine is not specific to a particular gas type. It doesn’t care whether fuel is being supplied by a CNG or LNG tank.
There are benefits and drawbacks to both storage solutions, particularly when discussing mobile equipment. CNG is stored as a gas at 2,900 to 3,600 psi, while LNG is stored as a liquid at a very low cryogenic temperature. With CNG, the compressed gas travels through a series of highly pressurized stainless steel lines until it reaches the regulator, where the pressure is reduced to around 115 psi to match the amount needed by the fuel injection system.
According to Paul Blomerus, senior director, High Horsepower Corporate Development, Westport Innovations Inc., the amount of natural gas an engine receives is regulated to the temperature and pressure required by the engine, typically 110 to 150 psi. The customer will pick the most appropriate tank solution.
Whether you use CNG or LNG, the natural gas storage tanks and more complex fuel delivery systems will add cost to the initial purchase price vs. a traditional petroleum-fueled vehicle. And fuel storage will require a larger footprint on a mobile vehicle than with an equivalent diesel fuel tank solution.
The storage volume required per unit of energy is larger for natural gas compared to diesel. “LNG requires a volume of about 1.7 times that of diesel to store an equivalent amount of energy,” says Scott Fiveland, GAS new technology manager, Caterpillar. “CNG systems, depending upon the degree of pressurization (usually 3,600 psi), require upwards of four times relative to diesel and 2.5 times relative to LNG. In key industries, LNG fuel systems will be designed to continue to meet key customer requirements while not sacrificing payload, etc.”
CNG has been more readily used in the past. “Up until now, it has been more CNG than LNG, but that is changing very rapidly,” says Blomerus. Many recent announcements tout additional capacity being added for LNG. “CNG is very popular on return to base vehicles.” This is because trickle compression filling is a very efficient way to fill a CNG tank. You plug in the compressor and fill the tank overnight when electricity is cheaper. “On an over-the-road haul truck, the LNG is going to be a better choice for fuel — better range, faster filling.”
Caterpillar works with customers to match fuel system storage requirements with customer needs, and executes the appropriate packaging on an application basis. “CNG does have some energy density storage challenges. But in some cases where CNG is directly available at a cheap cost, it can be utilized,” says Fiveland. “In this case, the appropriate storage technology and cycle times, derived via customer needs, will be designed into the products to maximize the customer value.”
LNG vs. CNG
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.
Yet, many industries need diesel engine efficiency. Dual-fuel and HPDI technology provide the solution. In a diesel/CNG dual-fuel engine, the diesel acts as an ignitor to get the explosion going. The engines can be run on diesel or a combination of diesel and natural gas. They are started as a pure diesel engine; as the inlet air temperature rises, gas is injected into the cylinder from an additional intake valve. As the gas is injected, the amount of diesel being injected is reduced. The average substitution in medium- and heavy-duty truck applications is often 50%.
This requires the engine to be certified as both a diesel and a gas engine, and it lengthens the payback period since it is still consuming 50% diesel. But it can be an attractive solution since the engine can run on 100% diesel when natural gas is not available. “A dual-fuel engine has all of the same emissions reduction technologies because there are certain conditions where a dual-fuel engine is going to be running 100% on diesel,” says Blomerus.
With HPDI technology, the engine runs on 90% to 95% natural gas. Gas is introduced at high pressure directly into the combustion chamber. Only a small amount of diesel is still needed, since this technology utilizes diesel pilot injection.
“A HPDI engine uses a small amount of diesel as an ignition source,” says Blomerus. “Typically, it is such a small amount that you can still get the benefit of the clean-burning gas, allowing you to reduce the cost and complexity of the emissions reduction equipment.
“Direct injection is directly comparable to diesel engine performance,” he adds. If an application requires low-end torque and responsiveness, then a diesel cycle with direct-injection gas technology is the best solution.
Fiveland agrees, noting, “Power density is defined appropriately as power/engine displacement. Engines burning on natural gas can match equivalent continuous or variable-speed diesel power requirements within the same engine envelope.”