Natural Gas Basics

The news that the City of Los Angeles recently retired the last diesel powered bus running on its roads would surprise no one living in any West Coast city. Inhabitants of cities from San Diego to Vancouver have become accustomed to seeing signage such as “Powered by Clean Natural Gas” on transit vehicles for a generation. In many cases, hybrid electrical power is used on the West Coast but unlike their counterparts in East Coast cities, the power source is more often a natural gas (NG) fueled- rather than a diesel-fueled engine.

The core constituent of NG is methane. Unlike diesel fuels which are typically composed of a complex chain of carbon and hydrogen atoms, methane has a simple chemical constitution consisting of a molecule in which a single carbon atom bonds with four hydrogen atoms. As a result, it tends to be relatively clean burning providing it is in a pure state. Although what we call NG is primarily methane, it can also contain other not so simple petro-chemicals such as propane, ethane, and butane – because NG is sourced from petroleum reserve or refinery boil-off. This chemical variability of NG can cause operational problems. The U.S. and Canada have sustainable NG resources: but outside of West Coast cities, the retail and distribution infrastructure is poor.

Methane in its pure form is a ‘clean’ fuel and depending from where it’s sourced, it can also be a ‘green’ fuel. It has three main sources: the first is NG, the second is from natural fermentation of vegetable matter (wetlands, etc), and the third is from landfill sites, where it is known as landfill gas (LFG). Because the U.S.A. and Canada are blessed with plenty of geographical space, we have the most primitive garbage handling infrastructure in the developed world: Most of our LFG emission from rotting garbage discharges to atmosphere where it can be defined as undesirable greenhouse gas (GHG). The Europeans and especially the Scandinavians have developed a garbage handling infrastructure which captures LFGs. Once captured, LFGs can be purified (removing sulfur and carbon dioxide) and the resulting methane used to fuel vehicles. It is likely to be some time before we have the capability to capture methane from LFGs because the investment in infrastructure and purification plants is significant and usually not within the budgets of our city corporations which handle most of our garbage. At present, it is easier to dump garbage on land that is perceived as having lesser value.

So, that leaves us with NG at least for the immediate future. It is commercially available in two forms. The first is liquefied natural gas (LNG). LNG is a cryogenic liquid which must be stored at temperatures of -238 degrees F (-150 degrees C) or less to prevent it boiling off as a gas. It is can be stored at pressures of around 230 psi (16 bar). The second is compressed natural gas (CNG). CNG is stored in gaseous form but this must be at pressures ranging from 3600 to 5000 psi (250 to 345 bar) requiring special high pressure storage vessels.

NG has low energy density in both its formats. For instance, diesel fuel has an energy density of 1058 Btus (267 kilo calories) per cubic foot – compared with 266 Btus (67 kilo calories) for CNG. This means that it’s unlikely that it will ever be truly practical as fuel for linehaul trucking operations: But that said, it is already used in some class 8 trucks operating short haul.

To this point in time, all commercial vehicle NG-fueled engines are adapted diesel engines. Because NG will not auto-ignite under normal diesel engine compression temperatures, some assistance is required. This can be provided in three ways:

• Diesel fuel pilot pulse
• Hot surface ignition (continuously energized glow plug)
• Spark ignition

Another problem with NG fuel usage is that while particulate matter (PM) is eliminated, it produces higher NOx emissions when combusted: this has actuallyreduced the number of manufacturers offering NG fuel systems since EPA 2010. Over the years, Westport Innovations has emerged as a leader in engineering NG fuel systems into commercial vehicle engines. Westport fuel injectors for both LNG and CNG fuel delivery are used by their partner OEMs, especially Cummins. Two current Westport injectors are:

• High-pressure direct injection (HPDI) for LNG: the HPDI is a common rail dual-fuel (diesel fuel is used for pilot ignition, LNG as the primary fuel) with piezo actuators.
• Mono direct injection (MDI) for CNG: MDIs inject the CNG charge and hot surface (usually a glow plug) ignition is required.

Another disadvantage of NG fuel usage at least from a trucking perspective, is that specialized engine lubes along with more frequent servicing is required. This, and the fact that NG degrades more rapidly than diesel fuel under storage conditions, reduces its appeal to any type of truck activity that operates beyond inner city boundaries. For the moment, NG has to be regarded as a fuel to be used for short haul, intra city vehicles.

Some technical data in this article is sourced from TMC S.11 Energy Conservation Group Tech Sessions. More detailed information on NG and other alternate fuel systems can be found in the 4th Edition of Medium/Heavy Duty Truck Engines, Fuel, and Computerized System Management available from Delmar Cengage Learning in February of 2012.