Ask An Engineer: Natural Gas For Dummies

Westport Innovations has just signed a second deal with General Motors to produce light duty natural gas engines, and it's probably not the last time we'll be seeing these kind of partnerships forming. Natural gas vehicles have been explored previously on TTAC, but the technology hasn't been fully explored in-depth, aside from some well-informed comments in various articles. As a fuel for vehicles (light duty as well as commercial vehicles), natural gas has a number of attributes which fit well with our current political narratives and economic realities Natural gas is 30-50% cheaper than diesel per unit of energy Abundant domestic supply Environmental benefits (lower GHG and tailpipe emissions) Significant reduction in CO2, CO, UHC, NOx, SOx and PM emissions versus conventional gasoline and diesel engines. Natural gas can be used across the full spectrum of spark ignition (gasoline type) and compression ignition (diesel type) engines with the appropriate enabling technologies. While spark ignition natural gas engines have been available for quite some time (such as the NG powered Honda Civic), compression ignition natural gas engines have required further development. The difficulty is that while natural gas burns cleanly, it is less likely to auto-ignite (octane rating of 120-130), unlike diesel, which has a lower octane number. This quality of natural gas is advantageous for a spark ignition engine as it prevents detonation and allows for higher compression ratios, but makes it detrimental for a compression ignition engine. Westport has devised a dual-fuel direct injection system to enable natural gas substitution in a compression ignition engine. The fuel injector at the heart of this system is able to inject both liquid diesel and gaseous natural gas in precisely metered quantities directly into the cylinder. In this system, the diesel fuel ignites as a result of compression as it would in a regular diesel engine. The combusting diesel fuel initiates the natural gas combustion. 93-95% diesel substitution is achievable according to public documentation. This innovation is directed at the heavy-duty diesel market which includes everything from transport trucks to locomotives. One of the main criticisms is the lack of infrastructure surrounding natural gas. Compressed natural gas (CNG) is easier to store and transport than liquefied natural gas (LNG) so it is the optimal choice for light duty applications. LNG has a greater volumetric energy density but is more expensive to store, transport and ultimately use in a vehicle as it must be kept cold and pressurized to remain a liquid. Vehicles like the Civic Natural Gas have a reduced range relative to a gasoline Civic, but commercial vehicles, like transport trucks, are emerging as one of the prime candidates for natural gas engines. Large transport trucks are a significant contributor to green house gas emissions and are on the road enough to make the conversion cost effective – though LNG, rather than CNG, would be the fuel of choice. A relatively small number of LNG filling stations placed along major transport corridors could meet their fueling needs and present a great way to thoroughly evaluate the technology. Less complex CNG stations could be added if the decision was made to target light duty vehicles. Going "all in" on CNG/LNG is a little premature at this point, but the adoption of natural gas as a transport fuel is a good first step in reducing our emissions while other alternative technologies reach maturity. More in-depth discussion is always welcome in the comments. "Ask an Engineer" is hosted by Andrew Bell, a mechanical engineer and car enthusiast. Andrew has his MASc in Mechanical Engineering from the University of Toronto, and has worked on Formula SAE teams, as well as alternative fuel technologies in Denmark and Canada. Andrew's column will explore engineering topics in the most accessible manner possible. from The Truth About Cars http://www.thetruthaboutcars.com

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