By on June 27, 2012

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

  1. Natural gas is 30-50% cheaper than diesel per unit of energy
  2. Abundant domestic supply
  3. Environmental benefits (lower GHG and tailpipe emissions)
  4. 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.

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38 Comments on “Ask An Engineer: Natural Gas For Dummies...”

  • avatar

    Interesting stuff. Thanks for that.

    So does this mean that transport trucks converted to natural gas would need larger tanks?

  • avatar
    schmitt trigger

    Thanks for the interesting article.

    Caterpillar has been manufacturing stationary compression ignition CNG-diesel engines for some time now.

    Its primary market -not surprisingly- is the oil and gas industry, but also the auxillary gen sets.

  • avatar

    Way back in the old days (1960’s) my Grandparents lived in the country and used propane to heat their home and cook with–a big tank 500-750 gallons. My Grandfather and uncle converted their pick-ups and tractors to run on propane exclusively. In a time of .25/gallon gasoline, propane was even cheaper. More importantly they had a storage and delivery system on hand when the nearest gas station was 30 miles away.

    One by-product of the propane operation was an extremely clean engine that seemed to run forever. My understanding is that in the days of the carburetor and low voltage ignitions one of the problems was fuel dilution of the crankcase oil leading to premature wear; hence the call for 3/month or 3,000 mile oil changes. (today the only people calling for 3k oil changes are quickie lube places, the uneducated and operators of older vehicles) Apparently propane was much better at not diluting the oil leading to longer engine life.

    Dual-fuel and propane conversions had a big push in the 70’s after the first oil shortage, but never really caught on big. I believe it truly was because it was so cheap the major oil companies had no incentive to promote. To this day natural gas associated with oil drilling is “flared off” if no suitable/nearby pipeline is available. Natural gas is going for around $2.25/mcf vs. a barrel of oil going for about $80. I understand that generally oil wells are profitable about $60/bbl and gas wells are profitable at about $3.50/mcf. Boone Pickens’ ideas/plans are not bad.

    • 0 avatar

      We use propane for heating and cooking. Since most of our propane is derived from natural gas, we’ve seen the price of a gallon of LPG drop by almost $2.00/gallon in the last year.

    • 0 avatar

      This is always an issue when discussion of CNG/LNG comes up. The public confuses it with LPG. While the molecular structure of methane propane and butane are very similar, physical properties are not.

      Propane is not exactly derived from natural gas, but is merely present, and therefore can be separated from methane if there is sufficient market at a higher price. Most natural gas delivered has a small percentage of propane and butane in it. The LPG term dates back to when almost all of it came from the atmospheric distillation tower at an oil refinery.

      LNG (propane and butane) is liquid at room temperature and very low pressure, hence the cheap and easy-to-handle BBQ tanks. LNG is a cryogenic liquid, hundreds of degrees below zero. CNG is dispensed and stored at up to 3600psi in special expensive tanks.

    • 0 avatar

      Its not uncommon here in Australia, for Taxis (mostly Ford Falcon’s, factory fitted to LPG) to reach 1,000,000km or 600,000 miles without a rebuild. It is partly because LPG (or CNG in your grandparents case) does not wash oil of the bores like petrol/gasoline does, and also that being a taxi it is always warm.

  • avatar
    George B

    Dumb question, but why not add spark ignition to allow up to 100% natural gas fuel vs. dual-fuel? If the end user wants to run on diesel part of the time dual-fuel makes sense, but once LNG is available for a city bus, garbage truck, etc, why add cost to handle the more expensive diesel? I would assume that the hardware for spark ignition is less expensive than the fuel tank, high-pressure fuel pump, fuel injectors, etc. for a diesel engine.

    • 0 avatar

      That’s what I was thinking too. They could keep the high compression and direct injection for efficiency. But if the NG won’t ignite on its own, just add a spark plug. I wonder why they wouldn’t do this?

    • 0 avatar

      The reason for not adding a spark plug is that this system is being retrofitted to a engine that is already in use, or an existing engine design. In a retrofit, a new cylinder head would be required at a tremendous cost. Or in a new motor, the head would have to be completely redesigned. Which a manufacturer is unlikely to do for such a limited market.

      • 0 avatar

        Because if you’re burning only CNG, you’re range-limited. CNG has about 30% of the energy density of diesel, so you need a lot more to do the same amount of work.

  • avatar
    DC Bruce

    So, what’s the deal with natural-gas powered city buses that are widespread in places like Washington, DC? I had assumed they were powered by compression ignition engines. Also, its pretty common in natural gas pipelines to have compressors powered by a bit of the gas being transported. Are all of those spark ignition engines?

    Natural gas powered buses sure are nice: no smoke, no smell.

    • 0 avatar

      DC also has a fairly significant fleet of light duty CNG vehicles that refuel at a different location than the buses. Mostly NG civics, I believe. City buses drive on closed loops and are able to re-fuel at one (or three?) central fueling depots, and have a large roof area that they fill with the low-pressure tanks to extend their range.

      None of these filling stations are open to the public, I believe.

    • 0 avatar

      Dc got several grants to build a CNG station.

      I like CNG buses as well. Notable pollution benefits.

      For a variety of reasons, WMATA is moving away from CNG buses and into hybrid-electric buses.

      The biggest is the techs don’t know how to maintain the engines, and the repair rates on the CNG engines is high as a result.

  • avatar

    Because you’re burning only CNG, you’re range-limited. CNG has aboutu 30% of the energy density of diesel, so you need a lot more to do the same amount of work.

  • avatar

    Here in Argentina about 30 to 40 % of the cars have cng, its ultra cheap ,with about 4 u$s you drive about 250km. The gas price is about 1,5u$s liter, so its kinda like driving for free.

    I actually work for a company that makes cng kits and parts, if you have any questions let me know!

  • avatar

    LNG could be a game changer in the trucking business: if your competitor can reduce their fuel cost by 50% that is a huge competitive advantage. If the LNG infrastructure takes shape the over the road trucking business could change radically, very quickly.

    Would it work for railroad locomotives as well? The old steam engines used to pull a coal car loaded with fuel; would today’s locomotives pull a LNG car?

    Interesting times.

  • avatar

    The sad part is the conversion would be so easy. The most important and expensive parts of the infrastructure are already in place (how much of the country has natural gas pipelines running for home and business use), in terms of CNG, it would require the individual stations adding a compression unit to take NG to CNG and that’s just about it (not compressing and then transporting, in this respect it would reduce costs even furthur than gas. However actually doing this would be an order of magnitude greater than the equiv. of getting the cigerette industry to admit that smoking is addictive and causes cancer (Think of what the oil industry would stand to lose to utilities)

  • avatar

    It’s happening in over the road trucks. Shell committed to 300Million to install LNG refueling stations.
    UPS is running a test fleet between LA and Las Vegas. All major truck engine companies are offering at least some engines for LNG. If natural gas keeps being developed this is a major improvement for energy use and domestic energy. Plus the diesel not needed can become jet fuel, helping that industry.

  • avatar

    I took a hard look at a Civic NG (mfg here in Indiana) when I purchased my last car. But the premium (including a home compression unit) is about 10 grand. I don’t drive nearly enough miles to pay this off in any reasonable time period.

    The trucks and trains look like a whole different scenario though.
    Of course if they help reduce demand on petroleum, that’s good for us too.

    • 0 avatar

      Yeah- that $10k premium makes no sense. That’s about the cost of an aftermarket conversion. You’d think a factory effort would be much more economical than an aftermarket conversion.

  • avatar

    Would it be suitable for locomotive engines?

  • avatar

    We are now on our fourth CNG automobile in the family. The home compressor sounds attractive until you get to the details. We tried it but the numbers dont work out. Public CNG stations are cheap and fast, just that they are only in limited geographic areas. Los Angeles is one of those places. With our current CNG/gasoline Cavalier there is no HOV lane sticker but it can go anywhere.

  • avatar

    If only our wise government had thrown the kind of money they did at boondoggles like Ethanol to natural gas instead.

    It really is a game changer, but it seems most environmentalists want 100% electric vehicles or nothing.

    Natural gas would buy the world a few decades until the all electric vehicles are cheap and practical enough for the masses. I’d even argue that natural gas powered vehicles likely pollute less than actual electric cars when you consider how much electricity in this country is produced by coal.

    • 0 avatar
      el scotto


    • 0 avatar

      > I’d even argue that natural gas powered vehicles likely pollute
      > less than actual electric cars when you consider how much
      > electricity in this country is produced by coal.

      1. Pollution is much better controlled at very expensive and permanently monitored power stations rather than on (relatively) very cheap and barely monitored cars.

      2. Power stations pollute areas with low population density. Cars pollute areas with highest possible population density.

      > It really is a game changer

      An exact opposite of it, unfortunately. It is just another fossile fuel to burn through. Burning through oil we have created peak oil plus numerous oil wars. Now, with no single conclusion at all, should we continue to burn what’s else left?

      > but it seems most environmentalists want 100% electric vehicles
      > or nothing.

      > If only our wise government had thrown the kind of money they
      > did at boondoggles like Ethanol to natural gas instead.

      Have you considered that you may be completely wrong?

      Ethanol at least makes an attemp towards sustainable energy source. It is badly flawed, though, mostly because of low energy return on energy invested.

      • 0 avatar
        Athos Nobile

        You’re also wrong. Methane only source doesn’t necessarily has to be fossil related. It can also come from biogas which happens to be more “sustainable” than ethanol.

      • 0 avatar


        I don’t hear about the butanol project any more. What happened? Butanol is a growable, renewable, carbon-neutral liquid fuel that has almost the same energy content as high-octane gasoline. It does not have the adverse chemical and water problems of EtOH. And it would allow all conventional engine technology still to be used, virtually unchanged.

        Seems to me it would side-step all the hassle with electric cars on one hand and ethanol on the other, while still reducing our consumption of more intensely polluting petroleum products.


    • 0 avatar

      +1 although i would like to caution that a lot of the domestic natural gas is coming from fracking. i don’t want to turn this thread into an environmental policy flame war but it is a fact that fracking is a controversial technique for natural gas extraction.

  • avatar
    el scotto

    Since this is a car blog, I have a couple of questions. 1. I worked at a place that had natural gas and gasoline powered forklifts. The natural gas ones were slower and and had less power. Is lower power an across the board occurrence? 2. Why couldn’t Shell, BP, etc install natural gas pumps at truck stops like Pilot, Truck stops America,etc or even at large gas station chains too?

    • 0 avatar

      No real reason they couldn’t but I’ve noticed here in Korea that LPG stations are separate from diesel/petrol stations. They’re all over the place.

      My leased an LPI Azera/Grandeur for me for a few months. Range sucked, but Korea is a small country so finding a fuel station was never a problem.

      • 0 avatar

        i noticed this, too. i went to refill my rental in korea and was completely confused until the gas station attendant explained to my korean wife that the car was lpg and i had to go to a special station. i think the deal is that only rentals and taxis are lpg in korea. my father-in-law told me that regular drivers can’t buy lpg cars because of some government policy. the only downside i could see to the lpg thing was that the trunk space was somewhat limited.

      • 0 avatar

        There’s a number of qualifications for owning an LPG vehicle in Korea. The two you mentioned, taxis and rentals, plus the majority of private sales which are people who are on any form of disability or are disabled.

        The popular conception here seems to be that LPG engines don’t last as long.

  • avatar
    Andy D

    What is the optimum compression ratio to run CNG or propane in a spark ignition engine? What other differences are there between an engine designed to run only on CNG and a regular gas engine? Yes I am aware of the injectors, fuel delivery ,etc. Other than that stuff are there structural differences?.

    • 0 avatar

      No structural differences for a spark ignition engine. The Motor Octane number of Methane is 130, 97 for Propane, so compression ratio can be very high. The fuel injection system is nearly identical. Its just the fuel tank(s) and the high pressure (3600psi) fuel delivery parts that are different.

  • avatar
    schmitt trigger

    When any gas expands, it cools tremendously. Gas-powered vehicles have to tap hot water from the radiator to a heat exchager otherwise the gas line would freeze.
    With some ingenuity, this could be modified for “free” air conditioning.

  • avatar

    I wonder how those 1980’s Soviet diesel KAMAZ trucks converted to natural gas worked.I was under impression they had regular diesel engines, maybe a bit modified.

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