In this story we take a look at some key indicators of natural gas as a road transport fuel: price, performance, range, and safety.
What it is
Natural gas exists in vast quantities in various geological formations. It can be in the form of natural gas (70% to 97% methane), or as methane hydrates found in the ocean bed and frozen tundra.
It is colourless, non-toxic, highly volatile and approximately half the weight of air. Methane itself is also odourless (but odorants may be added to make leaks more noticeable). Contrary to public perception, methane is difficult to ignite, having a very narrow flammability limit of between 5% to 15% fuel-to-air ratio under ambient conditions.
Most natural gas is used as fuel for heating and power stations, with significant quantities also used to manufacture ammonia fertilisers and other chemicals. Only a small percentage of this strategic fuel is currently used in transport - but that could change.
Comparing the retail cost of fuels worldwide is a nightmare. Base cost, currency exchange rates, quality standards, units of measure and tax rates are all different and sometimes highly variable. Double taxation of road fuels is common. Even on the same street, prices for natural gas may vary depending upon how much gas you use, your supplier contract and whether you are a household or business.
To help with this comparison, we have converted some (hopefully) representative figures from around the world in the spring of 2013 to common units of US Dollars per kilowatt hour. The natural gas figures are based on published household prices as delivered. They do not include the price of compression at your home for use in your car, which can add considerably to the cost and will in turn depend upon electricity prices. If you have access to a commercial filling station or business gas, supplies then prices may be lower.
Despite the many limitations and qualifications which must necessarily be applied to any price comparison like this (including any possible mistakes!), it should be immediately apparent that natural gas enjoys a considerable base (before tax) price advantage over other fuels. As you might expect, the lowest base prices on this list are found in the US, where they are currently enjoying a surplus from shale gas, and Australia, which is a regular exporter of gas to world markets.
On top of this base price advantage, natural gas enjoys considerable tax advantages in many countries where its use as a transport fuel is actively encouraged to reduce a range of harmful emissions.
Unless they are pressurised, gaseous fuels require large tanks. But there are some advantages to having your fuel under pressure - vapour lock isn’t a problem and you may not need a fuel pump. But that’s about it. Pressure tanks are more expensive, have inconvenient shapes, are heavy and may require periodic pressure testing to ensure they are still safe.
Natural gas does not liquefy at temperatures and pressures that are practical in most automotive applications (see breakout box on Liquefied Natural Gas at the end of this article). That means the amount of fuel you can carry is quite low and the tank will weigh much more than the gas in it! One maker of lightweight composite pressure tanks lists a 100 litre tank rated at 248 bar (3600 psi) as weighing 42kg and holding only 20kg of fuel.
The practical range between fill-ups will also be quite limited. According to Consumer Reports in the US, “The Civic Natural Gas sedan’s fuel tank also occupies more than half the trunk, and once filled, it holds the energy of just eight gallons of gasoline. Honda rates the car’s cruising range at 220 to 250 miles.”
Recently, some companies have started marketing low pressure Compressed Natural Gas (CNG) tanks which are filled with an adsorbent-like activated carbon (charcoal). However, ‘low pressure’ still means 34 to 41 bar (500 to 600 psi)! But because of the lower pressure, these tanks can be made into shapes that will fit more neatly into automotive applications. Capacity is still quite limited, but lower pressure means that the cost of compressing gas will be greatly reduced.
Quality and availability
Pipeline natural gas is widely available around the world and standards are well-developed, partly because of the need to manage condensate and corrosion in distribution networks. Natural gas also has to meet minimum standards for use in appliances. Good quality pipeline gas will contain around 94% to 97% methane but there are reports of low or inconsistent fuel quality in many parts of the world. The balance will be other hydrocarbons (ethane, propane etc), carbon dioxide, nitrogen and other trace gases.
Methane can also be sourced from biological sources. However, gas from landfills and anaerobic digestion plants contains many substances besides methane. Care should be taken in using supplies directly from these sources as, without further treatment, they will not normally meet appropriate standards. The correct specification of bio-product is bio-methane, which is almost 100% pure methane, the result off following a refining process.
Natural gas is widely used in many urban areas, but is usually delivered at very low pressure at the point of use. Direct access to high pressure pipelines is not normally allowed for safety reasons. This means you can fill your car up at home, but you will need an expensive compressor that uses a lot of electricity. You may find a CNG filling station near you if you live in an urban area such as Los Angeles where the local utility heavily promotes the use of CNG.
Operators of large fleets that undertake relatively short journeys, such as rubbish collection vehicles, can establish their own filling stations and recoup the cost through savings from cheaper fuel. For the rest of us, availability will mostly depend on government or corporate strategies in the areas where we live. With such a short range in normal automotive applications, the availability of refuelling stations will be critical for the development of this market.
Performance in Spark Ignition Engines
You can of course buy a kit to allow your petrol engine to run on CNG. Some manufacturers even sell cars already converted. That’s great, but most conversions just allow cars which were designed to run on petrol to use CNG as well. You can expect a significant reduction in power and efficiency with this simple approach. To recoup these losses, you will need to deal with the capabilities and limitations of this fuel.
Of any fuel you might consider using in a spark ignition engine, CNG has the highest resistance to auto ignition or detonation. With an octane rating of around 130, methane is well outside the scale that the rating system was intended for! This means you can run compression ratios and supercharging pressures which are well above what petrol will tolerate, for improved performance and efficiency.
The bad news is that methane gas is much less dense than evaporated petrol. That means that for stoichiometric mixtures in a spark ignition engine, it takes up more space in the incoming charge and will displace air. Less air means less oxygen, and that means less power.
To illustrate, let’s consider the filling of a single cylinder with a stoichiometric ratio of petrol and air at atmospheric pressure. Now petrol isn’t a single chemical, so we will use iso-octane to illustrate. The evaporated iso-octane will occupy about 10% of the cylinder swept volume, leaving 90% for air. Because it is much less dense, the stoichiometric ratio of methane in a cylinder charge will occupy about 36% of the cylinder, leaving only 64% of the volume available for air. Without supercharging, this will reduce peak power in a spark ignition engine by nearly 30% compared to petrol, simply because there is less air in the cylinder. Ouch!
To a large extent, this can be countered by higher maximum intake pressures in supercharged engines and the use of intercoolers to increase the intake air charge density. However, as we mentioned earlier, if you are thinking of using a simple adapter kit without engine modifications, then you can expect a significant loss of power.
Another small disadvantage is that CNG enters the engine as a vapour, rather than a liquid. With a petrol injected engine, the heat absorbed by evaporating fuel helps to reduce intake temperatures and therefore suppress detonation. That doesn’t happen with a fuel which is already a gas so we must rely on a combination of intercooling and methane’s considerable resistance to detonation.
There are a number of options if you want to use CNG in cars. Let’s take a look.
If you want good performance and efficiency, but really need petrol as a backup fuel, then you will need to consider your modifications carefully. For example, you might use a moderate compression ratio with a switchable supercharger, or turbo with variable boost. When running on CNG, the supercharger should easily allow you to recover the 30% power reduction, and the high resistance to detonation will help keep things civilised. When you switch to comparatively low octane petrol, the boost will have to be dramatically reduced or switched off entirely to avoid destructive detonation.
That may sound like a simple solution, but it probably isn’t. Under boost at high rpm and load you will be injecting large volumes of compressed methane into a manifold which is also full of compressed air. Precise control of the amounts injected under rapidly changing conditions will be critical.
It will probably be more straightforward to deliver good power and economy in a spark ignition engine that is dedicated to run only on CNG. This is because the high compression ratios (along with moderate boost pressures) possible with such a high octane fuel should allow you to achieve good efficiency and make up the power loss due to air displacement. But you should not expect spectacular horsepower figures. Unfortunately, if you try to run your high compression engine on normal petrol, detonation will be difficult to avoid.
A very different approach has been successfully applied to co-firing of natural gas with diesel fuel in compression ignition engines, mostly in large engines such as trucks and locomotives. With Dual-Fuel, a small amount of a pilot fuel (diesel) is direct injected into the cylinder normally. This ignites the natural gas which was premixed with air in the intake manifold. The injected fine droplets of diesel act like a thousand spark plugs to ignite the gas mixture. Such an ignition source is far superior to that of a single spark from a spark-plug and so Dual-Fuel combustion allows much leaner mixtures of natural gas to be ignited.
But you can’t just add natural gas to the intake and expect good results. Direct injection diesels have a compression ratio of between 14:1 and 20:1, which is well above what you can get away with without detonation, even with the high octane rating of natural gas. By operating at very lean mixtures, detonation is suppressed, but of course if the mixture is too lean then it will not ignite at all. With no throttle on the intake air, other ways must be found to maintain good ignition while avoiding destructive knock.
Depending on factors such as the compression ratio, intake air temperature and pilot fuel injection timing, the knock limit at full power will be somewhere around a lambda of 1.2. Lambda 1.0 is stoichiometric so we are talking about 20% excess air. This is a fundamental limit on the power output of a Dual-Fuelled gas engine and knock sensors are used to allow the addition of as much gas as the engine will tolerate at full power without detonation. However, because diesels normally operate with a very high lambda ratio at full power anyway, Dual-Fuel engines can usually provide equal power levels.
The rich knock limit restricts power at low rpm where turbo boost is low. This seems counter-intuitive for those of us who are used to limiting boost pressure in petrol engines to avoid knock. However, in a co-fired engine, the extra air supplied by the turbo keeps the air fuel ratio high (lean) and suppresses detonation. Of course, if the turbo is sized to give boost at low rpm, large amounts of bypass will be necessary to avoid over boosting and running too lean at high rpm.
At idle or low power levels, natural gas is turned off and the engine runs on just diesel. This is necessary because without an intake air throttle, the air fuel ratios would be extremely lean and result in misfire. The transitions between operating on just diesel to Dual-Fuel with gas are tricky to manage in a way which provides both efficiency and driveability.
Dual-Fuel with diesel is definitely not a simple solution in terms of controls, but it does involve relatively minor physical modifications to production diesel engines. That means the engine can be configured to continue operating as normal on diesel once you run out of gas.
Another way of using methane in a compression ignition engine is to inject methane directly into the cylinder at high pressure along with the pilot diesel fuel. This results in initial ignition of the first methane injected by the pilot fuel. Subsequent methane injected enters an already burning environment and ignites as it is injected. This approach provides a pressure curve in the cylinder which is much more like a conventional diesel engine, rather than a flame front rapidly working its way across the cylinder in the premixed example above.
There are a number of advantages to this approach:
The major disadvantage to the direct injection approach is that we have eliminated the conventional high capacity diesel injector, so diesel cannot be used as a backup fuel. Our direct methane injection engine may idle on just diesel, but that’s about it. Once you run out of gas (methane), you need a tow truck.
There are obvious hazards that come with the use of compressed flammable gasses, and most governments publish mandatory safety standards with associated schemes for trained technicians. While we encourage readers to explore ways of modifying their own cars, it is vitally important to be aware of local safety schemes and regulations. Any modifications to or installations of gas handling systems should at a minimum be reviewed and approved by qualified installers.
Having emphasised the need to abide by local safety rules, we should point out that CNG has an admirable safety record. Partly, this is due to the standards just mentioned. CNG is safe in practice because it is almost totally contained in normal operations and the primary component, methane, is non-toxic.
Compare that with petrol systems, which are not completely sealed and that use a fuel containing several per cent of volatile carcinogenic chemicals, such as benzene. In the US, petrol stations are fitted with vapour recovery systems. However, this is not the case in much of the rest of the world and you can get an unhealthy dose with each fill-up. When it can be avoided, I suggest that visits to the petrol station should not be a family affair!
Approved CNG tanks are rated to pressures much higher than they will be subjected to in normal use. Even in relatively severe collisions, the chances of a catastrophic tank failure or fire are low. We are not suggesting you should install tanks in vulnerable locations, but they are quite strong and can usually sustain pretty heavy damage before any major release. In the event of a leak, methane is lighter than air, so will tend to disperse upwards.
CNG is not suitable for everyone. However, if you consider the global outlook for motor fuels, it seems possible that there may be a CNG car in your future. If you live in a part of the world where infrastructure is already available, the price is right and filling up after each journey isn’t a problem, then you may be able to experience significant savings right now.
The outlook for fleets of light or heavy goods vehicles is even brighter. Fuel is a major cost for operators and dedicated refuelling facilities can be provided at depots. Local delivery or rubbish collection vehicles tend to have more under-body space for CNG tanks, take short journeys and return to base each night where they can be refuelled.
It is harder to judge the potential for car performance enthusiasts with unbounded creativity. The 36% displacement of intake air is a major limitation. On the other hand, the high octane rating should allow high compression ratios and boost pressures in a spark ignition engine.