Where are NGVs used now?

      Approximately 100,000 NGVs are on U.S. roads today. NGVs have a long-established record in Europe, Canada, New Zealand and Australia, as well. Italy has been using natural gas as a vehicular fuel since the 1940s, with more than 350,000 NGVs. In Canada, nearly 20,000 NGVs operate with a network of 220 public fueling stations. Argentina has 680,000 NGVs, and Russia has more than 30,000. Worldwide, nearly two million NGVs are in use, in countries now including Uzbekistan, Venezuela, Mexico, the Philippines and Indonesia. Click here for detailed statistics. Growing interest in NGVs led to the formation of the International Association for Natural Gas Vehicles in 1986, which now has more than 500 members from over 30 countries, and to the establishment in 1994 of the European Natural Gas Vehicle Association, with more than 120 members from 20 countries.

     How much do NGVs cost?

     Currently all alternative fuelled vehicles have a price premium over traditional fuelled vehicles (unless manufacturers have special promotional prices that they subsidise). With more vehicles coming on to the market, certain economies of scale will be achieved. The price of an NGV varies depending upon whether it is a petrol vehicle converted to run on natural gas or a factory-built vehicle. Different size vehicles also vary in price. Typical sedans are less expensive and trucks, which require more storage cylinders, are more expensive. NGV conversion equipment can be purchased for about US$4000 and installed by the fleet owner, who can receive training from the conversion companies or conversion kit manufacturers. Alternatively, NGV specialists can do the conversion, which adds about 25% to the vehicle cost. Larger vehicles require more fuel storage cylinders, and the price can increase depending how many cylinders and the type installed. Light duty NGVs from the factory can range in price from US$ 1000-6000 over the price of a traditional fuelled vehicle. Heavy duty engines, trucks and buses typically cost US$30,000- 50,000 more than standard diesel engines and vehicles. Much of the added cost can be attributed to the number of cylinders required to obtain the desired range of the vehicle. However, natural gas costs significantly less than gasoline and diesel. For many fleet customers, the up-front costs can be recovered over the life of the vehicle.

     Where can an NGV be fueled with natural gas?

     A growing number of public fueling stations are available in some countries. In the US more than 1,200 natural gas fueling stations operate in 46 states and the District of Columbia, and more than half of these are open or available to the public. Oil companies such as UNOCAL and Shell are involved in public NGV fueling stations. Many utilities provide compressor station equipment or compressed natural gas for on-site fueling of large customer fleets NGVs also can be fueled from a small dispenser directly connected to a home or business natural gas line. This is commonly known as a Vehicle Refuelling Appliance (VRA). The dispenser is operated by a small electrically driven compressor. Click here for more information on refuelling station numbers in various countries.

     What dedicated NGVs are being manufactured now?

     All major US car, truck and bus manufacturers have built dedicated prototype NGVs. Many NGVs are directly available from the original equipment manufacturers. Bus manufacturers like Blue Bird and Orion Bus Industries sells buses designed to run on natural gas. Major diesel engine manufacturers, such as Caterpillar, Cummins, Detroit Diesel, Mack and Deere Power Systems, are developing or producing heavy-duty natural gas engines for a wide variety of vehicular applications. Forty-two manufacturers produce more than 93 varieties of natural gas vehicles, engines and chassis, varying from light-duty passenger vehicles to school buses and forklifts. • NGV Purchasing Guide • Japanese Auto Development In other countries most vehicle manufacturers have an NGV programme.

     Can current NGV technology keep pace with the advances in the auto industry?

     Recent advances in NGV technology will keep the industry on track, with the most advanced technologies coming from major automotive manufacturers. The NGV industry is intently focused on new research and development in areas of infrastructure, vehicle and engine technology, and reductions in the emissions of NGVs. NGV conversion mechanicals are compatible with throttle body and multiport fuel-injected engines. Closed-loop, computer-compatible conversion kits are now being developed and marketed. These will improve bi-fuel NGV performance and further reduce their already-lower emissions.

     The only major difference between a gasoline vehicle and an NGV is the fuel system. Natural gas is compressed to between 3,000 and 3,600 pounds per square inch (200 bar) and is stored on board the vehicle in cylinders installed in the rear, undercarriage, or on the roof. When natural gas is required by the engine, it leaves the cylinders, passes through a master manual shut-off valve and travels through a high-pressure fuel regulator located in the engine compartment. The natural gas is injected at atmospheric pressure through a specially designed natural gas mixer where it is properly mixed with air. Natural gas then flows into the engine\'s combustion chamber and is ignited to create the power required to drive the vehicle. Special solenoid-operated valves prevent the gas from entering the engine when it is shut off.

     A bi-fuel vehicle can run on either natural gas or gasoline. Many are designed to switch automatically to gasoline when the natural gas fuel tank reaches empty. These vehicles get the same or slightly fewer miles per equivalent gallon of natural gas as do vehicles using gasoline only.

     NGVs are ideal for fleet operations. The industry is concentrating on high fuel-use commercial fleets, such as transit buses, airport shuttles and taxis, refuse haulers and over-the-road trucks. NGVs of all types are on the road now, an indication that the industry has moved beyond the developmental stage into commercialization and expanded applications. Many states view the emerging alternative-fuels industry as an economic development opportunity. These states support combining the use of incentives and the implementation of Environmental Protection Act fleet regulations to make the AFV industry sustainable. The Department of Energy\'s Clean Cities program is currently operating in 56 areas across the United States. More than 1,200 stakeholders have signed agreements to increase the use of AFVs in their localities. Natural gas is the only alternative fuel that has a presence in all Clean Cities locations except Hawaii

     Gasoline vehicles converted to natural gas are subject to a small power loss when running on natural gas; however, vehicles designed specifically to run on natural gas will have no loss of power and may even have greater power and efficiency. Natural gas has a 130 octane rating, compared with 87 to 96 octane rating of gasoline.

     In the US contact the marketing or NGV department of your local natural gas utility or the Natural Gas Vehicle Coalition at (703) 527-3022. In other countries contact IANGV members for the best sources of information. The 1997 Position Paper is an authoritative review of the current technology and industry.

     A direct answer to your question is that the energy content of natural gas (NG) is about 47 MJ/kg or 40 MJ/m3. (gross heating value). The values for a typical petrol are 60 MJ/kg and 44 MJ/litre. Another comparison on an energy basis is that 1 kg of NG is equivalent to about 1.33 litre of petrol or 1.22 litre of diesel. Or on a volume basis 1 m3 of NG is equivalent to about 1.10 L of petrol or 1.0 L of diesel. When making comparisons you may also need to take into account the relative energy efficiency of the engines that use the various fuels. Generally engines that are designed for natural gas fuel are slightly more efficient than a similar petrol engine (because they can run at a higher compression ratio). The NG and diesel engines of similar size will have a much the same thermal efficiency.

     If the NGV is an original Equipment Manufacturer (OEM) model, it will have been designed to make the most of the excellent properties of Natural Gas - eg it will have a higher compression ratio than the petrol model and different ignition timing - and you could expect to see an improvement in performance and fuel consumption on an energy basis. This might be about 5% or more. Of course you might then drive faster, and not have any advantage. If the car has been converted from gasoline to NG and you can choose to run on either fuel (ie a bi-fuel vehicle) then it is not possible to make the most of the higher octane rating of the NG. In this case the change in fuel consumption will depend very much on the vehicle and engine design and on the conversion equipment used and how it is tuned. In this case you might expect an increase of possibly 5% in consumption. However the tune may be optimised to a particular power and speed range and if you can hit this you might get a small improvement. There may be more scope to achieve this on a high capacity engine with reserves of power. On a smaller engine there may be a noticeable drop in power and your comsumption could increase if you try to match the old on-road performance.

     A pressurized gas cylinder is probably the strongest component on the vehicle. Vehicles that totally destroyed in collisions show the only discernible component being the intact gas cylinder. It is unlikely that cylinders will rupture due to collision impact. Regarding the danger of fire from leaking cylinders, all we have is the experience to date that indicates that such an event is unlikely to occur. In North America there was a problem with leaking type 4 designs from a particular manufacturer, but there has never been an ensuing fire. The risk of fire from leaking cylinders must be low since there are well over a million CNG vehicle installations worldwide that have not experienced such problems. It is worth pointing out that natural gas is lighter than air and in the unlikely event of a leak from piping or container the gas will dissipate upwards quite quickly. In the case of petrol and LPG the vapour given off is heavier than air and will tend to pool near the ground. This is where there is a strong risk of some ignition source. In general terms diesel ranks high in terms of safety, but most people rank Naturak Gas next.

     There is a problem with the standard mechanical petrol carburettor when driving at higher altitudes, where the air density is lowered, and that is that the engine runs progressively more rich. So that the power falls off both because the engine is breathing less oxygen (because of the decreasing air density with altitude) and also because a venturi actuated carburettor will run richer as the air density decreases. A conversion to natural gas fuelling, using a typical mechanical carburettor with a venturi metering system will suffer the same problems and so in this respect you will be no worse or better off. But recall that the power of a natural gas engine also falls off by about 12 to 14% because the gas occupies about 12% of the intake volume and so you have less air or oxygen (and of course the liquid fuel does not suffer this problem). On the other hand there is a possibility of using an electronic natural gas metering system operated by an oxygen sensor which will maintain a constant fuel/air ratio with altitude and this would solve the enrichment problem, but not the 12% loss. SO if you\'re driving is limited in terms of getting over the mountain quickly it might be better to stick to petrol - although if you convert to gas you can still switch back to petrol when hitting the high mountain passes.

     CNG Fueling Speed and Range

     A slow fill gets more gas into the tank than does a fast fill. The reason is that as the gas builds up the pressure in the tank it is in effect compressing the gas that is already there - and this causes a rise in temperature, which in turn lowers the density of the gas. As the tank cools the pressure will fall. If you use the slow fill approach there is time for the tank to come to equilibrium with the ambient air temperature and the result is higher density and a more complete fill. During a fast fill, at the point where the gas enters the cylinder (going from high pressure to to a lower pressure) it is expanding and chilling. At the far end of the cylinder the gas is compressing and heating. This temperature difference is observed for about 5 seconds until an equilibrium is reached and the temperature of the gas within the cylinder rises uniformly as it is compressed. Both the cylinder and the gas will be relatively warm at the end of a fast fill. As the cylinder and gas cools down to the ambient temperature the pressure correspondingly decreases.

     What are the factors which affect the fuel efficiency of CNG?

     In the first place let us list the energy content of the fuels you mention. Using units of MJ per kilogram, the net heating values of petrol, diesel, LPG and NG are about 45, 43, 46, and 44; the net heating value does not include the heat in the water vapour of the combustion products. If you look up the gross heating values - which do include this, the values are different (higher). So the differences between the fuels are not large. But the values will also vary quite a lot depending on the composition of the fuel - particularly for NG and LPG. We now need to consider the way in which different engines use the fuels - in particular the efficiency. The engine thermal efficiency is a function of many different factors but perhaps the most important one is the engine compression ratio. The higher the compression ratio the higher is the theoretical and also the actual efficiency. The maximum compression ratio (CR) different fuels can tolerate in fact defines the efficiency. Since diesel used in a compression ignition engine can operate at say 14:1 the diesel will be expected to have the highest efficiency - lets say 40% as an upper limit. The next highest efficiency in the fuels comes from CNG, which can operate at say 12:1. with an efficiency of say 35%. It is possible to run an engine on CNG at 14:1 but that is the very upper limit. We would not expect to be able to run petrol and LPG engines at much more than 9:1 and an efficiency of about 30%. These efficiencies are the upper limits and at full load - in normal operation they will be lower than the values quoted, but in much the same proportion. The main reason for the differences is the variation in limiting CR for the different fuels. So here is a starting point for a discussion of the differences in efficiency. As far as fuel energy comparisons go (and this does not take into account the different engine efficiencies), 1 kg of NG is equivalent to about 1.33 litres of petrol or 1.22 litres of diesel - but of course occupies a greater volume. Or 1 cubic metre of NG at atmospheric pressure is equivalent to 1.10 litres of petrol and 1.00 litres of diesel. Note that there are a lot of other factors that we have not considered - for example the diesel engine will be much heavier than the other engines, and the gaseous fuels will need pressure vessels to contain them. Having established how much energy you get from the different fuels and how efficiently the different engines can use the fuels, you will be able find out how much they cost and then work out a cost per km or mile. In many countries CNG will come out as best value and that diesel will be next, followed by LPG and then petrol. But prices do vary a great deal. Incidentally if you have a petrol engine that has been converted to use NG you will not achieve the high efficiency quoted above because the compression ratio will be fixed at the level required for petrol - you will only get the benefit of higher efficiency in an OEM.

     What are the safety issues with gaseous fuels?

     First of all the safety regulations for all fuels - whether liquid or gaseous - will generally ensure that the risk of a fire under normal operating conditions is very small indeed. So it is generally in the event of a crash or equipment failur that a hazard will occur. As with most fuels the main fire hazard comes from leakage - either during refuelling operations or during operation of the equipment, a vehicle crash etc. In any of these situations there needs to be all of three requirements before there is the potential for a fire or an explosion. First the leakage of the fuel, second a situation where a mixing of the fuel with air gives a mixture in the flammable range and third a source of ignition. Most gases have an oderant added so that leakage can be detected by people in the vicinity. Once a leakage occurs and a source of ignition is present - say a spark or a naked flame of sufficient energy - there must still be a mixture of the gas in the flammable range. The likelyhood of this flammable mixture occurring is less for natural gas (NG) than for LPG since the NG is ligher than air and ends to float away. LPG vapour is heavier than air and tends to form \'pools\' near the ground. It is generally accepted that the various automotive fuels range in safety from diesel (safest) to LPG as the most hazardous, with alcohol fuels, methane and gasoline lying in the missle of the range. But in all cases it needs an equipment failure or an accident to set up the conditions for a fire. The safety measures include a strict adherence to the regulations for installation and operation of the equipment and the use of care and common sense.

     Is driving around with cylinders full of gas under pressure dangerous?

     Thick-walled reinforced aluminum cylinders, steel cylinders or 100% composite materials are used to store compressed natural gas as a vehicle fuel. These cylinders are manufactured and tested in compliance with strict regulations, and have withstood severe abuse testing under conditions far more stringent than tanks designed for storing gasoline. Natural gas vehicles submitted to test crashes up to 52 miles per hour, which have been totally destroyed, but show little or no damage to the compressed gas cylinders. Bonfire and dynamite tests push cylinders to temperature and pressures exceeding specified limits showing that compressed natural gas cylinders are durable and safe. Of course, as with all fuel systems, these cylinders are not indestructible and should be inspected periodically to ensure that no surface damage has occurred.

     Does the size and added weight of the cylinders inhibit conversion of vehicles to use natural gas?

     All the alternative fuels -- natural gas, LPG, electricity and alcohols -- suffer from problems associated with fuel storage size and weight. For NGVs, the size of the cylinder is a factor in the conversion process. Installation of the cylinders in autos with severely restrictive truck space does inhibit conversion. Added weight also is a factor, particularly where vehicle carriage weight is a concern such as on transit buses and refuse lorries. (In some countries the vehicle tax is based upon weight, and this is an added disadvantage.) But there are many options to installing cylinders and the industry is gaining valuable field experience that is leading to improved placement of cylinders in vehicles. Development of \'cylinder packs\' to fit beneath the vehicle also have resulted in improvements of the on-board storage system. Conversion of large automobiles, speciality vans, trucks, forklifts and many other vehicles are not hampered because of cylinder size. LNG also is used to increase the fuel storage capacity on a vehicle.

     In terms of kilometres(km) per litre, a light duty NGV will get about the same km/hr equivalent litre of natural gas as it does on petrol. The range of each vehicle will depend, therefore, on a vehicle\'s performance (km/litre fuel), and the number of fuel storage tanks on board. In terms of power, bi-fuel natural gas vehicles lose about ten to eleven percent because the natural gas displaces oxygen in the engine\'s combustion chamber. The reduced power is less noticeable in larger capacity engines, although four cylinder engines perform successfully at high and low altitudes at all temperature extremes. In terms of acceleration, the natural gas octane rating of 130 helps ensure performance that is close to that of a normal petrol vehicle. In heavy duty engines, performance is slightly improved when running on natural gas due to the high octane fuel in higher compression engines, however, in heavy duty natural gas engines that are designed to run on an even fuel/air ratio (i.e. stoichiometric) engines lose some thermal efficiency. This translates into fuel consumption that, in some engines, has been as much as 25% higher than the diesel counterpart. But new approaches using lean burn (less fuel/more air) or high pressure fuel injection are helping to improve the fuel performance of these larger engines. As with diesel engine technology developments, heavy duty natural gas engines continue to improve as technology becomes more refined.

     It would be difficult to try. to \'improve\' the combustion of natural gas (NG) with an additive; it burns very well on its own, when mixed with the right amount of air. I assume you mean to \'control the speed of combustion\' because natural gas burns very well when, like any other combustible fuel, it is fully mixed with air in the right ratio (stoichiometric ratio). This ratio varies with gas composition but is about 10 to 1 (air to fuel or A/F ratio) for a typical natural gas. If you wanted to extend the range over which it will burn you might try to mix in some hydrogen which burns over a very wide A/F ratio but this would not really make sense. On the other hand you may want to try to increase the speed with which it burns, particularly if it is used in a lean burn situation - which slows down the burning speed. In this case there is a variety of techniques to be used. Generally it may be possible to have a richer mix close to the point of ignition - the spark plug - and have a leaner mix more remote from the plug; a stratified charge you might say. In general as the mixture is leaner the flame speed goes down and in this case you may be left with unburnt gas in the more remote parts of the cylinder. So the system requires a lot of research and development to optimise. Lean burn operation can increase the thermal efficiency of the engine and in some cases with special combustion chamber design a lambda value of 1.5 (implying an excess of air of 50%) can be achieved with significant gains in thermal efficiency of the engine. So, generally speaking, the flame speed depends on the A/F ratio, the temperature and the turbulence in the cylinder and the cylinder shape, and it is best to play around with these parameters. Or in a larger engine put in two spark plugs.

     Power requirements depend on the size and type of the compressor - the larger ones will be more efficient. The power demand decreases as the inlet (mains) pressure increases. Some rough figures are inlet pressure 2 bar, specific power (SP) in kW hours per cubic meter will be 0.32 for a small compressor, 0.28 for a large compressor. Inlet 5 bar SP 0.26, 0.22 (for small & large) Inlet 10 bar, 0.22, 0.18. Inlet 15bar, 0.18, 0.14.

     There is no doubt that methane is an excellent fuel and that very low emissions can be achieved with it. The IANGV is dedicated to furthering its use. So we all agree that it would be good to see the wider use of natural gas (NG) as a fuel, from the point of view of lower emissions, better resource use and higher engine efficiency. But there are many other factors to be taken into account. First the storage issue: this requires that gas be compressed to high pressures (or liquefied) for storage - this requires a network of costly refuelling stations. And the on-vehicle storage requirements are a significant weight and cost factor. It is clear that the early development of networks of stations are best concentrated on areas where heavy duty vehicles - buses and other service vehicles - are operating. We certainly support the use of biogas, which in its original form will contain a high proportion of CO2 which must be removed if storage costs are to be minimised and high engine performance is to be maintained. When it is cleaned up (eg sulphur content reduced) and the CO2 is removed, it has properties that are more or less identical to NG. But the processing is costly. All these problems can be overcome, but require careful planning. To all intents and purposes the problems involved in the use of biogas and NG are identical once this processing has been done - and we are dealing with a gas with a content of methane of 90 to 95% or more. Because of the high octane rating of methane the engine efficiency will - with a purpose designed engine - be significantly higher than for petrol - and about the same as for diesel. And the emissions can be improved by a small margin - but of course an exhaust catalyst must be used. I would note here that there is no magic way in which these improvements are to be made - the fuel control system must provide for very precise fuel/air mixture control and the catalyst must be matched to the needs of the methane fuel. In this way, since the fuel now is made up largely of a single and relatively simple chemical compound (CH4), the emissions are likely to be, for the same degree of sophistication of equipment, marginally better. But note that the emissions of NOx are largely dependent on the combustion temperature. And since the methane fuel can use a higher compression ratio than petrol (and thus achieve a better thermal efficiency), it will also have a higher combustion temperature than petrol and therefore higher NOx (before the catalyst). There are many difficult and complicated trade-offs to be made to achieve the best balance. It is by no means a simple problem and is worth some detailed study.

     Typical HD diesel city buses run about 300 km per day and consume 40 to 60 litres per 100 km. Thus diesel tanks of 250 to 300 L provide adequate supply for two days light duty (LD) and one day HD operation. The total weight of the on-board installation would be about 300 kg. When using CNG the same types of operation would result in a fuel consumption of 45 to 65 kg of gas per 100 km. To achieve the same range as the diesel vehicle the gas quantity would be about 300 kg with a margin of safety (remember there may not be many compressor stations available in case of running out of fuel). With standard steel 120 litre cylinders each weighing about 120 kg and each holding about 30 kg gas when fully charged - a total 10 cylinders is required, resulting in a total mounted weight of about 1.4 tonnes including fittings etc. In regard to power loss, this depends very much on how the engine is converted to gas. Since the process involves considerable modification if the diesel engine is converted to spark ignition with a lower compression ratio (perhaps 10 or 12 to 1) the efficiency may be slightly lower than the diesel and there may be a small power loss - or even gain. This is because the gas /air mixture with spark ignition can be at stoichiometric. This means that all the air is used to burn gas. In the case of the diesel engine with fuel injection the overall mixture is very lean in order to ensure comlete combustion. Quite a complicated picture. There is a great deal of this type of information in the IANGV Position Paper. The Paper covers 10 Chapters in 220 pages. They are still some available to individuals or companies that join the IANGV. See the web site on how to join IANGV.

     There are several different factors at work that determine the exhaust temperature of an NG engine as compared to a similar engine running on petrol. NG engines that are converted from petrol operation do generally run with hotter exhausts. There are several reasons for this - one of the main ones being that with petrol there is a cooling effect as it evaporates in the induction system and in the cylinder. This does not happen with gas. Also a gas mixture tends to burn more slowly than petrol and so may still be burning when it is exhausting through the valves. There are several things that must be taken care of when doing a conversion - all important. The cooling system must be in good order and clean on the water side and the radiator clean on the air side. The engine ignition timing must be correctly adjusted (usually advanced) for gas - which tends to burn more slowly and you have to avoid gas still burning as it passes through the exhaust system. These are some of the important issues.

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