Bus and Motor Coach Library

Alternate Fuels - CNG                                                         

Author – Brian Niddery (2001)

Particulate matter produces that brown haze or smog that is so visible in large urban centers.  Particulate matter does eventually settle out of our atmosphere; however, its assorted toxins will then go to work on our lakes, rivers and soil.

The oxides of nitrogen group of emissions often referred as NOX emissions, and sulfur emissions contribute to acid rain in the forms of nitric and sulfuric acids.  Many scientists believe that this group of acids may be responsible for long term biological damage to both plant and animal species. Already there is some indication that it is at least partly responsible for the growing incidence of lung problems, asthma, allergies, and cancers.

It is definitely a source of major damage to our forests and lakes. In Ontario, Canada, fish have ceased to exist in some 10,000 lakes in that province alone, due to acid rain.

Carbon dioxide and carbon monoxide is the third major result of vehicle emissions. There is evidence that supports the notion that we may have already changed the ratio of oxygen to carbon dioxide in our atmosphere over the course of the last hundred years.

Do you know that for every gallon of fuel consumed, about 10 pounds of carbon dioxide is expelled into the atmosphere?  Every hour of every day the cars, trucks and buses of this world are literally expelling millions of tonnes of carbon dioxide into our atmosphere, and slowly replacing existing oxygen levels.

It is little wonder then that individuals, consumer groups, scientists and politicians in every corner of the globe identify pollution as being one of the most important challenges that we face in the coming years. And it doesn't take a great deal of imagination to understand when these people look at a major urban center, and see the congestion and pollution that occurs in these cities, that they want solutions.

Government officials are demanding more efficient passenger transportation systems that will not only alleviate traffic congestion but will be far less polluting. Legislation is being passed that contain more stringent environmental and clean-air legislation. Although much of this legislation is directed at the urban problem, and affects transit vehicles directly, it is inevitable that in the future all forms of passenger and freight transportation will have to subscribe to very strict guidelines. The mandate is quite clear!

As a result major transit bus manufacturers in North America have diligently invested large sums of capital over the last decade into research and development programs to reduce toxic emissions.  Many concepts have been investigated, and tremendous research and development programs have been undertaken in recent years to determine the most viable options. 

The four primary options include CNG, an acronym for Compressed Natural Gas, the Hybrid Electric Diesel system, the Oxygen Hydrogen Fuel Cell system, and the development of "clean" diesel fuels. 

There are also a number of derivatives and combinations of the above forms, and hybrid or dual systems being employed that include liquefied natural gas, propane, bio fuels and there is even a hythane derivative, which involves mixing hydrogen with natural gas to produce a better bang for the buck.

Some readers may believe that we are missing a fifth - a most obvious form of alternative power, that being the electric powered vehicle.  After all, it may seem obvious that the electric vehicle is by far the cleanest form of motive power!  As a vehicle, yes; however, electricity is only as clean as the source of power used to generate this electricity in the first place. 

If it is generated by wind turbines or through a hydro-generating station, then it may very well be an obvious form of alternative energy.  However, in many parts of North America, electrical power is produced by coal, oil and gas generating plants and nuclear generating stations, thus in most regions the pure electric vehicle can be disqualified as a viable alternative.

Due to the sheer weight of the required batteries, electric vehicles by nature are quite heavy, and cannot transport a large amount of freight or passengers. In addition, electric vehicles have a relatively short operational range. Until technology can develop lighter batteries that can store more energy, electric powered vehicles have only a marginal application for vehicle use.

The attraction to CNG as a viable fuel alternative lies largely in the fact that it burns very cleanly.  It produces very little in the way of particulate matter, and only a fraction of NOX emissions compared to standard diesel, and reduced amounts of sulfur emissions.  However, it still produces similar quantities of carbon dioxide.            

CNG therefore addresses two out of the three major emission problems.

If we can reduce the particulate matter to a nominal amount then we may begin to reduce the often eternal brown blanket that are common to larger towns and cities.

If we can substantially reduce NOX and sulfur emissions, then we will go a long way in saving our forests and lakes, and our own health and quality of life.

Reducing carbon dioxide emissions is not so easy a matter. It is the direct result of burning fossil fuel. The power requirement directly dictates the amount of fuel consumption that in turn dictates the amount of carbon dioxide that is expelled into the atmosphere.  The only effective way to reduce carbon dioxide in a combustion engine is to reduce the power requirements.  Carbon dioxide is odorless, tasteless, and invisible. However it represents a major threat to the biological health of our planet.

Developing an efficient, safe, viable CNG vehicle has taken literally decades.  The development costs have been enormous.  A great deal of this development has had to do with the infrastructure; the developments of large storage and fuel line systems, fast-fill refueling systems and of course the development of the on-board systems to make the vehicle itself a safe and reliable unit.

The greatest barrier to acceptance by municipalities and transit properties has been the initial cost factor. CNG requires a relatively extensive infrastructure, which can be very costly. Not only is a dedicated fueling facility required, but also dedicated service and garage facilities are required.  This represents a high initial capital investment, which is sometimes difficult to justify in today's economic environment.  Once having built such an infrastructure, a municipality is committed to using CNG for a very long time. All repair facilities for CNG powered vehicles must be equipped with isolated electrical wiring systems, and sophisticated venting systems.

Unlike a conventional diesel tank, one cannot top up with fuel. It is compressed to 3,000 PSI, and at the end of the day, it must be carefully reloaded using very strict procedures. It must also be defueled whenever service is required.  If a CNG unit breaks down while on the road, mechanics will need special tools and take great care in following the correct procedures.

In addition a CNG powered bus can run upwards of $40,000-70,000 more than a conventionally powered unit.  A portion of this additional cost is associated with amortizing the original research and development costs of CNG technology.

A third barrier to acceptance is the perceived danger and of the high volatility of natural gases itself.  Although it has been proven beyond a doubt that CNG is a very safe fuel, this fear still persists for many people. Let's look at the results of a survey carried out by the American Gas Association. This survey reviewed more than 8,000 natural gas powered vehicles, which traveled a total of 280 million kilometers between 1987 and 1991.  Natural Gas fueled vehicles (NGV) were found to be much safer in accident situations, as the survey recorded no fatalities involving NGV's, compared with 2.2 fatalities per 100 million miles for gasoline vehicles.

These statistics may be attributed partly to the fact that NGV drivers may tend to be more responsible than the average driver is.  However, when due care and correct procedures are followed, CNG is nonetheless a relatively safe fuel.  Natural gas requires more than twice the heat factor as gasoline to ignite, 6500C, compared with 2600C for gasoline.  Natural gas also has a very narrow range of flammability, when mixed with air, between 5% and 15%. Below or above this percentage mixture with air, natural gas will not burn. Natural gas also is lighter than air, and so if there is a leak, it will rise and usually dissipate harmlessly into the atmosphere.

It is quite a different story for most other fuels, including gasoline or diesel fumes or propane which are heavier than air and tend to collect at ground level or within engine, baggage or passenger compartments in or around a vehicle, subjecting a vehicle to a greater risk of fire. Gasoline and diesel, being a liquid, if spilled will usually pool in or around a vehicle, also increasing the risk of fire. On the other hand, natural gas burns almost instantaneously and explosive-like, usually with little or no warning.  For that specific reason CNG use requires extraordinary care in its handling.

CNG fuel costs are substantially less than diesel fuel costs.  However, direct comparisons between fuel costs of CNG and diesel are very difficult. 

When one takes the equivalent energy between a liter of diesel fuel and the same equivalent for natural gas, we find that one-liter of diesel equals 1.044 M3 of natural gas.  In a cost comparison, a liter of diesel fuel may average nearly 40 cents per liter, while a cubic meter of natural gas may average just over thirteen cents. 

On the surface this seems to indicate a net savings of about 70% in favor of natural gas.  However this comparison is in itself misleading. Although natural gas is cleaner burning, it burns at a much higher temperature than diesel fuel.

Specifically modified diesel engines are used to handle CNG fuel. Engine compression ratios are less, and since natural gas burns at a higher temperature, more of the energy is given off as heat, resulting in less motive power being produced. This means that there is a reduction of power in an equivalent engine package.  Therefore the savings in actual fuel mileage are actually less than 70%.                 

There are many more variables when making such comparisons.  Diesel fuel and natural gas prices can vary widely from one region to another and from one point in time to another.  Since this article was written fuel prices have since escalated significantly, thus changing the comparables.

In addition, one must consider the quality of the natural gas.  It may contain some impurities, or have some water content, and in some areas may have some ethane content, which has a lower heat value.
When one calculates the savings in fuel costs of a CNG powered vehicle over its service life, as substantial as they are, most agree that these savings do not completely offset the initial high cost of the vehicle.

Engine life should be longer, as the fuel is cleaner burning and it has a lower compression ratio.  Due to many varying factors there is insufficient field data to yet unequivocally prove this argument.  Some operators believe that longer engine life coupled with fuel savings will offset the initial cost of the vehicle, while others say it will not quite offset this cost. 

Also a CNG engine tends to run quieter, providing perhaps a smoother ride for passengers.  Maintenance costs are considered to be a little higher than for a diesel-powered unit. It is possible that a portion of this cost may be attributed to the fact that since this is rather new technology, operators are monitoring systems more closely and perhaps incorporate more intensive inspections within their procedures.
Another cost factor concerns weight. An equivalent CNG powered 35-40' transit bus is about 2500-2800 pounds heavier than a diesel due to the larger tank systems that are required. 

This additional weight results in increased brake and suspension wear.  As the same basic bus models are used, A CNG powered model differs very little from conventional diesel unit.  A CNG system comprises fuel storage tanks, a fast fuel fill system, engine compartment components, fuel lines, and a fuel unloading system. 

Without going into a great amount of detail, there are also many safety-related devices and fail-safe mechanisms built into such a system.  Most CNG units have rooftop mounted tanks, anchored within a storage platform and consisting usually of 6 to 8 tanks, which are fabricated using steel or aluminum reinforced with fiberglass.  With a full fuel complement, a CNG powered bus has a range of approximately 400 miles.

The decision that a municipality usually takes in committing to CNG power tends not to be economically motivated; rather it is more often politically motivated.  Politicians, always subject to the will of the people, must be seen these days to be doing something positive for the environment.  The decision to go CNG tends to be highly visible, and is usually greeted with a great deal of approval by the public.  In addition its use directly reduces reliance upon imported oil, thus serving to reduce the US trade deficit.      

Just prior to the beginning of 1997, some 87 municipalities in the U.S. already had CNG facilities, and many more either are in the process of or have plans to develop such facilities. As of January 1st, 2000, the American Public Transportation Association (APTA) reported a survey of 64 transit systems that currently operate 2,966 CNG fueled buses in service.  As of this date, 16 transit properties have ordered 1,207 new transit buses, that were not yet delivered and some 29 transit properties have approved the future purchase of a total 3,497 new CNG fueled buses. 

CNG use is currently limited to generally urban transit use.  Intercity motor coaches cannot yet take advantage of this source of alternative power, due to limited fueling facilities; nor can most school bus and shuttle bus operators due to the high initial cost of a CNG facility. 

Having said that, however, there may be increasing opportunity!  If a transit operation makes the decision to set up a CNG facility, it might then become a more viable alternative for local school bus and shuttle operators, and perhaps some trucking or delivery operators.  As such, the transit operator may have the opportunity to be able to amortize the initial cost of the facility over a greater volume of CNG sales and perhaps even get involved in the servicing of these additional vehicles.

In summary, natural gas reserves in the US alone are estimated to be sufficient to meet projected demands for the next 200 years.