In response to high fuel prices and environmental concerns, compressed natural gas is starting to be used in light-duty passenger vehicles and pickup trucks, medium-duty delivery trucks, and in transit and school buses.

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Technology

A CNG powered high-floor Neoplan AN440A, operated by ABQ RIDE in Albuquerque, New Mexico.

CNG can be used in Otto-cycle (gasoline) and modified Diesel cycle engines. Lean-burn Otto-cycle engines can achieve higher thermal efficiencies when compared with stoichiometric Otto-cycle engines at the expense of higher NOx and hydrocarbon emissions. Electronically-controlled stoichiometric engines offer the lowest emissions across the board and the highest possible power output, especially when combined with EGR, turbocharging and intercooling, and three way catalytic converters, but suffer in terms of heat rejection and fuel consumption. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol.

CNG may be refueled from low-pressure (“slow-fill”) or high-pressure (“fast-fill”) systems. The difference lies in the cost of the station vs. the refueling time. There are also some implementations to refuel out of a residential gas line during the night, but this is forbidden in some countries.

CNG cylinders can be made of steel, aluminum, or plastic. Lightweight composite (fibre-wrapped plastic) cylinders are especially beneficial for vehicular use because they offer significant weight reductions when compared with earlier generation steel and aluminum cylinders, which leads to lower fuel consumption. The CNG cylinders generally follow the ISO 11439 standard. ^[1]

The equipment required for CNG to be delivered to an Otto-cycle engine includes a pressure regulator (a device that converts the natural gas from storage pressure to metering pressure) and a gas mixer or gas injectors (fuel metering devices). Earlier-generation CNG conversion kits featured venturi-type gas mixers that metered fuel using the Venturi effect. Often assisting the gas mixer was a metering valve actuated by a stepper motor relying on feedback from an exhaust gas oxygen sensor. Newer CNG conversion kits feature electronic multi-point gas injection, similar to petrol injection systems found in most of today’s cars.

Drawbacks

Compressed natural gas vehicles require a greater amount of space for fuel storage than conventional gasoline power vehicles. Since it is a compressed gas, rather than a liquid like gasoline, CNG takes up more space for each GGE (Gallon of Gas Equivalent). Therefore, the tanks used to store the CNG usually take up additional space in the trunk of a car or bed of a pickup truck which runs on CNG.

[edit] CNG compared to LNG

CNG is often confused with liquefied natural gas (LNG). While both are stored forms of natural gas, the key difference is that CNG is in compressed form, while LNG is in liquefied form. CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks. CNG requires a much larger volume to store the same mass of gasoline or petrol and the use of very high pressures (3000 to 4000 lbf/in², or 205 to 275 bar).

Worldwide

Canada

Canada is a large producer of natural gas, so it follows that CNG is used in Canada as an economical motor fuel. Canadian industry has developed CNG-fueled truck and bus engines, CNG-fueled transit buses, and light trucks and taxis. Both CNG and propane refueling stations are not difficult to find in major centres.

Europe

In Germany, CNG-generated vehicles are expected to increase to two million units of motor-transport by the year 2020. The cost for CNG fuel is between 1/3 and 1/2 compared to other fossil fuels in Europe.^{[citation needed]}

South America

Argentina and Brazil are the two countries with the largest fleets of CNG vehicles. Conversion has been facilitated by a substantial price differential with liquid fuels, locally-produced conversion equipment and a growing CNG-delivery infrastructure. A ‘Blue-network’ of CNG stations is being developed on the major highways of the Southern Cone (including Chile and Bolivia) to allow for long-haul transportation fuelled by CNG.

Asia

CNG Radio Taxi in New Delhi, India

One of the many CNG propelled autorickshaws on the streets of New Delhi, Delhi. A fleet of twelve also operates in Brighton, England.

A CNG powered Volvo B10BLE bus, operated by SBS Transit in Singapore.

In Asian Economies such as India, CNG costs are at Rupees 18.90(USD $0.46) per kg compared with Rs.50.00 (US$ 1.25) per liter of petrol. The cost saving is immense along with reduced emissions and environmentally friendlier cars.

CNG has been made mandatory for all public transport in the Indian capital city of New Delhi.

CNG has grown into one of the major fuel sources used in car engines in Iran, Pakistan, Bangladesh and India. The use of CNG is mandated for the public transport system of India’s capital New Delhi as well as for the city of Ahmedabad in the state of Gujarat. The Delhi Transport Corporation operates the world’s largest fleet of CNG buses. The government of Punjab, Pakistan, the most populous province of that country, has mandated that all public-transport vehicles will use CNG by 2007. Today many rickshaws as well as personal vehicles in India and Bangladesh are being converted to CNG powered technology, the cost of which is in the range of $800-$1000. In the Bangladesh capital of Dhaka not a single auto rickshaw without CNG has been permitted since 2003. As of July 2007 Pakistan is the largest user of CNG in Asia, and second largest user in the world.^[2]

According to the International Association for Natural Gas Vehicles, Pakistan has the second-largest number of natural gas vehicles.^[2] In the Middle East and Africa, Egypt is a top ten country in the world with more than 63000 CNG vehicles and 95 fueling stations nationwide. Egypt was also the first nation in Africa and the Middle East to open a public CNG fueling station in January 1996.^[3]

In Singapore CNG is increasingly being used by public transport vehicles like buses and taxis, as well as goods vehicles. However, according to Channel NewsAsia on April 18, 2008, more owners of private cars in this country are converting their petrol-driven vehicles to also run on CNG – motivated no doubt by fiercely-escalating petrol prices these days.

The initial cost of converting a regular car to bi-fuel at the German conversion workshop of C. Melchers-Galileo, for example, is around S$4,000 (US$2,300); with the promise of real cost-savings bi-fuel cars bring in the long term.

Singapore currently has three operating filling stations for natural gas. SembCorp Gas Pte Ltd runs the station on Jurong Island, and jointly with Singapore Petroleum Company, the filling station at Jalan Buroh. Both these stations are in the western part of the country. Another station on the mainland is in Mandai Link to the north and is operated by SMART Energy. SMART also plans a second station on Serangoon North Ave 5 which will be set up the 2nd half of 2008; so will two more – at Jalan Bukit Merah and Bedok in the central and eastern parts of the country.

As a key incentive for using this eco-friendly fuel Singapore has a Green Vehicle Rebate (GVR) for users of CNG technology. First introduced in January 2001, the GVR grants a 40% discount on the Open Market Value (OMV) cost of newly-registered green passenger vehicles.

Oceania

During the 1970s and 1980s, CNG was commonly used in New Zealand in the wake of the oil crises, but fell into decline after petrol prices receded.

Brisbane Transport and Transperth in Australia have both adopted a policy of only purchasing CNG buses in future. Transperth is purchasing 451 Mercedes-Benz OC500LE buses, including 58 articulated buses, while Brisbane Transport has purchased 216 Scania L94UB and 180 MAN 18.310 models as well as ordering up to 30 articulated CNG buses on MAN chassis’.

In the 1990s Benders Buslines of Geelong, Victoria trialled CNG buses for the Energy Research and Development Corporation.^[4] Recently Landi Renzo of Italy has set up a production subsidiary in Karachi to cater the growing demand of CNG Kits in Pakistan.OEM’s like Toyota Pakistan and Suzuki Pakistan is producing company fitted CNG cars.

Sorce

Stoichiometric

Stoichiometric combustion is when the chemically exact amount of fuel is added to the air so that when the combustion is completed the chemical formula for the fuel is completed:

CH4 + 2O2 = CO2 + 2 H20 i.e. methane + 2 oxygen = carbon dioxide + 2 water

This offers exceptionally clean combustion and exhaust gases. The downside is that the power output of the engine may be lower and its fuel consumption slightly higher when compared with a diesel engine. Due to the reduced efficiency of low sulfur diesels, this is becoming less of an issue.

Lean Burn

The lean burn system employs an air/gas mixture that has more air than the stoichiometric ratio in the combustion cylinder. Looked at the other way, it requires less fuel in the cylinder. This can result in lower fuel consumption compared to stoichiometric combustion with the power output usually maintained by turbocharging.

Carburettor

The carburettor is generally used in stoichiometric engines as it can deliver the right balance of fuel for the air entering the engine. Provided the carburettor is located in reasonably close proximity to the intake of the engine and there are not highly variable load demands, the system works very well.

Single Point Injection

Single point injection is, in essence, an electronically controlled carburettor. The advantage is that the gas is delivered more accurately in accordance with engine demand. Again the injector is still some distance from the inlet (like the carburettor) and so its response to quickly changing conditions is not ideal.

Multi Point Injection

Multi point injection has an injector for each cylinder, so the injectors can be placed in close proximity to the cylinder’s intake port. It also enables fuel to be delivered precisely as required to each individual cylinder (called sequential) and enables more sophisticated technologies such as skip-firing to be used. Skip-firing is when only some of the cylinders are operating (the other cylinders are being skipped). This enables even more efficient use of the fuel at low loads, further lowering fuel consumption and unburnt hydrocarbon emissions.

Carburettors v Injectors

As a point of comparison we can look at the fuel delivery systems used by cars to get an idea of the relative sophistication of the various delivery systems. It should be noted that no system is inappropriate, they each have the benefits and costs.

· Carburettors are no longer used on new cars today
· Single point injection is currently only used by low cost cars.
· Multi point injection is the system used by most cars today and is the most sophisticated system generally available.

Resource

Benefits of natural gas vehicles include:

• Reduced particulate and NOx (Nitrogen Oxides) emissions
• Reduced greenhouse gas emissions
• Widespread availability of natural gas
• Lower cost
• Can be derived from renewable sources (biogas)
• Technically proven
• Available now
• Can be used in all vehicle classes
• Minimal processing or refining requirements
• Safer than most liquid fuels
• Can be refueled at home or workplace
• Noise reductions of as much as 50%
• Reduced engine wear
• and more…

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