Readers interested in this topic would recall the previous article “To convert or not to convert” which discussed the pros & cons of whether to convert a petrol vehicle to diesel or gas.
This is the second article in that series which will answer the queries of how to choose between CNG & LPG.
What is CNG
Compressed Natural Gas or CNG is a mixture of hydrocarbons, mainly methane. This is the same gas which is delivered to our homes through pipelines and is usually referred to as Sui Gas. When this gas is compressed to increase its storage capacity, it is called compressed natural gas.
It is found in gas wells or produced in conjunction with crude oil, natural gas is a clean-burning, domestically produced fuel that generates significantly fewer emissions than fossil fuels when used to power vehicles.
What Types of Vehicles Run on Natural Gas?
Nearly all types of gasoline powered vehicles can be converted to run on CNG. Dedicated CNG vehicles are designed to run only on CNG, while bi-fuel CNG vehicles have two separate fueling systems that enable the vehicle to use either CNG or gasoline.
In year 2002, auto manufacturers produced 19 models of sedans, service vans, passenger vans, and light-duty pickup trucks capable of running on CNG.
How Do Natural Gas Vehicles compare to Gasoline Vehicles?
In CNG vehicles, the fuel is stored at pressures of 2,400-3,600 pounds per square inch (psi) in one or more cylinders located under the body or in the trunk of the vehicle. The filling valve is placed near the cylinder or in the engine compartment accessible by opening the bonnet.
When the CNG leaves the cylinder, it travels through high-pressure lines into the pressure regulator, where it is reduced to low atmospheric pressure. Unlike gasoline, which must be vaporized before ignition, CNG is already gaseous when it enters the combustion chamber.
Advantages
The following are key benefits of CNG as compared to petrol:
• 50% more economical to use
• High octane rating of 120 or more (commercially available petrol is 90) which means no “knocking” of engine
• Immense environmental benefits by reduction in following exhaust emissions thus improving the air quality:
1. Carbon monoxide emissions by 70%
2. Carbon dioxide emissions by 25%
3. Nitrogen oxide emissions of 60%
4. Non-methane hydrocarbon emissions by 75%
5. No particulate matter
Disadvantages
10-15% lower power output than petrol engines
Increased engine valves wear due to the drying effect of the gaseous fuel
High cost of conversion
The additional weight of CNG cylinders
1/3rd driving range for an equivalent gasoline vehicle
LPG
Liquefied petroleum gas (LPG) consists mainly of propane & butane in various mixtures. It is a by-product of natural gas processing and petroleum refining.
What Types of Vehicles Run on LPG?
LPG has been used in vehicles since the 1920s. Nearly all types of gasoline powered vehicles can be converted to run on LPG. Vehicles that run on LPG have engines and fuel systems that are optimized for gaseous fuel use.
Dedicated LPG vehicles are designed to run only on LPG, while bi-fuel LPG vehicles have two separate fueling systems that enable the vehicle to use either LPG or gasoline.
What are the Benefits of Driving a LPG Powered Vehicle?
LPG vehicles produce fewer exhaust emissions than cars powered by gasoline or diesel fuel but slightly higher than CNG .
Tests conducted by the U.S. Environmental Protection Agency show that LPG vehicles can produce 30% to 90% less carbon monoxide than gasoline engines and about 50% fewer toxins and other smog-producing emissions.
The cost of a gasoline gallon equivalent (gge) of LPG is generally less than that of gasoline, so driving a LPG vehicle saves money.
How Do LPG Powered Vehicles compare to Gasoline Vehicles?
In LPG vehicles, the fuel is stored as a liquid, usually in one tank, at low to moderate pressure that depends on ambient air temperatures. For example, at 70 degrees Fahrenheit, the pressure inside a LPG storage vessel is 127 psi; at 100 degrees Fahrenheit, the pressure is 196 psi. The LPG travels from the tank to a vaporizer/ pressure regulator, also called a converter, where it is transformed into vapor. The vapor travels to an air/fuel mixing device that feeds the engine.
Which CNG/LPG brand name to choose
There are numerous well known brands which deal in CNG & LPG kits. Most of them are Italian based.
The heart of any such system is the regulator OR converter. All regulators do one work i.e. reduce the pressure of the gas/liquid coming from the cylinder before it is introduced to the engine.
While choosing a particular brand ensure that the brand is a well established one with more than 10 years of experience. Avoid state-of-the-art brand new technology as you will be the test mule in this case.
Go for a brand and design that has been in the market for more than three years. Don’t follow the sheep herd mentality, all Italian kits are equally good so don’t spend more for a particular brand.
A list of govt. approved brand names is available at:
http://www.cbr.gov.pk/budg2003/sros/cu/2002sro373cu.pdf
Care in installation
Installation of kit and the final tune up are critical to performance. Most kit installers are aware of the basic installation principles but it is your safety and investment, so ensure that the following points are followed:
- The regulator/converter is installed absolutely vertical and parallel to the sides of the car i.e. perpendicular to the front of the car.
- The piping from cylinder to the engine compartment is run OUTSIDE the passenger compartment.
- The ignition timing of the car should be advanced by 12 degrees to ensure good performance on CNG.
- Don’t fiddle with the original gap setting of the spark plugs.
- The idle exhaust CO (carbon monoxide) should be adjusted to less than 1.5%. (Enercon tune-up centers and all good workshops have CO analysers, get your car checked there for this parameter).
- All the pipes in engine compartment should be secured with clamps. Engine fires result if this is not adhered to.
- The original packing of the kit contains clamps, screws and mounting brackets. They are not useless things and should almost be consumed when the kit installation is done.
- Once the kit is installed and you have taken your car home, then perform the overnight leak test by doing the following steps:
a. Note the pressure reading on the gauge installed on the regulator in the engine compartment.
b. Close the valve of gas on the cylinder.
c. Next morning i.e. after 8-10 hours, before opening the gas valve on the cylinder, check the pressure gauge again. The reading should be the same as last night. If it has dropped, it indicates a leakage in the system and have it addressed as soon as possible.
CNG and LPG Cylinder Safety Codes
Note: LPG cylinders in our country are for domestic use and not certified for transport use, hence posing a serious safety hazard.
Both CNG and LPG cylinders are manufactured according to rigorous safety standards which comply to either ANSI or DOT or ASME standards.
CNG cylinders are tested according to ANSI (American National Standards Institute) codes. They may include repeatedly over-pressurizing and depressurizing the tank thousands of times; placing the tank in a fire to be sure the pressure relief device works and the tank does not burst; and dropping the tank from a height of 6 feet. CNG cylinders must exceed a safety factor of at least 2.25-3.5 times the pressure of the tank.
CNG cylinders manufactured to DOT (Department of Transportation, USA) standards may need to be re-certified periodically: every 5 years for steel cylinders and every 3 years for steel composite and aluminum composite cylinders. CNG conversions may also use cylinders manufactured according to NGV2 standards; these have a 15-year useful life.
Because LPG is not stored at such high pressures, the testing for LPG cylinders is not as rigorous as that for CNG cylinders. Every LPG cylinder must be exposed to twice its service pressure, and one out of every 500 is exposed to four times its pressure, or about 960 psi.
Two types of containers are authorized for LPG: DOT cylinders and ASME (American Society of Mechanical Engineers) tanks.
COMPARISON OF CNG & LPG
Following is a comparative table about the various important parameters and how the two gases perform with respect to one another in a converted vehicle:
CNG (Methane) LPG (Propane)
Power loss 10-15 % 7%
Driving range 2-3 times the range of CNG
Safety Better than LPG because:
1. Lighter than air
2. In Pakistan, LPG cylinders are not certified for transport use
Conversion cost 2-3 times more expensive than LPG
Exhaust Emissions Slightly better than LPG
Maintenance cost Same as LPG
Availability Conveniently available Not easily available
Ease of Refueling As easy as gasoline Difficult
Leakage consequences Disperses in air quickly Settles down in low areas
Odour issues Almost odourless Bad odour which helps detect leakage quickly
Cost, Rs/kg (Dec 2002) 16.45 Not available
Fuel economy, (1000 cc car) km/kg 11 Not available
Calorific value, MJ/kg
(higher is better)* 48 46
Vehicles on road 5 times more LPG vehicles in the world vs CNG
Use in freezing temperatures Yes Difficult
Auto-ignition Temperature, C (higher is safer) 600 450
Research Octane number (RON)
(higher is better) 120 112
Flame speed, m/s 0.45 0.45
Flame temp, C
(higher is better) 1950 1925
Flammable range, volume %
(smaller is safer) 5-15 2-9
Ignition energy, mJ
(higher is safer) 0.30 0.26
* Dependent on the gas composition including Methane, Ethane, Propane & Butane.
Final question - Which one to Choose
LPG has a lot going in its favour as compared to CNG which can be summarized as below:
Lesser power loss
More driving range
Lower conversion cost
The only major problem is that its refueling infrastructure is not available. This is the biggest shortfall. So until some LPG refueling stations come up, CNG is the logical choice.
You could pose the question that if LPG is so good, how come every other car is converting to CNG. The answer is four main reasons:
- Easy refueling
- Better safety characteristics
- Environmentally better
- More abundant availability
Food for Thought
The refueling station for LPG is 10 times less expensive than that of a CNG, so it is a good business opportunity for anyone interested ! Government policies that favour both CNG and LPG are required to provide an equal opportunity to LPG also.
For the interest of readers, following are the typical heat content (Net calorific values) for commercial fuels in MJ/Kg (mega joules/kilogram) are, (higher number is better):-
Gross Nett
Hydrogen 120.0
Natural Gas 48.0
Liquefied petroleum gas 46.1
Aviation gasoline 44.0
Automotive gasoline 43.8
Kerosene 43.3
Diesel 42.5
Ethanol 26.68
Methanol 19.95