What are the Available Alternative Fuel to Petrol and Diesel now?
PETROL (GASOLINE) AND DIESEL VEHICULAR FUEL
In the early days of the automotive industry, choice of vehicular fuel was determined by its storage capacity against the mileage that a single refill can achieve. Petrol (Gasoline) and diesel fuel provides the highest storage capacity in a denser liquid state.
LIQUIFIED PETROLEUM GAS
In the late 1970’s and early 1980’s, the oil embargo and the resultant energy crisis in the World, spurred new interest in alternative fuels that could reduce dependence on fossil fuel. The immediate alternative fuel is Liquid Petroleum Gas (LPG). Currently, it is the third most common vehicular fuel after petrol (gasoline) and diesel. A large number of the world's vehicle fleets are using LPG as a vehicular fuel, which includes light- and heavy-duty trucks, buses, taxicabs, police cars, rental and delivery vehicles.
LPG is a petroleum derived, colorless gas consisting mainly of propane, propylene, butane, and butylene in various mixtures. Propane is safe. It is nontoxic and nonpoisonous and has a very small flammability range. It is produced as a by-product from two sources: natural gas processing and crude oil refining. LPG is a mixture of various hydrocarbons that are normally in a gaseous state at atmospheric pressure, but liquefy at higher pressures, approximately 200 psi or less. Typically, LPG has an octane rating of 105-110 as compared to petrol (gasoline) at 85-97.
BI-FUEL PETROL (GASOLINE) AND LPG VEHICLE PROFILE
Many of the fleets using LPG have reported 2-3 years longer service life and extended intervals between required maintenance. Spark plugs from propane vehicles can last for 80,000-100,000 miles and propane engines can last 2-3 times longer than gasoline or diesel engines. (Source: US National Propane Gas Association).
Propane is generally considered to reduce engine maintenance and wear in spark-ignition engines. The most commonly cited benefits are extended oil change intervals, increased spark plug life and extended engine life. Gasoline fueled engines particularly carbureted engines require very rich fuel mixtures during cold starting and warm up. Some of the excess fuel collects on the cylinder walls, effectively washing lubricating oil off the cylinder wall and contributing to accelerated wear during engine warm up. Gaseous fuels do not affect cylinder lubrication.
LPG FUEL TANK DESIGN & SPECIFICATION
LPG tanks are constructed of heavy gauge steel, in compliance with the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME), to withstand a pressure of 1000 psi. Normal working pressures within the tank vary depending upon ambient temperatures and the quantity of fuel in the tank. Common operating pressures are in the range of 130-170 psi. Propane tanks limit the liquid level to 80% of the total tank volume by using an auto-stop fill valve.
Tanks are equipped with a pressure relief valve that can release propane vapours to the atmosphere to prevent tank rupture under abnormally high-pressure conditions. Under normal operating conditions, the LPG system is essentially a closed fuel system without the typical vapour emissions associated with petrol (gasoline). Each tank also includes a manual shut-off valve. The propane fuel tank is installed along with a fueling port, fuel lines, and pressure safety valves. A filter removes particles and contaminants that may be present in the propane.
LPG FUEL DELIVERY AND VACUUM OPERATED VAPORIZER
The LPG system draws fuel from the bottom of the tank and sends liquid propane to the vaporizer. The vaporizer converts the liquid to a gas. The primary heat source for this vaporization is engine coolant flowing through specially designed water jackets cast into the vaporizer body. Many vaporizers include an internal pressure regulator to control the pressure of the fuel sent to the engine. Some early propane systems with mechanical mixers use a separate pressure regulator.
LPG FUEL METERING SYSTEMS
Early propane systems use a mixer, which operates as a conventional venturi device in a manner quite similar to a gasoline carburetor. Vaporized propane is drawn through a fixed orifice in response to engine air flow. As intake air enters the engine, a venturi effect is created through the mixer. This slight pressure drop acts on a spring-loaded diaphragm in proportion to the air flow. The result is a simple yet fairly accurate flow meter, which controls the volume of fuel to the engine as a function of air flow.
Like gasoline carburetors, the mixer is limited in accuracy. Changes in altitude, ambient weather conditions, and even temperature will cause significant variations in the fuel mixture that cannot be compensated for by using a mixer. Electronically controlled, closed-loop fuel injection provides a much more precise method of metering fuel. Based on sensor inputs, the electronic control module (ECM) determines the engine operating conditions, and then modifies the injector pulse width to maintain a stochiometric mixture.
NATURAL GAS (METHANE) - COMPRESSED FORM
Natural Gas has long been in use as a motor fuel, dating back as far as the earliest (spark ignition) Otto-cycle engines. Early use of natural gas as a vehicular fuel was curtailed by the inability to provide adequate on-board storage in sufficient quantity to meet the needed range of drivers. As a result, the now common fuels, gasoline and diesel, achieved widespread use due to their energy content being in a denser liquid state. Natural gas continued to be used as a fuel source on stationary equipment, where a continuous supply could be provided through underground pipelines.
In the 1970-80s oil crisis, new methods were developed, which would allow natural gas to be compressed and stored in containers fitted to an automobile. Compressor stations were built, which delivered Compressed Natural Gas (CNG) at pressures up to 3600 psi. Engine upfit systems were designed and installed, and the modern Natural Gas Vehicle (NGV) was born. There are now over 3 millions CNG Vehicle (commonly known as Natural Gas Vehicle or NGV), both dedicated and bi-fuel versions, with more on the way. Most recently, the lower emissions of natural gas has been a strong influence in its use. With reductions in hydrocarbons (HC), oxides of nitrogen (NOx), and lower amounts of carbon dioxide (CO2) natural gas is leading the way as an alternative fuel since year 2000 onwards.
Natural gas is primarily methane (CH4). This makes natural gas a very friendly fuel for the environment. Methane as a hydrocarbon is considered non-reactive. That means hydrocarbon emissions of natural gas do not react with sunlight to create smog. CNG is non-toxic, non-carcinogenic, and non-corrosive. Natural gas is also lighter in weight than air. This provides an increased safety factor as any leakage will quickly be dissipated into the atmosphere to reduce the risk of a potential explosion as compared to liquid fuels, which pool on the ground or pollute our ground waters. When used as an engine fuel, the Society of Automotive Engineers (SAE) has established a standard minimum methane content of 95% (SAE J1616).
To provide natural gas at pressures up to 3600 psi special compressor stations are necessary. The CNG station compresses, filters, stores and delivers fuel to a natural gas vehicle (NGV) in either of two ways, a fast fill or a time fill. Fast filling provides for quick and convenient refueling with a slight reduction in volume due to the heat rise of the gas during compression. Time filling allows for complete filling of the storage tank, but over a longer time up to eight hours. Small time-fill dispensers are available, which, when attached to a consumer’s household gas line can refill a vehicle overnight. Both types of fuel delivery systems are regulated by standards established by the ISO Organisation.
CHARACTERISTICS OF A NATURAL GAS VEHICLE (NGV)
As most of us are more accustomed to driving petrol (gasoline) or diesel powered vehicles, it may be helpful to point out some of the minor differences between liquid fueled engines and those that operate on a gaseous fuel.
First, most liquid fueled engines require some form of a fuel delivery pump to transport the fuel from the tank to the engine. When operating on CNG, a fuel pump is not used. Instead, system pressure provides for delivery of the fuel. CNG vehicles have at least one pressure regulator, which maintains a steady supply of fuel regardless of fluctuations in system pressure. Second, the ignition system must deliver a slightly higher voltage to ignite the gaseous mixture verses the liquid droplets that make up the normal charge of gasoline. Finally, CNG is high in octane. With octane ratings up to 130, ignition timing may be increased without resulting in spark knock.
Modern NGV’s use closed-loop computer controlled technology to provide lower emissions and excellent driveability. Some equipment manufacturers offer fuel injection versions to provide even more precise mixture control. With the latest certification processes implemented by the Road Transport regulatory agencies, systems are required to be calibrated for specific engine family applications. When a properly calibrated CNG system is correctly installed, many drivers will notice a difference in the vehicle’s driving characteristics.
VOLUME AND RANGE
Natural gas quantity is normally stated or measured in Standard Cubic Feet (SCF) while liquid fuels such as gasoline or diesel are sold in liquid liter. To provide a simple way to compare CNG fuel mileage with gasoline fuel mileage the CNG-NGV industry adopted a standard measurement called the Gasoline Liter Equivalent (GLE) The standard states that 124 scf of natural gas is equal to 1 gallon of liquid gasoline (135 scf for diesel). CNG storage tank manufacturers specify the GLE or Water Liter rating for each size of storage tank they produce.
OTHER FORMS OF NATURAL GAS
Natural Gas can be stored in a compressed gaseous state or in a liquid form. The liquefying of Natural Gas is called Liquefied Natural Gas (LNG).
NATURAL GAS (METHANE) - LIQUEFIED FORM
Liquefied natural gas, or LNG, is natural gas in its liquid form. When natural gas is cooled to minus 259° Fahrenheit (-161° Celsius), it becomes a clear, colorless, odorless liquid. LNG is neither corrosive nor toxic. Natural gas is primarily methane, with low concentrations of other hydrocarbons, water, carbon dioxide, nitrogen, oxygen and some sulfur compounds. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is primarily methane with only small amounts of other hydrocarbons. Liquefying natural gas results in the purest form of methane when heated back to a gas.
For heavy-duty applications requiring long-range capability and large volumes of on-board fuel storage, LNG provides all the benefits of a clean burning natural gas in liquid form. LNG vehicles are essentially natural gas vehicles that store methane as a liquid. LNG vehicles include a heat exchanger installed between the fuel tank and the engine to warm the liquid and convert the fuel back to a gaseous state.
LNG is commonly used as the best form in transporting natural gas worldwide. Special LNG tanker ships are built to transport LNG in crygenic storage tanks. It is still not widely used as a direct alternative fuel to Petrol (Gasoline) and Diesel in many countries.
Compiled and presented by Dr. Xander Thong and Suhimi Hussain
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