The origin of natural gas derives from the decomposition of plants and animals over a long period of time and under tremendous heat and pressure. When natural gas comes out of the ground, it commonly mixed with water, liquefied hydrocarbons, hydrogen sulfide and other solid matter. The gas is cleaned of the water and solid matter, then the other gases and liquids are stripped away. Because water content in natural gas would cause formation of ice or hydrates in the pipeline. Likewise, the amount of available hydrocarbons heavier than ethane should be taken into account to reduce the risk of blocking the pipeline due to accumulation of condensable liquids
Natural gas is a naturally occurring mixture of combustible hydrocarbon gasses found in porous formations beneath the earth's surface, usually in association with crude petroleum. When it is found, it can be
The main component of natural gas is methane (CH4) with minor amounts of heavier hydrocarbons and some nonhydrocarbons, Table1
| Component |
Volume (percent) |
Mass (percent) |
| Methane |
|
|
| Ethane |
|
|
| Propane |
|
|
| Butanes |
|
|
| Pentanes |
|
|
| Hexanes |
|
|
| CO2 |
|
|
| Nitrogen |
|
|
| Water |
|
|
| Total |
|
|
Source: [16]
There is growing interest in using natural gas in vehicles in the U.S and worldwide, Table 1.
The main factors which stimulates interest in NGVs in U.S.
The use of natural gas as a transportation fuel reduces operating cost and emissions. It is very accessible in countries where there are existing distribution infrastructures.
Table 2. Countries with the Greatest NGV Populations (1992)
| Country |
Number |
| Former Soviet Union |
300,000 |
| Italy |
235,000 |
| Argentina |
100,000 |
| New Zealand |
60,000 |
| U.S |
30,000 |
| Canada |
26,100 |
| Brazil |
700 |
| Australia |
626 |
| Netherlands |
303 |
Source: [16]
The U.S has a huge natural gas resource base and nationwide underground storage as well as 1.2 million mile pipeline/delivery systems already in place. Natural gas is also in abundant supply around the world.
A survey in U.S.A showed that the composition of natural gas varies significantly. According to this survey average natural gas contains 92.3 % methane in volume.
CNG is a mixture of several gases. Methane is the dominant component , but ethane and " heavier hydrocarbons" such as propane butanes, etc. are in natural gas up to a maximum of their equilibrium vapor pressure. Only saturated hydrocarbons ( alkane, in other word paraffins) are found in gas. The general formula for this series is CnH2n+2.
| |
Formula |
Molecular Weight |
Density kg/m3) |
Specific Gravity |
Boiling Point(C) |
Autoign. Temp.(C) |
Flamm. Limits |
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
Source: [16]
Paraffins are unable to combine with additional hydrogen atoms. They are also called "saturated". If the paraffin has more than three carbons, there is possibility of having more than one arrangement of the carbon and hydrogen atoms in molecule. Molecules which contain the same number and some kind of atoms, but differ in structure are called isomers. For example, three structural isomers of pentane (C5H12) are possible, Table 4.
|
|
n-pentane |
isopentane |
neopentane |
| Boiling Point |
36.1 |
27.9 |
9.5 |
| Melting Point |
-129.7 |
-159.9 |
-16.5 |
| Density (20 C) |
0.6262 |
0.6201 |
0.6135 |
The composition of natural gas has impact on emission, performance and safety. In terms of this issue, the effect of gas composition can be outlined as follows:
Water, sulfurcompounds, carbon dioxide, oxygen and other impurities in natural gas cause storage tank and fuel system corrosion and corrosion-fatigue cracking of the materials. The limitations of this impurities are important in terms of minimizing of corrosion and acceptable service life of the storage tank. Ice occurance in regulators and lines due to condensation of water content in natural gas should be considered. Besides this, clogging in fuel injectors and fuel filters cause rough engine operation.
The dew point indicates the point at which gas to become saturated with water vapor. At dew point, the mixture contains maximum amount of water. Relative humidity is 100 % at this temperature. When the natural gas is compressed from pipeline pressure of approximately 50 psi to tank pressure of 3600 psi, this increase causes higher dew point which is the function of pressure. Liquid water would precipitate out of the gas with the consequence of this increase. Dryers or desiccants are used to remove the water. Another method is the methanol injection in order to decrease the freezing point of the gas. Pipeline quality natural gas has a dew point of -28 F at 50 psi which corresponds 52 F at 3600 psi. In order to prevent occurrence of corrosion problems in vehicles, natural gas must be dried to a dew point under the minimum ambient temperature in which the vehicle run. Regarding this matter the dew point of the gas at the maximum operating pressure to be at least 10 F below the winter design dry bulb temperature will be required by the proposed regulations.[4]
Fuel stratification in the cylinder should be considered when the natural gas contains heavy hydrocarbons. At low ambient temperatures, as fuel draws from the storage tank, the pressure decreases. This decrease changes the composition of liquid and vapor states. For instance, propane/air peak shaving gas is not suitable for natural gas vehicles due to the variation of propane concentration more than 10 percent during the operation. Obviously, the variation of the fuel composition has a primary impact on engine performance, knock tendency, emissions and fuel economy. In order to handle the variation of fuel composition,
can be taken into consideration for natural gas vehicles
The high octane number of natural gas allows high compression ratio in optimized natural gas engine which means an increase in power and efficiency. However, an antiknock rating higher than that required for knock-free operation does not improve performance.
Cetane number is of primary importance in overall efficiency of diesel engine. For a given diesel engine, a fuel with higher cetane number provides shorter ignition delay period and smaller amount of fuel is collected in the combustion chamber when ignition occurs. As a result , high cetane number fuels generally provide lower rates of pressure rise and lower peak pressure which mean less combustion noise and more control in combustion. In other word, this characteristic defines engine efficiency and power output. In addition to these, easier starting especially in cold weather and faster warm-up are realized with high cetane number fuel.
CNG has a very poor cetane number . Because of this, some modifications are necessary in diesel engine for CNG applications.
The heating value in another word, (heat of combustion, calorific value,) of a fuel is the amount of heat produced when the fuel is burned completely. There are two values for the heating value. They are higher heating value and lower heating value. The difference between them is that higher heating value exceeds the lower heating value by the energy supplied by the water vapor in condensing.
A fuel with low heating value provides less heat on combustion which means less power than the same amount of fuel with high heating value. In order to maintain power output with low-heating value fuel , more consumption of it would be necessary. If natural gas is used in optimized engine, better fuel consumption will be obtained
Energy content based on volume, determines the vehicle range. To increase the range, a high density fuel is preferred because heating value per unit volume of fuel is greater. This explains why the liquid state is of primary interest for storage problem. According to this data vehicle running on CNG can travel only 26 % as far as it could on gasoline.
Vapor Pressure of a fuel is a prime importance of drivebility of vehicles under all conditions. One of the most common method to measure fuel volatility is Reid Method. In this method, the vapor pressure of liquid fuel is measured at 100 F in a chamber having 4:1 air to liquid ratio. There is no need to vaporize CNG, contrarily liquid fuels have to be vaporized before they are introduced into the engine. This characteristics of CNG makes the cold start problems and low temperature emissions due to the cold enrichment, minimum.
It defines the fuel energy delivered to the combustion chamber per unit mass of air inducted. In other words the lower A/F ratio means that chemical energy released per kg of stoichometric mixture burnt during combustion is greater. It is calculated by dividing the lower heating value of the fuel by the air/fuel ratio. This explains why gasoline has a greater heat release although it has a lower heat of combustion.
The heat of vaporization of a fuel effects the volumetric efficiency positively by decreasing the temperature of the fuel-air mixture which means makes the mixture dense. Although natural gas has a higher heat of vaporization, it is already in gaseous state when it is inducted into engine and it does not provide this cooling effect. On the other hand cold starting problems would occur with higher heat of vaporization in IC engines.
Thermal efficiency increases with lower flame temperature due to the reduced heat losses from an engine. The lower flame visibilty (luminosity) also decreases heat loss by radiation. Flame temperature is also a parameter in NOx . Lower flame temperature reduces NOx emission.
Flame speed defines the relative motion of the flame front towards the unburned mixture. Stoichometric A/F ratio strongly determines the flame speed. The fast combustion rates provides more efficient torque development. On the other hand , the increasing thermal and mechanical burdens along with increased combustion temperatures cause higher thermal losses, combustion noise and NOx emissions.
Wide flammability limit allow intense lean operation with extremely reduced NOx emissions. Also high diffusion coefficient affect lean operation positively by providing homogeneous charges. Wide flammability limits also may be useful because rich air / fuel ratios can be used when needed to maximize power by delivering more fuel per charge. However operating at higher levels in this way reduces efficiency. Gaseous fueled engines which operate in lean side suffer from low fuel conversion rates. Low quenching distance offers some improvement in this manner . It characterizes the penetration of a flame into smallest crevices. Flame of a fuel with lower quenching distance reaches to difficult chamber zones like quenching areas with high surface to volume ratios.
The temperature at which the precipitation occurs is called cloud point. At this point wax settles out and blocks the fuel system lines. The cloud point of a fuel indicates the temperature at which it may clog filter systems and restrict flow. More paraffinic fuel means the higher precipitation temperature and the less suitable the fuel for low temperature operation. Cloud point is especially important for fuels used in high speed diesel engines due to the trend towards finer filters. The finer filters more easily become clogged due to the small quantities of precipitated wax.
Flash point defines the temperature to which the fuel must be heated to produce an ignitable vapor-air mixture above the liquid fuel when exposed to an open flame. The flash point of a fuel does not affect the performance in an engine. Auto-ignition temperature, fuel injection and combustion performance are not influenced by the flash point. Flash point especially has an importance in point of safety. A low-flash point may be the reason of fire hazard subject to flashing , and possible continued ignition and explosion.
Wobbe number is especially an important parameter in open loop control system. Wobbe number defines the chemical energy that will flow through an orifice with a given pressure drop. It is calculated by dividing the heating value of the fuel by the square root of specific gravity.
To describe natural gas characteristics on the basis of its energy content, Wobbe number is used. Wobbe number is computed by dividing square root of the ratio between CNG and air specific gravity ; it represents the energy flow rate resulting from a certain pressure drop. Wobbe number increases when the content of the non-methane hydrocarbon increases. This increase results from the higher densities of the non-methane hydrocarbons. The Wobbe number decreases when the inert gases concentration rises.
In an open loop engine, different Wobbe number causes change in stoichometric A/F ratio which effects equivalence ratio. Equivalence ratio is of primary importance in engine operation.
In closed loop control engine, the affection of variations in Wobbe number is negligible. Because, engine control module responds the signal from the exhaust oxygen sensor and corrects for equivalence ratio change.
There are basically two types of conversion systems available, mechanical (carburetted) and electronic (fuel injected) types. Mechanical systems work on the same principles as gasoline carburetor do. The fuel is mixed with the intake air in a fuel/air mixture (carburetor). In electronic systems, injectors or flow control valves are used to meter the fuel into the intake air.
Source:[4]
A) Mechanical Conversion System :
Gaseous fuel is introduced into intake manifold. The mixer is placed upstream of the throttle valve in order to respond to manifold pressure to provide the adequate amount of fuel introduced into the manifold for engine load. Some mixers have air valve to control the air flowrate. Needle valve adjustments provide appropriate fuel / air mixture.
Air flow capacity requirement for the range of the necessary engine operation conditions is the selection criteria of mixers. The power obtained from engine is dependent of the size of mixer. The selection of small size of mixer causes the restriction of air entering to intake manifold after a certain point. Consequently engine is not be able to achive the desired operation with respect to increased throttle opening. Engine starting and low speed running will be problem with the oversize mixer due to inadequate and poorly controlled manifold vacuum. The engines which can not run with wide open throttle will probably operate with smaller size mixer. The advantage of slightly oversize mixer appears on over-the road applications. Because it achieves optimum performance at higher engine speeds. The formula which is below can be used in order to estimate required air flowrate.
In order to provide necessary control in CNG conversions, pressure regulations are achived in two stages. A single regulator with two stages or two separate regulators can be used for this purpose.
In first stage, the pressure is reduced from tank pressure to approximately 100 psi. It is necessary to reduce the pressure to a few inches of water to meter the fuel through the mixer and into intake manifold. There are two kind of two stage regulators available.
The piston regulators compared to diaphram type provides better control in terms of outlet pressure with varying tank pressure. In mechanical conversion, this pressure variation is compansated by the precise control of second stage regulator with respect to varying inlet pressure. However, for an electronic conversion without second stage regulator this pressure variation is of importance in the amount of fuel that meters through the injector.
It stops the flow of the natural gas when the engine does not run. The valve provides fuel flow from the tank to regulator when the engine runs. A filter can be available with this equipment to filter the fuel before entering the regulator.
It is a solenoid type valve which is controlled from the fuel selector switch in the vehicle. This valve shuts off the gasoline supply when the alternative fuel is selected. It is also required to cut the power to gasoline injectors when the engine runs on alternative fuel.
It responds to outputs from the conversion system computer in order to provide a richer or leaner fuel / air fuel mixture. In some applications control valve operates with the second stage regulator make the fuel / air ratio leaner during deceleration. The control valve is driven by the conversion system computer in order to determine fuel / air ratio by receiving outputs from oxygen sensor which is placed in engine exhaust.
Source:[19]
Oxygen sensor is installed into exhaust manifold. ZrO2 is coated with a platinum films on both sides. ZrO2 acts as a solid electrolyte, its output is strogly dependent of oxygen content of the exhaust gas. Small voltage is generated on the platinum surfaces due to difference between the oxygen content in atmosphere and exhaust. Measured value of the oxygen content in exhaust provides conversion system computer keep the A/F ratio within 0.05 tolerance.
The refueling receptacle should provide safe and easy using with minimum trapped gas or vapor release when disconnected. It must be a quick disconnect type. They are constructed of non-sparking material, usually brass with corrosion resistant iternal parts. NGV-1 (ANSI Standard) is available for receptacles. According to this :
Check valve leakege appear the common reason which is relaed to receptacle problems. Particulate contamination and ice or hydrate build up usually cause the problems with receptacles.
Electronic control units with mechanical conversion systems vary from the simple type device which mainly eliminate false Electronic Control Module (ECM) malfunction codes to smart equipments with adaptive memory which dynamically control the A/F ratio and ignition.
Basic type device provides prevent false EGR, knock and oxygen sensor malfunction codes by using open loop control strategy together with the Original Equipmenr Manufacturer (OEM).
Advanced type devices evaluate data from all the engine sensors to adjust A/F ratio, spark timing and other controllable parameters by using feedback control (closed loop control) strategy.This type devices also have a feature in order to achieve open loop control method for some conditons like cold starting and wide open throttle (WOT)
B) Electronic Conversion System:
The main difference in the application of electronic conversion method with respect to mechanical conversion is the use of solenoid driven injectors or proportional metering valves to supply the fuel into intake manifold instead of fuel air mixture. Fuel injection provides significant advantages over the use of mixer. These are:
When solenoid injectors used, the amount of fuel introduced into engine can be adjusted by varying the pulse width (the period which is valve open.). In case of proportional flow control valves are used instead of solenoid injectors, the fuel flowrate is metered by using a mass flow sensor which supplies data to to the alternative fuel microprocessor which achieves the control of the valve. The Alternative Fuel Microprocessor evaluates Original Equipment Manufacturer (OEM) speed density in order to drive the alternative fuel injectors. Another approach is to modify the injector actuation signal delivered by OEM Electronic Control Module. Ignition timing is also one of the parameters controlled by alternative fuel microprocessor.
The use of injectors or flow control valve provides very accurate and precise control of mixture ratio which is resulted in optimum operation conditions at all loads. These operation conditions can not be achieved by mechanical conversion systems. Electronic systems have first stage regulator to decrease 3600 psi of tank pressure to 50-100 psi of injector pressure. However, some electronic systems equipped with flow control valve require second stage regulator.
There are two strategies available in emission control of natural gas engines.
a) A Stoichometric Engine with three way catalyst (TWC)
Stoichometric approach is primarily used in light-duty vehicle engines.Complete combustion is achived by introducing the chemically correct mixture of fuel and air. A closed loop fuel system operates on equivalance ratio which is very close to 1. This approach cause high NOX , CO and HC emissions before the treatment of catalytic convertor. Due to this, Stoichometric approach is strongly dependent on three way catalytic convertor for the desired emission. Natural gas has a narrower equivalance ratio window for peak operating efficiency than that of gasoline. It is set at a slightly richer point compared to gasoline. It is necessary to keep the equivalance ratio within this window, otherwise catalyst efficiency and emission result dramatically worsen
TWC achieves the reduction in three products simulteneously. Both NO reduction and CO and HC oxidization can be achieved in one single catalyst bed. Enough oxygen for oxidizing the CO and HC and reducing gases for decreasing NO are provided. This type of catalyst is called three-way catalyst since it removes three pollutants at the same time. In order to realize high conversion efficiency for all components, it is necessary to monitor equivalance ratio over a very narrow range.
It is required moderate modifications on the ignition system. It provides very low emissions along with high Brake Mean Effective Pressure (BMEP) even in naturally aspirated engines.
Engine operation is stable with stoichometric approach. But, the emissions are strongly dependent of proper working of the oxygen sensor, catalyst and control system. Besides this, thermal loadings are high with respect to diesel and lean-burn applications. This drawback causes some restrictions for turbocharging. But excess power and knocking do not effect stoichometric engine, contarily to lean-burn engine. Slight efficiency penalty can appear with stoichometric operation comparedto lean-burn operation.
|
FUEL TYPE |
COMBUSTION STRATEGY |
EQUIV. RATIO |
ENGINE TYPE |
CATALYST TYPE |
APPLICATION RESULTS |
| Methane/Propane |
Stoichometric |
1 |
Naturally Aspirated |
Three-way catalyst |
Very Low Emissions |
|
|
|
|
|
|
Reasonable power output |
|
Methane/Propane |
Stoichometric |
1 |
Turbocharged |
Three-way catalyst |
Very Low Emissions |
|
|
|
|
|
|
Complicated engine design |
|
|
|
|
|
|
Knock problems with propane |
| Methane/Propane |
Lean-burn |
<1 |
Naturally Aspirated |
Oxidizing catalyst |
Moderate emissions levels |
|
|
|
|
|
|
Simple engine design |
|
|
|
|
|
|
Low power output |
|
|
|
|
|
|
Methane better than propane in this application |
|
Methane.Propane |
Lean-burn |
<1 |
Turbocharged |
Oxidizing catalyst |
Reasonable emissions level |
|
|
|
|
|
|
High power output |
|
|
|
|
|
|
Relatively simple engine design |
b) Lean-burn application
Lean-burn application is primarily used in heavy-duty vehicles. Thermal loadings are moderate with lean-burn application There is also a large power drop, roughly equivalent to the change in equivalence ratio, this makes turbocharging necessary. Lean-burn natural gas engines can often be made to run sufficiently lean (equivalance ratio of .55 to .75) in order to keep NOX emission at low level. Excess of air provides temperature reduction which resulted in low NOX rate along with low tendency to knocking.
However, when the engine operates close to lean flammability limit in other word misfire limit, variations in equivalance ratio can result in unstable combustion along with an increase in HC emission. Oxidizing catalyst is necessary in order to control the HC emission. In case of running on the rich side of flammability limit high NOX can be obtained due to the change in equivalance ratio.
Precise fuel metering has a great importance on lean-burn engine operation. Current lean-burn engines are turbocharged to provide high power and thermal efficiency, in order to approach diesel performance. Under these conditions the sensitivity of lean-burn engine to variation of fuel mixture and spark timing. Intake boost pressure exceeds safe levels in case of an increase in the equivalance ratio.This increase can lead extra mass air flow along with more fuel to the engine which causes engine parts failure due to the detonation or preignition. Knock tendency is also problem with rich fuel metering.
Continue to CNG Review - Part 2