CNG Engine System | Carsuffer

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The huge import bill declining oil resources and worsening Ecosystem have necessitated us to pursue for a cheap indigenous fuel.

Which is technically comparable to or better then conventional fuels in Terms of engine performance and emission.

Among the option that are currently available as alternate to petrol and has received a great deal of attention and has already been applied successfully to over a million vehicles in various parts of the world.

Such as Italy, Russia, Argentina and is gaining increasing acceptance particularly for city transport vehicles.

What is CNG(Compressed Natural Gas)?

What is cng

Compressed Natural Gas(CNG) is a mixture of gases such as Methane, Ethane, propane, butane, co2 etc.

The major constituent being Methane it is found dissolve in petroleum or in huge amount under Earth surfaces in oil and gas bearing areas.

In India its resources exist in Bombay-Offshore, Upper-Assam, Krishna and Godavari basins and Gujarat.

Refineries are a secondary source of natural has where the dissolved gases in crude oil are released during the refining process but in laser volumes.

Composition of CNG (%volume)

Carbon dioxide0.27
Moisture content2.0ppm

Increased power & efficiency in CNG Engine

Increased power & efficiency in CNG Engine

CNG is a safe fuel being lighter than air is diffuses easily into the atmosphere and does not form a sufficiently rich mixture from combustion to take place.

In this respect CNG is superior to LPG or propane or even petrol storage of propane on vehicle is not as difficult as CNG but the cost of propane is lighter than that of CNG.

The excellent knock resistance property of CNG allows use of a higher compression ratio for increased power output and fuel economy compared to petrol.

The energy content per kg of CNG is very similar to that the petroleum-based fuel but it has a lower energy content per unit of volume.

Due to its anti-knock property CNG can be safely used in engine with compression ratio as high as 12:1 compared to normal petrol (range of 7.5:12 to 10:1).

At this high compression ratio natural gas fitted engines have higher thermal efficiency than those fueled by gasoline.

CNG has higher Octane number than petrol and it is therefore possible for CNG engines to operate at a higher compression ratio then petrol engines without knocking.

The fuel efficiency of CNG engine is there for better than that of petrol engine.

However compare two diesel engines the compression ratio is lower for CNG engines and consequently the fuel efficiency of CNG engines is about 10 to 20% lower than that of diesel engine.

CNG present a more cost-effective emission reduction measures than quite a few another option that have been the subject of serious debate in recent month.

One of these option is the catalytic converter which cannot reduce the portion of the particular Ltd referred to as suit for diesel engines particularly.

Further and the reliability of diesel catalytic converter has yet to be proven because of the higher sulphur content in the Indian diesel fuel.

Particulated trap is a high cost device that is now under development of diesel engine and the process of generation of the trip to clean the disposal it carbon particle call of energy consumption and possibility of pollution during regeneration.

Thus particulate trap are costly to install and costly to operate and maintain CNG engines over a better solution to eliminate particulate.

The importance of cost effective efficient easy to maintain and user friendly solution to every come environmental problem cannot be over emphasised, and CNG meets this requirements.

The Urban Indian fuel

With its lower and mission level and other advantages over existing fuels.

CNG is ideally suited to become the fuel of urban India it is specially ideal for Indian cities because:

The pipeline system required for distribution is easier to provide in cities than in rural areas.

The major portion of Indian vehicle are in the cities.

Capital for providing CNG engine is also more easily available in urban setting.

There is adequate mechanical expertise available in the cities.

Introduction to exhaust emission combustion

Complete combustion of CNG result only in the harmless substance carbon dioxide (CO2) and water vapour (H2O).

Engine exhaust has also contains product of incomplete combustion such as carbon monoxide (CO) and partially combusted aur non combusted hydrocarbon (HC).

In addition two nitrogen in the induction combustion air the exhaust has also contains the oxidation product of Nitrogen (NOX).

The emission of solids particulate emission is very low on Spark ignition engine by comparison with diesel engine and is negligible.

Owing to the minimum sulphur content of the CNG only a very slight amount of harmful sulphur dioxide is contained in the exhaust gas of the spark ignition engine.

Characteristics of CNG Engine

  1. Higher Octane rating (120-130) excellent knock resistance.
  2. lower rate of combustion pressure rise and low peak cylinder pressure.
  3. Energy content per kg of CNG is similar to petroleum base fuels.
  4. Self ignition temperature is 730°C about 250°C higher than Petroleum.
  5. CNG has very narrow flammability limits.
  6. Lighter than air diffuses upwards.
  7. Clean burning fuel exhibits no measurable black smoke very low particulate and about 40% reduction in mass emissions.

Changes made to adopt CNG Engine

Module 1 (Engine)

Piston Combustion bowl volume increased to reduce the compression ratio to 11.5:1.
Cylinder headSpark plug introduced in place of injectors with proper threading arrangement.
Distributor driveDistributor introduced in place of fuel injection pump with adoption of drive arrangement.

Module 2 (Engine)

Gas feeding systemGas inducted into the engine bike carburetor after reducing the gas pressure the stoichiometric air/fuel ratio maintained by a closed loop system
Ignition system To provide electrical Spark initiate ignition or air fuel mixture
End speed Governing system Governs the maximum speed off the engine by controlling gas the quantity inducted

Gas storage & filling

Gas cylinder with individual manual shut off valve each cylinder has individual manual shut off valve (Dr fig.)


12*50l For DTC

8*50l, 4*95l For BEST

Working pressure 200kg/cm²

Test pressure 330kg/cm²

Filler valve

It is basically to feel the gas from filling station with a built-in non return valve.

Gas filling arrangement

For faster feeling of cylinder 3. Feeling arrangement adopted.

Pressure gauge

It is the analogue pressure gauge installed in the gas line graduate both in PSI and bar (0-3500 psi/ 0-250 bar).

Pressure sensor

It senses the gas cylinder pressure and communicate to an indicator on the drivers dashboard pressure indicators of dashboards consists of four green bulb and one red bulb the illumination of this bulb indicates the pressure level as given below:

Cng fuel indicators led

4Green full – 170-200bar

3Green 3/4 – 150-165bar

2Green 1/2 – 110-145bar

1Green 1/4 – 35-70bar

Red Nil – <30 bar

Caution:- If red bulb glows it is suggested that vehicle needs to be taken to nearest filling station for recharging cylinder with has to 200-bar pressure

Gas service components

Gas service components

First stage regulator

The high pressure regulator reduces the pressure of the gas from 200 bars to 12 to 12 to 30 bars.

Normally the high pressure regulator is set at the manufacturing and and should not be adjusted in the field.

improper installation may damage property/life.

Low pressure solenoid switch (24V)

Located between the regulator operated by ignition switch when the valve is open gas passes from 1st stage to 2nd Stage regulator.

Caution : No repair/dismantling is recommended replace the unit is necessary.

Second Stage regulator

It has three compartment:

(1) diaform chamber

(2) secondary value chamber

(3) primary wallpaper chamber


(A) Reduces pressure from 13 bar to 1.04 bar absolute (which is slightly above atmospheric pressure)

(B) Used for and speed governing of 2900/3000 RPM. by receiving signals from 3 way solenoid valve which is actuated by ESG unit.


All components of carburettor system except carburator and regulator second stage regulator requires no overhauling only placement.

Gas carburettor because of its simplicity is unlikely to give any problems when properly installed with an adequate supply of gas.

Oxygen sensor

Oxygen sensor is located between engine exhaust manifold and catalytic converter rich air fuel ratio supply fuel to mix with all the oxygen entering the engine.

Rich air fuel ratio create low oxygen levels in the exhaust because access fuel mixes with all the oxygen entering the engine.

when lean mixture are used oxygen level in exhaust also remains high.

The presence of unburnt Hydrocarbons in exhaust with oxygen would harm the life of the catalytic converter as well as result in emission.

The heated oxygen sensor sensor the oxygen available in the exhaust.

Closed loop control

The voltage from the oxygen sensor is fid to the engine management electronic control unit (ECU).

When the ECU is reading a high voltage indicating a rich mixture is respond by reducing the gas supply duration slightly to weaken the mixture.

As the mixture weakens the oxygen sensor voltage goes low and so the ECU enrich the mixture.

By constantly increasing and then reducing the mixture strength the air/ fuel ratio is kept at 17:1.

To prevent the mixture swinging between rich and lean the ECU contains an integrator circuit which has the effect of averaging out the oxygen sensor readings.


The commander basically a micro processor which gets two input signals (1) the RPM signal from the ignition coil negative and (2) another signal from the oxygen sensor in proportional to the oxygen availability in the exhaust pipe.

Depending upon the signals the fuel delivery into the engine will be verify by regulating the vacuum stage regulator by FCv-1.


Restrictor is incorporated in between 2nd Stage regulator and 3 way valve after the T piece.

As the name implies it restricted the flow of vacuum signal from carborator to 2nd Stage regulator for faster response.

This is necessary for reaching and effective signal from FCV 1 to 2nd Stage regulator which in turn respond in regulating gas supply to the carborator.

Vacuum control switch

The vacuum control which which census the air intake manifold vacuum and the vacuum levels reach below 3 inches of Mercury the Valve will become operational.

As the valve open the commander circuit will be bypass and 2nd fuel control valve will be actuated.

Ignition system

Ignition coil

Ignition coil consists of metal housing which accommodates primary and secondary coil on and off the primary current will induce a secondary voltage.


This is the contactless distributor which provides the RPM signal to the trigger box and also distributes High tension voltage to the individual spark plug.

The HT connection of the spark plug will be as per firing order.

Trigger box

That trigger box provides the making and breaking of the primary current of the ignition coil.

The sensing of RPM of the engine is from the signal from the ignition distributor.

This does the function of the contact breaker of the conventional distributor.

Spark plug

This is the ignition source off the air fuel mixture.

The secondary voltage developed by the ignition coil discharged in between the central and ground electrode of the spark plug as a spark.

The initiation of combustion cricket only be a spark plug.

Warning: in any cng engine fitted miss firing or miss firing expert in the cylinder will head to catalytic converter damage.

End speed governor-24V

Auto relay/ESG

The ESG unit senses the engine RPM from the ignition system signal and activates the Three Way valve for controlling the engine RPM on failures of ESG unit the engine cannot be started on Rises beyond buy on specified RPM.

Three way solenoid

The 3 way valve is part of an end speed governing circuit this valve is actuated by a solenoid which obtain signal from ESG unit depending upon the engine RPM.

Of the three ways, one is outlet the remaining two inlets are selectively connected to the outlet depending upon the ECG signal received.

Lubrication system

Description of the system : pressurized lubrication

Lubrication reservoir : Beneath crankcase

Feed system : by pump

Oil cooler : plate type

Electrical power circuit components

(A) Battery : The battery in the electrical system performs the function of a chemical storage device for electrical energy generated by the alternator the lead sulphuric acid battery is used for this purpose.

(2) Isolation switch : By switching off the switch both the battery terminals circuit in enterally cut off from the vehicle electrical circuit.

This situation is essentially required whenever a repair work is being carried out in the electrical system this is situated near the driver seat who can easily operate this manually.

In case a vehicle is not operating for more than a day then by switching off the regulations which battery discharge can be avoided.

(3) Ignition switch : Ignition switch when is on position supplies current for Spark generation.

(4) Starting and charging circuit : A starter switch relay a Starter Motor and alternator with charging circuit incorporated in CNG engine is similar to the other vehicle

(5) 24v/12V converter : This necessity of 12V supply for ignition and closed loop system components need the battery voltage of 24V converted to 12V the converter supply power to the following circuits.

(¡) Ignition system

(¡¡) closed loop system

(¡¡¡) gas level indicator

Do’s and Don’t for CNG Engine


(●) allow misfiring and misfiring to be analysed immediately and rectify.

(●) use open fire for detecting gas leakage.

(●) are the wires to check the electrical continuity/voltage.

(●) temper the fuel setting to carborator and 2nd Stage regulator.

(●) try to reassemble and reuse gas component except 2nd Stage regulator and carborator.

(●) set the idling RPM in cold condition.

(●) try to service filter valves.

(●) temper any electrical circuit particularly engine wiring harness.

(●) make any cut and join in the wires.

(●) Hammer on component for fitment particularly HT cable distributor and drive for distributor components.

(●) rise engine speed beyond 2000 RPM in vehicle.

(●) remove spark plug in hot/warm engine condition.

(●) apply grease/oil while assembling O2 sensor.

(●) open the gas line under pressure.

(●) adjust ignition timing without stroboscope.


(●) cng engine carry out all the maintenance as per the maintenance schedule.

(●) use correct spanners and tools to avoid damage to the engine related system.

(●) check all gas leak joint only with soap solution.

(●) check all electrical components and voltage only with multimeter.

(●) replace washers/ferrules rules of gas circuit on removal.

(●) maintain the ignition timing as per specification.

(●) avoid spillage/leakage of coolant lubricating oil.

(●) maintain correct coolant lubricating oil level and connection particularly horse clips intact.

(●) proper routing of wires (during reassembling of components) if possible in the original route.

(●) in use of cng engine correct procedure for dismantling and assembling of component unnecessary hitting or force fitting will damage the component.

(●) spark plug to be removed and fitted only with spark plug sleeve clamp in position.

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