Compressed Natural Gas (CNG) and Liquified Petroleum Gas (LPG) Vehicles (Automobile)


Compressed Natural Gas (CNG) and Liquified Petroleum Gas (LPG) Vehicles

Strict environmental norms and growing demand for fuel have quickened the quest for alternate fuels. Compressed natural gas (CNG) and liquefied petroleum gas (LPG) are receiving a great deal of attention and is being globally applied in vehicles. LPG and CNG have been used as motor fuels since the late forties in various parts of the world such as Italy, Argentina and Russia, particularly for city transport vehicles like taxis, buses, delivery trucks and garbage tippers. In India the usage started during mid 1990s. Over few millions of vehicles around the globe are running with CNG and LPG. In India over 0.3 million vehicles are running only on LPG. Both CNG and LPG systems offer environment friendly benefits over petrol engines. Historically the ratio of CNG to LPG vehicles is about 1:4, except in the case of Argentina where only a CNG conversion is not allowed. This is due to the following factors :
(i) Higher cost of transportation infrastructure is required for CNG filling facilities. CNG is normally piped from the oil field to refineries to the end user, unlike LPG which is easily transported by tankers and filled in small cylinders for end use. CNG is supplied to consumers mainly through the existing CNG stations. The dispensing units at the stations are supplied through storage cascade, called Mother Stations, in truck-mounted-cascades to the retail outlets called the Daughter-Stations. The on-board cylinder in the CNG driven vehicle is refuelled from the dispensing unit under pressure. There is a reduction in storage pressure with each successive filling at a CNG retail outlet, if it is not connected to gas pipeline. Normally one car on-board cylinder is refuelled in 3 minutes when cascade storage pressure is full. (ii) Higher cost of CNG conversion equipment in comparison to that of LPG (at present in
the ratio of 2:1 approximately). (Hi) Increase in the dead weight of the vehicle, thus affecting the mileage. (iv) CNG works on higher compression ratio 14.5:1 than for LPG that requires 10.5:1. Hence, in a duel fuels mode, a large drop in power is experienced.


If vehicles are not required to meet any emission standards, the gas systems consist of what are considered to be the first generation (Gi) installations. The carburation is affected by a pressure reducer vaporizer and a designed mixer.
Second generation (G2) LPG/CNG conversion units have open loop micro-processor, which regulates fuel mixtures by forward measurement of engine operating conditions but no meas­urement of end results and non-readjustment of mixture take place.
Third generation (G3) fuel system controls optimize engine performance by having a close loop electronic micro-processor system, which analyses engine performance results and adjusts the in-take conditions for optimum performance. The cost of G3 equipment is the highest and is generally about double to thrice the cost of Gi equipment. G3 can meet Euro II to Euro IV emission norms.
The compression ratio of LPG is closer to that of petrol than of CNG where ratios may be in the order of 14.5. This enables LPG to be a more powerful under bi-fuel application. Due to its anti-knock property, CNG can be safely used in engines with a compression ratio as high as 12:1 compared to normal gasoline (ranges from 7.5:1 to 10:1). At these high compression ratios, natural gas-fuelled engines have higher thermal efficiencies than those fuelled by gasoline. CNG has a higher octane number than petrol and it is, therefore, possible for CNG engines to operate at higher compression ratios than petrol engines without knocking. Hence the fuel efficiency of CNG engines is better than that of petrol engines. However, compared to diesel engines, the compression ratio is lower for CNG engines and consequently the fuel efficiency of CNG engines is about 10-20 percent lower than that off diesel engines.
LPG kit can be fitted only on petrol engine, but CNG kit can be fitted both in petrol and diesel engines without modification. In diesel engines, without modification LPG kit cannot be fitted. Natural gas is economical compared to petrol and diesel. It is a clean burning fuel, which reduces vehicle maintenance. Many CNG owners report that oil changes are needed only every 16000 to 32000 kiometres running. Standard spark plugs last as long as 1,20,000 kilometres running. CNG is lead free and its use substantially reduces harmful engine emissions. CNG represents a more cost effective emission reduction measure than quite a few options available for diesel engines. In diesel engines, the catalytic converter cannot reduce the portion of the particulate referred to as soot. Particulate trap is a high cost device and has to be regenerated to clean the deposited carbon particles to avoid exhaust choking.

CNG Conversion

Almost any petrol vehicle can be converted to operation on CNG. Vehicles with catalytic converters can also be fitted with a CNG kit without any difficulty as CNG does not contain lead. In petrol engine operated vehicles, bi-fuel operation reduces the engine fuel consumption. This feature combined with the lower price of CNG has made it attractive to convert in-service petrol cars to CNG/petrol operation.
Diesel engines can be converted to use 100 percent CNG fuel or to use part HSD and part CNG. In case of the latter mode, the quantity of HSD injected is reduced and natural gas is carburetted along with the intake air so that the engine power is kept the same. In diesel engines, for duel fuel mode, diesel provides combustion initiation for CNG burning. Kits can be retrofitted to existing vehicles for diesel/CNG operation mode. However under city driving conditions, the substitution of diesel with CNG is quite low and hence the gains in emission reduction are also low.

Components of a CNG Conversion Kit for Mono-fuel Operation.

CNG Cylinder. These are high pressure cylinders designed for storage of CNG at a pressure of 200 bar. A typical tank capacity is 50 litres. The number of cylinders required depends on the vehicle.
Vapour Bag Assembly. This is made of PVC and is designed to cover the cylinder valve. It is tubular in shape and has a threaded flange at one end screwed on to the cylinder neck threads and a screwed cap at the other end to give access to the cylinder valve.
Filling Connection/Valve. This valve is used for filling high pressure gas from the CNG compressor to the CNG tank.
Electronic Selector/Change-over Switch. This activates the electrical circuits in the system to automatically change the mode of operation from diesel or petrol to CNG.
Pressure Regulators. Two pressure regulators are used to reduce the gas pressure from 200 bar to just above atmospheric pressure.
Carburettor. It is a special air valve diaphragm carburettor.
Ignition System. Six-cylinder contact-less distributor ignition system with spark plugs is located in place of injectors.
Venturi. This is a gas and air mixing and metering device. It meters the gas flow proportionate to the engine speed.
End Speed Governor. This is a special electric governor, used to reduce gas flow at second stage regulator as the specified engine rpm is reached.
Components of a CNG Conversion Kit for Dual-fuel Operation in Diesel Engines.

CNG Cylinder. These are high pressure cylinders designed for storage of CNG at a pressure of 200 bar. A typical tank capacity is 50 litres. The number of cylinders required depends on the vehicle.
Filling Connection/Valve. This valve is used for filling high pressure gas from the CNG compressor to the CNG tank.
Pressure Regulator. Multi-stage pressure regulator reduces pressure from 200 bar to less than atmospheric pressure.
Electronic/Pneumatic Speed Control. Pneumatically operated safety valve is used to close gas supply as the engine rpm reaches beyond specified limits.
Linear Load Valve. This is connected to accelerator pedal and controls gas flow as per engine load.
Rack Limiter. This allows full load diesel flow up to certain engine rpm and then reduces to pilot value beyond the specified speed.
Venturi. This is a gas mixing and metering device located down stream of the engine air filter.

LPG Conversion

The LPG fuel/air mixture burns well, so problems with starting of engines are less common than that with liquid fuels, however the electrical system of the vehicle has to be efficient. The conversion kit to convert vehicle petrol to LPG contains the following components.
• Auto LPG tank
• Multifunctional valve
• LPG adapter
• LPG solenoid valve
• Petrol solenoid valve
• Electronic change over switch
• Vaporizer
Installing the regular domestic cylinder on automobiles for fuel has created many problems like improper mixing and less factor of safety. These problems led to the development of a new design, which facilitate proper clamping with high factor of safety. The tanks come in various shapes and sizes, suiting to all kinds of cars. Each tank is tested at a pressure of 3 MPa, whereas the working pressure of LPG is 1.66 MPa at LPG solenoid valve. The LPG solenoid valve is positioned between the tank and the pressure reducer valve (vaporizer) and cuts off the flow of LPG during petrol operation and with the engine switched off. This is also operated by the change over position. This is normally closed type solenoid valve and allows the flow during LPG operation.
The petrol solenoid valve is positioned between the petrol tank and the carburettor, and cuts off the flow of petrol during gas operation. This comes with the manual by pass system so that in case of the solenoid’s failure, the flow of the petrol can be made direct. The main function of the vaporiser is to convert the LPG to vapour.
Electronic change over switch enables the duel fuel operation of the car. The switch for carburettor type has gas/petrol selector with 3 positions and reverse level indicator gauge for the fuel. The switch has got gas starting with timed automatic system to enrich the mixture for starting. The middle position of the switch closes the gas and petrol solenoid valves at the same time. The position is used to drain the petrol from the carburettor while changing the petrol to gas. In case of a permanently fitted cylinder, it is possible to know the level of gas in the tank by means of an IED. The switch has an electronic safety device that cuts off supply to the LPG valves if the engine stalls. Electronic change over switch for multi-point fuel injection system is normally a two-position switch one for gas and the other one for petrol. In this system, the vehicle can normally start on petrol but change over a LPG once the engine attains the preset rpm.

Installing CNG/LPG Kit

Before conversion to CNG/LPG, the vehicle must undergo a pre-conversion check. This is to ensure that the engine is mechanically sound and properly tuned to the manufacturer’s specification. The check should include audit of the electrical system, ignition, valve clearances, cylinder compressions, exhaust gas analysis and the condition of the air cleaner. Also it is advisable to check fuel consumption, power output and vehicle performance on’starting, idling and running. Fuel emission should also be checked both before and after conversion on CNG as well as on petrol separately.
Sometimes there can be additional expenses initially for replacing batteresis, ignition circuits, etc. This is because the ignition temperature for CNG is much higher than for petrol and a much stronger spark is needed to ignite CNG, even though the earlier spark strength may have been acceptable for petrol running.

Cylinder Location.

Cylinders must be located in a protected position to minimize damage in the event of an accident and should never be mounted on the roof. Cylinders are usually mounted in the luggage space of vehicles. In station-wagons and hatchback vehicles, they are mounted behind the rear seat or, in some cases underneath the chassis. Specially designed steel clamps, which completely surround and grip the cylinders, are used to mount these on vehicles.

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