Automobile - CNG in Engines & Vehicles

ME 467 : Automobile Engineering Dr. Md. Ehsan © 2015 ME, BUET
CNG in Engines and Vehicles
Dedicated Engines
Automotive Engine
Less than 1% of the vehicle
population runs on
Natural gas
Power Generation
Industrial Gas Engine
(1-5 MW)

Changes expected in a Dedicated NGV
Increase in engine compression ratio, to use higher SIT of NG.
Stronger engine construction to withstand higher
compression ratio.
NG based ECU, programmed to be compatible with parameters like –
NG spark advance, Intake manifold vacuum, Throttle position
requirement.
Mixing chamber (for Carburetion) design would be in accordance to
gaseous fuels, relaxing some requirements.
Cooling capacity corrected for higher exhaust temperature.
Exhaust valve and other component life designed for the elevated
temperature.
For NGVs chassis design may be modified to accommodate safe and
space efficient placement of CNG cylinders.
AFratio biasing made compatible with the catalytic converter used.
Fuel system component life designed for dry gaseous fuel.

Retrofitted Engines
Bi-Fuel Engines : Only one fuel runs the engine at a time
Example – Petrol/CNG, Petrol/Alcohol, Petrol/LPG,
Diesel/Bio diesel
Dual Fuel Engines : Both fuels run the engine at a time
Example – Diesel+CNG, Diesel+LPG, Diesel+Biogas
Retrofitting SI Engines
Gas Fumigation System
Mixes gas with the intake manifold air at sub-atmospheric
pressure (using intake suction)
Gas Injection System
Mixes gas with the intake manifold air at above-atmospheric
pressure (using pressurized gas injection). This could be of
SPFI or MPFI type
Both the systems may be Open-loop or Closed-loop in
operation
Retrofitting CI Engines : Turning it in to SI engine
• Changing the Compression ratio
• Adding CNG/NG Fuel system
• Removing Diesel Fuel System
• Adding Spark Ignition System

Automotive CNG kit for Carburetor System
Automotive CNG kit for EFI System

Diesel Engine converted to run with CNG only.

Dual Fuel Engines : Some diesel used to initiate ignition
Major part of diesel replaced by another fuel
640 kVA MAN Diesel-NG Dual Fuel Generator
< Single Cylinder Diesel-NG Dual
Fuel Engine (16 hp, 2200 rpm)

CNG Engine Technology
CNG Conversion Components
A typical standard CNG conversion (retrofit) system essentially consists of the following components, the
locations are indicated in figure-1.
1. CNG Storage Cylinder 2. Fuel Selector Switch 3. High Pressure Piping
4. Refueling Valve 5. Pressure Regulator 6. Gas-Air mixing Chamber
7. Petrol Solenoid Valve 8. Gas Vapor Enclosure
Automotive CNG kit for Carburetor System
Figure-1 : CNG conversion
components of a vehicle

Pressure
Gauge
Solenoid
Valve
Pressure
Regulator
Filling
Receptacle
NZ type Spark Timing
Advance
LP
Tubing
Gas-Air
Mixer
Indicator
Resistance
Radiator
Vacuum
Diaphragm
for AC
Air
Cleaner
Flow
Adjustment
Valve
Main Components of a CNG Conversion System
HP
Tubing

A typical automotive CNG conversion (retrofit) system consists of the following
components:
1. The Cylinder is used to store CNG at a working pressure of 200 bar. It is
fitted with a shut-off valve and a safety burst disc. Cylinders may be of
seamless steel structure or reinforced carbon fiber composite. 50,60,90
water liter size cylinders are commonly used. At a pressure of 200 bar and
ambient temperature CNG is compressed to about 3.7 times of equivalent
petrol volume. Due to the cylindrical shape and wall thickness the packing
efficiency is poor and the real car space blocked by the cylinders is much
more. The shut-off valve with the cylinder may have overflow limiter
facilities built-in.
2. The Selector Switch is fitted at the dash board, enabling the driver to
choose either the CNG mode or the petrol mode of operation. The
electronics built in this unit also ensures safety by switching off the gas
solenoid whenever the engine is switched off. It also indicates the quantity
of CNG available in the cylinder.
3. The High Pressure Pipe connects the refueling valve to the CNG Cylinder
and Pressure Regulator.
4. The Refueling Valve is used to refuel the CNG cylinder.
5. The Pressure Regulator has a solenoid valve to shut-off gas supply to the
engine. The CNG stored at a high pressure in the cylinder, is reduced to
just below atmospheric pressure by this unit. This negative pressure is also
a safety feature that will not allow gas to pass through when the engine is
not running. Generally this is provided with a gas filter element.
6. The Gas- Air Mixer is a unique component, specially designed to suit
each engine model. It precisely meters gas fed into the engine.
7. Petrol Solenoid valve is used to cut off petrol supply to the engine when it
is run on CNG.
8. Fitted near the cylinder, the Vapour bag/enclosure is used to enclose the
cylinder valve and the pipes connecting it and is vented out of the car.
In addition to these basic components the conversion system may have few more
components, generally if the engine is equipped electronic fuel injection system
and electronic control unit (ECU). These include the following:

Selector Switch
Different Types of Gas-Air Mixers
Spark Timing Advance Processor (STAP/TAP) : The slower flame
propagation speed in methane-air mixture make it necessary to increase the
spark advance for CNG operation of a petrol engine. This unit allows
optional spark advances (eg. additional 6,9,12,15° BTDC), generally during
accelerating conditions. The processor modifies the spark advance signal to
the electronic ignition system accordingly. Use of this improves the peak
power and acceleration performance of the vehicle running on CNG, while
retaining the performance with petrol.
Emulator/Simulator : The EFI injectors are generally kept non-functional at
CNG operation. On the other hand, in some engines feedback signals from
the fuel injectors operation is fed back to the electronic control units(ECU).
Under CNG operation the emulator simulates dummy feedback signals to the
ECU, maintaining its normal operation as with petrol. This component may
not be necessary for all models of EFI engines.

Filling Charge Valve -Type_NZ TRD with Burst Disk
CNG Safety Devices
Burst Disc : Prevents excessive pressure to enter the cylinder during refilling.
PRD : Pressure Release Device, Relives excessive pressure in the cylinder
TRD : Thermally Activated Release Device, Relives excessive pressure in cylinder
Excess Flow Valve : Prevents excessive flow of gas from cylinder

Conventional Pressure Regulator
Regulator type: 3 stages with electronic starting
device and idling at negative prs.
Heating: Engine cooling circuit liquid
Test pressure: 300 bar
Inlet pressure: 220 bar
First stage adjustment pressure: 4 bar
Second stage adjustment pressure: 1.5 bar
Power supply: 12 V DC
High-pressure solenoid valve coil power capacity:
20W
Automotive device which reduces and stabilizes the natural gas pressure, in order to properly feed the
engine. The three reduction stages provide stability at both high and low pressures. The pressure
regulator is equipped with a pressure relief valve and a low pressure solenoid valve fitted upstream of
the third stage. The necessary operating heat is provided by a connection with the engine coolant
system. The engine idling gas flow is activated through the gas pipe main flow as a result of the
negative pressure generated by the engine. These may be equipped with an electronic starting device
with a built-in safety system which closes the gas solenoid valve when the engine stops.
Three Stage Pressure Reduction

Types of CNG Cylinders
• There are 4 basic types of tank designs.
• Which design to use depends on need to reduce weight and cost.
• All designs have equivalent safety, as all meet requirements of same
standards.
• Design type can also determine how a tank may be handled, and how it
may be filled.
Type 1 - All metal (aluminum or steel (Cr-Mb))
Cheap but heavy
Since the 1940’s seamless steel cylinders have been by far the most widely used for
storage of CNG on vehicles. The introduction of Italian high-strength lightweight
cylinders in the late 1970’s was a very significant step forward in the development of
the industry and since then this type of product has been adopted all over the world.
Some standards also allow welded steel cylinders, but require a higher safety factor.
Aluminium cylinders have also been used for on-board storage of CNG and can
provide a lighter weight alternative.
Suppliers of Steel CNG containers include - White Martins (Cilbras), Inflex and Faber.
Luxfer offers a small range of 7000 series aluminium CNG cylinders.

Type 2 - Metal liner reinforced by composite wrap (glass or carbon
fiber) around middle (“hoop wrapped”)
Liner takes 50% and composite takes 50% of the stress
caused by internal pressurization
Less heavy, but more cost
Type 2 fuel containers have a metal liner and a composite reinforcement on the
straight side only; a hoop overwrap. Products on the market steel or aluminium liners
and a glass, aramid, or carbon fibre reinforcement. Type 2 cylinders are designed to
have a liner with sufficient thickness and strength to contain the service pressure,
even without the composite overwrap. They provide a compromise between the low
cost of Type 1 cylinders and the lightest weight Type 3 and 4 cylinders.
Suppliers of Type 2 containers include - Faber, Mannesmann, and Luxfer.
Type 3 - Metal liner reinforced by composite wrap around entire tank
(“full wrapped”)
Liner takes small amount of the stress
Light-weight, but expensive

Type 3 CNG cylinders have a seamless metal liner over wound on all surfaces by a
composite reinforcement that provides between 75 and 90% of the strength to the
vessel. The liner provides the rest of the strength, plus acts as a rigid membrane to
hold the gas and provide extra impact resistance to the product. Type 3 cylinders are
used in a wide range of applications where weight reduction is important, for example
in transit buses and delivery trucks. Type 3 cylinders have also been used on various
OEM vehicles such as the Volvo and Volkswagen CNG cars.
Suppliers of Type 3 containers include - Luxfer, Dynetek, and Structural Composites.

Type 4 - Plastic gas-tight liner reinforced by composite wrap around
entire tank (“full wrapped”)
Entire strength of tank is composite reinforcement
Light-weight, but expensive
Type 4 fuel containers have a plastic liner and a full overwrap of carbon fibre or
mixed fibre construction. The liners of Type 4 tanks provide no structural strength to
the product and act only as a permeation barrier. Although these liners are not gas
tight, the rate of permeation has been found acceptable for use with CNG. As a rule
Type 4 CNG cylinders are fitted with impact protection on the domes, as the plastic
liner does not provide a rigid backing to the composite overwrap. Again, Type 4
cylinders are used where weight is important such as on buses, trucks and OEM
vehicles such as the Honda Civic.
Suppliers of Type 4 containers include - Quantum, Ullit, Lincoln Composites, and
Ragasco.
Comparison of Cylinder Types

Mandatory for CNG Driven Vehicles (Ref : RPGCL web site)
Essential steps for CNG driven vehicles:
1 Vehicles should be converted by RPGCL approved workshops with GOB
approved international standard NGV cylinders
2 No NGV cylinder manufactured in Bangladesh & nobody is given the
permission for manufacturing NGV cylinders in Bangladesh
3 NGV cylinders should be tested after every 5(five) years of use
4 Unauthorized cylinders like Oxygen cylinders, acetylene cylinders, LPG
cylinders or any other cylinders should not be used in CNG driven vehicles
5 Safety instruments like excess flow device, PRD etc & Conversion plate must
be attached to the converted vehicles. If any Conversion Workshop does not
attach conversion plate, then please inform it to RPGCL.
6 Conversion Certificate from the Workshop after Conversion must be issued
7 Do not receive or deliver CNG to the vehicles at or above 200 barg (3000 psig)
pressure
8 Repair works of CNG vehicles should be done by approved CNG workshops
9 No welding work should be done on high pressure pipe line
10 Repair works on CNG driven vehicles should be done after evacuating CNG
from the cylinders or removing CNG cylinders from the vehicles
11 CNG vehicles should be checked by approved CNG workshops after every
1000 km run
12 Maintenance works of CNG driven vehicles should be done by trained
mechanics (trained by RPGCL or BPI)
13 Government has made the use of `Electrically Operated Solenoid Valve with
Cylinder Shut-off Valveâ€™ compulsory for heavy vehicles run on CNG
14 For more information on CNG, please contact to RPGCL. Phone Nos: 8914682,
8917137-221, 01713-018117, 01717-106934

Automobile - CNG in Engines & Vehicles

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