CNG Leak Detection, Diagnosis and Fault Auto Healing on Small Bi-Fuel Vehicle.Chandrakant Parmar, Prakhar Bothra, Siddharth Deshmukh, Augustine S Arun, T. Sethuramalingam

ERC, Tata Motors Limited, Pune

ABSTRACT

An automobile running on CNG fuel, gas leakage prevention is the prime requirement to provide basic safety to the passengers. If there is leakage observed in the system, it needs to be detected properly by using some innovative, robust techniques followed by prevention of the leak by closing the regulating valves of CNG cylinders. In this work, the EMS is calibrated to detect two types of leakages (1) External Leakages (Leakages from CNG system to ambient) and (2) Internal Leakages (Leakages from CNG components to CNG system). Further, an external leakage is distinguished into two types, high pressure side & low pressure side leakage. Logic is defined to capture fine leakage & heavy leakage separately so that suitable actions can be taken as per the severity of leakage. The subject vehicle runs on Bi-fuel which means it is drivable with Gasoline as well as CNG fuel. CNG leakage behaviors are different when the vehicle is driven in respective fuel modes and hence the leakage strategy is defined in such a way that CNG gas leakage can be detected even when the vehicle is driven in gasoline mode. It is a very unique strategy designed to detect any gas leakage in the system which uses suitable counter measures like automatically converting the vehicle into gasoline mode by closing CNG regulating valves. In case of no gas leakage diagnosed, it switches the engine back to CNG fuel mode.

INTRODUCTION

CNG (Compressed Natural Gas) is emerging as an attractive alternative fuel due to its clean burning characteristic and very low amount of exhaust pollution. It is a mixture of hydrocarbons consisting of approximately 80 to 90 percent methane in gaseous form.  CNG (methane stored at high pressure) can be used in place of gasoline (petrol), Diesel fuel and propane/LPG. CNG combustion produces fewer undesirable gases and no significant aldehydes or other air toxins which are the concerns in gasoline and some other alternative fuels. In recent time, fall in price of natural gas in India based on Global Crude Oil Prices and Indian Rupee conversion rate make CNG vehicles more attractive proposition for consumers which offers savings of about 40-60% and 15-20% as compared to petrol and diesel respectively. According to analysts, the move not only benefits consumers by giving them a much cheaper alternative fuel to run their vehicles, but also works for companies too by translating into a jump in sales volumes and profit margins in the coming years. It also gives a boost to the Government’s initiative to push consumption of CNG by setting up green corridors across major highways in the next two years. The petroleum ministry is contemplating developing green energy corridors across the country. The corridors in the northern region will be between Delhi-Jaipur, Delhi-Chandigarh, Delhi-Haridwar and Delhi-Agra and similar green corridors are being proposed for Allahabad-Kanpur-Varanasi-Lucknow, Bengaluru-Mumbai-Pune and across a few cities of Andhra Pradesh.

CNG is one of the safest fuels available for vehicles. CNG has a high ignition temperature, about 650˚C compared with about 300 ˚C for gasoline. It also has a narrow range of flammability i.e. CNG gas concentration in air between 5 to 15 percent only burns. The high ignition temperature and limited flammability range make accidental ignition or combustion of CNG unlikely. Also, CNG has no known toxic or chronic physiological effects (it is not poisonous). It is non-corrosive and do not contaminate ground water. Human Exposure to a moderate concentration may result in a headache or similar symptoms in human being due to oxygen deprivation but it is likely that the smell would be detected well in advance of concentrations being high enough for this to occur. Natural gas is made up primarily of methane. In its natural state, it’s odorless. However, for practical purposes, it’s odorized with Mercaptan to enable easy leak detection. According to a safety article in the Natural Gas Vehicle Knowledge Base, the average person can detect odorized natural gas at concentrations as low as 0.3 percent. That concentration is about 16 times lower than the level which will support combustion. As mentioned earlier, for combustion to occur, natural gas requires a concentration of at least five percent. In other words, a driver will detect the smell long before combustion conditions are met.

CNG cylinder pressure - as CNG is stored at high pressure, it is often perceived that the high pressure of the product makes it more hazardous than other fuels. CNG fuel systems are "sealed," which prevents any spills or evaporative losses. CNG fuel cylinders are much, much stronger than gasoline tanks. The systems are also fitted with valves and other safety devices to prevent leakage and eliminate the risk of an explosion. In case of leakage, while a high pressure gas leak tends to make a lot of noise as the gas is escaping. The resulting high concentrations of gas and its tendency to dissipate upwards make it less likely for the gas in the immediate vicinity of the leak to ignite. Fuels such as diesel,

gasoline, or liquefied propane gas (LPG)/propane auto-gas are heavier than air, pool on the ground, creating a fire hazard. If a CNG leak occurs, the gas disperses rapidly upward into the atmosphere and dissipates.

Even with all above safety factors, the most common worries about CNG vehicle is to do with leaks. The high pressure does cause the gas to escape with some force, so obvious precautions are necessary. The CNG system which is discussed in this paper is developed for TATA NANO-The world’s cheapest car. It has achieved this tag due to frugal design dominated with compact systems integration, CNG system is one of them. The whole CNG system including 2 gas tanks is packaged in front passenger compartment below driver & co-driver seats and low pressure circuit is extended to rear of the vehicle in engine compartment as shown in figure 1. As the CNG system is packaged in driver compartment, leak detection became very important even though CNG is one of the safest vehicle fuels available. The vehicle specifications are tabulated in table 1.

Figure 1. CNG system packaging Layout in NANO vehicle.

Vehicle Specifications

Table 1. NANO Bi-fuel vehicle specification.

The present work is to determine the methodology to detect leak in the CNG system and to control the situation by stopping the gas flow and allowing the vehicle to run on other fuel (Petrol) available without stalling. In case of no leakage diagnosed auto-reconnection of the engine to its initial fuel (CNG) system. The vehicle which is used to define, develop and validate said work is equipped with Bi-fuel configuration (Petrol/CNG), wherein the vehicle can run on both the fuel modes. In case of Gas leak, it is necessary to have robust gas leak detection and protection strategy to avoid further consequences of leakage, as any gas leak can directly cause panic in customers. The Schematic Layout of the CNG system is shown in figure 2.

CNG Receptacle

Pressure indicator

Manual valve

Pressure sensor

Pressure Regulator

Temperature and Pressure sensor

Injectors

CNG Cylinder

Figure 2. Schematic Layout of the CNG system.

For Automobiles, running on CNG fuel, gas leakage prevention is the prime requirement to provide basic safety to the passengers. In case if leakage is observed in the system, it is required to be identified; followed by prevention of the leak by closing the valves through the receipt of electrical signal. This detection of leak is done by means of various sensors. We developed a system to detect CNG gas leakage scenarios and provide a security alert to drivers using the pressure sensors to detect any gas leakage. The gas pressure sensor sends out a signal to engine management system (EMS) as soon as it encounters a gas leakage. The EMS processes this signal and sends out a signal with required message details and an alerting indication to the driver so that they may handle the issue and look out for leakages. The methodology provided to confirm the leakage is determined by obtaining the drop in pressure and mass in the system. Here, the CNG system is divided into two systems, high pressure system (HP) and low pressure system (LP) as shown in figure 3, which calls for requirement of two different gas leakage detection strategies for HP and LP side.

Figure 3. CNG system layout showing LP side and HP side.

LEAK DETECTION STRATEGY

Gas leak detection and protection from further consequence is very important in CNG vehicles. Since any gas leak will directly cause panic in customer, it is necessary to have robust gas leak detection and protection strategy. Such strategy will help to avoid customer panic, ensure vehicle safety, avoid environmental pollution, and will assist rectification in case of physical leakage.

Based on system architecture of Nano, the gas leakage detection strategy is divided into two parts.

(1)External Leakages (Leakage from CNG system to ambient).

(a) Leakage in high pressure (HP) side (Upstream leak): Further divided as Major leakage and small leakage(b) Leakage in Low pressure (LP) side (Downstream leak): Further divided as Major leakage and small leakage(c) Pipe line leakage: Leakage observed from the CNG lines (unused fuel in the lines) when the vehicle is running in Gasoline mode and the CNG tank solenoid switch is off is termed as pipe line leakage.

(2) Internal (Component) leakage:

Flow of gas when the valves are closed (not activated) is referred here as Internal leak of valve. Internal leak of valve may lead to uninterrupted gas supply in lines even when valves are closed. This will be a safety issue and must be diagnosed and contained. The internal gas leakage through pressure regulator valve will lead to rise in rail pressure and consequently high rail pressure will cause injector shut off. Therefore, the functionality of the valves has to be checked once in a driving cycle.

(a) CNG Tank Valve leakage.(b) CNG HPR leakage

Post the detection of leak, in order to stop further leakage, EMS shuts-off the CNG Cylinder High Pressure Solenoid Valves, cut off CNG supply. Engine switch over to Gasoline mode, GIL (Gas Indicator lamp provided in the cluster as information to the driver about vehicle running in CNG mode) gets OFF indicating fuel switch over to Gasoline mode and DTC gets logged in EMS with SIL blinking indication to the driver.

Auto healing strategy is also developed in the EMS. In next driving cycle, the vehicle will run in gasoline mode. When the vehicle speed exceeds the set threshold, both tank valves and regulator valve will be opened for the set time with petrol mode continuing. System will monitor pressure in HP and LP lines through regulator pressure sensor and rail pressure sensor for the set time. If there is no major drop in pressure found, ECU confirms no leakage and vehicle switchover to CNG mode. CNG mode running is also monitored for leakage if any for set time. After reconfirmation of no live leakage, engine will permanently shift to CNG with DTC stored in ECU memory with SIL On (Service indication lamp is provided in the cluster to inform the driver about malfunction in the engine, thereby recommending the driver to visit to the service station for rectification of the fault). However, if the pressure drop is more than the threshold value, the system considers as leakage and vehicle will continue to run in petrol mode.

The matrix of different kind of leakages and detection strategy for different vehicle operations are tabulated in table 2.

Leakages\ Vehicle conditionGasoline Mode CNG Mode Gasoline/ CNG Mode

Idle/Vehicle running Idle/ Vehicle running Ignition ON During Power latch period

HP sideMajor leak Y Y Y Y

Minor leak Y Y Y Y

LP sideMajor leak Y Y Y Y

Minor leak Y N Y Y

Internal Leakages N Y(only in Idle) N N

Table 2. Leakages which are covered in EMS – CNG leak detection strategy

Gas leak detection strategy for HP system:

Leak detection based on drop in Pressure: - A Pressure sensor is provided in the HP circuit for monitoring the pressure difference. In an event of rate of drop in pressure is more than the threshold value, the system identifies it as a leakage and stops the supply by shutting off the solenoid switch. Such leakages are displayed as ‘MAJOR LEAK’.

Leak detection based on drop in mass:

Mass in the system is calculated using characteristic equation of a state of a perfect gas.

P V = m R T

Where P= pressure, V= System volume, m= mass of gas in system, R= gas constant, T= Temperature.

The calculated mass in the system is monitored continuously. In case of drop in mass is greater than the threshold value, the system identifies it as a leakage and stops the supply by shutting off the solenoid switch. Such leakages are displayed as ‘MINOR LEAK’.

Gas leak detection strategy for LP system:

Leak detection based on mass drop: The LP system strategy is similar to Gas leak strategy for HP system of MINOR LEAK. However the ‘MAJOR Leak’ and ‘MINOR leak’ differentiation in LP system is based on leakage rate i.e. mass drop with respect to time. Let ‘X’ be the rate of mass drop observed during maximum consumption of gas in running, hence any value greater than X is considered as Leakage. The major leak and Minor leak is distinguished based on the time taken for percentage mass drop.

Fuel mode Switchover during CNG leak detection Phase:

In conventional Bi-fuel vehicle running in CNG mode, the petrol mode is switched-off and hence the rail pressure will be less than standard operating pressure. When system detects a gas leak, it will cut off the gas flow and then switch over to petrol mode. Hence during the switch-over phase the possibility of vehicle stalling is more prominent as shown in figure 4.

Time in Secs

Pres

sure

, bar

Time, s

Figure 4. Conventional CNG system – fuel switch over from CNG to Petrol mode after detecting leakage.

To take care of the above issue, the logic has been developed in the Engine management system (EMS). Once the leak gets identified by the system in the HP and/or LP circuit, it activates the petrol fuel pump to supply petrol fuel thus

Time , s

Pres

sure

, bar

pressurizing the fuel rail. Simultaneously, the system monitors the consistency of standard specified petrol fuel pressure in the fuel rail for auto-shifting to petrol mode. At this transition phase both fuel supply systems work at operating pressure to assist smooth switchover. This transition phase terminates by switching off the CNG valves & regulator valve; hence prevent any further leakage of CNG gas through CNG circuit. Switchover phenomenon is explained through graph as shown in figure 5.

Figure 5. NANO Bi-fuel CNG system – fuel switch over from CNG to Petrol mode after detecting leakage.

The vehicle will continue to run in the petrol mode only till the start of next drive cycle. An Error code is logged in the system stating “Type of leakage (Major/ Minor)” and “Specifying the location of the leakage (In HP circuit/ LP Circuit)”

Healing Strategy:

In the next drive cycle when the engine is cranked, it will start in Petrol mode keeping the CNG valves closed hence eliminating the possibility of leakage if persistent. Once the vehicle speed exceeds a defined speed in kmph, the system pressurizes the CNG lines by opening the CNG tank valves for few seconds to check for persistence of leak in the system.If the system mass and pressure is constant during the phase for desired time then it auto-shifts to CNG mode by switching off the petrol fuel and allow the vehicle to run on CNG fuel with the same strategy. Please find attached the flow chart of Simultaneous leak detection in HP circuit and/or LP circuit and healing strategy in next drive cycle. (Refer figure 6)

Figure 6. NANO Bi-fuel CNG system – flow chart of Simultaneous leak detection in HP circuit and/or LP circuit and healing strategy in next drive cycle

Both the gas leak strategies provided for HP and LP circuit work simultaneously on identification of gas leakage through any of the above mentioned strategies, the system auto-shifts to petrol fuel mode in order to enable the vehicle to run without stalling.

It is designed in order to diagnose any leakage left in the system and in case of no-leak detected; the system should auto-shift to CNG mode and continue to run in the same condition. Such strategies will help to ensure vehicle safety, assist in rectification of physical leakage and avoids panic in customers, and reduce environmental pollution.

The leak detection and healing strategy has the following uniqueness:

(1) HP and LP leak detection with clear categorization as ‘Major’ and ‘Minor’ leak in the system and store the information in memory fault code.

(2) The healing strategy performs reconfirmation of persistence of leakage in the system during the next crank cycle and auto switch to CNG mode on the confirmation of no leakage.

ADVANTAGES OF NANO BI-FUEL CNG SYSTEM OVER CONVENTIONAL CNG SYSTEM

(1) Provides safety during gas leak by discontinuing the gas flow.

(2) Smooth switch over to other fuel mode thus avoiding abrupt stalling of vehicle.

(3) Utilizes dual strategy based system which improves its responsiveness.

(4) Reduces the service time by assisting in identification of the leakage area i.e. HP / LP in the system.

(5) Self-healing strategy helps to reconfirm the existence of leakage & utilizes the use of CNG fuel by auto shifting to CNG mode.

Once the strategy synopsis is finalized, baseline data is generated to define thresholds and boundary conditions.

BASE LINE DATA FOR DEFINING THRESHOLDS

The gas leak determination is based on drop in measured pressure and calculated mass. If CNG gas pressure/mass drops faster than the threshold rate, the system will determine it as leak.

Various factors considered contributing in gas-leak detection:

(1) Engine Gas consumption rate in full load conditions – the threshold defined higher than engines maximum consumption rate to avoid false leak detection

(2) Temperature effect -The pressure measurement depends on ambient temperature around gas cylinder. The variations of cylinder ambient were considered in various real life scenarios namely prolonged parking after soaking the vehicle in hot ambient, seasonal temperature variation, AC effect on cylinder surface temperatures etc.

(3) Pressure sensor variations - In ideal condition the gas pressure in the system should remain constant if not consumed; also during running the drop in pressure should be linear. However, it is seen that the pressure sensor output signal has certain variations.

(4) Refueling process - During refilling of gas in the tank, it is observed that once the desired quantity of gas is filled and the filling nozzle is removed, minor drop is observed in tank pressure value; this phenomenon occurs to achieve equilibrium in the complete system.

Gas leak detection strategy was developed with input & feedback from various internal customers.

TESTING AND VALIDATION:

Intentional leakage was created in the HP and LP circuit for testing of the strategy. The tests were conducted on three vehicles to ensure repeatability, consistency and robustness of strategy.

(1) Leakage tests HP side: - The joinery was made loose in order to generate a leakage to simulate the HP side leakage. Major and minor leakage was differentiated by regulating the opening of ball valve. The position of the lever where the gas starts to leak is considered as minor leak whereas opening of the lever above that point to complete opening of the valve is considered as major leakage.

(2) Leakage test LP side: - Major leakage simulated by piercing a needle (syringe) in the LP hose between regulators and filter. Minor leakage simulated in the LP hose by piercing a needle (syringe) of 0.43mm diameter.

(3) Physical leakages were also created by loosening the joinery of the HP side at receptacle, joinery near tank and at the joinery near the regulator during confirmatory tests.

(4) Validation of healing strategy.

Following test conditions were considered for validation of gas leak detection:

(1) During engine running in CNG mode.

(2) During engine running in petrol mode.

(3) In Ignition ON condition and engine not fired.

(4) When the ignition is turned off.

Gas Leakage test:

Gas Leakage was simulated by creating a physical leak at the joinery in the HP circuit:

Major cause of leakage is due to lose or improper fitment of joinery in the HP circuit. Hence to simulate this case of failure, the joinery was made loose in order to generate a leakage.

Following are the major locations considered for leakage as shown in figure 7.(1) Joinery near the receptacle side (Please refer photo below for location 1)(2) Joinery near the CNG tank (Please refer photo below for location 2)(3) Joinery near the Regulator side. (Please refer photo below for location 3)

Figure 7. NANO Bi-fuel CNG system – Simulation of physical leakage at the joinery in the HP circuit

Minor leakage in HP side:

(Mass based strategy): High Pressure line fine leakage detection is mass based strategy. As system do not have a tank temperature sensor, the calculated mass value is subjected to change with increase or decrease in temperature. The tank temperature is taken as minimum of atmospheric temperature, coolant temperature and constant 40˚C temperature, whichever is minimum.

Major leakage in LP side:

Major leak was created by creating a hole in the hose that connects the PR Valve & the rail as shown in figure 8. Low Pressure line leakage error is possible to be detected in Pure Gasoline Operation and Intermittent Gasoline Operation. A time threshold is defined, if the leakage gets detected within the defined threshold then it is considered as Major leakage if not it is considered as fine leakage.The possibility to detect a LP Major leak in intermittent gasoline operation is pretty guaranteed, at worst case scenario the intermittent gasoline operation takes a certain period, which will be enough to detect a Major leak in intermittent operation.

The measurement shows that when the Operating mode is changed from CNG to Gasoline, the mass drop is monitored. If the present mass has dropped below the threshold within the set time threshold then low pressure line Major leak error is logged.

Figure 8. NANO Bi-fuel CNG system – Simulation of physical leakage in the hose that connects the PR Valve & the rail in the LP circuit

Minor leakage in LP side: Minor leak was created by piercing the LP hose with a syringe needle in the hose that connects the PR Valve & the rail as shown in figure 9.

Figure 9. NANO Bi-fuel CNG system – Simulation of physical leakage using 0.43 mm syringe in the hose that connects the PR Valve & the rail in the LP circuit

LP hose detached from the regulator in:

Four joints are provided in the LP side. The regulator side, Filter inlet side, Filter outlet and the rail end. Spring band clamps are provided to ensure positive gripping of the hose on the ends. However, in-case of fitment of wrong clamps, there is possibility of hose getting detached from the end, thereby releasing gas in the atmosphere. Hence test was conducted to prove the strategy competent to identify this kind of leakage. (Due to safety concerns this test was done in stationery condition and ignition on only)

Internal Leakage Tank Valve error simulation: The Tank Valve internal leakage is simulated by connecting a dummy tank valve to the ECU and the actual tank valve is connected to the power supply to keep it always ON. Initially the Tank valves are closed and the slowly the tank valve is manually opened to create the leakage through the Tank Valve. As shown in figure 10.

Figure 10. NANO Bi-fuel CNG system – Simulation of Internal Leakage Tank Valve error simulation.

Validation of healing strategy:All test results showed the strategy is effective in detecting and containing gas leak in most of the static and dynamics events. During the leak detection periods, no thermal risk was perceived due to leaked gas. The healing strategy validated during each tests and observed that it functions correctly if the mode selector switch is in CNG mode. If switch is in Petrol mode, system solenoid auto-activates CNG tank switch for defined time and then monitor pressure for set time, however vehicle will run in gasoline only.

CONCLUSION:

This paper illustrates an innovative strategy for detection & containment of gas leak in the CNG system. It is very robust strategy which helps in detecting major as well as minor leakages on High & Low pressure sides and also helps further to control the situation by stopping the gas flow from high pressure cylinders and allowing the vehicle to run on the other available fuel (Petrol) mode by smooth switchover from CNG fuel without stalling the vehicle. In the next drive cycle, system is designed to detect any gas leak left in the system and in case of no gas leakage diagnosed, switch the engine back to CNG fuel mode.

ACKNOWLEDGMENTS

Our sincere thanks to the management of Tata Motors, for offering the challenging task and providing valuable guidance to perform the above said work. We are also thankful to our colleagues associated with the project for their support to the various activities.

REFERENCES

1. Hitendra Rawat, Ashish Kushwah, Khyati Asthana, Akanksha Shivhare,“LPG Gas Leakage Detection & Control System”, National Conference on Synergetic Trends in engineering and Technology (STET-2014) International Journal of Engineering and Technical Research ISSN: 2321-0869, Special Issue, 2014

2. Santosh A Jalihal and T S Reddy, “CNG: An alternative fuel for public transport”, Journal of Scientific and Industrial Research, Vol 65, May 2006, pp.426-431.

3. Government of India, Ministry of Petroleum and Natural Gas, Economics and Statistics Division, New Delhi, “ Indian Petroleum and Natural Gas Statistics 2014-15”.

4. Ministry of Heavy Industries and Public Enterprises, Government of India, “ Review of Automotive mission plan 2006-2016”.

CONTACT

Chandrakant ParmarERC, Tata Motors Limited, Pune, India. chandrakant.parmar@tatamotors.com

Siddharth DeshmukhERC, Tata Motors Limited, Pune, India. siddharth.deshmukh@tatamotors.com

T. SethuramalingamERC, Tata Motors Limited, Pune, India. t.sethu@tatamotors.com

Arun Augustine Paul S.ERC, Tata Motors Limited, Pune, India.arun.paul@tatamotors.com

DEFINITIONS, ACRONYMS, ABBREVIATIONS

Term Definition for the term

EMS Engine management system

ECU Electronic control unit

CNG Compressed Natural Gas

GIL Gas indication lamp

SIL Service indication lamp