Aftermarket CNG Kit in BS6 Phase 2 Cars: In India’s automotive sector, driven by ever-rising fuel prices, Compressed Natural Gas (CNG) has always been a craze among car owners.
In the past, people would pick up a cheap petrol car from the showroom and spend ₹35,000 to ₹50,000 installing an aftermarket CNG kit from a local vendor. This formula continued even with BS4 and early BS6 Phase 1 vehicles.
But if you’re buying a new BS6 Phase 2 (RDE / OBD-2 norms) car today and thinking of installing CNG from outside, stop! Following the new emission norms, the engine mechanics and electronics of modern cars have been completely transformed. In today’s times, getting CNG fitted from outside can prove to be a silent killer for your car’s engine.
In this detailed scientific report, we will understand what are the major technical blockers behind fitting modern aftermarket CNG kit in BS6 phase 2 cars, what impact it has on the micro-components of the engine, and how heavy it can be on your pocket.
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The Core Engineering Blocker: Real Driving Emissions (RDE) & OBD-2 Compliance
The biggest change in the BS6 Phase 2 rules is the mandatory implementation of Real Driving Emissions (RDE) norms and OBD-2 (On-Board Diagnostics) standards in every vehicle. Let’s put it simply:
Earlier System: Older vehicles only had their emissions checked in the laboratory, by running them through a standard test cycle.
New BS6 Phase 2 System: Vehicles now have an extremely smart, real-time diagnostic microchip (OBD-2). This chip checks emissions data every second of the vehicle’s life during real-world road, acceleration, and mountain testing.
The ECU Matrix Conflict
The Electronic Control Unit (ECU) of modern BS6-2 cars is much more advanced. It is designed for a closed-loop petrol system, where oxygen sensors and exhaust gas recirculation (EGR) valves change input controls within milliseconds.
Step 1: Aftermarket CNG Advancer Input – When the local installer installs a separate electronic advancer to adjust the vehicle’s timing.
Step 2: Altered Ignition Timing – This advancer alters the engine’s original crankshaft position sensor data to make the spark plugs fire earlier based on gas instead of petrol.
Step 3: ECU Detects Out-of-Spec Values ​​– The advanced ECU of the Modern BS6 Phase 2 checks for micro-voltages in milliseconds and immediately detects if ignition timings are going out of limits.
Step 4: Check Engine Light (CEL) Activation – As soon as this sensor data conflict is detected, the system activates safety mode and a permanent Check Engine Light starts glowing on your dashboard.
When you install a local mechanical aftermarket CNG kit in BS6 Phase 2 cars, they cut or bypass the original petrol fuel injection lines and install a secondary Sequential CNG ECU.
Since the new petrol ECU is continuously monitoring the fuel-air ratio matrix, it immediately understands that the fuel parameters have changed. This conflict causes the car to go into permanent Limp Mode or the Check Engine Light (CEL) on the dashboard to be on forever.
Factory CNG vs. Aftermarket CNG Fitment
There is a huge difference between factory-fitted sets (like Maruti Suzuki S-Sung, Tata King, Hyundai S-Sung) and local aftermarket sets:
| Engineering Parameter | Factory-Fitted CNG (Company Standard) | Aftermarket CNG Kit (Local Installation) |
|---|---|---|
| Engine ECU Setup | Dual Interdependent ECUs (Integrated Coding) | Piggyback ECU (Bypasses Petrol Inputs via Couplers) |
| Valve Seat Engineering | Hardened Stellite Coated Valves | Standard Factory Petrol Valves (Soft Material) |
| Suspension Calibration | Re-tuned Springs with Higher Coil Stiffness | Stock Petrol Springs (Leads to Rear Sagging) |
| Catalytic Converter Life | Calibrated for High exhaust temp (EGT) | Subjected to Premature Melting/Thermal Stress |
| OBD-2 Real-Time Monitoring | Integrated Fuel Trim Adaptations | Causes Continuous Misfire & Fuel Trim Lean Errors |
| Gas Injection Precision | Sequential Injection based on Cylinder Load | Generic Timing (Causes Jerks at Lower RPMs) |
The Thermal Nightmare: Catalytic Converter Degradation and Melting
Natural gas (CNG) has a very different chemical structure from petrol. CNG has a very high octane rating (approximately 120-130), which makes its combustion process slightly slower than petrol.
Exhaust Gas Temperature (EGT) Escalation
Due to its slow burning speed, half of the CNG fuel remains burning during the cylinder’s exhaust stroke. This process is technically known as delayed combustion. This causes the temperature of the gas exiting the exhaust manifold, i.e., Exhaust Gas Temperature (EGT), to increase by approximately 100°C to 150°C compared to petrol.
Step 1: CNG Delayed Combustion – Due to its high octane rating and slow combustion speed, CNG gas continues to burn inside the engine cylinder until the exhaust stroke begins.
Step 2: Exhaust Gas Temperature Spike – This delayed combustion causes the temperature of the gas exiting the exhaust manifold (EGT) to rise by approximately 150°C compared to petrol.
Step 3: Precious Metals Melting – Precious metals like platinum, palladium, and rhodium found inside the catalytic converter of modern BS6 Phase 2 vehicles cannot withstand this extreme thermal shock and begin to melt.
Step 4: Exhaust Blockage – Due to melting and choking of the substrate, the exhaust pipeline gets completely blocked, due to which the exhaust gases cannot escape and the engine starts spinning and choking badly.
Catalytic converters in modern BS6 Phase 2 vehicles are much more advanced. They have a very thin, precise layer of precious metals like Platinum, Palladium and Rhodium to block emissions.
When the exhaust system is continuously exposed to extra heat of 150°C after installing a local aftermarket CNG kit, the new catalytic converter cannot withstand that high thermal shock.
The substrate inside your exhaust pipelines slowly starts melting, causing the exhaust to get choked and the car engine to choke (extreme power loss). The cost of a new catalytic converter ranges from Rs. 40,000 to Rs. It goes up to Rs 90,000, which means huge losses are certain in the pursuit of cheap fuel.
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Mechanical Wear and Tear: Valve Recession and Gasket Blow-out
CNG is a completely dry fuel, whereas petrol contains natural lubricating agents that keep the engine’s moving parts running smoothly.
Valve Seat Recession (VSR)
When petrol is sprayed into the engine, it creates a micro-lubrication layer between the cylinder inlet and exhaust valves. However, running dry CNG destroys this lubrication.
Operating at extreme cylinder temperatures (due to high octane combustion), the valves constantly make metal-on-metal impact on the valve seats. This, over years, causes the valves to sink into the seats, a condition known as Valve Seat Recession (VSR) in engineering.
This slowdown causes:
- Cylinder compression pressure to leak.
- The vehicle begins to experience trouble starting in the morning.
- The engine begins to produce heavy vibrations at low RPM.
Cylinder Head Gasket Failure
BS6 Phase 2 engines are optimized to run on very lean fuel mixtures to reduce pollution. When an aftermarket CNG kit is installed on an engine with such tight tolerances, peak pressure within the combustion chamber spirals out of control.
This excessive mechanical fatigue directly ruptures the cylinder head gasket (gasket blow-out), causing the engine oil and coolant to mix and permanently seize the engine.
Electronic Conflicts: Advancers, Wiring Harness, and Couplers
Outdoor CNG fitters install a device called a timing advancer to correct the lagging or pick-up drop of a vehicle’s petrol engine. This device manipulates the signals from the engine’s original crankshaft position sensor to cause the spark plugs to ignite the gas earlier.
The Micro-Voltage War
Modern OBD-2-equipped cars’ sensors operate at micro-voltages (0.1V to 0.9V). When local installers make cuts in the wiring harness or install tight couplers, the electrical resistance of the entire circuit increases.
When the original ECU reads this altered voltage signal, it assumes the sensor is faulty. Consequently, the vehicle’s ABS, traction control, and even automatic climate control systems malfunction. Sometimes, local tape can cause a short circuit, posing a fire hazard.
Financial & Legal Implications: Warranty Void and Insurance Denials
Not just mechanical components, but you could also face a significant legal and financial setback:
Complete Manufacturer Warranty Cancellation: As soon as you install aftermarket CNG in your new BS6-2 car, the company will immediately void your entire 3-year or 5-year engine, electrical, and transmission warranty. After this, if even a small sensor in the engine fails, you will have to bear the entire cost out of your own pocket.
Insurance Claim Denials: If you have not got CNG endorsement from RTO on your RC (Registration Certificate) (which is almost impossible to get for aftermarket kits under the new rules), and God forbid the vehicle meets with an accident or fire breakdown, the insurance company will not pass even a single rupee claim.
Auto India Daily Verdict
Friends, most car buyers buy cars simply by looking at top speed stats and large, fancy touchscreens, but the true test of a car is on Indian roads, when its mechanical parts are subjected to extreme daily stress.
If you’re driving a highly calibrated, modern engine, it’s crucial to understand the engineering barriers and risks behind installing an aftermarket CNG kit in BS6 phase 2 cars, especially if your daily commute is on heavily trafficked metropolitan routes.
If your main objective is to have a fearless, low maintenance engine block that won’t overheat or lag on long journeys or sharp road turns, then choosing a factory-fitted CNG option or sticking to stock petrol fuel is the best and safest advice.
On the other hand, if you drive a company-engineered clean-energy car (Factory CNG) with good driving habits, then in the long term it will prove to be a better and superior choice for your daily driving needs.
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Raj Prajapati is a Senior Automotive Content Writer at AutoIndiaDaily. A B.Tech graduate in Computer Science and Engineering, he leverages over four years of experience covering Indian car and bike launches, EV tech, and market dynamics to break down complex automotive regulations into simple consumer guides. Specialising in Indian motor vehicle laws, IRDAI updates, and ownership costs, Raj translates technical auto policies into actionable advice for everyday drivers.