While aftermarket CNG conversions are not new, with the rise in fuel prices, there has been a noticeable spike in interest surrounding them. Although there are more factory-fitted CNG cars on the market than ever before, several models still do not give you the option. Not to mention, existing car owners are increasingly looking to convert their cars to CNG to reduce running costs.

Naturally, this increased interest brings up several questions about the technology, the installation, the legalities and more. We thus teamed up with Lovato CNG’s authorised distributor in Maharashtra, Blue Drive Traders, as well as Lets Go CNG and its authorised installer in Mumbai, M.S Motors, to answer questions about CNG conversions.

Know your CNG kit

There are three broad types of kits available on the market today. The oldest is the Venturi type, typically used for cars that are Bharat Stage 3-compatible and older. These use a Venturi mixer, similar to a carburettor, that mixes CNG with air before it is injected into the intake manifold. They are the most affordable, usually costing between Rs 30,000 and Rs 40,000, including installation and registration, but they are not the most efficient and bring a significant drop in performance. Since the cars they are compatible with are also reaching the end of their lifecycle, these kits are gradually being phased out.

Modern naturally aspirated MPFI cars require a sequential type kit. These use individual injectors that inject CNG directly into the intake manifold and feature their own ECU that controls timing, volume and other parameters based on RPM and driver inputs. These are much more fuel-efficient, and the drop in performance is nominal in everyday driving conditions. Like the older Venturi kits, these too require the car to be started on petrol before switching over to CNG. However, they automate the process by making the switch once a certain engine temperature is reached. Prices vary depending on the manufacturer and the specifications of each kit, ranging from Rs 40,000-Rs 50,000 for older BS4 cars, to Rs 50,000-Rs 65,000 for smaller BS6 cars, stretching to Rs 70,000-Rs 75,000 for larger cars that require an eight-injector kit.

The newest type of kit to hit the market is the direct-injection kit, used for cars with direct petrol injection. Like sequential kits, these too inject CNG into the intake manifold. However, direct-injection engines cannot run purely on CNG. The fuel injectors in a DI engine sit within the combustion chamber, so to prevent them from overheating, a small amount of petrol still needs to be injected through them even when running on CNG. Therefore, a turbocharged direct-injection engine can only run on a maximum 80:20 ratio of CNG and petrol. As a result, fuel efficiency gains are not quite as high as with a naturally aspirated engine, and kit prices range from Rs 1 lakh to Rs 1.5 lakh, thus pushing the breakeven point further away.

What’s in the box?

The largest and heaviest component of a typical CNG kit is the tank itself. Made from seamless steel, these can weigh close to 70kg empty, with volumetric capacities ranging from 60 to 65 litres. Metal-lined composite cylinders and carbon fibre-reinforced plastic cylinders are also now available, and while they are significantly lighter, they usually cost two or three times more. Attached to the tank is a valve with a manual shut-off.

The filling valve, through which CNG is filled, can either be placed under the bonnet or beside the petrol filler cap, depending on the vehicle. Transporting the gas from the tank to the pressure reducer under the bonnet is high-pressure tubing made from carbon steel and coated in polymers. The reducer has a pressure gauge and lowers gas pressure from about 200bar in the tank to a manageable 2-3 bar. From here, the gas is routed to the injector rail that sits atop the engine, which further distributes it to nozzles drilled into the intake manifold via reinforced rubber hoses. Between the reducer and the injector rail sits a Manifold Absolute Pressure (MAP) sensor to measure the pressure as well as the temperature of the gas. There is also an optional filter to remove any impurities in the line.

Controlling the injector rail is an Electronic Control Unit (ECU) that uses signals from the MAP sensor and communicates with the car’s original ECU to dictate the amount of CNG being injected and the timing. Modern kits also come with a comprehensive wiring harness that can plug into the car’s original wiring via OBD connectors. Older pre-BS6 cars, however, may require some wire splicing. Optionally available is an Advancer, which advances ignition timing when running on CNG and negates some of the power loss, particularly on smaller-capacity engines. Some sedans and hatchbacks also require a rear coil spring spacer to compensate for the added weight of the tank in the boot.

How a CNG kit is installed

A professional CNG installation takes around 4 to 5 hours to complete. Here’s what happens behind the workshop doors.

Health check

The engine is inspected to ensure it’s suitable for CNG conversion. A basic service is performed if required, with spark plugs and ignition coils replaced as needed and the throttle body cleaned.

Components positioned

The reducer and its pressure gauge are mocked up in the engine bay before mounting points are marked and drilled. The filler valve may be located alongside them or next to the petrol filler cap, depending on the vehicle.

Injectors installed

The CNG injector rail is mounted on its dedicated bracket close to the petrol injectors. This will send CNG to corresponding nozzles that are drilled and fitted into the vehicle’s plastic intake manifold.

ECU wired in

The CNG ECU is mounted alongside the factory ECU and connected to the vehicle’s wiring loom. The harness is neatly routed and secured away from heat and moving parts.

Plumbing completed

Reinforced rubber gas hoses are cut to length and connected between the reducer, injector rail and the intake manifold. A MAP sensor and a filter are also installed as part of the system.

Tank prepared

The CNG tank is temporarily mounted to a rig that holds it in place while the shut-off valve and associated fittings are installed before everything is torqued down.

Tank frame installed

The mounting frame is positioned inside the boot, holes are drilled through the floor, and reinforced brackets securely bolt it to the vehicle’s structure.

Tank fitted

The CNG cylinder is secured to the frame using heavy-duty metal straps and bolts before being covered with a carpeted shroud for a neat factory-like finish.

High-pressure line routed

Polymer-coated steel high-pressure pipes are routed beneath the car, connecting the CNG filler valve, the tank and the reducer under the bonnet.

Electrical connections

The remaining wiring is completed and a dashboard-mounted CNG switch is installed, allowing the driver to monitor fuel level and switch between petrol and CNG.

Leak test

Around 2kg of CNG is added and every connection is checked using an electronic gas detector and good old-fashioned soap solution to ensure the entire system is leak-free.

Calibration and road test

The system is calibrated using dedicated software to optimise fuel delivery and switchover settings. A final road test confirms performance before the vehicle is delivered to the customer.

Why go aftermarket?

On average, an aftermarket CNG installation costs around 25 to 30 percent less than the premium a factory-fitted CNG variant commands over its petrol counterpart. However, this alone is not enough to swing the scales in favour of the aftermarket due to the disadvantages we will get into later. The bigger advantage the aftermarket holds is the ability to convert models or variants that do not come with a CNG option. For instance, Maruti Suzuki only offers its factory-fitted CNG cars with a manual transmission, so if you want one with an automatic, an aftermarket CNG kit is your only option.

Similarly, popular SUVs like the Hyundai Creta and Kia Seltos are not available in CNG guise, which leads many owners to turn to the aftermarket. Owners of older cars, or those buying used cars, can also convert them to CNG for a fairly affordable price and enjoy significantly lower running costs. CNG is spark-ignited, thus only petrol cars can be converted to run on the gas. Since diesel cars use compression-ignition, they would need far too many modifications to make this possible. Strong hybrids are also not recommended for conversions, as here, the engine keeps switching on and off too often, which makes CNG calibration complex. Cars that are not in good mechanical shape should also not be converted to CNG, as the conversion can take a toll on the overall engine health.

For a car in generally good shape, aftermarket kits are proving to be reliable in the long run, provided they are well maintained. Most modern sequential kits from brands like Lovato and Mijo are also comparable in quality to OEM systems, and reputed, authorised installers are able to achieve near-OEM levels of fitment.

The catch

Fitting an aftermarket CNG kit – even if it is government-approved and fitted by an authorised installer – is grounds for the manufacturer warranty to be voided. While some brands may still honour warranty claims that are unrelated to the powertrain, it remains a grey area. Some dealer-fitted CNG kits do retain the factory warranty, while others provide a third-party warranty in its place.

Aftermarket installers have also begun offering third-party warranties in collaboration with insurance companies to provide coverage in place of the factory warranty. This, however, is in addition to the cost of the CNG kit. The kits themselves generally come with a warranty ranging from one to two years.

Packaging is also one area where the aftermarket trails OEMs. While Maruti, Tata and Hyundai are now offering underbody and dual-tank setups to retain a usable boot, the aftermarket still largely uses larger single tanks that take up more space.

Staying fit

As per law, any CNG tank – aftermarket or OEM fit – must be hydrotested every three years. For this, the tank must be removed from the vehicle and sent to a dedicated facility. While the process itself takes only a couple of hours, hydrotesting facilities are usually located outside major cities, adding transport time to the process. A hydrotest typically costs around Rs 3,000 per tank.

The CNG filter should also be replaced at the same three-year or 10,000km interval, whichever comes first. This typically costs under Rs 1,000 and is fairly straightforward. It is also recommended to drive on petrol at least 10 percent of the time to keep the fuel system healthy. Finally, it is important that the engine’s service schedule is strictly followed.

The legal bit

Aftermarket CNG installations are legal as long as the kit is government-approved, fitted by an approved installer and the registration certificate (RC) is updated accordingly. Most approved installers will handle the RC update at the Regional Transport Office (RTO) as part of the service. Once this is complete, the owner must also update the insurance provider.

The verdict

There is no denying the running cost advantage that CNG provides. However, when buying a brand-new car, a factory-fitted CNG version is still preferable to an aftermarket conversion simply because of the peace of mind. Once a car is out of warranty, though, a CNG conversion makes a great deal of sense. In most cases, the cost of the kit can be recouped within a couple of years, even with an average monthly running of around 1,500km.

However, the installer is just as important as the kit. Always opt for government-approved kits installed by approved installers with a strong reputation. Wherever possible, insist the installation is carried out without splicing into the original wiring and follow the recommended service schedule to ensure your car stays reliable.