CNG Analysis

A few numbers first. As of June 15,2012, petrol prices have soared to Rs 71.16 per litre (Delhi). At Rs 40.91 per litre, diesel is not all that cheap either, and there’s also the uncertainty on diesel car pricing in the future. So, CNG at Rs 35.45/kg doesn’t sound all that bad. Given the potential saving in running costs, could CNG be the fuel for you? Does it make sense to switch your car to run on CNG? Is it safe? And what are your options? Read on.


If you pass a CNG filling station you are sure to see a gaggle of cars from different vintages waiting for a tank-up. The mix includes everything from 30-years-old Premier Padminis (mostly taxis) and Daewoo Cielos to Chevrolet Sparks and Maruti SX4s. The downmarket image is a big turn-off for snooty customers, but perhaps the time of raising your nose to this fuel has gone. What’s clear is that almost any petrol car can also be made to run on the gas.

In the aftermarket, you choice include gas mixture-type kits and injection-type kits. Priced at about Rs 20,000, the gas maxture-type kits are very tempting, but this option can do more harm to your car than good. These kits operate at a low gas pressure, resulting in a noticeable loss of performance on modern fuel-injected engines, which is largely the source of bad press for cars converted to run on CNG. Lambda sensors and timing advancers have improved their functioning but cases of misfiring and engine stalling are still fairly common.

In that light, it makes sense to stretch your budget to invest in an injection-type kit (more commonly called a sequential kit). Injection-type kit prices range between Rs 50,000 and Rs 75,000. IN operation, the kit is similar to regular petrol injection. Gas stored in a boot-mounted cylinder is drawn to the engine compartment where a regulator reduces the pressure of this stored gas and feeds it to the CNG injectors on the intake manifold. The engine starts first on petrol but once revs reach a predetermined range (usually 2000rpm), the ECU switches from petrol to gas. Like in a regular engine, the CNG ECU also dictates the quantity of gas to be injected.

All components of the aftermarket kit come packed together, so in most cases CNG kit installation is nothing more than a simple bolt-on job. However, it is important to note many manufacturers do not honour warranties if a car has been modified to run on CNG by a third party. From that perspective, you’d be best off carrying out a conversion by the dealer only. Many manufacturers do have facilities for this. Hyundai dealers, for instance, office CNG kits on the Santro, i10 and Accent. Once the buyer decides to buy a CNG variant of a car, an aftermarket kit is fitted at the dealer’s end by a retrofitting partner chosen by the company. The car is only delivered after the fitment and registration as a CNG vehicle by the RTO.


Given the huge potential of CNG car sales, it comes as quite a surprise that most manufacturer do not offer factory-fitted CNG kits. Maruti has taken the lead here and offers CNG versions of its Alto, Eeco, Estilo, SX4 and Wagon R models, and we hear a CNG Ertiga is in the works too. Apart from these, the Chevrolet Aveo, Hindustan Ambassador and Tata Indigo XL are among the handful of cars that come with the options to run on CNG straight from the factory. So what’s the difference between an aftermarket kit and a factory-fitted kit? And should you pay more for a factory-fitted kit that uses effectively the same technology? If you value engine longevity, you just might need to.

Being a gas, CNG lacks the lubricating properties of liquid fuels like petrol and this can speed up wear and tear on engine components. To counter this, manufacturers, like Maruti, use special materials for faster-wearing parts like valves and valve seats on its CNG powerplants. The intake manifolds are also designed to incorporate CNG injectors from the outset. Aftermarket fitters, on the other hand, drill holes into the stock manifold for the same, which is a compromise. Maruti’s CNG models also use a different layout for the wiring harnesses to accommodate CNG kit components in the engine bay, the aim being to minimize risk of sparking here.

That brings us to the safety aspect, which remains the biggest question mark for first-time CNG car buyers. Here too, factory-fitted kits come with their advantages. Mounting points for the cylinder and other components are made before the car goes to paint, which provides the best protection from rust. This also serves to give it a more robust frame for the CNG kit. Another crucial difference we saw was in the piping that connects the gas cylinder to the engine compartment. To keep costs low, aftermarket suppliers mainly use cheaper and inferior carbon steel pipes. Despite being sheathed with PVC covers, these pipes are prone to corrosion (and hence leaks) and are not as sturdy as the pre-formed stainless steel pipes that Maruti’s CNG cars come with.

Like the pipes, all joints on the Maruti’s CNG cars are also tailor-made and are virtually leak proof. The cylinder too gets a solenoid valve, which keeps the gas out of the system when the engine is not running. Maruti engineers also claim that placing the gas receptacle near the petrol filler instead of in the engine compartment is a safer option, though we are yet to hear of any incidents to support this. To deal with the additional stress generated by the boot-mounted gas cylinder during acceleration, braking and cornering, the rear suspension parts are of a different rating and the floor is strengthened too. None of the aftermarket suppliers we checked with offer this. It’s also reassuring to know Maruti tests each of its CNG models for crash-worthiness in a rear impact.

A crucial difference among retrofitters and manufacturers is in the homologation process. While manufacturers have each model individually homologated, regulation allow aftermarket suppliers to homologate a particular CNG Kit. The homologated kit can then be used on cars with engine capacities within 20 percent of the tested vehicle. However, this practice is far from ideal because despite what a retrofitter may claim. It is very difficult to perfectly calibrate the same ECU to different engines. As a result, performance in CNG mode takes a hit. A wrongly tuned CNG vehicle will also emit unburnt methane (which constitutes CNG) that contributes to the greenhouse effect.


So how does this change of diet work in the real world? The few owners we spoke to were largely positive about the fuel. Almost all talked about the economies of running their can on CNG but complained of a small loss in power and, of course, losing boot space. However, there biggest grouse was about the availability of CNG, a problem which is compounded by the limited range on a tankful of CNG. A typical CNG cylinder can hold nine litres, depending on the pressure at the filling station and ambient temperature. So your real world range is about 120-150km (depending on how the car is driven and the engine size) before you need to refuel.

And what about service? Maruti’s CNG cars come with the same service intervals as their petrol siblings, but further investigation revealed you’ll have to spend Rs 1,100 more at the 20,000km mark for CNG kit maintenance. A service advisor at a Maruti workshop also told us that running the car for the first few kilometers on petrol in the morning is a good practice and keeps the engine lubricated. On the other hand, an independent workshop owner told us he had seen cars being brought into service only when the fuel efficiency had dropped. Regular air-filter cleaning and routine replacement are other suggestions to keep the car running smooth.


It’s not difficult to see where the poor image of CNG originates and over time it has only got worse. Owners want to save as much money as possible and going for the cheapest alternative is natural course for most. But then the driving experience tends to be poor and it’s the fuel that gets blamed. However, modern aftermarket kits are a lot better now and there’s an increasing array of factory-ready CNG cars to choose from. There’s also the reassurance in knowing CNG prices are and will remain more stable than those for petroleum, which further strengthens its case.

But perhaps the biggest road block for the wider acceptance of CNG as a fuel is centered on its availability. There are simply too few filling stations and these are concentrated in a few pockets across the country. A wider distribution base will not only better serve the needs of existing users but will also spread the message loud and clear that CNG is here, and it’s here to stay.


In petrol and diesel fuel prices, there’s been a lot of talk about whether it makes sense to invest in a more expensive diesel car. CNG adds another dynamic to this dilemma. For our calculations, we have use the Maruti SX4, which is available with all three fuel options.In the table below, we’ve assumed a typical city driver covers 35km a day. While the petrol car is the cheapest to buy, the CNG car is the cheapest to run. The saving in running costs vis-à-vis the petrol car mean a CNG car user will take only a year and 10 months to recover the additional purchase cost over the petrol model. The table also highlights another often overlooked fact – that diesel cars don’t make sense for low mileage users. Assuming price remain at their current levels, it will take the SX4 user here three years and nine months to make good his additional investment for the diesel car. Of course, the higher the daily run, the quicker the recovery on investment. However, this calculation doesn’t take into account the higher cost of diesel car service of the added interest burden for a financed car. The higher resale value of a diesel car could change the equation though.

MARUTI SX4                                       PETROL                                 DIESEL                                   CNG

Buying cost                                    RS 7.97 LAKH                      RS 9.24 LAKH                      RS 8.65 LAKH

FUEL efficiency                             15.5kpl                                 21.5kpl                                 21.4km/kg

Cost of fuel per unit                       Rs 71.16/litre                     Rs 40.91/litre                     Rs. 35.45/kg

Daily running cost (avg. 35km run) Rs 160.68                       Rs 66.59                                Rs. 57.97

Investment recovery period        -                                              3year 9 months                 1year 10 months


Source: Autocar July 2012

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