Petrol vs CNG: the maths that decides it, and the trade-offs that spoil it

Ask which fuel is cheapest per kilometre in India and the answer is not petrol, not diesel and, for most households, not electricity. It is compressed natural gas, pumped at a rate that has stayed below liquid fuels for as long as the network has existed. Ask why more cars do not run on it and the answer is about inconvenience rather than money.

What the kit actually is

A CNG car is a petrol car with a second fuel system bolted alongside it, and the engine itself is almost unchanged. What matters is a high-pressure cylinder in the boot, filled to many times tyre pressure; shut-off valves and solenoids between it and the engine; a regulator that drops the pressure to something an inlet manifold can use; and injectors that meter the gas into the air charge.

The distinction that decides whether the car is any good is how the gas gets metered. On a static mixer, the oldest arrangement, gas is drawn through a venturi and the mixture is set mechanically, which is why those cars lose power and run badly when the tuning drifts. On a sequential kit, the engine’s own computer meters gas through dedicated injectors once the engine is warm, and the driveability penalty shrinks to something you notice only as the cylinder runs down. Factory kits are sequential; CNG’s bad reputation in India was earned by aftermarket static mixers fitted in workshops that had other work to do.

The car starts on petrol, switches over once the engine is warm, and falls back to petrol when the gas runs out, usually so subtly that new owners miss it. There is a switch on the dash, and there is a boot you no longer entirely have.

The economics, honestly

The saving is real and it is proportional to distance. A car covering a couple of thousand kilometres a year saves a sum that will never justify anything. A car doing thirty or forty thousand — a taxi, a sales run, one car and two commuters — moves into a bracket where the kit pays for itself and then keeps paying.

What eats into that number is not the pump rate but everything around it:

• The purchase premium. A factory CNG variant costs more than the petrol car it is built on, and more than the mid-trim you would actually have bought.
• The boot. The cylinder lives in the luggage space, and it is the most common reason a household returns a CNG car within a year: on a hatchback that is most of the practical boot, on a sedan a shallow shelf.
• Resale. A used CNG car is a narrower market than a used petrol one, and buyers who cannot verify a cylinder’s certification history price that hesitation into the offer.
• Refuelling geography. CNG pays only where the network is dense enough that you never plan around it. In some metros it is a normal part of a route; elsewhere the nearest pump is a detour and the queue at it is an appointment.
• The efficiency give-back. Gas carries less energy per unit volume than petrol, so the car returns fewer kilometres per unit even before the extra weight. The saving comes from the price gap, not from the thermodynamics.

CNG does not make your car cheaper to buy or nicer to drive. It makes it cheaper to run, and only if you run it enough.

What it feels like to drive

A well-installed sequential kit is close to invisible below about 3,000 rpm, which is where Indian driving mostly happens. Above that, and particularly with the air conditioning on and four aboard, the difference appears: gas engines make slightly less torque, and a car that was already working at 4,000 rpm to pass a bus now needs a gear. An automatic hides most of it behind the converter.

The other change is in character rather than numbers. The engine runs more quietly and smells of nothing at a signal, and there is always a second tank underneath the first: when the gas gives out mid-journey the car simply continues on petrol. Range is the quiet limitation. A cylinder holds a modest amount of energy, so the gas-only distance between fills is well short of the petrol tank’s, and a trip has to be planned around the two added together.

The safety question, properly

The cylinder is engineered to a pressure well above working pressure, tested individually and stamped with a certification that expires. A shut-off solenoid closes when the engine is off, a crash-activated cut-off isolates the supply on impact, and gas is lighter than air, so a leak disperses upward rather than pooling the way petrol does — which is why an installation that blocks a ventilation path is the wrong way to do this job.

The failure mode in India is not engineering. It is a kit fitted outside the manufacturer’s network, into a car whose warranty then discovers a dispute, and a re-certification that quietly lapsed three years ago. Buy a factory car, keep the paperwork, and the argument about CNG safety is an argument about somebody else’s installation.

What to check before you commit

Our catalogue lists no factory CNG variants among the cars we publish figures for, which is itself informative: CNG makes most sense in the volume hatchback and sedan bracket, and Maruti offers gas options across its line-up in the wider market. So this is a check list rather than a pick, to apply to whichever showroom quote you are holding.

Ask which of the three systems it is — factory kit, dealer-fitted, or a recommended workshop. The warranty and servicing answers follow automatically.

Ask what remains of the boot. Then look, physically, at the cylinder in the car you are being quoted.

Ask how many pumps are on your two regular routes, not how many are in your city. The one you pass daily is the only one that will get used.

Ask what the kit does to the serviced price — the re-certification cycle, the valve-clearance interval, and whether the local network stocks the gas-specific parts at all.

Ask the annual-distance question out loud. Below roughly the middle of the private-car range in India, a CNG car is an exercise in patience for a saving that pays for one holiday every few years. Above it, and inside a city with a dense pump network, it is the cheapest way to run a combustion engine that anyone has found without a charging point.