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How does a CNG car work?

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Quick answer

A CNG car stores natural gas compressed to about 200 bar in a sturdy cylinder, usually in the boot. A regulator lowers that pressure, and injectors feed the gas into the engine's intake air. A spark plug ignites the mixture, driving the pistons just as petrol would, and most cars can switch back to petrol.

A CNG car works by storing natural gas, mostly methane, in a strong tank at about 200 bar and feeding it through a pressure regulator into the engine, where spark plugs ignite it like petrol. Most are bi-fuel cars that can switch between CNG and a petrol tank.

Step by step

  1. A filling station compressor pushes natural gas into the car's cylinder until it reaches about 200 bar.
  2. When the driver selects CNG, a solenoid valve opens and gas flows through a high-pressure line.
  3. The regulator, warmed by engine coolant, reduces the gas pressure to a low and steady level.
  4. The engine computer times the gas injectors so each cylinder receives the right amount of fuel.
  5. The gas mixes with air, a spark plug ignites it, and the expanding gases push the piston down.

Key things to understand

  1. Compressed natural gas, mainly methane, is stored in a thick steel or composite cylinder at around 200 bar.
  2. A high-pressure pipe carries the gas to a regulator, which drops its pressure to a level the engine can use.
  3. Engine coolant warms the regulator, because rapidly expanding gas cools sharply and could otherwise freeze its parts.
  4. Electronically controlled injectors spray the low-pressure gas into the intake, where it mixes with air before entering the cylinders.
  5. A spark plug ignites the gas-air mixture, and the expanding gases push the pistons just as in a petrol engine.

Labelled diagram

  1. Cylinder

    Thick-walled tank stores methane compressed to about 200 bar, usually in the boot.

  2. Solenoid valve

    Opens electrically when the driver selects gas, releasing fuel into the line.

  3. Regulator

    Cuts pressure to a low steady level, warmed by engine coolant to prevent freezing.

  4. Gas injectors

    Spray metered gas into the intake air under control of the engine computer.

  5. Engine cylinders

    Spark plugs ignite the gas-air mixture to push the pistons and turn the crankshaft.

Gas path inside a CNG car

Real-world examples

  • Delhi's public buses and auto-rickshaws were moved to CNG to cut the city's air pollution from diesel.
  • Many taxis in Mumbai and Delhi are factory-fitted bi-fuel cars that start on petrol and switch to gas.
  • Some city bus fleets in Europe and the United States run on CNG to reduce local soot emissions.
  • Car makers sell factory CNG versions of small hatchbacks and sedans with the cylinder under the boot floor.

Common mistake

✗ Myth: A CNG cylinder is a bomb waiting to explode in a car.

✓ Fact: Approved cylinders are far stronger than the pressure they hold, are tested before fitting and at intervals, and have relief devices. The real risks come from uncertified kits or skipped inspections, so fitting and testing should follow official rules.

People also ask

How does cng work in a petrol car?
A CNG kit adds a gas cylinder, a pressure regulator, gas injectors and a switch to a petrol engine. When CNG is selected, the petrol injectors stop and the gas injectors take over, spraying methane into the intake air. The same spark plugs ignite it, so the engine runs on either fuel.
Why do cng cars start on petrol?
Many bi-fuel cars start on petrol and switch to gas automatically once the engine warms up. The regulator relies on hot engine coolant to stop the expanding gas from freezing it, and a warm engine also vaporises and burns the gas mixture more smoothly than a cold one.
What is the pressure in a cng cylinder?
Vehicle CNG cylinders are usually filled to about 200 bar, roughly 200 times normal atmospheric pressure, and are designed and tested to withstand much more. That high pressure is needed because natural gas is a gas at room temperature, so it must be squeezed hard to carry useful range.
Is CNG the same as LPG?
No. CNG is mostly methane squeezed into a cylinder as a gas, while LPG is propane and butane stored as a liquid at much lower pressure. Methane is lighter than air and rises if it leaks, whereas LPG is heavier and tends to pool near the ground.
Why does a car feel less powerful on CNG?
Gas takes up space in the intake that liquid petrol droplets would not, so slightly less air enters each cylinder and each combustion stroke releases a little less energy. Most drivers notice a modest drop in pickup when running on CNG.
Is a CNG cylinder safe in a crash?
Approved CNG cylinders are built far stronger than petrol tanks and are pressure-tested before use and at regular intervals. They carry safety valves that release gas if the tank overheats, and leaking methane rises and disperses quickly in open air.
Why is CNG considered cleaner than petrol or diesel?
Methane has a simple molecular structure, so it burns with very little soot and lower carbon monoxide and hydrocarbon emissions. It also releases somewhat less carbon dioxide per unit of energy than petrol, although it is still a fossil fuel.

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