“I drive on CNG most of the time. Petrol is only for starting the car or emergencies. So why should E20 petrol matter to me?”

It sounds logical—but it isn’t always true.

If your car is not E20 compatible, leaving E20 petrol in the fuel tank for months can create problems that many CNG owners don’t even think about.

In this article, we’ll explain why this happens, what the risks are, and a simple habit that can help protect your car.

What Is E20 Petrol?

E20 petrol contains:

• 20% Ethanol
• 80% Petrol

Ethanol is an alcohol-based fuel that helps reduce dependence on fossil fuels and lowers emissions. That’s why India is gradually shifting towards E20 fuel.

However, many older vehicles were not originally designed to handle higher ethanol blends.

The Biggest Myth Among CNG Car Owners

Many CNG owners believe:

• “I hardly use petrol.”
• “Petrol just sits in the tank, so it doesn’t matter.”
• “I only need petrol to start the engine.”

This is where the problem begins.

Even though your car runs mostly on CNG, it still depends on petrol for cold starts, certain driving conditions, and as a backup fuel.

If petrol remains unused in the tank for several months, its quality can deteriorate over time.

Why Can Stored E20 Petrol Become a Concern?

One important property of ethanol is that it is hygroscopic, meaning it absorbs moisture from the surrounding air.

If E20 petrol remains stored for a long period—especially in humid conditions—it can gradually absorb moisture.

Under certain conditions, this may lead to phase separation, where the ethanol-water mixture separates from the petrol.

When this happens, the fuel is no longer in its ideal condition for engine operation.

What Problems Can It Cause?

Although not every vehicle will experience these issues, long-term storage of degraded fuel may increase the risk of:

1. Corrosion

Moisture and ethanol can increase the possibility of corrosion in certain metal components of the fuel system.

2. Fuel Pump Wear

Old or degraded fuel may affect the performance and longevity of the fuel pump.

3. Injector Deposits

Poor-quality fuel can contribute to deposits or clogging in fuel injectors.

4. Starting Problems

Since CNG cars usually start on petrol, stale fuel may lead to rough starts or starting difficulties.

5. Reduced Fuel Quality

Petrol doesn’t stay fresh forever. The longer it sits unused, the more its quality can decline.

Will Every Car Face These Problems?

No.

If your vehicle is E20 compliant, its fuel system has been designed to work with higher ethanol content.

However, if your vehicle is not E20 compatible, taking a few extra precautions is a sensible approach.

What Should CNG Car Owners Do?

Fortunately, the solution is simple.

✔ Fill Only 5–7 Litres of Petrol

Instead of filling the tank completely, keep only enough petrol that you can use within a few weeks.

✔ Use Petrol Regularly

Drive your car on petrol occasionally for a few kilometres.

This helps circulate fresh fuel through the fuel system and prevents petrol from sitting in the tank for months.

✔ Avoid Long-Term Fuel Storage

If you rarely use petrol, avoid keeping a full tank for extended periods.

✔ Follow Your Owner’s Manual

Always check your vehicle manufacturer’s recommendations regarding E20 fuel compatibility and maintenance.

Should You Avoid Filling a Full Tank?

Not necessarily.

If you regularly drive on petrol, filling the tank isn’t usually a problem.

However, if:

• Your car runs almost entirely on CNG,
• Petrol is used only occasionally,
• The same fuel remains in the tank for several months,

then keeping a full tank may not be the best practice.

A Small Habit Can Save You Big Repair Costs

If your CNG car is not E20 compliant, follow these simple habits:

• Keep only 5–7 litres of petrol in the tank.
• Use that petrol regularly.
• Don’t let the same fuel sit in the tank for months.
• Drive on petrol occasionally before switching to CNG.

These small precautions can help maintain fuel system health and reduce the chances of avoidable repairs in the future.

Final Thoughts

Owning a CNG car doesn’t mean petrol is no longer important.

Your fuel system still relies on petrol, and if your vehicle isn’t designed for E20 fuel, storing the same petrol for long periods may not be ideal.

Using fresh petrol, rotating the fuel regularly, and avoiding unnecessary long-term storage are simple habits that can help keep your car running smoothly for years.

Disclaimer: The likelihood of fuel degradation, phase separation, or corrosion depends on several factors, including storage duration, temperature, humidity, fuel quality, and vehicle design. These issues do not occur in every vehicle or every situation. Always follow your vehicle manufacturer’s recommendations and consult your owner’s manual for the most accurate guidance.