Why doesn't the petrol in my car ignite even when exposed to an open flame?

To say that petrol is flammable is a bit inaccurate. The fumes of petrol, the gas created when petrol evaporates is extremely flammable. The liquid is not. If it is very cold outside, your petrol might not be evaporating at all, which would make it difficult for you to light it on fire. Either way, I'd recommend that you stop trying to light the petrol in your car on fire with an open flame.