All of the car chargers I have seen are very simple devices designed to take the 12 volt DC input from an automobile socket and reduce it down to 5V DC and feed it out through a USB type A output socket. More expensive models may also limit the output amperage to prevent overloading some devices. But their main purpose is to provide an interface between the automobile and a
USB type A sockets.
Laptop power supplies (a.k.a. power bricks) are far more complex that, with appropriate socket adaptors, accept 100 - 240 volt, 50 - 60 cycle AC, convert it to the voltage and power your MacBook Air is designed to work with. I don't know what the voltage is — perhaps 5 volts but it could be more or less and a maximum power of 45 Watts and their output is via a dedicated power cord
and cannot be charged via the USB port. (
NOTE: Recent MacBook Pros with Thunderbolt3/USB C ports use that instead of a dedicated power connection.)
What you need is an AC car power
inverter similar to
this,
this, or
this and then plug your laptop power supply into that.