Variable voltage 110-220v input AC to DC 12v 15a 180W output power adapter with cigarette light socket
Can someone educate me how a variable voltage 110-220v AC input can be converted to identical 12 V, 15 A, 180 W output without having to flip a switch at the input end?


This is a common feature on many SMPS (switched mode power-supplies) units. A high-frequency switching circuit adjusts the high-voltage pulses fed into a small transformer to feed enough energy through to maintain the correct voltage on the output over a wide range of input voltages and output currents.


  Would the DC output be steady enough to power a tire pressure compressor without damaging it? 


Motors present a difficult load as the starting current is very high so we can't say.


  I have also seen products that works only with 110 V or 220 V (not variable) AC input to achieve same output results. Is the variable voltage input product making a false claim?


There is no reason to suspect this if buying from a reputable supplier.



Note: 'V' or 'volt', 'A' or 'ampere', 'K' or 'kelvin' (but 'k' for kilo). SI units named after a person are lowercase when spelled out but their symbols have the first letter capitalised.