Several undergraduate and graduate students and I are building a small engine powered by Compressed Natural Gas (CNG). It is a small and straightforward affair, similar to what is found in small equipment such as chainsaws and lawn mowers. I wanted to experiment with Liquefied Natural Gas (LNG) as fuel storage (due to it's 600x greater density), but I obviously ran into some logistical issues. I can easily purchase CNG, but LNG is obviously not on the menu in any quantity smaller than a supertanker, for obvious reasons. But I was wondering if there is a feasible way to condense gas from a CNG container into LNG for experimental purposes (I intend to produce only 1-2 L of LNG at a time). The cost or energy economics of the liquefaction process are not important as this is just an experimental rig. And this is not paid/funded research, or even official research at all, so keeping the costs below around five thousand USD is important. If it works it will likely develop into research, but that's a bit forward in the future. I'm just wondering if there's a SMALL machine I could purchase or a simple solution I could build myself. Thanks.