We have total 256 GB RAM for qliksense server. I can see by defailt engine is consuming 72GB without running any task so need to confirm how much RAM consumption by default take by engine service?
as soon as any app is open or still cached the ram usage will increase of the engine service will increase. If you are sure that no one is using a qlik sense app and no task is running that ram utilization shouldn't be that high.
Here is a pretty good community post on how to address high RAM usage.