How to run app on App Engine for free

There are 28 free instance-hours per day for frontend instances only when you are using one of the standard environments or runtimes(python, java, php and go) and not when you're using app engine's managed VMs. All managed VM environments are based on Compute Engine and are all billable.

For a nodejs app, you're certainly using a managed VM.

To enjoy the free instance hours you'll need to use one of the standard runtimes.