For 3.2, the default storage engine is wiredTiger, but it isn't really configurable (it's a string in the storage_options.h header).

This leads to the unfortunate reality that 32 bit builds of mongodb 3.2 don't actually run without having --storageEngine passed to them. It would be preferred to have the default engine always runnable (so make mmapv1 the default on 32 bit) and further, it makes sense to have it be configurable (so system packagers could make the call where we hadn't yet)