The upper nodes of the tree of principles of operation have no constructive details. The lower we go the more constructive details appear. There are nodes where most details of implementation are known, and nodes where most details are unknown. If one starts a new branch under a node, where most details of implementation are unknown, we say about revolutionary change in the principle of operation.
For example, Watt started his branch under a node where only the following details were known:
Diesel, conversely, started his branch of the internal combustion engines from a node where most details of implementation were known. Just the way the ignition takes place remained unspecified at this node. That is why the principle of operation of a diesel is not principally different from the principle of operation of a gasoline engine.
One may ask: what if a new branch is started from a node where it is not so easy to discern whether most details of implementation are known or not? Would this new branch be a revolutionary change in the principle of operation or not?
To answer this question in each particular case would require application of the theory of fuzzy sets. Basically experts in the known machines that dangle from this node have to be polled. They have to tell whether in their opinion the principle of operation of the new machine changed principally. Then votes for and against have to be counted and a decision is made.