Prior Art of Otto's four stroke engine
When checking the novelty of an invention, the invention, expressed in the claims of the patent application, is compared with the prior art existing before the day of the patent application has been filed.
What is prior art?
Prior art is any evidence that your invention is already known.
Prior art does not need to exist physically or be commercially available. It is enough that someone, somewhere, sometime previously has described or shown or made something that contains a use of technology that is very similar to your invention.
source: EPO inventor's handbook
It does not matter if the prior art was known to the inventor or not. It does not matter if it is disclosed in a language the inventor is familiar with. If an inventor wants to be sure that his invention is not disclosed already, he should conduct a novelty search first.
In the case of Otto's four stroke engine the following prior art was filed by the parties requesting a nullification of the patent DE 532. Apart from the Lenoir engine, of which he had read in the newspapers, we are not sure if Otto had knowledge of any of the other prior art:
(3) The Hock engine
An invention is not new when a single prior art document shows all technical features used by the invention.
An invention is not inventive, if any combination of two (not more!) prior art documents shows all features used by the invention. This is a rule of thumb, the topic of assessing inventive step is far more complex.