What is an engine rev? What is its speed?
Engine "Revs" are a measure of the engines speed. Engines are rotating machines and the speed is measured in revolutions per minute - that is how many full turns the engine does every minute. The faster the engine runs, the more complete rotations it does per minute and the more power it produces.
For a diesel generator, the engine normally runs at a fixed speed. 1500 Revolutions per minute (RPM) or 3000 (RPM) revolutions per minute for 50Hz generator and 1800 RPM or 3600RPM for 60hz.
See how the number of alternator poles relates to the engine speed:
Can you hear the engine speed?
While you cannot actually "hear" the speed engines will make a different noise at different speeds. While it isn't really possible to know the engine speed exactly from the sound, it is possible to hear the changes in speed. At a constant speed, the engine will make a constant noise, when it slows down, the frequency of the noise becomes lower. When it speeds up the frequency becomes higher.
When a generator has load applied, then engine speed goes down and when it has load removed it goes up and you can hear the effect. In a car you can also hear the effect, especially from the exhaust tail pipe which will be come much louder at higher speeds as the flow of exhaust gas speeds up.