Engines have three main stats in this game: Performance, Reliability and Efficiency. Each will affect your overall car performance in a different way. On top of that, Engines have Traits (special properties) and Engine Modes.
Performance will boost your overall car performance. Reliability, combined with other car parts, affects your overall car durability. Efficiency will dictate how much fuel consumption your engine has on each Engine Mode.
Traits will may give you additional points to certain driver stats, faster laps when certain race conditions are met, influence pit stop times and more.
Engine Modes affect your speed at the cost of fuel consumption and wear rate. Some modes will save lots of fuel but make your car slower while other will give you a huge speed boost at the cost of wearing your car much quicker.
There are 6 Engines available in the game but only one is available at the start. You need to progress through the Career Mode in order to unlock other Engines.
Mersault[]
This is the only engine available at the start of the game. Not very fast but has got good reliability to compensate. Doesn't excel at anything in particular but the presence of Medium Engine Mode means fuel management is rarely an issue here.
Kitano[]
You can unlock Kitano quite early in the game but the low reliability may put people off at the start.
Kitano excels at tyre management, especially after upgraded, but its Engine Modes mean you will need to do some micro management in order not to run out of juice mid-race.
Tectra[]
This Chinese manufacturer has got decent speed, superb fuel efficiency but very low reliability. In addition to that, it only has two Engine Modes, which means you will have to micro manage a lot during the race.
Tectra traits focus on performance on Out Laps so it may work best with multiple pit-stop strategies.
Hammer[]
American Engines have always been about raw power and Hammer is not any different. One of the fastest engines available but not very efficient.
The Durable Engine Mode available on Hammers remove the Red Zone of the engine, making it easier to manage car wear. This is very useful considering the below average durability of this engine.
Steinmann[]
High Reliability and decent Fuel Efficiency in exchange for Performance. Steinmann does not posses raw speed but it specializes in energy recovery, making it the best engine for ERS-based strategies.
As you would expect from German engineering, this engine is very reliable. So reliable it can even fix itself during pit stops.
Rossini[]
This is by far the fastest engine available to choose from. All this speed comes at the cost of Reliability though.
Although Rossini's Engine Modes may put some wear on the car, some traits can be unlocked which improve tyre wear while in Red Zone.