Hi PENKO,
So is an engine on the A321, with it's higher thrust rating more likely to fail than an engine on a 318?
No. It should have about the same reliability. The reason being, the higher thrust rated engine will be removed from service earlier. Hence the higher maintenance costs.
The CFM56-7B Turbofan Engine
"In 2007, CFM breathed new life into the CFM56-7B fleet by introducing the Tech Insertion production standard. Compared to the base CFM56-7B, this new configuration provides operators up to 1 percent better specific fuel consumption over the engine’s lifecycle, and
between 5 and 15 percent lower maintenance costs (depending on the thrust rating) through enhanced durability.