Now that you mention it I recall an article I read years ago about the engine development lab at Ford using a quartz or sapphire window in an experimental engine so that they could film the combustion wave while the engine was running.
Of course the same question arises how long the crystal window will last. With an experimental engine the cost of replacing the window is unimportant. The cost of replacing one in your daily driver is a different matter.
On your 'daily driver', it had better last the life of the engine. Replacement is almost guaranteed to be a giant pain in the neck. Industrial (artificial) sapphire and spinel are extremely durable materials, and very transmissive through the realm of reasonable LASER wavelengths:
