The following are measurements of the thermal efficiency of n=15 diesel engines made by a prominent manufacturer: 30.7, 35.0, 34.9, 33.6, 28.7, 32.1, 29.0, 31.4, 31.7, 31.8, 33.6, 29.7, 33.4, 28.2, 31.6.
Construct a 99% confidence interval of the average thermal efficiency of such diesel engines.