Your ancestor would have been a member of the Genie de la Garde Impériale. (The Engineers of the Imperial Guard). The Imperial Guard was a regiment of men who served directly under Napoleon, protecting his person. The Imperial Guard was a prestigious division of Napoleon's Grand Armee. To enlist, the soldiers had to be very experienced in battle, and at least 5'8" tall; an impressive stature at that time. There were several regiments within the Guard. My ancestor was a Chasseur a Cheval de la Garde Impériale. He was a mounted guardsman.

Here is the main site link: http://centjours.mont-saint-jean.com/index.php

Search under the unités tab, then click on France. Scroll through the types d'unités and select Genie. From there, they are divided by battalion. It depends where your ancestor ranked within the Guard. Was he an officer?