Engelhard was influenced by Munich's leading poster artist Ludwig Hohlwein (1874-1949), who I wrote about here. That influence seems to have been greatest before 1920. Thereafter, Engelhard's style became more versatile.
Examples of his work are below, with caption translations by me.
"Friemann Kaffee" - 1926 - by Ludwig Hohlwein
Compare to the next Engelhard posters below.
An Engelhard poster in Hohlwein's style.
And yet another. Not exactly Hohlwein, by very nice.
Near the end of the Great War. The airplane looks like a Fokker Eindecker of 1915 vintage.
The style is no longer strongly Hohlwein's.
More of a conventional representational style.
Here Engelhard does Art Deco, imitating French fashion illustration style.