This paper provides eight new generalized convolutions of the Hartley transforms and considers the applications. In particular, normed ring structures of linear space $L^1(\mathbb{R}^d)$ are constructed, and a necessary and sufficient condition for the solvability of an integral equation of convolution type is obtained with an explicit formula of solutions in $L^1(\mathbb{R}^d)$. The advantages of the Hartley transforms and the convolutions constructed in the paper over that of the Fourier transform are discussed.