Emulating book-class-sectioning using titlesec and modifiying it

Question

In short:

• Prerequisite: Need to emulate book-class for \part and \chapter using titlesec
• Change both to hang, start on same page
• Add titlerule to \chapter

---

I want to optimize my rather hacked template.

First, as it isn't possible to do the changes below using titlesec without defining the sectioning title/head as a whole, I need it to be exactly like predefined by the standard book-class. That needs to be the starting point.

Then I want to use titlesec to make \part and \chapter "hanged" (like \titleformat{\part}[hang]) and on the same page with the following chaptertext:

Not this:

      Part A     
     Parttitle

-------newpage--------
Chapter 1

Chaptertitle

but this:

A Parttitle

1 Chaptertitle

At last I need to replace the following newcommand

\newcommand{\addchapterunderline}{\vspace{-4ex}\hrule height 1.5pt \vspace{7ex}\thispagestyle{empty}}

for \chapter AND \chapter*, wich I utilized like this until now:

\chapter[Test]{Test}
\addchapterunderline

The final structure should look like this using \titlerule[1.5pt]:

A Parttitle

B Chaptertitle
________________________

Lorem ipsum ...

In this example I already tried my best (<-- still beginner). But I cannot prove whether this is book-class-like with only my changes above, as I could not find the heading-definitions in the book.cls:

\documentclass{book}

        \usepackage[ngerman]{babel}             
        \usepackage[T1]{fontenc}
        \usepackage{lmodern}
        \usepackage[ansinew]{inputenc}      
        \usepackage{tgheros}                            
        \usepackage[expert]{mathdesign}     
        \usepackage{amssymb}                            
        \usepackage{ae,aecompl}                     
        \usepackage{charter}                            

%\newcommand{\addchapterunderline}{\vspace{-4ex}\hrule height 1.5pt \vspace{7ex}\thispagestyle{empty}} %I don't want to use this

\usepackage{titlesec}

\titleclass{\part}{top}
\titleformat{\part}[hang]
  {\usefont{T1}{qhv}{b}{n}\selectfont\huge} % global formatting (number and title) 
    {\thepart} 
    {20pt} %spacing between number and title
    {\huge}
\titlespacing*{\part}
   {0pt}% left
   {1cm}% before
   {1.2em}% after
\titleclass{\chapter}{straight}
\titleformat{\chapter}[hang]
  {\usefont{T1}{qhv}{b}{n}\selectfont\huge} % global formatting (number and title)
    {\thechapter}  % label: number and its formatting
    {20pt} %spacing between number and title
    {\huge}[\vspace{2ex}{\titlerule[1.5pt]}]
\titlespacing*{\chapter}
   {0pt}% left
   {1cm}% before
   {6.2em}% after



\begin{document}
\part{This is a Testpart}
\chapter{This is a Testchapter}
\chapter*{This is a unnumbered Testchapter}
\end{document}

Answer 1

There's no information on the formatting for other chapters in the same part, so I assumed they should all have the same behaviour (in particular, they should not start on a new odd page); there was also no information on the vertical space before part titles, so I assumed some settings (which can be easily changed, if necessary). Here's one possibility:

\documentclass{book}
\usepackage{titlesec}

\renewcommand\thepart{\Alph{part}}
\titleclass{\part}{top}
\titleclass{\chapter}{straight}

\titleformat{\part}[hang]
  {\normalfont\huge\bfseries}{\thepart}
  {1em}{\Huge}
\titleformat{\chapter}[hang]
  {\normalfont\huge\bfseries}{\thechapter}
  {1em}{\Huge}[\vskip8pt{\titlerule[1.5pt]}]

\titlespacing*{\part}
  {0pt}{50pt}{40pt}
\titlespacing*{\chapter}
  {0pt}{0pt}{20pt}

\begin{document}

\part{Test Part One}
\chapter{Test Chapter One}

\end{document}

After a comment to the original question, only the first chapter should have the mentioned behaviour; other chapters of a same part should issue the standard \cleardoublepage command from book.cls; this can be automatically achieved using a conditional test and the etoolbox package:

\documentclass{book}
\usepackage{titlesec}
\usepackage{etoolbox}

\renewcommand\thepart{\Alph{part}}
\titleclass{\part}{top}
\titleclass{\chapter}{straight}

\titleformat{\part}[hang]
  {\normalfont\huge\bfseries\raggedright}{\thepart}
  {1em}{\Huge}
\titleformat{\chapter}[hang]
  {\normalfont\huge\bfseries\raggedright}{\thechapter}
  {1em}{\Huge}[\vskip8pt{\titlerule[1.5pt]}]

\titlespacing*{\part}
  {0pt}{50pt}{40pt}
\titlespacing*{\chapter}
  {0pt}{0pt}{20pt}

\pretocmd{\chapter}{\ifnum\value{chapter}>0 \newpage\thispagestyle{plain}\cleardoublepage\else\fi}{}{}{}

\begin{document}

\part{Charakterisierung des französischen Automobilwesens in Berlin} \chapter{Charakterisierung des französischen Automobilwesens in Berlin} \chapter{Charakterisierung des französischen Automobilwesens in Berlin}

\end{document}

An image of the first page containing the part title: