[Tex/LaTex] In IEEEtran, why does `fontspec` package automatically change the font type once loaded

font-encodingsfontsfontspecieeetranpackage-options

By default, the body text font for IEEEtran document class is like this:

enter image description here

When I load fontspec package, the font type changes:

enter image description here

What is the name of the new font? How can I retain the original IEEEtran main body font when loading fontspec? And how can I change it on demand to any type I desire (such as Bookman)?

\documentclass[]{IEEEtran}

\usepackage[T1]{fontenc}
\usepackage{fontspec}
\usepackage[]{mathtools}
\usepackage{bigfoot}


\begin{document}



    \title{Development of a Generalized PV Model in MATLAB/Simulink Using Datasheet Values}


    % Authors names and emails
    \author{Al-Motasem I. Aldaoudeyeh
        % Information about the authors (in the margins)
        \thanks{Al-Motasem I. Aldaoudeyeh is with the Department of Electrical and Computer Engineering, North Dakota State University, Fargo, ND, 58102 USA. email: almotasem.aldaoudeye@ndsu.edu}
    }



    % Make titles available (this command goes here for IEEE style)
    \maketitle
    % This command is important for both title and author names and information. Without it, none of the previously mentioned appears on the text.




    \begin{abstract}

        This paper proposes an improved single-diode modeling approach for PV modules suitable for a broad range of the PV technologies available today, including modules on tandem cell structures. After establishing the model (which has an overall of seven parameters), the paper devises a methodology to estimate its parameters using Standard Test Conditions (STC) data, Nominal Operating Cell Temperature (NOCT) data, and temperature coefficients values as provided in most manufacturers' datasheets. Simulation results and their comparison with a previous work show a very accurate prediction of critical points in the current-voltage characteristics curve. The precise prediction happens for both STC and NOCT conditions and the error in predicting maximum power point lies within $1\%$ limit, and the error in its corresponding voltage and current is almost always within $2\%$ limit. Further, for both maximum power point and open-circuit voltage, the statistical variance around manufacturer measurements due to temperature changes is demonstrated to be low for five various module technologies.

    \end{abstract}




    \begin{IEEEkeywords}

        temperature coefficients, thin film cells, tandem cells, photovoltaic arrays, I-V curves, evolutionary algorithms, parameterization

    \end{IEEEkeywords}





    % ===== End of IEEE Format =====
    % =========================================================
    % =========================================================
    %\end{comment}

    \section{Introduction}
    \label{section:introduction}



    \textit{Solar photovoltaics (PV)} is one of the fastest growing power industries in the world thanks to its appealing merits,like the widespread accessibility to natural solar resources, high reliability, easy integration into buildings and structures, fast installation, modularity, and predictable annual output. Between 2000 and 2013, total PV production has been experiencing annual growth rates between $40\%$ and $90\%$ reaching an overall increase of two orders of magnitude. By 2010, thin film PV technologies accounted for around $13\%$ of the PV market share. The values were distributed as follows: $5\%$ for a-Si, for $2\%$ Copper indium di-selenide/Copper indium gallium selenide (CIS/CIGS), and $6\%$ for Cadmium Telluride / Cadmium sulfide (CdTe/CdS). However, most of the efforts in modeling PV modules focus on modules on crystalline silicon (c-Si) cells technology which exhibits characteristics closer to ideal cells. Thus, it is necessary to develop generalized models suitable for different types of technologies \footnote{this is a footnote test}.


\end{document}

Best Answer

The IEEEtran class has

% The IEEE uses Times Roman font, so we'll default to Times.
% These three commands make up the entire times.sty package.
\renewcommand{\sfdefault}{phv}
\renewcommand{\rmdefault}{ptm}
\renewcommand{\ttdefault}{pcr}

Doing \renewcommand{\rmdefault}{ptm} for typesetting in Times has been deprecated for about 20 years, but apparently this is not a sufficient timeframe for realizing it. For instance, math formulas will be typeset in Computer Modern with a very bad final effect, because the two fonts are visually incompatible.

You can get Times with fontspec, though: add

\setmainfont{TeX Gyre Termes}

to your document preamble. Math will still be in Computer Modern, but apparently the people at IEEE is satisfied with bad typesetting.

If you want better typesetting, add

\usepackage{newtxmath}

Example:

\documentclass[]{IEEEtran}

\usepackage{mathtools}
\usepackage{newtxmath}
\usepackage[no-math]{fontspec}

\setmainfont{TeX Gyre Termes}

\begin{document}

\title{Development of a Generalized PV Model 
  in MATLAB/Simulink Using Datasheet Values}

% Authors names and emails
\author{Al-Motasem I. Aldaoudeyeh%
  % Information about the authors (in the margins)
  \thanks{Al-Motasem I. Aldaoudeyeh is with the Department 
     of Electrical and Computer Engineering, North Dakota State 
     University, Fargo, ND, 58102 USA. email: almotasem.aldaoudeye@ndsu.edu%
  }%
}

% Make titles available (this command goes here for IEEE style)
\maketitle

\begin{abstract}
This paper proposes an improved single-diode modeling 
approach for PV modules suitable for a broad range of 
the PV technologies available today, including modules 
on tandem cell structures. After establishing the model 
(which has an overall of seven parameters), the paper 
devises a methodology to estimate its parameters using 
Standard Test Conditions (STC) data, Nominal Operating 
Cell Temperature (NOCT) data, and temperature coefficients 
values as provided in most manufacturers' datasheets. 
Simulation results and their comparison with a previous 
work show a very accurate prediction of critical points 
in the current-voltage characteristics curve. The precise 
prediction happens for both STC and NOCT conditions and 
the error in predicting maximum power point lies within 
$1\%$~limit, and the error in its corresponding voltage 
and current is almost always within $2\%$ limit. Further, 
for both maximum power point and open-circuit voltage, 
the statistical variance around manufacturer measurements 
due to temperature changes is demonstrated to be low for 
five various module technologies.
\end{abstract}

\begin{IEEEkeywords}
temperature coefficients, thin film cells, tandem cells, 
photovoltaic arrays, I-V curves, evolutionary algorithms, 
parameterization
\end{IEEEkeywords}


\section{Introduction}
\label{section:introduction}

Some math just to see what happens: $a+2b=3c$.

\textit{Solar photovoltaics (PV)} is one of the fastest 
growing power industries in the world thanks to its 
appealing merits,like the widespread accessibility 
to natural solar resources, high reliability, easy 
integration into buildings and structures, fast installation, 
modularity, and predictable annual output. Between 2000~and~2013, 
total PV production has been experiencing annual growth rates 
between $40\%$ and $90\%$ reaching an overall increase of two 
orders of magnitude. By 2010, thin film PV technologies accounted 
for around $13\%$ of the PV market share. The values were 
distributed as follows: $5\%$ for a-Si, for $2\%$ Copper 
indium di-selenide/Copper indium gallium selenide (CIS/CIGS), 
and $6\%$ for Cadmium Telluride / Cadmium sulfide (CdTe/CdS). 
However, most of the efforts in modeling PV modules focus on 
modules on crystalline silicon (c-Si) cells technology which 
exhibits characteristics closer to ideal cells. Thus, it is 
necessary to develop generalized models suitable for different 
types of technologies\footnote{this is a footnote test}.

\end{document}

enter image description here

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