how to cite the numeric reference at beginning of sentence

I want to citing the numeric references using BibTeX. Problem arise when I try to cite a reference at the beginning of a sentence. e.g., when I try to cite \citet{TrHe02} at the beginning of sentence the I got "(author?)[1]". Actually the citation should be "Trampert and Heijst [1]". But when I cite the same at end or inbetween the sentence using "citep" or "cite" then I got citation correctly as "[1]". How to get the correct citation as "Trampert and Heijst [1]" at he beginning of sentence?. Below I have given my latex script with content of reference file and file which i have used inside script with the link.

\documentclass[preprint,12pt]{elsarticle}

\begin{document}

\section{Introduction}

\citet{TrHe02}, when I use "citet" then i get the problem, but if cite it as ''citep'' or ''cite'' then i just got the numeric reference.

\cite{TrHe02}, here i used just use cite at the beginning i just got the numeric reference as [1]. Actually i should get as  Trampert and Heijst \citep{TrHe02}. I have manually written Trampert and Heijst.


From here details of the file which i have use while running the script.

In the latex script all this file elsarticle and elasrticle-num are standard which i have taken from website. This is the link

\verb+http://www.elsevier.com/author-schemas/the-elsarticle-latex-document-class+

I am using elsarticle-template-num.tex, elsarticle-num.bst and
elsarticle.cls
\bibliographystyle{elsarticle-num}
\bibliography{refs}
\end{document}


% refs.bib is my reference file and it contain

 @STRING{SCI={Science}}

@article{TrHe02,
AUTHOR ={Trampert, J. and Heijst, H. J. V.},
TITLE  ={Global Azimuthal Anisotropy in the Transition Zone},
YEAR ={2002},
JOURNAL    =SCI,
VOLUME={296},
NUMBER={},
MONTH  ={},
PAGES  ={1297-1299}
}

Have a closer read of that Elsevier page, you need to use elsarticle-num-names not elsarticle-num