I have a R file running a Rmd file to generate a pdf (rmarkdown::render).
I use fancyhdr to tune the layout, header,... 
As it is dynamic, I also use parameters (params).
My question is, which command should I use to split the title in two lines ? 
I refer to that post, but none of the proposed solutions work. And I don't know if it is a question for R people or for those of Latex/Lualatex...
Here is my code in R, you may use your own path and fname (figure name) if you want to reproduce the code.
          rmarkdown::render("your path/Fig_generator.Rmd", 
                            output_file = file.path(your path, paste("test",".pdf", sep = "")),
                            encoding = "native.enc", 
                            params = list(
                              dyntitle = "This is a very very very very long line that I would like | to split where the | is",
                              dynsubtitle = "Some subtitle text",
                              dynfigno = "Fig. xx-x",
                              dynprojectname ="xxx / xxx",
                              dynimage = paste("your path/",fname,".","pdf",sep = "")))
Here is the code in Rmd:
---
#knit: (function(inputFile, encoding) { rmarkdown::render(inputFile, ) })
template: default-1.17.0.2.tex
output: 
  pdf_document:
    latex_engine: lualatex
    keep_tex: true
documentclass: article
mainfont: Arial
fontsize: 12pt
params:
  dyntitle:  !r dyntitle
  dynsubtitle:  !r dynsubtitle
  dynfigno:  !r dynfigno
  dynprojectname:  !r dynprojectname
  dynimage:  !r dynimage
title: "`r params$dyntitle`"
subtitle: "`r params$dynsubtitle`"
figureno: "`r params$dynfigno`"
projectname: "`r params$dynprojectname`"
image: "`r params$dynimage`"
---
\includegraphics[width=7.27in]{`r params$dynimage`}
Can someone help me with that, please ?