diff --git a/src/Spout/Writer/AbstractWriter.php b/src/Spout/Writer/AbstractWriter.php
index 57870f3..a7e9b31 100644
--- a/src/Spout/Writer/AbstractWriter.php
+++ b/src/Spout/Writer/AbstractWriter.php
@@ -69,18 +69,17 @@ abstract class AbstractWriter implements WriterInterface
      * By using this method, the data will be written to a file.
      *
      * @param  string $outputFilePath Path of the output file that will contain the data
-     * @param string $sheetName The custom name of the sheet
      * @return \Box\Spout\Writer\AbstractWriter
      * @throws \Box\Spout\Common\Exception\IOException If the writer cannot be opened or if the given path is not writable
      */
-    public function openToFile($outputFilePath, $sheetName = null)
+    public function openToFile($outputFilePath)
     {
         $this->outputFilePath = $outputFilePath;
 
         $this->filePointer = $this->globalFunctionsHelper->fopen($this->outputFilePath, 'wb+');
         $this->throwIfFilePointerIsNotAvailable();
 
-        $this->openWriter($sheetName);
+        $this->openWriter();
         $this->isWriterOpened = true;
 
         return $this;
diff --git a/src/Spout/Writer/Helper/XLSX/FileSystemHelper.php b/src/Spout/Writer/Helper/XLSX/FileSystemHelper.php
index daaa0dd..fddbb25 100644
--- a/src/Spout/Writer/Helper/XLSX/FileSystemHelper.php
+++ b/src/Spout/Writer/Helper/XLSX/FileSystemHelper.php
@@ -249,13 +249,10 @@ EOD;
     <Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml" PartName="/xl/workbook.xml"/>
 
 EOD;
-    
-    $escaper = new \Box\Spout\Common\Escaper\XLSX();
 
     /** @var Worksheet $worksheet */
     foreach ($worksheets as $worksheet) {
-        $worksheetName = $worksheet->getExternalSheet()->getName();
-        $contentTypesXmlFileContents .= '    <Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml" PartName="/xl/worksheets/' . $escaper->escape(strtolower($worksheetName)) . '.xml"/>' . PHP_EOL;
+        $contentTypesXmlFileContents .= '    <Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml" PartName="/xl/worksheets/sheet' . $worksheet->getId() . '.xml"/>' . PHP_EOL;
     }
 
     $contentTypesXmlFileContents .= <<<EOD
@@ -291,7 +288,7 @@ EOD;
         foreach ($worksheets as $worksheet) {
             $worksheetName = $worksheet->getExternalSheet()->getName();
             $worksheetId = $worksheet->getId();
-            $workbookXmlFileContents .= '        <sheet name="' . $escaper->escape($worksheetName) . '" sheetId="' . $worksheetId . '" r:id="rId' . $worksheetId . '"/>' . PHP_EOL;
+            $workbookXmlFileContents .= '        <sheet name="' . $escaper->escape($worksheetName) . '" sheetId="' . $worksheetId . '" r:id="rIdSheet' . $worksheetId . '"/>' . PHP_EOL;
         }
 
         $workbookXmlFileContents .= <<<EOD
@@ -319,13 +316,10 @@ EOD;
 
 EOD;
 
-        $escaper = new \Box\Spout\Common\Escaper\XLSX();
-
         /** @var Worksheet $worksheet */
         foreach ($worksheets as $worksheet) {
-            $worksheetName = $worksheet->getExternalSheet()->getName();
             $worksheetId = $worksheet->getId();
-            $workbookRelsXmlFileContents .= '    <Relationship Id="rId' . $worksheetId . '" Target="worksheets/' . $escaper->escape(strtolower($worksheetName)) . '.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"/>' . PHP_EOL;
+            $workbookRelsXmlFileContents .= '    <Relationship Id="rIdSheet' . $worksheetId . '" Target="worksheets/sheet' . $worksheetId . '.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"/>' . PHP_EOL;
         }
 
         $workbookRelsXmlFileContents .= '</Relationships>';
diff --git a/src/Spout/Writer/Internal/XLSX/Workbook.php b/src/Spout/Writer/Internal/XLSX/Workbook.php
index 060fbf8..1c521ea 100644
--- a/src/Spout/Writer/Internal/XLSX/Workbook.php
+++ b/src/Spout/Writer/Internal/XLSX/Workbook.php
@@ -62,23 +62,18 @@ class Workbook
     /**
      * Creates a new sheet in the workbook. The current sheet remains unchanged.
      *
-     * @param string $sheetName The custom name of the sheet
      * @return Worksheet The created sheet
      * @throws \Box\Spout\Common\Exception\IOException If unable to open the sheet for writing
      */
-    public function addNewSheet($sheetName = null)
+    public function addNewSheet()
     {
         $newSheetNumber = count($this->worksheets);
         $sheet = new Sheet($newSheetNumber);
-        if( !empty($sheetName) )
-        {
-            $sheet->setName($sheetName);
-        }
 
         $worksheetFilesFolder = $this->fileSystemHelper->getXlWorksheetsFolder();
         $worksheet = new Worksheet($sheet, $worksheetFilesFolder, $this->sharedStringsHelper, $this->shouldUseInlineStrings);
         $this->worksheets[] = $worksheet;
-
+        
         return $worksheet;
     }
 
diff --git a/src/Spout/Writer/Sheet.php b/src/Spout/Writer/Sheet.php
index d2a27d5..e1c00d6 100644
--- a/src/Spout/Writer/Sheet.php
+++ b/src/Spout/Writer/Sheet.php
@@ -33,7 +33,6 @@ class Sheet
      */ 
     public function setName($sheetName)
     {
-        $sheetName = str_replace(' ', '_', $sheetName);
         $this->name = $sheetName;
     }
     
diff --git a/src/Spout/Writer/XLSX.php b/src/Spout/Writer/XLSX.php
index 874ef06..1f4e44d 100644
--- a/src/Spout/Writer/XLSX.php
+++ b/src/Spout/Writer/XLSX.php
@@ -66,24 +66,15 @@ class XLSX extends AbstractWriter
     /**
      * Configures the write and sets the current sheet pointer to a new sheet.
      *
-     * @param string $sheetName The custom name of the sheet
      * @return void
      * @throws \Box\Spout\Common\Exception\IOException If unable to open the file for writing
      */
-    protected function openWriter($sheetName = null)
+    protected function openWriter()
     {
         if (!$this->book) {
             $tempFolder = ($this->tempFolder) ? : sys_get_temp_dir();
             $this->book = new Workbook($tempFolder, $this->shouldUseInlineStrings, $this->shouldCreateNewSheetsAutomatically);
-            if( empty($sheetName) )
-            {
-                $this->book->addNewSheetAndMakeItCurrent();
-            }
-            else
-            {
-                $worksheet = $this->book->addNewSheet($sheetName);
-                $this->book->setCurrentWorksheet($worksheet);
-            }
+            $this->book->addNewSheetAndMakeItCurrent();
         }
     }