XslCompiledTransform uses UTF-16 encoding

I have the following code, which I want to output xml data using the UTF-8 encoding format. but it always outputs data in UTF-16 :

        XslCompiledTransform xslt = new XslCompiledTransform();

            xslt.Load(XmlReader.Create(new StringReader(xsltString), new XmlReaderSettings()));

            StringBuilder sb = new StringBuilder();

            XmlWriterSettings writerSettings = new XmlWriterSettings();
            writerSettings.Encoding = Encoding.UTF8;
            writerSettings.Indent = true;

            xslt.Transform(XmlReader.Create(new StringReader(inputXMLToTransform)), XmlWriter.Create(sb, writerSettings));