We have overriden the default mapping by adding:

<indexConfigurations>
 <defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
    <fieldMap type="Sitecore.ContentSearch.SolrProvider.SolrFieldMap, Sitecore.ContentSearch.SolrProvider">
        <fieldNames hint="raw:AddFieldByFieldName">
              <field fieldName="vehiclename" returnType="string" indexType="string">
                <analyzer type="Sitecore.ContentSearch.LuceneProvider.Analyzers.LowerCaseKeywordAnalyzer, Sitecore.ContentSearch.LuceneProvider" />
              </field>
            </fieldNames>
       </fieldMap>
 </defaultSolrIndexConfiguration>
</indexConfigurations>

However, while querying on this field, it is case sensitive. Kindly help