When we talk of biodiesel fuels today, we are generally talking about fatty acid methyl esters (FAME). Neat vegetable oils or animal fats as such are not suitable for high-speed diesel engines. Biodiesel esters are usually made in a transesterification process from vegetable oils, such as soy, rapeseed, sunflower, etc. Fossil methanol is typically used as an alcohol in the process, and sodium or potassium hydroxide as a catalyst. Ethanol could be used to produce fatty acid ethyl esters (FAEE). The side-product of the process is glycerol. The process is quite simple and the product can be rather low in cost. The transesterification process is basically limited to oils and fats as feedstocks, and the product is always an ester. Due to certain end-use problems with FAME, the current maximum concentration is limited, e.g. up to 7.0% in the European EN590:2009 specification for diesel fuel. FAME biodiesel generally reduces CO, HC, and PM emissions, but increases NO_x emissions.