Nitrogen oxide (NOX) emissions can be reduced using internal engine technology by cooling some of the exhaust gas, which is then redirected back into the charge air. This results in the reduction of the combustion temperature and less nitrogen oxide is produced. This process is known as exhaust gas recirculation (EGR) and is one of the principal methods used to reduce nitrogen oxide emissions from diesel engines. mtu has been developing this important technology and the functions and components associated with it since the beginning of the 1990s. It was first introduced in series production in mid-2011 for Series 4000 Oil and Gas engines in hydro-fracking applications (EPA Tier 4 emissions standard). It was likewise intro - duced in rail engines subject to EU IIIB emissions regulations which came into force in 2012.