This article covers key and representative developments in the area of high efficiency and clean internal combustion engines. The main objective is to highlight recent efforts to improve (IC) engine fuel efficiency and combustion. Rising fuel prices and stringent emission mandates have demanded cleaner combustion and increased fuel efficiency from the IC engine. This need for increased efficiency has placed compression ignition (Cl) engines in the forefront compared to spark ignition (SI) engines. However, the relatively high emission of oxides of nitrogen (NOx) and particulate matter (PM) emitted by diesel engines increases their cost and raises environmental barriers that have prevented their widespread use in certain markets. The desire to increase IC engine fuel efficiency while simultaneously meeting emissions mandates has thus motivated considerable research. This paper describes recent progress to improve the fuel efficiency of diesel or Cl engines through advanced combustion and fuels research. In particular, a dual fuel engine combustion technology called "reactivity controlled compression ignition" (RCCI), which is a variant of Homogeneous Charge Compression Ignition (HCCI), is highlighted, since it provides more efficient control over the combustion process and has the capability to lower fuel use and pollutant emissions. This paper reviews recent RCCI experiments and computational studies performed on light-and heavy-duty engines, and compares results using conventional and alternative fuels (natural gas, ethanol, and biodiesel) with conventional diesel, advanced diesel and HCCI concepts. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license