Full-load Miller cycle with ethanol and EGR: Potential benefits and challenges

Abstract

The Miller cycle, also known as the over expanded cycle, is a well-known internal combustion engine process since 1940. The main purpose of the Miller cycle is to increase in efficiency based on longer expansion ratio. The longer expansion ratio can be achieved both by early or late intake valve closure enabled by distinct variable valve actuation mechanisms. Turbocharger and intercooler are used frequently to increase the intake air density and to recover the power lost due to reduced intake stroke. With the growing environmental concerns regarding climate change, the use of biofuels such as ethanol is a promising alternative. Compared to gasoline, ethanol has higher knock resistance and provides better charge cooling effect, which enables the use of higher compression ratio or increased boost pressure. Despite the potential benefits, this concept is still not widely available due to technological challenges. The need for special valve train configurations and high pressure ratio superchargers are some of the issues pointed out in this article. This paper presents a detailed 1-D simulation analysis of a full-load Miller cycle spark-ignited engine when running on hydrous ethanol in different configurations of valve train and supercharging. The effects of intake valve closure timing as well as camshaft profiles, charge dilution through EGR or excess air, burn duration and in-cylinder temperatures were investigated. A detailed assessment of the main losses was carried out and several possible arrangements were studied. Diesel like engine brake efficiency of more than 40% could be achieved when applying the Miller cycle concept.