**Marcin Noga**

Institute of Automobiles and Internal Combustion Engines, Division of Mechatronics, Cracow University of Technology, Al. Jana Pawla II 37, 31-864 Kraków, Poland; noga@pk.edu.pl; Tel.: +48-12-628-3688

Academic Editor: Jose Ramon Serrano Received: 1 February 2017; Accepted: 14 March 2017; Published: 17 March 2017

**Abstract:** The paper presents the results of research on the turbocharged spark ignition engine with additional exhaust expansion in a separate cylinder, which is commonly known as the five-stroke engine. The research engine has been constructed based on the four cylinder engine in which two outer cylinders work as the fired cylinders, while two internally connected inner cylinders constitute the volume of the additional expansion process. The engine represents a powertrain realizing an ultra-expansion cycle. The purpose of the study was to find an effective additional expansion process in the five-stroke engine. Cylinder-pressure indicating measurements were carried out for one of the fired cylinders and the additional expansion cylinder. The study was performed for over 20 different points on the engine operation map. This allowed us to determine a dependence between the pressure indicated in the fired cylinders and in the additional expansion cylinders. A function of the mean pressure indicated in the additional expansion cylinder versus a brake mean effective pressure was also presented. This showed a load threshold from which the work of the cylinders of additional expansion produced benefits for the output of the experimental engine. The issues of mechanical efficiency and effective efficiency of this engine were also discussed.

**Keywords:** spark ignition; efficiency; Miller cycle; Atkinson cycle; five-stroke engine; additional expansion process
