**1. Introduction**

Spark ignition engines, which have been used in motor vehicles for over 140 years, still have a relatively low effective efficiency of conversion of fuel combustion energy into mechanical energy of rotation, despite significant developments. The best spark ignition engines used in motor vehicles currently obtain a peak effective efficiency of about 40% [1], what means that in the best case scenario, 60% of the energy delivered with the fuel is lost. In the case of low engine load, the loss of energy may exceed 80% of the value resulting from fuel combustion. To solve this and other problems, the European Commission has initiatives aimed at obtaining automotive engines with significantly increased efficiency. A peak thermal conversion efficiency of more than 50% is expected for engines developed in the Horizon 2020 Program [2]. However, at the moment this is only one of the targets for R&D projects which will be finished in the years 2020 to 2021, so an implementation of the developed engines for mass production will undoubtedly take place in the next few years. There are many different methods to improve this situation. It is worth mentioning here the most important methods, such as direct injection, thermal insulation of the combustion chamber, turbocharging, downsizing, downspeeding, the implementation of exhaust energy recovery using thermoelectric generators [3], or the advanced mechatronic systems, such as variable valve timing or continuously variable valve lift.

Among the ways to improve the effective efficiency (fuel conversion efficiency) of an internal combustion engine, an interesting method is to increase the expansion ratio to be significantly higher than the compression ratio of the engine. This leads to a significant increase in the thermal efficiency of the theoretical comparative cycle of the engine [4], but also gives tangible benefits in the effective efficiency of the engine performing such a cycle [5]. Figure 1 presents the concept of using an increased expansion ratio to improve the efficiency of an internal combustion engine. In a particular case, the working medium pressure can be decreased to match the ambient pressure.

**Figure 1.** Potential of exhaust energy recovery using additional expansion.

Differentiation of the compression and expansion ratios of the engine may be accomplished in several ways. The first well-known solution was developed by the English engineer James Atkinson in the second half of the nineteenth century [6]. The engine concept by Atkinson was characterized by shorter intake and compression strokes than the power and exhaust strokes. This solution, however, was a significant disadvantage due to the sophisticated crank mechanism with respect to classical four-stroke engine. The increased interest in construction solutions of engines with increased expansion occurred in the second half of the twentieth century, when the concept by Ralph Miller [7] was developed. This concept was based on the classic four-stroke engine, but with suitably modified valve timing so as to ge<sup>t</sup> a reduced effective length of the compression stroke—Late Intake Valve Closing—or by reducing the degree of filling of the cylinder in a supercharged engine with an additional charge cooling by expansion in the intake stroke as a result of an Early Intake Valve Closing strategy. Engines performing an Atkinson/Miller cycle, but based on the classic four-stroke engine, have been used in motor vehicles since the mid-1990s [8]. Due to the specific characteristics of this type of solution, they often occur in vehicles with hybrid drive systems, diesel locomotives, and in industrial applications [9], where the engine operates with an average high load. The Atkinson cycle is also used in modern engines for conventional propulsion systems in order to reduce pumping losses in the low load region [10]. Research and development of engines performing Atkinson/Miller cycles are also carried out in Poland at several universities [11,12].

An entirely different approach to the problem of an internal combustion engine with an expansion ratio greater than the compression ratio was applied in the five-stroke engine developed according to the concept by Gerhard Schmitz [13,14]. In this engine, a significantly increased expansion is done after the exhaust process by a second expansion of the working medium in a separate cylinder. This cylinder has a displacement volume that is about twice as high in comparison to the cylinder where the combustion occurs. The engine provides practically unlimited possibilities in terms of establishing the compression and expansion ratios, without losing a significant part of the displacement volume of a working cylinder, as it is a classic engine implementing a Atkinson/Miller cycle.

### *Five-Stroke Engine Developed at Cracow University of Technology*

In the years 2012–2014 at the Cracow University of Technology (CUT), an engine which worked according to the concept of Gerhard Schmitz [13] was developed. The main difference of this engine is the fact that it was not designed from scratch as a new engine, but its design was based on the classic four-stroke engine [15]. The tested engine was based on a mass-produced four-cylinder turbocharged spark ignition engine and a displacement volume of 2.0 L [16]. The concept of retrofitting the classic engine to five-stroke is shown in Figure 2.

**Figure 2.** Scheme of the engine with an additional expansion process developed at Cracow University of Technology.

As may be seen in the figure above, the engine after modification has two fired cylinders operating in a classical four-stroke mode with a phase shift of 360 Crank Angle Degrees (CAD), while cylinders No. 2 and 3 are permanently connected by the channel in the cylinder head where the process of additional expansion of the working medium occurs. These cylinders work as one in the two-stroke mode and are filled by exhaust gasses alternately from cylinder No. 1 and cylinder No. 4. The research engine has 4 valves per cylinder.

During the development of the engine with the additional expansion of exhaust gas, a thermodynamic cycle taking into account the specific characteristics of the tested engine was proposed [17]. Extensive simulation studies of a similar engine, but with a single-cylinder of additional expansion, was also carried out by Li et al. at Shanghai Jiao Tong University [18,19]. Initial simulation studies on a five-stroke engine were also performed by Palanivendhan et al. [20].
