Various aspects of research of the SI engine with an additional expansion process

The paper presents an analysis of the results of the both experimental results and theoretical works on the SI engine with additional expansion of exhaust gases, also known as five-stroke engine. The engine like this was constructed at Cracow University of Technology as a retrofitted in-line four cylinder engine in which outer cylinders (1st and 4th) work as fired cylinders and inner cylinders (2nd and 3rd) work as volume for the additional expansion. The aim of development of such an engine is to gain higher energy recovery ratio of the combusted fuel through the second expansion of exhaust in a separate cylinder. The operating parameters of the engine in various versions were analyzed: as naturally aspirated, supercharged using a turbocharger with a waste-gate valve and a turbocharger with variable nozzle turbine. Selected results of the indicating measurements of the engine with special emphasis on the indicated thermal efficiency were presented. The results pointed out the directions of further optimization of the engine. These results are all the more important, because according to the author's knowledge, the research on the real object of this type are carried out in only one science center in the world besides Cracow University of


Introduction
The use of internal combustion engines to drive vehicles is inevitably associated with emissions of toxic exhaust gas and carbon dioxide into the atmosphere.The number of motor vehicles on the roads of the world is growing year by year.In order to reduce pollution of the natural environment, more stringent emission limits for toxic gases and CO 2 emissions are introduced.The vehicle approval testing procedures are changed to match the actual road conditions [1,2].However, despite the introduction of advanced technical solutions for the engines themselves as well as the exhaust aftertreatment systems, it is not possible to completely eliminate the problem of emissions from the powertrains with combustion engines.Consequently, interest in electric vehicles has increased over the past 20 years [3].The pure electric drive for vehicles has the advantage that the vehicle does not emit toxic components or CO 2 in the area where it is used.Some negative connotations are connected with the way of generating electricity in a given area [4].In any case, it is true that the use of electric cars in crowded city centers will significantly improve the quality of the air there.
However, electric drives for road vehicles encounter many problems in their development.The most important problem is the extremely limited capacity of electric energy storage system using contemporary batteries, which in turn causes very significant problems in obtaining a range acceptable to the user while maintaining a reasonable price for such a vehicle [5].The next constraint in development is the long battery charging time and the troubles of developing a common standard of charging device for all electric car manufacturers [6].Such a state of affairs makes that a reasonable solution for the coming years is to improve and further disseminate the various hybrid drive systems for road vehicles [7].First of all the use of a hybrid system gives the ability to reduce fuel consumption through energy recovery during braking and enables the engine to operate in a limited area of the operational map in which it is highly efficient.On the one hand, electrical systems are developed.Innovative high-performance electric motors are emerging, innovative electronic solutions such as SiC-based transistors are used in inverters and voltage converters.On the other hand, internal combustion engines for hybrid drive systems are also subject to continuous improvement [8].

Engines with an extended expansion process -the background
As with all automotive combustion engines, also for engines for hybrid vehicles, the development is now primarily driven by emission reduction and fuel consumption considerations.The engines of hybrid drive systems work in slightly different regime than in conventional propulsion systems.This causes that they can be optimized for a narrower area of the work map, to work mainly to make bett fuel that is lo is to use an gases in rela comparison o SI engine an to the ambien As it is extended exp of heat sup additional po significant i working acco The first engine includ the intake an power (expa presented a supercharged intake valve in addition t reduced the e is achieved b what was us 90s of the 20 expansion of which result compression expansion ra years, the p Miller cycle [18].Researc increased exp scientific cen However efficiency of not free of d maximum p volumetric ef air mixture compression not a loss-fre y in this optim ter use of the ost in the exha n increased ex ation to the of the thermo nd the cycle w nt pressure is odynamic cycle shown in the pansion of the pplied to the ortion of usefu increase in t ording to this attempts to im de the James A nd compressio ansion) strok a different d engine [ .One of the w rgy of the bur n a normal en o of the exh ratio [10][11][12] le of the class n of exhaust g a Figure 1.area of a ne e paper [13] ency of the pro ol Unit  -stroke engine hermal efficie of heat supp , the test e r with a boost alve (Fig. 9). 10 presents d engine with oost pressure tly feeding the he WG valve.y of the naturally process.
ed of 4200 rpm turn can be nomena in the w length to di course of η V ted in the brak nces between d the brake n the therma f the engine, ge of project.

eters of the arger with W ith VNT
al tests of th n indicated th respect to the hieved value w and, this is co ch work, on t velopment of rsion it wa s of additional increase the th should rem the engine, in tional expansi y lower than f e's theoretical ency along w plied, so that ngine was e t pressure con selected tes the turbochar (p bst ) was lim e boost pressu y aspirated eng m, η V starts to explained by e intake mani iameter ratio.research stag n-cylinder p sors were loca djacent additio h was carried valve.Meas oelectronic pre per [33] an an e with additio .The results f the engine not deviate f r a five-strok chanical effici than that of to utlines the re the fired cy cylinder asse brake mean e dicated mean expansion c he energy re ylinders and t from whic liver power t he analysis cylinders de t a load of ab ey receive pow e fact that, w expanded in ent pressure lculating the i f engine with ented in this pa g measuremen he engine wo Figure 15 • The use of a turbocharger with the WG valve enabled a significant increase in engine performance and a reduction in the BSFC to a level of the original 4stroke engine, however, the turbocharger did not provide sufficient boost pressure at low engine speeds.
• For the above reason, and because of the low exhaust gas temperature downstream the additional expansion cylinders, it was decided to use the VNT-type turbocharger.Initial tests of this solution have shown that the torque has increased significantly, but the BSFC was increased relative to the situation where the engine worked with the classic turbocharger with a WG valve.This is due to the increased back pressure of the exhaust gas in the additional expansion cylinders caused by the variable nozzle turbine.In the near future, a new turbocharger with a WG valve will be selected with a characteristic for the engine with a displacement of about 1.0 dm 3 , but with exhaust gases of lower energy than that of a conventional engine, due to the additional expansion.
• The indicated thermal efficiency η i of the engine with additional exhaust energy recovery is relatively high, is slightly higher than that obtained for the classic four-stroke engine.On the other hand, η m is lower, so the effective efficiency η e is now similar to that of the classical engine.
• Thermal efficiency would certainly increase thanks to the increasing of the cross section of the port connecting the cylinders No. 2 and No. 3, the reducing the volume of former combustion chamber in cylinders No. 2 and 3 and the shortening of transfer ports between fired and additional expansion cylinders.The simplest method is to increase the port cross-section between the additional expansion cylinders.would be slightly more difficult to reduce the chambers in additional expansion cylinders by using pistons with higher crown.However, the modification of the transfer ports between the fired and additional expansion cylinders is practically impossible for the cylinder head coming from an existing four-stroke engine.
• It is also planned to optimize the valve timing using the 0D/1D modeling and simulation.Currently the valve timing is adopted as for the original 4stroke engine, so an improvement of the efficiency of the engine with an additional expansion process is still possible.
• Once the amendments to the existing design have been made, an exhaust gas aftertreatment system will be selected taking into account the specifics of the operation of the engine with an additional expansion (i.e.mainly low temperature downstream the turbine).
The study results show that the five-stroke engine developed at Cracow University of Technology achieves good performance, especially taking into account that it has been developed on the basis of an existing fourstroke engine, what creates some constraints (e.g. two separate cylinders for the additional expansion processes and the cylinder head design).The best results are obtained when working at high load and at about 3000 rpm.The result is that the engine would not be a good source of propulsion for the vehicle with classic drive system, but it will be suitable for applications to vehicles with hybrid drive of series type or extended range electric vehicle, or for stationary applications.A further development the design is planned in the nearest future to increase the fuel conversion efficiency of the engine while maintaining the stoichiometric air-fuel mixture to allow the use of a simple system of a classic three-way catalytic converter.

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