Test of a New Low-Speed Compressed Air Engine for Energy Recovery

energies

Article Test of a New Low-Speed Compressed Air Engine for Energy Recovery

Mariusz Rz ˛asa 1 , Ewelina Łukasiewicz 2,* and Dariusz Wójtowicz 2

1 Department of Computer Science, Automatic Control and Informatics, Faculty of Electrical Engineering, Opole University of Technology, 45-758 Opole, Poland; [email protected] 2 Department of Thermal Engineering and Industrial Facilities, Faculty of Mechanical Engineering, Opole University of Technology, Prószkowska 76 Street, 45-758 Opole, Poland; [email protected] * Correspondence: [email protected]

Abstract: The paper presents a new design solution for the multi-cylinder compressed air engine, described in the PL 216801 patent. A characteristic feature of the engine is its double-piston operation with pistons working in pairs in opposition and a reciprocating movement in toroidal cylinders. The energy of compressed air was used more effectively in the described engine than in the solutions known so far. Comparing the engine built in accordance with the PL 216801 patent with the parameters of the MP165 and MP3000 engines, lower air consumption in relation to the power generated on the shaft is demonstrated. The described engine uses only one crankshaft and one straight complex shaft, which constitutes an innovative combination of pistons, while maintaining the same engine operation as in the case of two crankshafts operating with phase shift and working chamber shift. Such a solution results in a reduction in the harmful space occurring at the beginning of the power stroke to the value close to “zero”—the necessity to maintain the minimum distance between the pistons working in one cylinder when they are at their closest exists only to for the sake of collision-free operation—which is very desirable for the compressed engine operating with a shift of the working chamber. The mechanical efﬁciency of the engine has also been improved by guiding the pistons on the complex shaft, and the number of kinematic nodes was decreased by Citation: Rz ˛asa,M.; Łukasiewicz, E.; applying only three connecting rods supporting six pistons, which also makes it possible to improve Wójtowicz, D. Test of a New the power and mass relation by approximately 25% in comparison with the currently known engines Low-Speed Compressed Air Engine of similar power. for Energy Recovery. Energies 2021, 14, 1179. https://doi.org/10.3390/en14 Keywords: pneumatic motor; compressed air engine; energy recovery 041179

Received: 25 January 2021 Accepted: 19 February 2021 1. Introduction Published: 23 February 2021 One of the methods of storing and accumulating energy is compressing gases, most

Publisher’s Note: MDPI stays neutral often air. Compressed air engines are machines which use the energy of compressed air. with regard to jurisdictional claims in The basic advantage of compressed air engines is their relatively small mass in relation to published maps and institutional afﬁl- their power at relatively high permissible overloads. Even though there are many designs iations. of compressed air engines, there are far fewer low-speed engines [1–3]. The application of high-speed compressed air engines to drive machines and equipment often requires the application of speed reduction gears. Thus, it is appropriate to draw attention to the design of low-speed compressed air engines. The history of construction solutions for pneumatic motors dates back to the 17th Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. century. The ﬁrst pneumatic railway was built in 1678. The most dynamic development This article is an open access article of pneumatic motors began with the invention of the Polish engineer, Ludwik M˛ekalski. distributed under the terms and In the year 1870, M˛ekalskibuilt a compressed air piston engine which was used to power conditions of the Creative Commons locomotives in mines and drive trams in French cities, such as Vichy, Nantes or San- Attribution (CC BY) license (https:// Quentino [4]. The principle of its operation was based on the application of the check creativecommons.org/licenses/by/ valve, the opening of which was activated by a working piston (Figure1a). After the 4.0/). valve opened, the compressed air was pumped into the cylinder under high pressure, and

Energies 2021, 14, 1179. https://doi.org/10.3390/en14041179 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 15 Energies 2021, 14, 1179 2 of 15

valve opened, the compressed air was pumped into the cylinder under high pressure, and fromfrom that that moment moment the the work cyclecycle beganbegan (Figure (Figure1b). 1b). The The piston piston would would move move downwards downwardsunder under the force the force of expanding of expanding air. In air. the In ﬁnal the phase final ofphase piston of movement,piston movement, the outlet the channel, outlet channel,through through which the which excess the air excess escaped, air escaped, would open would (Figure open1 (Figurec). The second1c). The cycle second consisted cycle consistedof the piston of the returningpiston returning to the upperto the upper return return point, point, where where the next the working next working cycle would cycle wouldbegin. begin.

(a) (b) (c)

Figure 1.Figure Working 1. Working cycles of cyclesthe pneumatic of the pneumatic motor built motor by Ludwik built by M Ludwikękalski: M˛ekalski:( (a) the openinga) the of opening the of the inlet valve;inlet (b valve;) power (b stroke;) power (c stroke;) exhaust (c) stroke. exhaust stroke.

The historyThe historyof the application of the application of air comp of airressed compressed engines engines to drive to drive vehicles vehicles is, how- is, however, ever, muchmuch longer. longer. The Thesystem system of dynamic of dynamic tank pumping tank pumping during during the braking the braking of vehicles, of vehicles, developeddeveloped in 1892 inby 1892Robert by Hardie Robert Hardie[5,6], was [5, 6an], wasimportant an important invention. invention. It was based It was on based on invertinginverting the engine the work engine to work compressor to compressor work during work duringthe braking. the braking. Thanks Thanksto that, tosome that, some decompresseddecompressed air was air recovered was recovered and re-compressed. and re-compressed. One Oneof the of thelast lastpre-war pre-war achieve- achievements ments inin the the field field was was the the design design of ofJohannes Johannes Wardenier, Wardenier, which, which, according according to tothe the reports, reports, was was supposedsupposed to be to bethe the first first engine engine not not requiring requiring liquid liquid fuel fuel [7]. [7 ].Currently, Currently, solutions solutions in- involving volvinglow-speed low-speed enginesengines forfor vehiclesvehicles areare stillstill beingbeing developeddeveloped [ 8[8].]. In recentIn decades, recent decades, increasing increasing attention attention has been has beenpaidpaid to the to theimpact impact of ofvarious various in- industry dustry branches on thethe naturalnatural environment environment [ 9[9,10,10].]. Therefore, Therefore, the the ecological ecological factor factor is becomingis becomingincreasingly increasingly important important [11 [11,12].,12]. The The combustion combustion of of fossil fossil fuels fuels is oneis one of of the the main main causes of causes ofenvironmental environmental pollution pollution [13 [13,14].,14]. Environmentally Environmentally safe safe and and emission-free emission-free vehicles vehi- include cles includeelectric electric vehicles; vehicles; they, they, however, however, have ahave disadvantage—their a disadvantage—t batteriesheir batteries contain contain toxic elements toxic elementswhich maywhich expel may toxic expel fumes toxic [ 15fumes,16]. Thus,[15,16]. compressed Thus, compressed air engines air engines are also are within the also withinarea the of interest.area of interest. In 1991, In Guy 1991, N èGuygre etNègre al. developed et al. developed a double-energy a double-energy engine, en- powered gine, poweredby petrol by and petrol compressed and compressed air [17]. Moteurair [17]. Development Moteur Development International International (MDI) developed (MDI) developeda compressed a compressed air vehicle, air E. vehicle, Volution E. Volution [18]. The [18]. vehicle The is vehicle equipped is equipped with a 300-litre with tank a 300-litrefor tank compressed for compressed air, which air, which allows allows you to you travel to travel the distancethe distance of 200 of 200 km km at at a speeda of speed of96 96 kmph. kmph. The The vehicle vehicle is is charged charged at at a high-pressurea high-pressure air air charging charging station. station. Another Another interesting interestingarea area of research of research on the on usethe ofuse renewable of renewable sources sources of power of power are studies are studies conducted con- by the ducted byUniversity the University of Northern of Northern Texas andTexas University and University of Washington of Washington on powering on powering vehicles with vehiclesliquid with liquid nitrogen nitrogen [19,20 ].[19,20]. Compressed air engines have a number of advantages such as: convenient ways Compressed air engines have a number of advantages such as: convenient ways of of obtaining power source, high starting torque, easy control of speed and torque as in obtaining power source, high starting torque, easy control of speed and torque as in this this case it is sufficient to use only an air flow or pressure regulator. In addition, they are case it is sufficient to use only an air flow or pressure regulator. In addition, they are re- resistant to overloads, and in the heat balance the heat generated by the friction between the sistant to overloads, and in the heat balance the heat generated by the friction between internal elements is cooled as a result of gas expansion [21,22]. Qihui Yu et al. developed a the internal elements is cooled as a result of gas expansion [21,22]. Qihui Yu et al. de- compressed air engine (CAE), for which they conducted experimental tests and obtained veloped a compressed air engine (CAE), for which they conducted experimental tests and good economic results at low speed. For the supply pressure of 2MPa, the maximum obtained good economic results at low speed. For the supply pressure of 2MPa, the output power was 1.92 kW, and the maximum output torque was 56.55 Nm; the maximum maximum output power was 1.92 kW, and the maximum output torque was 56.55 Nm; efficiency was 25% [18]. the maximum efficiency was 25% [18]. Trajkovic presented a concept aimed at reducing fuel consumption [23,24]. Urban Trajkovictraffic ispresented associated a withconcept more aimed frequent at reducing acceleration fuel and consumption deceleration. [23,24]. While Urban decelerating, traffic isthe associated energy previously with more used frequent to accelerate acceleration is wasted and deceleration. mainly on the While heat generateddecelerating, by friction

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brakes. If the energy wasted in the internal combustion engine (ICE) could be saved, fuel consumption would improve. Currently, there are several solutions which will satisfy the need for better fuel efficiency, and pneumatic hybrids are among them. The idea of pneumatic hybrydisation consists of decreasing fuel consumption by using the braking energy which may be lost [25]. Heavy Scania engines, which were converted to operate as pneumatic hybrid engines, were tested in a study presented by Trajkovic. During hybrid pneumatic work, the engine can be used as a 2-stroke compressor for the generation of compressed air during the braking of a vehicle (compressor mode), and during its acceleration the engine may work as a pneumatic engine powered by the previously accumulated pressurised air (air-motor). Vehicle driving cycle simulations showed that fuel consumption of a conventional bus can be decreased by as much as 58% after converting it into a pneumatic hybrid bus [23]. Researchers Allam and Zakaria modified the petrol engine to run on compressed air. They conducted a comprehensive study on the behaviours of compressed air and an overview of compressed air engines, and they confirmed that compressed air is clean, safe and efficient. There are no exhaust emissions during the use of compressed air and the gas leaving the engine is non-flammable [26]. Wang et al. modified an ICE with a capacity of 100 cm3, converting it from a four- stroke engine into a two-stroke engine powered by compressed air. The engine was used to drive a motorcycle. It was demonstrated that the motorcycle with compressed air engine could be integrated with a conventional combustion engine with the use of the same system with a piston and cylinder [8,27]. Currently, there are few known designs of low-speed compressed air engines [28,29]. The paper presents a solution for low-speed compressed air pneumatic engines. This type of engine can be used to drive auxiliary equipment such as, for example, air conditioning. It can be particularly beneficial in public transport buses. Such vehicles frequently stop, which is also associated with their door frequently opening and closing. The mechanism of the opening and closing of the doors is very often controlled by pneumatic actuators. The compressed air in the actuator is often irretrievably lost after the work is performed. In combination with the recovery of braking energy, transport public buses have high potential of compressed air which can be used to power the air conditioning system. Currently, the combustion engine is a typical source of air conditioner compressor power. Such a solution causes an increase in fuel consumption, hence the idea to recover the energy of compressed air and use it to power the compressor of the air conditioning system, which enables a reduction in fuel consumption. In an air conditioner application, low speed engines of the order of several hundred revolutions per minute are desirable. This is due to the requirements of compressor drive systems used in air conditioners. The use of high- speed air engines, the designs of which are well known, require complex reduction gears, which greatly increases the cost of such a solution. Therefore, it is advisable to search for low-speed air engines that meet the above requirement. When such engines are used for air conditioners in public transport, it is advantageous to obtain a pressure as close as possible to the atmospheric pressure at the engine outlet. Thanks to this, maximum recovery of energy stored in compressed air is achieved. The study included tests of the compressed air engine described in patent no. PL 216801, and the determination of the basic parameters of a prototype compressed air engine [30]. On the basis of the conducted studies, the possibilities of using this type of engine to power equipment were determined. These results may constitute the basis for further development of the design and its adjustment to the needs associated with the implementation into industrial production. The scope of the studies covered measurements of the basic parameters of compressed air engine operation and an analysis of the obtained results. The research experiment consisted of conducting a number of tests at a specially prepared site for determining the basic movement characteristic of the compressed air engine. Energies 2021, 14, 1179 4 of 15

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2. The Design of a Typical Compressed Air Engine 2. The FigureDesign2 ofpresents a Typical the Compressed principle of Air operation Engine of a typical compressed air engine [ 31]. WorkFigure cycle 2 presents begins when the principle the piston of operatio passes itsn of upper a typical return compressed position, air and engine the inlet [31]. valve Workopens cycle with begins a certain when delay the piston (Figure passes2a). Enteringits upper thereturn cylinder, position, the and compressed the inlet valve air exerts opensforce with on the a certain piston causingdelay (Figure its movement 2a). Entering toward the thecylinder, lower the return compressed point. Before air exerts the piston forcereaches on the the piston point, causing the inlet its valve movement closes, towa andrd the the piston lower will return continue point. to Before move, the powered pis- by tonthe reaches expanding the point, air. After the inlet it passes valve the closes, lower and return the piston point, will the continue exhaust valveto move, opens pow- and the eredpiston by the returns expanding to the air. upper After return it passes position the lower (Figure return2b). point, During the theexhaust cycle valve the piston opens does andnot the perform piston work.returns to the upper return position (Figure 2b). During the cycle the piston does not perform work.

(a) (b)

FigureFigure 2. 2.TheThe principle principle of the of theoperation operation of a oftypical a typical pneumatic pneumatic engine: engine: (a) power (a) power stroke; stroke; (b) exhaust (b) exhaust stroke [31]. stroke [31].

TypicalTypical piston piston engines engines for forcrankshaft crankshaft rotati rotationon angle angle within within the range the range 0–45° 0–45 transfer◦ transfer thethe pressure pressure force force onto onto the the piston piston during during crankshaft crankshaft moment moment to a very to a verysmall small extent. extent. The The moment on shaft is the product of the component load pressure on the piston, Fm, re- moment on shaft is the product of the component load pressure on the piston, Fm, resulting sulting from the pressure in the cylinder and the radius of the crankshaft, r. For the shaft from the pressure in the cylinder and the radius of the crankshaft, r. For the shaft rotation rotation angle of less than 45° the force is relatively small (Figure 3a). The Fm component angle of less than 45◦ the force is relatively small (Figure3a). The F component increases increases significantly with an increase in the angle of shaft rotation (Figurem 3b). The force signiﬁcantly with an increase in the angle of shaft rotation (Figure3b). The force exerted exerted by the pressure onto the piston is used most effectively when the shaft rotation by the pressure onto the piston is used most effectively when the shaft rotation angle is angle is between 45 and 135°. For a typical solution, it constitutes merely 80° of the shaft between 45 and 135◦. For a typical solution, it constitutes merely 80◦ of the shaft rotation rotation angle [31,32]. In order to ensure even torque for full shaft rotation, the engines of thisangle type [31 are,32 most]. In orderoften tobuilt ensure as engines even torque with four for fullor more shaft cylinders, rotation, thewith engines a crankshaft of this type arrangementare most often ensuring built aspower engines strokes with for four various or more rotation cylinders, angles. with a crankshaft arrangement ensuring power strokes for various rotation angles. Typical piston engines for crankshaft rotation angle within the range 0–45◦ transfer the force on the connecting rod onto the moment on shaft to a very small extent. It results from the unfavorable angle between the connecting rod and the crankshaft. The force on the connecting rod is used most effectively within the shaft rotation angle range between 45 and 135◦. The proposed engine structurally uses the transfer of the moment for the most effective rotation angle, which makes it possible to use the energy of compressed air more effectively.

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(a) (b)

Figure 3. MomentFigure on 3. Moment shaft depending on shaft dependingon the rotation on the angle: rotation (a) angle angle: below (a) angle 45°; below (b) angle 45◦ ;(aboveb) angle above 45◦. 45°. 3. The Principle of the Operation of the Compressed Air Engine Described in Patent TypicalNo. piston PL 216801 engines for crankshaft rotation angle within the range 0–45° transfer the force on theThe connecting principle rod of onto the the operation moment of on the shaft engine to a very described small extent. in patent It results no. PL 216801, from the unfavorablepresented in angle Figure between4, is based the conne on twocting strokes, rod and power the andcrankshaft. return (exhaust) The force stroke on [ 30]. It the connectinguses rod reciprocating is used most pistons: effectively mainpistons within TG1,the shaft TG2 rotation and TG3 angle and auxiliaryrange between pistons TP1, TP2 45 and 135°.and The TP3. proposed The main engine pistons structurally are permanently uses the connected transfer toof thethe auxiliarymoment pistonsfor the by means most effectiveof the rotation elements angle, of thewhich complex makes straight it possible shaft. to Thisuse the way, energy they of form compressed three pairs air (TP1, TG2), more effectively.(TP2, TG3) and (TP3, TG1). The main pistons are connected to the crankshaft by means of connecting rods. They work in pairs, performing an opposed movement in the toroidal Energies 2021, 14, x FOR PEER REVIEW 6 of 15 3. The Principlesections of ofthe the Operation cylinders. of A the compression Compressed chamber Air Engine of varying Described capacity, in Patent common for both No. PL 216801pistons, is created in the cylinders. The principle of the operation of the engine described in patent no. PL 216801, presented in Figure 4, is based on two strokes, power and return (exhaust) stroke [30,33]. It uses reciprocating pistons: main pistons TG1, TG2 and TG3 and auxiliary pistons TP1, TP2 and TP3. The main pistons are permanently connected to the auxiliary pistons by means of the elements of the complex straight shaft. This way, they form three pairs (TP1, TG2), (TP2, TG3) and (TP3, TG1). The main pistons are connected to the crankshaft by means of connecting rods. They work in pairs, performing an opposed movement in the toroidal sections of the cylinders. A compression chamber of varying capacity, common for both pistons, is created in the cylinders.

FigureFigure 4.4.Construction Construction ofof thethe engineengine accordingaccording to to patent patent no. no. 216801.216801.

TheThe principleprinciple ofof thethe operationoperation ofof thethe three-cylinderthree-cylinderengine engine is is that that two two pistons pistons in in one one cylindercylinder workwork inin opposition,opposition, inin aa reciprocating reciprocating movement—the movement—the mainmain TGTG pistonpiston and and the the auxiliaryauxiliary TPTP piston.piston. TheseThese pistons,pistons, movingmoving closercloser andand furtherfurther apartapart fromfrom oneone anotheranother createcreate chamberschambers ofof differentdifferent capacity.capacity. EngineEngine workwork cyclecycle isispresented presented in in Figure Figure5 .5. The The description of the work cycle begins when the TG1 piston is in the upper return point (Figure 5a). The inlet valve is opened in the first cylinder, at the noxious space close to zero, the power stroke begins. In this position, TP1 is already moving and the capacity of the working chamber rapidly increases. The force from TP1 is transferred onto TG2 and further through the connecting rod onto the crankshaft, which is located in a favourable position for generating the torque, that is approximately 45° from Before Top Dead Cen- tre (BTDC). High torque is generated from the beginning of the power stroke, that is 0.7 of the value of the piston force from TP1. Under pressure, the TP1 piston moves up toward the external return point pulling the crankshaft upwards; at this point the torque is generated only from the piston force of TP1. TG1 changes the position to a very slight extent. When TP1 moves by an angle of crankshaft rotation of approx. 20° (Figure 5b), downward movement of TG1 toward the external return point begins, and the torque is now generated from TP1 and TG1, which is connected directly to the crankshaft by means of the connecting rod, and pushes it downwards. The closing of the inlet valve takes place at this stage of piston movement, performing the expansion in closed capacity (technical work). Both pistons are headed toward the external return points generating the highest torque from combined TP1 + TG1 piston forces (Figure 5c). TP1 reaches the external return point (Figure 5d) and TG1 continues movement generating the torque. TP1 begins movement toward the internal return point, TG1 is still moving toward the external return point, but the working space does not increase as TP1 moves in the same direction-a shift of the working chamber takes place and the outlet valve opens. TP1 and TG1 are headed toward their internal return points, decreasing the capacity of the working chamber. TP1 reaches the internal return point, stops and “waits” for TG1 (it results from the connection with the TG2 connecting rod), which approaches it at the minimum distance possible, and then both pistons move in the same direction—the working chamber shifts until TG1 reaches the internal return point the cycle repeats. Because the distribution of the crankpins on the shaft is even and equal to 120 degrees, the cycles repeat every 120 degrees in each cylinder successively, and a single cycle lasts

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description of the work cycle begins when the TG1 piston is in the upper return point (Figure5a). The inlet valve is opened in the first cylinder, at the noxious space close to zero, the power stroke begins. In this position, TP1 is already moving and the capacity of the working chamber rapidly increases. The force from TP1 is transferred onto TG2 and further through the connecting rod onto the crankshaft, which is located in a favourable position for generating the torque, that is approximately 45◦ from Before Top Dead Centre (BTDC). High torque is generated from the beginning of the power stroke, that is 0.7 of the value of the piston force from TP1. Under pressure, the TP1 piston moves up toward the external return point pulling the crankshaft upwards; at this point the torque is generated only from the piston force of TP1. TG1 changes the position to a very slight extent. When TP1 moves by an angle of crankshaft rotation of approx. 20◦ (Figure5b), downward movement of TG1 toward the external return point begins, and the torque is now generated from TP1 and TG1, which is connected directly to the crankshaft by means of the connecting rod, and pushes it downwards. The closing of the inlet valve takes place at this stage of piston movement, performing the expansion in closed capacity (technical work). Both pistons are headed toward the external return points generating the highest torque from combined TP1 + TG1 piston forces (Figure5c). TP1 reaches the external return point (Figure5d) and TG1 continues movement generating the torque. TP1 begins movement toward the internal return point, TG1 is still moving toward the external return point, but the working space does not increase as TP1 moves in the same direction-a shift of the working chamber takes place and the outlet valve opens. TP1 and TG1 are headed toward their internal return points, decreasing the capacity of the working chamber. TP1 reaches the internal return point, stops and “waits” for TG1 (it results from the connection with the TG2 connecting rod), which approaches it at the minimum distance possible, and then both pistons move in the same direction—the working chamber shifts until TG1 reaches the internal return point the cycle repeats. Because the distribution of the crankpins on the shaft is even and equal to 120 degrees, the cycles repeat every 120 degrees in each cylinder successively, and a single cycle lasts for approximately 160 degrees of crankshaft rotation, which means that the cycles overlap, providing a more even distribution of the torque in the crankshaft rotation function. An engine with three or a multiple of three cylinders is most beneficial for the presented solution. Very effective use of the compressed air energy for generating the torque on the engine shaft constitutes an advantage of the solution. It stems from the fact that when the greatest force acts on the pistons, the angle of the connecting rod in relation to the crankshaft is in its most favourable position. It can be noticed easily that one cylinder generates an effective torque through shaft rotation angle from 0 to 120◦, unlike in the case of the classic design, in which the moment is generated only for an angle between 45 and 135◦. This engine is characterized by the efficient use of energy stored in the compressed air. The simultaneous use of two pistons means that in the first phase of the stroke, the main piston performs the main part of the work and it transmits the main driving torque. In this phase, the energy of the compressed air is optimally used, causing the air to expand. Both pistons are headed toward the external return points generating the highest torque from combined TP1 + TG1 piston forces. After reaching this position, the main torque is transmitted by the piston TG2, while the auxiliary piston TP1, starting its stroke, supports the piston TG2, using the remaining energy stored in the compressed air, leading to almost complete expansion. In this way, the energy stored in the compressed air was used more efficiently. Energies 2021, 14, x FOR PEER REVIEW 7 of 15

Energies 2021, 14, 1179 7 of 15 for approximately 160 degrees of crankshaft rotation, which means that the cycles overlap, providing a more even distribution of the torque in the crankshaft rotation function.

(a) (b)

FigureFigure 5. 5.TheThe principle principle of operation: of operation: (a) beginning (a) beginning of the cycle; of the (b cycle;) gas injection; (b) gas injection; (c) the external (c) the external returnreturn point point of ofTG1; TG1; (d) ( dthe) the external external return return point point of TP1. of TP1.

4. TestAn engine Stand with three or a multiple of three cylinders is most beneficial for the pre- sentedA solution. test stand Very was effective built in use order of the to co determinempressed theair energy basic performance for generating parameters the torque of the on the engine shaft constitutes an advantage of the solution. It stems from the fact that compressed air engine, as shown in Figures6 and7. It was assumed that the maximum when the greatest force acts on the pistons, the angle of the connecting rod in relation to operating pressure would not exceed 6 bars. The test stand will be powered by a pneumatic the crankshaft is in its most favourable position. It can be noticed easily that one cylinder circuit. The value of the pressure supplied to the engine will be constant and regulated by generates an effective torque through shaft rotation angle from 0 to 120°, unlike in the a pressure reducer 3, the power supply system is additionally equipped with a particulate case of the classic design, in which the moment is generated only for an angle between 45 filter 2. In order to avoid the impact of the power supply pressure pulsation on the and 135°. measurement results, an equalisation tank 5 was built. The air from the tank will power the prototype compressed air engine 7 through lubricator 13. The air stream is measured with flow meter 4. The air outlet is directed to the equalisation tank with a throttle valve 9, which enables controlling the air stream flowing through the engine. There is a measuring channel connected to the Thermokon DPL6/V (6) pressure difference sensor behind the equalisation tank. The load on the engine shaft is regulated with the use of brake 12 installed on the Energies 2021, 14, x FOR PEER REVIEW 8 of 15

This engine is characterized by the efficient use of energy stored in the compressed air. The simultaneous use of two pistons means that in the first phase of the stroke, the main piston performs the main part of the work and it transmits the main driving torque. In this phase, the energy of the compressed air is optimally used, causing the air to expand. Both pistons are headed toward the external return points generating the highest torque from combined TP1 + TG1 piston forces. After reaching this position, the main torque is transmitted by the piston TG2, while the auxiliary piston TP1, starting its stroke, supports the piston TG2, using the remaining energy stored in the compressed air, leading to almost complete expansion. In this way, the energy stored in the compressed air was used more efficiently.

4. Test Stand A test stand was built in order to determine the basic performance parameters of the compressed air engine, as shown in Figures 6 and 7. It was assumed that the maximum operating pressure would not exceed 6 bars. The test stand will be powered by a pneumatic circuit. The value of the pressure supplied to the engine will be constant and regulated by a pressure reducer 3, the power supply system is additionally equipped with a particulate filter 2. In order to avoid the impact of the power supply pressure pulsation on the measurement results, an equalisation tank 5 was built. The air from the tank will power the prototype compressed air engine 7 through lubricator 13. The air stream is Energies 2021, 14, 1179 measured with flow meter 4. The air outlet is directed to the equalisation tank with8 of 15a throttle valve 9, which enables controlling the air stream flowing through the engine. There is a measuring channel connected to the Thermokon DPL6/V (6) pressure difference sensor behind the equalisation tank. The load on the engine shaft is regulated with common shaft with the tested engine. The torque is measured with the Megatron DFM2X the use of brake 12 installed on the common shaft with the tested engine. The torque is torque meter 10. The torque is measured with the Wobit MOK40-200/1224/BZ/K encoder measured with the Megatron DFM2X torque meter 10. The torque is measured with the 11. The input power is calculated on the basis of the measurement of the stream of the Wobit MOK40-200/1224/BZ/K encoder 11. The input power is calculated on the basis of ﬂowing air and operating pressure. the measurement of the stream of the flowing air and operating pressure.

Pwe = Q·∆p (1) P =Q∙∆p (1) where:where: Δ∆p-pressurep-pressure drop drop on on engine, engine, Pa; Pa; Q-air Q-air stream, stream, m m33/s./s.

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FigureFigure 6. 6. TestTest stand stand diagram. diagram.

Figure 7. Test stand,Figure where: 7. Test 1-brake; stand, 2-torque where: gauge;1-brake; 3-engine. 2-torque gauge; 3-engine.

Due to the factDue that ﬂowto the meters fact that are calibratedflow meters in standardare calibrated litres in per standard hour, it islitres required per hour, it is re- to scale the resultsquired of to the scale ﬂow the meter results indications, of the flow considering meter indications, operating considering pressure operating to be pressure to be higher than normal. The indication of the flow meter is corrected on the basis of the following formula:

ρ p Q =Q =Q (2) ρ p

where: Qm-measured stream, m/h; ρn–gas density under normal conditions (p = 1 bar and 3 3 T = 20°C), kg/m ; ρ–gas density, kg/m ; pn–normal pressure (pn = 1 bar); p-operating pressure, bar. Thus, after considering Equations (1) and (2), we obtain the formula for the input power, at controlled operating pressure value: ρ p P = Q ∆p = Q ∆p (3) ρ p

where: Qm-measured stream, m/h; ρn–gas density under normal conditions (p = 1 bar and 3 3 T = 20°C), kg/m ; ρ–gas density, kg/m ; pn–normal pressure (pn = 1 bar); p-operating pressure, bar; Δp-pressure drop on engine. The power on the shaft of the compressed air engine is determined on the basis of the measurement of the torque and load torque. The load torque is regulated with a brake, and the speed is regulated with valve 9, throttling the stream of the flowing air. The power of the compressed air engine is calculated from correlation: 2π P = n∙M (4) 60 where: M-torque, Nm; n-speed, rpm. The efficiency is calculated on the basis of the calculated power supply and power on the engine shaft, in accordance with correlation:

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higher than normal. The indication of the ﬂow meter is corrected on the basis of the following formula: r ρ r p QN = Qm = Qm (2) ρn pn

3 ρ p 2 P = n Q ∆p = n Q ∆p (3) we ρ N p m

where: Qm-measured stream, m/h; ρn–gas density under normal conditions (p = 1 bar and ◦ 3 3 T = 20 C), kg/m ; ρ–gas density, kg/m ; pn–normal pressure (pn = 1 bar); p-operating pressure, bar; ∆p-pressure drop on engine. The power on the shaft of the compressed air engine is determined on the basis of the measurement of the torque and load torque. The load torque is regulated with a brake, and the speed is regulated with valve 9, throttling the stream of the ﬂowing air. The power of the compressed air engine is calculated from correlation:

2π P = n·M (4) M 60 where: M-torque, Nm; n-speed, rpm. The efﬁciency is calculated on the basis of the calculated power supply and power on the engine shaft, in accordance with correlation:

P η = M 100%, (5) Pwe The input power consists of the power on the engine shaft and power dissipation. The main reason for power dissipation is resistance to motion, losses associated with leaks, power lost to engine lubrication systems as well as losses associated with gas changes.

5. Results A number of tests were carried out under various conditions of engine operation in order to determine the operating parameters of the compressed air engine. In order to determine the maximum speed of the engine, it was started without load with the outlet valve 9 completely open. Torque on shaft in this system is zero, so all the energy of the supplied air is used to overcome engine’s own resistance, and the stream of the supplied air will be directly converted into engine speed. Figure8 presents the dependence of the maximum achievable speed of the engine on the stream of the ﬂowing air. A linear dependence with a slope of about 0.75 rotation per stream was obtained for the model engine. Engine Friction Losses increase as the speed increases. Figure9 presents the characteristics of changes in speed depending on the decrease in pressure on the engine. The correlation is linear with an inclination of 0.8 rotation per moment value. The inclination of the characteristics indicates dynamic losses associated with engine operation. Energies 2021, 14, x FOR PEER REVIEW 10 of 15

η= 100%, (5) The input power consists of the power on the engine shaft and power dissipation. The main reason for power dissipation is resistance to motion, losses associated with leaks, power lost to engine lubrication systems as well as losses associated with gas changes.

Energies 2021, 14, x FOR PEER REVIEW 11 of 15

FigureFigure 8. 8. TheThe dependence dependence of of the the maximum maximum engine engine speed speed on on the the stream stream of of supplied supplied air. air.

Engine Friction Losses increase as the speed increases. Figure 9 presents the characteristics of changes in speed depending on the decrease in pressure on the engine. The correlation is linear with an inclination of 0.8 rotation per moment value. The inclination of the characteristics indicates dynamic losses associated with engine operation. If the engine is loaded with torque, some part of the supplied energy will be converted into mechanical power. The tests consisted of supplying the engine with a continuous stream of air of different working pressures and then changing its load. Figure 10 presents the correlation of change in speed on the load moment of the engine. This way, a very typical characteristic of compressed air engines was obtained, where the speed strongly depends on the engine load. In order to achieve constant speed on the shaft, it is necessary to regulate the stream of the supplied air. As the operating pressure increases, the impact of the load on the speed of the engine decreases. It indicates that it is more beneficial for this type of engine to operate at higher working pressures. It stems from the fact that the required energy for the transfer of the oil lubricating engine elements has a large impact on engine efficiency. It indicates the direction of the search for the solutions concerning the improvement of engine efficiency. Undoubtedly, the centrifugal lubrica- FigureFigure 9. 9.TheThe dependence dependence of ofengine engine speed speed on on the the load load for for various various values values of ofoperating operating pressure. pressure. tion system will consume much less energy. TheIf thepower engine on the is loaded engine with shaft torque, depends some both part on of the the operating supplied energypressure will and be the converted load. Figureinto mechanical10 presents power.the dependence The tests consistedof the power of supplying obtained theon enginethe engine with shaft a continuous on the stream of air of different working pressures and then changing its load. Figure 10 presents speed. Engine speed results from the change in engine load. As can be seen in the pre- the correlation of change in speed on the load moment of the engine. This way, a very sented characteristics, the power on the shaft reaches a certain maximum for a given typical characteristic of compressed air engines was obtained, where the speed strongly speed. As the operating pressure increases, the maximum is reached at a higher speed. depends on the engine load. In order to achieve constant speed on the shaft, it is necessary The shifting of the maximum power to the right along with the pressure increase in the to regulate the stream of the supplied air. As the operating pressure increases, the impact diagrams is caused by the better sealing of the engine. Engines of this type are charac- of the load on the speed of the engine decreases. It indicates that it is more beneficial for terized by the fact that in the end positions, the valves open and close. With less pressure, this type of engine to operate at higher working pressures. It stems from the fact that the the engine obtains less torque on the shaft. Hence, the greater percentage of power is required energy for the transfer of the oil lubricating engine elements has a large impact on used to overcome the engine’s own resistance, which results in a lower maximum speed. engine efficiency. It indicates the direction of the search for the solutions concerning the In convergence, the non-sealing time of the valves related to the rotation of the valve cam improvement of engine efficiency. Undoubtedly, the centrifugal lubrication system will is longer. This results in a greater percentage of air loss. As a consequence, lower rota- consume much less energy. tional speed and efficiency are obtained and this is the cause of the maximum power shift. Therefore, one of the parameters affecting the efficient use of the engine is the appropriate selection of the size and power of the engine for a specific operating pressure. The value of the pressure of the supplied air also has a significant impact on the obtained power on the shaft.

Figure 10. The dependence of the engine power on the speed at different operating pressures.

Energies 2021, 14, x FOR PEER REVIEW 11 of 15

Figure 9. The dependence of engine speed on the load for various values of operating pressure.

FigureFigure 10. 10. TheThe dependence dependence of of the the engine engine power power on on th thee speed speed at at different different operating pressures.

The power on the engine shaft depends both on the operating pressure and the load. Figure 10 presents the dependence of the power obtained on the engine shaft on the speed. Engine speed results from the change in engine load. As can be seen in the presented characteristics, the power on the shaft reaches a certain maximum for a given speed. As the operating pressure increases, the maximum is reached at a higher speed. The shifting of the maximum power to the right along with the pressure increase in the diagrams is caused by the better sealing of the engine. Engines of this type are characterized by the fact that in the end positions, the valves open and close. With less pressure, the engine obtains less torque on the shaft. Hence, the greater percentage of power is used to overcome the engine’s own resistance, which results in a lower maximum speed. In convergence, the non-sealing time of the valves related to the rotation of the valve cam is longer. This results in a greater percentage of air loss. As a consequence, lower rotational speed and efficiency are obtained and this is the cause of the maximum power shift. Therefore, one of the parameters affecting the efficient use of the engine is the appropriate selection of the size and power of the engine for a specific operating pressure. The value of the pressure of the supplied air also has a significant impact on the obtained power on the shaft. A significant operating parameter of compressed air engines is the amount of the used air in relation to the power generated on the shaft. Figure 11 presents the dependence of the power obtained on the engine shaft on the stream of the air supplied to it at various values of operating pressure. The engine was powered by air with constant pressure, and then regulated by engine load. A change in the load of the engine causes a change in the speed, which results in a change of the stream of the gas flowing through the engine. Higher operating pressure causes the engine to maintain the maximum power generated on the shaft within a greater range of speed. After exceeding the critical value of the load stream, the power decreases very rapidly. It is caused by a decrease in the speed. As the operating pressure increases, the greater part of the air stream escapes through the leaks without performing any work. The obtained results were compared with selected series-produced engines. Table1 presents a comparison of the results of two piston engines, MP165 and MP3000 produced by STAWMET, with the tested engine. Comparing the results for two commercial engines, the proposed solution shows much lower air consumption in relation to the obtained power. The much more powerful engine MP 3000 shows significantly less air consumption per watt of power. This is typical of high-power engines where the achieved efficiencies are usually higher. Compared to an engine with a power slightly higher than the proposed MP 165, much better results were obtained, which proves the purposefulness of building engines of this design. Comparing the parameters of the engines from Table1, the engine Energies 2021, 14, x FOR PEER REVIEW 12 of 15

A significant operating parameter of compressed air engines is the amount of the used air in relation to the power generated on the shaft. Figure 11 presents the dependence of the power obtained on the engine shaft on the stream of the air supplied to it at various values of operating pressure. The engine was powered by air with constant pressure, and then regulated by engine load. A change in the load of the engine causes a Energies 2021, 14, 1179 change in the speed, which results in a change of the stream of the gas flowing through12 of 15 the engine. Higher operating pressure causes the engine to maintain the maximum power generated on the shaft within a greater range of speed. After exceeding the critical value of the load stream, the power decreases very rapidly. It is caused by a decrease in manufacturedthe speed. As the in accordanceoperating pres withsure the increases, PL216801 the patent greater shows part lower of the air air consumption stream escapes in relationthrough tothe the leaks power without generated performing on the any shaft. work.

Figure 11. The dependence of the engine power on the gas ﬂowflow at different operatingoperating pressures.pressures.

The obtained results were compared with selected series-produced engines. Table 1 Tablepresents 1. Comparison a comparison of the of parametersthe results of of selected two piston compressed engines, air MP165 engines. and MP3000 produced by STAWMET, with the tested engine. Comparing the resultsEngine for two commercial engines, the proposed solutionParameter shows much lower air consumption in relation to the ob- MP 165 MP 3000 PL216801 tained power. The much more powerful engine MP 3000 shows significantly less air consumption per wattPower of power. [W] This is typical 367 of high-power 2131 engines where 280 the achieved efficiencies Speedare usually [RPM] higher. Compared 400 to an engine 200 with a power 245 slightly 6 bar Air consumption [NL/min] 360 2460 230 higher than thePower proposed consumption MP 165, ratio much [W/L] better results 407 were obtained, 173 which proves 298 the purposefulness of buildingEfficiency engines [%] of this design. 10.19 Comparing the 8.66 parameters 12.17of the engines from Table 1, the engine manufactured in accordance with the PL216801 patent Power [W] 294 1764 225 shows lower air consumptionSpeed [RPM] in relation to the power 400 generated on 200 the shaft. 225 5 bar Air consumption [NL/min] 300 1920 195 Table 1. ComparisonPower consumptionof the parameters ratio of [W/L] selected compressed 392 air engines. 183 259 Efficiency [%] 11.76 11.02 13.84 Engine ParameterPower [W] 220 1323 120 MP 165 MP 3000 PL216801 Speed [RPM] 400 200 230 4 bar AirPower consumption [W] [NL/min] 367 240 2131 1440 280 150 PowerSpeed consumption [RPM] ratio [W/L] 400 366 200 183 245 184 Air consumptionEfficiency [Nl/min] [%] 360 13.75 2460 13.78 230 12 6 bar Power consumption ratio 407 173 298 In order to determine[W/l] the efficiency of the engine, depending on the speed and operating pressure,Efficiency tests [%] consisting of supplying 10.19 the engine 8.66 with a stream of12.17 air with constant pressurePower were [W] carried out. Figure 29412 presents the1764 results for changing 225 engine load.5 bar The highest engine efficiencies coincide with engine speeds at which the highest Speed [RPM] 400 200 225 powers on the shaft for a given operating pressure were obtained. Increasing the operating pressure makes sense only up to a certain value. Further increases cause the efficiency to stall or even decrease. The optimum operating pressure in the model engine is the pressure between 4 and 5 bars. Energies 2021, 14, x FOR PEER REVIEW 13 of 15

Air consumption [Nl/min] 300 1920 195 Power consumption ratio[W/l] 392 183 259 Efficiency [%] 11.76 11.02 13.84 Power [W] 220 1323 120 Speed [RPM] 400 200 230 4 bar Air consumption [Nl/min] 240 1440 150 Power consumption ratio[W/l] 366 183 184 Efficiency [%] 13.75 13.78 12 In order to determine the efficiency of the engine, depending on the speed and operating pressure, tests consisting of supplying the engine with a stream of air with constant pressure were carried out. Figure 12 presents the results for changing engine load. The highest engine efficiencies coincide with engine speeds at which the highest powers on the shaft for a given operating pressure were obtained. Increasing the operating pressure makes sense only up to a certain value. Further increases cause the efficiency to Energies 2021, 14, 1179 13 of 15 stall or even decrease. The optimum operating pressure in the model engine is the pressure between 4 and 5 bars.

Figure 12. 12. EngineEngine efficiency efﬁciency depending depending on on the the sp speedeed at different at different operating operating pressures. pressures.

6. Conclusions Conclusions TheThe paper paper presents presents the the design design solution solution and and test test results results of the of theprototype prototype PL 216801 PL 216801 engine [30]. [30]. The The presented presented engine engine design design is much is much more more economical economical in terms in terms of air of con- air consumption compared compared to to typical typical designs. designs. Air Air consumption consumption per perunit unit of power of power is higher is higher in in relation to to engines engines with with a apower power many many times times greater greater than than that that of the of theprototype prototype PL 216801 PL 216801 engine, but but lower lower in in relation relation to to engines engines of ofco comparablemparable power. power. In high-power In high-power engines, engines, the the displacement is muchmuch larger,larger, hencehence aa lower lower percentage percentage of of energy energy stored stored in in the the compressed com- pressedair is lost. air Comparing is lost. Comparing the efficiency the efficiency of the prototype of the prototype PL 216801 PLengine 216801with engine commercial with commercialsolutions, the solutions, efficiency the of efficiency the PL 216801of the PL engine 216801 is aengine little higher.is a little This higher. confirms This con- the pur- posefulnessfirms the purposefulness of the assumption of the that assumption the structure that consistingthe structure of twoconsisting counter-rotating of two coun- pistons ter-rotatingenables more pistons efficient enables use ofmore the efficient energy ofuse the of expandedthe energy air.of the By expandingexpanded air. the By air ex- in the pandingcylinder the to a air pressure in the cylinder close to atmosphericto a pressure pressure,close to atmospheric thus recovering pressure, the energy thus recov- stored in eringthe compressed the energy airstored to a greaterin the compressed extent than air is theto a case greater with extent conventional than is solutions.the case with During conventionalthe tests, elements solutions. were During noticed the whose tests, designelements changes were noticed can further whose increase design the changes efficiency canof the further PL 216801 increase engine the efficiency and increase of the its PL efficiency. 216801 engine It was and noted increa thatse inits the efficiency. prototype It PL 216801 engine, leakage between the cylinders and pistons was a major source of losses. Changing the design of sealants to lip sealants will most likely increase the resistance to movement; however, the benefits of better sealing appear to improve the overall efficiency

of the engine. The prototype PL 216801 engine is lubricated by an oil pump that draws power from the engine shaft. Splash lubrication that can be used in this type of engine design is much less energy-consuming. These observations will be the subject of further research.

Author Contributions: Conceptualization, D.W. and M.R.; methodology M.R.; investigation, D.W. and E.Ł.; resources, D.W.; writing—original draft preparation, M.R. and E.Ł.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2021, 14, 1179 14 of 15

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