APPARATUS FOR COFFEE PERCOLATION

Abstract

The invention relates to a complete automatic coffee machine (100) comprising a coffee extraction unit (10) with: - a percolation chamber (1) for receiving coffee grounds, - an upper piston (2), - a lower piston (3) movable within said percolation chamber (1) between a maximum low position and a maximum high position - wherein the lower piston (3) comprises a piston rod (31), comprising an internal channel (311) extending from a channel inlet (311a) on the side wall surface of the rod to a channel outlet (311b) on the lower wall surface of the rod, and wherein the channel inlet (311a) is designed and positioned on the side wall surface of the rod such that the channel inlet (311a) overlaps the lower wall (11) of the percolation chamber only when the lower piston is in the maximum high position.

Description

APPARATUS FOR COFFEE PERCOLATION FIELD OF INVENTION The present invention relates to an apparatus for preparing coffee beverages. In particular, it relates to a fully automatic coffee machine. BACKGROUND OF THE INVENTION In fully automatic coffee machines, coffee is typically brewed in a percolation chamber designed to receive a quantity of ground coffee. This coffee is compressed by at least one movable piston, and pressurized water is then introduced to percolate the compressed coffee. Finally, the brewed coffee flows from an outlet in the percolation chamber into a drinking cup. Several types of percolation chambers have been developed for fully automatic coffee machines. One type is called an "upflow" percolation chamber, which refers to a percolation chamber where water is introduced through the bottom of the chamber and the percolated coffee beverage is evacuated through the top. This type of upward percolation chamber has a disadvantage resulting from the fact that the percolated coffee cannot be completely evacuated from the percolation chamber and a small amount of residual liquid remains in the percolation chamber and is collected at the bottom of the chamber. If not drained, this residual liquid affects the quality of the next brewed coffee, as it mixes with the subsequent coffee beverage prepared in the percolation chamber. The usual method for draining this residual liquid is through the water pipe configured to introduce water into the percolation chamber, typically located at the bottom of the chamber. This water pipe usually includes a three-way valve that allows the percolation chamber's water inlet to be connected to either a hot water supply or a liquid drain. This valve allows hot water from the hot water supply to be introduced into the percolation chamber during coffee percolation and then allows the residual liquid to be drained from the percolation chamber after coffee percolation. This configuration is implemented in document no.WO2016116981 acknowledges that this implementation creates additional problems. Specifically, because the residual liquid comprises coffee extract and particles, some residue settles in the pipe between the percolation chamber's water inlet and the three-way valve, eventually clogging the water line. Furthermore, this residue affects the quality of the hot water entering the percolation chamber and the quality of the coffee beverage. The cost of the three-way valve does not take these disadvantages into account. To evacuate residual liquid from an upward percolation chamber, EP3545801 treats the wastewater obtained from pressing percolated coffee beans. The solution consists of creating a drainage outlet between the piston rod and the hole for this rod provided at the bottom of the chamber. A major problem with this solution relates to the liquid that flows under the piston rod and the bottom of the chamber, splashing through the machine's internal parts. This technique doesn't explain how the residual liquid is collected inside the machine. Furthermore, there are doubts about whether the connection between the conical shape of the rod and the orifice at the bottom of the chamber is designed to create an airtight seal when the bottom is closed. It is expected that deposits of coffee extracts and varieties will accumulate on the seal and around the edge of the orifice in the lower chamber, and that airtightness will no longer be guaranteed without regular cleaning that requires complete disassembly of the chamber. When roasted and ground (R&G) coffee is percolated, the optimal quality of the coffee beverage is directly linked to the precise temperature of the water used during extraction. A well-known problem is the poor percolation of the first cup of coffee, and sometimes even the second cup, after a period of non-use of the machine. Due to the thermal inertia of the percolation chamber itself and the presence of stagnant water within the water pipes, the temperature of the first cup is never at the temperature required for the percolation process, resulting in poor quality first cups and, frequently, poor quality second cups as well. To solve this problem, various solutions have been proposed, such as dispensing one or two low-quality coffees to warm the chamber. However, such a solution results in spilled coffee and cannot be implemented in a self-service coffee machine; an operator must be present to pour the coffee down the sink. In another solution, a thermal medium is provided within the percolation chamber itself, as in document no. EP1009270, where an electric heating element is incorporated within the chamber body. This solution presents several drawbacks: First, it requires a complex chamber construction and makes maintenance more difficult. Second, this solution does not allow for the heating of residual water that stagnates in the water supply line, meaning the first cup is never brewed at the optimal temperature. Furthermore, the relatively high temperature of the chamber walls alters the viscosity of the coffee inside, directly affecting the quality of the brewed coffee. Finally, the energy consumption of such an electrically heated chamber is high. Another solution involves introducing hot water into the percolation chamber to heat the water pipe and percolation chamber to the required temperature, and then evacuating this heated water before the first cup of coffee is brewed. W02020082190 describes such a solution: heated water is evacuated through the water pipe and a three-way valve that discharges it to the drain, based on the same fluid line supply construction as WO2016116981 mentioned above. However, this solution again requires the use of a three-way valve, with its associated costs and maintenance. One objective of the invention is to address at least some of the drawbacks of ascending percolation chambers or at least provide an alternative to them. In particular, one objective of the present invention is to provide a complete automatic coffee machine comprising an upward percolation chamber that allows for the evacuation of residual liquid. It would be advantageous to provide a fully automatic coffee machine comprising an upward percolation chamber that allows for the evacuation of residual liquid without contaminating the water supply line. It would be advantageous to provide a fully automatic coffee machine comprising an upward percolation chamber that allows for the evacuation of residual liquid without the use of an expensive three-way valve. eQRfrnn / eznz / B / Yi It would be advantageous to provide a fully automatic coffee machine comprising an upward percolation chamber that allows the preparation of the first cups with the same quality as the other cups. It would be advantageous to provide a fully automatic coffee machine comprising an upward percolation chamber that allows the preparation of first cups with the same quality as the other cups without spilling coffee or electrical energy. BRIEF DESCRIPTION OF THE INVENTION In a first aspect of the invention, a complete automatic coffee machine is provided comprising a coffee extraction unit, said coffee extraction unit comprising: A percolation chamber for receiving roasted and ground coffee powder, said chamber comprising an upper rim, a bottom wall and a water inlet, an upper piston, the upper piston being configured to be movable through the upper rim of the percolation chamber and within the percolation chamber in addition to the relative movement of the percolation chamber and the upper piston, and said upper piston comprising a liquid outlet conduit, a lower piston movable within said percolation chamber between: a low maximum position where the lower piston extends into the bottom wall of the percolation chamber, and a high maximum position where the lower piston is at the upper edge of the percolation chamber, wherein the lower piston comprises a piston rod, said piston rod extending through the bottom wall of the percolation chamber, and wherein the piston rod comprises an internal channel extending from a channel inlet on the side wall surface of the rod to a channel outlet on the bottom wall surface of the rod, and wherein the channel inlet is designed and positioned on the side wall surface of the rod such that said channel inlet overlaps the bottom wall of the percolation chamber when the lower piston is only in the highest position. The coffee machine comprises a percolation chamber configured for upward extraction. The chamber is designed and oriented along a longitudinal axis. This axis can be substantially vertical or tilted, for example, at an angle of up to 30° from the vertical. During extraction, water is introduced through the bottom of the chamber, and the extracted coffee is distributed through the top of the chamber. The percolation chamber defines an internal volume configured to receive a dose of roasted and ground coffee powder, preferably roasted and ground coffee, to be extracted. Usually, the chamber has a cylindrical shape with a side wall extending from a bottom wall to a top edge. The longitudinal axis of the cylindrical shape is preferably oriented substantially vertically. The top of the percolation chamber opens to allow the introduction of the coffee powder to be extracted. eQRfrnn / eznz / B / Yi The percolation chamber includes a water inlet configured to introduce water into the chamber to extract the coffee grounds present inside. This water inlet is usually positioned within the side wall of the chamber, preferably near the bottom wall. The coffee machine comprises two pistons that can move through the internal volume of the chamber. The first piston can move through the top of the chamber and is called the top piston. This upper piston is designed to move across the top edge of the percolation chamber and within the chamber, in addition to the relative movement of the percolation chamber and the upper piston itself. This upper piston is designed to allow for the compression of the coffee grounds present within the chamber. This upper piston includes a liquid outlet channel. This channel allows the extracted beverage, preferably coffee, to be distributed from the chamber. Typically, this channel extends from the piston to a nozzle for dispensing the beverage into a cup. The second piston is called the lower piston. The lower piston can move within the percolation chamber between two end positions. One end position is the lowest possible position of the second piston within the chamber. In this lowest position, the head of the lower piston extends against the bottom wall of the percolation chamber. Generally, the lower piston is positioned in this lowest position during the coffee powder introduction operation and during the coffee powder extraction operation with water. The other final position is the highest position of the second piston within the chamber. In this highest position, the upper end of the lower piston head is at the top edge of the percolation chamber. The lower piston is typically positioned in this highest position during the evacuation of the coffee grounds extracted from the chamber. The lower piston comprises a head and a rod. Typically, the head features water openings configured to introduce water from the water inlet upwards and into the compressed coffee powder within the chamber between the upper and lower pistons. The lower piston is assembled to the percolation chamber so that its piston rod extends through an opening in the lower wall of the percolation chamber, with the free end of the rod located outside the chamber. The piston rod is configured to move up and down within the opening. This rod comprises an internal channel that extends from a channel inlet on the side wall surface of the rod to a channel outlet on the bottom wall surface of the rod. This channel creates a fluid passage for a liquid in contact with the side surface of the rod downwards to a location below the bottom end of the rod. The inlet of this channel is designed and positioned on the side wall surface of the rod so that the inlet overlaps the bottom wall of the percolation chamber when the lower piston is alone in the highest position. Consequently, when the lower piston is at its highest position, allowing the evacuation of the extracted coffee, the channel inlet creates an opening at the bottom of the chamber to evacuate any residual liquid collected there. In this highest position, the lower piston simultaneously allows the evacuation of the coffee cake and the evacuation of the residual liquid. The residual liquid is not evacuated through the water inlet, it does not remain in a part of the water supply pipe or at the bottom of the chamber, consequently, the next coffee beverage prepared in the percolation chamber is extracted with optimal quality. Usually, the machine comprises a waste liquid collector and the channel outlet on the bottom wall surface of the rod is connected to said waste liquid collector, for example, a drip tray. Consequently, the waste liquid flows directly to the waste liquid collector through a dedicated pipe connection and without contaminating other pipes or machine parts such as the rod contact surfaces and the opening at the bottom of the chamber or the external surface of the lower piston rod. The machine comprises a support frame on which the percolation chamber is mounted. The relative movements of the percolation chamber and the two pistons can be implemented interchangeably by: keeping the percolation chamber fixed to the support frame and actuating the pistons into and / or out of the chamber, and / or actuating the percolation chamber relative to the support frame while the pistons remain fixed or are passively driven by the movement of the chamber that holds them. In a preferred embodiment, the percolation chamber is made mobile by means of a drive mechanism. The drive mechanism may be a frame or any other known means. Within this modality, preferably: The upper piston is fixed, the lower piston rod comprises a free lower end located outside the percolation chamber and can move with said piston and said chamber, the apparatus comprises a fixed lower stop wall that can come into contact with the lower end of the lower piston rod, the movement of the lower piston depends on the movement of the percolation chamber and / or the contact of the free lower end with the lower stop wall or the contact of the lower piston with the upper piston. Preferably, in this preferred embodiment, the lower piston is free of guide means, retaining means and springs, which means that the lower piston is passive and there is no active means to change the position of the lower piston with respect to the percolation chamber. Preferably, in this preferred embodiment, the rod and / or lower piston are provided with friction means to control their movement relative to the internal walls of the percolation chamber. These means allow the lower piston to be maintained in a specific position relative to the percolation chamber. Preferably, in this preferred embodiment, the fixed bottom end wall comprises a channel configured to connect the channel outlet of the internal rod channel to a drain pipe. The drain pipe can be configured to convey the water to a drip tray or any other drainage part of the appliance. eQRfrnn / eznz / B / Yi In an alternative embodiment, the percolation chamber can be mounted fixedly on the machine and the upper and lower pistons can be made movable in and out of the percolation chamber by means of drive. Generally, the coffee extraction unit includes a discharge device for the extracted coffee grounds (coffee cake). This discharge device is designed to slide the grounds above the upper edge of the chamber through any rotational or translational movement when the lower piston is positioned at its highest point. Generally, a complete automatic coffee machine comprises a pressurized hot water fluid system consisting of at least a water supply, a water heater, and a water pump. This system connects to the water inlet of the brewing chamber. Consequently, hot pressurized water is supplied to the percolation chamber for the extraction of roasted and ground coffee powder. Generally, a complete automatic coffee machine comprises a control system configured to: to actuate the movement of the percolation chamber and / or the pistons, control the devices of the pressurized hot water fluid system. Secondly, a method is provided for preparing a coffee beverage using a fully automatic coffee machine as described above, wherein said method comprises the following steps: a- In a dosing position, where the lower piston is in the lowest position and the upper piston is a distance from the upper edge of the chamber and out of the chamber, supply a dose of roasted and ground coffee powder into the percolation chamber, then, b- move the chamber and the upper piston relatively towards each other to compress the dose of coffee powder, then, c- introduce hot water into the percolation chamber through the water inlet, extract the coffee powder and distribute the extracted coffee beverage through the beverage outlet of the upper piston, then, d- move the chamber, the upper piston and the lower piston relatively towards each other to: Position the upper edge of the chamber away from the upper piston, and position the lower piston at the upper edge of the percolation chamber, and discharge the coffee powder extracted from the lower piston, and distribute the residual liquid present at the bottom of the chamber through the internal channel of the lower piston rod. This method allows for the evacuation of any liquid still present in the percolation chamber, which is collected at the bottom at the end of the coffee brewing process. Liquid evacuation occurs during the evacuation of the coffee cake, meaning there is no additional time added to the overall coffee brewing process compared to the previous method. The evacuation step remains virtually invisible compared to standard brewing sequences. At the beginning of stage a), the percolation chamber is devoid of coffee. Preferably, the method comprises after step d), at least one additional step e) comprising the step of moving the chamber, the upper piston and the lower piston relatively towards each other in a closed position, wherein the lower piston is in the maximum low position and the upper piston closes the chamber. This closed position prevents a very rapid drop in chamber temperature after coffee preparation in stage c). Due to the closed configuration of the chamber, the chamber walls remain warm and, if a new coffee is prepared, the chamber temperature immediately adjusts for optimal brewing. Preferably, this closed position is maintained until a new coffee beverage is ordered and, when that is the case, the camera moves to the dosing position to implement step a). In a preferred embodiment, the method comprises at least one additional step a0) before step a) is implemented, this step a0) comprising: a0) move the camera, the upper piston and the lower piston relative to each other to: Position the upper edge of the chamber away from the upper piston, and position the lower piston at the upper edge of the percolation chamber, and introduce hot water through the water inlet to purge the hot water fluid line and distribute said hot water through the internal channel of the lower piston. During this stage (a0), which is implemented before stages (a) and (ae) of coffee preparation, the percolation chamber is positioned to allow the simultaneous introduction of water from the water inlet and the immediate evacuation of this water from the chamber through the internal channel of the lower piston rod. As a result, if no coffee has been extracted for a certain time and the temperature of the water that stagnates within the water pipe upstream of the water inlet has decreased, this water can then be purged and evacuated before brewing coffee. Subsequently, the next coffee, prepared by implementing stages (a) and (ae), will be extracted with hot water at the correct expected temperature instead of with a portion of cold water. The volume of hot water introduced into the percolation chamber depends on the dead fluid volume in the specific water fluid line, which is the volume of water that stagnates between the outlet of the water thermal device and the water inlet of the percolation chamber, and on the dead fluid volume at the bottom of the chamber. In another preferred embodiment, the method may comprise additional steps before step a) is implemented; these steps comprise: To assemble the chamber, the upper piston, and the lower piston relative to each other: Close the chamber with the upper piston, and position the lower piston inside the percolation chamber so that the channel inlet of the internal channel is positioned below the bottom wall of the chamber, and introduce hot water through the water inlet to fill at least a part of the internal volume of the percolation chamber defined between the upper piston and the lower piston, then a2- move the chamber, the upper piston and the lower piston relatively towards each other to position the lower piston at the upper edge of the percolation chamber, to distribute said volume of hot water through the internal channel of the lower piston, then a3- move the chamber, the upper piston and the lower piston relatively towards each other to the dosing position. These steps allow hot water to be introduced into the coffee-free percolation chamber to preheat it. Specifically, these steps can be implemented when no coffee has been brewed in the percolation chamber for a certain period, resulting in the chamber and the water in the supply line being cold. Any first or even second brew prepared under such cold conditions will not achieve the expected quality. This waiting period between brews can vary from machine to machine depending on the materials used in its construction and / or the external environment (in a coffee shop, outdoors, or depending on the geographical location, such as very cold or hot climates). In step a^, hot water is introduced into the empty chamber. The volume of hot water may depend on the materials from which the percolation chamber is made. This volume can be selected by the operator in a coffee machine configuration menu, taking into account the machine's environment or water conservation. Preferably, the internal volume of the percolation chamber, defined between the upper and lower pistons, is less than the maximum internal volume of the chamber, so the amount of hot water introduced for heating is not critical. This volume can be defined on a case-by-case basis, but it must be sufficient to heat the chamber to a temperature similar to that obtained if a freshly brewed coffee beverage had been prepared in the chamber. Once hot water is introduced, it can be retained in the percolation chamber for a certain time to efficiently heat the percolation chamber before step a2 is implemented. This time can be made selectable by the operator in a coffee machine configuration menu, taking into account the machine's environment or to accelerate the brewing time. In stage a2) the heating water is evacuated from the percolation chamber and, preferably, hot water is introduced into the chamber and immediately evacuated to purge the hot water line of stagnant water. In a preferred embodiment, stage a0) is implemented before stage a^ to ensure the best preheating of the percolation chamber. In this application, the terms “inner,” “top,” “bottom,” and “side” are used to describe the relative position of the features of the invention. It is understood that these terms refer to the devices in their normal orientation when positioned in a complete automatic coffee machine for the production of a beverage, as shown in the Figures, and in particular, in Figure 6. The foregoing aspects of the invention can be combined in any suitable combination. Furthermore, various features described herein can be combined with one or more of the foregoing aspects to provide combinations other than those specifically illustrated and described. Other objects and advantageous features of the invention will become apparent from the claims, the detailed description, and the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES eQRfrnn / eznz / B / Yi Specific embodiments of the invention will now be described in detail, by way of example, with reference to the following figures in which: - Figure 1 illustrates the hot water fluid system of a coffee extraction unit according to the state of the art, Figures 2 and 3 illustrate the coffee extraction unit of the complete automatic coffee machine according to the present invention in two different operating stages. - Figures 4A and 4B are a view and a cross-section of a lower piston according to the invention, - Figure 5 illustrates the preparation steps of a coffee beverage using a coffee extraction unit as described above, - Figure 6 illustrates the hot water fluid system with a coffee extraction unit according to the invention, Figures 7 and 8 illustrate the steps that can be implemented before the steps in Figure 5 when the coffee beverage has not been prepared for a certain period. DETAILED DESCRIPTION OF THE ILLUSTRATED MODALITIES Figure 1 illustrates the hot water supply line of the coffee extraction unit 10 according to prior art WO2016116981. The coffee extraction section corresponds to an upward configuration where water is introduced at the bottom of the percolation chamber 1 and the coffee beverage is distributed at the top through line 21. The line successively comprises at least: a hot water tank 101, a pump 102 eQRfrnn / eznz / B / Yi a flow meter 103, a final hot water fluid line 7 connected to the water inlet of the percolation chamber portion of the coffee extraction unit 10. The water pipe never drains completely due to the bend in the machine and is one of the identified causes of the very cold water entering the percolation unit. This section of pipe is estimated to hold approximately 2 ml of water. To evacuate the residual liquid retained at the bottom of the percolation chamber 1 after a coffee extraction, a three-way valve 106 is provided in the hot water supply line that allows the drainage of residual liquid through the final hot water fluid line 7 down to the three-way valve 106 and then to the drain 8. As mentioned in the previous section “Background of the invention”, this drainage through the final hot water fluid line 7 presents serious drawbacks. Figures 2 and 3 schematically illustrate the coffee extraction unit 10 of the complete automatic coffee machine according to the present invention in two different operating stages. In Figure 2, the coffee extraction unit 10 is depicted in a coffee extraction or chamber preheating stage, while in Figure 3, the coffee extraction unit 10 is depicted in a coffee cake evacuation stage at the end of the sequence of operations for preparing a coffee beverage. eQRfrnn / eznz / B / Yi The coffee extraction unit 10 comprises a percolation chamber for receiving a dose of coffee powder, usually roasted and ground coffee. Preferably, the percolation chamber 1 is a cylinder with an open top extending from a bottom wall 11 to an upper edge along a substantially vertical axis XX'. The chamber 1 comprises a water inlet 13 to which a hot water fluid line is connected. The water inlet 13 is preferably positioned near the bottom of the chamber, such as within the lower portion of the side wall as illustrated in Figure 2, or optionally in the bottom wall. The coffee extraction unit 10 comprises drive means 6 for vertically moving the chamber 1 up and down. For example, the chamber 1 can be mounted on a screw conveyor unit by means of a motor. The rotation of the screw conveyor causes the vertical translation of the chamber. The coffee extraction unit 10 comprises an upper piston 2. This upper piston 2 has a lower end portion with a cylindrical design and a diameter substantially equal to the chamber diameter, allowing the upper piston to pass through the upper edge 12 of the chamber and into the percolation chamber, while also allowing the percolation chamber to move upward in the direction of the upper piston. Consequently, the upper piston allows the percolation chamber to close during coffee extraction (Figure 2) and compresses a dose of roasted and ground coffee powder introduced into the percolation chamber. The coffee powder can be introduced into the chamber by means of a ramp (not illustrated) that guides the energy dose towards the open top of the chamber. In addition, the upper piston 2 comprises a liquid outlet conduit 21 extending from the bottom surface of the upper piston and connecting to a coffee beverage distribution line. The coffee extraction unit 10 comprises a lower piston with a head 32 comprising water outlets 34 configured to distribute water introduced through the water inlet 11 onto the surface of the compressed coffee grounds positioned above the lower piston head (Figure 2). This head acts as a water supplier. A water distributor (not illustrated) is positioned above it to distribute the water through the coffee grounds. This type of piston for upward extraction, where water is introduced from the bottom, creates a large dead volume at the bottom of the chamber, preventing complete drainage. It has been observed that up to 3 ml of water remain trapped at the bottom of the percolation chamber. The head is disc-shaped and has a diameter substantially equal to the inner diameter of the chamber cylinder. The lower piston 3 comprises a rod 31 fixed to the bottom of the disc-shaped head 32 and extending downwards. The piston 3 is assembled to the chamber by the rod 31, which extends through an opening in the bottom wall of the chamber. The lower piston rod 31 is free of guide means, spring retaining means where the lower piston can move coaxially with and within the percolation chamber. Usually, a mesh filter (not illustrated) is placed on the top surface of the lower piston head to retain coffee particles. The lower piston 3 can move within the percolation chamber 1 between two end positions: a low maximum position where the lower piston extends into the bottom wall 11 of the percolation chamber (Figure 2) eQRfrnn / eznz / B / Yi and a higher position where the lower piston is at the upper edge 12 of the percolation chamber (Figure 3). The coffee extraction unit 10 comprises a discharge device 4 for removing the extracted coffee cake (Figure 3). It is positioned slightly above the upper edge of the percolation chamber. The removed coffee cake is stored in a cake receiving section. In the present illustrated embodiment, the upper piston 2 is fixed. The lower piston 3 can move coaxially: with the percolation chamber 1, meaning that when the chamber 1 moves, it simultaneously takes the lower piston with it, and within the percolation chamber 1, meaning that the lower piston can move to different positions within the percolation chamber. The movement of the lower piston 3 within the percolation chamber is driven by the contact of the disc-shaped member 32 of the piston rod against the upper piston 2 when the percolation chamber moves upward and by the contact of the lower end 312 of the piston rod of the lower piston against a bottom stop wall 5 when the percolation chamber moves downward. When the lower piston 3 is stopped in its upward stroke by the upper piston 2, the continued upward movement of the chamber allows the relative movement of the piston 3 within the chamber, and the same occurs with the bottom stop wall during the downward movement of the chamber. eQRfrnn / eznz / B / YiAi The lower piston rod 31 comprises an internal channel 311 extending from a channel inlet 311a on the side wall surface of the rod to a channel outlet 311b on the bottom wall surface of the rod. This channel provides smooth communication from the channel inlet 311a down to the channel outlet 311b. The inlet of channel 311a is designed in size and shape and positioned on the side wall surface of the rod so that, when the lower piston 3 is in its highest position (Figure 3), the inlet of channel 311a crosses and is oriented toward the edge of the percolation chamber orifice into which the rod is attached, forming a liquid outlet. Consequently, in this highest position, any waste liquid present in the chamber can flow through the inlet of channel 311a and down channel 311 to the outlet of channel 311b. Instead of being evacuated through the water inlet 11, the waste liquid is evacuated through the piston rod, eliminating the problem of water supply contamination. The bottom top wall 5 comprises a channel 51 that cooperates with the outlet of the channel 311b of the rod connected to a tube to conduct the waste liquid to a waste liquid collector, such as a drip tray. The inlet of channel 311a is designed in size and shape and positioned on the side wall surface of the rod so that, when the lower piston 3 is in another operating position such as dosing, percolation, or coffee preheating, the inlet of channel 311a is positioned below the bottom wall 11 of the percolation chamber. Consequently, in other positions (such as the position illustrated in Figure 2), no liquid can be blown through the inlet of channel 311a. Figure 4A is a perspective view of such a lower piston 3. The disc-shaped head 32 comprises multiple water outlets 34 to distribute water to eQRfrnn / eznz / B / Yi through the surface of the previously placed compressed coffee. Usually, a circular seal is positioned in the circumferential groove 33 to provide airtightness during extraction. Figure 4B is a vertical cross-section of the piston rod: it makes evident that rod 311 is empty, which provides a channel 311 for waste fluid. Figure 5 illustrates the different stages of the method for preparing a coffee beverage with a complete automatic coffee machine comprising a coffee extraction unit as described above. Step a) corresponds to a dosing operation where the chamber is in position O) where the lower piston 3 is in its lowest position within the percolation chamber and the upper piston 2 is at a distance 12 from the upper edge of the chamber. In this position, a dose of roasted and ground coffee powder can be introduced through the top of the percolation chamber. In stage b) the percolation chamber 1 and the upper piston 2 move relative to each other to compress the dose of coffee powder. In stage c), hot water is introduced into the percolation chamber through the water inlet 71. This water is distributed through the lower piston head via the compressed coffee powder. The coffee powder is extracted, and the extracted coffee beverage 91 is distributed through the beverage outlet within the upper piston. Next, in step d), the percolation chamber 1 moves downward, which has the effect of withdrawing the upper piston 2 from the chamber. During the downward movement of the chamber, the lower end of the lower piston rod is stopped by the stop wall, and as a result, the lower piston 3 moves upward within the chamber to the upper edge. As illustrated, when the lower piston 3 is in its upper position in the chamber, the coffee cake 92 is positioned above the upper edge of the chamber and can be removed. Simultaneously, the inlet 311a of the internal channel of the rod 31 creates an outlet for the waste liquid 72 present in the chamber. This liquid 72 is evacuated through the rod and emerges from outlet 311b. In a final stage e), chamber 1, the upper piston 2 and the lower piston 3 move relative to each other to close the chamber with the result of keeping the chamber hot for the preparation of the next coffee beverage. Figure 6 schematically illustrates the hot water fluid system with a coffee extraction unit 10 according to the invention. The line successively comprises at least: a cold water tank 101, a flow meter 103, a pump 102, a heater 104, a valve 105, a final hot water fluid line 7 connected to the water inlet of the percolation chamber portion of the coffee extraction unit 10. The waste liquid retained at the bottom of percolation chamber 1 after coffee extraction is drained through the lower piston rod directly into a waste liquid collector. The discharge channel inlet created in the lower piston rod acts as a passive valve that opens automatically when the lower piston is at its highest position within the percolation chamber. All the water in the dead volume can drain into a waste liquid collector such as the drip tray. This passive valve allows the implementation of stages to improve the preparation of the first coffee drinks prepared after a long period during which no coffee drink has been prepared, with the consequence that the percolation chamber and the water retained in the water supply line are cold. Specifically, when coffee has not been brewed for a certain period, additional steps may be implemented for the first brew. This period could be, for example, 15 minutes, but the operator may be able to define this timeframe in a machine settings menu, as machines can differ in their environment (hot or cold) and construction. Figure 7 illustrates an additional step (a0) that can be implemented before brewing a new coffee to improve the temperature of the supplied water if no coffee has been brewed for a certain period. Starting from the dosing position (illustrated O) where the lower piston 2 is in the lowest position within the percolation chamber and the upper piston 3 is at a distance 12 from the upper edge of the chamber, the chamber 1, the upper piston 2, and the lower piston 3 move relative to each other to: - position the upper edge 12 of the camera away from the upper piston 2, and - Position the lower piston 3 on the upper edge 12 of the percolation chamber. Next, hot water 71 is introduced through water inlet 1 to purge the hot water fluid line 7. Because inlet 311a of the rod's internal channel 31 creates an outlet for the liquid present in the chamber, the introduced water 71 is immediately evacuated through the rod and emerges from outlet 311b. As a result, the cold water retained in the water supply line is removed before a new coffee beverage is prepared. Afterwards, coffee percolation can begin as illustrated by the stages in Figure 5 after placing the different elements of the percolation chamber in the dosing position. When no beverage is prepared for a certain time, the coffee machine control unit can be set to automatically move the percolation unit 1 to the position illustrated in stage a0) of Figure 7 so that it is ready to purge cold water from the water supply line 7 when a coffee is ordered. Figure 8 illustrates additional steps that can be implemented before brewing a new cup of coffee to improve the temperature of the supplied water if coffee has not been brewed for a certain period. These steps can be implemented after step a0 or independently; this may depend on the location of the coffee machine and, in particular, on the external environmental conditions. Starting from the dosing position illustrated O) where the lower piston 3 is in the lowest position within the percolation chamber and the upper piston 2 is at a distance 12 from the upper edge of the chamber, in a stage a-i) the chamber 1, the upper piston 2 and the lower piston 3 move relative to each other to: Close chamber 1 with the upper piston 2, and position the lower piston 3 inside the percolation chamber 1 so that the channel inlet 3 311a) of the internal channel is positioned below the bottom wall 11 of the chamber. In these positions of the different elements of unit 10, the percolation chamber is closed and no liquid can drain from the bottom. Next, hot water is introduced through the water inlet 13 eQRfrnn / eznz / B / Yi into the internal volume of the percolation chamber defined by the upper and lower pistons to fill at least a portion of the internal volume of the percolation chamber defined between the upper piston 2 and the lower piston 3. Next, the hot water 71 is kept inside the percolation chamber, preheating it. A small volume of hot water at a temperature between 80 and 95 °C can be kept in the chamber long enough to preheat it as if coffee had just been brewed. For example, 25 ml of water at 80–95 °C held for 15 seconds inside a 45 mm diameter plastic espresso chamber was sufficient to preheat it for brewing the first cup of coffee. After this pause, in stage a2), chamber 1, the upper piston 2, and the lower piston 3 move toward each other to position the lower piston 3 at the upper edge 12 of the percolation chamber, distributing the hot water volume through the internal channel 311 of the lower piston. Then, in this position, hot water is introduced through the water inlet 13 to purge the hot water fluid line 7, and this hot water is immediately evacuated through the internal channel 311 of the lower piston. Then, in a stage a3), chamber 1, upper piston 2 and lower piston 3 move relative to each other to the dosing position O), before the preparation of a coffee begins, as illustrated in Figure 5. The effect of heating the chamber with hot water and purging the hot water supply line from stagnant water dramatically improves the brewing of the first cups of coffee after the machine has cooled down and has not dispensed coffee for a certain time. eQRfrnn / eznz / B / Yi The preparation time for the first coffee, when using the preheating chamber sequence, increases the overall brewing time by less than 20 seconds. This first coffee has the same organoleptic properties as subsequent coffees prepared from the hot chamber. The present invention offers the advantage of allowing the modernization of existing machines by simply replacing the lower piston of the chamber with a new piston and adapting the software of the machine's control unit that manages the movement of the percolation chamber and the water distribution. This modernization does not increase the cost of the machine and can be easily implemented. Another advantage is that new fully automatic coffee machines can be produced with a simple one-way valve instead of the three-way valve in the hot water supply line, thus reducing production costs. Furthermore, the invention prevents wastewater from flowing back into this one-way valve, which could cause clogging or poor sealing over time. Another advantage is that removing waste liquid from the chamber via the piston rod does not create a splash of coffee into the brewed cup, as occurs when opening a three-way valve using previous techniques (due to pressure drop). The coffee drinker does not perceive the chamber draining. Although the invention has been described with reference to the above illustrated embodiments, it will be appreciated that the claimed invention is in no way limited by these illustrated embodiments. Variations and modifications may be made without departing from the scope of the invention, as defined in the claims. Furthermore, where known equivalents exist for specific features, such equivalents are incorporated as if specifically referenced in this specification. As used in this specification, the words “comprises,” “comprising,” and similar words should not be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to.” 5 List of references in the figures: Extraction unit 10 Percolation chamber 1 Lower wall 11 Upper edge 12 10 Water inlet 13 Upper edge 2 Outlet conduit 21 Lower piston 3 Piston rod 31 15 Internal channel 311 Inlet 311a Outlet 311b Head 32 Friction medium 33 20 Water outlets 34 Discharge device 4 Lower stop wall 5 Channel 51 Drive medium 6 25 Hot water fluid line 7 Water71 Residual liquid72 Liquid collector8 Roasted and ground coffee9 Coffee drink91 Coffee cake92 Coffee machine 100 Water tank 101 Pump102 Flow meter 103 Heater104 Valve105 Three-way valve eQRfrnn / eznz / B / YiAi106

Claims

1. A complete automatic coffee machine (100) comprising a coffee extraction unit (10), the coffee extraction unit comprising: a percolation chamber (1) for receiving roasted and ground coffee powder, the chamber comprising an upper rim (12), a lower wall (11) and a water inlet (13), an upper piston (2), the upper piston being configured to be movable through the upper rim of the percolation chamber and within the percolation chamber in addition to the relative movement of the percolation chamber and the upper piston, and the upper piston comprising a liquid outlet conduit (21), a lower piston (3) movable within the percolation chamber (1) between: a maximum low position characterized in that the lower piston extends into the lower wall (11) of the percolation chamber, and a maximum high position in which the lower piston is at the upper rim (12) of the percolation chamber,wherein the lower piston (3) comprises a piston rod (31), the piston rod extending through the lower wall (11) of the percolation chamber, and wherein the piston rod (31) comprises an internal channel (311) extending from a channel inlet (311a) on the side wall surface of the rod to a channel outlet (311b) on the lower wall surface of the rod, and wherein the channel inlet (311a) is designed and positioned on the side wall surface of the rod such that the channel inlet (311a) overlaps the lower wall (11) of the percolation chamber only when the lower piston is in the highest position.

2. Complete automatic coffee machine according to claim 1, characterized in that the machine comprises a waste liquid collector (8) and the channel outlet (311b) on the lower wall surface of the rod is connected to the waste liquid collector.

3. A complete automatic coffee machine according to claim 1 or 2, characterized in that the percolation chamber (1) is made movable by means of drive (6) 4. A complete automatic coffee machine according to the preceding claim, characterized in that: the upper piston (2) is fixed, the rod (31) of the lower piston comprises a free lower end (312) located outside the percolation chamber (1) and can move with the piston and the chamber, the apparatus comprises a fixed lower stop wall (5) configured to come into contact with the lower end of the lower piston rod, the movement of the lower piston (3) depends on the movement of the percolation chamber and / or the contact of the free lower end (312) with the lower stop wall (4) or the contact of the lower piston (3) with the upper piston (2).

5. Complete automatic coffee machine according to any of the preceding claims, characterized in that the rod (31) and / or the lower pistons (3) are provided with friction means to control their movement with respect to the internal walls of the percolation chamber.

6. Complete automatic coffee machine according to claim 4 or 5, characterized in that the fixed lower stop wall (5) comprises a channel (51) configured to connect the channel outlet (311b) of the internal rod channel with a drain pipe.

7. Complete automatic coffee machine according to claim 1, characterized in that the percolation chamber (1) is fixed and the upper and lower pistons (2, 3) can be moved in and out of the percolation chamber by means of drive.

8. A complete automatic coffee machine according to any of the preceding claims, characterized in that the apparatus comprises a discharge device (4) for spent coffee 9. A complete automatic coffee machine according to any of the preceding claims, characterized in that the apparatus comprises a pressurized hot water fluid system comprising at least a water supply, a water heating device and a water pump wherein the system is connected to the water inlet (13) of the percolation chamber.

10. A method for preparing a coffee beverage with a complete automatic coffee apparatus according to any one of claims 1 to 9, characterized in that the method comprises the steps of: a- in a dosing position, wherein the lower piston (3) is in the lowest position within the percolation chamber and the upper piston (2) is at a distance from the upper edge (12) of the chamber and outside the chamber, supplying a dose of roasted and ground coffee powder into the percolation chamber, then, b- moving the chamber (1) and the upper piston (2) relatively towards each other to compress the dose of coffee powder, then, c- introducing hot water through the water inlet (13), extracting the coffee powder and distributing the extracted coffee beverage through the beverage outlet (21) of the upper piston, then, d- moving the chamber (1),The upper piston (2) and the lower piston (3) are positioned relatively towards each other to: position the upper edge (12) of the chamber away from the upper piston (2), and position the lower piston (3) on the upper edge (12) of the percolation chamber, and discharge the coffee powder extracted from the lower piston, and distribute the residual liquid present at the bottom of the chamber through the internal channel (311) of the lower piston rod.

11. Method according to the preceding claim, characterized in that the method comprises, after step d), at least one additional step e) comprising the step of moving the chamber (1), the upper piston (3) and the lower piston (2) relative to each other in a closed position, wherein the lower piston is in the maximum low position and the upper piston closes the chamber.

12. Method according to claim 10 or 11, characterized in that the method comprises at least one additional step before step eQRfrnn / eznz / B / Yi a) is implemented, this step comprises: a0- moving the chamber (1), the upper piston (2) and the lower piston (3) relative to each other to: position the upper edge (12) of the chamber away from the upper piston (2), and position the lower piston (3) on the upper edge (12) of the percolation chamber, and introduce hot water through the water inlet (13) to purge the hot water fluid line (7) and distribute the hot water through the internal channel (311) of the lower piston.

13. A method according to any one of claims 10 to 12, characterized in that the method comprises additional steps before step a) is implemented, these steps comprising: a) moving the chamber (1), the upper piston (2), and the lower piston (3) relative to each other to: close the chamber (1) with the upper piston (2), and position the lower piston (3) within the percolation chamber (1) such that the inlet of the internal channel (311a) is positioned below the lower wall (11) of the chamber, and introducing hot water through the water inlet (13) to fill at least a portion of the internal volume of the percolation chamber defined between the upper piston (2) and the lower piston (3), whereupon, a2) moving the chamber (1), the upper piston (2), and the lower piston (3) relative to each other to position the lower piston (3) on the upper edge (12) of the percolation chamber,5. To distribute the volume of hot water through the internal channel (311) of the lower piston, then, a3. move the chamber (1), the upper piston (2) and the lower piston (3) relatively towards each other to the dosing position.

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