|Every espresso machine represents a set of engineering and design problems that have been solved in a certain way....some more successfully than others. Every espresso machine which has resulted in a viably marketed product is the culmination of engineering, design, materials science, thermodynamics, and sometimes simple patent potential and lawsuit protection. All modern espresso machines have evolved from the original parameters discovered and marketed by the pioneers and refined by the many that have come after. The lever machines all maintain a continuity of functional group design and are basically a large cast brass group containing a cylinder and a piston which supplies the hydraulic pressure to conpress the heated water in the cylinder to 9 bar, the essential pressure for crema espresso production. These machines use a lever to compress a spring which supplies the power to compress the water to the target pressure. The spring piston lever machines, though still a viable format, have largely been superseded by machines that supply hydraulic pressure by use of an electromechanical pump, but the principles of espresso production remain the same......each machine requires a method to supply water heated to the proper temperature, an apparatus to hold the ground coffee, and a hydraulic power source to supply the appropriate pressure needed for the production of crema caffe. We have recently completed a full restoration of a 1948 Gaggia Gilda manual lever machine...the first mass marketed home lever machine....and our fascination with this machine centered on the design solutions to the myriad of engineering problems involved in not only manufacturing this machine but actually controlling the factors of temperature and pressure, and to design this machine in a manner to make it a viable product on the market. The Bacchi brings such considerations again to mind.
Bacchi Espresso Machine, Andrea Bacchi, Designer
The machine is composed of 3 basic functional components: the frame consists of the base, cup holders and assembly mechanism----the group contains the piston and cylinder----the portafilter composed of the dispersion screen, filter basket and control valve plate.
Visualize the machine in this manner, with a standard portafilter below a standard hydraulic group. The frame has a closed base which acts as a reservoir for water. This water acts as the power source for the machine when it is placed on an electrical or gas heat source. The proper water level is indicated by a ridge around the base. The volume of water for powering the machine is approximately 30ml or 1 ounce.
Water is added to the base reservoir and the group is placed on the frame. There is a seal between the two components. Joining of the group to the base has begun to approach the first design challenge of the machine.....the creation of a small boiler which, upon heating, will generate pressurized steam to produce power to move the piston.
Inside the group assembly there are 3 valves. One valve is a standard pressure safety valve.. The second valve is a pressure bleeding valve which is set to release steam at a pressure lower than the safety valve.....this one hisses at near optimal pressure and as the steam increases emits the whistle signal....much like a tea kettle whistle.. The third valve is an anti vacuum valve which bleeds off the vacuum that forms naturally when the machine cools after use. The pressure safety valve and anti vacuum valve are identical parts though installed in opposite directions. The void space in the bottom of the frame and the area around the valves compose the boiler of the machine. This is the power boiler. The water in this boiler is used as a power source only, not for extraction of coffee.
The inner component of the group moves freely and is sealed at the outer dimension with a u cup seal.
An above view of the group shows the cylinder, which contains the brew water with the piston below.
The piston moves upward in the cylinder.
Surrounding the cylinder is a large spring. This spring does not function as the spring does in a traditional lever machine, as the power source, but it acts as a counterbalancing force to the steam powered piston. It has two functions. First, it resists the upward power stroke of the steam powered piston and provides the needed pressure profile as the pressure is applied from below. Second, it returns the piston to the down position whenever the machine is not in operation.
The group is the heart of the Bacchi. It contains the boiler, the cylinder, and the piston, as well as the safety valves and control indicators for the use of the machine. The parts of the machine are all made of cast and machined aluminum. Again, as with the Gilda, the use of aluminum is both an economical consideration and a functional necessity. One of the critical engineering concepts in the machine is the transfer of heat. Thermal transfer from the water filled base to the brew water in the cylinder is most critical for proper extraction temperature. Even though there is a seal at the group to base junction the metal parts are in contact continuity throughout. The heated water and steam in the boiler serves to heat the water in the cylinder to the proper temperature for extraction and the aluminum construction aids in this process. In a classic lever group machine, the heat stability of the group is of utmost importance and that the Bacchi is not sitting at a steady state for hour upon hour but hitting the proper brew temperature of the water in the cylinder in a kinetic manner is a most impressive feat.
The portafilter is composed of 3 parts: the base which houses the control valve, the portafilter basket (54mm inside diameter) and the top cap which houses the upper screen and spouts. This part of the machine is another design challenge.....the dispersion screen in an espresso machine group is above the coffee and in this case it is the bottom of the filter basket. The portafilter is self packing in that no tamping is required. The coffee in the basket is compressed between the dispersion screen (basket bottom) and the cap screen in the form of a short cylinder (the coffee puck). This is much the same as any espresso machine, but in the Bacchi it is on top of the group rather than on the bottom. Of course, the spouts turn it all back into proper orientation to let gravity do its work and deliver the espresso into the cups!
The outer frame and the upper assembly bolt and knob contain all of the elements of the machine and all components have gaskets and seals at the interface which completely seal the machine into one unit with no leaks. With the extraction cylinder filled to the brim with water and the coffee compressed between the bottom screen of the filter basket and the upper screen of the cap there is no air space to disturb the puck or to create a drop in pressure due to the compressibility of air.
In essence the Bacchi is a dual boiler hydraulic piston espresso machine with a self packing portafiltera and a manual control function. A short description of the function is that water is heated in the base of the machine which builds steam pressure under the piston and during the heating of the power boiler heat is transferred at a controlled rate to the water in the brew chamber above the piston. As the pressure in the steam boiler increases the piston is held in the down positron by the force of the large counterspring in the group. An audible signal indicates the the moment of steady state when the predetermined ideal thermal factors have been reached and steam power is adequate to begin the extraction of the coffee. When the control valve is opened the water in the brew cylinder is allowed to enter the coffee puck. The steam pressure then forces the piston upwards, compressing the water in the cylinder (and coffee puck) to the 9 bar extraction pressure need for espresso production.
From a design standpoint once can make numerous observations:
The machine is self maintaining:
The components of the machine are all assembled in the frame from the
start each time an espresso is to be prepared. After the espresso shot
is made, the machine is allowed to cool, disassembled, and rinsed in
clean water. Therefore all of the components and seals etc are clean
before the next use, the machine is at the beginning of the
predetermined thermal cycle, and most importantly any scale
formation or buildup on the pressure control valves is minimized. I
have asked myself the question...." why not just make the machine so you
fill the lower steam boiler through a cap instead of removing
the entire group each time?" The forced removal by the user of the
group from the bottom frame controls for the many users who would
possibly not be as careful or observant as the
designer would like. There are people who simply do not follow the
instructions, so safety procedures must be built into the machine to
control for this. When using the machine, only
about 5 ml of the original 30ml in the steam boiler is vented through
the pressure valves Most of the water remains, but how much? Rather
than build in a sight glass or other method monitoring the
water level (again, to be possibly ignored by the user) simply start
over each time....this assures the proper water level and also minimizes
the possibility that the user will place the machine on the stove assuming that it
is filled with water. This also minimizes the obvious problems
which would occur in the event of scale buildup on the valves.....rather
than instruct the user to "descale every 3 months to insure proper
function of the pressure valves" which might or might not be done
by all users this method simply is a fail safe against such misuse (we
have used our Bacchi with hard water daily for 3 months and have not
noted water scale on any parts of the machine). Devious plan but very
effective safety device to control for variances in what is sometimes
called "the nut that holds the steering wheel".
Espresso consistency is enhanced through the reduction of variables: Given that the user follows the instructions and uses the proper amount of water in the steam boiler and the proper amount in the brew cylinder each time, in a consistent manner, the barista controlled variables are reduced to 3.....amount of coffee in the filter basket, grind quality of the coffee in the basket, and level of heat application to the base of the machine. Once the filter basket preparation method is followed as instructed and the heat source and time of heating is perfected the variables of espresso production are reduced to ONE! THE FINENESS OF THE ESPRESSO COFFEE GRIND! The self tamping portafilter also has an internal control on the machine.....too little coffee in the basket results in an nasty espresso shot and too much coffee in the basket does not allow the top cap to seal on the machine properly so when, in pursuit of that over the top super shot the user packs and crams and overdoses, places the machine on the heat, hears the whistle signal, and gleefully turns the knob expecting heaven in a cup all one gets is a "PFFFFFFT" and all the pressure just sputs out the side of the cap. There is no dangerous spewing of superheated water, just a pffft of the air contained in the coffee itself and it is done. The self packing portafilter does deprive the user of what can be thought of as the joy of tamping, which to some is heresy but this functional design aspect serves to increase shot consistency....there is still plenty of operator pride remaining with the Bacchi to make up for the loss of the use of the tamper.
So there you have the Bacchi. Add water in the bottom for steam power, add water in the cylinder for extraction, add coffee to the basket, add heat to the base, and enjoy a very good cup of espresso!!
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