Although the descriptions are written in German, the diagrams in this Poccina brochure
are excellent, one of which I've excerpted below:Diagram of a typical lever machine (see page 8)
Jim's Home Barista's Guide to Espresso
introduces the history and basic designs, which explains the diagram above:
In 1901, the Italian inventor Luigi Bezzera came up with a workable solution. Pavoni manufactured these first espresso machines in 1905.
This machine was also steam powered. However, the steam does not come into contact with the coffee. Instead, steam pressure at the top of the boiler forces water at the bottom of the boiler through ground coffee. The coffee is held in a group consisting of a portafilter, a metal filter basket and removable brass mount, and a brew head into which the portafilter attaches. The piping and group were designed to act as heat radiators, so the temperature of the pressurized water dropped from 250°F (120°C) in the boiler to the correct brewing temperature at the grouphead. This brewing principle is still used in stovetop mochapots. Since the water was pressurized, the coffee could be ground finer than in a regular pourover brewer, reducing the minimum brewing cycle from about 4 minutes to 30 seconds. Espresso machines and their accompanying coffee grinders became the standard equipment for making coffee in Italy, Southern France, Spain and Latin America. In other parts of the world, it followed Italian immigrants who popularized it in each country they settled.
But technology moves on, and this method is no longer regarded as specifically espresso, although mochapots and other steam pressured brewers continue to be marketed under the name. In the 1920s through the 1940s, Italian engineers experimented with pumping devices to increase the brewing pressure. The first practical one was developed by Cremonesi in 1938 and manufactured by Achille Gaggia in 1946. It used a hand powered piston. On machines of this type, steam pressure in the boiler forces the water into a cylinder, but then it is pressurized further by a spring-powered piston to about 8 to 9 bar (120 to 135 PSI), or 8 to 9 times the pressure that had been developed by the steam machines. The spring that powers the piston is compressed by a lever forced down by the barista (Italian for barkeep)--the person making the coffee. As with the older generation machines, these lever groups are designed to cool the water from boiler to brewing temperature.
My own Espresso Machines 101
and Espresso Machines 201
cover the popular non-lever types of espresso machines.