Technically, it was Antoine Lavoisier that proved we have an internal combustion engine, but let's give the guinea pig credit. It nearly froze in Lavoisier's special calorimeter to prove something new about biophysics.
In 1774, Joseph Priestley managed to get a hold of a pure sample of the gas we now call oxygen. At last, people understood what exactly it was about the air that kept people alive, and what it was about the material produced by fire that caused people in an "air-filled room" to smother if it wasn't ventilated properly.
Antoine Lavoisier noticed that, just like a fire, people and animals seemed to warm themselves. Also like a fire, if people stayed in an unventilated room, they would eventually transform the air into a gas that smothered them. Lavoisier realized that the two processes, a fire burning and people breathing, were equivalent. Although he couldn't prove the chemistry involved, he could prove the basic physics. One of the main problems with his theory was that, unlike fire, animal-produced heat was so minor. How could it be measured?
To that end he built a Guinea Pig Calorimeter. A calorimeter is a device meant to show exactly how much heat is produced by some process. A guinea pig is a species of rodent, one member of which was unfortunate enough to belong to Lavoisier when he was in an experimenting mood. The calorimeter resembled an office water cooler, right down to the spigot on the bottom. In the "bottle" instead of water, was a series of layered chambers. The outermost chamber Lavoisier filled with snow. This was just insulation. An inner chamber was filled with ice. This was what Lavoisier was looking at most, because physicists knew the exact amount of heat it took for ice to turn to water. The innermost chamber contained the guinea pig.
The guinea pig sat in the innermost chamber, probably freezing its ass off, but saying alive and therefore breathing. The heat from its breath slowly melted the ice in the chamber around it. The ice dripped down and came out the bottom spigot. Lavoisier collected the melted water, and when he at last got about a kilogram of it, calculated that the melting of the ice required about 80 kilocalories of energy from the guinea pig. He believed that the energy came from a process known as respiration, which turned oxygen into carbon dioxide and heat, the same way a burning piece of wood might. It was the first step towards biophysics, and all thanks to one scientist and one guinea pig, both of which hopefully got to sit by a well-ventilated fire afterwards.