Combustion is the process of burning a fuel to generate a rapid change in temperature and pressure of a gas, so internal combustion, is the same process, but contained inside a heat engine.

As the gas expands, the kinetic energy of the gas molecules is converted into mechanical energy—either linear (in a piston and cylinder) or rotary (in a turbine)

To be honest, this concept has little meaning outside the usage internal combustion engine (see the write-ups in that node for a much more detailed explanation).

The phrase was coined in the late 1800s to distinguish a new type of engine—pioneered by Rudolf Diesel—from the more familiar steam engine.

In a steam engine, the steam is made by heating water in a boiler. Clearly, the boiler and associated combustion zone, are all outside the actual engine, even if they are physically connected to it. The hot, pressurised steam is made in the external boiler, and then allowed to flow into the engine, where it is made to work the pistons inside their cylinders.

Similarly with a steam turbine. Steam is generated in huge boilers, and fed into the turbine. The pressure of the steam acts on the turbine blades to cause motion. As the turbine speeds up, so the pressure and temperature of the gas falls. Hot, dry steam goes in one end of the turbine and cold, wet steam comes out the other. This cold, wet steam is pumped back into the boiler, where it gets a boost of energy, and the cycle continues.

The important thing to note in both engines is that the steam is basically a passive carrier of the heat energy, taking it from the boiler, into the engine, where the energy is extracted from the steam and converted into mechanical form.

With internal combustion, however, the driving force for the engine is the energy released during the combustion process. In a diesel engine, or a gas turbine, air and fuel are mixed, and pumped into the engine. Then, at a predetermined place within the engine, the fuel is ignited. Immediately, there is a huge increase in temperature and pressure, and this energy is used to act on the piston (in the case of a diesel or petrol engine) or the turbine blades (in the case of a turbine).

The process is much more active in the internal combustion, with the energy carried into the engine in the form of chemical energy latent in the fuel. That is transformed into heat and kinetic energy during the combustion, which is then converted into mechanical energy through the engine mechanism.

The lady on the exercise show
talks in calories burned
every hour. 413 for tennis
if you don’t have a partner;
236 for soothing Tai Chi.

I want to ask her the count
for smashing an electric dryer
on a hairy wet bathroom floor
blindly sobbing and screaming
until your vocal cords crack.

I bend to unplug the sparking remains
as a commercial clatters, dumb company
that tells me nothing about home repair.
The exercise lady glows and smiles,
bends herself into a clever yoga pose:
244 calories and the dog faces down.

In*ter"nal-com*bus"tion, a. (Mach.)

Designating, or pertaining to, any engine (called an Internal-combustion engine) in which the heat or pressure energy necessary to produce motion is developed in the engine cylinder, as by the explosion of a gas, and not in a separate chamber, as in a steam-engine boiler. The gas used may be a fixed gas, or one derived from alcohol, ether, gasoline (petrol), naphtha, oil (petroleum), etc. There are three main classes:
(1) gas engines proper, using fixed gases, as coal, blast-furnace, or producer gas;
(2) engines using the vapor of a volatile fluid, as the typical gasoline (petrol) engine;
(3) oil engines, using either an atomized spray or the vapor (produced by heat) of a comparatively heavy oil, as petroleum or kerosene. In all of these the gas is mixed with a definite amount of air, the charge is composed in the cylinder and is then exploded either by a flame of gas (flame ignition -- now little used), by a hot tube (tube ignition) or the like, by an electric spark (electric ignition, the usual method is gasoline engines, or by the heat of compression, as in the Diesel engine. Gas and oil engines are chiefly of the stationary type. Gasoline engines are largely used for automobile vehicles, boats, etc. Most internal- combustion engines use the Otto (four-stroke) cycle, though many use the two-stroke cycle. They are almost universally trunk engines and single-acting. Because of the intense heat produced by the frequent explosions, the cylinders must be cooled by a water jacket (water-cooled) or by air currents (air cooled) to give the maximum thermodynamic efficiency and to avoid excessive friction or seizing.


© Webster 1913

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