The inherent complexity of the steam engine - with its attendant furnace, boiler, pumps, coal bunkers, ash pans, and myriad other adjuncts - and the necessity of employing a licensed engineer full-time were defects obvious to users and manufacturers of steam power. The constant threat of boiler explosion was a source of anxiety to the general public too. As a result, inventors sought to harness energy sources based on some completely different principle - sources able at least to compete with steam, and ideally to supplant it. When suited to the particular situation and where available, water power was employed, and, to a far less extent, wind. Both had topographical and geographical limitations that excluded them from anything like universal use.
The most attractive alternative energy source was the explosive power locked in such combustible substances as gunpowder, gas, and a variety of liquid vapors. Systems were developed to burn such fuels directly in the cylinder of an engine based loosely on the steam engine, the pressure resulting from the combustion forcing a piston out of the cylinder - a notion suggested, of course, by the cannon. Throughout the 19th century a host of inventors struggled to find better fuels, to develop means for introducing just
the right amount of fuel into the cylinder in a cycle that would produce continuous motion, to discover the best way of igniting the fuel at precisely the right moment, to control the explosive violence of combustion, and to solve innumerable other problems that stood in the way of a successful internal combustion engine.
By the 1840's several engines had been invented that burned either illuminating (coal) gas or volatile liquids. They operated poorly. Not until 1860 was one invented that became commercially successful: a gas engine patented by J. J. E. Lenoir of Paris. Hundreds of Lenoir engines were built in France and England for light industrial applications. They were able to compete with steam engines - even though the latter were far more efficient - because they could be started instantly, they consumed no fuel when not actually working, they occupied much less space, and they were free from the danger of explosion.
It was not the Frenchman Lenoir, however, but the German Nikolaus August Otto who solidified the position of internal combustion as an important source of mechanical power. In 1867 an engine designed by Otto and his associate Eugen Langen was shown at the Paris Exposition. Tests revealed that its gas consumption was about half that of the Lenoir engine and another competing engine.
Although makers of steam engines and boilers scarcely had much cause for alarm, by the time of the Centennial internal combustion engines were being sold in considerable numbers. As at Paris nine years earlier, three engines were shown at Philadelphia: one French, one German and one American. The judges gave the Mignon & Rouart 'Noiseless Gas Motor' a somewhat left-handed award, praising its simplicity of construction, and declaring it capable of rendering 'good service in cases where economy of gas is not of the first importance.' To be taken more seriously, though, were the other two engines. One was the Otto & Langen, successful for nearly ten years and on the verge of giving way to Otto's four-stroke-cycle engine. The other was the Brayton Engine, the first practical liquid-fueled internal combustion engine. It was the dawn of an era.
While the gas engine delivered its users from the encumbrances of the steam engine, it was still, in large cities, tied to the gas mains. Matters were even worse in isolated areas, where anyone using a gas engine had to install a gas generator. This was as massive and potentially dangerous as any steam boiler, and thereby negated much of internal combustion's advantage. Thus there was a clear inducement to develop an internal combustion engine fueled with combustible liquid. Several inventors came up with designs, more or less successful, that burned the volatile petroleum fractions in the range of what today we know as gasoline. But the capricious behavior of such fuels did little to inspire confidence in these machines.
In 1872 George Brayton of Exeter, New Hampshire, patented an engine capable of burning the heavier, safer fractions in the kerosene range, then in wide use as illuminants. This oil was atomized and sprayed into the cylinder by a charge of compressed air. When ignited, the charge of burning gas exerted a practically constant pressure on the piston, much like the action in a steam engine. This cycle of constant cylinder pressure came to be known as the Brayton Cycle, and is that on which modern gas turbines operate.
Brayton engines were built in a number of forms, and sold by several firms under license. The chief selling point was safety and convenience, their fuel economy being about midway between the Lenoir and the Otto & Langen. The Centennial judges were impressed, describing the Brayton engine as 'a new system for generating power, of great interest and promise, and which is so far developed as to have attained a very satisfactory degree of practical success.' Since it could operate free of gas mains, attempts were later made to propel boats and streetcars with Brayton engines.
Nikolaus Otto achieved success by employing a concept that had attracted inventors from the first. He did not explode a charge of combustible fuel in the cylinder and harness the force of the piston as it was driven out. Rather, as a means of fully utilizing the expansion of the burning gasses within the cylinder, Otto embraced the 'atmospheric' principle. Here, a gas-air mixture, when ignited, drove a 'free piston' up a long vertical cylinder, as far as the explosion would carry it, doing no work on the upstroke. By the time the piston had reached the top of the cylinder, the gasses below had cooled to the extent that a partial vacuum was formed, into which the piston was then driven with considerable force by the atmospheric pressure above plus its own weight. These forces were harnessed, through a rack on the piston rod that engaged a pinion on the drive shaft, which was turned as the piston and rack descended.
The success of the Otto & Langen engine was due in large part to the ingenuity of its mechanical details, especially the slide valve that admitted a measured charge of gas and ignited it from an open flame outside the valve, and the clutch mechanism that not only allowed the piston to rise freely but also, in starting, raised it slightly to draw in the initial charge. Combining the inherent advantages of internal combustion with economy, reliability and excellence of design, the engine sold well. N. A. Otto & Co. of Cologne (later Gasmotorenfabrik Deutz) built more than 5,000, and many others were produced by the English licensees, Crossley Bros, of Manchester. The engine shown in 1876 is a Crossley, of about one horsepower capacity.
Even while his 'atmospheric' engines were successfully spreading the gospel of internal combustion power, Otto knew that it should be possible to extract more of the theoretical energy in the gas. This, obviously, would further improve the competitive position of internal combustion vis-a-vis steam, particularly in large-size engines where fuel economy was especially important.
In 1876, after fifteen years of experimentation-and just a bit too late to exhibit at the Centennial-he finished and patented a new engine. Its gas-air mixture was drawn into the cylinder, then compressed by the piston impelled by the momentum of the flywheel, and then ignited, driving the piston back again in a 'power stroke.' In a final inward stroke of the piston, the burned gasses were driven out of the cylinder or 'exhausted.' These four distinct processes, each occurring during one stroke of the piston, gave to the engine the term, 'four-stroke-cycle' (since shortened to 'four cycle'). The key to the engine's greater thermal efficiency was the compression of the fuel mixture, which greatly increased the force of the explosion.
The increase was sufficiently dramatic, in fact, that for the first time internal combustion was placed on a soundly competitive footing with steam. The Otto four-stroke engine was put immediately into production, and shown publicly at the Paris Exposition of 1878. Its obvious merits drove from the market all engines of the Lenoir and Otto & Langen types. An advantage nearly as important as its economy was its mechanical simplicity compared to the clattery Otto & Langen engine, although the slide valve and open-flame ignition system of the 'Otto Silent' were adapted from the Otto & Langen.
Licensees around the world built Otto engines for every conceivable light service. The engine displayed in 1876 is an early product of Schleic-her, Schumm & Co. of Philadelphia, the American licensee. Built in 1882, it produced four horsepower at 160 rpm, and is virtually identical to the engines of the period built by Gas motorenfabrik Deutz.