We asked the Library of Congress for a picture of the Otto engine, which is considered the first practical power unit among internal combustion engines. The picture shown here was sent. It is identified as illustrated in Scientific American, possibly in the 1870s.
Below is the article that appeared with the picture.
The successful gasoline engine should, first of all, be so constructed as to prevent any leak of gasoline either in vapor or fluid form, and it should besides be simple in design and reliable in operation for each function belonging to the cycle of work of the engine. The Otto Gas Engine Works, of Philadelphia, who have made a national reputation on their Otto gas engines, have endeavored to meet these conditions, and the engine herewith illustrated represents the smallest size of such an engine which they have recently placed on the market. No separate apparatus is used for producing vapors, but the gasoline is conveyed to the engine from a supply tank placed outside of building, and only mixes with air when it reaches the engine cylinder, where it is fired at once.
The igniting is done by a hot tube, which has been found so efficient a device with the modern Otto gas engines, and this tube is heated by a flame, similar to that produced in gasoline stoves, and surrounded with the same precautions for safety. The Otto gasoline engine is also fitted for electric ignition, and the engine is so arranged that it can be furnished with either form of igniter as desired.
Among the sizes offered by the Otto Gas Engine Works some are specially designed for electric lighting, running at high speed and adapted for use in country residences, hotels, public gardens and grounds, etc. Other sizes have been made of portable design and are available as farm or contractors' engines, for thrashing, hay baling, pumping for irrigation, etc. The size illustrated is of about four horse power, and this size has been in demand from grain dealers for running elevators, conveyors, feed mills, corn shellers, etc. The running expense is of course very low, and as compared with gas engines the cost corresponds to that of gas at 60 to 80 cents per 1,000 c. ft., gasoline being 8 to 10 cents per gallon.