Otto's gas machine has no other disadvantage than that it is too loud. With a consumption of 1 cubic meter illumination gas per horse power and hour it consumes still slightly more than the Otto-Langen gas machine [the atmospheric gas machine]. Otto's principle to achieve a gas machine running at a constant pressure than having an explosion engine is to compress the mixture of air and combustible gas to a pressure of 2 atmospheres before igniting the mixture. As shown in the [above] diagram the effect of this and the addition of a large amount of air is to mitigate the ignition of the mixture and to stretch it over a longer period of time. The section ab in the diagram shows this effect. Conversely the section bc is similar to the expansion phase of an ordinary steam engine. Dr. Slaby, lecturer for small power engines at the Imperial Trade Acadamy in Berlin [later the Berlin-Charlottenburg Technical High School and, today, the Berlin Technical University] explained the effect due to different kinds of gas layered in the cylinder where an explosion starts at the most saturated combustion gas which progresses step by step to less saturated gas layers. However, the diagram seems to indicate that the combustion is stretched over a longer time period. The similarity of this diagram with diagrams for the Hock petroleum engine and the Brayton petroleum and gas engine indicates that this is not a new principle, although in these machines the processing steps are distributed over different vessels. 

In our view, the main progress of the Ott gas machine is to unify the different processes in the same cylinder which simplifies the design. Unlike the steam engine, heat losses are prevented. As the machine needs two rotations to perform one working stroke should rather called a half action machine than a single action machine. 

The four process phases in the diagram can be seen as


stSuction phase
tacompression phase
abc*explosion and expansion phase
tsexhaustion phase

* in the original document tac, but I think it should be ab explosion phase and bc expansion phase =  abc during one stroke. ct probably is the start of the exhaustion. Depending on the timing of the valves shortly before or after the upper dead point.


According to the discussed sequence the sliding plate S must only move back and forth only one time during the four phases. For this purpose the sliding plate as actuated by a shaft w which has only half of the rotational speed of the fly wheel shaft.

Fig. 4 shows die starting position of the sliding plate at the moment of the explosion. The piston is at the dead point, the cam of the shaft for actuating the slider plate is 45° before its inner dead point. By the effect of the explosion the piston is moved outwards to its outer dead point. The shaft for actuating the slider plate performs half of its 180° cycle, which is 90°, so that at the begin of the cycle where the piston is returning, the slide plate is in the same position as shown in Fig. 4. The intake opening e of the cylinder therefore stays closed during the reverse stroke whereas the exhaust opening a is opened by a lever that is adjoined to a cam disc on the shaft w.

Air enters the gas machine by an air inlet l. The illumination gas enters the gas machine by a gas tube G. The access of the illumination gas to the cylinder may be interrupted by a gas valve d, which is normally kept fully open by a gas lever.  A regulator R is driven by the shaft w. If the speed of the gas machine is above the normal speed the regulator R pushes a sleeve on the shaft, which then deactivates the gas lever p by which the flow of the illumination gas is fully interrupted. Without fuel the gas machine will return to its normal speed and the regulator R will then allow the gas lever p to fully open the gas valve again. 


source: Wilman, "Otto's geräuschlose Gasmaschine" in "Dingler's polytechnisches Journal" 1878, Band 228, pages 201-205