Obviously the lean mixture of combustible gases, which was vital for achieving a controlled combustion, was difficult to ignite. Otto must have recognized that he could not create, as cloud of gas particles wanted to illustrate in his 532 patent, a higher concentration of combustible gas close to the initiation flame, but still was not ready to fully renounce on his layered gas theory. He filed another application with the title "Improvements of the gas engine patent N° 532", which was granted a patent N° 2735 on 4 August 1877. In the application Otto stated:

If the filling of the cylinder is not stacked in layers, but more or less homogenously mixed, so this mixture should be considered as a diluted gas mixture. This mixture may be ignited by a small gas flame, an electrical spark, etc., but the ignition is slower than with a layered stacked gas mixture, as a quick initiation of the combustion by the stronger mixture is absent.

However, it is possible to combust such a homogenous, diluted mixture with the required higher speed, when instead of initiating the combustion at a small spot the ignition is caused over a longer line simultaneously. 


The coloured version of Fig. 10a of the N° 2735 patent shows a the piston (in purple colour) is at its extented stroke, a combustion space is depicted in yellow colour, and a control slide for allowing air and combustion gases to enter the combustion space in green colour. The tube containing the undiluted gas is shown in red and was termed firing channel. For igniting the undiluted combustion gas an ignition flame was used. Various similar concepts of a firing channel are still today subject of patent applications for petrol or diesel engines.

As the  firing channel is filled during the suction period I assume the ultimate technical effect of the firing channel is that if it is dimensioned sufficiently long the undiluted combustion gas furthest away from the combustion space is "safe" from the compression stroke and thus stays undiluted. 

This invention was such an important improvement that the Gasmotorenfabrik Deutz not only build all models with this firing channel but also retrofitted the gas engines that were already in use at their clients.



  1. Cause the combustion of a diluted mixture of explosive gas by forcing a flame of a stronger explosive mixture into a space with a diluted mixture, substantially and for the purpose as described in [section] 1 [of the description].

In a nullification action the claimants tried to revoke the patent as finally admitted by the patent proprietor were sold with various lengths of firing channels. It was therefore important to investigate if the firing channel of the sold gas engines were sufficiently long to have already substantially achieved the technical effect that was claimed by the patent. The description did not give any hint how long the firing channel should be. Expert witnesses could show that for one specific model a length of 34mm and still for 90mm drop outs were observed but not for a length of 130mm. On the other hand, the tests also proved that the length of the firing channel is not suitable for controlling the duration or the intensity of the combustion.  

(Decision of the Imperial Court of 21 June 1890)