Otto was obsessed by his original idea that the spread of the combustion from gas particle to gas particle was slowed down by the layers of combustible gas mixture, where the combustion was initiated, then progressing to a layer of fresh air and finally extinguished by the most upper layer of combustion residuals of a previous combustion cycle. He ignored that by the compression these layers inevitably will mix.

Today, for perfect efficiency, we know that a homogenous mixture of combustion gas, fresh air and combustion residuals works best. Indeed, following the ignition a flame front is formed, the speed of which increases with increasing pressure and temperature as the gas particles move around more quickly and statistically come into close contact more often. Only at the end of the combustion the speed slows down. In average the flame front progresses at 15...25 m/s [1]. However, for lean-burn engines a layered charge, as envisaged by Otto, increases the engine's efficiency but is only feasible with direct fuel injection at the end of the compression phase, shortly before the ignition takes place [2]. 

 

 

Obviously the lean mixture of combustible gases, which was vital for achieving a controlled combustion, was difficult to ignite. His answer was another invention: The firing channel in which an undiluted mixture of air and combustible gas was retained in a supply channel of the gas mixture. Finally he had produced at least two layers of different kind of gas, one layer in the combustion space and one layer in the supply channel for the intake. Unfortunately, he filed the patent application after he had already sold several machines with this invention and the patent was eventually revoked.

 

 

Despite Otto was initially misguided, he did the right thing: He created in the cylinder a lean mixture by mixing combustible gas, air, and residual combustion gas. He had learned form experimenting with his model that he cannot dilute the combustion mixture too much, as it will not ignite if it is too lean. Today we define the fraction of added air mass to the theoretical amount of air mass for a complete combustion as the lamda factor [3]. To insure an ignition for gasoline lamda needs to be in a range of 0.75 ... 1.3 [4]. Otto also had experienced that for insuring an ignition it is favourable to compress the gas mixture.

 

 

[1] Ottomotor-Management, Robert Bosch (Hrsg.), Viewweg, 3. Auflage, 2005, page 18-19.

[2] Ottomotor-Management, Robert Bosch (Hrsg.), Viewweg, 3. Auflage, 2005, page 19.

[3] Ottomotor-Management, Robert Bosch (Hrsg.), Viewweg, 3. Auflage, 2005, page 18.

[4] Ottomotor-Management, Robert Bosch (Hrsg.), Viewweg, 3. Auflage, 2005, page 19.