Otto's miscomprehension or vision?
Otto displayed a strong fixation on his initial concept, which proposed that the introduction of various gas layers into the cylinder would impede the rapid spread of combustion and consequently mitigate the explosive impact on the piston. According to his theory, the combustion process would commence with a mixture of air and combustible gas, then advance to a layer of fresh air, and ultimately conclude in the last layer consisting of residual combustion byproducts from the previous cycle. However, Otto failed to recognize that the subsequent compression stroke after building the layers during the engine's suction phase would inevitably blend these distinct layers together.
In order to achieve optimal efficiency, it is now understood that a uniform mixture of combustion gas, fresh air, and combustion residuals is most effective. Upon ignition, a flame front is generated, which accelerates in speed with increasing pressure and temperature. This acceleration is due to the increased movement of gas particles, resulting in a higher likelihood of close contact. Towards the end of the combustion process, the speed of the flame front decreases as less combustible gases and oxygen remains in the cylinder. On average, the flame front progresses at a rate of 15 to 25 meters per second [1].
However, for lean-burn engines, the implementation of a layered charge, as initially proposed by Otto, can enhance the engine's efficiency. This approach, however, is only feasible with direct fuel injection at the end of the compression phase, just prior to ignition [2].
Otto demonstrated a visionary mindset, despite lacking the necessary technological resources to actualize his ideas.
The ignition of the lean mixture of combustible gases, crucial for achieving controlled combustion, posed a significant challenge to ignite. In response, Otto devised a solution known as the firing channel. This mechanism involved the containment of an undiluted mixture of air and combustible gas within a supply channel. Consequently, two distinct layers of gas were established: one in the combustion space and another in the intake supply channel. Regrettably, Otto submitted the patent application after having already sold multiple machines incorporating this innovation, resulting in the subsequent revocation of the patent.
Despite being misguided, Otto's proposal successfully prevented the rapid self-destruction of his engine. He achieved this by creating a lean mixture in the cylinder through the combination of appropriate amounts of combustible gas, air, and residual combustion gas. Through experimentation with his model, Otto discovered that the combustion mixture cannot be diluted excessively, as it will fail to ignite if it becomes too lean. In contemporary terms, we now refer to the ratio of added air mass to the theoretical amount of air mass required for complete combustion as the lambda factor [3]. For gasoline to ignite properly, the lambda factor needs to fall within the range of 0.75 to 1.3 [4]. Otto also observed that compressing the gas mixture was advantageous in ensuring ignition.
[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.
[5] Bryant, Lynwood. “The Origin of the Four-Stroke Cycle.” Technology and Culture, vol. 8, no. 2, 1967, pp. 178–98. JSTOR, doi.org/10.2307/3101967. Accessed 14 Sept. 2023.
Last update 14/09/2023