49.18. What causes the fuel to ignite in diesel engines?
UAЗа рахунок чого запалюється паливо в дизельних двигунах?
Question without image
This is an exam question on the technical training of a driver, which supplements knowledge of the Traffic Rules of Ukraine and helps to better understand the operation of a vehicle. On the theoretical exam, such topics are important because they affect the correct use of the car, assessment of its serviceability, and safe behavior on the road, even if the traffic rules themselves do not describe the principles of engine operation in detail.
In fact, it checks the understanding of the difference between a gasoline and a diesel engine, that is, the section which in training courses goes alongside the traffic rules as "Structure and operation of vehicles." In a diesel engine, ignition occurs through self-ignition: first, only air enters the cylinder, then it is compressed very strongly, which causes a sharp increase in pressure and temperature, and under these conditions, fuel is injected, which almost immediately ignites without a spark.
Among the answer options, "spark plug spark" is characteristic of gasoline engines, so for diesel this is an incorrect statement (it does not have conventional spark plugs). The option about "open flame from the previous portion" is also incorrect, because the process in the cylinder occurs due to controlled compression and injection, not the transfer of flame. Therefore, the correct explanation is about the sharp increase in pressure and temperature, which ensures the self-ignition of the fuel.
Clause 1.1
These Rules, in accordance with the Law of Ukraine "On Road Traffic," establish a unified procedure for road traffic throughout the territory of Ukraine. Other regulatory acts related to road traffic must be based on the requirements of these Rules.
Application to the question: the Traffic Rules of Ukraine do not contain specific clauses describing the structure of the engine or the physical process of fuel ignition in a diesel engine; this question pertains to technical knowledge about the vehicle, which is included in the exam alongside the Traffic Rules, but is not regulated by a specific provision of the Traffic Rules.
That is, the correct answer is "Due to a sharp increase in pressure and temperature," given that according to the definition in the Traffic Rules (clause 1.1), the Rules establish the procedure for road traffic, but do not describe the principles of engine operation, so ignition in a diesel engine is explained technically: self-ignition of the fuel from the high air temperature created by strong compression (a sharp increase in pressure and temperature).
In a diesel engine, fuel is ignited not by a spark, as in a gasoline engine, but due to the phenomenon of self-ignition. This means that the source of ignition is the high temperature in the cylinder, which occurs during strong compression of air.
Initially, during the intake, only air enters the cylinder, without fuel. Then the piston compresses this air with a very high compression ratio, so the pressure rises sharply, and along with it, the temperature increases dramatically. It is this heated air that creates the conditions for ignition.
At the end of the compression stroke, the injector sprays diesel fuel under high pressure into the already hot, highly compressed air. Under such conditions, the fuel ignites almost immediately, without any electric spark, because the temperature and pressure are sufficient for its self-ignition.
Therefore, the correct answer is "Due to the sharp increase in pressure and temperature," since in a diesel engine, the air is strongly compressed before the fuel is injected, which causes the pressure and temperature to rise sharply, and the fuel self-ignites without a spark.