​Do diesel engines have spark plugs?

Diesel engines do not have spark plugs. The diesel engine uses compression ignition. Because diesel engines use diesel fuel as fuel, diesel fuel has a lower auto-ignition temperature than gasoline, and is more viscous and not easy to evaporate. In addition, the compression ratio of diesel engines is greater than that of gasoline engines. Therefore, diesel engines can compress the mixture by compression. It is ignited, therefore, a spark plug is not needed for diesel engine ignition.

Working principle of diesel engine

The role of spark plugs

Reasons why diesel engines don't need spark plugs

Diesel spontaneous ignition point

Working principle of diesel engine

The working principle of the diesel engine is as follows: 1. The diesel engine sucks in pure air during the intake stroke. At the end of the compression stroke, the diesel oil is increased to over 10MPa through the fuel injection pump and injected into the cylinder through the fuel injector. It mixes with the compressed high-temperature air in a short time to form a combustible mixture; 2. Due to diesel engine compression The ratio is high (usually 16-22), so the air pressure in the cylinder can reach 3.5-4.5 MPa at the end of compression, and the temperature can reach 750-1000 K (while the mixed gas pressure of gasoline engine at this time will be 0.6-1.2 MPa, and the temperature can reach 600-700K), which greatly exceeds the spontaneous combustion temperature of diesel; circle 3. Therefore, after the diesel is injected into the cylinder, it immediately ignites and burns after being mixed with air in a short period of time. The air pressure in the cylinder rises rapidly to 6.9MPa, and the temperature also rises to 2000-2500K. Driven by high-pressure gas, the piston moves downward and drives the crankshaft to rotate to perform work.

The engines of large ships are basically diesel engines. Small sports boats/yachts have Otto cycle engines that use gasoline as fuel, and most of them are diesel engines. Diesel engines are fired by compression ignition and atomized diesel without spark plugs. The gasoline engine has a spark plug only by igniting the atomized gasoline.

The role of spark plugs

The spark plug is an important component of the ignition system of a gasoline engine. It can introduce high-voltage electricity into the combustion chamber and cause it to skip the electrode gap to generate sparks, thereby igniting the combustible mixture in the cylinder. The working voltage of the spark plug is at least 10000V, and the high-voltage electricity is generated by the ignition coil from 12V and then transmitted to the spark plug. Under the action of high voltage, the air between the center electrode and the side electrode of the spark plug will rapidly ionize, forming positively charged ions and negatively charged free electrons. When the voltage between the electrodes reaches a certain value, the number of ions and electrons in the gas increases like an avalanche, so that the air loses its insulation, the gap forms a discharge channel, and a breakdown occurs.

Reasons why diesel engines don't need spark plugs

What the diesel engine sucks is not the fuel mixture, but all the air. These gases are strongly compressed in the cylinder, and the temperature can reach 700C to 900C. (In gasoline engines, the temperature of the fuel mixture can only reach 400°C to 500°C). At the end of the compression step, a small amount of atomized diesel will be sprayed into the hot air through the nozzle. These oil mists start to ignite spontaneously because the temperature is too high. Therefore, diesel engines do not need a carburetor or a spark plug.

Diesel spontaneous ignition point

The ignition point refers to the minimum temperature required for a combustible substance to be heated and ignited, which is the "flame point" of the substance. Also called "fire point". The lower the ignition point of a substance, the easier it is to burn. When diesel is used in a diesel engine, it is generally believed that the auto-ignition temperature of diesel at a pressure of 3MPa is roughly 205C. The lower the spontaneous combustion temperature of diesel, the greater the risk of spontaneous ignition during storage and transportation. However, once this type of diesel is injected into the cylinder of a diesel engine, it can quickly self-ignite without requiring the air to reach a too high temperature, making the diesel engine easy to start.

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