June 12, 2025

Occurrence and effects of soot

Since combustion starts in diesel engines by injecting fuel into the the combustion chamber, there isn't enough time for air to mix with the fuel and because diesel fuel does not easily vaporize, it is difficult to mix with the air, causing pockets of concentrated of concentrated fuel. 
Because of this, parts have insufficient oxygen, causing incomplete combustion and generating soot (carbon). There is tendency for a large amount of soot to be generated when depressing the accelerator pedal heavily in particular, such as when starting or going up hills, as large amount of fuel is injected and pockets with insufficient oxygen are more likely to occur.
The ejected soot not only causes air pollution but also has adverse effects on the human body. Therefore, at the time of vehicle inspection, it is necessary to inspect whether the soot discharge is exceeding the standard or not.

This soot is not only ejected as exhaust gas, but is also absorbed into engine oil, degrading the oil. That's why engine oil for diesel engines contains many additives.
Additionally, if large amount of soot enters into the oil, wear of engine parts will accelerate and oil consumption will increase.

Therefore it is extremely important to inspect the oil on a daily basis.

Engine oil classifications for diesel engines
The diesel fuel contains sulfur and oil. Because of the sulfur content, sulfuric acid is generated through combustion.
This sulfuric acid is strong, corroding bearings, oxidizing oil and generating sludge. Therefore, diesel engine oil is required to have clean dispersiveness to prevent the generation of sludge and the ability to neutralize acid. Engine oil for diesel engines are classified into several standards, making it possible to know to which extent these performances are satisfied.

JASO classification
JASO is the Japanese Automotive Standards Organization. In Japan, API service classifications are commonly used as a quality standard for vehicle diesel engine oils, but due to design differences between Japanese and US engine, quality standards for engine oils more suitable to Japanese-made diesel engines were set.
There were newly set for vehicles equipped with post-treatment devices such as Diesel Particulate Filters (DPF) which complied with new short-term emission regulations, with DH-2 for trucks and buses, and DL-1 for passenger class vehicles.
 

Diesel engine description

Diesel engine are diesel fueled engines that are used in comparatively large vehicles, as they can produce a large amount of force at low speeds.
In addition, they have a good fuel economy and the fuel is cheap, making them economical and a popular choice for passenger cars as well.

In order to look at the characteristics of these diesel engines in detail, let's first compare gasoline engines and diesel engines.

The basic structure of both gasoline and diesel engines are almost the same, the combustion cycles are the same.  So what are the differences between gasoline and diesel engines ?
Let's look at each in detail.
  • Differences in the combustion method
    In gasoline and diesel engines, the method with which fire is applied to burn the fuel is quite different.
    In gasoline engines, a spark ignites the fire.
    In diesel engines, the fuel burns through self-ignition.
    Self ignition is the same as pouring cooking oil into a pan, setting a fire under it, and leaving it.
    As the temperature of the oil rises, it self ignites and begins to burn. This is the biggest feature of diesel engines.
  • Differences in the fuel used
    Diesel engines used for automobiles use diesel fuel, while gasoline engines use gasoline. Diesel fuel has a higher flash point than gasoline, as well as a lower ignition point. 
    In other word, gasoline will catch fire with a fire applied to it even at low temperatures, while diesel fuel will not at normal temperatures under the same conditions. The diesel fuel will be the first to ignite on its own when raising the temperature of the fuel itself.
    For this reason, ignition occurs in gasoline engines using spark plugs, while in diesel engines heat is applied to the diesel fuel directly for self-ignition.
    In diesel engines, heat is generated by compressing air. In self ignited diesel engines, it is necessary for the fuel to ignite easily, and so diesel fuel with good ignitability is convenient.