JP5052874B2 - Fuel oil composition for diesel engines - Google Patents

Description

  The present invention suppresses solidification / gelation that occurs when fuel is at a high pressure and maintains an appropriate combustion state in a high-pressure fuel injection diesel engine such as a common rail fuel injection device or a unit injector fuel injection device. And a fuel oil composition capable of avoiding mechanical damage to the injection device.

In recent years, reduction of exhaust gas emitted from automobiles and improvement of thermal efficiency have been demanded due to environmental problems. Diesel vehicles have better thermal efficiency than gasoline vehicles and have less CO ₂ emissions, but the impact on the human body due to particulate matter (PM) emissions and NOx emissions due to the diffusion combustion characteristic of diesel Concerns and adverse environmental impacts are a problem.

  In order to improve the combustion mode against the problem of PM and NOx discharged from these diesel vehicles, introduction of a common rail type fuel injection device and a fuel high pressure injection device represented by a unit injector method has been advanced. The high-pressure injection of fuel can improve the combustion mode by promoting atomization of the fuel injected into the engine cylinder and promoting mixing with the cylinder air. The maximum fuel injection pressure by the common rail type fuel injection device or the unit injector type fuel injection device is very high, about 160 to 200 MPa, compared to the injection pressure of about 50 MPa of the row type injection device which is a typical conventional vehicle. Is going further. In addition, these high-pressure fuel injection devices use electronic control to pool fuel that has already been increased in pressure rather than the fuel injection pressure corresponding to the engine speed so far, and determine the optimal fuel injection pressure based on operating conditions. In addition, since the injection is performed by adjusting the injection pressure, it is possible to maintain the optimum combustion state in the entire operation region. Further, in the above-described high-pressure injection type vehicle, the size of the injector nozzle injection hole is also smaller than before, which contributes to the miniaturization of the injected fuel.

  With the introduction of such a fuel injection device, the combustion mode has been greatly improved, and PM emissions and NOx emissions have been greatly reduced. And by reducing the exhaust gas from the engine out, it becomes possible to use various post-processing devices (DPF, DPNR, etc.) at the rear stage of the engine that could not be installed with the amount of PM and NOx exhausted from the conventional engine. A significant reduction in NOx has been achieved.

  On the other hand, as described above, high-pressure injection of fuel has a great effect on exhaust gas reduction and is a very useful technique, but there is little technical knowledge about the state of fuel in an ultra-high pressure state such as 200 MPa. There is no current situation. Under such circumstances, as a fuel oil corresponding to a high-pressure injection type vehicle, for example, a fuel oil that defines a density of light oil and a viscosity at 40 ° C. has been reported (for example, Patent Document 1).

Here, a compression ignition engine with a high pressure common rail fuel system can be operated with no loss of performance and substantially reduced emissions, provided that the density and viscosity at 40 ° C. are in a certain range. It is disclosed that it is possible.
JP-T-2004-514746

However, this technique aims to reduce emissions in high-pressure injection vehicles, and does not specify the state of fuel in an ultra-high pressure state.
Considering the three-state diagram of a substance, even if it is a liquid substance at normal temperature and normal pressure, when it is exposed to an ultra-high pressure state such as 200 MPa, the state changes to a solid. Compared to the conventional fuel injection system, the new fuel injection system is more than three times the maximum pressure and the minimum pressure is 50 MPa or more, so the fuel pooled under high pressure is solidified and gelled. Is concerned. It is clear that the solidification / gelation of the fuel has a great influence on the spray shape of the fuel injection designed on the assumption that the fuel is liquid, and the solid or gel state is 200 MPa. There is a concern that these parts may be damaged in a high-pressure injection type vehicle in which the nozzle hole of the injector is smaller when it is injected at a very high pressure.

  From the above technical background, the present invention can maintain the fuel state in a liquid state even under an ultra-high pressure such as that used in a high-pressure injector, and maintain an appropriate combustion state by maintaining the fuel spray form. An object of the present invention is to provide a fuel oil composition for a diesel engine that can be held and can avoid troubles such as breakage of a nozzle hole of an injector.

As a result of intensive studies, the present inventors have obtained the knowledge that enables proper fuel spraying under high pressure by using a fuel having an appropriate composition and properties, and have completed the present invention.
That is, the present invention provides a fuel oil composition suitable for a high pressure fuel injection type diesel engine such as a common rail type fuel injection device and a unit injector type fuel injection device having the following characteristics.
(1) 10% by volume distillation temperature is 180 to 225 ° C., 90% by volume distillation temperature is 315 to 350 ° C., has a sulfur content of 10 mass ppm or less, and a saturated content is 70 to 83. 3 % by volume, aromatic content is 16.7-30 % by volume, bicyclic aromatic content is 1.5% by volume or less, and polycyclic aromatic content of 3 or more rings is 0.5%. A diesel engine having a volume% or less, a naphthene content of 65 volume% or less, a polycyclic naphthene content of 35 volume% or less, and an isoparaffin content of 5 to 25.9 volume % Fuel oil composition.
(2) The fuel oil composition for a diesel engine according to (1), which is used for a diesel engine having a fuel injection pressure of 50 MPa or more.

Details of the invention are described below.
The distillation property of the fuel oil composition for diesel engines in the present invention has a 10 vol% distillation temperature of 180 to 225 ° C, preferably 183 to 222 ° C, and a 90 vol% distillation temperature of 315 to 350 ° C, preferably 317 to 347 ° C.
If the 10% by volume distillation temperature is 180 ° C. or higher, the flash point and kinematic viscosity appropriate for light oil can be maintained, and if it is 225 ° C. or lower, it has moderate volatility. And the like, and the PM derived from heterogeneous mixed combustion can be reduced. If the 90% by volume distillation temperature is 315 ° C. or higher, the kinematic viscosity can be maintained appropriately, and if it is 350 ° C. or lower, heavy components in light oil, particularly aromatic components, can be suppressed to a low level. This is preferable because good combustibility can be maintained.

Moreover, the sulfur content contained in the fuel oil composition for diesel engines in the present invention is 10 mass ppm or less, preferably 8 mass ppm or less. By setting the sulfur content to 10 mass ppm or less, it is preferable that the amount of sulfate, which is a component of particulate matter (PM) discharged from the engine, is reduced and the influence on the performance of the exhaust gas aftertreatment device is reduced.
The distillation properties in the present invention can be measured by the atmospheric pressure distillation test of JIS K2254, and the sulfur content can be measured by the microcoulometric titration method of JIS K2541.

The saturated content of the fuel oil composition for diesel engines in the present invention is 70% by volume or more and less than 85% by volume, preferably 75% by volume or more and less than 85% by volume, and the aromatic content is 15 to 30% by volume, preferably 15%. ~ 28% by volume. Among them, the content of bicyclic aromatics is 1.5% by volume or less, preferably 1.4% by volume or less, and the content of tricyclic or more polycyclic aromatics is 0.5% by volume or less, preferably 0.8%. 3% by volume or less.
Reduces PM and NOx emissions during combustion by reducing the saturation to 70% by volume and aromatics to 30% by volume, especially by reducing the levels of bicyclic aromatics and polycyclic aromatics with 3 or more rings to a low level. it can.
In addition, the composition ratio here is calculated | required based on JPI-5S-49-97 "petroleum product-hydrocarbon type test method-high performance liquid chromatograph method (HPLC)".

  The naphthene content in the fuel oil composition for diesel engines in the present invention is 65% by volume or less, preferably 60% by volume or less, and the content of polycyclic naphthenes having two or more rings is 35% by volume or less, preferably 30% by volume or less. Naphthenes and polycyclic naphthenes tend to be easily solidified under an ultrahigh pressure of 50 MPa or more because most of them have a molecularly close shape to a planar structure. Here, examples of naphthenes include cyclohexane and cyclopentylcyclohexane, and examples of bicyclic naphthenes include dicyclohexylethane and decalin. Examples of naphthenes having 3 or more rings include tetradecahydroanthracene. When the naphthenes are 65% by volume or less, particularly the polycyclic naphthenes are 35% by volume or less, the inclination of the quadratic curve of the solid-liquid equilibrium line (pressure-temperature curve) can be kept large, which is preferable.

  The naphthenes content ratio here is determined by gas chromatography-mass spectrometry after the fuel oil composition for a diesel engine is fractionated into an aromatic component and a saturated component by high performance liquid chromatography (HPLC). (GC-MS) and analysis according to ASTM D 2786 to calculate the ratio of naphthenes for each ring number, and the ratio obtained here is determined as JP-5S-49-97 "Petroleum products-hydrocarbon type". It is calculated | required by multiplying the saturation fraction calculated | required by "the test method-high performance liquid chromatograph method.

  The isoparaffin content of the diesel engine fuel oil composition in the present invention is 5% by volume or more and less than 85% by volume, preferably 10% by volume or more and less than 85% by volume. When the isoparaffin is 5% by volume or more, the slope of the quadratic curve of the solid-liquid equilibrium line (pressure-temperature curve) can be kept large, and if it is less than 85% by volume, the combustibility of the fuel oil composition is improved. An appropriate range can be maintained.

  The isoparaffin content ratio here is the sum of the naphthene content and the n-paraffin content from the saturated content determined by JPI-5S-49-97 “Petroleum products—Hydrocarbon type test method—High performance liquid chromatograph method”. Can be obtained by subtracting. The n-paraffin content can be obtained by gas chromatography (GC), and can be obtained by converting this to volume%.

The crystal oil deposition temperature at 50 MPa of the fuel oil composition for diesel engines in the present invention is 20 ° C. or lower, more preferably 15 ° C. or lower. In addition to this, the boundary between the solid state and the liquid state of the fuel oil composition The slope of the solid-liquid equilibrium line to be expressed is preferably −0.035 or more, and more preferably −0.03 or more.
If the crystal precipitation temperature at 50 MPa is 20 ° C. or less, the fuel can be kept in a liquid state at the pressure of the high-pressure fuel injection device immediately after the engine is started, and the engine malfunction immediately after the start can be prevented. If the slope of the solid-liquid equilibrium line is -0.035 or more, even if crystals are precipitated by an arbitrary pressure increase, the state changes from a solid phase to a liquid phase by a small temperature increase (a small amount of heat input). Therefore, it is preferable for maintaining the liquid phase state.

  The crystal precipitation pressure at 50 MPa and the solid-liquid equilibrium line can be obtained using the experimental apparatus shown in FIG. As shown in FIG. 2, the solid-liquid equilibrium line indicates the pressure at which crystals are precipitated in the sample chamber when the temperature in the high pressure chamber is fixed at an arbitrary temperature of −30 to 80 ° C. and the pressure is increased in steps of 5 MPa. The graph can be drawn as a quadratic function graph by plotting on a graph of horizontal axis: temperature, vertical axis: crystal precipitation pressure, and connecting a plurality of points.

  The light oil composition in the present invention is produced by appropriately distilling kerosene or the like into a light oil fraction that has been subjected to treatment such as hydrodesulfurization and aromatic extraction treatment by adjusting the distillation properties of various petroleum fractions by distillation. be able to. As JIS standard light oil, No. 1, No. 2, No. 2, No. 3, and No. 3 can be used in general, and are not particularly limited. Other diesel oil bases that do not contain kerosene fractions, or diesel oil that is fractionated after heavy oil is catalytically cracked, hydrodesulfurized, hydrocracked, dearomatized, and upgraded with heavy oil by coker, etc. The fraction can be used as long as its properties satisfy the above-mentioned properties, and is not particularly limited, but does not include naphthene or polycyclic naphthene that promotes solidification under ultrahigh pressure. It can be suitably produced by selecting a crude oil or selectively using a base material such as hydrogenating these substances in a hydrodesulfurization reaction.

  The low temperature fluidity improver is preferably added in an amount of 10 to 1000 ppm by volume, and preferably 50 to 700 ppm by volume, to the diesel engine fuel oil composition of the present invention. Addition of 10 ppm by volume or more of the low temperature fluidity improver is preferable because the clogging point (CFPP) and the pour point (PP) can be improved. Moreover, it is preferable that the addition amount of the low temperature fluidity improver is 1000 ppm by volume or less because aggregation of the additive itself can be prevented.

  Various low temperature fluidity improvers used in the present invention can be used, for example, alkenyl succinimide, ethylene-vinyl acetate copolymer, ethylene-alkyl acrylate copolymer, copolymer polymer such as polyethylene glycol derivative, Examples thereof include polymers such as chlorinated polyethylene and polyalkyl acrylate. These low temperature fluidity improvers may be used singly or in combination of two or more.

  Moreover, various other additives can be suitably mix | blended with the fuel oil composition for diesel engines of this invention as needed. Examples of such additives include lubricity improvers, cetane number improvers, surfactants, preservatives, rust preventives, defoamers, detergents, antioxidants, hue improvers, and other known fuel additives. Is mentioned. These can be added singly or in combination.

Next, the present invention will be described more specifically with reference to examples and comparative examples. In addition, this invention is not restrict | limited at all by these examples.
In Examples and Comparative Examples, the flash point, distillation properties, sulfur content, 30 ° C. kinematic viscosity, clogging point, and pour point were measured according to the method defined in JIS K 2204. In addition, 15 ° C. density was measured according to JIS K 2249, cloud point was measured according to JIS K 2269, nitrogen content was measured according to JIS K 2609, and oxygen content was measured according to JIS K 2536.

The ratio of the saturated component, the aromatic component, and the aromatic component according to the number of rings were measured based on JPI-5S-49-97. The apparatus configuration and analysis conditions of HPLC are shown below.
Equipment: Agilent 1100 Series (ALS: G1329A, Bin Pump: G1312A, Degasser: G1379A, Rid: G1362A, Colcom: G1316A)
Mobile phase: n-hexane Flow rate: 1.0 ml / min
Column: Silver nitrate impregnated silica column (4.6 ml. D. * 70 mm L. AgNO ₃ -1071-Y manufactured by Senshu Scientific)
Amine modified column (4.0 ml.D. * 250 mm L. 2 LICROSORB-NH ₂ manufactured by Senshu Scientific)
Column temperature: 35 ° C
Sample concentration: 10 vol%
Injection volume: 5 μl

The content of naphthenes and ring-based naphthenes, and isoparaffin analysis were performed by the following methods.
First, a sample was fractionated by a saturated component and an aromatic component by HPLC, and then a type analysis was performed on the saturated component by GC-MS. Based on the analysis result obtained here, the analysis was performed according to ASTM D 2786, and the content ratios of the paraffins, naphthenes and ring-based naphthenes in the saturated content were determined. The contents of naphthenes and naphthenes by number of rings were determined by multiplying the ratio of naphthenes and naphthenes by number of rings in the saturated content obtained here by the saturation ratio determined as described above.

For isoparaffin, the sum of the naphthenes obtained by the above method and the content of n-paraffin shown below converted to volume% is determined by JPI-5S-49-97 "Petroleum products-Hydrocarbon type test method- It can be determined by subtracting from the saturated content determined by “high performance liquid chromatography”.
The analysis conditions are shown below.
Apparatus: HP-6890 HP5973 Quadrupole mass spectrometer Column: DB-1: 30 m × 0.25 mm I.D. D. × 0.25μm
Oven temperature: 40 ° C. (1 min) → 10 ° C./min→280° C. (5 min)
Inlet temperature: 43 ° C. Even track mode ON
Interface temperature: 300 ° C
Carrier gas: He: 55 KPa Constant flow mode ON
Solvent Delay: 4.5min
Mass range: 50-500 Threshold = 100 Sampling # 3
Ionization voltage: 70 eV
Injection method: On-column injection 1.0 μl

The n-paraffin content and its distribution by carbon number were measured by gas chromatography (GC). The measurement conditions are shown below.
-About light oil composition and fuel oil composition for diesel engine Equipment: 5890 series2 (Agilent Technologies)
Column: Ultra 1 (Agilent) Crosslinked Methyl Silicone Gum, 50 m × 0.20 mm I.D. D. Film thickness 0.33μm
Detector: FID
Oven temperature: 60 ° C. (0 min) − (6 ° C./min)→340° C. (10 min) Run 56.7 min
Inlet: On-column
Inlet temperature: oven track mode (oven temperature + 3 ° C)
Detector temperature: 350 ° C
Carrier gas: He 280 kPa (constant pressure) 1.3 mL / min Linear velocity 29.7 cm / sec (at 60 ° C.)
Make-up gas: He
FID combustion gas: H ₂ 30 mL / min, Air 400 mL / min
Injection volume: 0.2 μl
Quantitative method: Internal standard method (Internal standard substance: Dibutyl phthalate)
-About paraffin composition Apparatus: 6890 (Agilent Technologies)
Column: DB-1 30m x 0.25mmI. D. Film thickness 0.25μm
Detector: FID
Oven temperature: 50 ° C. (1 min) − (5 ° C./min)→340° C. (20 min) Run 79 min
Inlet: Split (Back) 100: 1
Carrier gas: He 83 kPa (constant pressure) 1.0 mL / min Total 100 mL / min
Average linear velocity: 26 cm / sec
Make-up gas: He
FID combustion gas: H ₂ 30 mL / min, Air 400 mL / min
Injection volume: 0.1 μl
Sample dilution: 1/2 dilution with carbon disulfide Baseline: with correction

  The experimental apparatus in the present invention is shown in FIG. This experimental apparatus consists of a high-pressure chamber in which experimental samples are placed and a glass piston that moves up and down in response to pressure from a pressure medium (at normal temperature: water, at low temperature: ethanol). By pumping the pressure medium with a pump, the pressure in the high-pressure chamber can be increased from normal pressure to an arbitrary pressure of 300 MPa. At this time, the high pressure cell itself shown in the figure is filled with the pressure medium, so that the high pressure chamber and the outer wall of the piston can be observed without being deformed or damaged by the pressure. Observation in the high-pressure chamber can be optically performed by a microscope or the like through a glass piston through sapphire windows above and below the apparatus.

<Example 1>
75% by volume of desulfurized gas oil base obtained by hydrodesulfurizing a gas oil fraction having a boiling point range of 150 to 370 ° C. and a 90% distillation temperature of 340 ° C. obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less Fuel for diesel engines by mixing 25 vol% of desulfurized kerosene base material obtained by hydrodesulfurizing a kerosene fraction having a boiling range of 140 to 280 ° C obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less An oil composition was obtained. The resulting diesel engine fuel oil composition was mixed with a fluidity improver comprising an ethylene-vinyl acetate copolymer at 300 ppm by volume based on the total amount of the diesel engine fuel oil composition, and a lubricity comprising a long chain alkyl ester. The improver was added at 100 mg / kg to the total amount of the diesel engine fuel oil composition. Properties of the obtained diesel engine fuel oil composition are shown in Table 1.

<Example 2>
85% by volume of desulfurized gas oil base obtained by hydrodesulfurizing a gas oil fraction having a boiling point of 150 to 370 ° C. and a 90% distillation temperature of 350 ° C. obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less Fuel for diesel engines by mixing 15% by volume of a desulfurized kerosene base material obtained by hydrodesulfurizing a kerosene fraction having a boiling range of 140 to 280 ° C. obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less An oil composition was obtained. The resulting diesel engine fuel oil composition was mixed with a fluidity improver comprising an ethylene-vinyl acetate copolymer at 300 ppm by volume based on the total amount of the diesel engine fuel oil composition, and a lubricity comprising a long chain alkyl ester. The improver was added at 100 mg / kg to the total amount of the diesel engine fuel oil composition. Properties of the obtained diesel engine fuel oil composition are shown in Table 1.

<Example 3>
65% by volume of desulfurized gas oil base obtained by hydrodesulfurizing a gas oil fraction having a boiling point range of 150 to 370 ° C. and a 90% distillation temperature of 340 ° C. obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less Fuel for diesel engines by mixing 35% by volume of a desulfurized kerosene base material obtained by hydrodesulfurizing a kerosene fraction having a boiling range of 140 to 280 ° C. obtained by atmospheric distillation of crude oil to a sulfur content of 10 mass ppm or less An oil composition was obtained. The resulting diesel engine fuel oil composition was mixed with a fluidity improver comprising an ethylene-vinyl acetate copolymer at 300 ppm by volume based on the total amount of the diesel engine fuel oil composition, and a lubricity comprising a long chain alkyl ester. The improver was added at 100 mg / kg to the total amount of the diesel engine fuel oil composition. Properties of the obtained diesel engine fuel oil composition are shown in Table 1.

<Comparative Example 1>
A gas oil fraction having a boiling point range of 180 to 390 ° C. and a 90% distillation temperature of 350 ° C. obtained by atmospheric distillation of a naphthene-based crude oil having a higher naphthene and polycyclic naphthene content than the crude oil used in Example 1. N-paraffin mixture in which 90% by volume of desulfurized gas oil base obtained by hydrodesulfurizing and dearomatizing to a sulfur content of 10 mass ppm or less was adjusted to have a boiling point range of 250 to 350 ° C. using commercially available n-paraffin. The fuel oil composition for diesel engines was obtained by mixing 10 volume%. The resulting diesel engine fuel oil composition was mixed with a fluidity improver comprising an ethylene-vinyl acetate copolymer at 300 ppm by volume based on the total amount of the diesel engine fuel oil composition, and a lubricity comprising a long chain alkyl ester. The improver was added at 100 mg / kg to the total amount of the diesel engine fuel oil composition. Properties of the obtained diesel engine fuel oil composition are shown in Table 1.

<Comparative example 2>
Hydrogen gas oil fraction having a boiling point range of 180-390 ° C and 90% distillation temperature of 350 ° C obtained by atmospheric distillation of naphthenic crude oil with a high content of naphthene and polycyclic naphthene to a sulfur content of 10 mass ppm or less By mixing 95% by volume of the desulfurized gas oil base material subjected to hydrodesulfurization and dearoma treatment with 5% by volume of an n-paraffin mixture adjusted to have a boiling point range of 250 to 350 ° C. using a commercially available n-paraffin, A fuel oil composition for a diesel engine was obtained. The resulting diesel engine fuel oil composition was mixed with a fluidity improver comprising an ethylene-vinyl acetate copolymer at 300 ppm by volume based on the total amount of the diesel engine fuel oil composition, and a lubricity comprising a long chain alkyl ester. The improver was added at 100 mg / kg to the total amount of the diesel engine fuel oil composition. Properties of the obtained diesel engine fuel oil composition are shown in Table 1.

  In the table, regarding the maintenance of the liquid state at 15 ° C. and 50 MPa, if the fuel in the high-pressure chamber is in the liquid state as shown in FIG. 3, the determination of “◯” is observed, and crystal precipitation is seen as shown in FIG. If the fuel is solidified as described above, it was determined as “x”. In Comparative Example 1 having a slightly heavy distillation property, a high naphthene content and a small amount of isoparaffin, crystal precipitation was observed at 15 ° C. and 50 MPa.

  Since Comparative Example 1 has a high naphthene content, the slope of the solid-liquid equilibrium line is also small. When the pressure is further increased, the solid is solid in a considerable temperature range as compared with Examples 1 to 3. As a result, it became a state. In Comparative Example 2 where the content of naphthenes having two or more rings was higher than that in Comparative Example 1, crystals were precipitated at a considerably higher temperature at a lower pressure in the operating range of 50 MPa.

  FIG. 2 shows a solid-liquid equilibrium diagram of the fuel oil composition. The horizontal axis shows the sample temperature [° C.], and the vertical axis shows the crystal precipitation pressure [MPa] at a certain temperature. By plotting the crystal precipitation pressure at an arbitrary temperature at multiple points, the solid-liquid equilibrium diagram is usually It is drawn as a quadratic curve. The portion to the left of the solid-liquid equilibrium line indicates that crystals are deposited on the sample and are in a gel or solid state. Starting point of solid-liquid equilibrium diagram: The temperature of crystal precipitation in the sample when the pressure is 0 indicates a so-called cloud point. Even for samples with the same cloud point, the slope of the quadratic curve differs depending on the composition in the sample, and the solid state region of the sample varies greatly. In the figure, curve 1 represents Example 1 and curve 2 represents Comparative Example 1. In Example 1, it is in a liquid state at 15 ° C. and 50 MPa.

It is the schematic of the experimental apparatus used for the measurement of a crystal precipitation pressure and a solid-liquid equilibrium line. 2 is a graph in which solid-liquid equilibrium lines of Example 1 and Comparative Example 1 are plotted. It is the figure which observed the liquid state of the fuel oil composition with the microscope. It is the figure which observed the crystal precipitation state of the fuel oil composition with the microscope. It is the figure which observed the solid state of the fuel oil composition with the microscope.

Claims (2)

10% by volume distillation temperature of 180 to 225 ° C., 90% by volume distillation temperature of 315 to 350 ° C., a sulfur content of 10 mass ppm or less, and a saturation content of 70 to 83.3 % by volume . The aromatic content is 16.7 to 30% by volume, the bicyclic aromatics content is 1.5% by volume or less, and the polycyclic aromatics having 3 or more rings is 0.5% by volume or less. A fuel oil for diesel engines having a naphthene content of 65% by volume or less, a polycyclic naphthene content of 35% by volume or less, and an isoparaffin content of 5 to 25.9 % by volume. Composition. The fuel oil composition for a diesel engine according to claim 1, which is used for a diesel engine having a fuel injection pressure of 50 MPa or more.