JP2002047976A - Fuel injection controller of diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimize injection start time of pilot injection and improve the ignition property when a diesel engine is started at a low temperature. SOLUTION: A small amount of fuel is injected as pilot injection prior to main injection in accordance with an operation condition of the engine. Although gas temperature in a cylinder rises in accordance with a crank angle in a compression process, the gas temperature in the cylinder is reduced on the whole if, for example, the number of cranking revolutions is small. Crank angles (CAA, CAB) at which the gas temperature in the cylinder reaches a predetermined temperature Th are calculated based on a suction air temperature, and the time when the gas temperature in the cylinder exceeds the predetermined temperature Th is set as injection start time of pilot injection. By setting the predetermined temperature Th close to a low temperature oxidation reaction start temperature of fuel, the injected fuel is securely ignited and satisfactory ignition property and low temperature start property are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel injection control device for a diesel engine, and more particularly to an improvement in a fuel injection control device for injecting a small amount of fuel as pilot injection prior to main injection.

[0002]

2. Description of the Related Art Conventionally, in a diesel engine, a method of injecting a small amount of fuel as so-called pilot injection prior to main injection in order to reduce combustion noise and suppress NOx has been known.

In order to optimize the injection timing of the pilot injection, Japanese Patent Application Laid-Open No. H11-294228 discloses that the pilot injection timing is advanced in consideration of the in-cylinder temperature during warm-up of the engine. Is disclosed. Specifically, for example, the coolant temperature is detected as the engine temperature, the outside air temperature is detected, and the smaller the difference temperature obtained by subtracting the outside air temperature from the engine temperature, the larger the advance amount of the pilot injection timing with respect to the main injection timing is. The pilot injection timing is controlled as follows. In other words, immediately after the start, the water temperature matches the outside air temperature, and during the warm-up, the water temperature is higher than the outside air temperature. Even if the water temperature is the same, white smoke is generated in the latter. Therefore, by controlling based on the above-mentioned difference temperature, the pilot injection timing is more appropriately obtained as compared with the case where the advance amount of the pilot injection timing is controlled only from the water temperature.

[0004]

However, there is a relatively large error between the engine cooling water temperature and the in-cylinder temperature of the engine, and they do not completely match. That is, since the cooling water temperature changes after the cylinder wall temperature changes after the cylinder temperature changes, the cooling water temperature changes more slowly than the cylinder gas temperature. This error increases as the temperature change speed increases, particularly during startup or during warm-up.
For example, an error when the vehicle starts immediately after starting is larger than an error when the engine is warmed up slowly in the idle state.

Further, in the above prior art, the in-cylinder temperature is grasped as an average temperature as the warm-up progresses, and attention is paid to the actual in-cylinder gas temperature that changes according to the crank angle during a cycle. is not.

Therefore, when the start timing of the pilot injection is determined based on the temperature difference between the engine cooling water temperature and the outside air temperature as in the prior art, the timing is not always optimal, and the ignitability is not always appropriate. May worsen.

Further, when the present invention is applied to an engine having a low compression ratio, the in-cylinder pressure is low due to the low compression ratio and the in-cylinder gas temperature is low. If) is not increased, the ignitability of the pilot injection will deteriorate. In general, a method of securing the vaporization time by advancing the start timing of injection is adopted.However, the earlier the start timing, the lower the in-cylinder gas temperature at that time. The gas temperature becomes lower than the temperature required for vaporization (the temperature at which thermal decomposition occurs first), making it impossible to vaporize the fuel itself, deteriorating the ignitability. It is conceivable that HC is discharged as it is.

An object of the present invention is to improve the ignitability of pilot injection and to suppress HC emission.

[0009]

According to a first aspect of the present invention, there is provided a fuel injection control apparatus for a diesel engine, wherein the fuel injection means is capable of performing a pilot injection prior to a main fuel injection according to an operating state; The injection start timing of injection
And pilot injection timing setting means for setting the timing at which the in-cylinder gas temperature during the cycle becomes equal to or higher than a predetermined temperature.

That is, as the in-cylinder gas is compressed in the compression stroke, the in-cylinder gas temperature increases, but the temperature fluctuates due to various factors such as the engine speed and the intake air temperature. According to the first aspect of the present invention, the pilot injection is performed at the crank angle at which the sequentially changing in-cylinder gas temperature becomes equal to or higher than the predetermined temperature. Therefore, the injected pilot fuel surely reaches ignition combustion, and satisfactorily ignites and burns the fuel of the subsequent main injection.

Although the in-cylinder gas temperature may of course be directly detected, it is generally estimated from other engine parameters such as the intake air temperature, the engine speed, and the intake air amount.

A second aspect of the present invention is a more specific example of the first aspect.
In the invention, the pilot injection timing setting means sets the injection start timing of the pilot injection to a timing at which the in-cylinder gas temperature during the cycle becomes equal to or higher than the predetermined temperature when the engine coolant temperature is equal to or lower than the predetermined water temperature. .

That is, when the engine is cold, the in-cylinder gas temperature decreases as a whole, and unless the injection start timing of the pilot injection is set appropriately, an increase in the amount of unburned HC discharge and misfire tend to occur. In the present invention, even during such a cold period, pilot injection is performed at a crank angle at which the in-cylinder gas temperature is equal to or higher than a predetermined temperature, so that pilot fuel is reliably ignited and burned.

[0014] A third aspect of the present invention is a more specific example.
In the invention, the pilot injection timing setting means sets the injection start timing of the pilot injection to a timing at which the in-cylinder gas temperature during the cycle becomes equal to or higher than a predetermined temperature when the engine is started.

At low engine speeds due to cranking, the in-cylinder gas temperature also decreases as a whole,
In the present invention, even under such conditions, the pilot injection is performed at the crank angle at which the in-cylinder gas temperature is equal to or higher than the predetermined temperature, so that ignition combustion is reliably achieved and good startability is obtained.

Further, the invention of claim 4 is characterized in that the pilot injection timing setting means performs a minute advance correction of the injection start timing of pilot injection when the fuel injection pressure of the fuel injection means is low. I have.

If the fuel injection pressure is lowered, the degree of atomization of the fuel spray is deteriorated, and the ignition delay of the injected pilot fuel becomes large. On the other hand, if the injection start timing of the pilot injection is advanced slightly more than usual, the ignition delay due to insufficient atomization is compensated, and the ignition combustion is reliably performed at an appropriate timing. It is necessary that the in-cylinder gas temperature at the crank angle when advanced in this way is also equal to or higher than a predetermined temperature.

Further, the invention of claim 5 is characterized in that the pilot injection timing setting means corrects the injection start timing of pilot injection by a small amount when the engine speed is high.

As the engine speed increases, the actual time becomes shorter. On the other hand, if the injection start timing of the pilot injection is slightly advanced from that at the time of the low rotation, the ignition and combustion can be reliably performed at an appropriate timing by compensating for the atomization and the delay of the vaporization due to the reduction of the real time. It is necessary that the in-cylinder gas temperature at the crank angle when advanced in this way is also equal to or higher than a predetermined temperature.

The predetermined temperature is desirably a temperature at which thermal decomposition of the fuel first occurs, that is, a temperature near the low temperature oxidation reaction start temperature. If the actual in-cylinder gas temperature at the time of the pilot injection is equal to or higher than the low-temperature oxidation reaction start temperature, the ignition reaction reliably proceeds after the thermal decomposition of the fuel occurs. Below this temperature, the fuel is not ignited.

In the invention of claim 7, the in-cylinder gas temperature is calculated based on the following equation (1).

T = PV / (Mall × R) (1) where T: in-cylinder gas temperature, P: in-cylinder pressure obtained from the average specific heat ratio of gas, V: cylinder content that changes according to crank angle Product, Mall: Total number of moles of in-cylinder gas determined from intake conditions, R: Gas constant.

By calculating the in-cylinder gas temperature from the gas state equation as described above, the in-cylinder gas temperature at the injection start timing of the pilot injection can be more reliably compared to the case where the in-cylinder temperature condition is estimated from the cooling water temperature or the like. At the desired temperature.

The present invention is suitable for a low compression ratio engine having a compression ratio of 16 or less. In such a low compression ratio engine, the in-cylinder gas temperature generally decreases, but by setting the injection start timing of the pilot injection appropriately, good ignition performance can be ensured.

[0025]

According to the present invention, the pilot injection is performed at a time when the in-cylinder gas temperature of the engine becomes equal to or higher than the predetermined gas temperature, so that ignition after the pilot injection can be reliably performed, and the pilot injection can be performed. It is possible to reliably obtain the improvement of the ignitability, which is the main effect of (1).

In particular, according to the second aspect of the invention, when the cooling water temperature of the engine is equal to or lower than the predetermined water temperature, the pilot injection is performed at a time when the in-cylinder gas temperature becomes equal to or higher than the predetermined temperature.
Even during cold periods when the in-cylinder gas temperature of the engine is low overall, ignition after pilot injection can be reliably performed, and the main effect of pilot injection, ignitability, can be reliably improved. It is.

According to the third aspect of the present invention, the pilot injection start timing is similarly controlled when the engine is started, so that ignition after the pilot injection can be reliably performed even under a low rotational speed condition when the engine is started. And the startability is improved.

Further, according to the fourth aspect of the present invention, it is possible to compensate for insufficient atomization of fuel spray when the fuel injection pressure is low, and to ensure that ignition after pilot injection is performed. It is possible to reliably obtain the improvement of the ignitability, which is the main effect of the injection.

Similarly, according to the fifth aspect of the present invention, it is possible to compensate for the atomization and lack of vaporization of the fuel spray after the pilot injection due to the reduction of the real time when the engine speed is increased. By ensuring that the subsequent ignition is performed, it is possible to reliably improve the ignitability, which is a main effect of the pilot injection.

According to the sixth aspect of the present invention, the gas temperature in the cylinder at the time of pilot injection can be obtained higher than the temperature at which thermal decomposition of fuel first occurs, that is, the oxidation reaction start temperature. The fuel ignition reaction can be performed more reliably.

According to the seventh aspect of the present invention, unlike the case where the in-cylinder temperature condition of the engine is estimated from the coolant temperature or the like, the in-cylinder gas temperature itself of the engine can be calculated. It can be determined appropriately, and the ignitability is improved.

According to the eighth aspect of the present invention, it is possible to obtain good ignitability even in a low compression ratio engine in which the in-cylinder gas temperature is low as a whole.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows the overall configuration of a diesel engine fuel injection control device according to the present invention. First, the engine 1 and its fuel supply system will be described with reference to FIG.

The engine 1 is provided with a fuel supply pump 2 on the outside, and fuel is pumped by reciprocating motion of a plunger (not shown) inside the fuel supply pump 2. The fuel is introduced from the fuel tank 3 into the fuel supply pump 2 via the suction passage 4, and the pressure-fed fuel is accumulated in the pressure accumulating chamber 6 via the discharge passage 5 in a state where the pressure is increased.

The fuel supply pump 2 is provided with an effective stroke control valve 7 for controlling an effective stroke of the plunger, and starts fuel pressure feeding when the effective stroke control valve 7 is opened in the compression stroke of the plunger. The discharge amount of the fuel supply pump 2 is determined accordingly. The accumulator 6 is provided with a pressure sensor 8 for detecting a fuel pressure in the accumulator 6. The accumulator 6
Is supplied to the fuel injection nozzles 10 of the respective cylinders via the supply passage 9, and from the tip of the fuel injection nozzles 10, the cavity 12 formed on the top surface of the piston 11 in the engine 1 and the cylinder 13 Is injected into the combustion chamber.

The fuel pressure in the accumulator 6 is controlled by the control unit 14. Specifically, the opening and closing timing of the effective stroke control valve 7 is controlled by a control signal from the control unit 14 in accordance with the fuel pressure in the accumulator 6 detected by the pressure sensor 8, and the discharge amount of the fuel supply pump 2 is controlled. Is determined. The effective stroke control valve 7 opens during the expansion stroke of the plunger in the fuel supply pump 2 and closes only during a necessary stroke during the compression stroke for discharging fuel, thereby changing the fuel discharge amount. That is, when the fuel pressure in the accumulator 6 is lower than a predetermined value, the fuel pressure is increased by increasing the effective stroke amount, and conversely, when the pressure is high, the effective stroke amount is decreased to decrease the fuel pressure. It has become.

The control unit 14 includes a cylinder discriminating sensor 15 for discriminating a cylinder of the engine 1, a crank angle sensor 16 for detecting an engine speed and a crank angle, and an accelerator opening sensor 1 for detecting an accelerator opening.
7, detection signals are input. Further, an intake temperature sensor and a water temperature sensor (not shown) are provided, and their detection signals are also input to the control unit 14. Then, the control unit 14 determines operating conditions based on the accelerator opening Acc, the engine speed Ne, the intake air temperature Tint, the cooling water temperature Tw, and the like, and obtains the injection pressure and the injection amount from a preset operation map. ,
Each target value of the injection timing is searched, and the opening / closing timing of the control valve in the fuel injection nozzle 10 is set so as to become the target value of the injection amount and the injection timing, and the fuel pressure in the accumulator 6 is set to the target value. The opening / closing timing of the effective stroke control valve 7 is controlled in such a manner as to be as follows.

Further, by controlling the input signal to the control valve in the fuel injection nozzle 10 according to the operating conditions,
It is possible to divide the fuel injection into a pilot injection and a main injection. In this pilot injection, by injecting a small amount of fuel in advance before the main injection, the ignitability is improved, the NOx emission amount is reduced by reducing the initial combustion amount,
There are effects such as suppression of combustion noise by reducing the pressure rise rate. For this reason, for example, during cold periods, pilot injection is applied mainly to improve ignitability, and at medium load or the like, pilot injection is applied when it is desired to suppress combustion noise.

In particular, at the time of a cold start or a cold start, the outside air temperature decreases and the cooling water temperature Tw of the engine 1 decreases, so that the in-cylinder gas temperature also decreases. For this reason, during combustion, an increase in the amount of unburned HC emission is likely to occur, or misfiring is likely to occur, resulting in deterioration of startability. Therefore, based on signals such as the accelerator opening Acc, the engine speed Ne, and the coolant temperature Tw that are input to the control unit 14, the engine 1 is in a cold state or in a cold start state. When it is detected, the control valve in the fuel injection nozzle 10 is controlled to perform the pilot injection. Here, at the same time, the injection amount, the injection timing, and the injection pressure of the pilot injection and the main injection are optimally controlled.

In the present embodiment, especially when pilot injection is applied at a cold time or at a cold start, a cylinder in a combustion chamber constituted by a cavity 12 and a cylinder 13 formed on a top surface of a piston 11 in the engine 1 is used. The internal gas temperature is calculated, and the injection timing of the pilot injection is set at a time (crank angle) at which the in-cylinder gas temperature sequentially changing with the crank angle becomes equal to or higher than a predetermined temperature.

The in-cylinder gas temperature in the engine 1 fluctuates due to various factors such as the engine speed and the intake air temperature. FIG.
The graph shows a change in the in-cylinder gas temperature with respect to the crank angle when the engine speed is a low speed corresponding to cranking at the time of starting. As shown in the drawing, the in-cylinder gas temperature increases with the compression by the piston 11, but when the engine speed is higher at 50 rpm, the in-cylinder gas temperature is higher than when the engine speed is 20 rpm, which is lower. Increases as a whole, and the compression end temperature at the top dead center also increases. In the present embodiment, the injection start timing of the pilot injection is set so that the pilot injection is performed when the in-cylinder gas temperature that changes in this manner becomes equal to or more than a predetermined value. FIG.
Assuming that the predetermined temperature of the in-cylinder gas temperature is Th, at 50 rpm where the engine speed is high, this predetermined temperature Th
The injection start timing of the pilot injection is set at the crank angle CAA that reaches
Since the crank angle until the temperature reaches the predetermined temperature Th is delayed, the crank angle at the injection start timing of the pilot injection becomes CAB that is later than the CAA.

On the other hand, when such a pilot injection is performed, the injection timing of the main injection is generally set near or before the top dead center due to the requirements of exhaust performance and noise vibration performance. . However, when the in-cylinder gas temperature is low as a whole as described above (for example, the above-described 20r
pm), the pilot injection timing approaches the top dead center. Where the interval Δ between pilot injection and main injection
IT is limited by the capability of the control valve in the fuel injection nozzle 10 and is a minimum of about 5 to 10 ° CA. for that reason,
When the pilot injection timing approaches the top dead center due to the in-cylinder gas temperature, the injection timing of the main injection is corrected in consideration of the ΔIT.

Here, the predetermined value of the in-cylinder gas temperature for determining the injection start timing of the pilot injection is set, for example, at a temperature at which the fuel ignition reaction is likely to occur, that is, near the low temperature oxidation reaction start temperature of the fuel. You. This is a specific temperature determined according to the composition of the fuel, and is the temperature at which pyrolysis of the fuel first occurs. After the thermal decomposition of the fuel occurs, the ignition reaction proceeds. That is, the fuel is not ignited below the low temperature oxidation reaction start temperature. Therefore, by performing the pilot injection after the in-cylinder gas temperature of the engine 1 reaches this temperature, ignition after the pilot injection can be reliably performed. The startability at the time of starting the engine 1 can be improved.

Next, the flow of control executed in the control unit 14 will be described with reference to the flowchart of FIG. First, in step 401, the engine speed Ne, the accelerator opening Acc, the intake air temperature Tint,
Detection signals such as the intake air amount Qa and the cooling water temperature Tw are read. The operating state of the engine 1 is determined from these signals. In step 402, the cooling water temperature Tw detected by the water temperature sensor is compared with a predetermined value set in advance. Here, if the cooling water temperature Tw is lower than the predetermined value, it is determined that the engine 1 is in a cold state, and the process proceeds to step 403. This is the same in the case of starting in a cold state.
In step 403, a crank angle CAP at which the in-cylinder gas temperature of the engine 1 reaches a predetermined temperature is calculated based on the operating state at that time. Next, at step 405, the fuel pressure Pr in the accumulator 6 is detected based on the signal of the pressure sensor 8 provided in the accumulator 6, and the fuel pressure Pr is compared with a predetermined value. When the fuel pressure Pr is higher than the predetermined value, the process proceeds to step 407. Step 40
In step 7, the engine speed Ne is compared with a predetermined value. If the engine speed Ne is lower than the predetermined value, the routine proceeds to step 4.
Proceed to 08. In step 408, the injection start timing of pilot injection is determined by the crank angle CAP calculated in step 403.
And the process proceeds to step 410.

On the other hand, if the fuel pressure in the pressure accumulating chamber 6 is lower than the predetermined value in step 406, or step 407
If the engine speed Ne is lower than the predetermined value, the routine proceeds to step 409, where the set crank angle of the pilot injection start timing is set to a value advanced from the CAP by ΔCA, that is, CAP−ΔCA, and the routine proceeds to step 410. .

In step 410, the injection timing CAM of the main injection is set in consideration of the operation state of the engine 1 and the injection start timing (CAP or CAP-ΔCA) of the pilot injection. The control valve in the fuel injection nozzle 10
Control is performed according to these set injection start timings by another routine not shown.

If the cooling water temperature Tw is higher than the predetermined value in step 402, it is determined that the engine 1 is in the warming-up completed state, and the routine proceeds to step 404, where it is set in advance according to the operating state of the engine 1. Injection conditions (injection amount, injection timing, presence / absence of pilot injection) are set according to the operation map.

Here, a method of calculating the crank angle CAP performed in step 403 will be described.

The in-cylinder gas temperature T is obtained by the following equation (1).

T = PV / (Mall * R) (1) where P: pressure in the cylinder, V: volume in the cylinder, Mall: total number of working gas moles, and R: gas constant.

Therefore, the in-cylinder gas temperature required at the injection start timing (this is set to be slightly higher than the low-temperature oxidation reaction start temperature described above, or calculated by multiplying the low-temperature oxidation reaction start temperature by a certain safety factor) And the intake air temperature T
Int, the in-cylinder pressure calculated from the enthalpy balance using the average specific heat ratio of the working gas, and the state equation (1) ), The volume in the cylinder when the predetermined temperature is reached is calculated. Then, a crank angle CAP corresponding to the volume is calculated. The crank angle CAP obtained in this manner is a crank angle at which the in-cylinder gas temperature reaches a predetermined temperature, and is an injection timing at which pilot injection is to be performed.

As described above, the crank angle can be advanced in step 409 by setting the predetermined temperature of the in-cylinder gas temperature to be higher than the low temperature oxidation reaction start temperature of the fuel. However, ΔCA needs to be set to a range in which the in-cylinder gas temperature at the injection start timing does not fall below the low-temperature oxidation reaction start temperature due to the advance correction.

As described above, according to the present embodiment, the crank angle at which the in-cylinder gas temperature during the cycle becomes a predetermined temperature or more is calculated in accordance with the operating conditions of the engine 1, and the pilot injection is performed at the crank angle. In addition, since the injection start timing of the pilot injection is set, ignition after the pilot injection can be reliably performed, and the emission of unburned HC can be suppressed and the startability can be improved.

Further, when the injection pressure is reduced, the pilot injection timing is advanced to correct the angle, so that the atomization of the fuel spray is insufficient when the injection pressure is reduced, that is, the formation of the air-fuel mixture is delayed. It can compensate for the decrease in ignitability.

Similarly, even in a case where the actual time is shortened due to an increase in the engine speed, a reduction in ignitability can be avoided by advancing the pilot injection timing.

Further, an engine equipped with a variable valve mechanism for changing the intake air amount by changing the intake valve closing timing,
In an engine equipped with a variable compression ratio mechanism, the in-cylinder gas temperature may change with a change in the actual compression ratio. However, as in the present embodiment, the in-cylinder gas temperature is equal to or higher than a predetermined temperature. If the crank angle is calculated and pilot injection is performed at that crank angle, it is possible to set an appropriate pilot injection timing according to the change in the in-cylinder gas temperature accompanying the change in the actual compression ratio. Become.

Also, in the case where the compression ratio itself of the engine 1 is lowered in order to improve the maximum output, according to the present embodiment, the in-cylinder gas which becomes a problem in a cold state or a low-speed low-load region is used. It is possible to avoid a decrease in startability and an increase in unburned HC emission due to a temperature drop.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing one embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a change in a cylinder gas temperature with respect to a crank angle.

FIG. 3 is a flowchart showing a control flow of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Engine 2 ... Fuel supply pump 6 ... Accumulation chamber 10 ... Fuel injection nozzle 14 ... Control unit

Continued on the front page F-term (reference) 3G084 AA01 BA13 BA15 CA01 DA09 EA11 EB09 EC02 EC03 FA02 FA10 FA13 FA17 FA20 FA22 FA33 FA38 FA39 3G301 HA02 JA00 JA25 JA37 KA01 MA18 MA23 NA08 NC02 NE11 PA10Z PB03Z PB05Z PB08ZPE03Z05

Claims (8)

[Claims] 1. A fuel injection means capable of performing a pilot injection before a main injection of fuel according to an operation state, and an injection start timing of the pilot injection is determined by setting an in-cylinder gas temperature during a cycle equal to or higher than a predetermined temperature. And a pilot injection timing setting means for setting a timing at which the fuel injection is performed. 2. The pilot injection timing setting means sets the injection start timing of the pilot injection to a timing at which the in-cylinder gas temperature during the cycle becomes equal to or higher than a predetermined temperature when the engine coolant temperature is equal to or lower than a predetermined water temperature. The fuel injection control device for a diesel engine according to claim 1, wherein 3. The pilot injection timing setting means sets an injection start timing of the pilot injection to a timing at which the in-cylinder gas temperature during a cycle becomes equal to or higher than a predetermined temperature when the engine is started. Item 2. A fuel injection control device for a diesel engine according to item 1. 4. The fuel injection system according to claim 1, wherein the pilot injection timing setting means corrects the injection start timing of the pilot injection by a small amount when the fuel injection pressure of the fuel injection means is low. A fuel injection control device for a diesel engine according to any one of the above. 5. The pilot injection timing setting means according to claim 1, wherein when the engine speed is high, the injection start timing of the pilot injection is slightly advanced.
5. The fuel injection control device for a diesel engine according to any one of 4. 6. The fuel injection control device for a diesel engine according to claim 1, wherein the predetermined temperature is a temperature near a temperature at which thermal decomposition of fuel first occurs. 7. The fuel injection control device for a diesel engine according to claim 1, wherein the in-cylinder gas temperature is calculated based on the following equation (1). T = PV / (Mall × R) (1) where T: in-cylinder gas temperature, P: in-cylinder pressure obtained from the average specific heat ratio of gas, V: in-cylinder volume that varies according to crank angle, Mall : Total number of moles of in-cylinder gas determined from intake conditions, R: Gas constant. 8. The fuel injection control device for a diesel engine according to claim 1, wherein the fuel injection control device is applied to a low compression ratio engine having a compression ratio of 16 or less.