JP3028650B2 - Control unit for diesel engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control system for a diesel engine.

2. Description of the Related Art There is a fuel injection valve which performs fuel injection stepwise in order to realize a smooth combustion by reducing the ignition delay of a diesel engine. In this fuel injection valve, a first spring and a second spring that urge the valve of the fuel injection nozzle to close the valve are interposed in parallel, and the first spring operates at the stage where the needle is initially lifted, and when the needle passes the initial lift position. In addition to this, by operating the second spring, the needle is lifted stepwise with respect to the fuel injection pressure, a large amount of main injection is performed after the initial injection fuel is ignited, and there is little ignition delay, and therefore vibration and noise Low stable and smooth flammability is ensured. Even in a diesel engine having a fuel injection valve that injects such fuel in multiple stages, NOx
As means for reducing the amount of exhaust gas, control to delay the fuel injection timing or EG for recirculating a part of the exhaust gas to the intake system
It is still very effective to lower the maximum temperature and pressure during combustion by R control (see, for example, JP-A-61-55358). However, in the low-speed and medium-load range where the rotation speed of the fuel injection pump is low, the period during which the needle lift stays at the initial lift is relatively long, and the injection period of one cycle is longer than in the high-speed range. If an attempt is made to reduce NOx with a delay, in the vicinity of the end of injection, the piston drops too much, the in-cylinder pressure is low, and the amount of smoke and HC emissions increases. Therefore, there is a problem that the allowable amount for delaying the fuel injection timing is small in a low-speed and medium-load region. In addition, a large amount of exhaust gas can be recirculated in the low-load region because the excess air ratio is high, but in a medium-load region, the amount of exhaust gas recirculation is limited because the excess air ratio is relatively low. In contrast to the restriction, the NOx emission amount tended to increase in the low-speed and medium-load region as shown in FIG. The present invention focuses on the above points, and in a diesel engine equipped with a fuel injection valve that injects fuel in multiple stages, a fuel injection timing control and an EGR
An object of the present invention is to reduce the amount of NOx emission in a low-speed / medium-load region where control is limited.

According to the present invention, as shown in FIG. 1, a fuel injection valve is provided in which a needle is lifted in a stepwise manner in accordance with a fuel pressure fed from a fuel injection pump to inject fuel in multiple stages. In a diesel engine, a variable valve timing mechanism 44 that makes opening and closing timing of an intake valve variable.
Means 41 for detecting the operating state of the engine, means 42 for determining an operating range in which the amount of fuel injected at the initial lift position of the needle is relatively large, and a variable valve timing mechanism 44 in this operating range. A control means 43 for delaying the opening / closing timing of the intake valve is provided.

In the low-speed and medium-load range, the fuel injection pressure does not increase so much because the rotation speed of the fuel injection pump is low, so that the period during which the needle remains in the initial lift becomes relatively long.
Although the injection period of one cycle is longer than that in the high-speed range, the present invention reduces the effective compression stroke by delaying the closing timing of the intake valve in this low-speed, medium-load range, thereby reducing the pressure in the combustion chamber. The combustion temperature is reduced by the
Reduce Ox emissions. By lowering the compression ratio,
An increase in the fuel injection amount during the ignition delay period with respect to the entire injection period rather causes an increase in the NOx emission amount, but in a low-speed, medium-load region, the period during which the needle stays at the initial lift position is long, so Therefore, the amount of fuel injected during the ignition delay period is small, and NOx
The increase in the amount is kept small, and the effect of lowering the combustion pressure during a long injection period after ignition lowers the combustion temperature, so that the total NOx emission can be reduced. Furthermore, in the low-speed, medium-load range, the closing timing of each intake valve is delayed, so that the valve overlap that the intake valve opens at the same time as the exhaust valve near the top dead center of the piston is reduced, and the burned combustion remaining in the cylinder is reduced. By increasing the gas amount (internal EGR amount),
The effect of increasing the specific heat of the combustion gas while maintaining the gas temperature in the cylinder at the time of ignition high can be obtained.

Embodiments of the present invention will be described below with reference to the accompanying drawings. 2 and 3, reference numeral 10 denotes a syringe head,
Reference numeral 2 denotes a cylinder, 3 denotes a piston, and a cavity 4 is formed at the top of the piston 3 to define a combustion chamber 5. A fuel injection valve 6 is attached to the cylinder head 10, and an injection nozzle 7 of the fuel injection valve 6 is provided facing the center of the combustion chamber 5. FIG. 4 shows an example of the fuel injection valve 6, in which a needle 22 is disposed inside a nozzle holder 21, and the needle 22 has a first spring 23 and a second spring 24.
Urged in the valve closing direction. Fuel pressure sent from a fuel injection pump (not shown) is applied to the needle 22,
When this fuel pressure increases above a predetermined value the needle 22 is lifted by an initial lift L ₁ while compressing the first spring 23 via the push rod 26. This is the initial injection, and this state is maintained until the fuel pressure exceeds the initial spring load of the second spring 24 (second stage valve opening pressure). Then, while compressing the second spring 24 via the spring seat 27, the needle 22 is lifted up to a total lift amount L _2, the main injection for supplying a large amount of fuel starts, staged fuel injection is performed.
In the cylinder head 10, two intake ports 33 and two exhaust ports 34 are formed around the injection nozzle 7 so as to face each other. Each intake valve 8 and each exhaust valve 9 are provided with respective camshafts 11 and 12 provided on a cylinder head 10.
As each of the cams 11a and 12a is driven by each of the valve springs 15 and 1 via the respective rocker arms 13 and 14 as the cam 11a rotates.
6 is driven to open and close. Each camshaft 11, 12
The cam pulleys 35 and 36 are connected to their respective front ends, and a timing belt 37 is wound around each of the cam pulleys 35 and 36 and a crank pulley (not shown), and is rotated in synchronization with the engine rotation. The intake-side camshaft 11 that opens and closes each intake valve 8 is provided with a variable valve timing mechanism 44 that varies its rotation phase. The variable valve timing mechanism 44 is connected to the intake camshaft 11.
It is constituted by an actuator which makes the rotation phase between the motor and the cam pulley 35 variable by hydraulic pressure. The opening and closing timing of each intake valve 8 is adjusted by changing the rotation phase between the camshaft 11 and the cam pulley 35 by the actuator. In the present invention, the key point is that the opening and closing timing of each intake valve 8 is delayed by operating the variable valve timing mechanism 44 in an operation range where the amount of fuel injected at the initial lift position of the needle 22 is relatively large. Is controlled by the controller 30. The controller 30 is constituted by a microcomputer including a CPU of an operation part, a RAM and a ROM of a storage part, and an I / O of an input / output part. The controller 30 receives a detection signal from an engine speed sensor 31 and a lever opening sensor 32 that detects the control lever opening of the fuel injection pump as a representative of the engine load, and is set in advance as shown in FIG. An operating range where the amount of fuel injected at the initial lift position of the needle 22 is relatively large is determined based on the map, and the opening / closing timing of each intake valve 8 is delayed in this operating range as shown by a broken line in FIG. Next, the variable valve timing mechanism 44 is operated. Next, the operation will be described. FIG. 7 shows the lift characteristic of the needle 22 which changes according to the engine speed and the load in the lower part of each square, and the characteristic of the fuel injection rate in the upper part of each square. The arrow shown on the characteristic of the fuel injection rate indicates the ignition timing, and the black portion before this arrow corresponds to the fuel amount injected during the ignition delay period. FIG. 5
In the low-speed / medium-load region corresponding to the delay control region shown in FIG. 7, the fuel injection pressure does not increase so much because the rotation speed of the fuel injection pump is low, so that the period during which the needle 22 remains at the initial lift position becomes relatively long. The injection period of the cycle is longer than in the high-speed range. In the present invention, the effective compression stroke is reduced by delaying the closing timing of each intake valve 8 in the low-speed and medium-load range, and the pressure in the combustion chamber is reduced as shown in FIG. As a result, the combustion temperature is reduced, and the emission amount of NOx can be reduced. If the compression ratio is lowered, and the fuel injection amount during the ignition delay period is increased with respect to the entire injection period, the NOx amount is rather increased, but the period during which the needle 22 remains at the initial lift position is long in a low-speed and medium-load region. Therefore, the amount of fuel injected during the ignition delay period is small with respect to the entire injection period, and even if the actual compression ratio is reduced to some extent, the increase in the NOx amount is kept small, and the combustion pressure during the long injection period after ignition is reduced. Due to this effect, the combustion temperature decreases, and the total amount of NOx emissions can be reduced. further,
In the low-speed, medium-load range, the valve closing timing of each intake valve 8 is delayed, so that the valve overlap in which the intake valve 8 opens simultaneously with each exhaust valve 9 near the piston top dead center (TDC) is reduced. Burned gas remaining in the engine (internal EGR
As a result, the specific heat of the combustion gas is increased while the gas temperature in the cylinder at the time of ignition is kept high. In the low-speed low-load region, the opening timing of each intake valve 8 is returned to the normal timing. However, since the excess air ratio is high, a large amount of exhaust gas can be recirculated, and the NOx emission amount can be sufficiently suppressed. Also at the start, the opening / closing timing of each intake valve 8 is returned to the normal timing, so that the actual compression ratio is increased and good startability is secured. At low speed and high load,
By returning the opening timing of each intake valve 8 to the normal timing, the efficiency of charging fresh air is increased, and the amount of smoke discharged is reduced. In this low-speed high-load region, the NOx emission can be effectively reduced by delaying the fuel injection timing. In the high-speed low-load range, the rotation speed of the fuel injection pump is high,
Because the fuel injection amount is small, the amount of fuel injected during the ignition delay period increases, so by increasing the exhaust gas recirculation amount or delaying the fuel injection timing without delaying the closing timing of the intake valve, NOx emissions can be reduced. Next, as another embodiment, a means for detecting the lift amount of the needle 22 may be provided, and an operation range in which the initial lift period exceeds a predetermined value may be determined based on the detection signal to perform highly accurate control. It is possible. In this case, a piezoelectric element 28 is interposed between the spring seat 27 of the second spring 24 and the spacer 29 in FIG. The piezoelectric element 28 is the output voltage changes according to the pressure acting on this, the lift amount of the needle 22 with the to lift beyond the initial lift amount L _1, the second spring 2
When the fourth spring seat 27 moves away from the piezoelectric element 28, the output changes and it can be detected whether the needle 22 is at the initial lift position. Further, the lift detecting means of the needle 22 need not always be provided for all cylinders in a multi-cylinder engine.

As described above, the present invention relates to an intake valve for a diesel engine having a fuel injection valve in which a needle is lifted stepwise according to the fuel pressure fed from a fuel injection pump to inject fuel in multiple stages. A variable valve timing mechanism for varying the opening / closing timing of the engine, a means for detecting the operating state of the engine, a means for determining an operating range in which the amount of fuel injected at the initial lift position of the needle is relatively large, And control means for delaying the opening / closing timing of the intake valve via a variable valve timing mechanism. Therefore, in a low-speed and medium-load region where the delay control of the fuel injection timing and the EGR control are restricted, the NOx emission is compatible with HC and smoke. Emissions can be reduced effectively.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of the present invention.

FIG. 2 is a longitudinal sectional view of an engine showing an embodiment of the present invention.

FIG. 3 is a plan view of the cylinder head.

FIG. 4 is a longitudinal sectional view of the fuel injection valve.

FIG. 5 is a control characteristic diagram.

FIG. 6 is a valve lift characteristic diagram.

FIG. 7 is a graph showing a fuel injection rate and a needle lift characteristic.

FIG. 8 is a characteristic diagram of pressure in a combustion chamber.

FIG. 9 is a characteristic diagram of the NOx emission amount of the conventional device.

[Explanation of symbols]

 41 Operating state detecting means 42 Operating area discriminating means 43 Control means 44 Variable valve timing mechanism

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) F02D 13/02 F02D 41/38 F02M 45/08 F02M 61/10

Claims (1)

(57) [Claims] 1. A diesel engine having a fuel injection valve that lifts a needle in a stepwise manner according to a fuel pressure fed from a fuel injection pump and injects fuel in multiple stages.
A variable valve timing mechanism for varying the opening / closing timing of the intake valve, a means for detecting the operating state of the engine, a means for determining an operating range where the amount of fuel injected at the initial lift position of the needle is relatively large, A control device for a diesel engine, comprising: control means for operating a variable valve timing mechanism in an operating range to delay opening / closing timing of an intake valve.