Direct injection type diesel engine
DETAILED DESCRIPTION OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a direct injection type Diesel engine, and more particularly to a direct injection type Diesel engine which reduces noise, HC (hydrocarbon) and blue-white smoke 
     2. Prior Art 
     The direct injection type Diesel engine has the advantages such high combustion efficiency and low exhaust temperature, but on the other hand, it has the disadvantages of high explosion pressure, and high vibration and noise. 
     There is known a direct injection type Diesel engine employing a M-type (Mann-type) combustion chamber. The M-type combustion chamber is constructed in the following manner. Fuel injected from a fuel injection nozzle is deposited onto an inner wall surface of the combustion chamber to form a fuel film on the inner wall surface. The fuel film is heated by heat of the atmosphere in the combustion chamber and the inner wall to create a fuel vapor. The fuel vapor is mixed with suction air to form an air-fuel mixture for combustion and to cause self-ignition. 
     In the direct injection type Diesel engine employing the M-type combustion chamber, the fuel vapor sequentially vaporized from the inner wall of the combustion chamber is provided for combustion to slow the combustion and suppress rapid increase in pressure. Therefore, noise and vibration are relatively reduced. However, ignition lag is easily generated, and causes explosive combustion. Furthermore, performance in terms of HC and blue-white smoke deteriorates. This problem occurs under low load and at low temperature in particular. 
     Further, there is known another direct injection type Diesel engine employing a Pintaux nozzle for preliminary injection. In this case, a preliminary injecting fuel (auxiliary spray fuel) is injected to the center of the combustion chamber in such a manner as to be drawn by swirl S, thereby accelerating creation of an air-fuel mixture and improving ignitability and performance in HC and blue-white smoke. However, as the creation of the air-fuel mixture is instantaneously attained, combustion results quickly. As a result, the rate of increase in pressure becomes is high thereby causing a large amount of noise and vibration. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to overcome the aforementioned problems. It is an object of the present invention to provide a direct injection type Diesel engine which reduces the noise and decreases HC and blue-white smoke. 
     According to one aspect of the present invention, the direct injection type Diesel engine comprises a combustion chamber having an inner wall surface formed at a piston head of a piston reciprocated in a cylinder of the Diesel engine, and a fuel injection nozzle means mounted to a cylinder head of the Diesel engine, said fuel injection nozzle means having a main nozzle hole exposed to the combustion chamber and an auxiliary nozzle hole offset from the center of the combustion chamber and exposed to the inner wall surface of the combustion chamber at a position adjacent to a side portion of the combustion chamber, wherein the main nozzle hole is closed under low load of the Diesel engine to allow fuel to be injected from the auxiliary nozzle hole is allowed to collide with the inner wall surface of the combustion chamber and is atomized, while a residual part of the fuel is allowed to flow along the inner wall surface of the combustion chamber, thereby achieving premature ignition and slow combustion. 
     According to another aspect of the present invention, the direct injection type Diesel engine comprises a combustion chamber having an inner wall surface formed at a piston head of a piston reciprocated in a cylinder of the Diesel engine, and a passage formed at a cylinder head of the Diesel engine and communicating with the cylinder, said passage having an inner wall surface connected to the inner wall surface of the combustion chamber, and a fuel injection nozzle means provided in the passage, said fuel injection nozzle means having a mian nozzle hole exposed to the combustion chamber and an auxiliary nozzle hole exposed to the inner wall surface of the passage, wherein the main nozzle hole is closed under low load of the Diesel engine to allow fuel to be injected from the auxiliary nozzle hole only. The inner wall surface of the passage is formed between the fuel injection nozzle means and the inner wall surface of the combustion chamber. A part of the fuel injected from the auxiliary nozzle hole is allowed to collide with the inner wall surface of the passage and is atomized, while a residual part of the fuel is allowed to flow from the inner wall surface of the passage along the inner wall surface of the combustion chamber, thereby achieving premature ignition and slow combustion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a preferred embodiment of the present invention; 
     FIG. 2 is a sectional view taken along the line II--II in FIG. 1; 
     FIG. 3 is a sectional side view of another preferred embodiment of the present invention; 
     FIG. 4 is a plan view of the prior art; and 
     FIG. 5 is a sectional side view of the prior art. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Now, there will be described in detail a preferred embodiment of the present invention with reference to the accompanying drawings. 
     As shown in FIGS. 1 and 2, a piston head 1 of a piston adapted to be reciprocated in a cylinder of a Diesel engine is formed with a ball-like combustion chamber 2 having an inner wall surface 8. On the other hand, a cylinder head 3 of the Diesel engine is formed with a fuel injection nozzle means mounting passage 4 communicating with the cylinder at a position offset from the center of the combustion chamber 2. A fuel injection nozzle means 5 is provided in the passage 4 in a manner such as to be exposed to the combustion chamber 2 at the position offset from the center of the combustion chamber 2. 
     The fuel injection nozzle means 5 is of a type similar to a Pintaux nozzle. The fuel injection nozzle means 5 is formed with a main nozzle hole 6 for main injection at a center thereof, and is also formed with an auxiliary nozzle hole 7 for auxiliary injection at a side conical portion of the nozzle means. The auxiliary nozzle hole 7 is adapted to be opened and inject an auxiliary spray fuel when the amount of lift of a needle valve (not shown) is less than a predetermined value under low load, for example, and the main nozzle hole 6 is closed. On the other hand, the main nozzle hole 6 is adapted to be opened and inject a main spray fuel predominantly over the auxiliary nozzle hole 7 when the amount of lift of the needle valve is more than the predetermined value under high load, for example. In particular, an aperture of the auxiliary nozzle hole 7 is designed to be small, so as to atomize the auxiliary spray fuel. 
     As the fuel injection nozzle means 5 is offset from the center of the combustion chamber 2, the auxiliary nozzle hole 7 may be exposed oppositely to the inner wall surface 8 of the combustion chamber which surface is arranged adjacent to the hole 7 at a short distance. On the other hand, the main nozzle hole 6 may be exposed to the inner wall surface 8 of the combustion chamber 2 on a downstream side of swirl S created in the combustion chamber 2 at a relatively long distance from the wall surface 8. 
     In operation, the amount of fuel to be injected is small under low load such as at idling. Therefore, the amount of lift of the needle valve may be made small, and accordingly an auxiliary spray fuel f is injected from the auxiliary nozzle hole 7 only. The auxiliary spray fuel f injected collides with the inner wall surface 8 of the combustion chamber, and a part f 1  of the fuel is atomized by the power of such collision. Then, the atomized fuel is inwardly scattered in the combustion chamber 2, and is converted into an air-fuel mixture by the swirl S created in the combustion chamber 2 to contribute to ignition. 
     Most of residual fuel f is immediately deposited onto the inner wall surface 8 of the combustion engine after collision, and is allowed to flow in the state of film along the swirl S. In other words, premature ignition may be attained with the atomized fuel f 1  scattered by the collision against the inner wall surface 8 and mixed with air by the swirl S. Further, combustion after ignition may be slowed by the film-like fuel f flowing along the inner wall surface 8 of the combustion chamber. Thus, ignition lag may be prevented, and the rate of pressure increase in the cylinder may be suppressed to achieve reduction in the noise. 
     When the fuel is formed into a film and the film-like fuel is allowed to flow in the same direction of a stream of the swirl S, the mixing of fuel with air by the swirl S is suppressed. If vaporization of fuel and mixing of fuel with air are substantially reduced, the amount of HC and blue-white smoke is reduced. In the regard, according to the present invention, the auxiliary spray fuel f is atomized by making the aperture small, and the fuel film is made thin. As a result, vaporization of the fuel by heat in the atmosphere in the combustion chamber and from the inner wall of the combustion chamber may be accelerated. Under the accelerated condition, the partial fuel f 1  is atomized and scattered to obtain premature ignition. After the premature ignition, a fuel vapor is sufficiently supplied for combustion to attain smooth slow combustion and further decrease HC and blue-white smoke. 
     Consequently, according to the present invention, a slow combustion process is conducted under the accelerated condition of vaporization and mixing of the film-like fuel with air. Under such a good balance of accelerated vaporization of fuel and slow combustion, rapid increase in pressure is suppressed to reduce the noise and decrease HC and blue-white smoke. Especially, the premature ignition by atomization of partial fuel f 1  prevents ignition lag and explosive combustion, and further suppresses creation of HC and blue-white smoke. 
     Moreover, according to the present invention, the inner wall surface 8 of the combustion chamber is arranged in the vicinity of the auxiliary nozzle hole 7. Accordingly, it is possible to suppress the auxiliary spray fuel f being drawn by the swirl S until the fuel collides with the inner wall surface 8 of the combustion chamber. In this respect also, rapid combustion may be prevented. 
     On the other hand, under medium and high load, a combustion process similar to that under low load is conducted at the beginning of injection. Especially in this case, the needle valve is lifted highly, and then a main spray fuel F having a large spray penetration is injected from the main nozzle hole 6. Similarly, the main spray fuel F is injected in the same direction of the stream of the swirl S toward the inner wall surface 8 of the combustion chamber 2 on the downstream side of the swirl S, and is deposited in the state of film onto the inner wall surface 8 of the combustion chamber 2 by the energy of the swirl S. Then, the fuel F is vaporized by a flame or the like created by the premature ignition of the auxiliary spray fuel f. Accordingly, it is possible to reduce the noise rather than instantaneously mix the overall fuel with air. 
     FIG. 3 shows another preferred embodiment of the present invention. According to this preferred embodiment, in substitution for the aforementioned construction wherein the auxiliary nozzle hole 7 is exposed to the inner wall surface 8 of the combustion chamber as arranged adjacent thereto to allow the auxiliary spray fuel f to collide with and be deposited onto the inner wall surface 8 of the combustion chamber, the fuel injection nozzle means mounting passage 4 is formed with an inner wall surface 9 connected to the inner wall surface 8 of the combustion chamber, and the auxiliary nozzle hole 7 is exposed to the inner wall surface 9. The auxiliary spray fuel f is allowed first to collide with the inner wall surface 9, and is then allowed to flow from the passage 4 to the inner wall surface 8 of the combustion chamber. In this case, the auxiliary spray fuel f is prevented from being drawn by the swirl S until it collides with the inner wall surface 8 of the combustion chamber, thereby suppressing the rapid combustion. The main nozzle hole 6 in the preferred embodiment is similar so that in the previous embodiment. 
     Furthermore, according to the present invention, shortening of ignition lag (assuring of premature ignition) makes it possible to use fuel of a low cetane number to suitably operate the Diesel engine. 
     EFFECT OF THE INVENTION 
     As is mentioned above, the superior effects of the present invention are summarized as follows: 
     (1) Fuel may be satisfactorily burnt by the premature ignition and the slow combustion process to thereby reduce the noise and decrease HC and blue-white smoke. 
     (2) The auxiliary spray fuel is prevented from being drawn by the swirl until it collides with the inner wall surface to thereby suppress the rapid combustion. 
     (3) The use of fuel of a low cetane number is made possible to thereby suitably operate the Diesel engine by assuring premature ignition.