TECHNICAL FIELD

The technology disclosed herein relates to a combustion chamber structure of an engine.

BACKGROUND OF THE DISCLOSURE

JP6477849B1 discloses a compression ignition engine which combusts a portion of a mixture gas by compression ignition. This engine ignites the mixture gas by an ignition device. Flame propagation combustion begins, and unburnt mixture gas carries out self-ignition combustion by heat generated by the flame propagation combustion.

The engine is provided with a water injection device which injects water into the combustion chamber. As the water injection device injects water into the combustion chamber, the inside of the combustion chamber is cooled. This engine can reduce the occurrence of abnormal combustion.

In addition to the above-described purpose of reducing the abnormal combustion, the present inventors focused on an increase in an amount of operative gas by injecting the water (and steam) into the combustion chamber to increase a workload of a piston of the engine, thereby improving engine torque.

However, the present inventors found that combustion stability was lowered, if the water is injected into the engine combustion chamber for the purpose of improvement of the torque. This is because a large amount of water has to be injected into the combustion chamber when increasing the engine torque. That is, if the amount of water injected into the combustion chamber increases, the concentration of water near the spark plug becomes higher, and therefore, igniting the mixture gas by the spark plug becomes difficult.

SUMMARY OF THE DISCLOSURE

The technology disclosed herein enables ignition of a mixture gas, even if water is injected into a combustion chamber.

As a result of the examination of the above-described problem by the present inventors, they found a combination of a structure of the combustion chamber and an injection mode, which is capable of reducing a diffusion of the water. The technology disclosed herein is completed based on these new findings from the present inventors.

According to one aspect of the present disclosure, an engine system is provided, which includes a combustion chamber including a cylinder formed in an engine and a piston configured to reciprocate inside the cylinder, a spark plug disposed in a ceiling part of the combustion chamber, and a water injection device configured to inject water into the combustion chamber through a plurality of nozzle holes facing the inside of the combustion chamber. The piston has a cavity in an upper surface thereof. The water injection device injects water into the cavity in a compression stroke at a timing when an extension of axes of at least some of the nozzle holes intersects the cavity. The cavity has a bottom part where the water injected by the water injection device collides, and a raising part configured to raise the water spreading along the bottom part toward the water injection device.

During a compression stroke, a flow of gas inside the combustion chamber is weaker than that during an intake stroke. According to this configuration, by the water injection device injecting water in a compression stroke, diffusion of the water by the flow of gas is reduced. Note that the water injection device may inject water vapor inside the combustion chamber instead of water. “Water” as used herein includes water vapor which is a gas. Since the water does not consume combustion heat or consumes only a little if injecting water vapor into the combustion chamber, it is advantageous in improving thermal efficiency of the engine.

In this configuration, the water injection device injects the water into the cavity at the timing when the extension of axes of at least some of the nozzle holes intersect the cavity. The injected water reaches inside the cavity.

The cavity has the bottom part and the raising part which raises the water spreading along the bottom part toward the water injection device. According to the present inventors' examination, they found that by injecting water toward the cavity having the raising part, the water is suppressed from spreading to areas other than the cavity. The water is kept, or mostly kept, within an area in the cavity so that the concentration of water near the spark plug is kept low even if the water is injected into the combustion chamber. Thus, the spark plug can stably ignite the mixture gas.

Moreover, injecting water into the combustion chamber increases an amount of operative gas, and thus, the torque of the engine improves. An automobile mounted with this engine exhibits improved fuel efficiency.

Note that the phrase the “area in the cavity” as used herein refers to an area which is comprised of an area inside the cavity dented from the upper surface of the piston, and an area from a projected surface of the opening of the cavity which is projected toward the ceiling part of the combustion chamber in an axial direction of the cylinder, to the opening of the cavity. Moreover, the “areas other than the cavity” as used herein are areas other than the “area in the cavity.”

The bottom part may be comprised of a flat surface perpendicular to the axis of the cylinder, and the raising part may be comprised of a side wall perpendicular to the bottom part.

According to the present inventors' examination, they found that if the side wall perpendicular to the flat bottom part is provided, the water having entered the cavity is effectively raised toward the water injection device. By injecting water by the water injection device toward the cavity having this configuration, diffusion of the water inside the combustion chamber is further reduced.

A heat barrier layer configured to reduce heat transfer into the piston may be provided to the upper surface of the piston and the surface of the cavity.

The heat barrier layer reduces a cooling loss of the engine. Moreover, when the cavity is comprised of the flat bottom part and the side wall perpendicular to the bottom part as described above, the shape of the cavity is not complicated and the heat barrier layer is easily formed on the surface of the cavity.

At least some of the nozzle holes of the water injection device may be directed toward the raising part of the cavity at a timing of injecting the water.

The water injected inside the cavity is slowed down while spreading along the bottom part. If a flow velocity of the water is low when the flow reaches the raising part, the water becomes difficult to be raised. According to this configuration, at least some of the water injected by the water injection device reaches near the raising part of the cavity. Therefore, the raising part can raise the water, while the flow velocity of the water is high. By raising the water toward the water injection device, the water spreading to areas other than the cavity can be effectively suppressed.

The water injection device may be disposed in the ceiling part, and at least a part of an injection surface of the water injection device, where the nozzle holes open may be located at a position of the cavity in a direction perpendicular to the axis of the cylinder. The spark plug may be installed at a position offset from the cavity in the direction perpendicular to the axis of the cylinder.

At least the part of the injection surface of the water injection device is located right above the cavity with respect to the axis direction of the cylinder. Accordingly, even if the position of the piston changes, the water injection device can inject the water toward the cavity. In other words, even if the timing of the water injection device injecting the water changes, the water injection device injects the water toward the cavity. The injected water is kept, or mostly kept, within an area in the cavity.

Moreover, the spark plug is installed at a position offset from the cavity. Since the water is kept or mostly kept within the area in the cavity, the concentration of water near the spark plug is low. Thus, the spark plug can stably ignite the mixture gas.

The axes of all the nozzle holes of the water injection device may be parallel or substantially parallel to each other.

According to the present inventors' examination, if the axes of the plurality of nozzle holes are parallel or substantially parallel to each other, jet flows of the water injected by the water injection device become difficult to spread in a direction perpendicular to the injecting direction. Since the water flows injected into the combustion chamber from the plurality of nozzle holes move in the same or almost the same direction, the momentum of the water jet flows which move in this direction increases. When the momentum of the jet flow increases, a force which draws in the fluid around the water jet flows injected by the water injection device into the jet flows becomes stronger. Thus, by the drawing-in effect becoming stronger, the diffusion of the water is reduced.

Moreover, the water raised by the raising part toward the water injection device is drawn toward the jet flows by the drawing-in effect of the water jet flows described above. That is, the raised water is prevented from spreading to areas other than the cavity.

By the combination of the raising part of the cavity and the injection mode of the water injection device, the water is kept or mostly kept within the area in the cavity. Thus, the diffusion of the water inside the combustion chamber is reduced, and even if a large amount of water is injected into the combustion chamber, the spark plug can stably ignite the mixture gas.

A distance between the centers of the nozzle holes may be 5 mm or less.

If the distance between the centers of the nozzle holes is short, the density of the water in the jet flows injected by the water injection device increases. If the density of the water is higher, the drawing-in effect of the fluid around the jet flows becomes stronger due to the increase in the momentum. According to the present inventors' examination, they found that, if the distance between the centers of the nozzle holes is 5 mm or less, the diffusion of the water is reduced more effectively. That is, even if a large amount of water is injected into the combustion chamber, the spark plug can stably ignite mixture gas.

When the compression stroke is divided into three stages of an early stage, a middle stage, and a late stage, the water injection device may inject the water during a period from the early stage to the middle stage of the compression stroke.

As will be described later, increasing the temperature of the water injected into the combustion chamber causes the water to vaporize without using combustion heat, and it is advantageous in improving the thermal efficiency of the engine. During the period from the early stage to the middle stage of the compression stroke, the pressure inside the combustion chamber is relatively low. Since the boiling point of water reduces as the pressure inside the combustion chamber decreases, evaporated water is injected into the combustion chamber even at low temperatures. That is, by injecting the water during the period from the early stage to the middle stage of the compression stroke, the amount of heat required for heating the water can be saved. This configuration is advantageous in improving fuel efficiency of an automobile mounted with this engine.

When an engine load is low, an amount of water injected by the water injection device may be less than when the engine load is high. When the injection amount of water is small, the water injection device may delay a start timing of the injection more than when the injection amount of water is large.

If the water is injected into the combustion chamber at an early timing in a compression stroke, a compression resistance of the piston increases. Delaying the injection timing of the water is advantageous in improving the thermal efficiency of the engine. In a case where the injection amount of water is small and the injection period of water is short, the water injection device delays the start timing of the injection. Thus, the compression resistance of the piston reduces, which is advantageous in improving the thermal efficiency of the engine.

On the other hand, when the injection amount of water increases, the period in which the water injection device injects water extends. When the amount of water is large, the water injection device advances the start timing of the water injection. The water injection device completes the injection before the middle stage of the compression stroke. Therefore, the amount of heat required for heating the water can be saved as described above.

The water injection device may be provided with a heating device configured to heat the water injected by the water injection device.

According to this configuration, water vaporized by heating is injected into the combustion chamber. It is reduced that the injected water into the combustion chamber consumes combustion heat. This is advantageous in improving the thermal efficiency of the engine.

The heating device may have a heat exchanger attached to an exhaust pipe of the engine. The heat exchanger may heat the water by heat of exhaust gas of the engine before supplying the water to the water injection device.

According to this configuration, exhaust heat of the engine is collected via water and vaporized water is injected into the combustion chamber. Therefore, the thermal efficiency of the engine improves.

The water injection device may have a water injection valve, and a temperature increasing part provided between the water injection valve and the combustion chamber, and configured to raise temperature of the water injected by the water injection valve before supplying the water to the combustion chamber. The heating device may have a heat pipe configured to send the heat of exhaust gas of the engine to the temperature increasing part.

According to this configuration, exhaust heat of the engine is collected and vaporized water is injected into the combustion chamber. Therefore, the thermal efficiency of the engine improves.

The engine system may include a water supply device configured to supply water to the water injection device. The water supply device may include a condenser configured to condense water inside exhaust gas of the engine, a water tank configured to collect water condensed by the condenser, and a water pump configured to pressurize water inside the water tank and supply the water to the water injection device.

According to this configuration, a user does not need to feed water for injecting into the combustion chamber into the water tank. In addition, since water is pressurized by the water pump, the water is injected into the high-pressure combustion chamber in the compression stroke. At least a part of the injected water injected is evaporated by a decompression boiling effect. Therefore, the amount of heat required for vaporization of the water can be saved.

The engine system may include a fuel injection valve configured to inject fuel, the fuel injection valve being disposed in an intake port communicating with the combustion chamber.

According to this configuration, homogeneous mixture gas is formed inside the combustion chamber. Thus, the spark plug can stably ignite the mixture gas, even if water is injected into the combustion chamber.

The engine may be a compression ignition gasoline engine in which at least a portion of the mixture gas combusts by compression ignition.

The mixture gas is combusted more stably by compression ignition, even if water exists in the combustion chamber.

