Patent Description:
When ammonia is combusted, ammonia generates no CO<NUM> because it is carbon-free. Thus, the combustion of ammonia in an internal combustion engine (a diesel engine, in particular) has been attempted. Ammonia, however, is flame-retardant because its ignition temperature is <NUM>, which is higher compared, for example, to a heavy oil which have an ignition temperature of <NUM> to <NUM>. Thus, the combustion rate of ammonia is low, and currently, <NUM>% or more of the charged ammonia is emitted as unburned combustibles.

In order to solve such a problem, attempts to increase ammonia combustion efficiency have been made by reforming part of ammonia, for example, using a catalyst so as to produce hydrogen, charging the obtained hydrogen into a combustion chamber together with ammonia, and using the highly combustible hydrogen as an ignition source.

In Patent Literature <NUM>, for example, in order to enable combustion of high-temperature hydrogen-rich ammonia and combustion air, a carbon-free power unit is configured to produce a high-temperature hydrogen-rich gas by: feeding urea water as a raw material for hydrogen-rich ammonia; producing high-temperature ammonia from the urea water; and adding part of the ammonia to hydrogen andnitrogen. That is, Patent Literature <NUM> relates to a hydrogen-rich ammonia production reactor for producing a high-temperature hydrogen-rich gas by producing high-temperature ammonia from urea water and adding part of the obtained ammonia to hydrogen and nitrogen.

In Patent Literature <NUM>, ammonia is reformed to produce a reformed gas containing hydrogen, and the reformed gas, in addition to ammonia, is fed into a combustion chamber of an ammonia combustion internal combustion engine.

An object of the present invention is to provide a diesel engine that reduces CO<NUM> emission by increasing the combustion rate of ammonia with a relatively simple configuration.

According to an aspect of the present invention, there is provided a diesel engine including: a combustion chamber; and a fuel charging unit that charges a fuel oil, used as an ignition source, and gaseous ammonia into the combustion chamber. In this diesel engine, an ammonia premixed gas is formed in the combustion chamber by the fuel charging unit, and the ammonia premixed gas is subjected to mixed combustion. The diesel engine includes a control unit configured to control the formation of the ammonia premixed gas so that a concentration of ammonia in the combustion chamber becomes higher in an area closer to an inner side wall surface of the combustion chamber, than in a central portion of the combustion chamber.

This eliminates the need to subject ammonia to mixed combustion with, for example, hydrogen by providing, for example, a device for reforming ammonia into hydrogen with a special catalyst. Thus, a diesel engine that reduces CO<NUM> emissions by increasing the combustion rate of ammonia with a simple configuration can be provided.

As a second aspect, the control unit may preferably adjust an air excess ratio to be lower than an air excess ratio at a time of operation only with a fuel oil.

According to the invention, the combustion chamber includes a cylinder head, the cylinder head is provided with an ammonia feeding path, ammonia injection holes are provided at multiple locations closer to the inner side wall surface of the combustion chamber than the central portion of the combustion chamber, and vaporized ammonia is controlled so as to be charged into the combustion chamber in an intake stroke.

As a fourth aspect, in any one of the first to third aspects, a gas valve that adjusts a charged amount of ammonia and a fuel injection valve that adjusts a feed amount of the fuel oil may preferably be provided, and the control unit may preferably control at least one of the gas valve and the fuel injection valve.

As a fifth aspect, in any one of the first to fourth aspects, the charged amount of ammonia may preferably be adjusted so as to be <NUM>% or more and <NUM>% or less of the feed amount of the fuel oil in terms of a heat quantity ratio.

As a sixth aspect, in any one of the first to fifth aspects, a deviation of the ammonia concentration at the inner side wall surface of the combustion chamber may preferably be set to <NUM>% or more relative to an average concentration in the combustion chamber.

As a seventh aspect, in the second aspect, the air excess ratio may preferably be adjusted to be higher by <NUM> to <NUM> than the air excess ratio at the time of operation only with the fuel oil.

As an eighth aspect, in the second or seventh aspect, the adjustment of the air excess ratio may preferably be performed by adjusting an amount of air with an adjustment unit provided in an exhaust bypass of a supercharger.

An embodiment of the present invention will be described below in detail with reference to the drawings.

The present inventors took an analytical approach to study a method for a diesel engine provided with a mechanical fuel injector as shown in <FIG>. In the diesel engine such as this, a premixed gas of ammonia and air (also referred to simply as a premixed gas) is produced by spraying ammonia into an intakepipe <NUM> from an ammonia injection valve <NUM> (a gas valve) so that the ammonia is mixed with the intake for the engine, and in which engine the premixed gas of ammonia and air is subjected to mixed combustion with a fuel oil, which is a main fuel injected from a fuel injection valve <NUM>, such as a heavy oil or a light oil, being used as an ignition source.

In <FIG>, the reference numeral <NUM> denotes a cylinder, the such as a heavy oil or a light oil, being used as an ignition source.

In <FIG>, the reference numeral <NUM> denotes a cylinder, the reference numeral <NUM> denotes a piston, the reference numeral <NUM> denotes a cylinder head, the reference numeral <NUM> denotes an intake valve, the reference numeral <NUM> denotes an exhaust valve, and the reference numeral <NUM> denotes an exhaust pipe.

Specifically, a combustion chamber based on an actual engine (bore × stroke = <NUM> × <NUM>, four-stroke) was modeled, and combustion simulation was conducted. Ammonia in a gaseous state was fed into the intake pipe <NUM> at <NUM>% in terms of a heat quantity ratio, and the ammonia premixed gas was combusted through the injection of the main fuel oil (a heavy oil or a light oil) from the fuel injection valve <NUM>. Using an ammonia concentration distribution in the premixed gas, an injection pressure of the main fuel oil, an injection pattern, and an air excess ratio as parameters, their effects on the characteristics of exhaust gas, efficiency, and unburned ammonia were investigated. The results of the investigation are shown in Table <NUM>.

In a case (case <NUM>) where ammonia NH<NUM> was charged at <NUM>% in terms of a heat quantity ratio with an air excess ratio of λ = <NUM> (equivalent to that in a case (case <NUM>) operated by <NUM>% heavy oil without the supply of ammonia), the amount of unburned ammonia was <NUM>%, thus, only achieving a combustion rate of about <NUM>%. Calculation was then made for a case (case <NUM>) where the air excess ratio λ was adjusted to <NUM> (toward a richer mixture), and the amount of unburned ammonia was <NUM>%, thus confirming that the combustion rate was improved by about <NUM>%. Such improvement can be considered as the result of an increase in the combustible range of ammonia premixed gas due to an increase in combustion temperature as shown in <FIG>. In order to figure out reduction rates of the amount of unburned ammonia and changes in various performances when the air excess ratio λ was reduced, calculations when λ = <NUM> (case <NUM>), λ = <NUM> (case <NUM>), λ = <NUM> (case <NUM>-<NUM>), and λ = <NUM> (case <NUM>-<NUM>) were made. As calculation conditions, the premixed gas had a uniform concentration, and the main fuel oil injection timing was fixed at <NUM> deg.

High combustion temperature refers to a temperature sufficiently higher than the ignition temperature of ammonia and that is capable of raising the combustion rate, although such a temperature varies depending on the specification and ambient temperature of the diesel engine. As a guideline, the high temperature refers to a combustion temperature of about <NUM>, for example.

According to the results, it was found that the unburned ammonia can be reduced to <NUM>% by adjusting the air excess ratio toward a richer (fuel richer) mixture so as to be λ = <NUM> in the case <NUM>-<NUM>. In the range of λ = <NUM> to <NUM>, no significant changes were observed with regard to efficiency and NOx.

When the air excess ratio λ was adjusted to <NUM> toward a richer mixture in the case <NUM>-<NUM>, however, combustion delay rapidly occurred due to lack of air and reduction in compression pressure. The maximum combustion pressure was reduced by <NUM> MPa, and the efficiency deteriorated by <NUM>% accordingly. Note however that the combustion temperature was maintained high, and the amount of unburned ammonia was <NUM>%, that is, substantially equivalent to that when λ = <NUM>. The maximum combustion temperature was higher by <NUM> than that when λ = <NUM> (in the case <NUM>-<NUM>) and maintained for a long period of time, thus resulting in an increase of the NOx value by <NUM> ppm. Furthermore, although the efficiency reduction and the amount of unburned ammonia were equivalent to those when λ = <NUM> (in the case <NUM>-<NUM>), the reduction rate of CO<NUM> deteriorated by about <NUM>%. Based on these findings, it was concluded that an air excess ratio λ around <NUM> is optimal for <NUM>% ammonia-mixed combustion.

<FIG> shows changes in combustion temperature distribution caused by changes in air excess ratio λ. <FIG> shows the comparisons of temperature distributions and in-cylinder pressures (with the horizontal axis representing engine crank angles (deg. ), and the vertical axis representing in-cylinder pressures) when λ = <NUM> (in the case <NUM>-<NUM>) and λ = <NUM> (in the case <NUM>-<NUM>). <FIG> shows comparison among in-cylinder pressures caused by changes in air excess ratio λ.

The influence of fuel injection timing was investigated next.

When the air excess ratio λ is varied with a constant fuel injection timing, the smaller the value of λ, the lower the compression pressure. Thus, with a constant main fuel injection timing, the maximum combustion pressure is also on a downward trend. In order to maintain the efficiency and keep the combustion temperature, fuel injection timing needs to be adjusted in accordance with λ (the amount of air, or boost pressure). Comparisons were made between a case with λ = <NUM> and injection timing of <NUM> deg. BTDC and a case with λ= <NUM> and injection timing of <NUM> deg. The maximum combustion pressures had a difference of around <NUM> MP, but those had values equivalent to that of an ordinary diesel engine. For the case of the latter adjustment, it can be recognized, based on its heat generation rate, that satisfactory combustion was obtained. Accordingly, the amount of unburned ammonia also decreased to <NUM>%, i.e., decreased by half. Based on this, it was found that adjusting fuel injection timing in accordance with respective combustion conditions and keeping the maximum combustion pressure at a level equivalent to that of a diesel engine with the standard specification are desirable.

The influence of degrees of ammonia stratification was investigated next.

Since combustion temperature rises in the vicinity of a cylinder inner wall surface (hereinafter, also referred to simply as a wall surface) in combustion by means of main fuel oil injection, we made a hypothesis that increasing the concentration of ammonia in the vicinity of the wall surface, within the premixed gas of ammonia and air, which is flame-retardant, would be effective to reduce the amount of unburned ammonia. In order to verify the validity of the hypothesis, combustion calculations were made for three cases where a concentration difference (referred to as a degree of stratification) between a central portion of the cylinder combustion chamber and the wall surface portion was set to ±<NUM>% (case <NUM>-<NUM>), ±<NUM>% (linear concentration gradient: case <NUM>-<NUM>) and ±<NUM>% (lean at the top of the combustion chamber: case <NUM>-<NUM>), as shown in Table <NUM>. Note that in these calculations, λ = <NUM> and fuel injection timing of <NUM> degree, BTDC were common to all the cases.

In the case <NUM>-<NUM> with a degree of stratification of ±<NUM>%, the amount of unburned ammonia was <NUM>%, which shows little reduction as compared to the case of the uniform mixture. When the degree of stratification was raised to ±<NUM>% (in the case <NUM>-<NUM>), the amount of unburned ammonia was <NUM>%, thus achieving a significant improvement effect. In the case <NUM>-<NUM> with a degree of stratification of ±<NUM>%, the amount of unburned ammonia was substantially equivalent to that in the case <NUM>-<NUM> under the conditions of ±<NUM>%, but the combustion efficiency was slightly better, which indicates an efficiency improvement by <NUM>%. For the other exhaust gas components, both of the cases <NUM>-<NUM> and <NUM>-<NUM> contained nitrous oxide N<NUM>O at <NUM> ppm and particulate matters PM at <NUM>/KWh. A significant reduction was exhibited in PM, in particular.

<FIG> shows, in a comparative manner, in-cylinder pressures and heat generation rates according to the degrees of ammonia stratification when λ = <NUM> with the horizontal axis representing engine crank angles deg.

In the production of the premixed gas of ammonia and air, in order to obtain a uniform premixed gas, these methods were employed: providing the ammonia injection valve <NUM>, which is a gas valve, in the intake pipe <NUM> located in front of the cylinder head <NUM> shown in <FIG> and feeding ammonia into the intake pipe <NUM>, so that mixing is performed in the intake pipe <NUM>; and feeding the premixed gas in a more uniform state to the cylinder <NUM> through the intake valve <NUM>.

In such a conventional method of producing a premixed gas, however, limitations are imposed around <NUM>% for the creation of non-uniformity (the degree of stratification) in the premixed gas even when swirl is created by a gas valve equipped with a nozzle. In this case, the amount of ammonia to be combusted in a high-temperature part of the combustion chamber is insufficient, thus leaving about <NUM> to <NUM>% ammonia unburned.

According to the present invention, ammonia is fed simultaneously with air through a gas valve into a diesel engine that mainly uses a fuel oil while conducting flow control so that the concentration of ammonia is rich around a cylinder inner wall surface where the main combustion of the fuel oil combustion occurs and where the combustion temperature becomes high, in order to improve the combustion efficiency.

For the degree of ammonia stratification (the degree of concentration difference, or imbalance), a significant deviation of <NUM>% or more is provided in the high-temperature part of the diesel main combustion region. This cannot be achieved when a gas mixer is installed before a combustion chamber as in the typical production of premixed gases, or when ammonia is charged in the vicinity of the intake pipe <NUM>, in each cylinder as shown in <FIG>.

In view of this, according to the present embodiment, an ammonia feeding path (referred to as an ammonia line) <NUM> is provided in a cylinder head <NUM> in an ammonia combustion diesel engine as shown in <FIG>. Vaporized ammonia is charged in an intake stroke through ammonia injection holes <NUM> provided at multiple locations (two locations illustrated by way of example in <FIG> and <FIG> in the present embodiment) closer to an inner side wall surface of a combustion chamber, rather than to a central portion of the combustion chamber. This enables the achievement of a degree of stratification of <NUM>% or more, which has been impossible to achieve with the conventional method.

Note that the locations where the multiple ammonia injection holes are formed, as used herein, assuming that a distance from the center of the combustion chamber to the inner side wall surface of the combustion chamber corresponds to <NUM>%, preferably fall within a distance range of <NUM>% to <NUM>%.

In addition, in order to raise the combustion temperature higher, the air excess ratio is set to be richer than the conventional value by Δλ = <NUM> to <NUM>.

The ammonia line <NUM> is configured to include an ammonia tank <NUM>, a gas shut-off valve <NUM>, a gas pressure regulating valve <NUM>, a gas valve <NUM>, a pipe <NUM>, a check valve <NUM>, a connector <NUM>, and the ammonia injection hole <NUM>.

In the figure, the reference numeral <NUM> denotes a fuel oil line including a fuel oil tank <NUM>, a fuel feed pump <NUM>, and a solenoid valve <NUM>. The reference numeral <NUM> denotes a intake and exhaust system including: an intake line <NUM>; a supercharger <NUM> equipped with an intake turbine 64B to be rotated by an exhaust turbine 64A; an exhaust line <NUM>; and an exhaust bypass valve <NUM> provided in a bypass line <NUM> connecting between the front and rear sides of the exhaust turbine 64A along the exhaust line <NUM>.

The solenoid valve <NUM> is controlled by a device <NUM> for controlling fuel oil injection. The gas valve <NUM> is controlled by a device <NUM> for controlling an ammonia gas valve. The device <NUM> for controlling fuel oil injection, the device <NUM> for controlling an ammonia gas valve, and the exhaust bypass valve <NUM> are controlled by a centralized control device <NUM>.

The device <NUM> for controlling fuel oil injection, the device <NUM> for controlling an ammonia gas valve, the exhaust bypass valve <NUM>, and the centralized control device <NUM> together constitute a control unit.

Although ammonia is retained in liquid form in the ammonia tank <NUM>, the ammonia is vaporized and supplied in a gaseous state. The gas pressure thereof is regulated by the gas pressure regulating valve <NUM>, and the flow rate thereof is controlled by the gas valve <NUM> so as to achieve a predetermined heat quantity ratio (around <NUM>%). Such ammonia is fed to the engine at predetermined intake timing illustrated by way of example in <FIG>. The gas valve <NUM> is provided in each cylinder <NUM>. Ammonia controlled by the gas valve <NUM> is distributed by the pipes <NUM> shown in <FIG> and sent to the plurality of ammonia injection holes <NUM> through the connectors <NUM>. The portion of the connector <NUM> is provided with the check valve <NUM> so as to prevent the back flow of combustion gas at the time of combustion.

As shown in <FIG> in detail, the check valve <NUM> is a double-poppet check valve having two poppets 50A and 50B. The reason why the double-poppet check valve is employed here is to prevent the possibility that a sealing portion becomes worn with long-term use and thus leakage occurs.

As illustrated in an example of the poppet 50A in <FIG>, holes 50C are provided on the side surface of each of the poppets 50A and 50B. The poppets 50A and 50B move in a horizontal direction in the figure as in when NH<NUM> is charged (shown in <FIG>) and when the back flow is prevented (shown in <FIG>) to prevent the back flow of NH<NUM>. That is, at the time of charging NH<NUM> (shown in <FIG>), the poppets 50A and 50B move in the right direction in the figure by being pushed by the pressure of NH<NUM> until the poppets 50A and 50B abut against a stopper 50D. Thus, NH<NUM> flows in the right direction through an opening of a seating part 50E. At the time of preventing back flow (shown in <FIG>), on the other hand, the poppets 50A and 50B are pushed in the left direction by in-cylinder pressure. Consequently, the poppets 50A and 50B are seated on the seating part 50E, and sealing is thus achieved.

Since the double-poppet check valve is employed in the present embodiment, leakage can be reliably prevented from occurring. Note that a type of check valve is not limited thereto.

Ammonia thus charged through the ammonia injection holes <NUM> in the intake stroke can have a degree of stratification of <NUM>% or more on the wall surface. Note that the number of the ammonia injection holes <NUM> is not limited to two. The number of the ammonia injection holes <NUM> may be four as shown in <FIG> with broken lines, or six, for example.

The diesel main fuel is subjected to flow control by the device <NUM> for controlling fuel oil injection so as to have a predetermined heat quantity ratio (around <NUM>%) and injected via the fuel injection valve <NUM> at predetermined timing.

The air excess ratio λ is adjusted to a predetermined value by controlling an amount of exhaust to be passed via the exhaust bypass valve <NUM> provided on the exhaust turbine 64A side of the supercharger <NUM>. That is, in order to make the air excess ratio λ rich by reducing an amount of air, the exhaust bypass value <NUM> is closed a little to lower the number of revolutions of the intake turbine 64B, and thereby reduce the amount of air.

As illustrated by way of example in <FIG>, the degree of stratification of the premixed gas is set to around -<NUM>% in the central portion and to around +<NUM>% in the wall surface portion relative to the concentration average value of the premixed gas of ammonia and air.

The combustion temperature becomes high in the wall surface portion. Thus, the combustion rate of ammonia increases by concentrating ammonia near the wall surface portion.

Although the mechanical fuel injector is employed in the above-described embodiment, an electronic injection common-rail fuel injector can be employed instead. Moreover, the configurations of the ammonia line and the intake and exhaust system are not limited to those in the embodiment.

Although the present invention is applied to a diesel engine for ships in the above-described embodiment, the application target of the present invention is not limited thereto. The present invention can be similarly applied also to a diesel engine that uses a light oil as a fuel, or an engine that combines compression ignition as in a diesel engine with spark ignition as in a gasoline engine.

Claim 1:
A diesel engine comprising:
a combustion chamber in a cylinder head(<NUM>);
a fuel charging unit that charges a fuel oil, used as an ignition source, and gaseous ammonia into the combustion chamber, so as to form an ammonia premixed gas in the combustion chamber, and the ammonia premixed gas is subjected to mixed combustion,
characterized in that the diesel engine further comprises a control unit(<NUM>,<NUM>,<NUM>) configured to control the formation of the ammonia premixed gas so that a concentration of ammonia in the combustion chamber becomes higher in an area closer to an inner side wall surface of the combustion chamber, than in a central portion of the combustion chamber, and wherein
the cylinder head(<NUM>) is provided with an ammonia feeding path(<NUM>), ammonia injection holes(<NUM>) are provided at multiple locations closer to the inner side wall surface of the combustion chamber than the central portion of the combustion chamber, and vaporized ammonia is controlled so as to be charged into the combustion chamber in an air supply stroke.