Patent Application
20070039598
The present description relates to control of a lean burn internal combustion engine, such as a hydrogen fueled internal combustion engine, using exhaust gas recirculation.
Conventionally, hydrocarbon based fuel is widely used for internal combustion engines such as automotive engines. One of disadvantages of using such fuel for the internal combustion engines is emission of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) as well as carbon dioxide (CO2). To reduce such emission, lean-burn engines are known to combust the fuel at an air-fuel ratio leaner than the stoichiometric air-fuel ratio of the given fuel. In other words, the lean-burn engine is supplied with excess amount of air to combust the given amount of fuel to produce the desired amount of torque. Consequently, the fuel is almost fully oxidized into CO2 and H2O, thereby decreasing the emission of HC and CO. Further, the engine is not throttled as much as it would be at the stoichiometric air-fuel ratio, thereby effectively decreasing pumping loss of the engine so as to improve operating efficiency of the engine or fuel economy. However, combustion in the lean-burn engines may occur at relatively high temperatures, and may generate NOx.
In recent years, hydrogen fueled internal combustion engines have been developed, as they are considered inherently not to emit CO2 which is considered by some to be a major cause of the global warming. Although combustion of hydrogen fuel in an internal combustion engine generates little HC or CO, it may still generate NOx during the lean burn operation.
To decrease the NOx generation from the lean-burn engines, a method is known and presented, such as in Japanese patent application publication 2004-340065. This method re-circulates a portion of the exhaust gas back to the engine intake system. The exhaust gas combines with fresh air and inducted into the engine cylinders. This is known as exhaust gas recirculation (EGR). The re-circulated exhaust gas, which is inert gas, may reduce the combustion speed in the combustion chamber and the combustion temperature, thereby decreasing the NOx generation during the lean burn operation.
In the meantime, when an excess air to fuel ratio λ (=1 at the stoichimetric air-fuel ratio) is around 2.0 for hydrogen fuel, the NOx generation may be minimal. In other words, the EGR may not be necessary above certain level of λ.
The method described in the above Japanese patent document also has disadvantages. For example, when the engine is operated at a reduced torque level and with an air-fuel ratio that is leaner than λ=2, higher concentrations of EGR can excessively decrease the combustion speed. Consequently, there may be some deterioration of engine performance that may result from reduced combustion stability. This may lead to drivability degradation, fuel economy degradation or emission degradation such as emission of un-combusted fuel.
The inventors herein have recognized these disadvantages and have developed a method to improve the engine performance during the lean-burn operation.
Accordingly, there is provided, in one aspect of the present description, a method of controlling an internal combustion engine. The method comprises supplying fuel and fresh air with substantially no re-circulated exhaust gas into a combustion chamber of the internal combustion engine, so that an air-fuel ratio in the combustion chamber is a first ratio leaner than the stoichiometric air-fuel ratio, during a first engine operating condition. The method further comprises supplying fuel, fresh air and re-circulated exhaust gas into the combustion chamber, so that an air-fuel ratio in the combustion chamber is a second ratio leaner than the stoichimetric air-fuel ratio, the second ratio being richer than the first ratio, during a second engine operating condition at which time the desired torque is greater than that during the first engine operating condition.
In accordance with the method, during the second operating condition when desired torque is greater, NOx emission from the engine is decreased, by supplying the lean air-fuel mixture with the EGR into the combustion chamber. This allows the engine to operate with reduced emissions as well as increased fuel economy as a result of the lean burn. During the first operating condition when desired torque is reduced, by supplying the further lean air-fuel mixture with substantially no EGR into the combustion chamber, the EGR generation may be minimal because of the further lean air-fuel ratio, and the combustion stability is maintained because of the substantially no EGR. Therefore, the method may improve the engine performance such as drivability, fuel economy and emission over the range of lean-burn engine operation.
The supplied fuel may be hydrogen such as gaseous hydrogen. In that case, the emission of CO2, HC and CO can be eliminated, thereby leading to a further emission performance improvement.
The EGR may be adjusted by controlling an EGR valve configured to open and close an external EGR passage connecting between the intake air passage and exhaust gas passage, which can control the EGR in a desired manner. Further, the air-fuel ratio may be adjusted by controlling a throttle valve and a fuel injector, which can control it in a desired manner. During the second operating condition, the air-fuel ratio may be made richer and the EGR may be increased as the desired engine torque increases. Although the NOx generation may be increased as the air-fuel ratio is richer during the second operating condition, the EGR, which is increased corresponding to the air-fuel ratio, may suppress the NOx generation accordingly.
At least fuel and air may be supplied so that the air-fuel ratio in the combustion chamber is the stoichiometric air-fuel ratio during a third engine operating condition at which time the desired engine torque is greater than the desired torque during the first and second engine operating conditions. During the third operating condition, since the air-fuel ratio is at the stoichiometric, the NOx generation is relatively low and, NOx, if any, can be easily purified with a conventional three way catalyst.
EGR may be supplied during the third operating condition, particularly in the case of the hydrogen being fuel. Since the hydrogen is more likely to be ignited than the hydrocarbon based fuel, it may be ignited prior to the normal ignition timing or spark ignition timing during the compression stroke (pre-ignition), which may cause engine damage. By supplying the EGR to the stoichiometric mixture during the third operating condition, the pre-ignition can be eliminated because of the inert property of the EGR. In the case of using the external EGR passage, the EGR may be cooler, thereby further suppressing the pre-ignition property of the fuel.
The advantages described herein will be more fully understood by reading an example of embodiments in which the above aspects are used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:
The embodiment of the present description will now be described with reference to the drawings, starting with
The combustion chamber is supplied with fresh air through an intake system or intake passage 2 for combusting the hydrogen in the combustion chamber. There are provided, on the intake system 2 sequentially from the upstream side of the airflow, an airflow sensor 3, a throttle valve 4, and a fuel injector 5 which is preferably provided for each cylinder. The airflow sensor 3 detects a flow rate of the intake air through the intake passage 2 and outputs an air flow signal AF to an engine controller 15 which is described in more detail below. The throttle valve 4 regulates the intake airflow when it is at least partly closed as is known in the art. Its opening is adjusted by an actuator not shown which is controlled by the engine controller 15 with a signal TVO. The fuel injector 5 injects fuel into the intake air passage 2, particularly an intake port of the engine 1, when it opens in a particular engine cycle. Its opening, particularly an opening duration, is controlled by a pulse width signal FP from the engine controller 15 as is known in the art.
The fuel injector 5 is supplied with the hydrogen, particularly gaseous hydrogen, from a hydrogen source 6 through a hydrogen supply passage 7. The hydrogen source 6 may be, for example, a high pressure gas tank which stores high pressure gaseous hydrogen, a liquid tank which stores low temperature liquid hydrogen, or a hydrogen storage which is filled with hydrogen storage material inside. When the hydrogen storage material (for example, metal hydride) in its cooled state contacts with gaseous hydrogen, it may adsorb the hydrogen with one thousandths of the volume. On the other hand, when the storage material is heated, it will release the stored hydrogen with a higher pressure.
The exhaust gas or combustion gas generated by the combustion of the hydrogen in the combustion chamber of the engine 1 flows out through an exhaust passage or exhaust system 8. There are provided, on the exhaust system 8 sequentially from the upstream side of the exhaust gas flow, an exhaust gas oxygen (EGO) sensor 9 and an exhaust gas purification apparatus 10. The EGO sensor 9 detects an oxygen concentration in the exhaust gas, that is an excess air ratio λ or an air-fuel ratio of mixture of the fuel (hydrogen) and the air supplied to the combustion chamber. When the excess air ratio λ is equal to 1, the mixture is in a stoichimetric condition, that is, the oxygen amount is not excessive or short compared to the fuel amount. The EGO sensor 9 outputs a signal EGO to the engine controller 15 which outputs the signal FP to the fuel injector 5 to adjust the air-fuel ratio in the combustion chamber. The exhaust gas purification apparatus 10 is preferably, a three way catalyst. It can reduce the NOx generated from the combustion of the hydrogen with the un-combusted hydrogen there when the air-fuel ratio is adjusted at the stoichiometry. It is not limited to the three way catalyst, alternatively it may be a lean NOx trap, which can store the NOx therein, when the air-fuel ratio is lean. It releases the stored NOx and reduces it with oxidant such as fuel or hydrogen, when the air-fuel ratio is rich or stoichiometric.
The engine system comprises an EGR passage 11 for re-circulating a portion of the exhaust gas in the exhaust passage 8 to the intake passage 2 (exhaust gas recirculation: EGR). The EGR passage 11 connects a part of the exhaust system 8 upstream of the EGO sensor 3 with a part of the intake passage 2 upstream of the fuel injector 5 and downstream of the throttle valve 4. There is provided, on the EGR passage 11, an EGR valve 12 which adjust a flow rate of the re-circulated exhaust gas. The opening of the EGR valve 12 is controlled to be the desired by the engine controller 15 with a signal dEGR input to an actuator of the EGR valve 12. The opening is detected by an EGR valve opening sensor 13 which outputs a signal EGRo to the engine controller 15 for a feedback control of the opening of the EGR valve 12 which is ranged between 0 and 100%.
The engine controller 15, as known in the art, has a memory storing a program and data and a microprocessor executing instructions included in the program based on the data in the memory and the inputs from the various sensors described above including the intake air flow signal AF, the EGR valve opening signal EGRo and the signal EGO from the EGO sensor 9. In addition to the above inputs, an engine rotational speed signal RPM from an engine speed sensor 14 and a position of an accelerator pedal that is indicating a desired engine power dP are input to the controller 15. Based on these inputs, the controller 15 outputs the signal TVO to the actuator of the throttle valve 4 and the signal FP to the fuel injector 4 collectively to control the air-fuel ratio, and the signal dEGR to the actuator of the EGR valve 12 to control the EGR amount.
The air-fuel ratio and EGR control by the engine controller 15 will now be described with reference to a flowchart shown in
The routine proceeds to a step S2, where a target operation mode is determined based on the engine torque TQ and the engine speed RPM determined at the step S1. The mode determination may be made, for example, by referencing a two dimensional map indexed with the engine torque and speed, such as an engine operation mode map shown in
As shown in
In the lower torque range, the desired fuel amount is smaller. So, to reduce the pumping loss of the engine 1, the air-fuel ratio is set to be lean of stoichiometry, for example, the greater than λ=1.8, by relatively opening the throttle valve 4. As illustrated in a graph of
In the intermediate torque range, considering the EGR described later, the air-fuel ratio in the combustion chamber is set supposedly to be lean of the stoichiometry, for example λ=1.05 through 1.8 for the decrease of the pumping loss of the engine 1 by relatively opening the throttle valve 4 or in some cases, fully opening it. In this range of λ without EGR, NOx is generated through the combustion and can not be well adsorbed by the three way catalyst 10, so that the NOx emission is substantial as illustrated in the graph of
In the high torque range, the air-fuel ratio is set to be the stoichiometric air-fuel ratio (in other words, λ=1), then the operation mode would be called a stocihiometric mode. At the stoichiometric air-fuel ratio, although the NOx is still generated through the combustion, the three way catalyst 10 can be fully operative and effectively reduce the NOx into nitrogen, as indicated in the graph of
Referring back to the flowchart of
Further then the routine proceeds to a step S5, where the EGR valve 12 is fully closed, since as described above, the EGR is not needed for preventing the NOx generation. In this instance, the controller 15 may compute the desired EGR valve opening dEGR to be 0%. If it is already fully closed, the fully closed state is maintained or the dEGR is maintained to be 0%. Although, the EGR is not necessary for the NOx prevention, it may be useful for preventing pre-ignition of hydrogen in the combustion chamber before a normal ignition timing, which may cause an engine damage. Therefore, instead of fully closing the EGR valve 12 in the stoichiometric mode, the valve may be slightly opened, for example, by setting the dEGR to be 20% of the full opening.
If it is not determined that the current operation mode is the stoichiometric mode (NO) at the step S3, the routine proceeds to a step S6, where the throttle valve 4 and the fuel injector 5 are controlled to adjust the air-fuel ratio to be lean of the stoichiometry while the engine 2 generates the desired torque. In this instance, the throttle valve 4 and the fuel injector 5 may be controlled so as to adjust the air-fuel ratio supposedly to be λ=2 without considering the EGR, which can be called an initial λ. The air-fuel ratio adjustment may be made, for example, by referring to a two dimensional map stored in the memory of the controller 15, indexed to the desired engine torque dTQ and the engine speed RPM and mapped with sets of values of the target throttle opening TVO and the fuel injection amount FP. However, at a relatively higher torque region in the intermediate torque range, the TVO is set 100%, since λ=2 can not be realized in that region because the fuel injection amount FP is a half or more of that corresponding to the maximum torque. In other words, the air-fuel ratio would fall within a range of substantial NOx generation as shown in
Then, the routine proceeds to a step S7, where it is determined whether the current operating condition is within the intermediate torque range, in other words, whether the operation mode is the lean-burn with EGR mode or not. If it is determined that the operation mode is the lean-burn with EGR mode (YES) at the step S7, as described above, the air-fuel ratio may fall within the range of the substantial NOx generation as shown in
The EGR map shown in
On the other hand, if it is not determined that the operation mode is the lean-burn without EGR mode (NO) at the step S7, the routine proceeds to a step S9. In this case, the current operating condition falls within the lower torque range. At the step S9, the EGR valve 12 is fully closed. In this instance, the controller 15 may compute the desired EGR valve opening dEGR to be 0%. If it is already fully closed, the fully closed state is maintained or the dEGR is maintained to be 0%. In other words, the operation mode has entered the lean-burn with EGR mode illustrated in
During the engine operation in the lower torque range at the air-fuel ratio λ=2.0, the pressure in the intake passage 2 is substantially lower than that in the exhaust passage 8. In other words, the pressure difference is greater. If the EGR were supplied, the opening of EGR valve 12 would be needed to be very precisely controlled by the controller 15. Otherwise, proportion of the EGR in the inducted mixture into the combustion chamber would be varied by relatively large extent so that the EGR might excessively decrease the combustion speed leading to deterioration of combustion stability. Rather, the control routine stops the EGR in this instance to improve the drivability, the emission, and the fuel economy.
Now a second embodiment of the present description will be described mainly with reference to a flowchart of
Steps S11 through S15 are same as the steps S1 through S5 in
Then, the routine proceeds to a step S18, where it is determined whether the excess air ratio λ is smaller than 2.0 or not. The excess air ratio can be determined by directly detecting it with the EGO sensor 9 or by calculating it based on the airflow rate AF, the engine speed RPM and the fuel injection amount FP, or by any other way pertinent in the art. If it is determined that the excess air ratio λ is smaller than 2.0 at the step S18 (YES), which corresponds to the lower torque range in
At the step S19, the routine controls the EGR valve 12 with the desired opening dEGR, which is determined based on the detected excess air ratio λ determined at the step S18, so that the dEGR corresponds to the expected NOx emission at the given air-fuel ratio as shown in
On the other if it is not determined that the λ<2.0 at the step S18 (NO), which means the NOx generation is substantially zero as shown in
Although the second embodiment is different from the first embodiment in terms of controlling the air-fuel ratio, it may improve the drivability, the emission and the fuel economy as the first embodiment may do.
Although, in the above embodiments, the hydrogen fuel has been mainly referred to, the fuel may be gasoline, alcohol or any other fuel pertinent to this control method.
It is needless to say that the invention is not limited to the illustrated embodiments and that various improvements and alternative designs are possible without departing from the substance of the invention as claimed in the attached claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-237699 | Aug 2005 | JP | national |
