DUAL COMBUSTION MODE ENGINE
In an engine in which a first portion of engine cylinders operate in HCCI combustion mode only and a second portion of engine cylinders operate in SI combustion mode only, two EGR ducts and valves are provided: a first EGR duct adapted to provide flow from an exhaust of HCCI engine cylinders to an intake of HCCI cylinders; a first EGR valve disposed in the first EGR duct; a second EGR dust adapted to provide flow from an exhaust of SI cylinders to an intake of HCCI cylinders via a first EGR valve; and a second EGR valve disposed in the second EGR duct. By adjusting the EGR valves, the intake temperature to HCCI cylinders is controlled. During cold start, the second EGR valve is opened to pull SI exhaust through HCCI cylinders for the purpose of preheating HCCI cylinders.
The present invention relates to an internal combustion engine in which a portion of cylinders exclusively operate under a spark-ignition combustion mode and remaining cylinders exclusively operate under a homogeneous-charge, compression-ignition mode.
BACKGROUND OF THE INVENTIONHomogeneous-charge, compression-ignition combustion is known to those skilled in the art to provide high fuel efficiency and low emission operation in internal combustion engines. However, HCCI operation is feasible in a narrow range in engine torque, approximately one-third of the torque range of a conventional spark-ignited engine. Thus, most HCCI engines being developed are dual mode engines in which HCCI is used at low torque conditions. When a higher torque is desired, operation is transitioned to an alternative combustion mode, such as spark-ignition combustion or heterogeneous, compression-ignition combustion (diesel). Challenges accompanying such transitions include: torque matching (providing driver demanded torque during the transition interval), maintaining emission control, and robustly returning to HCCI combustion, to name a few. Another difficulty encountered in engines which transition from one combustion mode to another is that the combustion system geometry cannot be optimized for either combustion mode, but is instead a compromise. For example, a desirable compression ratio for HCCI combustion is about 15:1 and about 10.5:1 for spark-ignition combustion.
A disadvantage of HCCI combustion is its inferior transient response to a demand for a change in torque, orders of magnitude slower than SI combustion. The inventors of the present invention have recognized that HCCI operation cannot provide a vehicle operator with the responsiveness that they have come to expect from a SI engine.
In U.S. patent application 2004/0182359, an 8-cylinder HCCI/SI engine is described in which HCCI to SI transitions are made one cylinder at a time, i.e., at a lower torque demand all 8 cylinders are operating in HCCI combustion mode and as torque demand exceeds what HCCI combustion can provide, cylinders are individually switched to SI operation. The inventors of the present invention have recognized that it would be desirable to have an engine which provides the desired range in output torque at the high efficiency of HCCI combustion without having to undergo a combustion mode transition in any given cylinder because of the compromises inherent in designing a cylinder to operate robustly and efficiently in both HCCI and SI combustion modes over a wide operating range.
SUMMARY OF THE INVENTIONIn an engine in which a first portion of engine cylinders operate in HCCI combustion mode only and a second portion of engine cylinders operate in SI combustion mode only, two exhaust gas recirculation ducts and valves are provided: a first exhaust gas recirculation duct adapted to provide flow from an exhaust of the first portion of engine cylinders to an intake of the first portion of engine cylinders; a first exhaust gas recirculation valve disposed in the first exhaust gas recirculation duct; a second exhaust gas recirculation duct adapted to provide flow from an exhaust of the second portion of engine cylinders to an intake of the first portion of engine cylinders via a first exhaust gas recirculation valve; and a second exhaust gas recirculation valve disposed in the second exhaust gas recirculation duct. An electronic control unit coupled to the first and second portions of engine cylinders and the first and second exhaust gas recirculation valves commands a first position to the first exhaust gas recirculation valve and a second position to the second exhaust gas recirculation valve based on a desired intake temperature. Alternatively, the electronic control unit coupled to the first and second portions of engine cylinders and the first and second exhaust gas recirculation valves commands a first position to the first exhaust gas recirculation valve and a second position to the second exhaust gas recirculation valve based on a signal from a combustion sensor. Feedback control of valve position can be based on crank angle of peak pressure in the first portion of engine cylinders.
The first and second portions of engine cylinders are mutually exclusive, have separated intakes and exhausts and have different compression ratios, at least 2 ratios higher for the HCCI cylinders.
In one embodiment, the first portion of engine cylinders is greater than the second portion of engine cylinders. Further, the first portion of engine cylinders may comprise all but one cylinder on each bank of cylinders of the engine.
Also disclosed is a method for controlling an internal combustion engine, in which a valve position of a crossover exhaust gas recirculation valve is adjusted. The crossover exhaust gas recirculation valve is disposed in an exhaust gas recirculation duct between an intake of a first portion of engine cylinders (HCCI) and an exhaust of a second portion of engine cylinders (SI). The method further includes adjusting a valve position of a second exhaust gas recirculation valve which is disposed in an exhaust gas recirculation duct between an exhaust of the first portion of engine cylinders and the intake of the first portion of engine cylinders. In one embodiment, the adjustments are based on a desired intake temperature of gases provided to the first portion of engine cylinders. In another embodiment, valve adjustments are based on a signal from a combustion sensor. Feedback control of combustion parameters, EGR valve positions being one example, can be based on providing a desired crank angle of peak pressure from the combustion sensor, the desired crank angle of peak pressure in a range of 5 to 20 after top center of the piston.
The crossover EGR valve is open fully during starting of the engine during which time fuel is provided to the SI cylinders and no fuel to the HCCI cylinders. This provides the advantage of preheating HCCI cylinders in preparation for initiation of combustion therein.
An advantage of the present method is that control of the EGR valves can be feedback controlled on combustion in the HCCI cylinder. By doing this, efficient HCCI combustion is ensured.
A further advantage is that by providing two controllable exhaust gas streams of different temperatures to the intake of HCCI cylinders, intake temperature to HCCI cylinders can be controlled. Intake temperature is a key factor in controlling ignition timing with HCCI combustion.
BRIEF DESCRIPTION OF THE DRAWINGSThe advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:
In
Again, for clarity, only one of the three spark plugs for each of the cylinders of bank 12 is shown. Bank 14 cylinders may also have spark plugs. Although the bank 14 cylinders are HCCI cylinders, which indicates that combustion is initiated by compression ignition, it is known to those skilled in the art that at some operating conditions, it is useful to employ spark assist to initiate combustion. Alternatively, another ignition assist device such as glow plugs, plasma jet igniters, catalytic assisted glow plugs, as examples, could be used for ignition assist in HCCI. In SI combustion, a spark initiates a flame kernel and a flame front travels throughout the cylinder. In spark assisted HCCI, a spark initiates a flame kernel at the location of the spark plug. However, the mixture in the cylinder is too weak (not enough fuel or too much burned gases in the mixture) to sustain a flame front traveling through the cylinder gases. The flame kernel combusts the fuel-air mixture near the spark plug. The release of energy by the combustion of the mixture near the spark plug increases the pressure in the cylinder thereby causing the gases away from the spark plug to attain their ignition temperature and to self-ignite. When spark assist HCCI is contemplated, all HCCI cylinders are provided with a spark plug 58.
Engine 10 is shown to be a 6-cylinder with bank 12 being SI and bank 14 being HCCI by way of example. This is not intended to be limiting. Engine 10 has any number of cylinders greater than one and in any configuration: in-line, V, W, radial, opposed, or any other cylinder arrangement. The HCCI and SI cylinders need not be segregated by banks. There could be HCCI and SI cylinders on any given bank. However, as mentioned above, the intake gases to the HCCI cylinders and SI cylinders remain separated and exhaust gases coming from HCCI cylinders and SI cylinders also remain separated. Thus, such arrangements may require complicated manifolding to maintain the separation. An expected arrangement is that every other cylinder in the firing order is alternately HCCI and SI.
SI engines are typically produced with a 9.5-10.5:1 compression ratio, which is the ratio of the volume in the cylinder above the piston when the piston is at the top of its travel divided by the volume in the cylinder above the piston when the piston is at the bottom of its travel. HCCI combustion occurs more favorably with a higher compression ratio: 13-15:1. In prior art engines in which combustion mode is transitioned, the compression ratio selected is a compromise between the two compression ratios. According to the present invention, however, because each cylinder is optimized for a single combustion mode, the engine is produced with the compression ratio appropriate for the particular combustion mode. Thus, unlike prior art engines, the engine according to the present invention has some cylinders with a substantially higher compression ratio than other cylinders.
HCCI combustion occurs in a dilute mixture, either very lean of stoichiometric with excess air and/or with a very high level of exhaust dilution. It is well known to those skilled in the art to provide exhaust dilution by either recirculating exhaust gases into the engine intake, known as exhaust gas recirculation (EGR) sometimes referred to as external EGR, or to retain exhaust gases in the cylinder from a prior combustion event to mix with the combustion gases of an upcoming combustion event, commonly known as internal EGR. The latter is often achieved by valve timing adjustments. Typically exhaust gases are routed from an exhaust duct to an intake duct via a control valve (EGR valve). The present invention provides for an alternative configuration for EGR in which gases exhausted from the SI cylinder bank 12 are routed to the intake of the HCCI cylinder bank 14 via valve 39. In
Continuing with
In
The signal from an exhaust gas oxygen sensor 6o is commonly used for air-fuel ratio feedback control of SI combustion. Analogously, HCCI combustion timing is controlled by adjusting intake temperature, according to one alternative embodiment. Adjustment of intake temperature is feedback controlled based on a combustion parameter such as crank angle of peak pressure. Examples of sensors from which crank angle of peak pressure can be ascertained include: head bolt strain gauge, in-cylinder pressure sensor, ionization sensor, a head gasket sensor, sensor measuring instantaneous flywheel speed, etc. For stoichiometric SI combustion, it is well known by those skilled in the art that the crank angle of peak pressure corresponding to peak efficiency operation (at a given speed/torque condition) occurs roughly at 15 degrees after top dead center. Alternative combustion systems, particular lean burn, tend to have the crank angle of peak pressure occur at a somewhat earlier time, e.g., 12 degrees after top dead center to achieve peak efficiency. Furthermore, there are other objectives, besides achieving peak efficiency, such as emission control, which cause the desired crank angle of peak pressure to be other than that providing peak efficiency. It is expected that a desired crank angle of peak pressure is in a range of 5 to 20 degrees after top center. Various combustion control parameters, such as: intake temperature, EGR valve position, throttle valve position, flow through an intake heat exchanger, and pressure charging, can be feedback controlled based on crank angle of peak pressure, particularly for the HCCI cylinders.
Because HCCI combustion is dilute, the peak torque capable from a given cylinder is much less than peak torque from a SI cylinder. To increase the amount of torque from a HCCI cylinder, compressor 34 increases the intake manifold pressure on the HCCI cylinders, allowing for an increased amount of fuel delivery while maintaining a high dilution. As shown in
In
Continuing to refer to
Referring to
Also shown in
According to an aspect of the present invention, half of the cylinders are operated with HCCI combustion and half of the cylinders are operated with SI combustion, the effect of such operation on torque and thermal efficiency being shown in
To make up for the lesser torque of the engine illustrated in
If a turbocharger were employed in place of a supercharger, no increase in torque range with HCCI only operation is possible because the SI cylinders are deactivated, thus no exhaust to drive the turbocharger.
Because achieving a sufficiently high temperature to cause autoignition is paramount in HCCI combustion, providing a robust cold start presents a serious hurdle for HCCI combustion. Those skilled in the art discuss starting on SI combustion and transitioning to HCCI combustion after the engine has achieved a suitable operating temperature. However, with the present invention, the cylinders are adapted to operate in only one combustion mode. To overcome, the inventors of the present invention contemplate starting on SI cylinders. During the period of SI combustion, air can be delivered to HCCI cylinders through heat exchanger 38. By blowing warm air through the HCCI cylinder bank 14, the engine surfaces can be preheated and ready for HCCI combustion. In addition, the engine coolant is heated by the SI cylinders and preheats the HCCI cylinders.
According to an aspect of the present invention, the SI cylinders are equipped with valve deactivators (not shown). During HCCI only operation, the SI cylinders are deactivated by closing off the intake and exhaust valves. The piston continues to reciprocate, but the gas in the cylinder at the last combustion event remains trapped in the cylinder. If the valves were allowed to remain active, the flow of air through SI cylinder bank 12 would flow into exhaust after treatment device 20. If device 20 is a three-way catalyst, oxygen would be absorbed onto the surfaces and when the SI cylinders were reactivated, the three-way catalyst would be unable to reduce NOx until such oxygen is removed from device 20. Furthermore, the flow of air through SI cylinder bank 20 cools the engine down, thereby making restart more difficult.
In one embodiment, valve deactivators are provided for the HCCI cylinders (not shown in
Referring to
In
In
In another embodiment, the SI cylinders remain active at all times. In one example of this embodiment, an 8-cylinder is started on 2 SI cylinders. The remaining 6 cylinders are HCCI cylinders, which are turned on when they reach a suitable temperature which supports robust HCCI combustion. In this embodiment, the SI cylinders remain operational even after HCCI combustion has been achieved in the 6 HCCI cylinders.
Referring now to
Because HCCI combustion is very dilute, HCCI combustion gases are at a much lower temperature than SI combustion gases. By controlling the proportion of EGR gases coming from bank 12 and from bank 14, the temperature in HCCI cylinders is controlled. As mentioned above, one of the ways, known by those skilled in the art, for controlling HCCI combustion timing is by adjusting the temperature of the gases in the HCCI cylinder. By continuing to operate SI cylinders while HCCI cylinders are operating, the exhaust gases from SI cylinders is available for recycle to the HCCI cylinders for controlling temperature in HCCI cylinders.
While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.
Claims
1. An internal combustion engine, comprising:
- a first portion of engine cylinders;
- a second portion of engine cylinders;
- a first exhaust gas recirculation duct adapted to provide flow from an exhaust of said first portion of engine cylinders to an intake of said first portion of engine cylinders;
- a first exhaust gas recirculation valve disposed in said first exhaust gas recirculation duct;
- a second exhaust gas recirculation duct adapted to provide flow from an exhaust of said second portion of engine cylinders to an intake of said first portion of engine cylinders via a first exhaust gas recirculation valve; and
- a second exhaust gas recirculation valve disposed in said second exhaust gas recirculation duct.
2. The engine of claim 1 wherein a compression ratio of said first portion of engine cylinders is at least two ratios higher than said second portion of engine cylinders.
3. The engine of claim 1, further comprising: an electronic control unit coupled to said first and second portions of engine cylinders and said first and second exhaust gas recirculation valves, said electronic control unit commanding a first position to said first exhaust gas recirculation valve and a second position to said second exhaust gas recirculation valve based on a desired intake temperature in the said first portion of engine cylinders.
4. The engine of claim 1 wherein said first portion of engine cylinders and said second portion of engine cylinders are mutually exclusive.
5. The engine of claim 1 wherein said exhaust of said first portion of engine cylinders is separated from an exhaust of said second portion of engine cylinders.
6. The engine of claim 1 wherein said intake of said first portion of engine cylinders is separated from an intake of said second portion of engine cylinders.
7. The engine of claim 1 wherein said first portion of engine cylinders is greater than said second portion of engine cylinders.
8. The engine of claim 7 wherein said first portion of engine cylinders comprise all but one cylinder on each bank of cylinders of the engine.
9. A method for controlling an internal combustion engine, comprising:
- adjusting a valve position of a crossover exhaust gas recirculation valve, said crossover exhaust gas recirculation valve being disposed in an exhaust gas recirculation duct between an intake of a first portion of engine cylinders and an exhaust of a second portion of engine cylinders wherein said first and second portions are mutually exclusive, gases passing through said exhaust gas recirculation valve are provided exclusively from said exhaust of said second portion of engine cylinders, and said gases are provided exclusively to said first portion of engine cylinders.
10. The method of claim 9 wherein said adjustment is based on a desired intake temperature of gases provided to said first portion of engine cylinders.
11. The method of claim 9, further comprising: adjusting a valve position of a second exhaust gas recirculation valve, said second exhaust gas recirculation valve being disposed in an exhaust gas recirculation duct between an exhaust of said first portion of engine cylinders and said intake of said first portion of engine cylinders.
12. The method of claim 9 wherein said valve is adjusted to a fully open position during starting of the engine.
13. The method of claim 9 wherein said exhaust gas recirculation duct is coupled to said exhaust of said second portion of engine cylinders downstream of an exhaust after treatment device coupled to said exhaust of said second portion of engine cylinders.
14. A computer readable storage medium having stored data representing instructions executable by a computer, comprising: instructions to adjust a valve position of a crossover exhaust gas recirculation valve, said crossover exhaust gas recirculation valve being disposed in an exhaust gas recirculation duct between an intake of a first portion of engine cylinders and an exhaust of a second portion of engine cylinders wherein said first and second portions are mutually exclusive wherein intakes of said first and second portions of engine cylinders are separated and exhausts of said first and second portions of engine cylinders are separated.
15. (canceled)
16. The medium of claim 14, further comprising: instructions to determine a desired intake temperature for said first portion of engine cylinders wherein said adjustment of said crossover exhaust gas recirculation valve is based on providing said desired intake temperature.
17. The medium of claim 16, further comprising: instructions to adjust a valve position of a second exhaust gas recirculation valve, said second exhaust gas recirculation valve being disposed in an exhaust gas recirculation duct between an exhaust of said first portion of engine cylinders and said intake of said first portion of engine cylinders.
18. The medium of claim 14, further comprising: instructions to fully open said crossover exhaust gas recirculation valve during starting of said engine.
19. The medium of claim 18, further comprising: instructions to provide fuel to said second portion of engine cylinders and to provide no fuel to said first portion of engine cylinders during said starting.
20. The medium of claim 18 wherein said first portion of engine cylinders comprise all, less one, engine cylinders in each bank of the engine.
21. The medium of claim 14, further comprising: instructions to determine crank angle of peak pressure for said first portion of engine cylinders wherein said adjustment of said crossover exhaust gas recirculation valve is based on providing a desired crank angle of peak pressure.
22. The medium of claim 21 wherein said desired crank angle of peak pressure occurs between 5 and 20 degrees after top center.
Type: Application
Filed: Feb 17, 2006
Publication Date: Aug 23, 2007
Inventors: John Brevick (Livonia, MI), Diana Brehob (Dearborn, MI)
Application Number: 11/276,217
International Classification: F02M 25/07 (20060101); F02B 47/08 (20060101); F02B 33/44 (20060101); F01N 5/02 (20060101); G06F 17/00 (20060101);