Abstract:
A method is disclosed for making a transition from fueling an engine with hydrogen to another fuel. That other fuel may be gasoline, a gasoline and alcohol mixture, or gaseous fuels, as examples. The other fuel has the capability of providing higher BMEP than the hydrogen because of better air utilization and because the other fuel occupies less volume of the combustion chamber. Because a desirable equivalence ratio to burn hydrogen is at 0.5 or less and a desirable equivalence ratio to burn other fuel is at 1.0, when a demand for BMEP that leads to a transition change from hydrogen fuel to the other fuel, the amount of air supplied to the engine is decreased to provide more torque and vice versa. During a transition in which liquid fuel supply is initiated, it may be desirable to continue to provide some hydrogen, not leaner than 0.1 hydrogen equivalence ratio.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 11/855,439 filed Sep. 14, 2007, now abandoned. 
    
    
     BACKGROUND 
     Field of the Invention 
     A method to operate an internal combustion engine which is supplied with both hydrogen fuel and another fuel is disclosed. 
     Because of concerns about greenhouse gases that are emitted from carbon-containing fuels, such as gasoline, diesel, and alcohol fuels, there is keen interest in fueling motor vehicles with hydrogen, which produces water upon combustion. Hydrogen-fueled internal-combustion engines suffer from a low power output compared to gasoline or diesel powered engines due to hydrogen being a gaseous fuel which takes up much of the volume in the cylinder, particularly when compared to dense fuels like gasoline or diesel fuel. Furthermore, hydrogen combustion is limited to operating at an equivalence ratio of about 0.5 or less due to increasing combustion harshness and, if it is a concern, rapidly increasing NOx emission. An equivalence ratio of one is a stoichiometric ratio meaning that the proportion of fuel to air is such that all the oxygen and fuel could burn completely. An equivalence ratio of 0.5 is a lean ratio in which the amount of air supplied is double that needed to completely consume the fuel. Such a limit in equivalence ratio results in about half the fuel delivery as could be consumed by the amount of air in the chamber, and consequently about half of the torque developed by the engine than if at a stoichiometric proportion. 
     Equivalence ratio is defined as the mixture&#39;s fuel to air ratio (by mass) divided by the fuel to air ratio for a stoichiometric mixture. A stoichiometric mixture has an equivalence ratio of 1.0; lean mixtures are less than 1.0; and, rich mixtures are greater than 1.0. 
     SUMMARY 
     The inventors of the present invention have recognized that by operating on two fuels: hydrogen and gasoline, as an example, the engine could be operated on hydrogen at low torque levels and on gasoline at higher torque levels. Hydrogen combusts readily at very lean equivalence ratios and is well suited to burning robustly at very low torques with at most, a minimum of throttling. Gasoline is well suited to providing high torque because of its high energy density and ability to operate at stoichiometric. The inventors of the present invention propose a bifuel engine in which transitions are made between operating on hydrogen and another fuel. 
     The high torque fuel can be a hydrocarbon, such as natural gas, propane, gasoline, or alcohols, such as methanol or ethanol. Furthermore, combinations of the gaseous fuel or combinations of the liquid fuels may also be used, such as E85, a mixture of 85% ethanol with 15% gasoline. High torque fuels contain carbon, which upon combustion reacts to form carbon dioxide, a greenhouse gas. Because hydrogen produces only water as the product of combustion, it does not form a greenhouse gas. Thus, it is desirable to operate on hydrogen when possible and using the carbon containing fuels as needed to provide the desired torque. 
     A normalized engine torque commonly used by one skilled in the art is BMEP, brake mean effective pressure, which for 4-stroke engines is 2*P/(V*N), where P is brake power, V is displaced volume, and N is engine rpm. A method for making a transition from a first to a second operating mode is disclosed in which the air supply is decreased, supply of a first fuel is decreased, and supply of a second fuel is initiated at the start of the transition. The first fuel is substantially 100% hydrogen and the second fuel is primarily comprised of hydrocarbons, gasoline or gasoline and alcohol mixtures, as examples. Alternatively, the second fuel is a gaseous hydrocarbon. During the transition, the amount of hydrogen is continuously decreased so that at termination of the transition, hydrogen is no longer being supplied to the engine. Concurrently, the amount of the second fuel is increased during the transition in coordination with the decrease of hydrogen. The transition is initiated when a demand for torque causes the equivalence ratio of hydrogen fuel to exceed a threshold, which threshold is approximately 0.5. The air supply decrease is accomplished by closing the engine&#39;s throttle valve with the air supply decrease being in the range of 30-60% during the transition. In one embodiment, the transition is further initiated in response to the engine piston speed exceeding a threshold. Engine piston speed is computed as 2*S*N, where S is stroke and N is engine rpm. The piston speed is not constant through the revolution; the piston speed computed here is average piston speed. 
     Also disclosed is a method to transition between two operating modes in an internal combustion engine in which air supply is increased substantially, supply of hydrogen is initiated and supply of a second fuel is decreased, all occurring roughly at the initiation of the transition. The transition is initiated in response to a demand for a torque decrease below a threshold BMEP: that BMEP being 3.5 to 5 bar for a naturally aspirated engine and between 6 and 8 bar for a pressure charged engine. During the transition, air supply increases in the range of 30-60%. The supply of hydrogen to the engine upon transition initiation causes the equivalence ratio with respect to only the hydrogen fuel to be at least 0.1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The 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: 
         FIG. 1  is a schematic of an engine having two fuel supplies; 
         FIGS. 2   a - b  show engine operating maps of BMEP and piston speed, showing operating zones for two fuels; 
         FIG. 3  shows an engine operating map of BMEP and catalyst temperature, showing operating zones for two fuels; and 
         FIGS. 4 and 5  show timelines of transitions from hydrogen to gasoline. 
     
    
    
     DETAILED DESCRIPTION 
     A 4-cylinder internal combustion engine  10  is shown, by way of example, in  FIG. 1 . Engine  10  is supplied air through intake manifold  12  and discharges spent gases through exhaust manifold  14 . An intake duct upstream of the intake manifold  12  contains a throttle valve  32  which, when actuated, controls the amount of airflow to engine  10 . Sensors  34  and  36  installed in intake manifold  12  measure air temperature and mass air flow (MAF), respectively. Sensor  31 , located in intake manifold  14  downstream of throttle valve  32 , is a manifold absolute pressure (MAP) sensor. A partially closed throttle valve  32  causes a pressure depression in intake manifold  12  compared to the pressure on the upstream side of throttle valve  32 . When a pressure depression exists in intake manifold  12 , exhaust gases are caused to flow through exhaust gas recirculation (EGR) duct  19 , which connects exhaust manifold  14  to intake manifold  12 . Within EGR duct  19  is EGR valve  18 , which is actuated to control EGR flow. Hydrogen fuel is supplied to engine  10  by fuel injectors  30 , injecting directly into cylinders  16 , and port injectors  26  injecting a liquid fuel into intake manifold  12 . This arrangement is shown by way of example and is not intended to be limiting. In other embodiments include having port injectors  26  supplying hydrogen fuel and direct injectors  30  supplying liquid fuel. Alternatively, both fuels are supplied through direct fuel injectors. In yet another embodiment both fuels are supplied by port injectors. The fuel other than hydrogen, in another embodiment, is a gaseous hydrocarbon fuel such as methane. Each cylinder  16  of engine  10  contains a spark plug  28 . The crankshaft (not shown) of engine  10  is coupled to a toothed wheel  20 . Sensor  22 , placed proximately to toothed wheel  20 , detects engine  10  rotation. Other methods for detecting crankshaft position may alternatively be employed. 
     In one embodiment, the engine is pressure charged by a compressor  58  in the engine intake. By increasing the density of air supplied to engine  10 , more fuel can be supplied at the same equivalence ratio. By doing so, engine  10  develops more power. Compressor  58  can be a supercharger which is typically driven off the engine. Alternatively, compressor  58  is connected via a shaft with a turbine  56  disposed in the engine exhaust. Turbine  56 , as shown in  FIG. 1 , is a variable geometry turbine; but, may be, in an alternative embodiment, a non-variable device. In another embodiment, the engine is naturally aspirated, in which embodiment elements  56  and  58  are omitted. Downstream of turbine  56  is three-way catalyst  66 . Three-way catalyst  66  can alternatively be place upstream of turbine  56  for faster light-off. Alternatively, catalyst  66  is a lean NOx trap or lean NOx catalyst having the capability to reduce NOx at a lean equivalence ratio. 
     Two fuel tanks,  60  and  64 , supply the two fuels. In the embodiment shown in  FIG. 1 , tank  60  contains liquid fuel and tank  64  contains hydrogen. However, as described above, the inventors of the present invention contemplate a variety of possible fuel combinations, with the appropriate fuel storage container included. In tank fuel pump  62  pressurizes liquid fuel. Fuel tank  64  is under high pressure. Typically, no pressurization is required, but a pressure regulator may be used. 
     It is known in the prior art to make transitions between engine operating modes. For example, in stratified charge gasoline engines, transitions between lean, stratified to premixed, stoichiometric operation are known to pose a challenge because the equivalence ratio changes from lean to rich abruptly, with the fuel remaining constant. In the present invention, the equivalence ratio also changes abruptly when switching fuels because the best combination of hydrogen operating characteristics are achieved at an equivalence ratio less than 0.5; whereas, desirable fuel and emission operating characteristics are achieved with other fuels (hydrocarbons, alcohols, etc.) at an equivalence ratio of 1.0. Fuel transitions can be accomplished in a single cycle, whereas air lags thereby causing challenges during the transitions. The present invention differs from prior art transitions in stratified charge engines because in the present invention the fuel changes as well as the equivalence ratio. 
     It is known in the prior art to operate bi-fuel engines in which transitions are made between two fuels, such as between gasoline and propane or between gasoline and ethanol. However, most known fuels (gaseous hydrocarbons, liquid hydrocarbons, and alcohols) have a narrow range of flammability, equivalence ratio (roughly 0.65 lean limit and 1.7 rich limit) compared with hydrogen fuel (roughly 0.10 lean limit and 3 rich limit). Because most fuels cannot combust robustly at very lean equivalence ratios, their stable, lean operation occurs in a region in which high NOx is produced. Thus, most fuels, except hydrogen, are operated at stoichiometric, i.e., equivalence ratio of 1. Because very lean mixtures of hydrogen combust robustly, the amount of NOx produced is small allowing such lean operation without a great emission concern. Even though hydrogen can be combusted in a wide range of equivalence ratios, in an internal combustion engine, it is used in the 0.15 to 0.5 equivalence ratio range because when operating richer than 0.5 equivalence ratio harsh combustion and autoignition of the hydrogen results, conditions which are to be avoided. Thus, a bi-fuel engine, in which one of the two fuels is hydrogen, when making a transition from hydrogen to gasoline, a switch from an equivalence ratio of about 0.5, or leaner, to 1.0 occurs. 
     In summary, the present invention distinguishes between the prior art transitions between stratified, lean operation and stoichiometric operation, as discussed above, in that both a transition in equivalence ratio and fuel type occurs. The present invention distinguishes between the prior art bi-fuel transition because when one of the fuels is hydrogen, according to the present invention, switching among combustion modes results in an increase in both fuel type and equivalence ratio; whereas, in the prior art in which neither of the two fuels is hydrogen, the equivalence ratio does not substantially change when the fuel type changes. 
     Gaseous fuels that are delivered by an electronic fuel injector can be turned on, off, or anywhere in between in a single cycle with the only transient issue being inventory of fuel in the intake manifold in the case of the fuel injector being located in the intake port. Liquid fuels that are supplied directly to the combustion chamber (direct injected) can be affected in a single cycle. However, liquid fuels that are supplied into the intake port (port injected) present some difficulties due to fuel films that form on port surfaces. That is, when activating injectors, some of the fuel sprayed wets manifold walls and does not enter the combustion chamber directly. When deactivating liquid, port injectors, the fuel films on the walls remaining on intake port walls are removed and are inducted into the combustion chamber; this fuel inventory takes several intake events to empty. For example, changing the amount of air being inducted into a cylinder abruptly presents an issue as it takes several engine cycles for a manifold to fill or empty. Thus, the transition from one fuel to the other takes at least several engine cycles. In one embodiment, a switch between fuels is accomplished over tens of cycles. 
     In one embodiment, both fuels are delivered during the transition period while the supplied air is adjusted to the new operating condition. It is known to those skilled in the art that hydrogen, when used to supplement gasoline (or other hydrocarbon fuel) facilitates combustion at a substantially leaner equivalence ratio than would be possible with gasoline alone. 
     In  FIG. 2   a , it is shown the fuel  2  is used when the threshold BMEP is exceeded. This threshold is associated with an equivalence ratio of the hydrogen which is greater than a desirable level, e.g., 0.5. That is, to produce more than the threshold BMEP, the hydrogen equivalence ratio would exceed 0.5. In  FIG. 2   b , an additional constraint is placed on hydrogen operation in that when the piston speed exceeds a certain threshold, the engine transitions to fuel  2 . 
     When cold, the engine starts on hydrogen fuel, which presents no cold start vaporization and mixing issues such as a liquid fuel. In  FIG. 3 , fuel  2  is only used when both the catalyst has attained its light-off temperature and the threshold BMEP has been exceeded. 
     In  FIG. 4 , one embodiment of a transition from hydrogen to gasoline is shown in a timeline. Before the transition, hydrogen is used; after the transition, gasoline is used; and during the transition, a combination of the two fuels is used. In the top graph, a, torque is increasing. In the bottom graph, e, the equivalence ratio, Φ, is less than 0.5 prior to the transition. As discussed above, a transition from hydrogen to gasoline is desirable when the hydrogen equivalence ratio approaches 0.5; thus, the transition is initiated. In graph c, the amount of hydrogen provided increases prior to the transition to provide the increased torque of graph a. Prior to the transition, the air delivery rate, dm a /dt of graph b, remains constant with the additional torque provided by increasing hydrogen. At transition initiation, the throttle is partially closed and the amount of air is decreased. Air supply decreases such that the air supplied by the end of the transition is that required to provide Φ=1.0, which is the desired equivalence ratio for all fuels, except hydrogen. One of the reasons that there is a transition period is that air delivery cannot be changed in one engine cycle. Instead, even when the throttle is opened rapidly, it takes several engine cycles for the manifold to fill and the desired amount of air to be provided to the engine. Because the air is more than desired right after the start of the transition, hydrogen supply is continued. It is known by those skilled in the art, that by supplementing a conventional fuel with hydrogen, that the conventional fuel can robustly combust at an equivalence ratio at which it is unable to do so without the presence of hydrogen. Thus, the hydrogen continues through the transition period, until the equivalence ratio achieves the desired 1.0, at which time the hydrogen supply is discontinued. Alternatively, but not shown in the Figure, the hydrogen supply could be discontinued when the equivalence ratio reaches a ratio that the conventional fuel, e.g., gasoline, can robustly combust, such as greater than 0.8. Gasoline supply is initiated at the start of the transition. However, as discussed above, because the air cannot be reduced as quickly as desired, the hydrogen is continued into the transition period to ensure the combustion. Through the transition period, the gasoline is increased and the hydrogen decreased, as well as the air decreasing, so that by the end of the transition period, the gasoline operation takes over with no hydrogen assistance. 
     In  FIG. 5 , an alternative embodiment is shown in which the initial portion of the transition is similar to that shown in  FIG. 4 . However, at a point during the transition, the equivalence ratio is bumped up to 1.0 and maintained at 1.0 for the remainder of the transition. This is done to avoid the high NOx region of 0.85-0.90 phi. However, during this transition period of 1.0 equivalence ratio, the hydrogen supply is continuously being decreased and the gasoline supply is increased. At the end of the transition, hydrogen supply has ceased. 
     In the above discussion, a hydrogen-to-gasoline transition is described. However, the reference to gasoline is provided by way of example and is not intended to be limiting. Furthermore, the transition occurring at Φ=0.5 is also by way of example. The actual transition may occur at slightly lower or higher equivalence ratios than exactly 0.5. 
     A transition from a higher torque to a lower torque in which gasoline (or other fuel) operation is transitioned to hydrogen operation can be accomplished in the reverse of what is shown in  FIGS. 4 and 5 . If the fuel other than hydrogen is a liquid fuel and is port injected, the inventory of the fuel in the intake manifold is accounted for to provide the desired fuel into the combustion chamber. 
     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-describe embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.