US 20090071453 A1
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 desirable to continue to provide some hydrogen, not leaner than 0.1 hydrogen equivalence ratio.
1. A method to transition from a first to a second operating mode in an internal combustion engine, comprising:
decreasing air supply substantially, said decrease starting at transition initiation; decreasing an amount of a first fuel supplied to the engine at transition initiation;
initiating supply of a second fuel to the engine at transition initiation; and increasing supply of the second fuel abruptly in an amount to cause equivalence ratio to increase abruptly to 1.0 wherein the increasing is performed in response to equivalence ratio exceeding a threshold equivalence ratio.
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decreasing continually a supplied amount of said first fuel during said transition wherein at termination of said transition said first fuel is no longer being supplied to the engine; and.
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10. A method to transition between two operating modes in an internal combustion engine, comprising:
conducting the transition in a transition initiation phase followed by a transition completion phase wherein an equivalence ratio of 1.0 is maintained during the transition initiation phase and equivalence ratio decreases during the transition completion phase;
the transition initiation phase comprising:
increasing air supply;
initiating supply of a first fuel, hydrogen, to the engine; and
decreasing supply of a second fuel to the engine; and
the transition completion phase comprising further decreasing supply of the second fuel abruptly in an amount to cause equivalence ratio to decrease abruptly to an equivalence ratio below a threshold equivalence ratio.
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17. A method to transition from a first to a second operating mode in an internal combustion engine, comprising:
conducting the transition in a transition initiation phase followed by a transition completion phase with an equivalence ratio of 1.0 being maintained during the transition completion phase and maintaining an equivalence ratio less than a threshold equivalence ratio being maintained during the transition initiation phase wherein said threshold equivalence ratio is an equivalence ratio at which NOx production exceeds a corresponding threshold;
the transition initiation phase comprising:
decreasing air supply to the engine;
decreasing an amount of hydrogen supplied to the engine; and
initiating supply of a liquid fuel to the engine; and the transition completion phase comprising:
abruptly increasing supply of the liquid fuel; and
decreasing supply of hydrogen continually through the transition completion phase.
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21. A method for controlling an internal combustion engine during transitions between hydrogen fuel and a hydrocarbon fuel, the method comprising:
supplying hydrocarbon fuel to the engine in an amount to produce an equivalence ratio that is either below a first threshold equivalence ratio or above a second equivalence ratio.
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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'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.
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 s 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'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.
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:
A 4-cylinder internal combustion engine 10 is shown, by way of example, in
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
Two fuel tanks, 60 and 64, supply the two fuels. In the embodiment shown in
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.
When cold, the engine starts on hydrogen fuel, which presents no cold start vaporization and mixing issues such as a liquid fuel. In
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
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.