WO2019123129A1 - Controlling heat release within an engine cylinder - Google Patents

Abstract

A method for controlling an engine involves an engine control unit determining that a three-stage heat release in a cylinder in the engine should be performed. Responsive to the determination that the three-stage heat release will not occur within the cylinder, the engine control unit adjusts conditions within the cylinder in such a manner to cause the three-stage heat release within the cylinder. The engine control unit monitors conditions in the cylinder during a second stage heat release of the three-stage heat release. The engine control unit adjusts conditions in the cylinder during the second stage heat release to terminate reactions in the cylinder at an end of the second stage heat release.

Description

CONTROLLING HEAT RELEASE WITHIN AN ENGINE CYLINDER

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/599,913, filed on December 18, 2017, entitled“METHOD TO REDUCE C02 EMISSIONS WHILE GENERATING WORK AND SYNTHESIS GAS FROM AUTOMOTIVE COMBUSTION ENGINES,” and U.S. Provisional Patent Application No. 62/730,858, filed on September 13, 2018, entitled“CONTROLLING HEAT RELEASE WITHIN AN ENGINE CYLINDER,” the disclosures of which are incorporated herein by reference in their entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the disclosed subject matter generally relate to engines, and in particular to controlling combustion within engine cylinders so as to terminate reactions within a cylinder at the end of a second heat release stage of a three-stage heat release and/or to avoid or minimize a three-stage heat release.

DISCUSSION OF THE BACKGROUND

[0003] Most fuel consuming engines, such as gasoline or diesel consuming engines, produce combustion products that are considered dangerous to the environment. In order to comply with various governmental regulations, these fuel consuming engines are typically equipped with exhaust after-treatment systems that reduce the amount of dangerous chemicals and gasses released into the environment. Another way to reduce emission of dangerous combustion products is to tune the fuel consuming engines to operate at optimum conditions by adjusting, for example, the fuel-air equivalence ratio (ø), which also increases fuel economy. The fuel-air equivalence ratio quantifies the deviation of the actual fuel-air ratio from the stoichiometric fuel/air ratio, where all the oxygen available in air is consumed in the reaction. Lean fuel-air equivalence ratio (f < 1) has more air than fuel compared to a rich fuel-air equivalence ratio (f > 1 ). Because of the availability of air to oxidize the fuel injected into the engine cylinder using a lean fuel-air equivalence ratio compared to a rich fuel-air equivalence ratio, the amount of dangerous combustion products generated as the result of the combustion within an engine cylinder injecting a lean fuel-air equivalence ratio are typically less than when injecting a rich fuel-air equivalence ratio.

[0004] Governmental regulations regarding emission of environmentally dangerous combustion products continue to become stricter over time, which requires more advanced and expensive emission control systems. The increased costs for development and manufacture of these more advanced emission control systems are typically passed onto the consumer. This is particularly problematic for vehicle manufacturers because it increases the overall sales cost of the vehicle for features that do not necessarily provide a readily identifiable benefit to the vehicle purchaser.

[0005] One relatively low cost way to reduce the amount of environmentally dangerous combustion products from being emitted into the atmosphere, as well as to avoid engine damage, is to control the fuel-air auto-ignition combustion property during engine operation. Auto-ignition determines the knock limits of spark ignition engines and the efficiency and emissions of compression ignition engines. Aero engines and stationary power generation turbines control auto-ignition for safety reasons and/or to extend operability limits.

[0006] Hydrocarbon auto-ignition at high temperatures is governed by reactions between atomic hydrogen with molecular oxygen, while hydrogen peroxide (H2O2) decomposition drives auto-ignition at intermediate temperatures. Complex low-temperature termination, propagation, and branching pathways forming alkenes, cyclic ethers, and keto- hydroperoxides (KHP), respectively, control heat release rate, raise system temperature, and advance formation and decomposition of H2O2. The chemical kinetics of low-temperature oxidation is strongly related to fuel molecular structure, and can be linked to interesting combustion phenomena such as cool flames, negative temperature coefficient (NTC) behavior, and fuel anti-knock quality (i.e., octane numbers).

[0007] Two-stage heat release is a feature of auto-ignition that has attracted significant research interest in both fundamental and practical applications. This phenomenon is characterized by a low-temperature heat release (LTHR) regime that raises the system temperature and results in the first stage (ti) ignition delay time (IDT). Low-temperature heat release is driven by chain branching auto-oxidation reactions that result in the formation and decomposition of highly oxygenated intermediates (e.g., ketohydroperoxides). The region following low-temperature heat release exhibits a net decrease in the net heat release rate (HRR) due to negative temperature coefficient (NTC) chemistry. A subsequent transition to high- temperature heat release (HTHR) results in the total (tr) IDT, which is characterized by H2O2 decomposition, H+O2 chain branching, and CO oxidation reactions.

[0008] Thus, characteristics of the two-stage heat release feature of auto ignition can provide a lower cost way to optimize efficiency and reduce

environmentally dangerous combustion products compared to additional exhaust treatment systems. This, however, requires an accurate understanding of the both stages of the two-stage heat release. Although the first stage is currently well- understood, the second stage is not completely understood.

[0009] Thus, it would be desirable to reduce environmentally dangerous combustion products while minimizing the costs associated with such reduction by controlling the characteristics of heat release.

SUMMARY

[0010] According to an embodiment, there is a method for controlling an engine, which involves an engine control unit determining that a three-stage heat release in a cylinder in the engine should be performed. Responsive to the determination that the three-stage heat release will not occur within the cylinder, the engine control unit adjusts conditions within the cylinder in such a manner to cause the three-stage heat release within the cylinder. The engine control unit monitors conditions in the cylinder during a second stage heat release of the three-stage heat release. The engine control unit adjusts conditions in the cylinder during the second stage heat release to terminate reactions in the cylinder at an end of the second stage heat release.

[0011] According to another embodiment, there is a system, which includes an engine comprising at least one cylinder having an intake valve configured to receive at least air and fuel, and at least one exhaust valve configured to expel gasses from the cylinder, and an engine control unit coupled to the engine. The engine control unit is configured to determine that a three-stage heat release in a cylinder in the engine should be performed, adjust, responsive to the determination that the three- stage heat release will not occur within the cylinder, conditions within the cylinder in such a manner to cause the three-stage heat release within the cylinder, monitor conditions in the cylinder during a second stage heat release of the three-stage heat release, and adjust conditions in the cylinder during the second stage heat release to terminate reactions in the cylinder at an end of the second stage heat release.

[0012] According to a further embodiment, there is a method for controlling an engine, which involves an engine control unit monitoring operational parameters of the engine, wherein the operational parameters comprise an amount of air and amount of fuel injected into a cylinder of the engine, a temperature inside the cylinder of the engine, and a pressure inside the cylinder of the engine. The engine control unit determines, based on the monitored operational parameters, whether or not a three-stage heat release will occur within the cylinder. Responsive to the determination that the three-stage heat release will occur within the cylinder, the engine control unit adjusts conditions within the cylinder in such a manner to reduce or prevent the three-stage heat release within the cylinder. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

[0014] Figure 1 is a flowchart of a method of terminating reactions within a cylinder at the end of a second heat release stage of a three-stage heat release according to embodiments;

[0015] Figure 2 is a flowchart of a method of preventing or minimizing a three- stage heat release according to embodiments;

[0016] Figure 3 is a block diagram of a system according to embodiments;

[0017] Figures 4A and 4B respectively illustrate the ignition delay times versus temperature for fuel-air equivalence ratios of 1.0 and 0.3 according to embodiments;

[0018] Figure 5A is a graph illustrating the temperature and heat release rate versus time according to embodiments;

[0019] Figure 5B is a graph illustrating the species mole fractions and combustion efficiency versus time according to embodiments;

[0020] Figure 6 is a graph of pressure and carbon monoxide mole faction profiles in a rapid compression machine (RCM) according to embodiments;

[0021] Figure 7 is a block diagram of the key reactions involved in three-stage ignition of n-heptane according to embodiments; [0022] Figures 8 and 9 are graphs of homogenous charge compression ignition (HCCI) simulation results at compression ratios of 11 .5 and 12.1 , respectively, according to embodiments;

[0023] Figures 10A-1 OC are graphs of volumetric heat production rate of n- heptane at different pressures and fuel-air equivalence ratios while fixing

temperature at 500° K according to embodiments;

[0024] Figures 1 1 A-1 1 C are graphs of temperature effect on the third-stage heat release at different initial conditions according to embodiments;

[0025] Figures 12A-12C are graphs of volumetric heat production rate of n- heptane at different temperatures and fuel-air equivalence ratios while fixing pressure at 10 bar according to embodiments;

[0026] Figures 13A-13C are graphs of pressure effect on the third-stage heat release at different initial conditions according to embodiments;

[0027] Figures 14A and 14B are graphs of volumetric heat production rate of n-heptane at two different pressures according to embodiments;

[0028] Figures 15A and 15B are graphs of fuel-air equivalence ratio effect on the third-stage heat release at different initial conditions according to embodiments;

[0029] Figures 16A and 16B are graphs of pressure and temperature effect on the third-stage heat release at fuel-air equivalence ratio of 0.3 for different n- paraffinic hydrocarbons according to embodiments; and

[0030] Figures 17A and 17B are graphs of pressure and temperature effect on the third-stage heat release at fuel-air equivalence ratio of 0.3 for different hydrocarbons according to embodiments. DETAILED DESCRIPTION

[0031] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of fuel burning engines.

[0032] Reference throughout the specification to“one embodiment” or“an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases“in one embodiment” or“in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0033] As discussed above, heat release is conventionally considered a two- stage process. As detailed below, the inventors have recognized that under certain conditions a three-stage heat release process occurs, the prevention or minimization of which can reduce the environmentally dangerous combustion products generated by engine cylinders. The inventors have also recognized that terminating the chemical reactions occurring during a three-stage heat release at the end of the second stage produces gasses that are useful for exhaust emission control systems and/or can be used for exhaust gas recirculation (EGR), as well as reduces the amount of carbon dioxide (CO2) produced during the combustion process.

[0034] Figure 1 is a flowchart of a method of terminating reactions within a cylinder at the end of a second heat release stage of a three-stage heat release according to embodiments. Initially, an engine control unit determines that a three- stage heat release in a cylinder in the engine should be performed (step 1 10). The engine control unit then adjusts, responsive to the determination that the three-stage heat release will not occur within the cylinder, conditions within the cylinder in such a manner to cause the three-stage heat release within the cylinder (step 120). If it is determined that conditions within the cylinder indicate that a three-stage heat release will occur, then step 120 can be omitted. The engine control unit monitors conditions in the cylinder during a second stage heat release of the three-stage heat release (step 130) and adjusts conditions in the cylinder during the second stage heat release to terminate reactions in the cylinder at an end of the second stage heat release (step 140).

[0035] The manner of adjusting the conditions within the cylinder, and thus the heat release characteristic, to reduce or prevent three-stage heat release within the cylinder will be described in more detail below. Briefly, the conditions within the cylinder, and thus the heat release characteristic, can be adjusted by

a. adjusting one the operational parameters, such as

i. changing the amount of fuel relative to the amount of air to achieve a different fuel-air equivalence ratio, ii. terminating the chemistry of fuel oxidation by moving the charge to milder temperature and pressure conditions (e.g., by keeping an exhaust valve of the cylinder closed until there is full combustion of the fuel in the cylinder),

iii. changing the amount of pressure within the cylinder (e.g., by boost pressure of air fed to the engine and/or changing the compression ratio of the engine), and/or

iv. increasing the temperature in the cylinder when the temperature in the cylinder is below a predetermined temperature (e.g., by adjusting a temperature of the air, an inert gas, and/or an inert liquid injected into the cylinder) and/or by changing the temperature of air and/or fed to the engine, and/or

b. adjusting a timing of the injection of the air, inert gas, and/or inert liquid and of the fuel into the cylinder (e.g., by delaying the injection of the air, inert gas, and/or inert liquid and of the fuel into the cylinder.

[0036] It should be noted that the timing of the ignition of the three-stage ignition depends upon the auto-ignition quality of the fuel used (i.e., fuels with higher ignition delay times could possess the three-stage ignition at later times in the combustion cycle), and thus the adjustment of the timing of injection of the air, inert gas, and/or inert liquid and of the fuel into the cylinder will depend on the auto ignition quality of the fuel.

[0037] The adjustment of the conditions within the cylinder to terminate reactions at the end of the second stage heat release can involve opening an exhaust valve of the cylinder at the end of the second stage heat release and/or injecting an inert liquid (e.g., water) or inert gas (e.g., argon) into the cylinder. The injection of the inert liquid and/or inert gas into the cylinder quenches a temperature of the mixture of air, fuel and one or both of the inert liquid and inert gas.

[0038] As discussed in detail below, a complete three-stage heat release results in an increased production of environmentally undesirable gasses, such as carbon dioxide (CO2). Thus, the recognition that a three-stage heat release can occur under certain conditions allows for adjusting these conditions to minimize or avoid three-stage heat release conditions so as to reduce the amount of carbon dioxide (CO2) produced during the combustion process, as will be described below in connection with Figure 2. However, as also detailed below, it was recognized that desirable gasses, such as carbon oxide (CO) and hydrogen (H2) are formed during the first and second stages of a three-stage heat release. These desirable gasses, commonly referred to as syngasses, can be fed back into one or more of the engine cylinders using exhaust gas recirculation or can be used as burnable fuels by other units. Alternatively, these syngasses are also useful for exhaust after-treatment systems and are used by such systems to reduce environmentally undesirable gasses. Producing syngasses during engine combustion for use by exhaust after- treatment systems is particularly advantageous because it can reduce or avoid the need for certain components in the exhaust after-treatment system, such as oxidizers, and accordingly this can reduce the cost of the exhaust after-treatment system, as well as the installation space required by such a system. Accordingly, by terminating reactions during a three-stage heat release at the end of the second stage allows for the generation of these desirable gasses while minimizing the amount of undesirable gasses that would be produced if the third stage heat release were to occur.

[0039] Figure 2 is a flowchart of a method of preventing or minimizing a three- stage heat release according to embodiments. Initially, an engine control unit monitors operational parameters of the engine (step 210). The operational parameters can include an amount of air, inert gas (e.g., argon), inert liquid, and/or amount of fuel injected into a cylinder of the engine, a temperature inside the cylinder of the engine, and a pressure inside the cylinder of the engine. The engine control unit then determines, based on the monitored operational parameters, whether or not a three-stage heat release will occur within the cylinder (step 220). Finally, the engine control unit, responsive to the determination that the three-stage heat release will occur within the cylinder, adjusts an ignition delay time within the cylinder in such a manner to reduce or prevent the three-stage heat release within the cylinder (step 230).

[0040] The manner of adjusting the ignition delay time to reduce or prevent three-stage heat release within the cylinder will be described in more detail below. Briefly, the ignition delay time can be adjusted by

a. adjusting one the operational parameters, such as

i. changing the amount of fuel relative to the amount of air to achieve a different fuel-air equivalence ratio, ii. terminating the chemistry of fuel oxidation by moving the charge to milder temperature and pressure conditions (e.g., by opening an exhaust valve of the cylinder prior to full combustion of the fuel in the cylinder),

iii. changing the amount of pressure within the cylinder (e.g., by boost pressure of air fed to the engine and/or changing the compression ratio of the engine),

iv. decreasing the temperature in the cylinder when the

temperature in the cylinder is above a predetermined

temperature (e.g., by adjusting a temperature of the inert gas injected into the cylinder and/or changing the temperature of air fed to the engine), and/or

v. injecting an inert liquid (e.g., water) and/or inert gas (e.g., argon) into the cylinder, and/or

b. adjusting a timing of the injection of the air and/or inert gas and of the fuel into the cylinder (e.g., by delaying the injection of the air and/or inert gas and of the fuel into the cylinder.

[0041] The injection of inert liquid and/or inert gas into the cylinder quenches a temperature of the mixture of air, fuel, and one or both of the inert liquid and inert gas.

[0042] Figure 3 is a block diagram of a system according to embodiments.

The system 300 includes an engine 305 coupled to an engine control unit (ECU)

355. The engine control unit 355 can be configured to perform the methods described above in connection with Figures 1 and 2 by suitable programming. An engine control unit refers to a well-known structure used for controlling the operation of an engine. The engine control unit 355 disclosed here, however, is configured to cause, control, or prevent a three-stage heat release, whereas conventional engine control units are not because the existence of a three-stage heat release has not previously discovered.

[0043] The engine 305 includes a cylinder 310 in which combustion occurs (also known as a combustion chamber). Although an engine likely has more than one cylinder, a single cylinder is illustrated for ease of understanding the operation of the system. The cylinder includes a piston (not illustrated), an intake valve 315, and an exhaust value 320. The intake valve 315 is fluidically coupled, via an exhaust gas recirculation line, to an intake manifold 325 for supplying a combination of air, an inert gas, other gasses (e.g., exhaust gas recirculation of combusted gasses from a cylinder), an inert liquid, and/or a fuel (e.g., gasoline or diesel) to the cylinder 310 when the intake valve 325 opens. The exhaust gasses can include molecules produced during the first and second stage of the three-stage heat release from carbon, oxygen, and hydrogen atoms within one or more cylinders. The fuel can alternatively be injected directly into the cylinder 310. The exhaust valve 320 is fluidically coupled to an exhaust manifold 330, which receives the combustion products from the cylinder 310 when the exhaust valve 320 is open.

[0044] The intake manifold 325 is coupled to a fuel source to receive fuel 335, to an inert gas source to receive inert gas 340, an inert liquid source to receive inert liquid 341 , and to an air source to receive air 342. The use of inert gas 340 and/or inert liquid 341 is optional and the present invention does not require inert gas 340 and/or an inert liquid 341 . However, an inert gas, such as argon, can be used to achieve a three-stage ignition even when the fuel-air ratio is near stoichiometric conditions and an inert liquid can be used to control the temperature of the mixture of gasses within the cylinder. In order to adjust the temperature of the air and/or inert gas to cause, control or prevent a three-stage heat release, the inert gas 340, inert liquid 341 , and/or air 342 can pass through a heater and/or cooler 345 prior to being provided to the intake manifold 325. Alternatively, the inert gas 340, inert liquid 341 , and/or air 342 can be used to cause, control, or prevent a three-stage heat release without being heated or cooled, in which case the liquid or gas would not pass through the heater and/or cooler 345.

[0045] The exhaust manifold 330 can be fluidically coupled to an exhaust aftertreatment system 350 or can be fed back to the inlet using an exhaust gas recirculation process. If three-stage heat release is the intended operation of the system 300, the exhaust aftertreatment system 350 can be replaced by a container for storing the combustion products or be coupled directly to another device that uses the combustion products. Although Figure 3 illustrates a cylinder with two valves, the present invention can also be employed with engines having more than two valves per cylinder.

[0046] As discussed above, current research indicates engines exhibit a single or two-stage heat release during a combustion cycle. The inventors, however, have recognized that under certain conditions, engines exhibit a behavior having three distinct heat release stages. The conditions giving rise to a three-stage heat release and those which prevent or reduce the third stage heat release will now be described in connection with Figures 4A-17B. [0047] The graphs of Figures 4A, 4B, 5A, and 5B were produced using constant volume batch reactor simulations in CHEMKIN PRO.

[0048] Figures 4A and 4B respectively illustrate the ignition delay times versus temperature for fuel-air equivalence ratios of 1.0 and 0.3 according to embodiments. These graphs are based on simulations in which n-heptane is the fuel and the pressure in the cylinder is 40 atm. It will be recognized that the temperature referred to on the X-axis in Figures 4A and 4B is the temperature within the cylinder at the beginning of a combustion cycle.

[0049] As illustrated in Figure 4A, a fuel-air equivalence ratio of 1.0 at 40 atm exhibits either a single stage or two-stage heat release over temperature in the range of temperatures between 900 and 1 ,600 K. A notable two-stage heat release is exhibited in the temperature range of 800-950K, which is not readily visible in the figure because ignition delay time of the first stage and the total delay are close in value. For example, at 833 K (i.e., 1000/1.2), the first stage heat release

(represented by the dashed line) will occur at an ignition delay time of approximately 0.9 ms and the second stage heat release (represented by the solid line) will occur at an ignition delay time of approximately 0.6 ms.

[0050] In contrast, as illustrated in Figure 4B, a fuel-air equivalence ratio of 0.3 at 40 atm exhibits a three-stage heat release over a temperature range of 625-950K. The first-stage heat release resulting in n is up to 7 times shorter than the third stage rr, and the second-stage heat release resulting in n is up to 1.3 times shorter than TT. As will be discussed below, the ratios TI _:TT and T2:TT depend on pressure, temperature, and fuel-air equivalence ratio. [0051] Figure 5A is a graph illustrating the temperature and heat release rate versus time according to embodiments. This graph was produced using a constant volume simulation assuming a pressure in the cylinder of 40 atm, a temperature of 720K (i.e., the temperature within the cylinder at the beginning of a combustion event) and a fuel-air equivalence ratio of 0.3. In Figure 5A, the dashed line represents temperature, the values of which are reflected on the left-hand side Y- axis, and the solid line represents heat release rate, the values of which are reflected on the right-hand side Y-axis. Thus, the heat release rate and temperature correspond to the simulated heat release rate and temperature over time.

[0052] The volumetric heat production rate (FIPR) and the trace of relevant species including CO, C02, and H2 were monitored simultaneously as the reaction advances. In Figure 5A, the ignition delay time (IDT) of each stage is defined as the time corresponding to the peak of the volumetric heat production rate, which in Figure 5A corresponds to the times of the peaks in the heat release rate. Thus, in Figure 5A, the ignition delay time of the second stage occurs at the peak labeled tå, h2, and the ignition delay time of the third stage occurs at the peak labeled T3, h3.

The selected chemical kinetic models to explore the presence of three-stage ignition in hydrocarbons were comprehensively validated against fundamental combustion experiments. The simulations covered pressures of 10-60 bar, temperatures of 550K-900K, and fuel to air ratio from stoichiometry (equivalence ratio) of 0.3-1.0.

[0053] In order to examine the conditions in which fuels exhibit three-stage ignition, parameters to quantify the scale of the three-stage heat release rate are needed to establish a basis for comparison. Two parameters have been calculated for each case, which are obtained from the heat release curve. These variables are the ratio of the second heat release peak to the third stage (h2/h3) and the ignition delay time difference between the third and second stages (t3-t2), using the values from the graph of Figure 5A.

[0054] Figure 5B is a graph illustrating the species mole fractions and the combustion efficiency versus time according to embodiments. In Figure 5B, the Hcombustion curve is measured against the Y-axis on the right-hand side of the graph and all other curves are measured against the Y-axis on the left-hand side of the graph. The vertical lines in Figure 5B correspond to the times of the second and third heat release stages, which correspond to the timing of the heat release rate peaks in Figure 5A. As illustrated in Figure 5B, CO and H2 increase at a steady rate as the reaction advances until they reach a local maxima at t2 from which they decline. CO2 starts to be produced by the end of the first stage and has low concentration by the end of the second stage. As the reaction advances to the third stage, the CO starts to slowly convert to CO2. The combustion efficiency, which is defined as the ratio of the heat released to the fuel’s heating value, jumps to 80% by the first stage. The combustion efficiency then increases to around 90% by the end of the second stage where the fuel’s heating value is fully utilized by the end of the third stage. As illustrated in Figure 5B, terminating the chemistry (i.e., terminating the reactions within the cylinder) by the end of the second stage results in low CO2 trace with peak traces of ignitable CO and H2 components without compromising the combustion efficiency much. [0055] As will be appreciated from Figure 5A, upon termination of reactivity at the second stage, the production of carbon dioxide is reduced significantly at the expense of high yields of carbon monoxide (i.e., the mole fraction of carbon dioxide is approximately 0.005 and the mole fraction of carbon monoxide is approximately 0.025). Further, upon termination of reactivity at the second stage, hydrogen concentration reaches a global maximum but with small overall fraction as the majority of it has been converted to water vapor. The unbalance in concentrations between carbon monoxide and hydrogen poses a challenge to convert these species into alcohols such as methanol or ethanol. This can be addressed by supplying additional hydrogen to the exhaust to react with carbon monoxide to produce alcohols in the presence of a suitable catalyst. Alternatively, carbon monoxide can be captured by utilizing its strong affinity to transition metals. Iron powder can be reacted with carbon monoxide to create iron penta-carbonyl that exists in liquid phase. In addition, the exhaust has sufficient proportions of carbon monoxide and water vapor to react them with an appropriate catalyst and the right temperature and pressure conditions to create formic acid, which has extensive applications. The presence of carbon monoxide and water vapor with a nickel-containing catalyst can also be utilized for acetylene carbonylation by producing acrylic acid.

[0056] Rapid compression machine (RCM) experiments were conducted to further corroborate the existence of three-stage heat release in the auto-ignition of lean n-heptane/air mixtures. The rapid compression machine has a dual piston design with six ports available in the center of the combustion chamber for implementing sensors. The time-resolved pressure profile was measured using a Kistler 6045A pressure transducer. Two of the ports were fitted with sapphire windows to provide optical access for laser absorption measurements. The laser system used laser at 4.89 pm to quantify CO mole fraction. The Beer-Lambert relation was used to convert measured absorbance profiles to CO mole fraction (. XCO ) profiles with spectroscopic parameters taken from the high-resolution transmission molecular absorption (HITRAN) database. The post-compression temperature profile was inferred from the measured pressure, P, using the isentropic gas relation. Experimental conditions in rapid compression machine experiments are typically chosen such that the first-stage ignition delay is at least 3 ms after the end-of-compression (EOC) to minimize reactivity during the compression stage. However, in the interest of observing three-stage ignition behavior, certain end of compression conditions were chosen so that first-stage heat release occurs shortly after end of compression. Experiments were conducted at compressed pressure of 21.6 atm, equivalence ratio of 0.3, and compressed temperatures ranging 700 to 900 K.

[0057] Figure 6 is a graph of pressure and carbon monoxide mole faction profiles in a rapid compression machine at 780 K. The upper curve reflects pressure over time (i.e., the left-hand y-axis and the x-axis) and the lower curve reflects CO mole fraction over time (i.e., the right-hand y-axis and the x-axis). As illustrated, an initial increase in pressure occurs shortly after end of compression, which corresponds to n. This is followed by a delay period and subsequently a second- stage heat release resulting in tå. Finally, a third-stage heat release elevates the system to its final pressure and fully burned condition. The CO profile shows similar behavior as predicted by the constant volume simulations for the three-stage heat release case. The CO concentration profile increases at the end of n, then gradually rises to its maximum at the end of T2, and slowly decays during the third- stage heat release regime. Overall, the three-stage heat release behavior predicted by the computer simulation (i.e., kinetic modeling) is in excellent agreement with experimentally observed features.

[0058] Figure 7 is a block diagram of the key reactions involved in three-stage ignition of n-heptane according to embodiments. The three-stage chemistry was investigated using computational singular perturbation (CSP) tool by identifying reactions that contribute to the explosive time scale. The initial heat release stage follows the same reaction pathways as the low temperature heat release in a typical two stage ignition. The ignition is promoted through intra-molecular isomerization of RO2 radical to QOOH, which then oxidizes and isomerizes to produce OH radical. The aforementioned pathway competes with these two reactions QOOH ® RO2 and R0₂ ® Olefins + HO2 that opposes the explosive nature.

[0059] In the second heat release stage, the ignition propensity is controlled by a competition between hydrogen and carbon related chemistry. The two OH radicals produced from H2O2 can go either termination route through H2O2 + OH ® H2O + HO2 and then to HO2 + HOå® H2O2 + O2 or explosive carbon root through C2H₄ + OH ® products, in which its exothermic nature drives the system to thermal runaway. The hydrogen termination root, however, dominates at the second stage supported by this reaction OH + HO2 ® H2O + O2 that inhibits the reactivity of the second stage. [0060] Hydrogen and CO-to-CC>2 related chemistry dominate the third heat release stage. The radical termination pathway OH + HO2 H2O + O2 from the second stage still dominates throughout the third stage. This reaction competes with the exothermic reaction CO + OH CO2 + H and the endothermic reaction HO2

H + O2, which are both possessing explosive character. The slow decay of CO throughout the third stage is attributed to the inhibition nature of CO oxidation. The reaction that contributes the most to reactivity of the third stage is H + O2 O + OH that elevates the temperature of the system. These reactants, however, can take chain termination pathway through H + 02 H02.

[0061] Modeling combustion in closed homogenous batch reactor is not representative of a transient operation device such as the internal combustion engine. Unlike the constant volume batch reactor, the temperature and pressure are varying with time in such devices. Accordingly, the pre-defined model for

Homogeneous Charge Compression Ignition (HCCI) engine embedded in CHEMKIN PRO was used to explore the presence of three-stage heat release in practical combustion devices. The HCCI model is a zero dimensional single-zone adiabatic model that has been examined by CHEMKIN PRO HCCI Engine Tutorial using real data from single-cylinder HCCI test engine. The selected engine specifications for simulation was inspired by the test engine with modifications implemented to reduce the compression ratio and engine speed. The table below compares the engine specifications of the test engine to the simulations. Test Engine Simulations

Compression ratio 16.5 1 1 .5 - 12.1

1086-1 143

Swept volume 1601 cm³ cm³

Clearance volume 103.3 cm³ 103.3 cm³

Ratio of connecting rod to crank

3.714 3.714

radius

Piston bore 12.1 cm 12.1 cm Stroke 14.0 cm 9.5-10 cm

Engine speed 1000 rpm 600 rpm

[0062] The HCCI engine model is only applicable when the system does not experience any gas exchange through the engine valves. This represents the time between inlet valve closure (IVC) and exhaust valve opening (EVO) where the engine undergoes compression and expansion strokes. The valve phasing for the simulation was selected to be the same as the test engine having inlet valve closure of 142 crank angle degrees before top dead center (TDC) and exhaust valve opening of 1 15 degrees after top dead center. Hence, the total simulation duration is 257 crank angle degrees, and in terms of time scale is ~70 ms for the engine running at 600 rpm. The air fed to the engine is at T = 300 K and slightly boosted from ambient at P = 1 .05 bar.

[0063] One of the most important indicators to evaluate the performance of internal combustion engines is the thermodynamic efficiency. Because the predefined model of HCCI engine used here is zero dimensional and adiabatic, the model neglects any fluid flow interactions or heat transfer across the engine walls, making the thermodynamic efficiency an inappropriate parameter for analysis. The combustion efficiency is more suitable parameter to quantify the conversion of the available fuel energy to heat. It can be obtained from the mass fraction of the exhaust components with positive heating value to the mass of the fuel, according to the following equation:

[0064] Where Mi molar mass of unburnt species in the exhaust, M_p molar mass of the fuel, Xi concentration of unburnt species, QLHV lower heating value of unburnt species, A/F is the gravimetric air/fuel ratio, and QLHVJ is the lower heating value of the fuel.

[0065] The combustion efficiency was calculated throughout the three heat release stages to study the extent of fuel consumption through each stage. The unburned species considered in equation (1 ) are unburned hydrocarbons, hydrogen, and carbon monoxide. The heating value of unburned hydrocarbons was assumed to be the same as the fuel’s heating value.

[0066] Figures 8 and 9 show the simulation results of the two HCCI cases at different compression ratios (CR 1 1 .5 and CR 12.1 ). The X-axis represents the crank angle degree (CAD). The right-hand side Y-axis is the volumetric heat production rate for the dashed line, all other lines are measured against the left-hand side Y-axis. As illustrated, the increase of compression ratio advances the combustion phasing where the three-stage ignition occurs at earlier crank angles. The magnitude of the third stage relative to the second stage heat release is suppressed with compression ratio elevation. Terminating the chemistry (i.e., terminating the reactions within the cylinder) by the end of the second stage (e.g., by opening the engine exhaust, injecting water, etc.) yields to a local maxima of CO and H2 with low CO2 trace and relatively good combustion efficiency. Further, as discussed above, the carbon monoxide present by the end of the second stage can be converted into alcohols by introducing additional hydrogen into the exhaust or alternatively can be captured using transition metals arranged in the exhaust path.

[0067] A comprehensive analysis was performed to study the extent of the auto-ignition phenomenon. A sweep of different conditions (temperature, pressure, and equivalence ratio) were simulated in a constant volume batch reactor. From that, the IDT difference between the third and second heat release (t3-t2), and the heat production rate of the second to third stage (h2/h3) were determined to quantity the prominence of the three-stage ignition for each condition. The conditions examined are pressures of 10-60 bar, temperatures of 550K-900K, and equivalence ratios of 0.3-1.0.

[0068] The effect of temperature elevation on the three-stage ignition prominence varies based on the exerted pressure and the fuel/air ratio. The behavior can be explained further by examining the volumetric heat production rate of n-heptane auto-ignition at the same temperature (550 K) but different pressures and equivalence ratios, which can be performed using the graphs of Figures 10A- 10C. In Figure 10A, the pressure is 10 bar and the fuel-air equivalence ratio is 0.3, in Figure 10B, the pressure is 30 bar and the fuel-air equivalence ratio is 0.3, and Figure 10C, the pressure is 30 bar and the fuel-air equivalence ratio is 0.4. As will be appreciated by comparing Figures 10A-10C, the pressure elevation and fuel enrichment (i.e., increasing the amount of fuel versus the amount of oxidant injected into the cylinder) suppress the magnitude of the third stage relative to the second stage at low temperatures. For the three cases illustrated in Figures 10A-10C, when the temperature is elevated, the third stage heat production rate increases at the highest rate compared to the other two stages. The only outlier is in the graph illustrated in Figure 10B, where the first stage also increases at similar rate as the third stage with temperature increase.

[0069] The analysis of temperature effect is further explored in Figures 1 1A- 1 1 C where parameters relevant to the three-stage ignition are examined. In Figures 1 1 A-1 1 C, the solid line corresponds to the ratio of second to third heat release stage (h2/h3), which is measured using the Y-axis on the right-hand side of the figures, and the dashed line corresponds to the difference in ignition delay time of the third and second stages (i.e., T3- T2).

[0070] Both ignition delay time difference and heat release rate ratio between the second and third stage decrease with temperature elevation. The slope in which T3-T2 and h2/h3 decrease with temperature remains relatively the same for different predefined pressure and equivalence ratio values. The temperature elevation eliminates the inhibiting radical termination reactions at the second stage heat release and drives for thermal runaway.

[0071] The effect of pressure on the three-stage heat release was performed using a similar methodology to temperature analysis, the results of which are illustrated in Figures 12A-12C. Figures 12A-12C illustrate the volumetric heat production rate of n-heptane auto-ignition at the same pressure (P = 10 bar) but different temperatures and equivalence ratios. Specifically, in Figure 12A the temperature is 550K and the fuel-air equivalence ratio is 0.3, in Figure 12B the temperature is 700K and the fuel-air equivalence ratio is 0.3, and in Figure 12A the temperature is 550K and the fuel-air equivalence ratio is 0.4. As illustrated in Figure

12A, the second stage heat release dominates at the standard case, and as illustrated in Figures 12B and 12C, the dominance is shifted towards the third stage when either the temperature or fuel/air ratio is increased. Pressure elevation causes an increase in the volumetric heat production rate of all stages with the second stage having the highest rate of increase.

[0072] The pressure effect on the three-stage ignition parameters is illustrated in the graphs of Figures 13A-13C. In Figures 13A-13C, the solid line corresponds to the ratio of second to third heat release stage (h2/h3), which is measured using the Y- axis on the right-hand side of the figures, and the dashed line corresponds to the difference in ignition delay time of the third and second stages (t3- Tå), which is measured using the Y-axis on the left-hand side of the figures. The rapid increase rate of the second stage with pressure elevation is translated into an increase in the h2/h3 ratio for all cases. The delay between the third and second stages (t3-t2) remains relatively invariant regardless of the exerted initial temperature and equivalence ratio with the only exception is the first case (Fig 13A) where t3-t2 slightly reduces. [0073] It was found that fuel enrichment by increasing the fuel/air ratio has the strongest effect in suppressing three-stage ignition phenomenon. The temperature and pressure combination from which three-stage ignition is present at near stoichiometric fuel/air ratio is limited. Figures 14A and 14B illustrate the volumetric heat production rate of n-heptane at equivalence ratio of 0.3 and temperature 550 K but with different pressures (10 bar in Figure 14A and 30 bar in Figure 14B). The elevation of pressure, as discussed above, increases the heat production of the second stage at greater rate than the other two stages.

[0074] The fuel-air equivalence ratio effect on the three-stage ignition parameters is illustrated in the graphs of Figures 15A and 15B. In Figures 15A and 15B, the solid line corresponds to the ratio of second to third heat release stage (h2/h3), which is measured using the Y-axis on the right-hand side of the figures, and the dashed line corresponds to the difference in ignition delay time of the third and second stages (t3- Tå), which is measured using the Y-axis on the left-hand side of the figures.

[0075] As illustrated in Figures 15A and 15B, the fuel enrichment causes an exponential increase in the third stage heat release where it causes the h2/h3 ratio to drop exponentially. The exponential drop is also observed for the ignition delay time difference between the third and second stage (t3-t2) with equivalence ratio increase. The fuel enrichment increases the availability of free radicals that pushes the explosive nature of the system.

[0076] The analysis of three stage heat release has been extended beyond n- heptane by exploring potential components exhibiting this phenomenon. N-paraffins with shorter chain lengths than n-heptane, namely n-hexane, n-pentane, and n- butane, were examined in this study. Similar analysis to n-heptane has been implemented where a sweep of different temperature, pressure, and equivalence ratio conditions were simulated in a closed homogenous batch reactor. C1 -C3 n- paraffins have been eliminated from this analysis due to their low reactivity especially at extremely lean conditions.

[0077] Since the analysis has three independent variables (temperature, pressure, and equivalence ratio), the results have been presented in 3-D plots by fixing one variable and varying the other two. Figures 16A and 16B illustrate the temperature and pressure effect on t3-t2 and h2/h3 at equivalence ratio of 0.3 for C₄- C7 n-paraffins. For all the fuels, the h2/h3 ratio peaks at low temperature/high pressure point at which point it begins to gradually drop. N-butane has substantially higher h2/h3 ratio at the peak conditions compared to the other n-paraffins and then they all collapse together at conditions away from low temperature and high pressure. In contrast, t3-t2 reaches global maxima at low pressures while maintaining the low temperature conditions. The ignition delay time difference is highest for n-butane and lowest for n-pentane at peak conditions with n-hexane and n-heptane lie in between having the same t3-t2 magnitude.

[0078] The presence of three stage heat release was further explored in other hydrocarbon classes, namely iso-paraffins (isooctane and 2-methylhexane) and naphthenes (cyclopentane). For each fuel, simulations have been performed for a sweep of temperatures (550K- 900K) and pressures (10-60 bar) while fixing equivalence ratio at 0.3 to find the heat release ratio h2/h3 and IDT difference t3-t2. [0079] Similar to the analysis to the n-paraffins was implemented to study the other hydrocarbons where the three-stage ignition parameters have been presented in 3-D plots. Figures 17A and 17B show the temperature and pressure effect on T3- T2 and h2/h3 at equivalence ratio of 0.3 for different hydrocarbons. For all the fuels, the h2/h3 ratio peaks at low temperature/high pressure point at which point it begins to gradually drop. Isooctane has substantially higher h2/h3 ratio at the peak conditions compared to the other components and then they all collapse together at conditions away from low temperature and high pressure. In contrast, t3-t2 reaches global maxima at low pressures while maintaining the low temperature conditions. The ignition delay time difference is highest for n-heptane and lowest for isooctane at peak conditions with 2-methylhexane and cyclopentane lie in between having the same t3-t2 magnitude.

[0080] Thus, one skilled in the art can use the data in the graphs of Figures 10A-17B to determine temperature, pressure, and fuel-air equivalence ratio conditions that can prevent or cause a three-stage heat release, as well as when and how to terminate a second heat release stage of a three-stage heat release. These parameters can then be used by the engine control unit to adjust the ignition delay time within one or more cylinders to prevent or cause a three-stage heat release.

For example, a three-stage heat release can be prevented by feeding the cylinder a fuel-rich composition (e.g., f = 1 ) in most temperature and pressure conditions, whereas a lean composition (e.g., f = 0.3) will result in a three-stage heat release an extended range of temperature and pressure conditions. [0081] Although Figures 10A-17B analyze the effect of adjusting one of the temperature, pressure, and fuel-air equivalence ratio, it will be recognized that the information from these graphs can be used to cause or prevent a third stage heat release based on a combination of two or more of temperature, pressure, and fuel- air equivalence ratio. Specifically, the discussion above demonstrates that by adjusting one or more of these parameters results in an adjustment of the conditions within the cylinder and whether or not a third stage heat release occurs depends on conditions within the cylinder. Accordingly, instead of adjusting the pressure of the mixture of air and/or fuel injected into the cylinder, the temperature of the mixture of the air and/or fuel injected into the cylinder, and/or the fuel-air equivalence ratio of the combination of fuel and air injected into the cylinder, the injection of the fuel-air mixture can be delayed (if a third stage is to be reduced or prevented) or advanced (if the third stage is to be implemented). Those skilled in the art will recognize that the delaying or advancing of the injection of the fuel-air mixture affects the pressure and temperature within the cylinder, which, as discussed above, affects whether or not a third stage heat release will occur.

[0082] The disclosed embodiments provide methods and systems for controlling operation of an engine so as to produce or prevent or minimize the formation of a three-stage heat release within cylinders of the engine. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0083] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0084] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

1. A method for controlling an engine, the method comprising:

determining (1 10), by an engine control unit (355), that a three-stage heat release in a cylinder (310) in the engine (305) should be performed;

adjusting (120), by the engine control unit (355) responsive to the

determination that the three-stage heat release will not occur within the cylinder (310), conditions within the cylinder (310) in such a manner to cause the three-stage heat release within the cylinder (310);

monitoring (130), by the engine control unit (355), conditions in the cylinder (310) during a second stage heat release of the three-stage heat release; and

adjusting (140), by the engine control unit (355), conditions in the cylinder (310) during the second stage heat release to terminate reactions in the cylinder (310) at an end of the second stage heat release.

2. The method of claim 1 , wherein the adjustment by the engine control unit to terminate the reactions in the cylinder comprises:

opening an exhaust valve of the cylinder at the end of the second stage heat release.

3. The method of claim 1 , wherein the adjustment by the engine control unit to terminate the reactions in the cylinder comprises: injecting an inert liquid or an inert gas into the cylinder at the end of the second stage heat release to quench a temperature of a mixture of the air, fuel, and at least one of the inert liquid or inert gas.

4. The method of claim 1 , wherein during a first stage and the second stage of the three-stage heat release, molecules are produced from carbon, oxygen, and hydrogen atoms within the cylinder, the method further comprising:

recirculating exhaust gas from the cylinder, which includes the produced molecules, into the cylinder or another cylinder of the engine.

5. The method of claim 1 , wherein the adjustment of the conditions within the cylinder comprises:

adjusting at least one of the operational parameters.

6. The method of claim 5, wherein the engine control unit is configured to adjust the at least one operational parameter by:

decreasing the amount of fuel relative to the amount of air to achieve a fuel-air equivalence ratio, f, below 1.0.

7. The method of claim 5, wherein the adjustment of the at least one operational parameter comprises:

increasing an amount of pressure within the cylinder.

8. The method of claim 1 , wherein the adjustment of the conditions within the cylinder comprises:

adjusting, based on an auto-ignition property of the fuel, a timing of the injection of the air and of the fuel into the cylinder by advancing the injection of the air and of the fuel into the cylinder.

9. The method of claim 1 , wherein the termination of the reactions in the cylinder at the end of the second stage heat release produces exhaust gas having a mole fraction of carbon dioxide of 0.005 or less.

10. A system, comprising:

an engine (305), comprising at least one cylinder (310) having an intake valve (325) configured to receive at least air and fuel, and at least one exhaust valve (330) configured to expel gasses from the cylinder (310);

an engine control unit (355) coupled to the engine (305) and configured to determine (1 10) that a three-stage heat release in a cylinder (310) in the engine (305) should be performed;

adjust (120), responsive to the determination that the three-stage heat release will not occur within the cylinder (310), conditions within the cylinder (310) in such a manner to cause the three-stage heat release within the cylinder (310);

monitor (130) conditions in the cylinder (310) during a second stage heat release of the three-stage heat release; and adjust (140) conditions in the cylinder (310) during the second stage heat release to terminate reactions in the cylinder (310) at an end of the second stage heat release.

1 1. The system of claim 10, wherein the engine control unit is configured to adjust conditions within the cylinder to terminate the reactions by:

controlling an exhaust valve of the cylinder to open at the end of the second stage heat release.

12. The system of claim 10, wherein the engine control unit is configured to adjust conditions within the cylinder to terminate the reactions in the cylinder by:

controlling injection of an inert liquid or inert gas into the cylinder at the end of the second stage heat release to quench a temperature of a mixture of the air, fuel, and at least one of the inert liquid or inert gas.

13. The system of claim 10, wherein during a first stage and the second stage of the three-stage heat release, molecules from carbon, oxygen, and hydrogen atoms are produced within the cylinder, the system further comprising:

an exhaust gas recirculation line configured to recirculate exhaust gas from the cylinder, which includes the produced molecules, into the cylinder or another cylinder of the engine.

14. The system of claim 10, wherein the engine control unit is configured to adjust the conditions within the cylinder by:

adjusting at least one of the operational parameters.

15. The system of claim 10, wherein the engine control unit is configured to adjust the conditions within the cylinder by:

adjusting a timing of the injection of the air and fuel into the cylinder by delaying the injection of the air and fuel into the cylinder.

16. A method for controlling an engine (305), the method comprising:

monitoring (210), by an engine control unit (355), operational parameters of the engine (305), wherein the operational parameters comprise an amount of air and amount of fuel injected into a cylinder (310) of the engine (305), a temperature inside the cylinder (310) of the engine (305), and a pressure inside the cylinder (310) of the engine (305);

determining (220), by the engine control unit (355) based on the monitored operational parameters, whether or not a three-stage heat release will occur within the cylinder (310); and

adjusting (230), by the engine control unit (355) responsive to the

determination that the three-stage heat release will occur within the cylinder (310), conditions within the cylinder (310) in such a manner to reduce or prevent the three- stage heat release within the cylinder (310).

17. The method of claim 16, wherein the adjustment of the conditions within the cylinder comprises:

adjusting at least one of the operational parameters.

18. The method of claim 17, wherein the adjusting of the at least one operational parameter comprises:

increasing the amount of fuel relative to the amount of air.

19. The method of claim 17, wherein the adjusting of the at least one operational parameter comprises:

decreasing an amount of pressure within the cylinder, by opening an exhaust valve of the cylinder prior to full combustion of the fuel in the cylinder;

adjusting the temperature in the cylinder by adjusting a temperature of the air injected into the cylinder; or

injecting an inert liquid or inert gas into the cylinder.

20. The method of claim 17, wherein the adjustment of the conditions within the cylinder comprises:

adjusting a timing of the injection of the air and fuel into the cylinder by delaying the injection of the air and fuel into the cylinder.