EP3347580A1

EP3347580A1 - Internal combustion engine and auto-ignition control method

Info

Publication number: EP3347580A1
Application number: EP15760449.7A
Authority: EP
Inventors: Konstantinos GKAGKAS; Peter LINDSTEDT; Frank BEYRAU
Original assignee: Toyota Motor Europe NV SA; Imperial College of London
Current assignee: Imperial College of London; Toyota Motor Corp
Priority date: 2015-09-08
Filing date: 2015-09-08
Publication date: 2018-07-18
Anticipated expiration: 2035-09-08
Also published as: EP3347580B1; WO2017041833A1

Abstract

The present invention is directed to a method of enhanced combustion of hydrocarbon fuel by oxygen in an internal combustion engine wherein: - both oxidiser and a hydrocarbon fuel are introduced into the combustion chamber of an internal combustion engine; - optical energy is provided in the combustion chamber to the oxidiser- fuel mixture and/or reaction products of oxidiser/fuel mixing, using irradiation at one or more wavelengths greater than 200 nm and less than 310 nm and/or at wavelengths of at least 345 nm and at most 365 nm. Combustion apparatus that can be used in such a method of enhanced combustion is also described.

Description

Internal combustion engine and auto-ignition control method

Field of the Invention

The present invention relates to a method of enhanced combustion of hydrocarbon fuel by oxygen in an internal combustion engine through targeted ultraviolet irradiation of dissociating species, as well as combustion apparatus for use in such a method.

Motivation

The internal combustion engine is broadly used in transportation due to the performance and driving range that can be achieved in combination with the high energy density of liquid hydrocarbon fuels. Its efficiency and exhaust emissions have continuously improved over decades through the application of advancements in many fields such as electronics and metallurgy. In order to further improve efficiency by a significant degree, a more accurate control of the chemical processes occurring during the combustion of the fuel can be beneficial. More specifically, by fine control of the chemical processes, the heat release rate can be adjusted so that the maximum amount of chemical energy from the fuel is converted into mechanical energy through the piston motion.

Background Art

In the natural process of auto-ignition for a fuel/oxidiser mixture, a time length is necessary from the beginning of the reaction until the combustion. This time length is defined as ignition delay time (IDT). The beginning of the reaction can be roughly defined as the point where the liquid fuel is evaporated and locally mixed with the oxidiser. Consequently, during the auto-ignition process of fuel/oxidiser mixtures inside the combustion chamber of an internal combustion engine, the start points vary considerably. Further, the IDT depends on the local temperature, turbulence and stoichiometry. An example of ignition delay times for two different fuels is shown in Figure 1.

During this "delay" time, there is the "pre-ignition phase", where chemical reactions are happening, and breaking down the fuel molecules into smaller hydrocarbons and intermediate species (such as OH, H2O2, CH₂0 etc.). There is no significant heat release at this stage. Once a critical balance is reached, the main combustion event will occur with the conversion of intermediate species to (mainly) C0₂ and H₂0 and large heat release. This event should occur close to the Top Dead Centre point for maximum efficiency. The Top Dead Centre (TDC) point is conventionally defined as the position of a piston in which it is farthest from the crankshaft.

Patent publications JPH-10-196471 and JPH-10-196508 teach the use of laser light for starting combustion by production of OH or CH radicals. However, it may firstly be observed that the method of producing of OH and CH groups only achieves ignition under some specific conditions. To accurately control the combustion process inside the engine cylinder, it is necessary to affect any potential step of combustion chemistry and adjust it to the inhomogeneity of the real application. Secondly, by using a coherent light source (laser with a single wavelength) the target is only a specific radical and only a very limited effect can be achieved. Thirdly, by exciting OH groups or CH groups, the respective radicals cannot be generated.

Summary of the Invention

In one aspect, the present invention relates to a method of enhanced combustion of hydrocarbon fuel by oxygen in an internal combustion engine wherein:

- both fuel and oxidizer are introduced into the combustion chamber of an internal combustion engine; the "oxidizer" refers in particular to a gas mixture containing oxygen. This is typically atmospheric air, or atmospheric air mixed with recirculated exhaust gas (EGR).

- optical energy is provided in the combustion chamber to the oxidiser- fuel mixture and/or reaction products of oxidiser/fuel mixing, using irradiation at one or more wavelengths greater than 200 nm and less than 310 nm and/or at wavelengths of at least 345 nm and at most 365 nm.

In some advantageous embodiments, irradiation is carried out at one or more wavelengths of at least 240 nm and at most 295 nm, more preferably of at least 260 nm and at most 290 nm, and still more preferably of at least 270 nm and at most 280 nm.

In another aspect, the present invention relates to a combustion apparatus comprising a combustion chamber, a device for compressing a fuel/oxidiser mixture in said combustion chamber, and one or more light sources operatively linked to the combustion chamber so as to be able to impart to the compressed fuel/oxidiser mixture ultraviolet light irradiation at one or more wavelengths greater than 200 nm and less than 310 nm and/or at wavelengths of at least 345 nm and at most 365 nm.

In some advantageous embodiments, the light sources allow for irradiation to be carried out at one or more wavelengths of at least 240 nm and at most 295 nm, more preferably of at least 260 nm and at most 290 nm, and still more preferably of at least 270 nm and at most 280 nm.

In some advantageous embodiments, the light sources are light emitting diodes (LEDs).

In some advantageous embodiments, at least one light source is mounted in a wall of the combustion chamber. Also, at least one light source may be external to the combustion chamber but coupled thereto using optics such as optical fibres. The present invention provides internal combustion engines in which the timing and location of the combustion process is controlled, allowing high efficiency, low pollutant emissions and improved control. Brief Description of the Figures

Figure 1 shows a comparison of experimental data for stoichiometric n- heptane (open square symbols) mixtures at 40 atm and stoichiometric iso- octane (round open symbols) mixtures at 42 atm pressure. The n-heptane experimental data is from Ciezki and Adomeit [Combust. Flame 93:421-433, 1993] and the iso-octane data from Fieweger et al. [Combust. Flame 109:599- 619, 1997]. Iso-octane and n-heptane represent the lower and upper ignition delay time limits for the two primary non-aromatic gasoline reference fuel components.

Figure 2 shows adsorption coefficients in the wavelength range 200 to 350 nm for a number of candidate species that can be used as fuel additives or are naturally generated during combustion reactions and which may function as targets for UV irradiation in the present invention.

Figure 3a shows an absorption coefficient - wavelength profile for hydrogen peroxide (H2O2). Figure 3b shows the calculated OH yield based on irradiation at different wavelengths.

Figures 4 to 10 show schematic illustrative and non-limiting embodiments of combustion apparatus according to the present invention.

Figure 11 is a schematic diagram showing the functioning at each stage in the cycle i.e. for each stroke in a four-stroke engine, and Figure 12 shows an engine assembly with full pistons and crankshaft, as an illustration of a possible context in which the present invention may be used (Figures 4 to 10 only show part of the pistons of an overall engine assembly).

Figure 13 shows pressure traces versus crank angle degree after top dead centre (CAD ATDC) for (curve 1) an experimental engine, with no combustion occurring; (curve 2) the same experimental engine, with combustion; (curve 3) example simulation results for one specific lambda value (3.7) with photo-dissociation of H₂0₂; (curve 4) example simulation results for one specific lambda value (3.7) without photo-dissociation of H₂0₂. Figures 13a, 13b and 13c are different presentations of the same data.

Detailed Description of the Invention

The enforced dissociation process of the present invention aims to control the length of the pre-ignition phase and shorten it or lengthen it as needed in order to always achieve the optimal point of heat release, in terms of timing and localisation, everywhere in the combustion chamber. According to prior art references JPH-10-196471 and JPH-10-196508, optical energy is introduced at the top dead centre and there is instantaneous ignition. By contrast, in the present invention, optical energy is not necessarily introduced at TDC. In a preferred and non-limiting embodiment, the light energy is introduced in the pre-ignition phase, before TDC. This light energy is designed to force specific chemical reactions and therefore change the chemical path towards ignition and complete combustion. As a result, the combustion process can be accelerated or decelerated. The precise timing will depend on detailed experiments and simulations regarding each specific engine. Thus, a "mapping" of light introduction can be defined for different engine speed and load, in a similar fashion to the current spark plug ignition mapping and fuel injection mapping. It may be noted that current engine technology using computer control allows piston movement to be tracked on a millisecond timescale, so that precise coordination of light introduction with the engine cycle can be achieved.

In the present invention, one more UV light source(s) is (are) used to dissociate intermediate combustion species. These species are created naturally during the pre-ignition phase or can be provided externally as an additive. The dissociation of these species controls the balance between the different chemical reaction paths (e.g. endothermic cracking, exothermic pathways) and can accelerate or decelerate the ignition/combustion process. By this means, high efficiency and low emission operation is possible by achieving controlled auto-ignition of very lean and/or very stratified oxidiser/fuel mixtures in the cylinder.

In the present invention, optical energy is provided in the combustion chamber to the oxidiser-fuel mixture using irradiation at wavelengths greater than 200 nm and less than 310 nm and/or at wavelengths of at least 345 nm and at most 365 nm.

Generally speaking, photo-dissociation is triggered in the near UV region, with wavelengths between 200 nm and 400 nm. Above 400 nm the photons do not carry enough energy to cleave chemical bonds and below 200 nm, air becomes absorbing (but will not dissociate). In the latter case the light energy would be transferred into heat. This might lead to accelerated ignition as well but the mechanism would be quite different to that at the heart of the present invention.

In cited prior art references JPH-10-196471 and JPH-10-196508, optical energy is targeted in particular to OH groups, i.e. the O-H groups in hydroxy- containing intermediates which may generally be described as R-OH. However, in preferential embodiments of the present invention, it is proposed to target instead the bond between the R group and the OH.

In the present invention, the ignition process of the fuel-oxidiser mixture in the cylinder of an internal combustion engine may be initiated and controlled through the process of photo-dissociation by dissociating species that have previously been generated by low to medium temperature chemical reactions during the compression of the mixture or via the dissociation of suitable fuel additives. This dissociation generates highly reactive radical species through the breakage of targeted molecular bonds, for example the oxygen bond in structures such as -R-O-O-R', where R and R' can be a H atom or a hydrocarbon group. Such a dissociation, for example the case where R and R' are hydrogen atoms, will strongly promote the second stage combustion process involving the majority of the heat release. The use of suitable wavelengths to target carbon-carbon bonds in structures such as R-C-C-R', where R and R' are typically hydrocarbon groups, can further be used to promote thermal decomposition reactions that can inhibit ignition in the negative temperature coefficient region of combustion.

The origin of the photons for the photo-dissociation to be carried out in the invention is not restricted to a particular type of light source. For example, the photons can be generated by a laser source or an incoherent source, such as an LED. In a preferred embodiment however, an LED source may be used, in particular deep-UV LEDs. LEDs with emission spectra between 250 and 310 nm are commercially available. Deep-UV LEDs of this type are available which can deliver one millisecond pulses with a repetition rate of up to 1 kHz. As an illustrative and non-limiting example of an LED semiconductor material that may appropriately be used in the context of the present invention, one may cite materials based on AIGaN. Such materials are, for example, available from manufacturers such as Sensor Electronic Technology, Inc. (USA). Other materials may be contemplated as possible UV emitters as well and generally speaking LED research is a field in development, new materials becoming available with emission characteristics that would render them suitable for use in the invention.

In the research leading up to the present invention, the inventors sought to identify candidate molecular species that efficiently absorb light at a suitable wavelength, dissociate efficiently and yield reactive radicals. The molecular species may appropriately also be chemically stable enough so that they can be used as fuel additives. If not, the species should appropriately be ones which occur naturally from the chemistry taking place during the fuel-oxidiser mixture compression.

In this respect, the following suitable candidate species were identified which can used as additives:

^■ benzoic acid C₆H₅COOH

^■ formic acid HCOOH

^■ acetic acid CH₃COOH

^■ propionic acid C₂H₅COOH

^■ hydrogen peroxide H₂0₂

Moreover, H₂0₂ and other species below are also generated in situ from the chemistry during the compression phase:

^■ hydrogen peroxide H₂0₂

^■ formaldehyde H₂CO

^■ hydroperoxyl radicals H0₂

Figure 2 shows adsorption coefficients in the wavelength range 200 to 350 nm for the above-mentioned species. It may be noted here that, apart from formaldehyde, most species can be excited between 200 nm and 260 nm. Additionally, formaldehyde can be dissociated, with the use of wavelengths below 310 nm yielding reactive radicals. For the particular case of formaldehyde, a wavelength of irradiation around 355 nm, for example, between 345 and 365 nm (355 nm ± 10 nm) is also a suitable dissociation wavelength.

Based on the molecular candidate species absorption and LED emission characteristics, it is considered that more preferred targets for irradiation are benzoic acid and hydrogen peroxide as additives, and hydrogen peroxide, the hydroperoxyl radical and formaldehyde as naturally occurring species arising during natural fuel combustion.

Concerning in particular hydrogen peroxide (H₂0₂), the absorption cross- section decreases with increasing wavelength, as shown in Figure 3a. In practice, based on current LED performance, LED lifetime and peak optical output increases with increasing wavelength. Figure 3b shows the calculated OH yield based on irradiation at different wavelengths. Based on the calculated OH yield, 275 nm LED appears to be an optimal choice for irradiation wavelength based on current LED performance characteristics.

In terms of the fuel type that can be implied in the present invention, this may be chosen among gasoline, diesel and bio-fuel (oxygenated fuel).

In terms of fuel injection method, either port injection or direct injection may be used.

In terms of fuel/oxidiser mixture conditions, fully premixed combustion is possible. Partially premixed conditions may be used, and stratified combustion is a preferable embodiment.

In terms of engine operating conditions, an appropriate compression ratio is greater than 10. A preferable range is from at least 13 to most 20. The UV-enhanced process of the present invention is appropriately used in a compression ignition engine type. Appropriate fuel-oxidiser ratios are from stoichiometric to very lean, with a preferable global lambda of at least 1 and at most 10. A generally appropriate engine speed ratio is from at least 600 to at most 8000 rpm.

In the context of the present invention, it is advantageous for the light sources to be small enough to fit into the cylinder walls of an engine in an automobile, such as a passenger car. Deep-UV LEDs are available which do satisfy the size characteristics. For example, systems with cross-sectional widths / diameters of 1 cm or less are available. Multi-emitter packaged deep UV LEDs are commercially available which are still small enough to be mounted in the cylinder walls of a passenger car, for example in a cylindrical format where the diameter of the package is 8.5 mm. With such sizes, several of the devices may be mounted around the cylinder. As an example, the use of six such multi- emitter LEDs may result in more than 10¹⁴ photons per 1 ms pulse. In the present invention, one light source can be used for imparting optical energy to the fuel-oxidiser mixture and/or reaction products therefrom, or two or more light sources can be used. It is of importance in the present invention to be able to introduce light with potentially different properties (power/mean wavelength/wavelength distribution), at different locations in the combustion chamber. In order to achieve this, multiple light sources may be used, or alternatively a single one with corresponding optics in order to achieve the desired light distribution. In one preferred embodiment of the present invention, multiple light sources are used. Light with different properties, as regards power, mean wavelength and/or wavelength distribution may be provided at different locations in the combustion chamber.

Concerning the spatial distribution of light sources, it is useful in the present invention to be able to access different locations in the combustion chamber. It is potentially possible to use different types of distributions, such as planar, linear, conical, cylindrical etc. As an alternative to direct incorporation of light sources, such as LEDs, in the walls of a cylinder, UV light emitted by the light sources may be guided to each cylinder by optics such as optical fibres. In all configurations, a single wavelength or more than one wavelength of the radiation may be used.

In terms of the total optical energy imparted, a preferred maximum value would be 1000 mJ per combustion event, more preferably 100 mJ. In a 4- stroke combustion engine, there is one complete thermodynamic cycle (e.g. Otto or Diesel) per cylinder with 2 revolutions of the crankshaft, as shown schematically in Figure 11. The "combustion event" is essentially occurring during the "compression" and "expansion" stages.

Figures 4 to 10 show schematic illustrative and non-limiting embodiments of combustion apparatus according to the present invention. In the first of these illustrative and non-limiting embodiments, Figure 4, only the top of the piston is shown. In the embodiment of Figure 4, the injection method is direct injection, there is one light source per cylinder and one light point per cylinder. No optical fibres are present The reference numerals correspond to the following parts of the apparatus (and are conserved for Figures 5 to 10): 1: internal combustion engine

2: cylinder

3: piston

4: cylinder head

5: combustion chamber

6: oxidiser inlet

7: inlet valve(s)

8: exhaust outlet

9: exhaust valve(s)

10: fuel injector

11: UV light source(s)

12: optical fibres

Figure 5 shows a different embodiment in which the injection is port injection, although here again there is one light source per cylinder and one light point per cylinder, and no optical fibres are present. Figure 6 shows a direct injection embodiment, with more than one light point per cylinder. Figure 7 shows a port injection embodiment with more than one light point per cylinder. Figure 8 shows a close-up top view of an embodiment with a single light source in a cylinder. Figure 9 shows a close-up top view of an embodiment with multiple light sources in a cylinder in order to cover different locations and, optionally, different frequencies. Figure 10 shows an example of a single light source with an optical fibre for guiding the UV light to cylinders.

Within the practice of the present invention, it may be envisaged to combine any features or embodiments which have hereinabove been separately set out and indicated to be advantageous, preferable, appropriate or otherwise generally applicable in the practice of the invention. The present description should be considered to include all such combinations of features or embodiments described herein unless such combinations are said herein to be mutually exclusive or are clearly understood in context to be mutually exclusive.

Examples

The calculation method used in the example below was based on the transported probability density function approach [Pope, Prog. Energy Combust. Sci. 11:119-192, 1985] that has been shown to reproduce direction kinetic effects in turbulent diffusion flames [Lindstedt et al, Flow Turbul. and Combust. 72:407-426, 2004], turbulent premixed flames [Lindstedt and Vaos, Combust. Flame 145:495-511, 2006] and ignition in turbulent flows [Gkagkas and Lindstedt, Proc. Combust. Inst. 31:1559-1566, 2007; Combust. Theory Model. 13:607-643, 2009] including the influence of the chemistry of peroxides. The applied chemistry is based on a dimensionally reduced form that accounts for the small hydrocarbon chemistry [Lindstedt and Meyer, Proc. Combust. Inst. 29:1395-1402, 2002] with the higher hydrocarbon chemistry based on a characteristic site argument following the work of Curran et al. [Combust. Flame 114:149-177, 1998] for n-heptane and Curran et al. [Combust. Flame 129:253-280, 2002] for iso-octane. The resulting mechanism reproduces ignition delay times with good accuracy as exemplified in Figure 1.

The calculation procedure used features several adaptations in order to represent the "compression" and "ignition/expansion" phases of an internal combustion engine operation with reasonable accuracy. Specifically, it calculates the combustion chamber conditions from the moment the inlet valves close (IVC) and compression starts, to the point that the exhaust valves open (EVO) after the expansion of the combusted fuel-oxidiser mixture. Thus the fuel/oxidiser mixture is compressed according to the piston motion, heats up due to compression and auto-ignites. Heat losses to cylinder walls are also taken into account.

Using this calculation procedure, a reaction kinetics study was carried out in order to determine the effect of irradiation during combustion. Fully premixed, homogeneous conditions were assumed in the model, with a fuel mixture of 65% n-heptane and 35% iso-octane. Only induced photo- dissociation of hydrogen peroxide (H2O2) was simulated in the calculations. The compression ratio (CR) was chosen to be 15. An example of the calculated pressure traces is shown in Figure 13, which shows pressure traces for (curve 1) an experimental engine, with no combustion occurring; (curve 2) the same experimental engine, with combustion; (curve 3) example simulation results for one specific lambda value (3.7) with photo-dissociation of H2O2; (curve 4) example simulation results for one specific lambda value (3.7) without photo- dissociation of H₂0₂. The comparison of curves 3 and 4 shows that induced photo-dissociation of H₂0₂ had a significant impact on the heat release process.

In addition, by varying the relative concentration of fuel and oxidiser, the following results were obtained for combustion efficiency: Table 1

λ (lambda) ratio 3.5 3.8 4.7 5.4

without dissociation 100% 89.5% 80.2% 54.5% with dissociation 100% 100% 91.1% 73.5% Here, "without dissociation" means without simulated photo-dissociation of H₂O₂, and "with dissociation" means with simulated photo-dissociation of H₂0₂. The air-fuel equivalence ratio, λ (lambda), is the ratio of actual Air-Fuel-

Ratio (AFR) to the stoichiometric AFR needed to convert the entire hydrocarbon quantity into CO₂ and H₂0 for a given mixture. At stoichiometric conditions λ= 1.0, for rich mixtures λ < 1.0, and for lean mixtures λ > 1.0.

It was observed that induced photo-dissociation of H₂0₂ had a significant impact on combustion efficiency, leading to more complete conversion of fuel into combustion products for very lean mixtures.

Claims

1. Method of enhanced combustion of hydrocarbon fuel by oxygen in an internal combustion engine wherein:

- both oxidiser and a hydrocarbon fuel are introduced into the combustion chamber of an internal combustion engine;

2. Method according to claim 1, wherein irradiation is carried out at one or more wavelengths of at least 240 nm and at most 295 nm, more preferably of at least 260 nm and at most 290 nm, and still more preferably of at least 270 nm and at most 280 nm.

3. Method according to claim 1 or 2, wherein the irradiation targets one or more of the following molecular species: benzoic acid C6H5COOH, formic acid HCOOH, acetic acid H₃COOH, propionic acid C₂H₅COOH, hydrogen peroxide H₂0₂, formaldehyde H₂CO, and hydroperoxyl radicals H0₂.

4. Method according to any of claims 1 to 3, wherein one or more of benzoic acid C₆H₅COOH, formic acid HCOOH, acetic acid H₃COOH, propionic acid C₂H₅COOH, and hydrogen peroxide H₂0₂ are incorporated as fuel additives.

5. Method according to any of claims 1 to 4, wherein the oxidiser is atmospheric air, or atmospheric air mixed with recirculated exhaust gas (EGR).

6. Method according to any of claims 1 to 5, wherein the combustion chamber is in the form of a cylinder whose upper and lower sides are closed at a cylinder head and a piston can be moved up and down in the cylinder, wherein the oxidiser-fuel mixture is compressed by the piston.

7. Method according to any of claims 1 to 6, wherein the optical energy is introduced in the pre-ignition phase, before TDC (top dead centre).

8. Method according to any of claims 1 to 7, wherein multiple light sources are provided at different locations in the combustion chamber.

9. A combustion apparatus comprising a combustion chamber, a device for compressing a fuel/oxidiser mixture in said combustion chamber, and one or more light sources operatively linked to the combustion chamber so as to be able to impart to the compressed fuel/oxidiser mixture ultraviolet light irradiation at one or more wavelengths greater than 200 nm and less than 310 nm and/or at wavelengths of at least 345 nm and at most 365 nm.

10. The combustion apparatus according to claim 9, wherein the light sources allow for irradiation to be carried out at one or more wavelengths of at least 240 nm and at most 295 nm, more preferably of at least 260 nm and at most 290 nm, and still more preferably of at least 270 nm and at most 280 nm.

11. The combustion apparatus according to claim 9 or 10, wherein the light sources are light emitting diodes (LEDs).

12. The combustion apparatus according to claim 11, wherein the light emitting diodes (LEDs) comprise AIGaN semiconductor.

13. The combustion apparatus according to any of claims 9 to 12, wherein at least one light source is mounted in a wall of the combustion chamber.

14. The combustion apparatus according to any of claims 9 to 13, wherein at least one light source is external to the combustion chamber but is coupled thereto using optics such as optical fibres.

15. The combustion apparatus according to any of claims 9 to 14, wherein the device for compressing said fuel/oxidiser mixture in the combustion chamber is a reciprocating piston.

16. The combustion apparatus according to any of claims 9 to 15, wherein the combustion apparatus is an internal combustion engine.