System for Improving Combustion in an Internal Combustion Engine

This invention pertains generally to apparatus and a method for increasing efficiency and/or decreasing exhaust emissions of an internal combustion engine.

The concern over air pollution and the dwindling of petroleum resources has resulted in legislation which has caused a shift in emphasis from powerful, high compression engines to small, low compression ones. As the degree of pollution which an automobile introduces into the air is measured in parts per mile, a smaller, lower compression engine, burning a leaner mixture (i.e., a higher ratio of air to fuel) can more readily satisfy the pollution requirements.

It is known on the one hand that the level of CO (carbon monoxide) produced by the internal combustion engine decreases as the air-fuel ratio is increased, and continues to decrease beyond the "chemically ideal" ratio of 14.7, and the decrease extends to the "lean limit", i.e., the limit at which flame speed drops to zero and at which the air-fuel mixture does not ordinarily ignite. The production of NO.sub.x (oxides of nitrogen), on the other hand, is most sensitive to the time at which the spark is fired (given in degrees before top dead center, BTDC). The production of NO.sub.x is parts per mile, jumps from approximately 1,000 to 3,000 parts when the spark timing is advanced over a 20.degree. range. In order to reduce carbon monoxide, oxides of nitrogen and also other hydrocarbons, therefore, one must operate the internal combustion engine with an air-fuel ratio lying at the lean end of the scale, and ignite the mixture as close to TDC as possible. The difficulties associated with these conditions are two-fold: firstly, as the mixture is made leaner, it will become increasingly more difficult to ignite with the spark, since the spark constitutes a constant external energy source of approximately 0.1 joule/spark energy capacity, and secondly, the resultant drop in flame speed along with spark timing near TDC will result in late combustion of the mixture and hence reduced efficiency as well as increased discharge of unburnt hydrocarbons through the exhaust. (On the other hand it is known that in order to increase engine efficiency as well as decrease exhaust emissions it is very desirable to ignite and sustain combustion of a lean mixture in an internal combustion engine.)

Among the prior art references are references teaching the utilization of microwave energy to study piston motion and combustion processes in piston-type internal combustion engines. Examples of such prior art references are Merlo U.S. Pat. No. 3,589,177 and Merlo U.S. Pat. No. 3,703,825. These references, however, are concerned with obtaining resonances in an engine cylinder between the engine cylinder head, cylinder wall, and the piston face so that the motion of the piston and the constituents of the cylinder can be analyzed. Since these references have a diagnostic procedure as their object, it will be appreciated that very low power microwave energy is employed in order to not substantially perturb the system under study.

In view of the foregoing it is a principal object of the present invention to provide a system which increases the efficiency, and also reduces the exhaust emissions of an internal combustion engine, which can be installed in existing internal combustion engines, with a minimum of engine modification, and is relatively cheap and easy to manufacture and install, and requires relatively low power in operation.

Other objects are to enhance combustion and increase flame speed in the combustion chambers of internal combustion engines and to provide an improved ignition support system for an internal combustion engine.

Other objects and advantages of the invention will become apparent from the following description of particular preferred embodiments of the invention when read in conjunction with the accompanying drawings.

Briefly, the invention features a system for use with an internal combustion engine having a combustion chamber of predetermined shape, means for producing a combustible mixture therein, and means for igniting the mixture. The system comprises means for generating, and for conducting to the combustion chamber at substantial power levels, electromagnetic energy at an operating frequency, f.sub.o, which (a) is of the order of the plasma frequency of a species of charged particles of the mixture, and (b) excites at least one resonant mode of the combustion chamber continuously during the conduction of energy to the combustion chamber. Preferably, for a combustion chamber that is cylindrical in shape, f.sub.o is such that a cylindrical resonant cavity mode of the type TM.sub.lm0 is continuously excited, whereby resonance can be maintained in said combustion chamber independent of its length; the cylindrical resonant cavity mode is the TM.sub.010 mode; and the internal combustion engine is a piston engine having a plurality of combustion chambers with a moveable piston in each and the means for igniting comprise a spark plug for each chamber, each spark plug also connected to deliver the energy at frequency f.sub. o to its associated combustion chamber.

Other objects, features and advantages of the invention will appear from the description below, taken together with the accompanying drawing which is a generally schematic illustration of a four cylinder piston engine incorporating the features of the present invention.

The present invention is concerned with coupling microwave energy to igniting and/or combusting air-fuel mixtures in internal combustion engines so as to enhance the breakdown processes and to increase the speed of combustion reactions.

In order to more effectively couple microwave energy to the flame plasma (and spark plasma where applicable), it is proposed to maintain high electric fields in the vicinity of the flame plasma. It has been realized that this can be accomplished quite easily by operating at electromagnetic wave frequencies with corresponding wavelengths of the order of, and less than, the dimensions of the combustion chamber, where the chamber is constructed of electrically conductive material.

Typical combustion chamber dimensions lie in the 1 cm to 1 meter range. Frequencies corresponding to this length range lie in the 3 .times. 10.sup.8 Hz to 3 .times. 10.sup.10 Hz range. Hence, since wavelengths should be of the order of and less than the 1 cm to 1 meter range, a practical working frequency range for energy supplied to the combustion chamber is 10.sup.8 Hz to 10.sup.12 Hz.

Another criterion for effective coupling of microwave energy to flame plasmas is based on the realization that a plasma responds differently at different frequencies. Generally speaking, when the angular electron plasma frequency is of the order of (i.e., within one order of magnitude) the electron neutral collision frequency, one obtains optimum coupling of microwave energy to the plasma by operating at a frequency of the order of the plasma frequency. The angular electron plasma frequency is defined by

where N.sub.e and m.sub.e are the electron number density and mass, respectively; e is the electronic charge; and .epsilon..sub.o is the dielectric constant of free space.

According to the present invention, it has been realised that the electron plasma frequency f.sub.p of electrons in hydrocarbon-air flames at atmospheric pressures where f.sub.p = W.sub.p /2.pi., is of the order of 10.sup.10 Hz, a number well in the frequency range that was specified above as being ideal for more effective coupling to flames in engines. Hence, a metallic combustion chamber is an ideal environment for coupling of microwaves to hydrocarbon flame plasmas.

For combustion chambers of arbitrary shape or changing shape, one can optimize coupling of the microwave energy by operating at frequencies with corresponding wavelengths smaller than the chamber dimensions. In this way microwave energy can be radiated out to the flame, and also one or more standing waves, or cavity modes, can be set up which permits the maintenance of continuous high electric fields. Generally speaking, the chamber acts as a storage system of electrical field energy, and an equilibrium is maintained between the microwave power that is absorbed by the flame plasma (and walls) and that which is fed to the chamber by the microwave source. In the chamber, the power stored will be many times that dissipated in the flame plasma (and walls), and is directly related to the Quality Factor (Q) of the chamber, where Q is: ##EQU1##

For combustion chambers with some degree of symmetry, one can attempt to excite one particular cavity mode. This may be advantageous for at least two reasons:

1. It will allow one to predetermine the electric field configuration in the cavity and hence pick that particular mode which optimizes coupling of microwave energy to the flame plasma; and

2. It will allow one to operate at a lower microwave frequency, which may permit using power microwave solid state sources. These are currently more readily available at frequencies below 5.times.10.sup.9 Hz. Microwave solid state sources are typically powered by low voltage DC, such as 12v DC (the standard automobile voltage).

For chambers with cylindrical symmetry (or even merely circular symmetry, such as combustion chambers of jet engines, gas turbines, etc.), one can excite cylindrical transverse magnetic TM.sub.l,m,n modes or transverse electric TE.sub.l,m,n modes, where the subscripts l,m,n denote the number of standing waves (half wavelengths) in the angular direction, radial direction, and axial direction, respectively. The electromagnetic field components associated with these various modes are known to those skilled in microwave engineering. For example, the transverse magnetic mode TM.sub.0m0 has the following non-zero electromagnetic field components: E.sub.z (r), H.theta.(r), where r, .theta., z are the radial, angular and axial position variables, E.sub.z is the axial electric field, H.sub..theta. is the angular magnetic field. E.sub.z (r), H.sub..theta. (r) vary as a function of radius but are constant in the angular and axial directions. The TM.sub.0m0 mode will have (2m-1) half wavelength variations in the radial direction.

The TM.sub.lm0 modes are particularly interesting in that they can be continuously excited and maintained, in a conventional cylindrical piston-type engine with a fixed frequency of electromagnetic energy while the engine is running, since the mode does not depend upon the axial displacement. Only the Q of the combustion chamber will vary significantly with piston position; the resonant frequency for TM.sub.lm0 modes, for practical purposes, remains constant.

As is known, there is a spark plasma associated with the high voltage breakdown fields generated by the spark plugs of a conventional piston-type internal combustion engine. In order to optimize coupling of microwave energy to the spark plasma (as well as flame plasma), one can ground the spark to the piston face when firing occurs near "top dead center" in the piston's cycle. In this way, the larger resonant cavity chamber electric fields (the E.sub.z (r) field) are available and can be dumped into the spark plasma (with obvious lowering of cavity Q) to increase both the spark magnitude and duration. As the microwave-enhanced DC spark dissipates, the resonant field E.sub.z (r) builds up again (cavity Q increases) and microwave energy is transferred to the initial flame plasma to maintain lean mixture flame propagation and increase flame speed.

For illustrative purposes, there is shown in the drawing a schematic illustration of a four-cylinder piston-type internal combustion engine incorporating features of the present invention. Referring now to the drawing, there is shown a high frequency power oscillator or source 10, which may be one of many commercial CW magnetrons. The source 10 may be powered by an automotive power system (not shown). A remotely actuated coaxial relay switch 12 is coupled to the source 10 via coaxial cable 14. A distributor 16 provides the timing for introducing the DC electrical energy into each cylinder.

Coaxial cables 18a-d electrically couple the output of switch 12 with spark plugs 20. (To simplify the drawing only a single spark plug 20, cylinder 22, and piston 23 are shown.) Suitable spark plug designs for receiving, and for conveying to the combustion chamber 22, high frequency energy are described in the above-mentioned U.S. Pat. No. 3,934,566. High voltage DC blocks 24a-d are provided in the coaxial lines 18a-d between the sources 10 and the spark plugs 20 to insure that high voltage does not reach the microwave sources 10, while allowing the microwave energy to propagate with small reflection. The distributor 16, which distributes the DC high voltage to each cylinder, is coupled via coaxial cables 26a-d to cables 18a-d above the spark plugs. Power high frequency filters 28a-d are provided in cables 26a-d between distributor 16 and cables 18a-d to insure that high frequency power does not reach the distributor and the environment, but are chosen to carry without breakdown the high voltage DC. Lines 30a-d couple the switch 12 to the distributor 16, which provides the timing for the operation of switch 12.

Typically, the cylindrical combustion chamber 22 will have dimensions dictated by conventional design criteria for internal combustion engines. For the particular combustion chamber dimensions of any particular engine, the high frequency chosen is one which excites at least one of the resonant cylindrical cavity modes, as discussed above. As also discussed above, if a TM.sub.lm0 mode is to be excited, the movement of the piston 23 will not "de-tune" the cylindrical cavity 22 despite its reciprocating motion which continuously changes the length of the cylindrical cavity. Thus, higher levels of electric field can be maintained within the cavity 22 than would be the case if no resonant mode were being excited. These higher field levels, of course, indicate that more high frequency energy is available in the combustion chamber for coupling to the plasma in the flame front of a combusting air-fuel mixture.

Substantial levels of electric field intensity are important to successful operation of the present invention. Indeed, the microwave energy delivered to the combustion chamber should be at a power level sufficient to enhance combustion reactions. As is explained in greater detail below, it is presently believed that a power level of the order of 100 watts (i.e., the range 10 watts .ltoreq. power level .ltoreq. 1000 watts) is important. This range of power level is derived from the properties of combustion in internal combustion (IC) engines, from the properties of the flame plasma, from the energy requirement to severely perturb the flame front electron plasma, and from the work of H. C. Jaggers and A. von Engel, "The Effect of Electric Fields on the Burning Velocity of Various Flames", Combustion & Flame, 16, 275-285 (1971). As a matter of definition, a severe perturbation of the flame front electron plasma is one that produces a rise in flame front electron temperature .DELTA.T.sub.e equal to or greater than the initial electron temperature T.sub.e.sup.i, where T.sub.e.sup.i is typically 2000.degree. to 4000.degree. K.

The necessity for severely perturbing the electron plasma in order to enhance combustion reactions is based on the realization that only when electron temperatures are raised from the 2000.degree. K. to 4000.degree. K. range to the 5000.degree. K. to 10,000.degree. K. range do in-elastic electron-molecule collisions become important, and the electrons are capable of internally exciting substantial numbers of molecules (at the flame front).

Given this background, one can determine the power levels that are required to severely perturb flame front electrons in IC engines. However, it must be recognized that the required power levels can only be specified within a range, and one can only speak of an order of magnitude power level (order of 100 watts, as will be shown) for the following reasons:

1. The flame front plasma properties, and specifically the plasma frequency (or electron density) of a lean atmospheric hydrocarbon-air laminar flame, is believed to lie in the range 10.sup.9 .ltoreq. f.sub.p .ltoreq. 10.sup.10 Hz and a more precise value has not been determined yet.

2. Given the differing conditions in an IC engine (principally higher pressure and turbulent flame propagation), uncertainty will be introduced in extrapolation of the laminar atmospheric flame parameters to those of an IC engine.

3. The required power levels to enhance combustion will vary depending on the extent to which enhancement is desired. If enhancement of the weak, initial flame front only is desired, then power levels in the range 10 to 100 watts are required, while if enhancement of the entire combustion is required, then power levels in the 100 to 1000 watts will be required (for moderate size combustors as found in IC engines).

The general case can now be worked out. An expression for the increase in electron energy k .multidot. .DELTA.T.sub.e (k is Boltzman's constant) is given by: ##EQU2## V.sub.e is the collision frequency between electrons and neutral particles;

V.sub.ei is the collision frequency between electrons and neutral particle species "i";

r.sub.i is the fractional loss of electron energy per collision when an electron collides with a neutral particle of species "i";

W is the angular operating (microwave) frequency.

Since the electric field is related to the input power P according to:

it follows that the previous expression is the required relation between the resulting heating of the flame front plasma and the input power level P to the flame front.

However, this is only part of the picture, since the electric field that can be maintained in the combustion chamber depends strongly on the electrical properties of the flame front plasma. If, for example, we take the example of a typical automobile combustion chamber, which is excited in the TM.sub.010 mode, then we can write:

where E.sub.o is the electric field at the center of the cylinder (in the region of the initial flame front); d is the height of combustion chamber; and C is the radius of combustion chamber. Substituting for .pi., .epsilon..sub.o, W, and typical values of d, C, this reduces to:

where P is in watts.

The effect of the flame plasma on the electric field is contained in Q. If the plasma is tenuous, Q will be high; if it is dense, Q will be low. To obtain a constant E.sub.o.sup.2, and hence a constant .DELTA.T.sub.e, the power level must be made inversely proportional to Q.

Therefore, on this basis alone, it is apparent that P must be specified within a range of values. Since the parameters r.sub.i, V.sub.ei, V.sub.e are relatively insensitive to the flame properties (assuming an average neutral density Nn), we can substitute typical values for these and write: ##EQU3## V is the cavity (combustion chamber) volume where V.sub.ei, V.sub.e corresponding to a three-atmosphere density is used. ##EQU4## a is the inner radius of flame front; b is the outer radius of flame front

where (b.sup.2 -a.sup.2) is proportional to the flame volume. ##EQU5## If the required power level is defined as that which will produce k .multidot. .DELTA.T.sub.e equal to 2 .multidot. kT.sub.e.sup.i, then we can write: ##EQU6## which is the desired relation.

Although this equation contains features specific to an automobile combustion chamber excited in the TM.sub.010 mode, relatively small errors will be introduced if it is applied to other combustors. We can take this expression as defining, for a typical combustion chamber, the power level necessary to enhance combustion reactions.

As an example, consider:

C = 4 cms

d = 1 cm

b .perspectiveto. a = 1 cm

b - a = 0.1 cm (8)

Before substituting values, we note that the previous result can be written as:

If N.sub.e is expressed in units of electrons/cm.sup.3, then b, d, (b-a) should be expressed in cms.

or in more practical units

where N.sub.e .about. electrons/cm.sup.3

for example, if N.sub.e.perspectiveto. 10.sup.12 /cm.sup.3, T.sub.e.sup.i .perspectiveto. 3,000.degree. K., then:

P = 60 watts

Equations (9) and (10) can be taken as approximate definitions of the power that must be absorbed by the flame front electrons in order to enhance combustion reactions. For automobile engines, (10) suffices, while for other combustors such as jet engines with larger flame fronts, (9) gives the approximate expression.

In order to arrive at a practical range of values, we can take as the range of N.sub.e expected to be encountered in automobile applications as:

which according to (10) leads to:

and if T.sub.e .perspectiveto.1,800.degree. K., then

consistent with the original assumption.

While particular preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawing, other embodiments are within the scope of the invention and the following claims.