ELECTROSTATIC IGNITION GENERATOR

A. F. KALGANOV, E. N. NEVSTRUEV, V. P. SHCHERBININ

Introduction

Since the appearance of internal-combustion engines, an electric spark has been used as the source for igniting the working mixture. Of the many methods for producing and delivering a spark to the cylinders, two ignition systems have at present become most widespread: battery ignition and high-voltage magneto ignition. Both systems are electromechanical systems and differ only in the methods of supplying the primary low-voltage current. This difference is manifested in the characteristics of the ignition systems, which express the dependence of the high voltage at the system output on the engine speed (Fig. 1).

As the characteristics show, the high voltage does not remain constant as the engine speed changes. In the case of battery ignition, as the engine speed increases the high voltage decreases (curve a), which is a serious obstacle to the use of battery ignition on multi-cylinder and high-speed internal-combustion engines. When operating with a magneto, the high voltage drops sharply at low speeds (curve b), and this greatly worsens the starting capabilities of the engine. Since the speed range extends from starting speeds to 9–11 thousand revolutions per minute [1], it turns out that neither of these systems is fully satisfactory for such a range.

Fig. 1. Operating characteristics of modern ignition systems: a) battery; b) magneto.

Existing ignition systems also have another particularly important drawback. The rise time of the high-voltage pulse obtained by means of an ignition coil is relatively large, amounting to \(10^{-5} \div 10^{-4}\) sec. Under these conditions, and in the presence of the large shunting conductivity of the spark plug, which is formed as a result of carbon deposits and other analogous causes, a large leakage current arises. Part of the pulse energy, during its build-up, is dissipated in the shunting-

with a resistance. At a certain value of the resistance the expenditure of energy is so great that the breakdown voltage is not reached. Misfiring occurs, or even complete cessation of sparking [1, 9].

The deposition of carbon on spark plugs is inevitable. This may occur because of a temporary decrease in the temperature of the insulators, which is significantly promoted by the nature of the combustion process in certain modes of engine operation. When the temperature of the insulator rises, the carbon deposit, as a rule, disappears and the resistance is restored. But for this it is necessary that, during the temporary drop in insulation resistance, spark formation should not cease.

In ignition systems with an induction coil, when the insulation resistance is below 200,000 ohms, breakdown of the spark gap of the plug ceases, there is no combustion in the cylinder, and deposition of carbon on the cooling plug increases. Under these conditions “self-cleaning” of the plug does not occur. In a cylinder with such a plug there is no combustion, which leads to a considerable overconsumption of fuel and may affect the operation of the plugs of other cylinders [9]. There are reports that more than 50% of plug failures occur because of the appearance of deposits and carbon [9, 10].

It is believed that, when an ignition coil is used to ignite the working mixture, an insignificant fraction of all the potential energy stored in the field of the coil is involved [1, 3]. This part of the energy, the so-called “capacitive” part, is determined by the magnitude of the distributed capacitance of the secondary circuit of the coil. The capacitive spark has little energy but a high instantaneous power, sufficient for ignition. The remaining part of the potential energy, which is usually called the “inductive part” of the discharge, is released subsequently in the form of an arc. The arc, possessing a large energy, contributes to strong heating of the plug electrodes, which, along with other factors, may cause premature ignition of the mixture (glow ignition). This phenomenon limits the possibility of increasing the compression ratio and, consequently, the possibility of improving engine efficiency.

The above-noted and a number of other shortcomings make existing ignition systems insufficiently reliable. This compels the search for new, more reliable methods of ignition. The efforts of researchers are mainly directed toward increasing the efficiency of existing systems. These include, for example, the development of an ignition system using semiconductors, the creation of new spark plugs, etc. [11, 12, 13]. Often, some improvements lead to still greater complication and increased cost of the engine electrical equipment and do not eliminate the fundamental shortcomings of existing ignition systems.

In recent years reports have appeared on the use of electrostatic generators as high-voltage sources for igniting the mixture in the cylinders of internal-combustion engines. As follows from the few reports [4, 5, 6, 8], the electrostatic ignition system has proved highly effective. Since detailed reports on this subject, in particular in the domestic literature, are absent, we have developed a design of an electrostatic ignition generator (ESIG) with carrier-conductors.

Operating principle of the generator

The operating principle of the electrostatic ignition generator is as follows [7]. Within the volume formed by four metallic inductors having the shape of segments of a hollow cylinder, two metal rotor plates rotate; they are electrically connected to each other and are arranged diametrically opposite on the insulated body of the rotor (Fig. 2). Each diametrically arranged pair ...

of the arranged inductors is connected to the corresponding pole of the excitation source \(G\). Together with the rotor plates there rotates a metal finger 5 connected to them, which, during one revolution, each time at the moment of greatest capacitance between one of the pairs of inductors and the rotor plates, passes in the immediate vicinity of one of the electrodes 1—4, connected to the spark plugs \(1^1—4^1\). At this moment an electric spark jumps to the electrode and the corresponding spark plug.

Fig. 2. Schematic diagram of the ESGZ generator device.

The process of spark formation is illustrated in Fig. 3, which presents a linear development of the process in two ignition positions and one intermediate position [8].

Fig. 3. Linear development of the generator operating process.

In Fig. 3, \(a\), the mutually connected rotor plates \(B\) are located under the negative inductors \(A\), and finger 5 is opposite electrode 1. At this moment the negative charge induced by the inductors on the rotor plates is led off through the spark gaps \(1—1^1\). Then the rotor plates, on which a positive charge remains, move farther, and their potential increases (Fig. 3, \(b\)). At the instant shown in Fig. 3, \(c\), the rotor plates have the smallest capacitance with respect to the negative inductors and, consequently, the maximum potential; at the same time finger 5 passes by the next system of spark gaps \(2—2^1\). The positively charged rotor plates discharge and now become negative, since at this moment they are located under the positive inductors. Subsequently the process repeats.

The inductors are maintained at the corresponding potentials by means of an auxiliary excitation generator, constituting

a single whole with the ignition generator. In this case the inductors form the stator system of the excitation generator. Excitation is effected by means of four additional rotor plates, connected in pairs and located in the gaps between the main (working) rotor plates. The length of all the rotor plates is the same, whereas the width of the additional plates is less than half the width of the main rotor plates.

The complete diagram of the design and electrical connections of the ignition generator is shown in Fig. 4, a. As can be seen, the additional rotor plates and inductors form an ordinary electrostatic generator with cylindrical-type carrier conductors, operating—

Fig. 4. Diagram of the commutation and electrical connections of the generator plates.

…with self-excitation. The arrangement of the brushes and collector plates is chosen so that one pair of additional rotor plates charges one pair of inductors with electricity of one sign, while the other pair of additional plates charges the second pair of inductors with charges of the opposite sign. The compact arrangement of the rotor plates, which substantially reduces the overall dimensions of the generator as a whole, is due to the fact that the additional rotor plates must carry only the minimal charges that ensure compensation for current leakage from the inductors through the insulation.

In the model we constructed, the generator circuit is somewhat modified. Numerous parallel connections, at different potentials and intersecting one another, have been eliminated (Fig. 4, b). In this case the generator has twice the number of brushes, with 4 of them located at one end of the rotor and the other 4 at the other end. In the figure, the different positions of the brushes and collector plates are indicated by different line styles.

Calculation of the Generator

Like any electrostatic generator, the ignition generator is intended to operate in a compressed-gas medium. This provides the necessary energy in the spark with small generator dimensions.

The principal task of the calculation is to determine the dimensions of the generator needed to provide the corresponding potentials and capacitances.

Initial data: discharge voltage \(U_2 = 30\ \text{kV}\) and spark energy \(W = 0.015\ \text{J}\).

Denoting the potential of the inductors by \(U\), the maximum capacitance of the working rotor plate by \(C\), and taking the width of the rotor plate as equal to 1 (Fig. 5), one may write the equation of the transient process associated with raising the potential of the rotor plate:

Fig. 5. Calculation diagram of the transporter-inductor.

\[ C(1-x)(U_2+U)+Cx(U_2-U)+zCU_2=CU, \tag{1} \]

where \(x\) is the distance traveled by the rotor plate; \(z\) is the coefficient of the parasitic capacitance of the rotor plate to ground; \(CU=q\) is the maximum charge of the system. From (1) we find:

\[ U_2=\frac{2}{1+z}\,Ux. \]

The maximum value of \(U_2\) is reached at \(x=1\). Hence

\[ U_2=\frac{2}{1+z}\,U,\quad \text{or}\quad U=\frac{1+z}{2}\,U_2. \]

The voltage difference in the gap between the rotor and the stator must not exceed the value

\[ U_0 = U_2 + U. \tag{3} \]

Substituting the expression for \(U\) in terms of \(U_2\) from (2) into (3), we have:

\[ U_0=\frac{3+\alpha}{2}\,U_2. \]

Then the gap between the rotor and the stator is determined as

\[ d_1=\frac{U_0}{E}=\frac{3+\alpha}{2}\cdot\frac{U_2}{E}, \tag{4} \]

where \(E\) is the operating field strength in the gaps.

Without predetermining the choice of the working gas, we take the operating field strength to be \(250\ \mathrm{kV/cm}\). Further taking \(\alpha=0.4\) (the experimental value of the parasitic-capacitance coefficient in generators of this type [2]), from (4) we have:

\[ d_1=\frac{3+0.4}{2}\cdot\frac{30\cdot10^3}{250\cdot10^3}\simeq 0.2\ \mathrm{cm}. \]

The thickness of the generator plates \(d_2\) must not be less than the gap. We take \(d_2=3\ \mathrm{mm}\). The use of plates of smaller thickness would make it difficult to machine them in order to obtain the required profile of the frontal parts.

The constructed generator is enclosed in a steel cylindrical casing with an internal diameter \(D=82\ \mathrm{mm}\). The gap between the casing and the stator is \(3\ \mathrm{mm}\). Then the outside diameter of the rotor is \(D_p=66\ \mathrm{mm}\).

The energy carried by the conveyor is

\[ W=qU_2=CUU_2=C\frac{1+\alpha}{2}U_2^2=0.7CU_2^2, \]

whence the capacitance belonging to the two working plates of the rotor must be equal to

\[ C=\frac{W}{0.7U_2^2}=\frac{0.015}{0.7(30)^2\cdot10^6}=23.8\cdot10^{-12}\ \mathrm{F}=23.8\ \mathrm{pF}. \]

Under these conditions \((D_p=66\ \mathrm{mm},\ d_1=2\ \mathrm{mm},\ C=23.8\ \mathrm{pF})\), the length of the rotor plates is \(60\ \mathrm{mm}\).

Let us now consider the operation of the excitation generator. The connection of the additional plates to the high-voltage brushes must occur at the moment when the potential of the plates becomes equal to the potential of the inductors, which have equal magnitudes but opposite signs:

\[ U=\frac{1+\alpha}{2}U_2=\frac{1.4}{2}\cdot30=21\ \mathrm{kV}. \]

Returning again to Fig. 5 and writing the equation of the transient process for the present case, analogously to equation (1), we find the displacement \(x\) at which all potentials become equalized:

\[ x=\frac{1+\alpha}{2}. \]

Assuming, as before, \(\alpha = 0.4\), we have: \(x = 0.7\), in accordance with which the arrangement of the high-voltage brushes is determined, taking into account the arrangement of the collector plates, whose angular dimension is approximately half the angular dimension of the additional rotor plates. The grounded brushes are arranged so that the additional plates “break away” from them at the moment of their maximum capacitance with the inductors.

To maintain the specified values of the inductor potentials, a current must be diverted from them, the value of which is determined by the formula [2]:

\[ I = 2n(1-\alpha)C_1 U. \]

Taking the rotational speed of the rotor to be \(n = 2100\) rpm \(= 35\) rps, we have

\[ I = 2 \cdot 35(1-0.4)\cdot 4.10^{-12}\cdot 21\cdot 10^3 = 3.6\cdot 10^{-6}\ \mathrm{a} = 3.6\ \mu\mathrm{a}. \]

Here \(C_1 = 4 \cdot 10^{-12}\ \mathrm{F}\) is the maximum capacitance of the additional rotor plate relative to the inductor. In this case the average power of the excitation generator is determined as

\[ P_v = 4UI = 4 \cdot 21\cdot 10^3 \cdot 3.6\cdot 10^{-6} = 0.3\ \mathrm{W}. \]

As can be seen, in the case of an electrostatic ignition generator the power of the excitation source is negligible.

The power of the ignition generator proper is

\[ P_g = 4Wn = 4 \cdot 0.015 \cdot 35 = 2.1\ \mathrm{W}. \]

Overall, the generator must develop a useful power

\[ P = P_v + P_g = 0.3 + 2.1 = 2.4\ \mathrm{W}. \]

A significant problem is the removal of charges from the inductors. If the presence of the corresponding ohmic conductivity is provided, this will lead to an increase in the initial excitation time of the generator, which is irrational in the present case. To remove the charges we used the corona-discharge phenomenon; for this purpose, through the wall of the casing opposite the inductors there are passed corona points, whose position relative to the inductors can be adjusted without impairing the tightness of the system.

Structural Design of the Generator

The external appearance of the generator without the casing is shown in Fig. 6. All generator plates are made of duralumin. The inductors, which exceed the rotor plates in length, are fastened on two flanges made of insulating material. The rotor shaft, mounted in bearings installed in the flanges, is insulated from ground. It is connected to the working plates of the rotor and carries the contact finger of the distributor. The generator casing has two removable covers. Through one of them, made of insulating material, pass four electrode projections, connected externally by a “magneto” wire to the spark plugs. Through the other cover passes the drive shaft, connected to the rotor shaft by means of a cross-shaped dielectric coupling designed for full insulation of the output voltage taken from the rotor shaft through the contact finger.

The internal cavity of the generator is filled with gas at a pressure of \(20\ \mathrm{kg/cm^2}\). In our model, as the gas dielectric we used

air. The lower cover has a special seal for the rotor drive shaft, preventing gas from leaking out of the housing.

The generator described above was made for operation with a four-cylinder engine. The lower cover of the housing and the rotor drive shaft are designed with provision for installing it in place of the R-23 breaker-distributor column of the M-21 “Volga” engine.

When the ESGZ generator is installed on the engine, there is no need for all the devices inherent in a battery ignition system.

Fig. 6. External view of the generator (without casing).

Testing of the Generator

The test was carried out on a test stand that made it possible to vary smoothly the speed of the generator shaft within the range of 50–3000 rpm.

The first tests were conducted in order to determine the excitation time of the generator. Here, the excitation time is understood as the time that elapsed from the beginning of rotation of the rotor to the moment when a breakdown voltage of 20 kV was obtained at the arrester.

A record made with a loop oscillograph showed that the excitation time of the generator is determined by a certain number of revolutions (starting revolutions), depending on the rotational speed of the rotor during this period. Low rotational speeds (up to 20 rpm) were produced by turning the shaft by hand. In this case breakdown of the spark gap of the arrester occurred after 5 revolutions of the rotor. Since the shaft rotated slowly, this number of revolutions could easily be counted. At higher rotational speeds the generator developed the maximum voltage already after the second revolution. This indicates that the “starting” number of revolutions of the generator under the most unfavorable operating conditions (engine starting) will not affect the operation of the latter, since from the beginning of cranking until the working mixture enters the cylinders the engine makes many more revolutions than are necessary to excite the generator.

The diagram characterizing the dependence, established as a result of bench tests, of the voltage developed by the generator,

on the speed of rotation of the rotor (Fig. 7), was constructed from data obtained when the generator was operated with a standard three-electrode spark gap.

Unlike the characteristics of battery and magneto ignition (Fig. 1), the voltage developed by the ESTZ generator remains practically constant over a wide range of rotational speeds. This property of the generator follows from the principle of its operation. Indeed, as was shown earlier, the generator voltage is determined by the capacitance between the inductors and the transporters and by the field strength between them. For a given gaseous dielectric both of these quantities are constant. Therefore, with sufficiently high quality of insulation and constancy of the charges on the inductors, the maximum value of the pulse voltage should not depend on the speed of rotation of the rotor.

Fig. 7. Operating characteristic of the generator.

Returning to the operating characteristic of the generator, we note that the decrease in voltage becomes noticeable at 10–15 rpm, i.e., this decrease occurs at speeds lower than the minimum starting speeds of gasoline engines, determined by carburetion, and for most engines amounting to 40–50 rpm [1]. Therefore, the decrease in generator voltage at very low speeds will not affect the starting properties of the engine.

At elevated speeds the relative leakage of charges from the inductors decreases, and the quantity of charges on the inductors rapidly increases. The resulting increase in the field voltage of the inductors leads to breakdown inside the generator. After breakdown, the potential of the discharged inductor decreases so much that sparking at the corresponding spark gaps may cease for a certain time necessary for re-excitation of the inductor.

To eliminate this phenomenon, by means of the provided corona system, the corresponding adjustment was made. When the generator was operating at maximum speed, the distance between the inductor and the point of the needle was varied. The distance was considered optimal when breakdowns inside the generator ceased, which was monitored through the transparent upper cover. After such adjustment, the excess charge from the inductors flowed off along the corona points to ground, and their potential did not rise above a certain value. Confirmation of this is the horizontal course of the generator operating characteristic.

The installation of the above-described adjustable stabilizing device was required only as a temporary measure necessary for establishing the optimum dimensions of the excitation plates, since

In calculating a generator intended for operation over a wide range of rotation speeds, it was difficult to take into account the influence of parasitic capacitances and charge leakage through the insulation. It should not be considered a necessary accessory of the generator, since in the production model regulation will not be required, and the necessary leakage can easily be created inside the generator.

The most important feature, in our view, of the electrostatic ignition generator is its ability to operate without interruption with spark plugs that have a very large current leakage through the insulation. Spark plugs that have failed because of the appearance on the insulator of a conductive layer of carbon deposit, moisture, or deposits of lead salts prove to be serviceable with the ESG.

Modern ignition systems ensure uninterrupted operation of the spark plug when the resistance of its insulation drops no lower than 200,000 ohms, whereas the ESG generator steadily produces a spark with a shunting resistance a thousand times smaller.

Figure 8 gives the operating characteristics of battery ignition with a B-15 coil and of the electrostatic generator for three values of the shunting resistance.

Fig. 8. Operating characteristics of battery and electrostatic ignition when operating at various \(R_{\mathrm{sh}}\).

From the graphs it is evident that, in the presence of a shunting resistance even of 500 ohms, the ignition system with the ESG will operate reliably.

This phenomenon is explained by the form and duration of the electrical pulse produced by the generator.

In contrast to the pulse of the ignition coil, the voltage pulse of the ESG generator has a very short duration with a very steep front¹.

As a result, the maximum voltage created by the generator is applied to the spark gap almost instantaneously, which ensures—

ensures normal operation of the spark plug even in the presence of high conductivity through the insulation.

This is the enormous advantage of the ESGZ over all ignition systems containing inductance. This advantage may prove very important in solving, for example, the problem of the glow plug, which, with each passing year, in connection with the boosting of engines, is becoming ever more urgent.

In the operation of internal-combustion engines, the accuracy of spark delivery to the cylinders is very important. This determines the uniformity of operation of all cylinders and detonation phenomena in the engine.

To determine the magnitude of spark scatter with respect to the angle of rotation of the rotor, a rotating spark gap was used, which made it possible, during generator operation, to observe on a degree scale the instants at which sparks jumped.

The angular distribution of sparks among the cylinders was determined mainly by the accuracy of the placement of the electrodes in the generator cover and occurred after \(90^\circ\) of rotation of the rotor shaft. The synchronism pattern of each of the sparks was somewhat poorer. The scatter field was different for each of the four sparks. For one spark it fit within \(1^\circ\); for the second spark the drift exceeded \(4^\circ\), while the other two had an intermediate value. Upon inspection of the fixed electrodes of the upper cover and of the rotating electrode (the distributor), the cause of this became obvious. It turned out that, in manufacturing, deviations from the spherical shape had been allowed in the surfaces of the electrodes. These deviations produced strong distortions of the electric field between them, which led to either delayed or premature breakdown of the spark gap. By correcting the shape of the electrode ends, it was possible to reduce the scatter field of all the sparks to \(1^\circ\). More careful manufacture of the electrodes between which the spark must jump (inside the generator), and spacing them over as large a diameter as possible, will make it possible to obtain a spark-scatter field considerably smaller than in modern ignition systems.

The presence of air under pressure in the ESGZ housing should be regarded as a certain drawback, since this requires reliable sealing of the rotor drive shaft. However, even the first seal used, consisting of self-sealing rubber rings, made it possible to maintain constant pressure in the generator housing for several weeks of its operation.

According to available data, seals in similar generators operate quite effectively for a long time [6]. Therefore, the presence of gas under pressure does not make operation of the ESGZ at all difficult and does not introduce an element of unreliability into its operation.

Conclusion

The creation and investigation of a prototype electrostatic ignition generator make it possible to draw certain conclusions. Only the principal conclusions, which fundamentally distinguish electrostatic ignition from existing systems, are given below.

1. The generator produces a spark of purely capacitive origin, of high instantaneous power, capable of igniting a gasoline–air mixture. The short duration of the spark discharge eliminates heating of the spark-plug electrodes, thereby not promoting their erosion and reducing the possibility of glow ignition.

2. The complete autonomy of the generator makes it independent of malfunctions of other units of the electrical-equipment system, such as

...storage battery, generator, interrupter, etc. This, in turn, increases the reliability of engine operation as a whole.

1. The horizontal course of the generator’s operating characteristic makes the electrostatic ignition system universal. This universality has very broad limits, since the high voltage remains stable at any speed, and also does not depend on the medium in which the generator is located.

2. The electrostatic ignition system operates reliably under substantial current leakage due to failure of the spark-plug insulation. This will make it possible to increase the reliability of highly supercharged engines and to reduce malfunctions in the operation of ordinary engines associated with temporary failure of spark-plug insulation.

REFERENCES

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3. D. Morgan. Principles of Ignition. Mashgiz, 1960.

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5. N. Felici, M. Point. Journal de la Societe des ingenieurs de L’Automobile. Oct., 1950.

6. N. Felici. Electrostatische Hochspannungs Generatoren. G. Braun, Karlsruhe, 1957.

7. U. Neubert. ETZ—B, No. 6, 199, 1954.

8. U. Neubert. Electrostatic in der Technik, München, 1954.

9. Julien H. P., Neblett R. F. Spark—plug misfiring—mechanism studies. SAE Annual Meeting, Preprints, s. a., No. 1231.

10. Lenz L. R., Nostrant R. F., Graver R. T. Some problems of spark plug operation in two—cycle engines. SAE Annual Meeting, Preprints, s. a., No. 123v.

11. Spaulding G. E. Transistor switched ignition systems. SAE Annual Meeting, Preprints, s. a., No. 132A.

12. Was sind Mehrstoff—Electroden? Kraftfahrzeug—Kurier, No. 1, 1960.

13. Gouler G. A. Improvements is and relating to multy—cylinder engine ignition systems. Patent No. 798455.

1. The duration of the voltage pulse of the ESG was measured by means of an OK-19M oscilloscope and proved to be \(2.5 \cdot 10^{-9}\) sec. ↩