US2111601A - Double ignition combustion chamber - Google Patents

Description

March 22, 1938.

H. RABEZZANA ET AL I 2,111,601

' DOUBLE IGNITION COMBUSTION CHAMBER Filed Nov. 5, 1934 2 Sheets-Sheet mm fZe vkm him .54 f/ecioi @zfiezzmzw March 1933- H. RABEZZANA ET AL DOUBLE IGNITION COMBUSTION CHAMBER Filed Nov. 5, 1954 2 Sheets-Sheet 2 2O FLAME TRAVEL-L.

'COMBUSTION TIME E TIME T Q l mnsmmmna a I mmDmmmmn CDMBUSTION TIME -T flap/m7 1121112. & J/ecioi Wnfiezzmm Patented Mar. 22, 193s PATENT OFFICE DOUBLE IGNITION COMBUSTION CHAMBER Hector Rabezzana and Stephen Kalmar, Flint, Mich., assignors to General Motors Corporation, Detroit, Mich, a. corporation of Delaware Application November 5, 1934, Serial No. 151,457

4 Claims. (01.123-191) This invention relates to internal combustion engines equipped with means for igniting at two points each fuel charge regularly introduced into the combustion chamber or chambers during op- 5 eration. More specifically it contemplates the presence in each combustion chamber of two firing devices, such as spark plugs.

i The purpose of the invention is to make possible burning of the fuel charges in such manl her as to realize the highest efficiency of the impulses exerted on the pistons by the expanding gases, thereby deriving from the engine good idling performance, maximum output with minimum detonation tendency, and smoothness of l6 operation.

The invention consists in an engine comprising a cylinder block having one or more cylinder bores, and a cylinder head having one or more valved combustion chambers, equipped with plum) ral ignition devices, such as spark plugs, so spaced one from another and from the chamber walls. and so positioned with respect to the piston and valves as to produce within the chambers during a combustion period. an initial rate of pressure 25 rise, a maximum rate of increase (acceleration) of pressure rise, and a maximum rate of pressure rise, all as predetermined in order to produce a highly efficient application of the pressure of the burning gas upon the pistons and smoothness of 30 operation. I

By cylinder block is meant a casting or other rigid metal structure having one or more than one cylinder bore therein. By cylinder head is meant the structure that closes the end of one or more 35 cylinder bores in a cylinder block, whether formed integral with the cylinder block or as a separate attached part.

In the drawings, which disclose one embodiment of, the invention, Fig. 1 is a section through 40 a cylinder block vand cylinder head. illustrating a combustion chamber equipped with two ignition devices; Fig. 2 is an underside plan view of a i: fragment of the cylinder head shown in Fig. l, I and showingin broken line circles the relative eie zlo'cation of the cylinder and valve ports; Fig. 3

is a chart of. theoretical pressure-time curves comparing a curve derived by an indicator from firing a charge in a combustion chamber" of the form shown by an advantageously placed single 60 spark plug and one derived from firing a charge in I the same chamber by two spark plugs arranged as shown in Fig. 2; Fig. 4 is a chart of two curves indicating percentage of increase of volume of burned gas plotted against percentage of flame .4 travel in a combustion chamber of the form i1- lustrated wherein the charges are fired, respectively, by one and by two spark plugs; Fig. 5 is a chart of two pressure-time curves respectively derived from actual operation of an engine fired by one and by two spark plugs in a combustion 5 chamber of the form illustrated;

In Fig. 1 of the drawings the reference numeral' ll indicates an engine cylinder block having one or more cylinder bores l2, and one or more pistons l4 adapted to reciprocate therein. In the exemplary construction illustrated the block is formed at one side of each cylinder with valved fuel gas inlet and burned gas outlet passages, one of which is indicated at l6, communieating with the combustion space or spaces of the engine.

A cylinder head l8, preferably removably secured to the face of blockll, with a gasket 2! interposed between them, has formed therein one or more relatively voluminous combustion chamber cavities 22, each in communication with a cylinder bore l2 and with the passages l6, which, in the construction illustrated, open into the cavity 22 through the face of the block.

When the piston M is at the end of a com- 25 pression or scavenging stroke as indicated in Fig.

1 the pressure receiving face thereof approaches closely to a portion of the inner surface of the cylinder head that. constitutes the roof of the combustion cavity therein. During the periods when the piston is in the position illustrated the combustion cavity consists of the thin space 221 between the piston and roof portion 228. of the combustion cavity and the deeper communicating space 22 consituting the remainder of said combustion cavity. The complete combustion chamber, it is apparent, is bounded by the floor consisting of the pressure face of the piston, the valves, the face of the block around the valves, by the roof and side walls of cavity 22, the roof 22a. of space 22b and the inner edge of gasket 2|. The combustion chamber illustrated, as seen in Fig. 2, is of greater linear dimension measured inone direction than in thedirection at right angles thereto, the valves and the space 22!: be-

at that side which is most remote from the space 22band are so located that the flame front of a charge ignited by the spark plugs will reach the space 22b last during the progress of combustion. If the combustion space be divided transversely midway of the longitudinal center line 11-11, the spark plugs are in that portion remote from the cylinder and piston; and in the preferred form disclosed herein both spark plugs and valve ports are disposed in a portion of the chamber which is wholly beyond a tangent to the cylinder circumference at the longitudinal center line of the chamber. The points of both spark plugs are shown located on the same side of the longitudinal center line of the chamber and in a straight line oblique to said center line.

In order to aid in understanding the invention,

reference is made to the theoretical pressure-time curves shown in Fig. 3. The curve represented by the solid line is a pressure-time curve obtained by the combustion of a fuel charge in an internal combustion engine fired at one most advantageous point; that in dotted lines indicates the modification obtained by firing at two points as in the illustrations Figs. 1 and 2. The five characteristic components of any pressure-time curve are as follows:

(1) The component indicated by the section extending from A (the point of ignition) to B,

which represents the nearly uniform or slowly increasing pressure rise at the beginning of combustion, and will be called the initial pressure rise.

(2) The component indicated by the section extending from C to D, which, represents that phase of the reaction within the combustion chamber during which the rate of pressure rise becomes fairly uniform for an appreciable duration of time and likewiseattains the maximum rate of pressure rise in the cycle.

(3) The component indicated by the section extending from B to C, representing that period of time during whichthe pressure rise changes from a nearly uniform or slowly increasing initial rate to the fairly uniform maximum rate of rise and the rate of increase of the pressure rise becomes maximum which will be called maximum rate of increase of pressure rise.

(4) The time interval E from ignition to the point where combustion is completed (not always at peak pressure), which will be called the combustion time.

(5) The highest valuein the pressure curve, which will be called maximum pressure" (at F).

Each of the components of the pressure-time curve has a definite effect upon the capacity of the burning charge to apply'torque to the crankshaft.

Maximum pressure and fcombustion time determine jointly the indicated power.

Maximum rate of pressure rise and maximum rate of increase of pressure rise determine jointly the smoothness of the turning effort or torque.

Maximum pressure, maximum rate of pressure rise and combustion time jointly affect, to a certain degree, detonation.

Ideal conditions fulfilling satisfactorily all requirements would be represented by a pressuretime curve having a high "maximum pressure value, short combustion time (giving high in dicated mean-eifective pressure), low combined value of maximum rate of pressure rise and maximum rate of increase of pressure rise (giving smoothness of engine operation); but a relatively high maximum rate of pressure rise (giving minimum detonation).

Ideal conditions cannot be had from a charge in the combustion chamber ignited at only one point, because both high indicated mean effective pressure and minimum detonation require short combustion time and a high maximum rate of pressure rise. smoothness, on the other hand, requires that the combined values of maximum rate and maximum rate of increase of the pressure rise should be low. smoothness therefore can be achieved by keeping the rate of increase (acceleration) of the pressure rise very low, so as to compensate for the high maximum rate of pressure rise which is necessary to secure high output and reduce detonation tendency.

Inspection of Fig. 3 reveals that low rate of increase of pressure rise requires (on the chart) a large radius in the phase represented on the chart by the region including section BC of the pressure-time curve; if the maximum rate of pressure rise remains the same, the radius can increase only if the initial rate of pressure rise is increased. By firing a fuel charge from two points disposed in the valve region of a combustion chamber as shown, the radius (R) of the curve (B-C), representing rate of increase of pressure rise, is increased to radius R, curve (B C) indicating a much lower rate of increase of pressure rise in this region, thus giving greater smoothness of engine operation.

By igniting the fuel charge simultaneously at two points, as shown in Figs. 1 and 2, the initial rate of burning (flame spread) and consequently the initial rate of pressure rise can be increased any amount up to twice the initial rate obtained by iginiting the charge at one point only. For satisfactory results the initial rate only of burning should be increased. Consequently the charge in the combustion chamber should not be ignited at opposite ends, or diametrically opposite portions of the chamber, because in that case not only the initial rate of burning or flame spread would be doubled, but also during the whole reaction the rate would be approximately double the rate of burning when ignited at one point near one end of the chamber.

It is possible to translate the pressure-time characteristics of a combustion chamber into terms of percent of volume of charge burned (V) against percent of flame travel (L). Fig. 4 shows two VL curves, the curve in solid line representing the VL characteristics of a chamber fired at one most advantageous point, and the curve in broken line representing the characteristics of a chamber fired from two points, as in the chamber illustrated. Both curves show that the rate of burnt volume increase per unit of flame travel continuously proceeds until a maximum rate of increase is reached at M in the case of single ignition and at M in the case of dual ignition, as indicated.

The result of this gain in initial rate of burnt volume increase of the V--L curve due to dual ignition, as herein disclosed, upon the PT (pressure-time curve) is indicated in Fig. 5, showing indicator curves derived from actual operation of an engine. The solid curve is derived from an operating engine having a combustion-chamber of the form illustrated with single spark plug in the best position ascertained by tests; the broken line curve is derived from an operating engine having a combustion chamber and dual ignition as described and shown herein. The initial pressure rise (A-B') Fig. 5, is higher for double ignition than the rise (A-B) for single ignition.

The maximum rate of pressure rise for double ignition (C'D) is slightly lower than (I-D, forone another and the chamber wall as to cause maximum volume increase of burnt gas per flame travel (the point M in Fig. 4) to be reached as early as possible in the movement of the flame front. From extensive analysis of many combustion chamber types it hasbeen ascertained that maximum volume increase of burnt gas (point M, Fig. 4) should be attained when the flame front has moved 10 to 25% of its maximum length of travel within the chamber. From the geometry of the flame front propagation it can be determined that maximum volume increase of burnt gas per travel of the flame front occurs when the two flame fronts (propagated from the two ignition points) merge and when at the same time the chamber walls have not substantially cut off flame spread.

Referring again to Fig. 2, the curved broken lines I indicate successive positions of the spherical flame front. These lines are marked for convenience i0, 20, 30 up to Hill in concentric arcs, I00 indicating the position of the flame front at extinction or end of combustion period. Each successive arc proceeding from an ignition point indicates an advance of 10% of the extreme linear'distance to be traveled by the flame front. When the flame, spreading from ignition point 24, has reached the spherical area represented by broken line are it has advanced 20% of its total possible linear advance and the two flame,

fronts spreading from points 24 and 26 have merged. Line 12-11:, Fig. 2, represents 'a plane normal-to a line joining the firing points 24-26 at a point midway between said firing points and dividing the chamber into two parts, the gas on one side of said plane being ignited by firing point 26 and the gas on the other side of said flame being ignited by the firing point 24. At 20% of the flame travel in a chamber of the form illustrated in Fig. 1, a small part of the flame spread, preferably not more than 30%, has been intercepted by the wall of the chamber as indicated at 28.

If the chamber roof is parallel with the chamber floor, as shown in Fig. 1, the projected area of the burnt gas is roughly proportional to its volume. If the roof is not parallel with the floor,

ignitionpoints) corresponding to 10% of maximum flame travel and 20%, is larger than the increase in volume burned between flame fronts corresponding to 20% and 30% of maximum travel. If the flame front intervals were taken as 1% of maximum travel instead of 10% it could be ascertained with reasonable accuracy where maximum volume increase of burnt gas per extent of flame travel occurs. By calculation and trial it has been ascertained that it is possible to arrange two firing points in a chamber of the type disclosed so that when or after the flame fronts merge, and the chamber walls contact with thefiame-front, the rate of volume increase of burnt gas per distance of flame travel is maximum. In the drawings Fig. 1, this ratio is reached when the flame front has traveled approximately 20% of the greatest distance of flame travel.

Having disclosed a preferred form of our invention, explained the principle thereof and the best way now known to us for utilizing it,. what we claim is:

1.*An internal combustion engine comprising a cylinder block having therein a cylinder bore, a cylinder head having therein a combustion chamber communicating with and partly overlying the cylinder bore and partly offset therefrom, there being valved fuel inlet and outlet passages communicating with the offset portion, and ignition means having a total of two firing pointsv disposed entirely within the offset portion of the chamber, a straight line connecting saidfiring points forming an oblique angle with a plane passing between the inlet and outlet passages and perpendicular to a plane normal to the cylinder axis.

2. An internal combustion engine compris- 4 ing a cylinder block having therein a cylinder bore, a cylinder head having therein an elongated combustion chamber communicating with the cylinder bore and having one end portion overlying it and the opposite portion offset therefrom, there being valved fuel inlet and outlet passages communicating with the chamber, and ignition means having a total of two firing points disposed in the oifset end portion of the chamber at the same side 'of the longitudinal center plane of the chamber perpendicular to a plane normal to the cylinder axis, the point of greatest distance from said firing points to the wall of the combustion chamber being in that portion of the wall that terminates the end portion of the combustion chamber that overlies the cylinder bore.

3. An internal combustion engine comprising a cylinder block having therein a cylinder bore, a cylinder head having therein a combustion chamber partly overlying the cylinder bore and partly offset therefrom, there being valved fuel inlet-andoutlet passages communicating with said chamber, and ignition means having a total of two firing points disposed in the offset portion of the chamber, one nearer to the axis of the cylinder bore than the other, and positioned so thatthe two firing points are at the centers of two equal intersecting spherical zones the radii of which are within 10 and 25% of the maximum distance between the firing point nearest the cylinder bore axis and the chamber wall, when, at the same time, thewall of the chamber does not intercept more than 30% of the volume be-- tween said intersecting zones.

4. An internal combustion engine comprising a cylinder block having therein a cylinder bore, a

cylinder head having therein a combustion chamber communicating with the cylinder bore and having a portion overlying it and a portion offset therefrom, there being valved inlet and outlet passages communicating with said chamber and ignition means having a total of two firing points disposed in the ofiset portion of the chamand the chamber wall on the same side of a ber so that the length of each radius of the nonplane normal to a straight line connecting the intersecting spherical surfaces of largest area two firing points at a point midway of said conthat can be generated within the chamber about 'necting line. said two firing points is from 10 to 25% of the. HECTOR RABEZZANA.

maximum distance between either firing point STEPHEN KALMAR.

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