US3064417A - Exhaust systems for gas producing units - Google Patents

Description

' Nov. 20, 1962 D, w, TRYHORN HAL 3,064,417

EXHAUST SYSTEMS FOR GAS PRODUCING UNITS Filed June 20, 1958 '7 Shee'tsSheet 1 .IlZ/U enters DJsZTr hqpzz/ @1176, 5nd

Nov. 20, 1962 D w TRYHORN EI'AL 3,064,417

EXHAUST SYSTEMS FOR GAS PRODUCING UNITS 7 Sheets-Sheet 2 Filed June 20, 1958 Nov. 20, 1962 D. w. TRYHORN ETAL 3,064,417

EXHAUST SYSTEMS FOR GAS PRODUCING UNITS '7 Sheets-Sheet 3 Filed June 2O, 1958 Nov. 20, 1962 D. w. TRYHORN El'AL 3,064,417

EXHAUST SYSTEMS FOR GAS PRODUCING UNITS 7 Sheets-Sheet 4 Filed June 20, 1958 Fig.9.

Nov. 20, 1962 D. w. TRYHORN EI'AL 3,064,417

EXHAUST SYSTEMS FOR GAS PRODUCING UNITS Filed June 20, 1958 7 Sheets-Sheet 5 Nov. 20, 1962 D w TRYHORN EI'AL 3,

ExHAusT SYSTEMS FOR GAS PRODUCING UNITS 7 Sheets-Sheet 6 Filed June 20, 1958 M Qz'e fi z i w- Nov. 20, 1962 D. w. TRYHORN EI'AL 3,064,417

EJG-LAUST SYSTEMS FOR GAS PRODUCING UNITS 7 Sheets-$heet 7 Filed June 20, 1958 United States Patent EXHAUST SYSTEMS FOR GAS PRODUJENG UNITS Donald Wilfred Ti'yhorn, Chalfout St. Peters, and James John Stewart Smith, Maidenhead, England, assignors to Sir W. G. Armstrong Whitworth & Company (Engineers) Limited, London, England Filed June 20, 1958, Ser. No. 743,256 Claims priority, application Great Britain July 2, 1957 6 Claims. (Ci. 60-13) The invention relates to a gas producing unit of the type in which combustion takes place in an enclosed space and the gaseous products of combustion are periodically discharged at high temperature and under pulsating pressures to drive a turbine or other machine operated by the expansion of the said gaseou products. Included in such types of units are two-stroke cycle internal combustion engines provided with an exhaust gas driven turbo-charger, and gas generators of the free piston or crankshaft type. More particularly, the invention relates to improvements in the exhaust systems of such units.

The conditions of the gas supplied to a turbine affect its efficiency of operation, because it is designed to work with an expansion ratio which is substantially constant. Thus the potential energy in a pulsating gas flow can be converted into mechanical energy in the turbine shaft, with only relatively low efliciency. Nevertheless, for a given gas supply and suitable turbine, it is possible for a pulsating gas flow in a known exhaust system comprising a short duct between the point of gas supply and the tur bine to be more etficient than a substantially constant gas flow in another known exhaust system comprising an expansion chamber between the same two points, but the reason is that the high utilisable energy, and low turbine efiiciency, of the former case, are more effective than the lower utilisable energy and relatively higher turbine efiiciency of the latter case. The inefficiency of a turbocharger system at any unit or engine performance rating is shown by the turbine inlet temperature which is required to make it operate satisfactorily. The lower the efiiciency, the higher the exhaust temperature must be for satisfactory operation, and therefore the nearer the engine is to its maximum performance rating. In other words, improved methods of operation which enable a given performance rating to be obtained with a lower exhaust temperature are very advantageous, and permit an increase in the performance rating of the engine.

An object of the invention is to harness a high proportion of the energy of the pulsating gas flow and to deliver the gases to the turbine at more nearly constant pressure. The efficiency of a turbine increases when, for the delivery of the same total energy, the frequency of the pulsations increases and the pressure of their peaks decreases, relative to the frequency and pressure of the initial pulses from the gas producing unit.

Thus it is advantageous to arrange that each pulse of exhaust gases entering the system is divided with a minimum loss of total energy, and that the distances to be travelled by the different portions of the pulse, are such that the latter reach the turbine at different times in the gaseous cycle of the unit.

In such units the enclosed space or cylinder in which combustion takes place, requires to be scavenged and provided with fresh charge during each cycle of operation. With a known short exhaust duct system, the initial pressure pulse fills the duct for a large portion of the scavenging and charging period, thus leaving only a short time for the latter actions and therefore a high pressure difference between the scavenging medium and the exhaust gases is required. When an expansion chamber incorporated in the exhaust duct of another known system the scavenge pressure is low but the utilisable energy of the 3,064,417 Patented Nov. 20, 1952 ice turbine is also low. With both these exhaust systems it is only when the engine load is high and the temperature and pressure of the exhaust gases are high, that the turbine is capable of converting the energy in the exhaust gases into mechanical energy with a efiiciency high enough to provide sufiicient power to enable the compressors to supply the necessary quantity and pressure of the scavenging and charging medium. At all lower loads the attainment of the desired conditions requires the assistance of an engine driven or separately driven compressor. It is desirable that the scavenging and charging should be carried out with a minimum expenditure of energy and therefore it is advantageous to arrange for a low pressure to be present in the exhaust system adjacent the exhaust outlet from the cylinder during the scavenging and charging period. In this way, the necessary pressure difference between the scavenging medium and the exhaust gases can be obtained with a low pressure of the former. It will be appreciated that direct employment of the exhaust energy, to give the desired low pressure in the cylinder and duct, by means of waves of rarefaction, ensures that the energy is applied over a short interval of time, and thus it is more effective than passing a slightly higher pressure pulse to turbine, and the turbine and compressor converting this energy into a slightly higher scavenge pressure, which would be spread over a long interval of time.

As already described it is necessary at this time to have a relatively high pressure of exhaust gases in the said exhaust system adjacent to the turbine, to drive the latter. Preferably the pressure in the exhaust system adjacent the exhaust outlet from the cylinder should increase towards the end of the charging period, to reduce the quantity of charge escaping to the exhaust system, and to raise the pressure within the cylinder.

For example, in a two-stroke cycle internal combustion engine, in which the inlet and exhaust orifices are open for a large portion of the scavenging and charging period, and in which a turbine, driven by the exhaust gases from the engine, provides the scavenging, charging and supercharging medium, an exhaust system arranged in accordance with the above described requirements, enables an engine to give a high output, with low exhaust temperature and fuel consumping, and enables the said medium to be provided in the required quantities and pressures to suit all loads and speeds of the engine. Thus an additional engine driven or separately driven compressor is not essential, but one may be used to assist starting or where very high engine acceleration is required, and in. such cases the advantages of the system described are not lost, since the power required to drive said additional compressor is much less than with the known systems.

To obtain the desired effects, the increases or decreases in the cross sectional area of the exhaust system, which cause positive or negative reflections of the gaseous pressure pulses, must be restricted by careful design of sizes and lengths, to those which are necessary to give the required pressure distribution throughout the exhaust system for the complete scavenging and charging period. In a normal exhaust system, reflections which are not advantageous to the gaseous cycle frequently occur at junctions of pipes of different cross-sectional area, or where pipes join or leave a manifold, such junctions and manifold being arranged to give a simple and conventional exhaust system, without proper regard to their effect upon the pulses of pressure traversing the ducts.

An exhaust system complying with the invention and with the above described requirements has a first duct of a cross-sectional area substantially equal to the area of the exhaust orifice, and this area is constant for a predetermined length. At the end of this length of duct the exhaust pulse is divided into partial puises, namely, a primary and a secondary pulse, at the junction of one or more pipes, hereinafter called wave pipes. The expansion of the said gaseous products occurring at the junction initiates a wave of rarefaction which travels back to the exhaust orifices and causes a low pressure thereat, during a substantial part of the scavenge period. The magnitude and duration of the low pressure phase is controlled by the arrangements for expansion of the said gaseous products at the junction. If the combined crosssectional areas of the pipes into which the pulse is divided is approximately equal to that of the first duct, little expansion will occur, but if, say, two pipes are of the same cross-sectional area as the first duct, considerable expansion will occur, and a satisfactory wave of rarefaction will be propagated back to the cylinder. In most cases an abrupt change of cross-sectional area will be satisafctory, but the transition from one cross-sectional area to the next may be made more gradual by the tapering of the ducts, so that the reflections therefrom will be increased in duration and decreased in amplitude. An increase in the amplitude of the rarefaction Wave will be accompanied by a decrease in the ampitude of the partial pulses employed to drive the turbine, and as will be understood, it is essential to preserve a balance between these two requirements. At the same time a partial pulse travels outwards along the second duct from the junction to the turbine to drive the latter, designated as the primary pulse, and a partial pulse designated as a secondary pulse, along the wave pipe. The magnitude of the pressure pulse in each duct is reduced in proportion to the cross-section area of the ducts and pipes into which it is divided. The portion of pulse in the wave pipe may travel to the turbine along a longer path than the second duct, so that it arrives at the turbine after the partial pulse which travelled along the second duct has substantially finished employing its energy in driving the turbine. Alternatively, the partial pulse in the Wave pipe may be reflected at the closed end of said pipe without change of sign, so that a positive pulse travels back along the wave pipe to the junction, where it is divided, and a portion travels along the second duct to drive the turbine while another portion travels back to the cylinder and raises the pressure therein towards the end of the charging period. Moreover, as the turbine nozzles operate as a partially closed end and offer restriction to flow, some of the energy of all pulses reaching the turbine is reflected back to the cylinder and to the wave pipes.

A first duct of substantial length as herein proposed has a further advantageous eifect, in that the cool scavenging medium passing into the duct remains adjacent the exhaust orifice until the latter re-opens on the next cycle. This action is particularly beneficial when a valve is employed to control the exhaust orifice, because the presence of the cool scavenging medium for a large portion of the cycle, reduces the temperature of the exhaust valve.

In this way, a combination is achieved between the advantages of the known short duct, and expansion chamber systems, and the overall advantages and efliciency are greater than with either of the known systems.

The invention consists in a gas producing unit of the type which includes a combustion chamber from which the gaseous products of combustion are periodically discharged at high temperature and under pressure, thus producing pulses of gases, the expansion of which is employed to drive a machine and in which the movement of said pulses of gases in their passage from said unit to said machine are controlled in such manner that their frequency is increased and their amplitude is decreased, relative to the frequency and amplitude of the original pulses of gases produced by the discharge of gases from said chamber.

The invention also consists in that the said machine provides the medium for scavenging and charging the said unit, and that during a substantial part of the scavenging period a low pressure occurs adjacent the exhaust orifices of the combustion chamber to facilitate scavenging and a high pressure adjacent the machine to drive the same.

The invention still further consists in that the control of the exhaust pulses is achieved by providing an exhaust system between said combustion chamber and said machine, so arranged that each original pulse of pressure gases produced on exit from said chamber is divided into portions which are constrained to travel different distances to reach the said machine, so that they arrive there at difierent times in the cycle of the said unit. The divisions of the pulse may occur at a junction of at least two ducts forming the exhaust system and the latter may comprise a first duct leading from the exhaust orifices of the combustion chamber to the said junction, a second duct from the junction to the said machine, and at least one wave pipe which leads from the junction to a closed end, or to the machine by a longer route than the second duct. In a multi-cylinder unit, the said wave pipe for one cylinder may be formed by the first duct of one or more other cylinders.

According to the invention the machine may be mounted close to the combustion chamber and at the end of very short first and second ducts, while the wave pipe extends beyond the machine.

According to another feature of the invention the wave pipe may be formed by a pipe of larger cross-sectional area than the first or second ducts and may surround said first or second duct.

Two or more ducts may be formed by arranging helical passages within a single casing.

The invention will now be described, by way of example, only, with reference to the accompanying drawings, in which:

FIGURE 1 shows a diagrammatic representation of a simple exhaust system,

FIGURE 2 indicates the pressure variations close to the exhaust orifices and to the turbine in the exhaust system of FIG. 1.

FIGURE 3 shows a diagrammatic representation of a modified form of exhaust system.

FIGURE 4 indicates the pressure variations close to the exhaust orifice and to the turbine in the exhaust system of FIG. 3. 7 FIGURE 5 shows a diagrammatic represenation of a known short duct exhaust system.

FIGURE 6 indicates the pressure variations close to the exhaust orifice and to the turbines in the exhaust system of FIGURE 5.

FIGURE 7 shows a diagrammatic representation of a known exhaust system containing an expansion chamber.

FIGURE 8 indicates the pressure variations close to the exhaust orifice and to the turbine in the system of FIG. 7.

FIGURE 9 shows a diagrammatic representation of another modified form of exhaust system.

FIGURE 10 indicates the pressure Variations close to the exhaust orifice and to the turbine in the system of FIG. 9.

FIGURE 11 shows a form of wave pipe system suitable for a 3-cylinder engine.

FIGURE 12 indicates the pressure variations close to the exhaust orifice and to the turbine in the system of FIGURE 11.

FIGURE 13 shows a diagrammatic representation of. another form of wave pipe system suitable for a 3-cy1- inder engine and having a modified arrangement of wave pipe.

FIGURE 14 indicates the pressure variations close to the exhaust orifice and to the turbine in the system of FIG. 13.

FIGURE 15 shows a diagrammatic representation of a further form of wave pipe system suitable for a 3-cylin der engine and having a further modified arrangement of wave duct.

FIGURE 16 shows a side elevation of an arrangement of a six-cylinder engine and a suitable exhaust system in accordance with the invention.

FIGURE 17 shows a plan of the engine and exhaust system of FIGURE 16.

The invention may be applied to a supercharged twostroke cycle internal combustion engine and the general arrangement of the engine may be substantially in accordance with any known type, in which the fresh charge is provided by a compressor driven by an exhaust gas turbine, which may also be of any known types. The normal arrangements for conveying the fresh charge from the compressor to the cylinders may be employed. The total quantity of fresh charge entering each cylinder should be sufficient to scavenge, charge, and supercharge the cylinder to the required pressure, and to provide the excess quantity which passes to the exhaust system and remains therein at the end of the charging period, thus taking no part in the ensuing combustion process.

Exhaust systems will now be described with reference to diagrammatic arrangements of the exhaust system and to pressure time diagrams showing the sequence of the variation in pressure in the exhaust orifice and to the machine, throughout the exhaust, scavenging and charging periods. Only the major fluctuations in pressure are shown, since it is these which differentiate the processes in the various exhaust systems.

In all the figures illustrating this invention, the pressure diagrams are drawn in line with the position at which the pressure records have been taken, so that the change of cross-sectional area from which reflection has taken place can :be identified. Within the limits of the diagram matic representations, they are distance/time diagrams and the slope of the lines joining equivalent occurrences, are a measure of the velocity at which the pulse or wave moved along the duct. The size of the initial pulse will vary with the type and output of the engine, and the relative sizes of the reflected pulses Will vary with the form of the exhaust system. It is well understood that the length of the pressure pulse hereinafter mentioned relates to the length of duct of the chosen cross-sectional area which will contain the gases forming the pulse, and the method of calculating this length is well known. A check on the length of the pulse is readily made on an operating engine, and the behavior of pressure pulses, sometimes called positive waves, and of waves of rarefraction, sometimes called negative waves, under the conditions described, is well understood by those skilled in the art. To enable the various embodiments of the invention to be readily understood, the figures have been selected to show simply and clearly the sequence of events, and therefore they are to a certain extent indicative and diagrammatic.

The abbreviations employed have the following meanmgs:

EO=exhaust opening,

AO admission opening,

AC=admission closing,

EC exhaust closing and BDC=bottom dead centre, i.e. when the piston is at its furthest away point from the combustion chamber.

A simple form of the wave pipe type of exhaust system, in accordance with the invention, is that required for a single cylinder engine, as shown in FIGURE 1. Assume the engine to be towards the end of its power stroke, so that the cylinder 1 is full of high temperature high pressure gases. When the exhaust orifice 2 is opened, the gaseous contents of the cylinder 1 will form a pressure pulse which moves away from the orifice along the first duct 3. When the pulse reaches the junction 4, it will divide with consequent reduction in magnitude in relation to the cross-sectional areas now occupied and one part of it will continue along the second duct 5 to the machine 10, and another part of it will enter the wave pipe 6. The expansion occurring at the junction 4 creates a wave of rarefaction in first duct 3 which travels back to cylinder 1 and lowers the pressure of the gases therein, thereby assisting the scavenging of the cylinder by the fresh charge entering the ports 7 when they are opened by the piston 8. Meanwhile the pulse in second duct 5 has raised the pressure at the machine nozzles to drive the machine 10. As the machine nozzles offer a restriction to flow, some of the energy of the pulse is reflected back along second duct 5 and part enters pipe 6 and part passes back to thecylinder 1, while a small wave of rarefaction returns to the machine.

The wave pipe 6 is closed at its end 9, so that the pressure pulse therein is reflected without change of sign and returns to the junction 4 where part of it passes along second duct 5 to the machine 10 and part of it returns along first duct 3 to the cylinder. By reason of the greater distance travelled, the pulse passing along wave pipe 6 reaches the machine 10 later than the pulse travelling directly along second duct 5. The lengths of the ducts and pipes are chosen such that the scavenging of the cylinder has been substantially completed before the return pulse from the turbine or the closed end 9 of the wave pipe 6 reaches the exhaust orifice.

Use of the wave of rarefaction to assist scavenging enables the major part of the scavenging to take place before BDC of the engine piston, and the advantages of this will be described in greater detail later. It is a further advantage of the wave pipe type of exhaust system that the lengths thereof can be arranged so that a pressure pulse arrives back at the exhaust orifice shortly before the admission ports are closed, and if theexhaust orifice is closed after the admission po'rts but before the pressure pulse reflected from within the cylinder can escape, at least part of the pressure pulse can be trapped within the cylinder, so that the pressure of the fresh charge is increased. This latter feature may not be essential to obtain a high engine output, since with the exhaust system of the invention, high eliiciency, high pressure turbo-chargers can give sufficiently large quantities of fresh charge 'at a pressure high enough to ensure that the quantity retained in the cylinder exceeds the minimum necessary for good combustion. Moreover, when a high charging pressure is available these reflected pulses may add little to the total charge. FIGURE 2 shows the pressure variations close to the exhaust orifice of the system shown in FIGURE 1 for a single cylinder substantially constant speed engine. 11 is the initial exhaust pressure pulse leaving the exhaust orifice 2 and 12 the wave of rarefaction caused by the expansion of the pulse at the junction 4, while 13 is a portion of the pressure pulse 11 reflected from the machine nozzle ring, and of the coincident reflection from the closed end 9 of the wave pipe 6. The time of arrival of wave 12 at the cylinder depends upon the length of duct 3, that of pulse 13 upon the lengths of ducts 3 and 5 and duct 3 and pipe 6, and the lengths of 5 and 6 are equal. In the pressure diagram at the machine, the pulse 14a is the portion of the initial pulse 11 that has travelled along the duct 5 to the machine. Pulse 14b is the reflection of the other part of the initial pulse 11 at the closed end 9 of wave pipe 6, and as it does not occur at the machine it is shown in dotted line, but it serves to show the origin of part of the pulses 13 and 15. The pressure pulse 15 is the portion of the pulse 141) that has reached the machine following reflection at the closed end 9 of wave pipe '6, and division at junction 4, and this pulse 15 is further reduced by the coincident but smaller wave of rarefaction which passes back to the machine when pulse 14a in travelling back to the cylinder expands at the junction 4. In this simple example the length of the first duct 3 from the exhaust orifice 2 to the junction 4 should be approximately half the length of the pressure pulse, in which case, the head of the wave of rarefaction I I I 12 reflected from the junction 4 will reach the exhaust orifice about the time when the pressure in the cylinder has fallen to the scavenging pressure. If the duct 3 is made shorter the reflected wave will reach the exhaust orifice earlier and will cause a more rapid fall of pressure in the cylinder, but although the first part of the wave will thus assist in starting the scavenging period earlier it will not actually efiect scavenging, by which is defined, the removal of the gases in the cylinder after their pressure has fallen to the pressure of the charging The second duct 5 and pipe 6 should be of such length that the pulse 13 reflected from the machine 10 and the closed end 9 reaches the exhaust orifice 2 after the rarefaction wave 12 has completed its eflect upon the contents of the cylinder. Thus second duct 5 and wave pipe 6 should be about the length of the pressure pulse, that is about twice the length of first duct 3. If a pressure pulse at the exhaust orifice as it closes is not desired, the duct 5 and pipe 6 must be made longer or shorter. The cross-sectional areas of all the ducts and pipes may be made approximately equal to the area of the exhaust orifices, but in some systems difierent areas may give a further improvement. Provided the required conditions can be obtained, small cross-sectional areas and lengths will conserve the energy of the pulses by permitting less expansion than large areas and lengths, so that more energy will be delivered to and reflected from the machine.

In the modified arrangement shown in FIGURE 3, the wave pipe 6a leads directly to the machine 10, so that the energy of the pulse in this wave pipe is delivered directly to the turbine, without further division, and without the losses of an intermediate reflection. The length of the wave pipe 612' should be about three times the length of second duct 5.

The variations of pressure in the duct 3 close to the exhaust orifice 2, and in the ducts 5 and 6a close to the machine 10, are shown in FIGURE 4. The initial pressure pulse 11 leaving the exhaust orifice 2 travels along the duct 3 and is divided at the junction 4, so that as already described, a wave of rarefaction 12 passes back to the exhaust orifices. One portion of the pressure pulse 11 travels along the duct 5 to drive the machine and is indicated by the pulse 14a, which is reflected from the machine and returns to the junction 4 where it is again divided, and the negative reflection returning to the machine is indicated at b. A portion of 14a passes into wave pipe 6a, and the other portion returns to the exhaust orifice 2 as pulse 13a. The portion of the initial pulse 11 which travels along the longer wave pipe 6a, reaches a separate entry of the turbine 10 later than pulse 14a, and is shown by a dotted line as pulse 15a.

For the known system mentioned earlier, in which a short pipe connects the exhaust orifice 2 with the machine 10, the diagrammatic arrangement is shown in FIG. '5 and the pressure variations close to the exhaust orifice 2 and to the machine 10 are shown in FIGURE 6. In FIGURE 6, the initial exhaust pulse 16 issuing from the cylinder and its reflections 17 from the turbine nozzle ring become superimposed to form a large pulse which decreases in pressure relatively slowly. Satisfactory operation of an engine with this system necessitates a compromise, in that an exceptionally long period must be allowed before the admission ports are opened, or back flow of exhaust gases will occur through the admission ports when they are opened.

FIGURE 7 shows the other known exhaust system mentioned earlier, in which an expansion chamber 18 is provided between the exhaust orifice 2 and the machine 10, and which employs a constant pressure machine. The pressure variations close to the exhaust orifice and to the turbines are shown in FIG. 8. The exhaust pulse occupies approximately the normal time or crank angle period for a super-charged two stroke cycle engine. When the pulse enters the expansion chamber, the exg pansion occurring creates a wave of rarefaction in the exhaust pipe which returns to the cylinder as shown by the wave 20, and the pressure variations are suitable for good scavenging. The pressure diagram at the turbine shows that the-latter receives much less utilisable pressure energy than from the system recorded in FIGURE 6. v The wave pipe type exhaust system of the invention obtains a combination of the high utilisable energy at the turbine of the first mentioned known system, with the good scavenging conditions of the second mentioned known system, which leads to greatly improved overall performance of the engine.

As shown in FIGURE 9, the known short pipe system mentioned earlier may be adapted to the system of the invention by employing a short duct 3a and placing the turbine 10 at the end of a short branch duct 5a while the duct 3a is continued to form a wave pipe 6b. In this system, the provision of a wave pipe and the position of the machine-prevent the formation of the reflected pressure pulses 17 from the machine, which are shown occurring very early in FIG. 6 and are detrimental. The short branch duct 5a forms an opening and the machine a partial opening in the side of the duct 3a, which allow expansion, and nullify the pressure pulse which would otherwise be reflected back to the cylinder from the turbine. The pressure fluctuations at the exhaust orifice are shown in FIG. 10, in which 21 is the initial pulse leaving the exhaust orifice2, and 22 is a portion of the reflected pulse from the closed end 9 of wave pipe 6b. The pressure diagram at the turbine is also shown in FIGURE 10, in which 23 indicates the arrival of a portion of the initial'pulse 21 at the turbine, 24 indicates a portion of the reflected pulse from the closed end of the wave duct 6b, and 25 indicates a portion of the reflected pulse from the cylinder when the exhaust orifice is nearly or fully closed.

The above descriptions relate mainly to single cylinder constant speed engines, and the additional requirements for multi-cylinder and variable speed engines will now be described.

In systems in which numerous occurrences take place in rapid succession, and in which compromises have to be made, it is not uncommon for preliminary calculations to be employed to give the approximate dimensions of various parts, and to ensure a practical result, and for the optimum performance to be obtained by adjustments based on the results obtained.

Now that the requirements have been clearly stated, the preliminary calculations and compromises necessary in the present case will be readily appreciated by those skilled in the art, and also the adjustments necessary to obtain optimum results.

It is well understood that actions, such as valve openings and closings, whose period of operation are controlled by the mechanical cycle of an engine, take place in shorter times in high speed engines than in low speed engines, while the gaseous actions in the exhaust system, such as the movement of waves, take place in substantially constant time and depend mainly on the temperature and pressure of the gases. The necessity of making allowances for these occurrences is fully appreciated in connection with engines that are required to operate over a wide speed range.

It will thus be understood that a slow speed engine, which is usually a large engine, will require longer ducts and pipes in an exhaust system according to the invention, than a high speed engine, which is usually a small engine. The rate of opening of the exhaust orifices, in conjunction with the area of the orifices, will control the length of the pressure pulse for any given condition of the gases in the cylinder.

In constant speed engines a close approximation to the ideal lengths of ducts and pipes can be achieved, as described earlier, but further compromises are necessary for engines operating over a speed range.

If pressure curves of the type described earlier are plotted on a base of time and the opening of the exhaust orifice is taken as the fixed point, then the marks representing the opening and closing of the admission port and the closing of the exhaust port will all move (diagrammatically) closer to the fixed points as the engine speed being considered is increased, and further away from the fixed points as the engine speed is decreased.

If the pressure curves are plotted on a base of crankshaft degrees of rotation, and the orifice opening and closing points are considered fixed, then the various pressure pulses and negative waves will be spread out over the diagram as the speed considered is increased and contracted as the speed is decreased.

In normal engines, the scavenging and charging periods occupy a longer time than the duration of the pulses, and one compromise necessary is to arrange the length of the first duct so that the wave of rarefaction occurs about the middle of the scavenging period when the engine is operating at about the middle of its speed range, in order that the wave will not move out of the scavenge period at some higher speed, or be largely nullified by moving under the initial pressure pulse at some lower speed. The portion of the total admission period allotted to scavenging varies with the desired performance of the engine, as will be understood by those skilled in the art, but as stated earlier with the aid of the wave of rarefaction it can be completed about BBC. The point in the speed range at which maximum torque is required must also be considered. In one type of engine it may be advantageous at low speed to allow some portion of the returning wave of rarefaction to reach the cylinder before the pressure therein has fallen to the scavenging pressure, in order that at high speed the wave reaches the cylinder at a time which is not so late in the charging period that it is not effective. If it is so late that it arrives when the orifices are almost closed it will be detrimental. In another type of engine the operation at low speed may be of relatively greater importance, so that it is necessary to ensure that the wave has its full effect at low speed.

Similar considerations apply to the arrangement of the lengths of the second duct and wave pipe in order that the reflected pressure waves from the machine or the closed end of the wave pipe, do not reach the cylinder too early in the charging period, or alternatively after the exhaust orifice is closed, if the assistance of back charging is desired. If the second duct and wave pipe are made of different lengths, the reflected pressure pulse is decreased in magnitude but increased in length and time,

so that it is easier to arrange that some portion of it reaches the cylinder at the desired time.

Thus the sizes of the various ducts and pipes will vary with the speed range of the engine, the selected timing and areas of the inlet and exhaust orifices of the engine, the type of machine employed, the maximum temperature and pressure permissible within the cylinder, and the maximum output required.

There are a variety of exhaust pipe systems which can be designed to employ the wave pipe principle of this invention and when these are permutated with the possible different numbers of engine cylinders, the numbers of machine entries, and machines, it is impossible to give examples of all of them. The lengths of the various ducts and pipes, and the grouping of the exhausts from various cylinders, should be such that major pulses from one cylinder do not detrimentally aflect the scavenging and charging of any interconnected cylinder.

A popular arrangement of two-stroke cycle engine has three cylinders in line, and operates over a wide speed range. As shown in FIGURE 11 the three individual first ducts 26 from the exhaust orifices 2 of the three cylinders may be connected together .at a single junction 4a, with a single second duct 5b from the junctiona to the machine 10. In this novel arrangement, when one cylinder is exhausting and being scavenged, the exhaust orifices of the other two cylinders connected to the junction 4a are closed, so that the ducts 26 connected to these two latter cylinders can form the wave pipes of the former cylinder. In this example the ducts 26 are required to function as both first ducts and wave pipes, and the length of these ducts 26 must be a compromise between the ideal lengths required for each purpose.

As shown in FIGURE 12 a satisfactory compromise in the pressure fluctuations at the exhaust orifice and the machine can be obtained by making the first ducts 26 three quarters of the length of the pressure pulse, that is one and a half times the length of the duct 3 of FIGURE 2 and making the second duct 5b half the length of the pressure pulse, that is half the length of duct 5 of FIGURE 2. The initial exhaust pressure pulse 27 and the wave of rarefaction 28 are similar to the equivalent feature 111 and 12 described in relation to FIGURE 2, except that Wave 28 occurs a little later. The two pulses 29 shown by dotted lines in FIGURE 12 is the positive reflection of a portion of pulse 27 from the closed exhaust orifices of the other two ducts 26, and although this pulse does not occur at the exhaust orifice of the cylinder being scavenged, it is shown in FIGURE 12 to enable the origin of pulse 30 to be followed, which is that the pulse 29 has been reflected Without change of sign from the closed orifices of the two cylinders not being charged, has moved back along the two ducts 26 to the junction 4a, where it has divided, and a portion has travelled along the duct 26 of the cylinder being charged to the exhaust orifice thereof. Pulse 3 1 is a portion of the positive reflection of pulse 33 from the machine It and pulse 32 is a portion of the reflection of pulse 34 from the machine It and also a portion of pulse 33 which has travelled along the two ducts 26 of the cylinders not being charged, has been reflected without change of sign from the closed orifices of those cylinders, and has travelled back to the junction 4a where it has divided, and a portion has moved along the duct 26 of the cylinder being charged to the exhaust orifice thereof. In the pressure diagram at the machine 10, the pulse 33 is a portion of the initial pulse 27 arriving at the machine, and pulse 34 is a portion of the reflection of pulse 29 from the closed exhaust orifices of the other two pipes. Only small effects are caused by the wave of rarefaction moving back to the turbine when pulse 33 on its passage back to the cylinder, expands at the junction 4a.

The pressure fluctuations shown in FIGURE 12 are suitable for an intermediate speed of a variable speed engine. The compromise is such that increases or decreases of speed throughout a normal speed range will not move the wave of rarefaction 28 out of the scavenging period, or all the pulses 3 1, 30, 32, out of the charging period.

FIGURE 12 also shows one efiect of making the wave pipe longer than the second duct leading to the machine, in that the relatively intense pulse 13 of FIGURE 2 is replaced by pulses 31, 30 and 32 of reduced magnitude but of longer total duration. Where circumstances permit a similar result may be obtained by making the second duct longer than the wave pipe.

A three-cylinder engine may be treated as three single cylinders, as shown in FIGURE 13. In this embodiment the wave pipes 35 are shown as pipes of larger crosssectional area, surrounding each first duct 26, and each second duct 5b is led to a separate turbine entry. The pressure fluctuations at the exhaust orifices and at the turbine are shown in FIGURE 14. As the ducts and pipes have substantially the same length as those of FIG- URE 11, the pressure pulses and waves will occur at the same times, but as they are divided in two at the junction 4 instead of in three at the junction 4a, their magnitude will be diflerent. In FIGURE 14 the various occurrences have been given the same index numerals, as in FIGURE 12, but with a added.

In another embodiment, shown in FIGURE 15, each first duct 3 leads to a junction 4 where each wave pipe 36 is formed by a pipe of larger cross-sectional area, surrounding each second duct 5. Each pipe 36 terminates in a closed end, and each duct 5 leads to a separate entry of the machine it). The ducts and pipes have substantially the same length as those in FIGURE 1 and the pressure fluctuation will be similar to FIGURE 2 for each cylinder.

A six cylinder engine may be treated as two three cylinder engines, in which the six cylinders are collected together in two groups of three cylinders, in accordance with any of the previously described embodiments. An example of one suitable embodiment is shown in FIG- URES 16 and 17. The ducts and pipes may have the configuration necessary to obtain thedesired equal lengths of each type of duct or pipe while reducing the overall bulk of the system, and to bring the machine close to the engine. For this purpose a helical exhaust duct may be used, that is a single duct containing three helically arranged sections, one of which is coupled to each cylinder. This arrangement has the advantages of a long pulse travel distance for a given length of outer duct, while the heat losses are less than for separate pipes.

In FIGURES l6 and 17 the first duct for each cylinder comprises one of the separate ducts 3b and one of the sections of the helical duct 30. The wave pipes for each cylinder are formed by the first ducts of the other two cylinders in each group of three. Expansion occurs at the junction 4b, as described earlier, and the gases are led to the machine along duct 30.

The application of the invention to gas generators of the free piston or crankshaft type need not be described separately, except to state that the exhaust system of the invention is placed between the combustion cylinders and the power machine. The cycles of events in such gas generators are very similar to those in a two-stroke cycle engine, and the invention does not relate to the mechanical arrangements of the various gas producing units. The cyclic operation of the pistons and the gaseous occurrences in the exhaust system are similar in all the types mentioned, and the cyclic frequencies can also be similar. Thus the design considerations described earlier are applicable to gas generators of the free piston and crankshaft types.

In the examples described above attention has been given to maintaining equal lengths of ducts and pipes for each cylinder of a multi-cylinder engine and this is the optimum arrangement for a constant speed engine, but practical engine tests have shown that when this type of exhaust system is employed on an engine operating over a range of speeds, some departure from the optimum and equal lengths may be permitted to suit economical and practical requirements, while still obtaining a very high performance from the engine. For variable speed engines it is satisfactory to design the wave pipe system to suit the speed at which maximum torque is required.

Only the behavior of the major pulses has been described in detail in the above, and it will be appreciated that minor pulses must be present, including reflections of pulses that have passed through the cylinder and been reflected from the inlet orifices and piston or cylinder head. Again practical engine tests show that such occurrences do not detract appreciably from the improved eificiency of the engine, provided care is taken to ensure that such minor pulses are as small as possible if their effect is detrimental, or that they occur at points in the cycle where their effect is helpful.

The arrangements described may be extended, in a manner which will be clear to those skilled in the art, to ensure that the machine receives more than two major pulses for every single pulse from a cylinder, for example, by increasing the number of wave pipes 6a of FIG- URE 3 and varying their lengths.

In the case of engines having exhaust ports in the cylin der walls, or more than one exhaust valve in the cylinder head, more than one exhaust system per cylinder may be employed. Such exhaust systems may have diflerent lengths, or be joined together at points which ensure that the delivery of pressure gases to the machine is spread over a still longer period, while the waves reflected back to the exhaust orifices may be spread over larger portions of the charging period.

As a result of extensive engine testing, the applicants have found that when employing an exhaust system in accordance with the invention, exhaust gas-driven turbocharger may be used as the only source of fresh charge. Moreover, the overall efficiency of the complete unit is such that at the reasonable turbine inlet temperature of 500 to 600 C., the turbocharger can, with increases above a certain load, produce the same percentage increases in air flow as the percentage increases in fuel delivered to the combustion chambers. Thus, above this certain load the air to fuel ratio becomes constant and neither the exhaust temperature nor the specific fuel consumption, rise with further increases in load over a considerable range. The engine on which the tests were carried out was designed for about one third of the output now'obtained, and it is this feature which enables the new high output to be carried with substantially unchanged major components of the engine. The change in character of the unit means that the normal rating limits are no longer applicable, :as there is no limiting exhaust temperature or exhaust state to determine the full load. The limit to the output of the unit could be the mechanical limitations of the engine or the turbine rotor speed.

If an engine is designed with adequate strength, so that the mechanical limitation is the turbine rotor speed,'then for a constant turbine nozzle area the output of the unit would be limited, approximately, to a constant gas horsepower of the exhaust gases. If the engine is considered as a simple orifice in the system, this constant gas horsepower will be given by constant exhaust heat flow, which in turn will result from burning fuel at a constant rate in the engine regardless of its speed of rotation. The system of the invention will, therefore, give a near approach to the constant horsepower unit, and although this is the ideal case, it ensures that in practice a steeply rising torque curve is obtained with decreasing load, over at least the upper part of the speed range. Such a torque curve characteristic is very desirable for many applications of power units.

What we claim is:

1. An exhaust system for a variable speed gas producmg unit that comprises a plurality of combustion chambers each having an exhaust valve that opens and closes cyclically during operation of the unit and through which the gaseous products of combustion are periodically discharged in predetermined sequence at high temperature and under pressure, thus producing initial timed pulses of gases, a driven machine, said exhaust system including a first duct leading from each of the exhaust valves of said combustion chambers to ,a junction where each initial pulse of gases in succession is divided into primary and secondary pulses, a second duct leading from said junction to said driven machine to provide a passage for successive primary pulses, so that the gases may drive the machine by expansion, wave pipes for said secondary pulses closed at the end remote from the junction and in which said secondary pulses are reflected from said closed end, said wave pipes being provided by the first ducts of the other of said plurality of combustion chambers so that said secondary pulses travel back along said wave pipe and are again divided at said junction, and one portion of said reflected secondary pulses travels along said second duct to said machine, the distance travelled being so much longer than the direct path travelled by said primary pulses along said second duct that this portion of the reflected secondary pulses reach the machine after each primary pulse that travelled directly along the 13 second duct have substantially completed the expansion of their gases in driving the machine.

2. The device as claimed in claim 1, in which the wave pipe closed at the end remote from said junction the length of which is made such that when each reflection of a secondary pulse returning from the closed end is divided at the junction, the other portion of said secondary pulse travels back along the first duct to the exhaust orifice of the combustion chamber and reaches the latter when the scavenging thereof is substantially completed.

3. The unit as claimed in claim 1, in which the length of the first duct of the exhaust system is between one half and three quarters of the length of the initial pulse of gases.

4. In a gas producing unit comprising a plurality of combustion chambers, each being provided with at least one exhaust orifice from which the gaseous products of combustion are periodically discharged at high temperature and under pressure, thus producing successive pulses of gases, the expansion of which is employed to drive a machine, an exhaust pipe system including a plurality of first ducts leading from the exhaust orifices of each combustion chamber, a junction positioned at the end of said first ducts and joining said first ducts together, a second duct leading from said junction to said machine, each of said first ducts operating as a wave pipe for the others of said first ducts extending away from said junction whereby each pulse from each said combustion chamber is split into a primary pulse travelling directly to said machine and a secondary pulse travelling to said machine by way of said first ducts acting as wave pipes, said first ducts acting as wave pipes providing a passageway for said secondary pulses longer than said second duct along which each pulse travels from said junction to said machine, the length of said first ducts operating as wave pipes being of a length to lead a secondary pulse to said machine between primary pulses.

5. An exhaust system for -a gas producing unit of the type comprising a plurality of combustion chambers having valved exhaust orifices that open and close in sequence at predetermined points in the cycle of operations of said unit and from which the gaseous products of combustion are periodically discharged at high temperature and under pressure, a driven machine, an exhaust system in communication with said exhaust orifices and including a junction, a first duct leading from each exhaust orifice of said combustion chambers to said junction where each pulse of gases originating at that orifice is divided into primary and secondary pulses, a second duct leading from said junction to said driven machine so that by expansion of said primary pulses of gases passing therethrough the machine is driven, said first ducts each operating as a wave pipe connected to said junction to provide passage means through which the secondary pulses of gases pass, said first ducts and said second duct providing a passage for said secondary pulses so much longer than the direct passage of said primary pulses along said second duct that each of said secondary pulses reach said machine to aid in its operation after said primary pulses of gases, which travelled directly along said second duct, have each substantially completed the expansion of its gases in driving the machine.

6. The device of claim 5, in which the length of travel of said secondary pulses in travelling from said junction to said machine by way of said Wave pipe is not less than twice the length of travel of said primary pulse.

References Cited in the file of this patent UNITED STATES PATENTS 2,130,721 Kadenacy Sept. 20, 1938 2,542,756 Draminsky Feb. 20, 1951 2,581,668 Kadenacy Jan. 8, 1952 2,583,430 Kadenacy Jan. 22, 1952 2,602,291 Farnell July 8, 1952 FOREIGN PATENTS 1,062,309 France Dec. 2, 1953 350,712 Great Britain June 18, 1931

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