TWO-STROKE INTERNAL COMBUSTION ENGINE

Abstract: A two-stroke internal combustion engine (1), mainly intended for a working tool, preferably a chain saw or a trimmer, and provided with a muffler device (2). A pipe (3), which either is straight or bent, and having an adapted or tuned length and a closed outer end (4), is arranged in connection to the engine's exhaust port (5) as well as an outlet (6), which leads the exhaust gases to the surrounding air. By this arrangement of the pipe (3) and the outlet (6) the scavenging losses are reduced and thereby the specific fuel consumption is reduced and cleaner exhaust gases are achieved.

Description: TWO-STROKE INTERNAL COMBUSTION ENGINE Technical field The subject invention relates to an internal combustion engme of two- stroke type, mainly mtended for a working tool, preferably a cham saw or a trimmer, and provided with a muffler device Background of the invention For working tools run by internal combustion engmes generally two- stroke engmes are used, mainly due to their low weight and simple design Also, the crankcase scavengmg enables a lubrication system mdependent of position, m which the engme is lubricated by oil which is added to the air/fuel mixture scavenged through the crankcase The all-position lubrication system is necessary e g for cham saws, smce they are to be used m a lot of different working positions Two-stroke engmes for mopeds and motor-cycles generally have a so called tuned exhaust system Reflected pressure pulses from the exhaust system will press scavengmg gases back mto the cylmder so that the engme 's scavengmg losses are reduced In total this means that both the power output and fuel consumption can be unproved in comparison with a non-tuned exhaust system However, m order to function well the tuned exhaust system requires large lengths of pipe as well as large cross-section areas m the exhaust duct Such a muffler for a cham saw would be at least half a meter long and consist of a first comcally expanding duct section by approximately 8 degrees, and a second comcally narrowing section by approximately 12 degrees Between these conical parts there should be located a straight part having a diameter, or actually a cross-section area, which is many times larger than the exhaust port If e g the exhaust port would have a diameter of 30 mm then the straight part would have a diameter of approximately 60-100 mm, that is almost 10 times larger cross-section area Thereafter an absorption muffler should be connected in order to reach reasonably low sound-levels As mentioned above such a muffler is based on reflected pressure pulses as well as a low total pressure drop Regarding working tools it has turned out that such a muffler will be far too large and heavy, even if the pipe system is provided with a lot of curves This is due to ^ the large cross-section areas For, a working tool must be very light-weight, compact and handy m order to serve its purpose Consequently, tuned exhaust systems are normally not used for working tools Instead they have light-weight and compact mufflers m which the sound mainly is damped by throttling m the muffler A larger cylmder volume is used to reach the preferable effect Owmg to 10 the fact that there are great differences between the layout of two-stroke engmes with tuned exhaust systems and without tuned exhaust systems, it is difficult to transfer experiences from one field to the other A well-known problem of two-stroke engmes is their relatively high fuel consumption caused by high-scavenging losses, l e scavengmg gases which flow 15 straight out mto the exhaust system This also results m high emissions, especially of hydrocarbons As mentioned above, the difficulties to overcome this problem are especially big for two-stroke engmes with light-weight and compact mufflers, l e with non-tuned exhaust systems The high emissions of hydrocarbons also results m certain problems when usmg a muffler with catalytic 20 conversion For, the very high energy content of the exhaust gases leads to a very high heat generation m the catalytic converter as well as in the surrounding muffler This high extent of heat generation could mean that the conversion ratio in the catalytic converter must be kept down Consequently, the high scavengmg losses mcrease the fuel consumption at the same time as they complicate a co- 5 operation with an exhaust catalytic converter Purpose of the invention The purpose of the subject invention is to substantially reduce the above outlmed problems for a two-stroke internal combustion engme, provided with a "0 silencing device Summary of the invention The above-mentioned purpose is achieved m a device m accordance with the invention, having the characteristics appearing from the appended claims The two-stroke internal combustion engme m accordance with the invention is thus essentially characterized m that a pipe, which is either straight or bent, and having an adapted or tuned length and a closed outer end. is arranged m connection to the exhaust port of the engme, as well as an outlet, which lead the exhaust gases to the surrounding an When the exhaust port is opened a powerful pressure pulse is created which runs mto the pipe and is reflected by its closed outer end If the pipe is given a correctly tuned length for a given rotational speed, for example maximum power rpm, the reflecting pressure pulse from the pipe will mcrease the exhaust pressure outside the exhaust port before it will be closed, so that the pressure becomes higher than the pressure mside the cylmder Thereby further scavengmg gases are prevented to flow out and some of the scavengmg gases could even be pressed back mto the cylmder Therefore the scavengmg losses are reduced and the engme 's power can be increased and its specific fuel consumption be reduced The exhaust emissions, especially of hydrocarbons, will be reduced substantially, with the result that also the heat stram on an eventual following exhaust catalyser will be reduced, so that its co-operation with the engme will be simplified Even if the mvention primarily is mtended for an engme for a working tool it might as well also be used for other land of motor applications Further characteristics and advantages of the mvention will become more apparent from the detailed description of preferred embodiments and with support of the drawing figures Brief description of the drawing The mvention will be descπbed m closer detail in the following by way of various embodiments thereof with reference to the accompanying drawing figures, m which the same numeral references m the different figures denote corresponding parts Figure 1 illustrates schematically m cross-section an internal combustion engme of two-stroke type m accordance with the mvention It shows a muffler device that compπses a pipe and an outlet accordmg to the mvention The outlet takes place through a conventional muffler Figure 2 shows a cross-sectional view of the upper part of the engme according to figure 1 but equipped with a somewhat different muffler device only consisting of a pipe and an outlet Those parts of the pipe which are lying above the plane of the paper are shown by dash-dotted lmes Figure 3 shows a third embodiment of the muffler device, where the outlet takes place through a muffler with catalytic converter Description of embodiments In the schematic figure 1 numeral reference 1 designates an internal combustion engme of two-stroke type It has a cylmder 9 with a piston 10, a crankcase 11, an inlet 12 with a fuel supply device, such as a carburettor 13, and furthermore scavengmg ducts 14 and a spark plug 15 All this is conventional and will therefore not be descπbed in closer detail An exhaust pipe 16 connects to the engme' s exhaust port 5 at its inner end and at its outer end it connects to a muffler 7 What is characteristic is that a pipe 3 with an adapted or tuned length and a closed outer end 4 is arranged m connection to the engme 's exhaust port 5 Smce the outer end 4 is closed exhaust gases can not flow through the pipe 3 Instead they will flow out through an outlet 6, which is arranged via the conventional muffler 7 The outlet 6 can also contam a catalytic converter 8, which is shown m figure 3, or could be just an outlet mto the air without any surrounding muffler, as shown m figure 2 The pipe could be arranged so that it connects directly to the exhaust port 5 without any intermediate part, as shown in the figures 2 and 3 However, there can also be an exhaust pipe 16 or similar, as shown m figure 1 What is essential for the function is that the pipe 3 as well as the outlet 6 are arranged m connection to the engme 's exhaust port 5 Hereby a powerful reflected pressure pulse is created m the pipe For. the pressure wave that is created when the exhaust port is opened will travel to and fro in the pipe With an correctly tuned length of the pipe the pressure wave will at a suitable rotational speed turn back to the exhaust port exactly before it is closed and will hold back the outflow and even push back scavengmg gases, 1 e air/fuel mixture, into the cylmder The arrangement can be regarded as a three-legged intersection created m connection to the engme's exhaust port 5. with one branch to the port, one branch out to the pipe 3 and one branch through the outlet 6 The angles between the branches can be varied very much, e g the pipe can be located straight out from the port at an angle of 180°, or crosswise, l e at an angle of 90° 0 Consequently, when calculatmg the length of the pipe also the distance from the exhaust port to the mouth of the pipe must be considered If this part should have another cross-section area than the pipe, then it has to be calculated as one with the pipe co-operatmg oscillation pipe Also the length of the outlet 6 can affect the oscillation somewhat, and this can also be counted as a participant m the total oscillation The outlet can also consist of several following parts which to some extent will affect the oscillation m the pipe The outlet 6 shall have a smaller cross-section area than the pipe 3 Preferably 0, 1 - 0,7 times the cross-section area of the pipe This is important m order to achieve a satisfactory pressure pulse mto the pipe 3 The outlet can be o arranged as a cavity or as a tube, which connects to the air or to a muffler The pipe 3 will change the conditions of the flow so that a cavity or a tube with a smaller cross-section area can be used without mcreasmg the pressure fall over the muffler device One can say that the pipe has a buffer function that on the one hand changes the exhaust flow over the time period and on the other hand 5 decreases the outflow of the scavengmg gases It is advantageous if the smallest cross-section area m the outlet 6 is located early m the outlet Preferably the three throttlings in the outlet 6 as shown in figure 1 should be of approximately the same size It is also important for the function that the outlet 6 is located adjacent the exhaust port 5 Preferably the outlet is located at a distance from the exhaust o port that is less than half the length of the pipe 3, and preferably less than a quarter of its length By a location close to the exhaust port there is less risk that the returning pressure pulse will be soft down before it reaches the exhaust port It is also an advantage that any exhaust flow m the pipe will be reduced to a shorter part of the pipe closest to the exhaust port Preferably the pipe has an essentially constant cross-section area along its entire length However, it could also have some local change of the area, for example a comcal section, over its length, or have a slightly comcal section over its entire length It is also conceivable that the pipe is provided with a volume connected to its outer end, and that the length of the pipe and the size of the volume are adapted to obtam the correct tuning Preferably the cross-section area of the pipe is of the same order of size as the area of the exhaust port, 1 e 0,3 - 3,0 times the cross- section area of the exhaust port The area of the pipe could thus be varied rather widely and its cross-section area has importance for how strong the returning pressure pulse will be A very small cross-section area would give a negligible effect while the effect would mcrease with an mcreasmg area up to a maximum It is thus not an advantage to have a very large cross-section area and the pipe differs m two completely determmmg ways from a so called tuned pipe m a tuned exhaust system In the tuned exhaust system there is a flow through the pipe and this pipe has a very large cross-section area to enable a powerful reflecting pressure pulse from the outer end of the pipe even though this is provided with a through-flow aperture Obviously, the size of the through-flow aperture may not be so small that the muffler provides an unacceptably great through-flow resistance It means thus that the pipe 3 essentially has no through- flow and has considerably smaller cross-section areas than a pipe m a tuned exhaust system has The design and way of function are thus very different As a rule it is desnable to adapt the length of the pipe 3 so that a considerable fuel saving effect is achieved at max power This is valid particularly for a cham saw or similar tool, which during a great deal of its running time is running at max power speed For such a high-speed engme also the pipe becomes shorter than for a more low-speed engme For, the time peπod between opemng and closing of the exhaust port is shorter at a higher rotational speed In such a high-speed engme the pipe will be approximately 4-5 decimetres long and will have a cross-section area approximately as large as the cross- section area of the exhaust port It means that the total volume m the pipe 3 is relatively limited and the pipe can therefore be rolled up, so that it requires quite a limited space, e g the pipe accordmg to figure 1 could be rolled up to a coil around the exhaust pipe 16 This coil could also be placed mside the muffler 7, e g by drawing this more close to the engme 's cylmder than what is shown m the ; figure The coil could either be rolled up mto one plane or could have a pitch, such as a thread, which is shown in the figures 2 and 3 The pitch is here somewhat exaggerated for the sake of clarity The determmmg fact is that the pipe is given the correct length The test results obtamed with the mvention are very interesting Smce the i outflow of scavengmg gases were reduced also the fuel consumption and the exhaust emission were reduced at the same time as the output of the engme was mcreased This is valid for the speed range, which the length of the pipe is tuned for, e g the speed range of maximum power of the engme Test runs with such an engme have thus shown that the specific fuel consumption could be reduced by approximately 10 % and the engme output could be mcreased by approximately 10 %, while the emissions of hydrocarbons were approximately halved at the same time as the emissions of nitrogen oxides were reduced considerably However, besides that a substantial reduction of the sound level of the engine is achieved The pipe 3 simply contributes considerably to silencing of noise of the engme The volume m the pipe 3 serves as a buffer volume, which is advantageous considering silencing This can be used m many different ways One way is to reduce the sound level of the engme considerably by maintaining the same muffler 7 as that of a corresponding engme without pipe 3 Another way is to reduce the volume of the muffler 7 and still achieve a sound level that is as high or lower than that of a corresponding engme with a conventional muffler An example of such a solution is shown m figure 3 The muffler 7 is there given a very limited volume Figure 2 shows the most far-reaching example where the muffler 7 is missing completely, so that the pipe 3 alone serves as a muffler device 2 It is important to note that there is essentially no flow in the pipe 3, since the outlet 6 is located far up-stream close to the exhaust port 5 It means that the pipe 3 becomes considerably cooler than the outlet 6 or the muffler 7 By wmdmg the pipe 3 around the hot parts it will thus protect from contact with these For the sake of clarity the pipe 3 is shown with very few winding turns m the figures 2 and 3 Consequently, by means of the pipe 3 the emissions of particularly unburned hydrocarbons can be reduced considerably This preferably occurs at maximum power speed of the engme Thereby the heat stram on a catalytic converter element 8 and on the muffler 7 mto which the element is mounted will be reduced considerably It means that a more effective conversion can be utilized without a too high temperature The mvention thus improves the work conditions for an exhaust catalyser A catalytic converter element could also be placed m the pipe 3, so that it alone or m combmation with at least one catalytic converter element m the outlet 6 is cleaning the exhaust emissions Also, due to the returning pressure pulses from the pipe 3 a stirring effect is created m the exhaust pipe 16 This is valuable for all two-stroke engmes but particularly advantageous if the engme 1 is of so called air-head type In such an engme the scavengmg duct 14 will be filled with fresh an, which at first is scavenged mto the combustion chamber and then forces the exhaust gases ahead out through the exhaust port It means that mto the exhaust pipe 16 will come first exhaust gases, then an and thereafter an/fuel mixture which is lost out through the exhaust port The mvention could thus, on the one hand reduce the loss of an/fuel mixture out through the exhaust port, but also on the other hand create a more even distribution of the an/fuel mixture and the air m the exhaust gases m the muffler, so that a following catalyser could function better Furthermore, the fact is that the pipe is coolmg down the exhaust gases somewhat It means that the exhaust mixture becomes cooler This is particularly advantageous m connection with an air-head-engine, smce both the demand for exhaust homogenisation as well as cooler and leaner exhaust gases are greater for such an engme than for a conventional two-stroke engme For, the increased surplus of oxygen m the exhaust gases leads to an mcreased πsk of after- oxidation m the muffler for such an engme than for a conventional two-stroke engme The risk is particularly great if a catalyser is used for the whole or a part of the exhaust flow in the muffler An after-oxidation m the muffler leads to that all combustible exhaust components will be oxidized so that an unacceptable heat generation takes place As mentioned above the pipe can be tuned for a suitable rotational speed of the engme, usually the maximum power speed of the engme However, m combination with an an-head-engme it could also be mterestmg to tune agamst a lower rotational speed For, the purpose of the an-head system as such is to reduce the engme' s emissions and its timing is preferably earned out so that the engme 's emissions will be reduced especially much at the maximum power speed of the engme A tuning m order to achieve maximum reduction of the emissions at a considerably lower rotational speed would lead to deterioration of the engme performance In this regard the pipe is more flexible and m combmation with an-head it could then be tuned agamst a lower rotational speed than the engme 's maximum power rotational speed This means that the engme will get a considerably reduced level of emissions at this lower rotational speed E g the emissions could be more than halved at a speed of 100 rotations per second by the aid of a pipe that is tuned for this engme speed m an an-head- engme with a maximum power speed of approximately 160 rps At the higher rotational speed the pipe will have a very small effect on the emission degree, perhaps even a negative effect, smce its pressure pulse will occur too late, but at the lower rotational speed it will thus have a sigmficant effect Accordmgly, hereby the engme 's characteristics within a wider range of rotational speeds will be improved m that the an-head technology gives an improvement that is greatest at the higher rotational speed while the pipe will improve it further at the lower speed Obviously, both techniques will provide improvements also beside the tuned rotational speed, but to a decreasmg degree In this way the engine could thus cover a wider speed range regarding exhaust emissions However, it would also be possible to utilize this effect from a kind of coordinating point of view A chamsaw engme of air-head type, e g tuned for the higher speed 160 rps, could be provided with a pipe tuned e g for 100 rps for use as a blower engme running at a very constant lower rotational speed Hereby a very low degree of exhaust emissions can be achieved at this lower rotational speed even though the engme m other respects is not fully tuned for it Preferably the length of the pipe m this case is tuned for a rotational speed that is essentially lower than the maximum power speed of the engme, 1 e 50-90 % of this, and preferably 55-75 % of this It must be pomted out that m most portable working tools it is extremely difficult or almost impossible to get space for a perhaps 40-70 cm long pipe In particular, it seems most easy m a blower

Claims: CLAIMS 1 An internal combustion engme (1) of two-stroke type, mainly intended for a working tool, preferably a cham saw or a trimmer, and provided with a muffler device (2), characterized m that a pipe (3), which is either straight or bent and having an adapted or tuned length and a closed outer end (4), is arranged in connection to the engme's exhaust port (5), as well as an outlet (6), which leads the exhaust gases to the surrounding an 2 An internal combustion engme (1) accordmg to claim 1, characterized in that the outlet (6) is located at a distance from the exhaust port that is less than half the length of the pipe (3) and preferably less than a quarter of its length 3 An internal combustion engme (1) accordmg to claim 1 or 2, characterized m that the outlet (6) is arranged m form of a muffler (7) 4 An internal combustion engme (1) accordmg to any one of the precedmg claims, characterized m that the pipe has an essentially constant cross-section area along its entire length 5 An internal combustion engme (1) accordmg to any one of the precedmg claims, characterized m that the cross-section area of the pipe (3) is of the same order of size as the area of the exhaust port, I e 0,3 - 3,0 times the cross-section area of the exhaust port 6 An internal combustion engme (1) according to any one of the preceding claims, characterized in that the outlet (6) has a smaller smallest cross-section area than the pipe (3), 1 e 0,1-0,7 times the cross-section area of the pipe 7 An internal combustion engme (1) accordmg to any one of the preceding claims, characterized m that the pipe (3) is arranged in form of a coil, which extends around the outlet (6) or the muffler (7) 8 An internal combustion engme (1) accordmg to any one of the precedmg claims, characterized m that the pipe is arranged m form of a coil and is placed mside an adapted muffler (7) 9 An internal combustion engme (1) accordmg to any one of the precedmg claims, characterized m that the outlet (6) or the muffler (7) is provided with at least one catalytic converter element (8) 10 An internal combustion engme (1) accordmg to any one of the precedmg claims, characterized m that the engme is of a so called airhead type 11 An internal combustion engme (1) accordmg to claim 10, character l z e d in that the length of the pipe is tuned for a rotational speed that is essentially lower than the maximum speed of the engme, l e 50-90 % of this, and preferably 55-75 % of this

Inventors: BLAIR GORDON P, GUSTAFSSON REINE, BLAIR, GORDON, P., GUSTAFSSON, REINE
Applicants: BLAIR GORDON P, ELECTROLUX AB, GUSTAFSSON REINE, AKTIEBOLAGET ELECTROLUX, BLAIR, GORDON, P., GUSTAFSSON, REINE
Application year: 2000
Main classification: F01N1
IPC codes: F01N1, F02B1, F02B27, F02B33, F02B75