GB2189293A

GB2189293A - Two stroke engine exhaust port and duct

Info

Publication number: GB2189293A
Application number: GB08704152A
Authority: GB
Inventors: Udo Ktazula; Wolfram Gerbig
Original assignee: Schwermaschinenbau Kombinat Ernst Thalmann VEB
Current assignee: Schwermaschinenbau Kombinat Ernst Thalmann VEB
Priority date: 1986-04-17
Filing date: 1987-02-23
Publication date: 1987-10-21
Anticipated expiration: 2007-02-23
Also published as: GB8704152D0; CS175687A1; DE3701338A1; GB2189293B; FR2597549A1; DD260409A3; CS269794B1

Abstract

A two stroke engine has an exhaust duct (8) which reduces exhaust noise and improves engine efficiency in terms of its speed-torque curve and fuel consumption. The exhaust duct (8) extends from cylinder (1), the duct (8) having a cross section which increases non-linearly in the direction of exhaust gas flow. The duct has an outwardly convex upper duct wall (10) which in the section shown, forms at the point where it joins the cylinder an angle of not more than 30 DEG with the longitudinal axis of the cylinder. The invention has particular application to motorcycle engines. <IMAGE>

Description

SPECIFICATION Two stroke engine exhaust port The invention relates to a two stroke engine having an improved exhaust duct the design of which enables the exhaust noise to be reduced and, atthe same time, the speedtorque curve to be improved. It can particularly be used in motorcycles with engines having one or more cylinders.

It is known from DR-PS 316922 and DE-PS 2551 823 to give the exhaust ports of a two stroke engine an appropriate shape to reduce graduallythe pressure in the combustion chamber in orderto control the release of exhaust gases and to reduce exhaust noise. Forthis purpose the exhaust ports are enlarged beyond their usual upper boundary in the shape of a slot ora wedge in the direction ofthe vertical axis of the cylinder, so that when the piston moves downwards it gradually uncovers the exhaust cross section. As a result, it is possible to achieve a protracted pressure decrease in the cylinder and prevent high pressure amplitudes in the exhaust.

Afurther method for reducing the noise is known from DE-OS 3 100 851, in which the exhaust port is enlarged at its top in the shape of a slot our a wedge in the direction ofthe vertical axis of the cylinder, so thatthehigh pressure amplitudes which give rise to noise can be decreased by gradually uncovering the port on the downward stroke.

The disadvantage ofthese known solutions lies in thefactthatthe gradual equalization of pressure between the combustion chamber and the exhaust pipe is only controlled bytheshapeofthe cross-sectional area of the exhaust port in the cylinder and the pressure distribution can only be considered as a function ofthe uncovered aperture area of the port. However, other influential factors which have a considerable effect on the pressure-time-curve, such asthethree-dimensional shape of the exhaust duct and the change in cross-sectional area ofthe exhaust duct in the direction of expansion of the escaping gases, are not taken into account in the aforementioned prior art documents. The exhaust noise reduction effect in accordance with the prior art has only been demonstrated with low engine speeds.Furthermore, the engine imposes limits on the enlargements ofthe exhaustportatthetop,andthereductionofthe original port height, as disclosed in the prior art, usuallyentailsa decreaseinthevolumetric efficiency ofthe cylinder.

An object of the invention is to reduce the exhaust noise in port-controlled two stroke engines by constructing the exhaust duct such that sudden gas expansion phenomena are prevented and, at the same time, the speed-torque-curve is improved and the fuel consumption is reduced.

Another object of the invention is to prevent a sudden expansion of the combustion gases in the exhaust duct and, by appropriately constructing the exhaust duct, to obtain a pressure-time-curveforthe gases which is such that the gas exchange efficiency is increased.

According to the present invention there is provided atwo stroke engine, comprising a cylinder containing a piston,the cylinder defining a combustion chamber, inlet means forthe combustion chamber, and an exhaust duct from the cylinderforthe combustion chamber, the exhaust duct having a cross section which increases in a non-linear manner in the direction of exhaust gas flow from the combustion chambr, the opening to the exhaust gas duct being opened progressively as a function of movement of the piston outwardly from the combustion chamber, the duct having a wall closest to the combustion chamber which is outwardly convex when viewed in a transverse section containing the longitudinal axis of the exhaust port and a line parallel to the longitudinal axis of the cylinder, and a tangentto said wail in said section at the point where the wall joins the cylinder forming an angle of not more than 30 with said axis of the cylinder.

In order to increase the flow cross section in the exhaust duct in the vicinity of the bottom dead centre, the lower boundary ofthe exhaust port is preferably disposed at a lower level than the upper leading edge ofthe piston in the bottom dead centre.

The lower duct wall adjacent the lower boundary of the exhaust port preferably extends in a straight line.

On the downward stroke of the piston almostthe entire width ofthe exhaust port, which is preferably not enlarged beyond its upper boundary, simultaneously opens, so that the combustion gases enter an expansion section formed by the convex wall,the sidewalls of the exhaust duct and the piston, without any abrupt change in the cross section, and expand proportionally to the flow cross section which steadily increases in the flow direction.

The size of the flow cross section is essentially determined by the convex duct wall. When the piston descendsfurtherthe cross section ofthe expansion section formed by the upper duct wall, the side walls of the exhaust duct and the piston steadily increases, although preferably to a lesser extent than the cross-sectional area ofthe exhaust port which is uncovered by the piston. The pressure wave forming during the expansion of the combustion gases in the exhaust duct consequently has a pressure-time-curvewhich is morefavourablefor supporting the gas exchange than that obtained in the case ofthe aforementioned known exhaust duct constructions.

If the exhaust system is adapted accordingly, it is possible for the returning pressure wave to reach the cylinder before the piston closes the exhaust port on the upward stroke. Preferably the axis ofthe duct leads into the vicinity of the cylinder at an angle of at most 30 with respect to the longitudinal axis ofthe cylinder, with the result that a substantially greater proportion of the kinetic energy of the returning pressure wave can be used than in the known solutionsforadditional supercharging, as a result of which the speed-torque-curve is improved and the fuel consumption is reduced.

Owing to this kind of arrangement and construction of the exhaust duct, the gas expansion process in the exhaust duct is protracted and the pressure-time-curve is largely determined by the change in cross-sectional area in the exhaust duct.

As this construction of the exhaust duct does not increase the exhaust resistance or require the exhaust opening process to commence earlier, the field of application of the solution according to the invention can be extended to engines with a high specific output, the exhaust noise of which is to be reduced in conjunction with an improvement in the speed-torque-curve and a reduction in fuel consumption.

The invention is described in detail in the following with reference to an embodimentthereof.The accompanying drawing shows a longitudinal section through a cylinder of a port-controlled two stroke engine. A cylinder liner 2 is fixediy held by an interference fit in cylinder body 1. The cylinder liner2 isfixed intheaxial position bya linercollar3 attached on one side. A recess 4formed in the cylinder liner 2 connects an inlet 5 to the engine's crankcase. At least one transfer port 6 connects the crankcase (notshown)to a combustion chamber in the upper part of the cylinder liner 2. An exhaust port 2 connects the combustion chamber to an exhaust duct 8.The exhaust port 7 has an upper boundary 9 which is preferably straight and disposed at a right angletothelongitudinal axis ofthe cyiinder, to actas a guiding edge forthe exhaust-gas flow. The exhaust duct 8 adjoins the upper boundary 9 ofthe exhaust port 7. The exhaustduct8 has an upperductwall 10 which has a convex curvature and preferably straight side wails, which form a continuous, non-linear enlargement ofthe exhaust duct 8 in the direction of flow of the exhaust gases. The lower boundary 11 of the exhaust port7 is disposed below the upper leading edge ofthe piston 11 in bottom dead centre.

The lower duct wall 13, which is preferably straight, adjoins the lower boundary 11 of the exhaust port 7. Together with the upper duct waIl 10 and the side walls ofthe exhaust duct 8, the piston 12 forms an expansion section in the vicinity ofthe opening 15 into the combustion chamber, the flow cross section of which is changed by the piston 12 as afunction of the stroke.

The position of the upper duct wall 10 is such that an (imaginary) plane, which in the section shown in the drawing formsatangenttothewall 10 atthe upper boundary 9, is so arranged that the tangent forms an angle of 30" or less with the longitudinal axis ofthe cylinder.

In the direct vicinity ofthe opening 15 of the duct into the combustion chamber, the axis 14 of the exhaust duct 8 forms an angle of 30 or less with the longitudinal axis ofthe cylinder.

Claims

1. Atwo stroke engine, comprising a cylinder containing a piston, the cylinder defining a combustion chamber, inlet meansforthe combustion chamber, and an exhaust duct from the cylinderforthe combustion chamber, the exhaust duct having a cross section which increases in a non-linear manner in the direction of exhaust gas flow from the combustion chamber, the opening to the exhaust gas duct being opened progressively as a function of movement of the piston outwardly from the combustion chamber, the duct having a wall closest to the combustion chamber which is outwardly convex when viewed in a transverse section containing the longitudinal axis of the exhaust port and a line parallel to the longitudinal axis ofthe cylinder, and a tangent to said wall in said section atthe point where the wall joins the cylinder forming an angle of not morethan 30'with said axis ofthe cylinder.

2. An engine according to claim 1 wherein the wall of the exhaust duct opposite to said convex wall joins the cylinder at a point below the upper leading edge ofthe piston in bottom dead centre.

3. An engine accordingto claim 1 or2wherein the cross section ofthe expansion section formed by the convex wall, side walls of the exhaust duct and the piston increases steadily as the exhaust port opens and to a lesser extentthan the respective cross-sectional area of the exhaust port which is uncovered by the piston.

4. An engine according to any preceding claim wherein the longitudinal axis of the exhaust duct at the point where the ductjoinsthe cylinder intersects the longitudinal axis of the cylinder at an angle of not more than 30".

5. Atwo stroke engine substantially as hereinbefore described with reference to the single figure of the drawing.

6. A motorcycle with an engine according to any preceding claim.