Controlled two-stroke internal combustion engine

The invention concerns a two-stroke internal combustion engine and a method of operating a two-stroke internal combustion engine which includes at least one cylinder (10) with a piston (20) reciprocating therein and with at least one inlet (40) and exhaust duct (70), wherein the inlet duct (40) is arranged in relation to the exhaust duct (70) in accordance with the uniflow scavenging process in such a way that the inlet duct (40) opens into the cylinder in the region of the combustion chamber (30) and wherein a regulating element (50) is provided in the mouth opening region of the inlet duct (40). In that arrangement the regulating element (50) of the inlet duct (40) can be adapted to be controllable and a controllable regulating element (80) can also be arranged in particular in the mouth opening region of the exhaust duct (70). As a result the internal combustion engine can be operated in such a way that the exhaust duct (70) can be closed in dependence on an operating parameter of the internal combustion engine.

DESCRIPTION 
The invention concerns a two-stroke internal combustion engine as set forth 
in the classifying portion of claim 1 and a method of operating a 
two-stroke internal combustion engine as set forth in the classifying 
portion of claim 13. 
German patent specification No 583 885 discloses a two-stroke internal 
combustion engine and a method of operating same, in which both an inlet 
duct for air and also an inlet duct for a fuel-air mixture open into a 
combustion chamber of a cylinder. An exhaust duct for the exhaust gases 
opens at the cylinder wall in such a way that this known two-stroke 
internal combustion engine can be scavenged by means of unidirectional or 
uniflow scavenging. Opening and closing both of the inlet duct for the 
scavenging or fresh air and also the inlet duct for the fuel-air mixture 
is effected by way of non-controllable valves whereas the exhaust duct is 
opened or closed by way of the reciprocating piston. Opening of the valves 
is effected by virtue of the spring force of the respective valve spring 
being overcome by means of the medium which is introduced into the 
combustion chamber. An asymmetrical control diagram can be obtained with 
that known two-stroke internal combustion engine for the feed of the fresh 
mixture by virtue of control, which is not described in greater detail, in 
respect of the blower for the fresh air and the dimensioning of the 
valves. In contrast, it is only possible to provide a symmetrical control 
diagram for discharge of the exhaust gases, which results in an increased 
fuel consumption and a comparatively high level of pollutant emission. 
German patent specification No 410 695 also disclosed a two-stroke internal 
combustion engine wherein a pressure oil-operated slider is disposed in an 
inlet duct for scavenging air. German patent specification No 410 695 does 
not disclose whether and in what way the slider is controlled. The exhaust 
duct is also arranged in relation to the inlet duct in such a way that 
this known two-stroke internal combustion engine can be scavenged by means 
of uniflow scavenging. Opening and closure of the exhaust duct are 
effected by way of the reciprocating piston. It is also to be noted that 
this known two-stroke internal combustion engine is operated by means of 
Diesel fuel, for which purpose an injection nozzle arranged within the 
rotary slider injects Diesel fuel into the combustion chamber at a given 
time. 
Finally DE 40 12 474 A1 discloses a two-stroke internal combustion engine 
in which both the inlet duct and also the exhaust duct can be opened and 
closed with a respective valve. In that case scavenging is effected by 
means of the cross-flow scavenging method which has the disadvantage in 
relation to the uniflow scavenging method that exhaust gases can remain in 
the cylinder, in spite of the supply of scavenging air of fresh mixture. 
As in addition the exhaust duct opens into the cylinder in the region of 
the combustion chamber, thermal problems occur in that respect. 
The object of the present invention is to provide a two-stroke internal 
combustion engine and a method of operating a two-stroke internal 
combustion engine of the kind set forth in the opening part of this 
specification, which, with a higher degree of efficiency, permits a 
reduced fuel consumption and a lower level of pollution emission. 
In a two-stroke internal combustion the foregoing object is attained in 
that the regulating element of the inlet duct is adapted to be 
controllable and that a regulating element which is also controllable is 
arranged in the mouth opening region of the exhaust duct. In this respect 
controllable regulating element means that the moment of opening or 
closing, the opening or closing time and/or the opening or closing speed 
can be controlled in dependence on one or more operating parameters of the 
internal combustion engine, in particular the rotary angular position of 
the crankshaft. The proposed construction makes it possible that an 
asymmetrical control diagram can be provided both for the inlet range and 
also the exhaust range, which in turn permits a particularly advantageous 
fuel consumption and a reduction in the level of pollution emission. 
At the same time use of the uniflow scavenging method provides for 
particularly fast and thorough cleaning of the combustion chamber or 
cylinder, in respect of the exhaust gases. In addition the outlet which 
can be opened and closed by the controllable regulating element affords a 
possibility for the engine designer freely to determine the position of 
the exhaust duct in the cylinder wall. Thus in that way for example the 
exhaust duct can be moved closer to the combustion chamber so that it is 
possible to achieve a higher discharge flow speed for the exhaust gases 
and to empty the combustion chamber or cylinder of the exhaust gases more 
quickly. At the same time or as an alternative thereto, the outlet can be 
opened earlier by the controllable regulating element, in comparison with 
an uncontrolled outlet, so that the exhaust gases are overall cooler, 
thereby possibly avoiding a thermal problem which may arise. It is also to 
be noted that the drive for the controllable regulating elements can be 
produced for example by way of one or more camshafts. If those camshafts 
are electrohydraulically adjustable, not only is it then possible to 
achieve an asymmetrical control diagram, but it is also possible to 
provide for displacement of the entire opening time both of the inlet and 
also the outlet valves. The valves themselves used may be the most widely 
varying elements such as for example poppet or mushroom-type valves with 
valve elements opening into the cylinder, rotary slide or disk valves or 
the like. It is also to be noted that the controllable regulating element 
in the exhaust duct does not have to be arranged directly into the mouth 
opening region of the exhaust duct into the cylinder chamber, but can be 
disposed at a certain spacing relative thereto. In that way it is possible 
to avoid thermal problems for the exhaust valve, which may possibly occur, 
so that the amount of material used for the exhaust valve is reduced. With 
suitable tuning of the engine there is then the possibility of producing 
an additional charging effect in the feed of the fresh mixture as after 
closure of the exhaust duct by means of the controllable regulating 
element, the fresh mixture penetrates into the exhaust duct and is thrown 
by the closed controllable regulating element back into the combustion 
chamber where further fresh mixture has already accumulated. It is finally 
also to be noted that it is also possible only to provide one controlled 
regulating element for the exhaust duct and one uncontrolled regulating 
element for the inlet duct. If a plurality of inlet ducts are provided in 
that case, there is the possibility of arranging an uncontrolled or a 
controlled regulating element in the individual inlet ducts, for example 
depending on the function of the respective inlet duct. 
If the inlet duct is so arranged that it opens tangentially into the 
combustion chamber of the cylinder, the fresh air or the fresh mixture 
passes into the combustion chamber in a circling movement along the wall 
of the cylinder. As the cylinder wall is cooled by a suitable cooling 
arrangement, the fresh gases are accordingly also cooled down so that 
filling of the combustion chamber is improved and the compression ratio 
can be increased without the risk of self-ignition, which at the same time 
means a higher power output and a saving on fuel. 
One and also two or more inlet ducts may open into the combustion chamber. 
If there are two inlet ducts, it is particularly advantageous if they are 
arranged on both sides or symmetrically relative to the longitudinal axis 
of the cylinder so that when the two-stroke internal combustion engine is 
used as a gasoline-burning internal combustion engine, the sparking plug 
can be arranged on the longitudinal axis of the cylinder, that is to say 
in the middle of the cylinder. In that respect the central arrangement of 
the sparking plug is advantageous as the flame front can be uniformly 
propagated. If the two-stroke internal combustion engine is provided as a 
Diesel-burning internal combustion engine, an injection nozzle can be 
provided in place of the sparking plug, whereby the same advantages are 
achieved as in the case of a gasoline-burning two-stroke internal 
combustion engine, in particular combustion taking place uniformly. 
In addition the combustion chamber and the piston may each have a for 
example annular squish or squeeze surface, which surfaces are of such a 
configuration that there is an annular squish or squeeze gap along the 
cylinder wall in the top dead center position, whereby the fresh mixture 
is conveyed at high speed and possibly with a circling movement into the 
combustion chamber, in particular into the center of the combustion 
chamber. That provides a high compression effect without self-ignition of 
the fuel-air mixture occurring. If the sparking plug is arranged centrally 
combustion in that arrangement can take place uniformly in all directions. 
A further improvement in the efficiency of the two-stroke internal 
combustion engine can be achieved by the piston being provided at its 
upper piston surface with a mixture guide element, preferably a raised 
portion or dome. Upon being compressed the fresh mixture is accelerated by 
the mixture guide element in the direction of the center of the 
correspondingly shaped combustion chamber, and there concentrated. If in 
its center the mixture guide element has a part-spherical or the like 
shaped recess which can form a part of the combustion chamber, the mixture 
or the fresh air can be arranged in highly concentrated manner in front of 
a sparking plug or an injection nozzle. 
The above-indicated object is attained in terms of the method in that the 
exhaust duct is opened or closed in dependence on an operating parameter 
of the internal combustion engine, in particular the rotary angular 
position of the crankshaft. In this case also one again it is possible to 
achieve an asymmetrical control diagram for the exhaust region. In this 
case the inlet duct may also have a controllable regulating element. 
Besides the operating parameter `speed of rotation ` already used in the 
case of four-stroke internal combustion engines, it is also possible to 
use other operating parameters as control values. 
If at least two inlet ducts or more are provided for the two-stroke 
internal combustion engine, then an injection valve can be arranged in 
each inlet duct. In that way it is possible to achieve a so-called layered 
charge in the combustion chamber, that is to say, the two-stroke internal 
combustion engine can be operated in such a way that, fore each working 
stroke, firstly a lean mixture passes into the combustion chamber, in 
particular just before reaching the bottom dead center position, and then 
a rich mixture passes into the combustion chamber. A mode of operation of 
that kind which is referred to in the technical literature as lean-burn 
engines results in a high saving in terms of fuel. In this case also the 
fresh air or the fresh mixture can be introduced into the cylinder with a 
circling motion. 
If the fresh mixture is accelerated by means of the piston which is moving 
from the bottom dead center position to the top dead center position, in 
particular just before reaching the top dead center position, that makes 
it possible to achieve a high compression effect, without in that case the 
temperature of the fresh mixture initially rising greatly, that is to say, 
it rises with a time delay. That in turn results in an increase in the 
power output of the two-stroke internal combustion engine. 
If the two-stroke internal combustion engine is operated for example as a 
Diesel fuel-burning internal combustion engine, it is advantageous if the 
fresh air in the cylinder chamber is accumulated in the center of the 
combustion chamber, by means of the piston as it moves from the bottom 
dead center position to the top dead center position. That permits 
particularly gently combustion as the Diesel fuel accumulates inter alia 
in the form of a thin film on the wall of the recess. 
So that the amount of fresh gas or fresh air which is caused to pass into 
the cylinder for operation of the internal combustion engine is only as 
much as is required for the respective load range (idle, part load, etc), 
it can be provided that the amount of fresh gas required is regulated by 
way of a variable scavenging pressure which is set in dependence on the 
load applied to the internal combustion engine. That can be achieved for 
example by virtue of the provision in the fresh gas duct of a sensor for 
detecting pressure, which sensor is arranged upstream of the controllable 
regulating element and is connected to an electronic regulating system or 
control system for the internal combustion engine. In addition, it is also 
possible to dispose upstream of the controllable regulating element in the 
fresh gas duct a pressure relief valve which is controlled by the 
electronic control system. In dependence on the load applied to the 
internal combustion engine or the load transported thereby, the pressure 
relief valve is opened or closed by the electronic regulating system. In 
that way it is possible to provide for quantitative control and regulation 
of the amount of fresh gas supplied. That quantitative control can also be 
effected by the provision in the exhaust duct of a drivable throttle flap 
or valve which is arranged downstream of the controllable regulating 
element. That throttle flap is also actuated in dependence on load by a 
control motor or other drive, while in this case also there is provided 
and electronic control system which converts the load-dependent data into 
drive values for the drive of the throttle flap. This ensures that only as 
much exhaust gases pass into the open air, as fresh gases are required. 
In both cases exhaust gas residues remain in the cylinder, whereby exhaust 
gas recycling is eliminated and thus the combustion procedure is cooler, 
which reduces the emission of pollution. It is also to be pointed out that 
the above-described possibilities may also be combined with each other. 
It is also to be noted that, besides Diesel and gasoline, other gaseous 
fuels may also by used for the invention.

In FIG. 1, reference numeral 10 denotes a cylinder and reference numeral 20 
denotes a piston which can reciprocate from a bottom dead center position 
(=UT) to a top dead center position (=OT). Two inlet ducts 40 open on both 
sides of a longitudinal axis L of the cylinder 10 into the combustion 
chamber 30 of the cylinder head 12, the combustion chamber being of a 
frustoconical configuration. In this case the inlet ducts 40 are so 
arranged that the fresh mixture issuing therefrom can pass into the 
cylinder 10 with a circling motion along the wall of the cylinder, as is 
illustrated by appropriate arrows in FIG. 1. The two inlet ducts 40 can be 
opened and closed by means of inlet valves 50. In this case the two inlet 
valves 50 are so designed that upon opening of the respective inlet duct 
40 they project into the combustion chamber 30, as illustrated in FIG. 1. 
The two valves 50 can be pre-loaded into their closed position by means of 
a spring (not shown). As can be seen from FIG. 2a, a sparking plug 60 can 
be arranged in the center of the frustoconical combustion chamber 30 in 
such a way that it sits on the longitudinal axis L of the cylinder 10. 
Alternatively, instead of the sparking plug 60, there may be provided an 
injection nozzle 60', in particular a multi-hole nozzle, as can be seen 
from FIG. 2b. Referring to FIG. 1, disposed beneath the inlet ducts 40 is 
the exhaust duct 70 which can be opened and closed by way of a 
controllable regulating element, such as for example a valve 80. As can be 
seen from FIG. 1 the exhaust valve 80 is arranged at a certain spacing 
relative to the mouth opening region of the exhaust duct 70 into the 
cylinder 10. That makes it possible to avoid thermal overloading of the 
exhaust valve 80. As can also be seen from FIG. 1 the exhaust duct 70 is 
arranged relative to the two inlet ducts 40 in such a way that scavenging 
of the cylinder 10 is effected by means of uniflow scavenging. Both the 
exhaust valve 80 and also the two inlet valves 50 are respectively 
controllable in dependence on an operating parameter of the internal 
combustion engine. 
As can be seen from FIGS. 2a and 2b the piston 20 is provided with a 
frustoconical raised portion or dome 22 which forms a mixture or fresh air 
guide element and the shape of which is substantially adapted to the 
frustoconical shape of the combustion chamber 30. Provided in the center 
of the frustoconical raised portion 22 is preferable part-spherical recess 
24 in which the fresh air or the mixture to be fired can be collected, 
opposite the sparking plug 60 or the injection nozzle 60'. In addition, as 
can also be seen from FIGS. 2a and 2b, formed between an annular surface 
26 of the piston 20 and the cylinder head surface 14 which is opposite the 
piston 20 when it reaches the top dead center position is a squeeze gap 90 
which displaces the fresh air or the mixture which has accumulated at that 
location during the compression procedure, and accelerates it through the 
frustoconical raised portion 22 of the piston 20 in the direction of the 
sparking plug 60 or the injection nozzle 60'. 
As the two inlet ducts 40 and the exhaust duct 70 are controlled by way of 
controllable regulating elements 50, 80 in dependence on an operating 
parameter of the internal combustion engine, it is possible to produce an 
asymmetrical control diagram, as can be seen from FIG. 3. In FIG. 3, 1 
represents the working stroke of the piston, 2 represents the discharge 
flow of burnt gases, 3 represents the intake flow of fresh gases, 4 
represents scavenging, 5 represents additional charging, and 6 represents 
the compression stroke of the piston. 
As can be seen from FIG. 4, the fresh mixture can also be produced by means 
of injection of fuel into the fresh air which is supplied through the 
inlet ducts 40 by means of a blower (not shown). For that purpose, an 
injection valve 42a and 42b respectively is disposed in each of the inlet 
ducts 40. Instead of two inlet ducts 40 each having an injection valve 42a 
and 42b respectively, it is also possible to provide a single injection 
inlet duct 40 having a step injection nozzle (not shown). It is also 
possible to provide two injection valves 42a and 42b respectively in one 
inlet duct 40. 
The mode of operation of the internal combustion engine according to the 
invention is described hereinafter: 
In the first stroke the piston 20 is working, that is to say it moves from 
the top dead center position to the bottom dead center position. Before it 
reaches the bottom dead center position, the exhaust duct 70 is opened on 
the one hand by the piston 20 but also by the controllable regulating 
element 80. Shortly thereafter the inlet ducts 40 are opened by the two 
valves 50. The fresh air or the fresh mixture which is driven by a blower 
can pass by way of the inlet ducts 40 into the combustion chamber 30, the 
fresh mixture or the fresh air flowing with a circling motion along the 
wall of the cylinder 10. In the second stroke the piston 20 is moving from 
the bottom dead center position to the top dead center position. In this 
case the exhaust duct 70 is closed both by the valve element 80 and also 
by the piston 20. Then the inlet ducts 40 are also closed by the valves 50 
and the fresh mixture or the fresh air is compressed. Shortly before the 
piston reaches the top dead center position the fresh mixture is fired by 
means of the sparking plug 60 or, when the two-stroke internal combustion 
engine is operating as a Diesel-burning internal combustion engine, fuel 
is injected. It is to be noted that the exhaust duct 70 can be opened or 
closed independently of the position of the piston 20, by means of the 
controllable regulating element 80, that is to say, the exhaust duct 70 is 
already closed or opened before the piston 20 has assumed a corresponding 
position in the cylinder 10. 
A so-called charge layering effect or layered charge can also be achieved 
with the two-stroke internal combustion engine according to the invention. 
In that case the feed of the fresh mixture occurs through the two 
injection valves 42a, 42b in the two inlet ducts 40. When the piston 20 
moves from the top dead center position to the bottom dead center 
position, little fuel is added to the fresh air from the first injection 
valve 42a so that a lean mixture is produced. In the additional charging 
region, that is to say when the piston 20 is moving from the bottom dead 
center position to the top dead center position, a great deal of fuel is 
added to the incoming flow of air, by the second injection valve 42b, thus 
resulting in a rich mixture. By virtue of the piston 20 being of a 
particular configuration, the rich mixture collects in the combination 
chamber 24, which is of a part-spherical configuration, of the piston 20, 
where it is fired by the centrally arranged sparking plug. 
Instead of two injection valves 42a, 42b in each respective inlet duct 40 
or in the case of a single inlet duct 40, it is also possible to use a 
step injection valve. Likewise two injection valves can be provided in 
relation to a single inlet duct. 
Finally it is also to be pointed out that the drive of the control elements 
or valves for the inlet and exhaust ducts can be adjusted 
electrohydraulically, whereby the control times can be adapted to the 
respective requirement involved. 
FIG. 5 shows a working diagram for a five-cylinder two-stroke in-line 
engine. The engine has a selected firing order of cylinders 1, 4, 2, 5, 
and 3. In FIG. 5, the number 1 represents the compression stroke, 2 
represents scavenging, 3 represents additional charging, and 4 represents 
compression. It can be directly seen from this working diagram that the 
internal combustion engine according to the invention not only has a high 
level of efficiency but it is also smooth in operation as the overlap of 
the individual working strokes is comparatively great.