DE102008038915B4 - Two-stroke engine and method of operating a two-stroke engine - Google Patents

Abstract

Two-stroke engine (2) with external mixture formation in a carburetor (34) for diesel fuel or kerosene operation, which is designed for operation with crankcase scavenging, and which comprises a crankcase (14) limited by a crankcase (14), wherein
the carburetor (34) on the air inlet side upstream of a suction funnel (36), wherein in the region of an inlet opening (46) of the suction funnel (36) an insert (48) is arranged, wherein in the flow direction (50) of the sucked in during operation of the engine air flow is the Insert (48) initially continuously tapered and then expanded suddenly, and the cross section of the suction funnel (36) between insert (48) and carburetor (34) decreases continuously, characterized in that
a heating device is provided for heating the crankcase (14), the heating device comprising a number of electric heating elements (44) and comprising an exhaust manifold (42) or silencer (60) bypassing the crankcase (14).

Description

The invention relates to a two-stroke engine with external mixture formation in a carburetor. It also relates to an associated operating method.

Conventional-type two-stroke engines usually have what is known as crankcase scavenging, in which the aspirated fuel-air mixture is first conducted into the crankshaft receiving crankcase, which is limited to the outside by the crankcase. There, the fuel-air mixture is pre-compressed by the piston sliding downwards in the cylinder during the power stroke and then, when the piston releases an associated overflow channel in the cylinder wall, flows into the combustion chamber. In the next cycle, the piston slides up and compresses the overflowed mixture in the combustion chamber, while at the same time fresh mixture is sucked into the crank chamber. The terms "top" and "bottom" refer to an upright cylinder arrangement with overhead cylinder head.

In a two-stroke gasoline engine, the ignition of the fuel-air mixture compressed by the piston takes place in the combustion chamber through an active ignition device in the form of a spark plug with associated power supply and tripping electronics when the piston passes through top dead center. The combusted mixture or exhaust gas is discharged at the end of the power stroke by an then released from the piston exhaust passage from the combustion chamber and usually passed through an exhaust manifold in the exhaust or Schalldämpfertrakt.

Two-stroke engines may be configured for internal mixture formation with direct injection of the liquid fuel into the combustion chamber or for external mixture formation in a gasifier. In the carburetor, the liquid fuel is injected and atomized into the sucked or injected via a charger airflow. A crankcase scouring is usually combined with an external mixture formation.

A two-stroke engine according to the preamble of claim 1 is known from US 5,097,814 known. Another two-stroke engine is from the US 3,054,604 A known. Two-stroke engines with heaters are from the US 6,374,790 B1 and the DE 644 933 A known. A two-stroke engine, which is heated by exhaust gases, is from the US 1,366,530 A known. Two-stroke engines with external mixture formation and active ignition devices are in DE 37 30 090 C2 and the US 3,817,227A disclosed.

Usually, such a two-stroke engine is supplied as fuel, a gasoline-oil mixture, the oil additive is the engine lubrication. Recently, due to appropriate financial incentives, there has been a desire to avoid kerosene instead of (normal) gas, which is normally used as fuel for gas turbine engines, or even diesel fuel.

Corresponding attempts to make a two-stroke gasoline engine for a diesel or kerosene operation, for example, from US 5,855,192 known. To improve the ignition behavior, in particular in the phase of engine start, heating of the cylinder head is provided there by means of a glow plug which is used there and is independent of the spark plug. The cylinder head is thermally relatively high load, which requires the use of correspondingly high-quality, high-temperature resistant materials and, for example, ceramic insert components in the cylinder. Nevertheless, the running behavior of such modified engines can not be convincing overall.

The present invention is therefore an object of the invention to provide a two-stroke engine of the type mentioned with the least possible and cost-effective design changes for a supply of kerosene as a fuel without the running culture and durability suffer. Furthermore, an associated operating method should be specified.

With regard to the engine, the object is achieved according to the invention by the features of claim 1.

The invention is based on the consideration that the known from the prior art, taken over by the diesel engine heating of the cylinder head by means of separate glow plug or the like more problems than advantages, so this concept should be discarded. Rather, an ignitable fuel-air mixture, as has surprisingly been found, even when using kerosene be provided as fuel by an insert of the type mentioned in the inlet funnel of the carburetor is used, the flow vortex generated in the intake air flow and the air flow through increased the carburetor.

In other words, it is expedient for the carburetor to be preceded on the air inlet side by an intake funnel tapering towards the carburetor inlet. In the region of the inlet opening of the intake funnel, a preferably annular insert is arranged, which initially tapers continuously in the flow direction of the air flow sucked in during engine operation and then expands in an abrupt manner.

The insert may for example be introduced as a retrofit part in a conventional intake funnel. Alternatively, the intake funnel can also contain the desired inner contouring from the outset. In particular, a suitably modified intake funnel can also be produced as a one-component, integral component, for example in a casting process.

Due to the Venturi effect of the narrowing cross-section of the flow channel of the sucked air flow is first accelerated, thus increasing its flow velocity. At the transition point, where the cross-section then suddenly or suddenly expands to then narrow again to the carburetor inlet back continuously according to the predetermined funnel shape, takes place - accompanied by a speed reduction - a strong turbulence of the air flow. As has been found, such a turbulence of the air flow just before the entry into the carburetor especially in the atomization of kerosene is extremely advantageous and leads - especially in combination with a subsequent preheating of the kerosene-air mixture in the crank chamber (see below) - for better combustion.

The carburetor is preferably designed as a membrane carburetor. This has the advantage that the carburetor can work perfectly regardless of position and even under great vibration. The fuel-air mist thus produced is advantageously sucked via an inlet channel in the upward movement of the piston in the crank chamber of the two-stroke engine and pre-compressed there during the subsequent downward stroke of the piston. Appropriately, in the lying between the crank chamber and the carburetor inlet passage, for example, designed as a flutter valve check valve is arranged, which prevents a backflow of the fuel-air mixture from the crank chamber in the direction of the carburetor during the downward movement of the piston (precompression phase).

Furthermore, it has been found that a particularly good running behavior in a two-stroke engine with crankcase scavenging can be achieved in particular when the fuel-air mixture before its passage into the combustion chamber - but after the atomization in the carburetor - in the crank chamber to a suitable operating temperature , in particular beyond the achievable with the pre-compression level, is heated.

For this purpose, a comparatively large-surface heating of the crankcase by an associated heating device is provided according to the invention, which is preferably arranged on the side remote from the crank chamber outside of the crankcase. This ensures that takes place during engine operation, a heat transfer from the heated outer wall of the crankcase on the inner wall to the coming of the carburetor, flowing into the crank chamber fuel-air mixture.

The thermal load of the crankcase keeps within manageable limits, while an overuse of the already loaded by the combustion processes in the combustion chamber cylinder head, as it could be caused by the use of glow plugs or the like, is consistently avoided. Due to the relatively long residence time of the fuel-air mixture in the crank chamber, at least in comparison to the residence time in the combustion chamber before the ignition, the now provided heating is very effective. The fuel-air mixture enters preheated into the combustion chamber. The further heating by the compression process during the upward movement of the piston then does not have to be so strong. This ensures excellent ignition properties, especially when using kerosene as fuel.

According to the invention, the heating device is designed primarily as a cold start aid and comprises a number of electrical heating elements. For a good heat transfer to the crankcase these are preferably designed as heating rods or heating mats running elements advantageously directly on the outside of the crankcase, so are in intimate thermal contact with this. The attachment on the outside has the advantage that the electrical leads must not be passed through the crankcase, and that the heating elements are not exposed directly to the chemically aggressive fuel-air mixture. The heating elements are acted upon, especially with a cold start of the engine via an external power source with heating current, so as to heat the fuel-air mixture as it flows through the crank chamber. The number of heating elements and their heating power depend in particular on the volume of the crankcase surrounded by the crankcase and the desired, required for the achievement of a certain temperature level warm-up. For example, to make a kerosene-air mixture flammable for cold starting, the crankcase temperature at the interior wall facing the mixture should be about 80 ° C to 90 ° C.

In an alternative embodiment, the crankcase is advantageously designed double-walled, wherein the inner and the outer wall enclose a liquid-filled gap. To operate the engine, in particular during the starting process, the gap with a Liquid filled, which is then heated via a suitable electrical heating system, for example by arranged in the space or outside the outer wall heating elements.

In order not to have to operate the electric heating system permanently, but to turn it off after starting and possibly after a certain minimum time (warm-up phase) of the engine and can decouple from the Heizstromquelle, is advantageously a heating of the crankcase and thus the crank chamber released by the operation hot combustion residues or exhaust gases are provided. For this purpose, according to the invention, at the combustion chamber outlet, an exhaust manifold opening into a downstream exhaust or muffler system is guided around or along the crankcase or, if appropriate, also integrated into the crankcase in that it is heated by heat radiation and / or by heat conduction from the exhaust manifold. For an intimate thermal contact of the exhaust manifold is advantageously at least in a section on the outside of the crankcase on; Preferably, the exhaust manifold is fixedly connected there to the crankcase, in particular welded or soldered. Instead of a cohesive connection or in addition to but could also be provided a frictional and / or positive connection, for. B. by screwing, riveting, etc.

Alternatively, according to the invention, instead of an exhaust manifold which opens into a silencer pot at the end facing away from the combustion chamber outlet, a silencer itself may also be guided around the crankcase in the manner described.

In an advantageous embodiment, a thermal insulation between the - advantageously heated by a heater - crankcase and the carburetor is provided so that as little as possible or only a small heat transfer from the crankcase to the carburetor housing and the carburetor is kept as cool as possible. Conveniently, the seated between the crankcase and the carburetor body valve housing, which encloses the inlet channel for the atomized fuel-air mixture with the check valve therein, made of a compared to the crankcase significantly poorer heat-conducting, heat-insulating material, in particular of a heat-resistant, mechanically and chemically highly stressed plastic, z. B. from polyphenylene sulfide (PPS) or a similar plastic. Furthermore, the exhaust manifold is preferably arranged at a sufficient distance from the valve housing, so that no appreciable heat transfer takes place by heat radiation.

In a preferred embodiment, the two-stroke engine is designed as a gasoline engine, so has an electric ignition system with a built-in cylinder head spark plug - controlled depending on the piston position - ignition of the compressed by the piston fuel-air mixture. It would also be conceivable alternatively to design the two-stroke engine as a glow starter motor with a glow plug glowing permanently in operation, such as a wire coil coated with platinum-iridium.

With regard to the method, the object mentioned at the beginning is solved by the features of claim 9.

By "kerosene" is meant in particular present a fuel from the light middle distillate oil refining, namely a light petroleum, which is otherwise commonly used in aviation for the operation of gas turbine engines (aircraft turbines). The usually wide, flat boiling line of kerosene is between that of heavy fuel and diesel fuel. Suitable types of kerosene are sold, for example, under the trade designations JP-1 (Jet Propellant-1) or JET A-1 (formerly JP-1A) or JP-8 or JET B or TS-1. Kerosene of the JET A-1 grade is particularly widespread and readily available. For the purpose of engine lubrication, the kerosene may also be admixed with a lubricating oil and / or other additives, in particular about 4% synthetic two-stroke oil.

As an alternative to kerosene, a diesel fuel can also be supplied to the two-stroke engine described here.

According to the invention, the method is used in a two-stroke engine with crankcase scavenging, wherein the kerosene-air mixture or diesel fuel-air mixture produced in the carburetor is conducted via the crank chamber into the combustion chamber. The mixture is preheated by flowing through the crank chamber by a heater and ignited after passing into the combustion chamber by an active ignition device.

The crankcase is heated by an electric heater during a starting operation, particularly during a cold engine start. After a certain startup time, the electric heating device is switched off and the crankcase is heated by heat conduction and / or thermal radiation from an exhaust manifold or silencer, which is connected to the combustion chamber outlet of the two-stroke engine and through which hot combustion exhaust gases flow.

By the measures described operated with kerosene two-stroke engine runs similarly well and "round" like a two-stroke petrol engine. The better combustion makes the engine run cooler, the fuel consumption is reduced. The average temperature in the region of the cylinder head housing is then, for example, only 160 ° C to 190 ° C compared to about 220 ° C in a conventional gasoline engine same displacement. It is also not necessary to choose a higher compression ratio compared to gasoline engines. The compression ratio, ie the ratio of the entire combustion chamber space before compression to the remaining space after compression, on the contrary by the measures described may be slightly lower than gasoline operation and is for example only 8: 1 to 10: 1. This leads to a reduction of the vibration level and thus to an extension of the service life of the crankshaft and of associated ball or needle bearings.

The design changes necessary for the kerosene operation touch the actual engine tract - cylinder, piston, crankcase - not or only insignificantly. Rather, they are predominantly limited to peripheral components - exhaust manifold, valve body, carburettor and intake manifold. They can thus be relatively easily retrofitted with existing engines of hergebrachter design.

An embodiment of the invention will be explained in more detail with reference to a drawing. Show:

1 a longitudinal section through a two-stroke engine (the crank axis is in the cutting plane),

2 a cross section through the two-stroke engine according to 1 (The crank axis is perpendicular to the cutting plane), and

3 a cross-section through an alternative embodiment of a two-stroke engine.

Identical parts are provided in both figures with the same reference numerals.

The in 1 and 2 illustrated two-stroke engine 2 is of predominantly conventional design and is used, for example, for propulsion of model airplanes or chainsaws, lawn mowers, etc. With a corresponding displacement size but it could also be provided for example for driving a passenger aircraft or a passenger car or a motorbike or the like.

The two-stroke engine 2 includes one in a cylinder 4 sliding, by periodic combustion processes in the combustion chamber 6 driven piston 8th , Over a connecting rod 10 becomes the linear movement of the piston 8th on the crankshaft 12 transferred and converted into a rotary motion. The crankshaft 12 is in a crankcase 14 stored and opens on the drive side in a connectable to the drive train of a driven machine or be equipped with a propeller drive shaft 16 , The up through the piston 8th closed space between the crankshaft 12 and the crankcase 14 is called a crank chamber 18 designated. The crank chamber 18 is on the outside of the cylinder housing 20 limited overflow channel 22 with the combustion chamber 6 connected, taking in the combustion chamber 6 opening overflow opening 24 essentially only in the bottom dead center position of the piston 8th is released. In this piston position is also to the combustion chamber 6 connected exhaust duct 26 (please refer 2 ) Approved. In the cylinder head 28 is an electric spark plug 30 arranged.

On the from the drive shaft 16 opposite side of the crankcase 14 Close a valve housing 32 , a carburetor 34 and a suction funnel 36 at. About the intake funnel 36 is sucked in the engine operation ambient air and the carburetor 34 fed. In the carburetor designed as diaphragm carburetor 34 is fed via a supply line (not shown) supplied liquid fuel in the sucked air stream and atomized.

The fuel-air mixture occurs during the suction phase by the suction of the upwardly sliding piston 8th over the valve body 32 enclosed entrance channel 38 in the crank chamber 18 one. During the subsequent downward movement of the piston 8th the mixture is in the crank chamber 18 pre-compressed, with one in the inlet channel 38 inserted, designed as a flutter valve check valve 40 a reflux to the carburetor 34 prevented. At the end of the compression phase, the fuel-air mixture flows over the overflow channel 22 in the combustion chamber 6 at the same time pushing the combustion residues (exhaust gases) remaining from the previous combustion process via the outlet channel 26 from the combustion chamber 6 out. During the subsequent upward movement of the piston 8th The mixture is first in the combustion chamber 6 compressed and finally by a spark on the spark plug 30 ignited; the working cycle begins (the processes described are of course partly parallel).

The two-stroke engine 2 is equipped with a series of design measures for a kerosene plant:
In particular, the is to the outlet channel 26 connected, at the other end, for example, in the resonant pot of a resonance muffler (not illustrated) outlet manifold 42 in the manner of an arc around the lower part of the engine block. The plane in which the arc extends is in the embodiment substantially perpendicular to the crank or drive axle (see 2 ). The exhaust manifold 42 lies in a lower section on the outer wall 43 of the crankcase 14 and is welded there with this. Thus, during engine operation, effective heat transfer occurs from the exhaust manifold heated by the hot exhaust gases from the inside 42 on the crankcase 14 and finally on the crank chamber 18 incoming, from the carburetor 34 coming fuel-air mixture, which is thus preheated.

In an alternative variant, the in 3 is shown, no or only a very short Auslasskrümmerstück is provided. Rather, the silencer closes here 60 or silencer pot directly to the outlet channel 26 and follows in the manner of an arc of the contour of the cylinder 4 and the crankcase 14 , whereby the desired warm-up function is ensured.

For a cold engine start is also an electrical heating of the crankcase 14 by several mounted on the outside, optionally inserted in corresponding recesses and applied to the housing wall electrical heating elements 44 , here in the form of heating rods, provided. The electrical connection cables 45 the heating elements 44 If necessary, can be connected to an external Heizstromquelle not shown here. In the exemplary embodiment, the rod-shaped heating elements 44 essentially at the bottom of the crankcase 14 arranged, parallel to the drive shaft 16 aligned and evenly around the crankcase 14 distributed so as to uniformly heat the interior, ie the crank chamber 18 to enable. The concrete design and arrangement could also deviate from it.

A heating of the cylinder head by additional glow pins or the like is advantageously not provided. A preheating of the carburetor 34 supplied fuel is not necessary and advantageously not provided.

In contrast to the usually metallic crankcase 14 and to the likewise metallic exhaust manifold 42 That's the crankcase 14 on the other side with the carburetor housing of the carburettor 34 connected valve housing 32 made of a poor thermal conductivity material, here z. B. of a heat-resistant, dimensionally stable high-performance plastic, which is resistant to the flowing past in the kerosene-air mixture. A suitable material is, for example, the polyphenylene sulfide-based glass fiber-reinforced plastic known under the trade name Ryton R-4 (registered trademark of Chevron Phillips Chemical Company LP). Of course, however, other materials that meet the requirements mentioned, can be used. Conveniently, the valve housing 32 on one side with the crankcase 14 and bolted to the carburetor body on the other side.

Finally, the carburetor 34 inlet funnel upstream of the air intake side 36 insofar modified compared to previously common variants, as in its wide inlet opening 46 a cross-sectional narrowing, vortex-generating insert 48 is arranged. Through the annular insert 48 is the free flow area for the intake air in the flow direction 50 initially steadily and monotonically narrowed and then at the narrowest point preferably suddenly (discontinuously) expanded. The diameter at the bottleneck 52 of the insert 48 is depending on the engine type and individual case preferably 55% to 75%, more preferably 60% to 66% of the diameter of the inlet opening 48 , In the flow direction 50 seen directly after the bottleneck 52 then the diameter jumps again approximately to the value that is in the region of the inlet opening 48 is present (100%), to finally decrease continuously to the carburetor inlet. Such a "funnel-in-funnel" arrangement serves to increase the pressure of the carburettor 34 inflowing oxygen content due to the induced vortex and has proved to be particularly advantageous in a kerosene atomization.

In particular, in a combination of the measures described, the two-stroke engine without elaborate redesign and redesign of the central components cylinder, piston and crankcase or the ignition device for operation with kerosene or even with diesel fuel ertüchtigt, for all conventional displacement sizes, for example, 30 cm ³ or less to 700 cm ³ or more. Of course, the concept described also in multi-cylinder engines, z. B. in multi-cylinder inline engines, V-engines, boxer engines or radial engines realize. Depending on the location of the exhaust ports and the "accessibility" of the crankcase, it may be necessary to vary the course of the exhaust manifold (s). Under certain circumstances, a single exhaust manifold can then also be provided for heating a plurality of crankcase sections assigned to the individual cylinders. Such modifications, however, will be apparent to those skilled in the art in light of the above description and are covered by the present claims.

LIST OF REFERENCE NUMBERS

2

Two-stroke engine

4

cylinder

6

combustion chamber

8th

piston

10

connecting rod

12

crankshaft

14

crankcase

16

drive shaft

18

crank chamber

20

cylinder housing

22

overflow

24

overflow

26

exhaust port

28

cylinder head

30

spark plug

32

valve housing

34

carburettor

36

Velocity Stacks

38

inlet channel

40

check valve

42

exhaust manifold

43

outer wall

44

heating element

45

connecting cable

46

inlet port

48

commitment

50

flow direction

52

bottleneck

60

silencer

Claims (9)

Two-stroke engine ( 2 ) with external mixture formation in a gasifier ( 34 ) for diesel fuel or kerosene operation, which is designed for operation with crankcase scavenging, and one of a crankcase ( 14 ) limited crank chamber ( 18 ), wherein the gasifier ( 34 ) on the air inlet side a suction funnel ( 36 ), wherein in the region of an inlet opening ( 46 ) of the intake funnel ( 36 ) an insert ( 48 ) is arranged, wherein in the flow direction ( 50 ) of the air flow sucked in during engine operation, the insert ( 48 ) is first continuously tapered and then expanded abruptly, and the cross section of the suction funnel ( 36 ) between use ( 48 ) and carburetor ( 34 ) continuously decreases, characterized in that one for heating the crankcase ( 14 ) is provided, wherein the heating device comprises a number of electrical heating elements ( 44 ) and one on the crankcase ( 14 ) passed and abutting him exhaust manifold ( 42 ) or silencers ( 60 ). Two-stroke engine ( 2 ) according to claim 1, wherein the heating device is the crankcase ( 14 ) from an outer wall ( 43 ) Heated ago. Two-stroke engine ( 2 ) according to claim 2, wherein the heating elements ( 44 ) on the outside wall ( 43 ) of the crankcase ( 14 ) issue. Two-stroke engine ( 2 ) according to claim 1 with double-walled crankcase ( 14 ), wherein two walls of the crankcase ( 14 ) enclosing a liquid-fillable intermediate space, and wherein the heating elements ( 44 ) are provided for heating the liquid. Two-stroke engine ( 2 ) according to one of claims 1 to 4, wherein the exhaust manifold ( 42 ) or the silencer ( 60 ) with the crankcase ( 14 ), in particular positively or materially connected, in particular welded. Two-stroke engine ( 2 ) according to one of claims 1 to 5 with a cylinder housing ( 20 ) and a crankcase ( 14 ) comprising a thermal insulation between the engine block and the carburettor ( 34 ) is provided. Two-stroke engine ( 2 ) according to claim 6, wherein between the crankcase ( 14 ) and the carburetor ( 34 ) a valve housing ( 32 ) is arranged made of heat-resistant plastic. Two-stroke engine ( 2 ) according to one of claims 1 to 7, which is designed as a gasoline engine with an active ignition device. Method for operating a two-stroke engine ( 2 ), the two-stroke engine ( 2 ) one of a crankcase ( 14 ) limited crank chamber ( 18 in which kerosene or diesel fuel in a gasifier ( 34 ) is atomized and the resulting kerosene-air mixture or diesel fuel-air mixture in a combustion chamber ( 6 ), wherein a carburetor ( 34 ) supplied air stream before entering the carburetor ( 34 ) is guided by a first continuously narrowing, then suddenly expanding and again continuously narrowing portion of a flow channel and wherein in the carburetor ( 34 ) produced kerosene-air mixture or diesel fuel-air mixture via the crank chamber ( 18 ) in the combustion chamber ( 6 ) and in the combustion chamber ( 6 ) is ignited by an active ignition device, and wherein the kerosene-air mixture or the diesel fuel-air mixture when flowing through the crank chamber ( 18 ) is preheated by a heater, wherein the crankcase ( 14 ) is heated during startup by an electric heater and wherein the crankcase ( 14 by heat conduction and / or by thermal radiation from an exhaust manifold connected to a combustion chamber outlet ( 42 ) or silencers ( 60 ) is heated.

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