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DE-102024128373-B4 - two-stroke engine

DE102024128373B4DE 102024128373 B4DE102024128373 B4DE 102024128373B4DE-102024128373-B4

Abstract

A two-stroke engine (1) has a cylinder (2) in which a piston (5) is mounted to move reciprocally. The piston (5) defines a combustion chamber (3) formed in the cylinder (2). The piston (5) drives a crankshaft (7) rotatably mounted in a crankcase (4). The two-stroke engine (1) comprises an intake port (9) which opens into a crankcase interior (14) via an intake port opening (10), and an air port (11) which opens into a cylinder bore (13) of the cylinder (2) via at least one air port opening (12). The two-stroke engine (1) comprises at least one transfer port (17, 18) which opens into the cylinder bore (13) via at least one transfer port (19, 20) and fluidically connects the crankcase interior (14) to the combustion chamber (3) in the region of the bottom dead center (BDC) of the piston (5). The piston (5) has at least one piston pocket (16) that connects the at least one air duct opening (12) at least in a region of the piston's (5's) top dead center (TDC) with the at least one transfer port (19, 20). A fuel supply device (15) is provided for supplying fuel to the intake port (9). To ensure a sufficient fuel supply, the at least one piston pocket (16) has at least one section (23) designed and arranged such that, during the downward stroke of the piston (5), a fluidic connection exists between the at least one piston pocket (16) and the at least one air duct opening (12) up to a crankshaft angle (α) of at least 40° before bottom dead center (BDC).

Inventors

  • Simon Hummel
  • Michael Grether
  • Lukas Dürrwächter
  • Marvin Wahl

Assignees

  • ANDREAS STIHL AG & CO. KG

Dates

Publication Date
20260513
Application Date
20241001

Claims (8)

  1. Two-stroke engine with one cylinder (2) in which a piston (5) is mounted to move reciprocally, wherein the piston (5) defines a combustion chamber (3) formed in the cylinder (2), wherein the piston (5) rotatably drives a crankshaft (7) rotatably mounted in a crankcase (4), with an intake port (9) opening into a crankcase interior (14) via an intake port opening (10), and with an air port (11) opening into a cylinder bore (13) via at least one air port opening (12), with at least one transfer port (17, 18) opening into the cylinder bore (13) of the cylinder (2) via at least one transfer window (19, 20) and fluidically connecting the crankcase interior (14) to the combustion chamber (3) in the region of the bottom dead center (BDC) of the piston (5), wherein the piston (5) has at least one piston pocket (16) which which connects at least one air duct opening (12) at least in the region of the top dead center (TDC) of the piston (5) with the at least one transfer port (19, 20), with a fuel supply device (15) for supplying fuel into the intake port (9), characterized in that the at least one piston pocket (16) has at least one section (23) which is designed and arranged such that during the downward stroke of the piston (5) a fluidic connection exists between the at least one piston pocket (16) and the at least one air duct opening (12) up to a crankshaft angle (α) of at least 40° before bottom dead center (BDC).
  2. two-stroke engine after Claim 1 , characterized in that the section (23) has a width (a) measured in a development of the piston (5) perpendicular to a longitudinal central axis (29) of the cylinder bore (13) which is less than 50%, in particular less than 80%, of the maximum width (b) of the piston pocket (16) measured in a development of the piston (5) perpendicular to the longitudinal central axis (29).
  3. two-stroke engine after Claim 1 or 2 , characterized in that the section (23) has a height (c) measured parallel to the longitudinal central axis (29) which corresponds to at least 3%, in particular at least 5% of a stroke (h) of the piston (5).
  4. Two-stroke engine according to one of the Claims 1 until 3 , characterized in that the section (23) lies in a circumferential region (40) of the piston (5) which in no piston position is in overlap with a transfer window (19, 20).
  5. Two-stroke engine according to one of the Claims 1 until 4 , characterized in that the section (23) extends along an upper edge (22) of the piston pocket (16).
  6. Two-stroke engine according to one of the Claims 1 until 5 , characterized in that section (23) is designed as a groove.
  7. Two-stroke engine according to one of the Claims 1 until 6 , characterized in that the flow cross-section of the fluidic connection produced by the section (23) is less than 50%, in particular less than 80%, of the maximum flow cross-section of the connection between piston pocket (16) and air duct opening (12).
  8. Two-stroke engine according to one of the Claims 1 until 7 , characterized in that the piston (5) has at least one piston ring groove (34, 35) to which the piston pocket (16) has a distance (d) measured parallel to the longitudinal center axis (29) of the cylinder bore (13) which is less than 8 mm, in particular less than 5 mm.

Description

The invention relates to a two-stroke engine of the type specified in the preamble of claim 1. From the DE 10 2010 045 016 A1 A hand-held work device with a two-stroke engine of the type is known. Fuel is supplied via a carburetor depending on the vacuum in the intake manifold. The DE 10 2005 002 013 A1 , the DE 10 2020 000 989 A1 and the DE 103 21 571 B4 They also show two-stroke engines of the same type. It has been shown that in known two-stroke engines, insufficient fuel supply can occur, particularly in certain speed ranges. This has been especially evident at high speeds. The invention is based on the objective of creating a two-stroke engine of the generic type with which a sufficient fuel supply can be achieved in all operating conditions. This problem is solved by a two-stroke engine with the features of claim 1. It has been shown that insufficient fuel supply can result from excessively high pressure in the intake manifold at least within a certain engine speed range and for at least a certain period. This can prevent the necessary pressure differential between the fuel supply system and the intake manifold, particularly in fuel systems operating at low or near-zero pressure, from being present. It has been observed that this slight pressure differential can occur in known two-stroke engines before the point in an engine cycle when the intake manifold connects to the crankcase interior via the intake manifold opening, i.e., during the timing of the intake manifold opening. To reduce the pressure in the intake manifold before it connects to the crankcase interior via the intake manifold opening, the invention provides for maintaining the connection between the piston pocket and the air duct opening for a longer period than usual during the piston's downward stroke. It has been shown that closing the piston pocket early during the piston's downward stroke can result in a comparatively high pressure within the piston pocket. During the piston's downward stroke, the transfer ports to the piston pocket are typically closed first, followed by the air duct opening to the piston pocket. The pressure at the air duct opening is the same as the pressure in the air duct at that time. This can be the same pressure as the pressure in the intake manifold. The comparatively high overpressure that can exist in the piston pocket during the downward stroke of the piston in known two-stroke engines when the piston pocket closes can lead to a pressure increase in the air duct when the piston pocket reopens to the air duct. This increased pressure can, for example, be transmitted into the intake manifold via a fluidic connection between the intake manifold and the air duct. According to the invention, the fluidic connection between the air duct opening and the piston pocket is maintained until a crankshaft angle of at least 40° before bottom dead center. In the region of bottom dead center, and even before, the air duct and the intake port are typically at approximately ambient pressure. By maintaining the connection between the piston pocket and the air duct opening for a longer period than usual, the overpressure in the piston pocket can be gradually reduced at an early stage. When the piston pocket reopens to the air duct during the subsequent upward stroke, this reduces the pressure in the air duct and, in particular, the pressure in the intake port. This pressure reduction can be sufficient to lower the pressure level in the intake port to such an extent that fuel can be supplied to the intake port at low pressure. Fuel is supplied, in particular, via a carburetor or a fuel valve. The fuel supplied to the intake manifold is under low pressure. Specifically, the fuel overpressure is at most 1 bar, and in particular, at most 500 mbar above atmospheric pressure. Specifically, the fuel overpressure is at most 200 mbar, and in particular, between 50 mbar and 150 mbar above atmospheric pressure. Fuel systems that meter fuel under very low pressure can be simple in design. In particular, a fuel pump in the system is driven by the rotating crankshaft of the two-stroke engine. This results in a simple design. Specifically, the two-stroke engine includes a carburetor, and fuel is drawn into the intake manifold due to the vacuum created there. The carburetor may have an electrically actuated valve, especially an electromagnetic valve, to further control the fuel supply. Alternatively, the two-stroke engine may have a fuel valve, for example, an electromagnetic valve, that delivers the fuel. The section of the piston pocket that establishes the fluidic connection between the piston pocket and the air duct opening serves solely to allow pressure equalization between the piston pocket and the air duct up to a desired point during the downward stroke of the piston. For this purpose, this section has a particularly small flow cross-section. In particular, the intake duct and air duct are fluidically connected. In particular, the