Abstract
The present invention concerns a method in a two-stroke engine comprising at least one cylinder (1) with a reciprocating piston (2), a delimited combustion space (5), at least one outlet port (7) and an inlet port (9) which are both uncovered at the bottom dead center position of the piston, an actuator (8) which activates a valve (17) to open and introduce combustion air via an inlet pipe (6), a control system (15) which controls the actuator to open the valve in order to introduce combustion air via the inlet port. The invention is characterized in that the inlet port is closed by the piston after the outlet port has been closed, thus the opposite compared to the two-stroke engines of today.
Claims
1. A method of operating a two-stroke engine, the two-stroke engine comprising at least one cylinder with a reciprocating piston, a delimited combustion space, at least one outlet port and an inlet port disposed above the outlet port, which ports are both uncovered when the piston is at a bottom dead center position, and an actuator arranged to open a thereto connected valve in order to introduce combustion air into the combustion space via an inlet pipe and the inlet port, the method comprising: ordering, by a control system and after the piston moves from the bottom dead center position, the actuator to open the valve such that the valve is brought to open in connection with the outlet port being closed by the piston.
2. The method according to claim 1, wherein opening of the valve takes place in connection with the piston reaching the inlet port.
3. A two-stroke engine comprising at least one cylinder and a reciprocating piston therein which delimits a combustion space, wherein a lower portion of the combustion space is provided with at least one outlet port and above the at least one outlet port is provided with at least one inlet port, wherein the engine further comprises: an actuator for opening of a valve via which combustion air is introduced via an inlet channel disposed above the inlet port, and a control system comprising sensors and software controlling the actuator to open and close the inlet channel, wherein the piston forms part of a slide valve arrangement such that the piston, after being at a bottom dead center position, reaches the at least one inlet valve after the at least one outlet valve has been closed by the piston, and wherein the control system is configured to order the actuator to open the valve in connection with the outlet port being closed by the piston.
Description
DESCRIPTION OF FIGURES
[0015] FIG. 1 shows a two-stroke engine according to the invention comprising at least one cylinder 1 with a cylinder head 11, a piston 2 reciprocating in the cylinder, which piston covers the exhaust port 7 when the inlet port 9 is closed. Combustion and expansion stroke are to be initiated. A flywheel 3 is mounted on a crankshaft 4 on which a piston rod with said piston is arranged in a conventional manner. A combustion space 5 is disposed between the piston and the cylinder head. Combustion air is introduced to the combustion space from an inlet pipe 6 via a controllable valve 17 which is controlled to be open and closed by an actuator 8. On the crankshaft, a sensor or crankshaft probe 19 is arranged whose signals are read by a motor control system 15, which thereby registers the position of the piston in the cylinder, and orders the actuator to open and close said valve at the appropriate time. The piston is shown at its top dead center position where it covers an outlet port 7. In the exhaust pipe, an oxygen sensor 22 is arranged, whose signals are read by the engine control system 15. Using the oxygen detector, the ratio between fuel and combustion air can be controlled to be optimal based on the present exhaust aftertreatment system. It should be mentioned that the oxygen detector in some cases is not appropriate, for example in smaller engines such as in chainsaws, which must be lightweight and either operate at idle or at full throttle. Any spark plug 16 is arranged for engines where the fuel is to be ignited by means of a spark ordered by the engine control system at an optimal timing with regard to engine efficiency. In diesel engine applications, 16 represents a diesel fuel injector. The shown embodiment is appropriate also in smaller engines, such as garden machines, chain saws, motorcycles amongst others. It should be noted that the figure shows principally what is needed to apply the invention.
[0016] FIG. 2 shows the bottom dead center position of the piston in the cylinder, where an exhaust port 7 has been uncovered, whereby pressurized exhaust gases, often in a pulsating manner, flows out through said port to an exhaust system. If there is ambient pressure in the exhaust system, and the cylinder pressure is more than two bars as the port is uncovered, the exhaust gases initially flow out at the speed of sound, so called critical flow, and a short term negative pressure is created in the cylinder. The exhaust gases can propel an exhaust turbo by means of which the engine can be overloaded, which normally does not work well in a conventional two-stroke engine. In the area around the bottom dead center position of the piston, the piston speed is low, and at said bottom dead center the piston is stationary for a moment. This means that the exhaust area can be made smaller compared to today's two-stroke engines since there is more time for evacuation of the exhaust gases. If the exhaust area further is distributed over several low exhaust ports, the working stroke becomes longer, resulting in improved efficiency.
[0017] FIG. 3 illustrates the piston just having closed the outlet port 7 and reaching the uncovered inlet port 9 during the compression stroke. The engine control system has ordered the actuator 8 to open the valve 17, whereby combustion air, with or without fuel, is introduced to the combustion chamber via the inlet port. The short-term negative pressure described above with reference to FIG. 2 is used during introduction of combustion air. It should be mentioned that the space outside the inlet port 9 with the valve 17 can be embodied in many different ways to minimize the volume outside the inlet port.
[0018] FIG. 4 shows the cylinder according to FIG. 1, where the piston has closed the inlet port and compression of introduced combustion air has started while the controllable valve is still open. It is consequently realized that the most important aspect of the controllable valve 17 is that it can, by means of the actuator 8, be opened quickly in order to introduce combustion air, Closing of the inlet port takes place automatically by means of the piston 2.
[0019] FIG. 5 shows the cylinder according to FIG. 1 during a compression stroke.
[0020] FIG. 6 shows the cylinder according to FIG. 5 with a lengthened, so called tuned, inlet pipe 6, which corresponds to known so called tuned exhaust pipes. The purpose is to increase the mass of introduced combustion air at high speeds caused by an amplified pulse of introduced combustion air by means of the whole air mass in the inlet pipe being set in motion, which is achieved at existing negative pressure in the cylinder as the piston reaches the inlet port and the controllable valve is opened. Consequently, a greater mass of air is introduced to the cylinder compared to the mass of air which would be introduced to the cylinder using a short inlet pipe 6 in accordance with FIG. 5. If the crank case is used to introduce crank case compressed air into the cylinder, a lengthened pipe can instead be connected to the crank case to achieve a corresponding effect.
[0021] FIG. 7 is intended to show the cylinder wall 30 (dotted) with the piston 2 at the bottom dead center with an exhaust port 32 completely uncovered, an exhaust pipe 34 with a slideable exhaust throttle 33 with an opening 35 to control the exhaust flow through the exhaust port. The exhaust throttle is controlled by control means being connected to the engine control system 15 (not shown) based on the current need for torque, in this case maximum torque.
[0022] FIG. 8 shows the situation in FIG. 7 with the exhaust throttle which, by means of the engine control system based on the current need for torque, has been controlled to place the opening 35 such that the possible exhaust flow to the exhaust pipe is approximately halved.
