Ignition Device of an i.c. Engine

The present invention relates to an ignition system of an i.c. engine provided with a special protection against over-speeding of the engine.

A great over-speeding of the engine will give a high strain on its movable parts and is disagreeable to the environment due to the sound effect. Over-speeding occurs in an engine which works on a high load which suddenly vanishes. Such a case occurs by a motor saw at the moment the sword just cuts through a work piece. Before the operator manages to release the throttle control the engine speeds up to its highest possible r.p.m.

In the art of ignition circuitry protection arrangements are known which include coils and magnet poles which cooperate upon the exceeding of a predetermined r.p.m. in order to eliminate the ignition spark and thereby prohibit the increase of r.p.m. Such arrangements require a good adjustment and are sensitive to external stress.

In the present invention a microcomputer is used for the control of the ignition function and therewith also provides a limitation of r.p.m. by an accurately developed procedure of delayed ignition at a predetermined r.p.m. The use of a microcomputer has in this respect also the advantage of having space in its static memory for storing several functions of delayed ignition. In this connection the microcomputer can be used in a great assortment of engines which have different limitations of r.p.m., whereby the selection of a proper function of a special engine is made by performing permanent connections or disconnections on separate inputs of the computer which reads the inputs and choses the appropriate function.

An embodiment of an ignition device according to the invention is described in the following with reference to the accompanying drawings which show in:

FIG. 1 a wiring diagramme of the ignition device,

FIG. 2 a diagramme of the function of ignition time/r.p.m.,

FIG. 3 a primary voltage curve.

A wiring diagramme of the ignition device is shown in FIG. 1, the device including a microcomputer 10. The current supply to the electronic circuits and the computer is derived from the negative half-waves of the primary voltage (FIG. 3) from an ignition generator, 11 the circuit being connected to keep the capacitor 12 charged to an operating voltage. A transistor amplifier 13 is connected to output a pulse at the time of occurrence of a valve on the voltage curve, this valve in the present case being 0.6 V before the curve passes zero on the ascending part thereof. The pulse is supplied to the microcomputer as a start signal of a procedure which is herewith described in broad outline.

The input to which the signal is supplied is scanned and the time A (FIG. 3) is stored as a reference point. This storing is possible since the microcomputer has a timer working on a fixed frequency. At every reference point the number of pulses passing after the preceeding reference point is registered. The number of pulses corresponds to 360.degree. of the crankshaft. By dividing the number of pulses by a predetermined number, e.g. 16, a number of pulses remains which corresponds to an ignition angle 360/16=22.5.degree.. This number is called the reference number and is a memory data stored in the static memory of the computer. The reference number can be dependent of the r.p.m. and is at low r.p.m. inversely proportional (a straight horizontal line in FIG. 2). When the number of timer pulses reaches the said reference number (by comparing the numbers in an AND-circuit) the ignition is initiated by an output 14 of the computer. The timer is set to zero every time the reference point passes and a counting to the reference number takes place for every spark. At low r.p.m. ignition takes place at the point B on the curve in FIG. 3, since the ignition angle then is constant, whereby ignition is locked in phase to the voltage curve. At higher r.p.m. the reference number is dependent on r.p.m., making an ignition timing curve according to FIG. 2. The number of time pulses between A-A directly or indirectly forms an address to a position in the memory of the computer where the reference number corresponding to the ignition time is stored.

In FIG. 2 the relation between ignition time (angle) and r.p.m. is shown in a diagramme. The limitation of r.p.m. is effected at the sloping part of the curve, and brought about by an accurately developed procedure of delaying the ignition in two steps. In the first step, 15-16, a momentary retarding of the spark takes place through an angle of from 1.degree.-10.degree. on the crankshaft. Such a delay brings about an essential decrease of the torque of the engine so that the r.p.m. remains at about N.sub.1 if the torque is not enough for a further increase of r.p.m. In the second step, 16-17, a further retardation of the spark takes place along the sloping curve between these points and between the corresponding r.p.m. N.sub.1 and N.sub.2. Since ignition sparks in the range are still generated, here combustion takes place in the engine, so that no combustible gas is exhausted. In order to make sure that a limitation of r.p.m. takes place at the highest allowable r.p.m. N.sub.2, no sparks are generated above this r.p.m.

In order to obtain an enlarged field for use of an ignition device with such a limitation of r.p.m. this device has been provided with additional circuits which make the device adaptable to a great assortment of engines, in which different demands for ignition angle and limitation of r.p.m. exist. Several ignition timing curves 18, 19 can be stored in the computer which has special inputs for a code giving information of the selection of a specific curve. Such a selection can also include a limitation of highest r.p.m. according to one of the curves 20,21,22,23. The said code is produced by permanent connections or disconnections 24,25,26 of the inputs to ground. Each engine has such a code attached to the engine when it is manufactured, and the code always accompanies the engine.