GB2062756A

GB2062756A - An Arrangement for Adjusting the Ignition Timing Angle in an Internal Combustion Engine

Info

Publication number: GB2062756A
Application number: GB8035637A
Authority: GB
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 1979-11-10
Filing date: 1980-11-06
Publication date: 1981-05-28
Also published as: DE2945578C2; FR2469574B1; FR2469574A1; GB2062756B; IT1134192B; IT8025795A0; DE2945578A1

Abstract

A cam shaft (1) of the engine and a shaft (5) of a distributor (4) interengage via respective worm wheels (2 and 3) in such a manner that axial, thermal expansion of the camshaft (1) brings about an adjustment in the angular position of the distributor shaft (5) at any instant, thereby to advance ignition when the engine cools and retard ignition when the engine warms up. <IMAGE>

Description

SPECIFICATION An Arrangement for Adjusting the Ignition Timing Angle in an Internal Combustion Engine The present invention relates to an arrangement for adjusting the ignition timing angle of an internal combustion engine in the sense of relatively advancing the ignition timing when the engine cools and relatively retarding the ignition timing when the engine warms up.

In one such arrangement described in German Patent Application DE AS 2,051,956, the adjustment of the ignition angle is effected by a temperaturecontrnlled change-over member which acts on the distributor breaker plate. In one construction the change-over member is a magnetically actuated push-rod which is deenergised abruptly by a thermal switch when the internal combustion engine reaches a predetermined operating temperature. In another construction, the change-over member is itself constructed as a thermostat insert and is directly subject to the operating temperature of the engine. This change-over member effects the transition from advanced ignition to retarded ignition smoothly.Both constructions are complicated, while an additional adjustment of the ignition timing angle, for instance as a function of the engine load, is either not possible or requires considerable further structural outlay.

An aim of the present invention is to provide an arrangement as set out in the opening paragraph of the present specification which is of relatively simple construction and which is relatively simple to manufacture.

Accordingly, in an arrangement according to the present invention, a shaft of the ignition distributor and the engine camshaft are in drive communication via helical-tooth worm wheels, the camshaft is mounted in a locating bearing which restrains axial movement of the camshaft at a position remote from the distributor shaft and is mounted in a bearing which permits axial movement of the camshaft at a position closer to the distributor shaft, the camshaft and the camshaft casing have different thermal coefficients of expansion, the distributor is secured to the camshaft casing, and the direction of rotation, helix angle, direction of pitch and relative position of the camshaft and the distributor worm wheels are selected to be such that the temperaturedependent relative change in length between the camshaft and the camshaft casing brings about an advance in the ignition timing when the engine cools and a retarding of the ignition timing when the engine warms up.

In such an arrangement the desired adjustment of the ignition timing angle can be obtained automatically without the need for additional components. The relative change in the angular position of the distributor shaft at any instant resulting from the relative thermal change in length of the camshaft provides the desired setting of the ignition timing angle. Changing over from advanced to retarded ignition may ensue smoothly during the entire warming-up phase and in such a way that the ignition angle has an optimum value for every operating temperature.

The contact breaker plate does not have to be obstructed and may have its setting adjusted by means of an optionally two-stage vacuum control device in dependence upon the engine loading.

Many of the individual features of an arrangement according to the present invention have already been proposed. However, hitherto the direction of rotation and the direction of pitch of the camshaft and distributor worm wheels have been selected in such a way that the desired ignition timing angle adjustment is not obtainable.

If one of the previously proposed arrangements were adapted in such a way, for example, that the ignition angle has an optimum value when the engine has reached its normal operating temperature, then the ignition angle would be retarded too much, when the engine is cold, thus making starting much more difficult. Also, the socalled enriching required during the warming-up phase, i.e. the additional fuel required during this phase, would have to be greater with an overall increased consumption of fuel.

In contrast to this, an arrangement in accordance with the present invention can be made so that the setting of the ignition timing angle is at a predetermined optimum value also when the engine is cold. The setting of the ignition angle will then automatically be at least approximately at an optimum value when the engine has reached its normal operating temperature.

Conversely, the ignition timing angle can be set precisely at the predetermined optimum value with the engine at its normal operating temperature. Under these circumstances the angle will then be at least close to the optimum value when the engine is cold. By utilising the differential thermal coefficient of expansion of the camshaft and the camshaft casing, it is possible to set the ignition timing angle to be at an optimum value both when the engine is cold and when it is at its normal operating temperature.

This can be achieved merely by selecting a suitable diameter and/or suitable pitch of the camshaft and the distributor worm wheels.

An example ofan ignition timing angle adjusting arrangement in accordance with the present invention is illustrated in the accompanying drawings, in which: Figure 1 is an explanatory diagram showing the principle of operation of the arrangement which varies the ignition timing angle in dependence upon the operating temperature; and Figure 2 is a side, partly axially sectioned, view of the arrangement of Figure 1, viewed from the right of Figure 1.

A camshaft 1 of an internal combustion engine (not shown fully) is shown in diagrammatic form in Figure 1. On one end of the camshaft there is a helical worm wheel 2. This wheel 2 engages a helical worm wheel 3 of a distributor 4 (shown in Figure 2), the shaft 5 of which extends at right angles to the camshaft 1. The other end of the camshaft 1 extends in a fixed or locating bearing 6, which positively locates that end of the camshaft 1, while its first-mentioned end extends in a floating bearing 7 which allows relative axial movement between itself and the camshaft 1.

The camshaft is driven by a sprocket wheel 8.

The distributor 4 is secured by screws 10 to that part of the cylinder head which forms the camshaft casing 9. The two worm wheels 2 and 3 are toothed to have a left-hand pitch. The distributor drive 3 arranged behind the camshaft 1 in Figure 2 is moved anticlockwise as shown by the arrow round the distributor shaft casing when the camshaft 1 is turned in the direction of the arrow drawn on it, i.e. it turns in a left-hand or anti-clockwise sense viewed from the right in Figure 2.

As a result of a differential thermal expansion between the camshaft 1 made of, for example, grey cast iron, and the camshaft casing 9 made of, for example, aluminium, a relative change in length Al between the two occurs as the temperature of the engine changes between its warm and cold states. As a result there is a corresponding change of Aa in the angular position of the worm wheels driving the distributor at any given instant. The stated choice of the directions of pitch and the directions of rotation of the camshaft and distributor worm wheels 2 and 3 results in the change in length of the camshaft 1 causing the relative ignition timing to be retarded as the engine progressively warms up.Starting with an ignition timing angle for a cold engine (with its temperature at 2530K) of, for example, 80 ahead of the upper dead centre, an appropriately chosen diameter D of the two worm wheels of, for example, 23.3 mm, and a helix angle of the two worm wheels 2 and 3 of 450, then a differential change in length Al between the camshaft and its casing of 0.6 mm leads to an ignition timing angle approxlmately 5 ahead of the upper dead centre when the engine has reached a normal operating temperature of 363 or. Thus the ignition timing angle is retarded by30K.

The same temperature-dependent basic adjustment of the ignition timing angle can be obtained with related changes to the direction of rotation of the distributor shaft 5 and/or the camshaft 1, or of the direction of pitch, and the helix angle and/or the relative position of the distributor and camshaft worm wheels 2 and 3.

As viewed in Figure 1, this is accomplished, for example by the use of a distributor 4 rotating in an anticlockwise sense and distributor shaft and camshaft worm wheels 2 and 3 which have a lefthanded pitch, and by arranging the distributor 4 to be on the other side of the camshaft 1 viewing the device as in Figure 2.

By utilising the relative change in length of the camshaft 1 occurring as a function of the engine operating temperature, an optimum basic ignition timing angle is obtainable in a particularly elegant manner for all engine operating temperatures.

Claims

1. An arrangement for adjusting the ignition timing angle of an internal combustion engine In the sense of relatively advancing the ignition timing when the engine cools and relatively retarding the ignition timing when the engine warms up, in which a shaft of the ignition distributor and the engine camshaft are in drive communication via helical-tooth worm wheels, the camshaft is mounted in a locating bearing which restrains axial movement of the camshaft at a position remote from the distributor shaft and is mounted in a bearing which permits axial movement of the camshaft at a position closer to the distribution shaft, the camshaft and the camshaft casing have different thermal coefficients of expansion, the distributor is secured to the camshaft casing, and the direction of rotation, helix angle, direction of pitch and relative position of the camshaft and the distributor worm wheels are selected to be such that the temperatur4dependent relative change in length between the camshaft and the camshaft casing brings about an advance in the ignition timing when the engine coils and a retarding of the ignition timing when the engine warms up.

2. An arrangement according to Claim 1, In which the setting of the ignition timing angle when the engine is cold corresponds to a predetermined optimum value.

3. An arrangement according to Claim 1 or Claim 2, in which the setting of the ignition timing angle when the engine has warmed up corresponds to a predetermined optimum value.

4. An arrangement for adjusting the ignition timing angle of an internal combustion engine, substantially as described herein with reference to the accompanying drawings.