Glow plug for internal combustion micro-engines for modelling

DESCRIPTION

The invention relates to a glow plug for internal combustion micro- engines for modelling.

In the sector of modelling, the radio controlled models provided with internal combustion engines are used both for the purpose of mere entertainment and for real competitions. For example, racing car models are used for racing competitions that are very much alike a real Grand Prix, including pit stops, tyre replacement, refuelling, etc...

Usually, two-stroke engines fed with fuel mixture are used, having very reduced capacity and very high rotation speed up to 40.000 RPM or more in racing engines. Because of the growing request of higher performances especially by the model makers taking part to racings, said engines are object of very accurate studies and design.

More in detail, the ignition system of said engines is usually provided with a glow plug.

Basically, the glow plug comprises a metallic body to screw on the head of the cylinder, with an axial through opening in which a metallic pivot is housed, isolated from the external body. A filament, forming the electrode, is welded to the lower edge of the glow plug body and, inside, to an end of the pivot. The opposite end of the pivot is projecting from the upper part of the plug body, enabling the connection to an ignition device.

A glow plug for internal combustion micro-engines with a similar structure is disclosed, for example, in US 6.346.688.

Basically, the electrode is completely inserted in the plug body, apart from a short part of the end that is welded to the lower edge of the same body. Said short part represents a "hot point" that, in normal conditions, is sufficient to trigger the combustion of the compressed mixture.

Usually, the igniter gets connected only as the engine is ignited; thus, the engine works only if the filament is kept at a sufficiently high temperature.

It is not a problem if the engine is in steady state, considering the very high rotation speed and, consequently, the very high frequency of the explosions in the combustion chamber. Nevertheless, the internal combustion engines for modelling are subject to sudden switching off, especially in case of slow running and/or when the engine is cold. During the races, the switching off is more frequent during pit stops, and the stops for refuelling purpose.

The applicant found out that said phenomenon is caused by the inlet of quite cool mixture in the combustion chamber (that the glow plug fails to ignite).

For example, during the refuelling phase the tank is refilled with room-temperature mixture, cooler than that already contained in the same tank (that warms-up thanks to the proximity to the engine).

When the car leaves the pit and speeds up, a relevant amount of quite cool mixture suddenly flows in the cylinder; in similar conditions, the

electrode inside the glow plug may fail to ignite the engine.

As a result, the engine could miss or even switch off, impairing the race.

The scope of the invention is to eliminate the above said disadvantage. In detail, the invention is aimed to realize a glow plug for internal combustion micro-engines for modelling to prevent from sudden switching off, especially in case of refuelling stops.

A further aim of the invention is to improve the performances of internal combustion micro-engines for modelling, still ensuring the highest reliability, as more and more required by competitors.

The aims are reached by a glow plug for micro-engines for modelling, comprising at least a plug body, a pivot housed in the plug body and an electrode having a first end connected to said plug body and a second end connected to said pivot, characterized in that the plug body comprises a lower edge having a protruding portion, and said first end of the electrode is connected to said protruding portion of the plug body edge, so that, when the plug is assembled to an internal combustion engine, the electrode turns out to be partially protruded inside the combustion chamber of the engine. Preferably, said protruding portion comprises a dentil projecting from the lower edge of the plug body. When the glow plug is assembled to the engine, said dentil turns out to be facing the combustion chamber.

According to a further aspect of the invention, the electrode is composed by an alloy filament made of Platinum, Ruthenium,

Rhodium and Iridium. More preferably, said alloy is composed by: 50% Platinum, 15.5% Ruthenium, 19.5% Rhodium, 15% Iridium. An alloy of this kind turns out to be suitable for bearing the thermal stress in the combustion chamber. Further objects of the present invention are also an internal combustion engine for modelling comprising the described glow plug, and a radio-controlled model, if needed in assembling kit, provided with said engine.

The advantages of the invention will be more evident from the following detailed description and with the help of the figures, in which:

Fig. 1 shows a section view of a glow plug for micro-engines for modelling according to the invention;

Fig. 2 shows a schematic view of an internal combustion micro- engine provided with the plug of Fig. 1. With reference to the figures, a glow plug for internal combustion micro-engines is usually indicated by 1. The plug 1 basically comprises a plug body 2, a pivot 3 and an electrode comprising a filament 4.

The plug body 2 has an internal through opening, in which the pivot 3 is housed as shown in the Figure. The lower portion of the plug body 2 is provided with a thread 5 to screw the plug to the head of the engine.

The plug body 2 and the pivot 3 are made of electrically conductive material, typically steel. The pivot 3 is electrically isolated from the plug body 2, since two

gaskets 6, 7 and a retaining washer 8 are provided. The axial block of pivot 3 in the plug body 2 is obtained, for example, through a calking 9 of the upper edge of the body.

The pivot 3 has an upper end 10, projecting from the plug body 2, for the connection of an igniter. This latter (not shown) enables to establish an electric circuit between the pivot and the plug body, so that a proper electric current can flow and make the filament 4 incandescent.

The features up to now described have been already disclosed by the prior art and are well known for a skilled man; thus they will not be further described.

According to the invention, the lower edge of the plug body 2 has a protruding portion, to which filament 4 is connected. In the example of Fig. 1 , said protruding portion of the lower edge of the plug body is composed by a dentil 11 , projecting more or less 1 mm from the edge surface of the plug body 2. Consequently, the filament 4 has a first end welded to pivot 3, inside the plug body 2, and an opposite end welded to dentil 11.

It is clear that, when the glow plug is assembled to the engine, the filament 4 turns out to be partially protruded in the combustion chamber. Fig. 2 shows a schematic section of an internal combustion micro-engine, in which the piston P, the combustion chamber C and the glow plug 1 are indicated. Due to the connection to the protruding dentil 11 , a portion of the filament 4 extends in the combustion chamber C, instead of remaining completely inside the plug body, as

disclosed by the prior art.

The portion of filament 4 extending in the combustion chamber C is subject to relevant thermal stress. Satisfactory solutions to this problem are achieved by the alloy thread 4 composed by: Platinum 50%, Ruthenium 15.5%, Rhodium 19.5%, Iridium 15%.

Said solutions enable to trigger the combustion in a more efficient way, even in presence of quite cool mixture in the combustion chamber.

Furthermore, performances turn out to be improved.