Method for silencing and stabilizing the flame of gas burners fed via pulse width modulation-controlled electromagnetic valves

The method consists of providing a portion of greater cross-section in the gas feed conduit between the electromagnetic valve and the diffuser in a gas appliance, preferably for cooking or heating food.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for silencing and stabilizing the flame 
of gas burners fed via pulse width modulation-controlled electromagnetic 
valves. 
The invention also relates to the means for implementing the method. 
2. Description of the Related Art 
It is known, for example from EP-A-0550340, to feed a gas burner via a 
pulse width modulation-controlled electromagnetic valve. 
The valve throughput is a function of the duty cycle of the valve opening 
signal, this (variable) duty cycle being set by the user, for example by 
acting on a knob or on keys, in relation to the thermal power to be 
provided by the burner. 
In other simpler words the flame is set by the user by fixing the 
percentage of a predetermined time during which the valve is maintained 
open. If this percentage is 100%, maximum flame is obtained, the flame 
being smaller at lower percentages. 
At the lowest operating percentages, the consequent throughput fluctuations 
generate a pulsating flame at the burner with consequent increase in the 
so-called "flame noise" and in the possibility that the flame becomes 
unstable and finally extinguished. 
SUMMARY OF THE INVENTION 
The main object of the present invention is therefore to provide a method 
and means for stabilizing the flame, while at the same time reducing 
"flame noise". 
This and further objects which will be more apparent from the detailed 
description given hereinafter are attained by the method and means in 
accordance with the accompanying claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the figures the reference numeral 1 indicates the gas feed pipe from any 
suitable source, which can be the gas distribution mains or a cylinder. To 
the pipe 1 there is connected a known electromagnetic valve able to 
alternately interrupt gas passage under the control of variable-width 
pulses. 
The outlet of the valve in question is connected by a conduit 4 to a 
conventional injector 3 which, as such, comprises a nozzle 5 and a venturi 
6. The injector is in fluid communication with a burner 7. 
The electromagnetic valve is controlled on the basis of signals emitted by 
a modulator device 8 and amplified by an amplifier 9. The modulator device 
8 is connected to any direct current source 10 (such as a full wave 
rectifier provided with filters and connected to the electrical mains 
supply). 
A regulator system 11 comprising an electronic card connected to analog or 
digital control members is connected to the modulator 8. These control 
members can be analog sliders or potentiometers, or digital keys or 
contacts which enable the user to preset the thermal power emitted by the 
burner according to his requirements of the moment. Hence said members act 
on the card to change the state of the system until it corresponds to the 
required gas throughput. On the basis of this state the modulator device 8 
determines the duration of closure/opening of the valve 2 by modulating 
the signal fed to it. 
According to the invention, in order to reduce the "flame noise" in general 
and the flame pulsations when the burner is at low thermal power, within 
the conduit 4 there is provided a portion 12 having a cross-section 
greater than that of said conduit. The portion 12 can for example be 
formed by modifying the conduit 4 by mechanical operations or by inserting 
additional components into the conduit and fixing them thereto by welding, 
by ring nuts or by other known means able to ensure a perfect seal at the 
points of juncture. The volume enclosed by said portion 12 can, in the 
case of burners for cooking hobs such as that shown, vary preferably from 
15 to 350 cm.sup.3 with a ratio of the diameter of its cross-section to 
that of the conduit 4 of preferably between 3 and 6. The cross-section 
through the portion 12 is preferably circular but can be of other shapes 
and/or positioned closer to the nozzle 5. 
Any material allowed by current safety regulations can be used to construct 
the portion 12 and any components connected to it. Advantageously, in 
order to reduce pressure drops and relative turbulent motion to a minimum, 
both at the inlet and at the outlet of the portion 12, connection pieces 
12A and 12B of gradually increasing and respectively decreasing 
cross-section (for example conical) are provided as shown in FIG. 1. 
The advantageous results of using the portion 12, which can in fact be 
considered a silencer/stabilizer, can be seen from FIGS. 2 and 3, which 
show the variation in gauge pressure .DELTA.P against time, the value 
.DELTA.P being, in the case of FIG. 2, measured at any point of that 
conduit, without the silencer/stabilizer. which in conventional 
arrangements connects the electromagnetic valve to the diffuser, whereas 
in the case of FIG. 3 the value .DELTA.P is measured at the exit of the 
silencer/stabilizer 12 of adequate dimensions in the arrangement of the 
invention. The .DELTA.P measurements are obviously made under the same 
conditions and hence for equal duty cycles. As can be seen on comparing 
the graphs, the use of the silencer/stabilizer 12 results in a substantial 
reduction in pulsations, which can in practice reach annulment. 
It should be noted that where the gas enters the portion 12 there is an 
energy conversion in the sense that part of the kinetic energy of the 
gaseous stream is converted into pressure energy (piezometric), and vice 
versa at its exit from said portion.