OBJECT OF THE INVENTION
The present invention relates to a heater module for the
admission gases of an automobile thermal engine.
The object of the invention is to provide the heater module
with protection means against overheating, as well as to incorporate a control in
the heating means and for these means to allow a closed-loop regulation of the heater
temperature.
BACKGROUND OF THE INVENTION
Patent
PCT/ES03/00511
describes the problems related to designing an admission gas heater for
automobile thermal engines, wherein a heater of this type can be used for the following
functions:
- ■ Aiding cold starts
- ■ Aiding regeneration of the particles filter
- ■ Aiding the reduction of polluting emissions while complying with the
following requirements:
- ■ Minimal load losses
- ■ Short response times
- ■ High power dissipation capability.
This document explains how these design requirements make
it necessary to design heaters with dissipation resistors that must work at very
high temperatures, so that in order to operate such a heater without overheating
risks it is necessary to incorporate sensors for measuring the temperature of its
support, which may be in contact with plastic materials that must not exceed a certain
temperature, which for the plastics commonly used in the industry is on the order
of 150 °C. This heater module with an incorporated electronic temperature control
allows managing the dissipated power efficiently according to said measurement and
eliminates the risk of deformation or fire in the plastic parts in contact with
the heater.
However, an analysis of the failure modes of this heater
module reveals the following risks:
- 1. In case of short-circuit of the power switch in the control circuit, the
resistor will be constantly connected to the battery; if the heater module is mounted
on a plastic collector nothing will prevent the temperature from rising continuously,
deforming said collector due to overheating or setting fire to it, in turn resulting
in an engine fire.
- 2. The control embodied as described in the cited patent
PCT/ES03/00511
, is effective because it is based on the measurement of the support temperature
and limits the power dissipated by the heating resistors in order to keep said temperature
under a given safety value; however, this control has the added risk of the support
being heated due to power dissipation in the power switches, so that the management
of the power supplied to the air flow will be erroneous.
As regards the measurement of the resistor temperature,
invention patent
PCT/ES02/00369
describes a "system for controlling the temperature of the admission air
in Diesel internal combustion engines" which claims a heater with resistors made
by soldering to strips of different metal alloys such that they will dissipate power
while, as they are made from the union of two different metals, their thermocouple
function will allow knowing the resistor temperature. However, a heater manufactured
with resistors of this type does not reduce the aforementioned risk of a poor operation
of the control circuit and overheating, with the resulting risk of fire.
DESCRIPTION OF THE INVENTION
The heater module object of this invention proposes to
eliminate the above-described risks, increasing the safety and reliability of modules
of this type implemented in engines, by incorporating a heat switch in series in
the module heating resistors and mounting resistors with a positive thermal coefficient.
To eliminate the risk of overheating due to a short-circuit
of the heater module power switches or a malfunction of the control electronics
related to the temperature measurement, the control signal, etc., it is foreseen
that the module incorporate a thermal switch in series with the heating resistor,
whether at its earth connection or at its power connection, which cuts off current
to the resistor in case of said control circuit malfunction, this is, when the temperature
of the thermal switch, which is the same as that of the support of the heater module
in which it is integrated and to which it is thermally connected, is higher than
the actuation temperature of the fuse.
On another hand, to eliminate the risk of the support overheating
due to an excessive power dissipation not by the resistors but by the power switch
itself (degraded operation of the power switch), and of an incorrect power supply
to the air flow because the control believes that the overheating is due to an excessive
power dissipation by the resistors that cannot be evacuated by the air flow, a system
is implemented that measures the temperature of said resistors.
To measure this temperature, what is proposed is specifically
to manufacture the resistors using a material with a non-constant thermal coefficient
or by joining to segments, one with a resistance that does not change with the temperature
and another which does, so that measuring the ohmic value of each resistor allows
knowing its temperature.
The use of resistors with a positive thermal coefficient
(PTC) allows, on one hand, knowing the temperature of the resistors (by measuring
their ohmic value) and on another hand automatically limits the dissipated power
(as the resistor temperature increases its ohmic value rises and the dissipated
power falls).
Fitting thermal switches in series with the resistors ensures
that in case of a control circuit failure (such as a short-circuit of the power
switches) so that the resistors are permanently connected to the battery without
air flow (engine stopped), the temperature of the hottest points of support will
not exceed the working temperature of the plastics that may be in contact with it
(when the heater is mounted on a plastic collector).
The thermal switch is located on the resistor support and
is thermally connected to it so that its temperature is the same as that of the
support. The switch cuts off current to the resistors when its temperature exceeds
a predetermined value.
Several types of thermal switches are available in the
market. These can be divided into two groups: reversible switches, among which are
PTCs and bimetallic switches: and non-reversible switches, such as thermal fuses.
Switches of both of these types are devices that do not dissipate power, and therefore
their temperature increase is due exclusively to the increase in ambient temperature,
as opposed to standard fuses, which are power-dissipating elements.
From a safety point of view, a heater that includes both
of these innovations (heat switches in series with the resistors and PTC type resistors)
does not require that the control circuit be mounted on the same support as the
resistors, although in absence of a direct measurement of the temperature of said
support the responsibility for its control will fall on the thermal switch.
In this case the system may consist of a heater with one
or more resistors, depending on the engine requirements, and an electronic control
module electrically connected to the heater by cabling through which the resistors
are powered from the control board. This scheme would be appropriate for cases in
which space limitations or other restrictions in its physical implementation in
the engine prevent using a heater with incorporated electronics.
It is worth mentioning and emphasising that even by adding
the two improvements described above, a module with incorporated control will have
several advantages, among which are the following:
- ■ As the support temperature measurement is available, it is possible
to control the power dissipated before the heat switch is actuated, unlike the solution
that incorporates only a thermal fuse, which is an irreversible device that renders
the part useless and requires the vehicle to be serviced in a garage.
- ■ The cost of a module including the heating resistors and the control
module is lower than that of a system incorporating the components separately: heater,
control circuit and cabling. This is truer considering that the cabling will require
sealed power connectors.
- ■ The cost of assembly in an engine is higher for a system with three
components (heater, cabling, control module).
In short, it is proposed to manufacture heaters either
with the control electronics integrated in the same support or otherwise, incorporating
in any case a thermal switch in series with each heating resistor that can be interposed
between the resistor and its earth connection or between the resistor and its connection
to the power cable. In addition, if the heating resistors have a PTC behaviour,
a closed-loop control can be established using as a variable the temperature of
the resistors themselves.
For the resistors to have said PTC behaviour, they must
be either manufactured with a single wire or ribbon segment of a PTC type material
or with two or more segments of wire or ribbon connected to each other, with at
least one of them being of a PTC type material.
Several possible embodiments of the heater are considered,
in all of which the thermal switch is soldered in series at one of the ends of the
resistor and embedded in the metallic support of said resistors, which are electrically
insulated from said support by ceramic insulators which simultaneously act as a
thermal union between the resistor and the support. The resistors may consist of:
- a) A single resistive segment in the form of a wire or ribbon, of a metal alloy
with a resistivity that does not change with the temperature;
- b) A single resistive segment in the form of a wire or ribbon, of a metal alloy
with a resistivity that does change with the temperature;
- c) Two resistive segments in the form of a wire or ribbon, soldered to each
other, one made of a metal alloy with a resistivity that does not change with the
temperature and the other made of a metal alloy with a resistivity that does change
with the temperature.
The resistors included in the heater do not necessarily
have to be of the same type. For example, a heater with 4 resistors may incorporate:
- A first resistor of an alloy whose resistance does not change with the temperature;
- A second resistor of an alloy whose resistance does change with the temperature;
- A third resistor made of two segments soldered to each other, one made of a
metal alloy with a resistivity that does not change with the temperature and the
other made of a metal alloy with a resistivity that does change with the temperature;
- A fourth resistor identical to one of the above three; Incorporating a thermal
switch soldered in series with each resistor and embedded in the metallic support
of said resistors.
DESCRIPTION OF THE DRAWINGS
To complete the description being made and in order to
aid a better understanding of the characteristics of the invention, according to
a preferred example of embodiment, a set of drawings is accompanied as an integral
part of the description where, for purposes of illustration only and in a non-limiting
sense the following is shown:
- Figure 1 represents an elevation view of the heater module for the admission
gases of an automobile engine that shows the thermal switch placed between the resistor
and its earth connection.
- Figure 2 represents an elevation view of the heater module for the admission
gases of an automobile engine that shows the thermal switch placed between the resistor
and its power connection.
- Figure 3 represents a perspective view of a resistor that is provided with a
thermal switch on one of its ends consisting of a ribbon that can have a positive
or constant thermal coefficient.
- Figure 4 represents a perspective view of a resistor that is provided with a
thermal switch on one of its ends consisting of a ribbon with a constant thermal
coefficient combined with a wire of positive thermal coefficient.
- Figure 5 represents a perspective view of a resistor provided on one of its
ends with a thermal switch consisting of a ribbon with a constant thermal coefficient
combined with a ribbon with a positive thermal coefficient.
- Figure 6 represents a group of resistors of several types, each of which provided
with a thermal switch.
PREFERRED EMBODIMENT OF THE INVENTION
The heater module for the admission gases of an automobile
engine that constitutes the object of this invention is of the type incorporating
a frame (1) and a power control circuit that is provided with at least one power
switch (2) that controls the power supply (3) to at least one heating resistor (4)
mounted on a ceramic support (6) that heats the engine admission gases, and is provided
with an overheat protection system and/or closed-loop regulation based on the temperature
measurement of the frame (1) and/or the temperature of the resistor (4).
Based on this basic design, the heater module mainly stands
out in that it incorporates at least one thermal switch (5) in series with the heating
resistor (4) placed on the frame (1) of said heating resistors (4) which cuts off
current to the resistor (4) when a short-circuit occurs in the power switch, said
thermal switch (5) located between the heating resistor (4) and the earth connection
of the frame (1), as shown in figure 1, or between the heating resistor (4) and
the power supply (3), as shown in figure 2.
The heating resistors (4) can consist of a single resistive
segment in the form of a wire or ribbon of an alloy whose resistivity may or may
not vary with the temperature, as shown in figure 3.
Preferably, a heating resistor (4) will be used with a
positive thermal coefficient (PTC), this is, one whose resistivity changes positively
with the temperature, allowing a closed-loop control using as the control variable
the temperature of the heating resistors (4).
Similarly, the heating resistors (4) can consist of two
resistive segments in the form of a wire or ribbon, soldered to each other, one
of which is made of a metal alloy whose resistivity does not change with the temperature
and the other is made of an alloy whose resistivity does change with the temperature,
as shown in figures 3 and 4.
The heater can incorporate combinations of heating resistors
(4) of different types, such as resistors (4) with a resistivity that changes with
the temperature and others whose resistivity does not change with temperature, each
of which may consist of one or several segments.
As the heating resistors (4) incorporate a thermal protection
in the form of the thermal switches (5) disposed in series, the possibility is considered
that the heater incorporate the power control circuit (2) separate from the frame
(1) and connected to it by a cable.
