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BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to power units comprising an internal combustion
engine having variable-volume combustion chambers supercharged by a
turbocompressor having a turbine which receives the engine exhaust gases
and comprising by-pass pipe means which may be permanently open during
operation of the engine, designed to convey the air not drawn by the
engine from the compressor to the turbine with a pressure drop which, if
appreciable, is substantially independent of the flow rate and increases
with the compressor outlet pressure. An auxiliary combustion chamber is
disposed upstream of the turbine and receives at least part of the air
which has travelled through the by-pass pipe.
The invention applies to engines, having rotors as well as to an engine
having reciprocating pistons, and to engines ignited by sparking as well
as by compression. It is specially advantageous, however, in the case of
an installation comprising a motor constituting a displacement machine
(i.e., more particularly a four-stroke engine as opposed to a two-stroke
engine). In this case also, the invention is of particular advantage in
the case of power units comprising a four-stroke engine having a low
volumetric ratio (below 12 and even possibly below 6), supercharged by a
turbine-compressor unit whose compressor has a high compression ratio (up
to 6 or even more) and operating near its surge line so that its
efficiency is also high.
The term "turbocompressor" or "turbine-compressor unit" is to be construed
as covering the case where there are a number of turbine units and/or
stages or where there are a number of compressor units, the air being
cooled if required between successive compressor units.
In the power units of the aforementioned kind which is described in French
Patent Specification 2,179,310 and in the corresponding U.S. Pat. No.
3,988,894 the by-pass pipe is provided with throttle means which subject
the air flowing from the compressor to the turbine to a pressure drop
which is substantially independent of the flow rate and increases in
linear proportion with the compressor outlet pressure (and usually
represents 5 to 15 % of the last-mentioned pressure). The by-pass pipe
enables the turbocompressor to operate like a gas turbine with high
efficiency, near its surge line.
On the other hand, some of these power units have a limitation.
The supercharging turbocompressor is selected to match the engine when the
latter operates at the rated point, i.e., at maximum power (maximum torque
and maximum speed). The matching is generally such that, at the rated
point, the compressor supplies the flow rate of air drawn by the engine
plus an additional 5 or 15% of the flow rate taken in by the engine, the
additional flow being used for:
Maintaining an air flow in the by-pass pipe so as to maintain the pressure
difference between the compressor and the turbine,
And/or supplying the oxygen needed for a pilot burner in the auxiliary
combustion chamber, if such a chamber is provided,
And/or feeding circuits for cooling hot engine components (e.g. the exhaust
manifold, the spark-plugs if any, etc.),
And/or supplying reserve air for covering variations in the ambient
conditions or progressive clogging of the air filters in use.
For decreasing the manufacture and operation costs, the compressor is
typically selected to meet but not to exceed the aforementioned
requirements.
If now the air intaken by the engine is at a substantially constant
temperature, the line representing engine operation at constant speed (the
flow-rate/pressure characteristic) is approximately a straight line
passing through the origin, at least in the case of a four-stroke engine
constituting a volumetric machine. On the other hand, the operating point
of the turbocompressor moves along a curve which is concave towards the
pressure axis and which extends through (a) a rated matching point
corresponding to the rated pressure and a flow rate between 5 and 15%
higher than that taken by the engine and (b) a point representing a zero
flow rate for a pressure ratio equal to unity.
Consequently, the two characteristics inevitably intersect, whatever the
engine speed (the only effect of which is a decrease in the slope of the
engine flow-rate/pressure characteristic when the engine speed is lower).
If the pressure ratio is allowed to drop to and below the point of
intersection, the air flow in the branch pipe will first stop and then
reverse, thus preventing normal operation of the engine and, more
particularly, any acceleration without load, as we shall see hereinafter.
One obvious method of overcoming the difficulty would be to inject fuel
into the auxiliary combustion chamber at a rate controlled by regulating
means preventing the supercharging pressure from falling below a value
slightly above that corresponding to the intersection of the curves. In
most engines, however, this method is unsatisfactory since it
substantially increases the total fuel consumption when the engine is
idling or under low load.
It is an object of the invention to provide an improved power unit of the
above-mentioned type, wherein the aforementioned disadvantage is at least
partially overcome.
It is another object of the invention to take advantage of the fact that in
power units as disclosed in the aforesaid U.S. Pat. No. 3,988,894 the
pressure difference upstream and downstream of the by-pass pipe is
independent of the flow rate through the branch pipe (the difference being
the same as between the compressor outlet and the turbine inlet) so as to
solve a problem which is common to all supercharged engines having a low
compression ratio and is rendered more acute if the intake is throttled;
and which consists in starting and operating the engine under idling and
low-power conditions when the ambient temperature is very low.
According to an aspect of the invention, there is provided an internal
combustion engine having variable volume combustion chambers, a
supercharging turbocompressor unit having a compressor and a turbine, the
inlet of said turbine being connected to receive the exhaust gas of said
engine,
pipe means connecting the outlet of the compressor to the intake of the
engine,
by-pass conduit means having an inlet and an outlet connected to flow the
air delivered by said compressor and not drawn by the engine to the inlet
of said turbine with a pressure loss which is substantially independent of
the ratio of the flow rate traversing said by-pass conduit means to the
air flow delivered by the compressor and increases with the output
pressure of the compressor,
an auxiliary combustion chamber connected to receive at least part of the
air which circulates along said by-pass conduit means and the exhaust gas
of said engine and having an output connected to the inlet of said
turbine,
adjustable air flow throttling means diposed in said pipe means downstream
of the junction of said bypass conduit means with said pipe means,
control means having means responsive to the load of the power unit and
operatively associated with said throttling means for limiting the rate of
air flow taken by the engine when the unit delivers a low amount of power
and maintaining said rate of air flow at a value lower than the air flow
delivered by the compressor.
Advantageously the automatic actuating means are responsive to engine
operating parameters so as to maintain a constriction such that the flow
rate through the by-pass pipe is sufficient to establish a well-defined
pressure difference between the turbine inlet and the compressor outlet
and to provide the oxygen necessary for fuel burning in the auxiliary
chamber, and also so as to ensure that the engine exhaust temperature does
not exceed a permissible limit.
The power unit preferably comprises recycling pipe means communicating with
the output of the auxiliary combustion chamber and with the intake of said
engine and non-return valve means in said pipe means which open when the
pressure in the intake is lower than the pressure at the output of the
auxiliary combustion chamber for recycling combustion gas from said output
to said intake.
Thus, the engine intakes a fraction of its exhaust gases and of the
combustion gases from the auxiliary chamber together with the air required
for fuel combustion during start-up at low temperature.
Supercharged Diesel engines provided with a throttle valve at the engine
intake are known (U.S. Pat. No. 2,633,698 Nettel). In these engines,
however, the throttle valve fulfills a completely different purpose from
that contemplated by the invention, and does not give the same result.
More specifically, the only purpose of the valve in such prior art engines
is to render starting the engine at low ambient temperature easier; when
the flow of air from the compressor is stopped by throttling the valve,
the air is heated by compression before it enters the engine (e.g., see
col. 1, lines 52-55, and col. 2, lines 1-6).
In such prior art supercharged Diesel engines, a pipe can also be provide
for recycling exhaust gases, in which case the pipe must be provided with
a throttle valve for adjusting the ratio between the exhaust-gas flow rate
and the rate of air arriving through a wide open duct. The throttle valve
is usually manually controlled and it can be only very approximately
adjusted. Starting at very low temperature remains difficult, either
because too much exhaust gas is recycled so that combustion becomes
incomplete and the engine may choke and stall, or because not enough is
recycled, thus preventing starting. Furthermore, the throttle valve is
immersed in high-temperature gases and therefore rapidly deteriorates.
It should be noted that the invention does not consist simply in changing
the position of a throttle valve disposed downstream of the compressor so
as to facilitate starting of supercharged engines in accordance with a
prior-art feature, for the same purpose as in the case of an engine
according to French specification 2,179,310. The invention consists in a
combination of throttle means and means for controlling them and which
come into action when the engine is idling or slightly loaded so as to
fulfil a function which was not described (and had no reason to be
described) in the prior art.
A fundamental difference between the results sought by the two methods is
also clear from the fact that, apparently, the only result of a prior art
valve is in a power unit comprising a permanently open pipe is to
aggravate the problem which the valve is intended to solve in U.S. Pat.
No. 2,633,698, i.e., when the load is low, the throttle means tend to
direct air down the by-pass pipe.
In the power unit according to the invention, the difficulty is overcome by
combining a by-pass pipe producing a well-defined pressure drop and
wide-open recycling means, so as directly to regulate, not the ratio of
the two flow rates, but the flow rate of air to the engine, which is
maintained at a value such that there is no exhaust overload. In the
by-pass pipe there are two pressure levels which may be called "upstream"
and "downstream" and are both determined by the upstream pressure only.
When the throttle means at the engine intake provide an air flow
cross-sectional area which is insufficient for the intake manifold
pressure to be greater than the downstream level, recycling occurs and
restores the downstream level such that a hot atmosphere, at the same
pressure as downstream of the engine, prevails in the intake manifold.
Since the "downstream" pressure level is not influenced by the extent to
which the throttle means are opened, the flow rate of fresh air to the
engine is dependent only on the flow cross-sectional area provided by the
throttle means. Thus, the cross-sectional area can be adjusted to fulfil a
given condition, e.g. maintaining the engine intake temperature at a
constant value, without a complete feed back loop.
The recycling means are typically arranged to recycle the engine exhaust
gases and gases from the auxiliary chamber to the intake. Then the
regulating means can be simplified and the gases taken at the auxiliary
chamber outlet contain little or no unburnt hydrocarbons, unlike the
exhaust gases. Finally, the combustion gases are much hotter, and can
therefore be recycled at smaller mass flow rates.
The recycling means can consist of a pipe having a large cross-section (so
as not to produce an appreciable pressure drop) and provided with
non-return means (e.g. a check valve having a light spring basing it
toward its seat). In that case, the system for regulating the throttle
means can be designed to operate only when the engine load falls below a
value which is just above the value at which the auxiliary chamber comes
into action so as to maintain the compressor outlet pressure at a
threshold level necessary for self ignition of the engine.
SHORT DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following description of a
power unit constituting an embodiment given by way of example. The
description refers to the accompanying drawings in which:
FIG. 1 is a simplified diagram of the power unit, which comprises a
hydromechanical system for actuating the throttle means;
FIG. 2 shows the flow-rate pressure characteristics of an engine and a
turbocompressor which can be included in the power unit of FIG. 1;
FIG. 3 includes curves showing the variation in power in an engine for use
in the power unit in FIG. 1, in dependence on its speed rpm for various
exhaust temperatures Te, up to a maximum permissible value which is
650.degree. C; and
FIG. 4 diagrammatically shows a modified embodiment.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown a power unit comprising an engine 10,
hereinafter assumed to be a four-stroke Diesel engine having a low
volumetric ratio (less than 12and a supercharging turbocompressor unit
comprising a turbine 11 and compressor 12 whose rotors are connected by
shaft 13. An electric cranking motor 15 coupled to the shaft by a clutch
16 is mounted in the air intake of compressor 12, which is of centrifugal
type. Compressor 12 has a high compression ratio, typically greater than
6. Ratios of this magnitude can be obtained with existing single-stage
supersonic compressors. An air cooler 17 provided with means (not shown)
for putting it out of operation is disposed in the air path provided by
the connecting pipe 18a running between the air compressor 12 and the
engine intake manifold 18. An auxiliary combustion chamber 19 provided
with a fuel supply 20 can reheat the gases coming from the exhaust
manifold 21 via engine exhaust pipe 21a before they enter turbine 11, when
the energy contained in the exhaust gases is insufficient.
A by-pass pipe 22, permanently open, in operation supplies the turbine with
whatever air is provided by compressor 12 and not taken by engine 10. Pipe
22 connects to pipe 18a at the upstream end of cooler 17 and rims to the
auxiliary combustion chamber 19. Pipe 22 contains a throttle device 23
which subjects the air travelling through pipe 22 to chamber 19 to a
pressure drop .DELTA. P = P.sub.2 - P.sub.3 which increases substantially
in proportion with the compressor outlet pressure P.sub.2 and which is
independent of the flow rate through the pipe. Device 23 may inter alia be
as described in French Patent Specification 2,179,310 or the corresponding
U.S. Pat. No. 3,988,894.
With reference to FIG. 2, the problem of operating the power unit at low
pressures and low surpercharging flow rates, will be described along with
the solution provided by the invention.
Since engine 10 is a volumetric machine and, via cooler 17, receives air
which is usually at an almost constant temperature (approx. 100.degree.
C), its characteristic (the variation in the mass flow rate of intaken air
Q in dependence on the relative supercharging pressure P.sub.2 /P.sub.1)
at a constant speed N is substantially a straight line passing through the
original and having a slope which decreases with the speed. For example,
the characteristics A.sub.0, A.sub.1 and A.sub.2 of a typical 800 hp
diesel V8 engine at N = 2000 rpm, N = 2500 rpm and N = 2,800 rpm overspeed
are shown in chain lines on FIG. 2; curve A.sub.1 extends through the
rated point P.sub.M.
Since, owing to the permanently open branch pipe 22, the turbocompressor
operates as a gas turbine, the compressor 12 has a single characteristic
curve B passing through the point corresponding to P.sub.2 /P.sub.1 = 1
(P.sub.1 denoting atmospheric pressure) and Q = 0 (Q denoting the mass
flow rate of the compressor). The characteristic (B) is a concave line, an
example of which is given in FIG. 2. Its exact shape depends on the cross
sectional area offered by the nozzles of the turbine and there is no
simple way of modifying it for a particular turbocompressor in operation.
If compressor 12 is to be satisfactory matched to engine 10, the rated
point P.sub.C of the compressor generally corresponds to a flow rate
between 5 and 15% higher than the flow rate corresponding to the rated
point P.sub.M of the engine.
As can be seen, curve B inevitably passes below curve A near the origin
(for a value .pi..sub.1 = 1.7 of P.sub.1 /P.sub.2 at 2500 rpm in the case
illustrated) and operation below thie point is impossible since it would
result in a reverse flow in pipe 22 and the extinction of chamber 19. A
pressure drop .DELTA. P proportional to P.sub.2 - P.sub.3 would not be
obtained any longer. If the injection of fuel into the auxilary chamber is
simply controlled by device 20 so as to maintain the pressure in manifold
18 (detected by a pick-up 24) at a sufficient level for self-ignition, and
if this level corresponds to a value .pi..sub.2 P.sub.2 /P.sub.1 below
.pi..sub.1, it can be seen that it is impossible to accelerate under no
load conditions up to the rated speed. If, for example .pi..sub.2 = 1.5
(as illustrated in FIG. 2) it is impossible to accelerate under no load up
to 2000 rpm, to say nothing of higher speeds; the auxiliary chamber will
be extinguished before 2000 rpm, owing to lack of air, and since the
turbocompressor cannot operate only on the low-temperature exhaust gases
from engine 10, the engine also will stall.
This situation is liable to occur and produce faulty operation during
double declutching on a vehicle or when coating as well as if a ship's
propeller gets out of water or is subject to cavitation.
A simple remedy is to increase the threshold value for it to be greater
than .pi..sub.1, but this considerably increases the consumption under
partial load and during idling. Another solution is to provide a
compressor which, at the rated point P.sub.C, gives a much higher flow
rate than required by the engine; in this case matching is poor, since the
flow rate along pipe 22 will be too high, at least under partial load, and
the engine exhaust gases will be cooled by dilution, necessitating either
increased heating in chamber 19 (i.e., an increase in total fuel
consumption) or an increase in the engine exhaust temperature and
consequent thermal overloading.
The power unit of FIG. 1 comprises means which obviate the last-mentioned
fault without increasing fuel consumption during idling and under low
load, and retaining satisfactory compressor engine matching. For that
purpose, there are provided:
Means 25 for throttling the flow of air to engine 10, the means being
represented in FIG. 1 by a butterfly valve which can be replaced by any
other means which is not subjected to subtantial pressure forces tending
to open or close it; and
A system for automatically actuating means 25, which maintains means 25
fully open when the engine load is greater than a given fraction (e.g.
20%) of its rated load and, below this threshold, partially closes means
25 so as to limit the air flow to motor 10 to a value which is
sufficiently low for the flow rate in pipe 22 to enable chamber 19 to
operate and sufficiently high for the temperature of a critical component
of engine 10 (usualy the exhaust temperature) not to exceed a given
limiting value.
This limitation has no disadvantage for the engine. In the absence of means
25, the engine is under low power and takes in much more air than is
needed for fuel combustion, and the exhaust temperature is much lower than
the maximum permitted temperature. At a given power, the exhaust
temperature is a very rapidly decreasing function of the speed N (the
air/fuel ratio increasing substantially in proportion to N). This property
is shown in FIG. 3, which shows the variation, at a predetermined exhaust
temperature T.sub.e, of the fraction F of the rated power of a typical
engine in dependence on the speed N, referred to the rated speed N max.,
for an installation having an auxiliary combustion chamber 19 which comes
into operation when F is less than 20%, so as to maintain P.sub.2 /P.sub.1
at the value .pi..sub.1 and ensure self-ignition. It can be seen that, at
low load and high engine speed, the operating conditions are still such
that the exhaust temperature T.sub.e is low and can be increased by intake
throttling without risk of overheating.
Consequently, the automatic actuating means can be very simple; it can be
limited e.g. to an element for moving means 25 associated with a
temperaure pick-up disposed on the exhaust manifold and a circuit which
operates the element so as to maintain the gas outlet temperature at a
value which is either constant or dependant on N/N.sub.m as long as F does
not exceed 20%. Whatever method of actuation is used, the result is to
modify the engine characteristics at a constant speed. By way of example,
the chain-dotted line in FIG. 2 shows the modification of characteristic
A.sub.1, the lower part of which has the same characteristic as A.sub.O
and thus intersects curve B below .pi..sub.1.
However, the auxiliary combustion chamber cannot operate if the engine is
allowed to operate at overspeed (from 2800 rpm) irrespective of the engine
load (since curve A.sub.2 is above curve B). Consequently, a device for
relighting the auxiliary chamber should be provided, or no load overspeed
operation should be prevented.
In the embodiment shown in FIG. 1, means 25 and their actuating system are
associated with a hot-gas recycling pipe 26 which, when means 25 are put
into operation, re-establishes the downstream pressure P.sub.3 at the
engine inlet by supplying hot gases from the outlet of the auxiliary
combustion chamber 19. Pipe 26 connects the downstream end of chamber 19,
which is at pressure P.sub.3, to the engine intake manifold 18. Pipe 26
contains a non-return check valve 27 which closes the pipe during the time
when fresh air after travelling through means 25 is intaken by the engine
at a pressure higher than P.sub.3.
Since .DELTA. P = P.sub.2 - P.sub.3 is dependent only on P.sub.2, the flow
rate of air through means 25 is dependent entirely on the flow
cross-section offered by means 25. Accordingly, an examination of the
operation of the engine shows that the system for regulating means 25 can
be very simple, like the system shown in FIG. 1.
During low-power operation (when P.sub.2 corresponds a fraction F which is
less than 20% in the case illustrated in FIG. 3), the air cooler 17 being
inoperative, the auxiliary combustion chamber 19 operating and butterfly
valve 25 being partly closed, there is the following approximate relation
between the temperature T.sub.5 at which the air-exhaust gas mixture
enters the engine, the compressor outlet temperature T.sub.2, the recycled
gas temperature T.sub.3, the sucked in air flow-rate q.sub.a and the total
flow rate q.sub.m taken in by the engine;
T.sub.5 = (q.sub.a /q.sub.m) T.sub.2 + (1 - q.sub.a /q.sub.m) T.sub.3.
assuming that the pressure drop .DELTA. P = P.sub.3 - P.sub.2 produced by
means 23 is proportional to P.sub.2, and that T.sub.2 and T.sub.3 depend
only on P.sub.2 (i.e., the compressor has a single characteristic line) a
simple calculation shows that T.sub.5 remains substantially constant (at a
given ambient temperature) if the cross-sectional area S provided by the
throttle means 25 is proportional to the engine speed N, provided the
auxiliary combustion chamber operates so as to maintain P.sub.2 at the
bottom value.
Under these conditions, a constant intake temperature during the operating
periods at which chamber 19 intervenes to maintain the bottom level
.pi..sub.2, can be maintained simply by actuating the means 25 so that S
is proportional to N.
The system in FIG. 1 comprises an automatic regulating device which is
designed to fulfil the last-mentioned condition and to take account of
changes in ambient temperature, which result in proportional changes of
temperature T.sub.3 (for a given value of .pi..sub.2) and the intake
temperature, as long as cooler 17 is inoperative. As will be seen, the
compensation is produced by modifying the proportionality ratio between
the cross-section and the speed of rotation.
The actuating system can be regarded as comprising:
A source of oil or any other hydraulic fluid at a pressure proportional to
the square of the engine rotation speed N;
An actuator operatively associated to the throttle means 25; and
A switching or distributor valve 45, which is sensitive to the pressure
P.sub.2 upstream of the throttle means and applies the oil pressure to the
actuator, either completely or after reduction, depending on the pressure
P.sub.2.
Oil under pressure is supplied by an oil pump 28 driven by engine 10 and
supplying a flow rate Q.sub.h proportional to the speed N;
q.sub.h = k.sub.1 N
the pump draws oil from a tank 29 and delivers it to a pipe system 30
comprising a calibrated valve 31, the only purpose of which is to protect
the hydraulic circuit. The pump delivery pressure P.sub.h is adjusted by
one or more leak circuits containing restricted nozzle means having a
cross-section which is fixed or dependent on that operating parameter,
which is to be taken into consideration.
In the embodiment illustrated, two leak circuits are provided in parallel
flow relation.
The first circuit comprises a duct 32 leading back to the dump tank 29 and
comprising a nozzle 33 having a fixed cross-section s.sub.1, and
The second circuit comprises a duct 34 leading back to the tank and
comprising a nozzle 35 having a flow cross-section s.sub.2 dependent on
the position of a needle 36 carried by a temperature sensitive capsule 37
subjected to ambient temperature T.sub.O.
Thus, when the first circuit is open, the pressure P.sub.h in pipe 30 is:
P.sub.h = k.sub.2 N.sup.2 / (s.sub.1 + s.sub.2).sup.2
k.sub.2 is being a constant.
If pressure P.sub.h can reach 50 bars at the rated speed, valve 31 will
e.g. be calibrated at 60 bars.
The actuator is a two-stage hydraulic jack 38 comprising a cylinder 39 and
two pistons axially movable in the cylinder, namely:
A main piston 40 having a rod connected to a lever secured to the butterfly
valve 25, one surface of which is subjected to atmospheric pressure
P.sub.O and the other surface of which is subjected to the pressure
P*.sub.h in a relay chamber 42, whose action is opposed by that of a
return spring 41; and
A control piston 43 which is urged in a direction away from piston 40 by
the pressure P*.sub.h in chamber 42 and a calibrated return spring 44, and
is urged in the other direction by the delivery pressure P.sub.h of pump
28.
Last, the switching valve 45, by "on-off" operation gives the following
values to P*.sub.h :
The value P.sub.O when the supercharging ratio P.sub.2 /P.sub.0 is less
than a predetermined value (.pi..sub.3 on FIG. 2) thus actuating butterfly
valve 25 via piston 43 (in which case piston 40 is substantially pressure
balanced); and
the value P.sub.h when P.sub.2 /P.sub.0 is greater than the threshold
value, .pi..sub.3, thus moving piston 42 to its limit position (towards
the left in FIG. 1) corresponding to full opening of butterfly valve 25.
Switching valve 45 comprises a casing disposed in duct 32 downstream of the
place where chamber 42 is connected, and contains a slide valve 46 which
is acted upon in one direction by a force exerted by pressure P.sub.2 and
in the other direction by a calibrated spring 47.
Spring 47 is calibrated so that, when P.sub.2 /P.sub.0 is less than
.pi..sub.3, slide valve 46 occupies a position (full lines in FIG. 1) in
which it connects duct 32 with the discharge tank) whereas when P.sub.2
/P.sub.0 exceeds this value, valve 46 shuts off the connection (the
position shown in broken lines).
If the ambient temperature T.sub.0 can vary within wide limits, .pi..sub.3
should preferably be given a value which increases when T.sub.0 decreases.
For this purpose, as indicated by broken lines in FIG. 1, it is sufficient
to pick-up the pressure acting on valve 46 at a point in leak duct to
atmosphere from the intake of engine 10, that point being located between
a nozzle 48 having a constant cross-section and a nozzle having a
cross-section controlled by a needle 49 of suitable profile, borne by a
thermometer capsule 50.
The operation of the power unit will be briefly described, assuming that
the pressure value .pi..sub.3 is not adjusted in dependence on the ambient
temperature T.sub.0, but the pressure P.sub.h is adjusted in dependence on
temperature T.sub.0.
For simplicity, we shall assume that the turbocompressor is first started
up and then brought to the operating rotating speed, using cranking motor
15 and auxiliary chamber 19. Pressure P.sub.h is then zero and the
supercharging pressure is low. Springs 41, 44 return pistons 40, 43 to
their abutment position (towards the right in FIG. 1) which is selected so
as to correspond to complete closure of butterfly valve 25. Slide valve 46
connects the relay chamber 42 to the tank 29.
Next, the cranking motor 51 of engine 10 is actuated. As long as the engine
operates with the starter in operation, its speed is low (less than 250
rpm for a rated speed of e.g. 2500 rpm). Pressure P.sub.h remains below
the value (e.g. 0.5 bar) at which piston 43 begins to move after
overcoming the force of spring 44 (P*.sub.h remaining equal to P.sub.0).
Under these conditions, engine 10 starts by directly intaking the gases
supplied by combustion chamber 19, which receives an excess of air which
is quite sufficient to burn the fuel injected into the engine. After the
motor has started, but is still under idling or low-load conditions, and
until the value .pi..sub.3 of P.sub.2 /P.sub.0 is reached and causes
valve 45 to switch over, actuator 38 progressively opens the butterfly
valve 25 so that the air flow cross-section (and consequently the air flow
rate) is substantially proportional to the speed N, the proportionality
coefficient depending on T.sub.0.
To this end, it is sufficient if the cross-section of valve 25 and the
cross-section of the pipe section in which the valve moves are such that
the flow cross-section varies in proportion to the square root of the
travel of piston 40 from the completely-closed position.
Finally, after passing the value .pi..sub.3 (which is advantageously
slightly greater than the value for which the auxiliary chamber 19 is cut
off), the slide valve 46 moves into the position shown by broken lines in
FIG. 2, and P*.sub.h becomes equal to P.sub.h (which simultaneously
increases owi | | |
