FR2504631A1 - Hydraulic power transmission circuit - has gas to oil and oil to oil heat exchangers to preheat oil from pump - Google Patents

Abstract

The i.c. engine (17) drives a hydraulic pump (3) which pumps high pressure oil to the hydraulic motors (6). The oil returns in the low pressure circuit (7). The exhaust gases (G) from the engine (1) are fed to a gas-to-oil heat exchanger (10) where heat is given up to the high pressure oil (F) to heat it to 180 degrees. An oil-to-oil heat exchanger (11) reoperates heat from the oil leaving the hydraulic motors (6) to preheat the high pressure oil from the pump (3). A final air-to-oil heat exchanger reduces the return oil temperature to 50 degrees C. The heated air can be used to heat the vehicle cab.

Description

Method and device for improving the efficiency of a transmission

hydraulic power.

  The present invention relates to a method for

  improve the efficiency of a hydraulic power transmission of the type according to which the mechanical energy of a heat engine is transmitted to a hydraulic pump sending a high pressure fluid to the input of a hydraulic motor, the output of which is connected to the

  pump by a circuit comprising a low pressure tank.

  More particularly but not limited to, the present invention relates to a hydraulic transmission of the type of

  that described in the French patent application N 080 15416.

  We know from this aforementioned request that the efficiency of a hydraulic transmission can be optimized by always operating the hydraulic motors in the vicinity of their displacement.

  maximum, if the motors are with axial pistons and tilting plate.

  To satisfy this condition, and knowing that the vehicle operates at extremely variable powers, it is necessary to increase the pressure with the power, so that the inclination of the engine plates is in the vicinity of their maximum value However, this is not sufficient for the very low speeds of the vehicle, and as it is supposed to be equipped with several hydraulic motors, (one per wheel, for example), it is necessary to decouple one or more motors, so that those which remain in operation work at their maximum displacement . The aim of the present invention is to propose a complementary optimization of the hydraulic transmission, increasing

  the efficiency of the propulsion unit, including the heat engine.

  This object is achieved in accordance with the invention by the fact that the hydraulic fluid is heated at the inlet of the hydraulic motor by using part of the calories evacuated by the

thermal motor.

  Thus, the hydraulic power delivered to the hydraulic motor is increased by the thermal expansion effect of the fluid.

  (production of an additional flow using these calories.

  This is made possible by the fact that certain oils, in particular silicone oils, which can be used as hydraulic fluids, have the advantage of having a high coefficient of thermal expansion, of the order of 10/0 C, and are

  stable over a wide range of temperatures, (-50 to + 300 ° C).

  If the normal operation is at a temperature

  rature O c ,, it results from a 6 Q increase in temperature an ac-

  proportional increase in flow rate relative to the initial fluid flow rate, Q 0, and relative to the expansion of the latter, and therefore a

  increase in transmitted power.

  Advantageously, part of the calories are used

  discharged into the exhaust gases of the heat engine for heating

  iron the hydraulic fluid at the inlet of the hydraulic motor.

  Advantageously, part of the calories from the fluid are recovered at the outlet of the hydraulic motor in order to preheat the inlet fluid before heating by the calories of the heat engine. Advantageously, the fluid is cooled before its

back into the pump.

  Advantageously, part of the calories removed by cooling the heat engine are used to heat the

  hydraulic fluid at the inlet of the hydraulic motor.

  Advantageously, the temperature of the fluid at the inlet of the hydraulic motor is optimized as a function of the speed of said motor. The invention also relates to a device for

  implementation of the process of the invention, in particular thanks to exchanges

  judiciously arranged thermal geurs.

  Advantageously, the means for optimizing the time

  fluid temperatures include an engine speed sensor, a

  function generator giving an optimum operating temperature

  male as a function of the speed of the motor, a thermal sensor measuring the effective temperature of the fluid at the input of the hydraulic motor, a threshold and hysteresis amplifier controlling, as a function of the difference between the effective temperature and the optimum temperature of the fluid, the opening of a solenoid valve interposed on the exhaust pipe of the heat engine and leading the exhaust gases

  either to the heat exchanger or to the atmosphere.

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  The advantages and characteristics of the invention will be

  better understood after the description of a particular embodiment

  tion made with reference to the attached drawing in which:

  Figure 1 is an overall diagram of a trans-

  hydraulic mission and the device of the invention; Figure 2 is a variant of Figure 1;

  FIG. 3 represents the curves giving, for different

  different values of the viscosity of the fluid, the optimal temperature

  depending on the speed of the vehicle to which the invention applies.

  We see in Figure 1 a thermal engine trans-

  putting through a mechanical connection 2 its 4 opening to a pump 3 to

  constant power In fact, the pump can be linear or rota-

  according to the type of engine 1 The pump 3 sends a high pressure fluid into the high pressure circuit 4 comprising a high pressure accumulator 5 The high pressure circuit supplies one or more hydraulic motors 6, of volumetric type, from which the fluid leaves under

  low pressure and is brought back by a low pressure circuit 7 comprising

  as a low pressure accumulator 8 towards the pump 3 Arrows F

  indicate the direction of circulation of the fluid, in general of the oil.

  More detailed explanations of this type of trans-

  mission are given in French patent application 80 15 416.

  The heat engine 1 has an exhaust duct 9 in which the gases circulate in the direction of the arrows G.

  In accordance with the invention, the high circuit pres-

  sion 4 and the exhaust duct 9 are arranged on one of their

  parts so as to form a gas-fluid heat exchanger 10.

  Although shown in the simplest form of a countercurrent exchanger, the exchanger 10 can adopt multiple configurations of known type thanks to the heat exchange which takes place therein.

  product, the temperature of the fluid entering the hydraulic motors

liques 6 can reach 1800 C.

  At the outlet of the hydraulic motors 6, the temperature

  ture of the fluid is still high (about 170 'C) and it is interesting-

  sant to provide a fluid-fluid heat exchanger 11 against

  current, between the low pressure circuit 7 just downstream of the hydraulic motors 6 and the high pressure circuit 4 upstream of the exchanger

gas-fluid 10.

  This makes it possible to recover the calories from the hot fluid leaving the motors and which must be cooled, and to transfer them to the cold fluid, leaving the pump which, for its part,

must be reheated.

  An exchanger 12 is finally placed between the fluid leaving the exchanger 11, and circulating in the direction of the pump 3, and the ambient air It makes it possible to cool the fluid completely, and to reduce its temperature to the value of 50 C which is that of

usual operation.

  In the case of a cooled engine 1 (engine

  non adiabatic), we can recover the c 1 lories evacuated by

  cooling through a heat exchanger 19 (Figure 2) between the hydraulic fluid and the engine 1 However, this

  solution does not offer the advantages of heat exchanger 10, including

  with regard to the possibilities of simple regulation which

will be exposed later.

  The operation of the system is as follows. It will be described in the context of its application to a vehicle as proposed in French patent application N O 80 15 416, it being understood

  that it is in no way limited to this application.

  With the vehicle operating at constant speed, the pump

  pe 3, driven by the heat engine 1, provides a flow rate Q, At the pressure P, which corresponds to the power PQ necessary to make the vehicle run at the chosen speed The hydraulic fluid leaving the exchangers 11 and 10 has been brought to the temperature Q = Q + A, o O, usual operating temperature, is for example 500 C. Its mass flow rate remained constant, but its volume flow rate was increased by a value proportional to the coefficient of thermal expansion, P, and when the temperature rises, to 9 If the temperature at the inlet of the hydraulic motors 6 is 1800 C, for example, it will be slightly lower at the outlet, (160 to 1700 C approximately), Because of the expansion adiabatic of the fluid, and thermal losses by conduction in the motors 6 To cool the fluid, it is

  first passes through the exchanger Il which also receives counter

  running the fluid leaving the pump 3, which heats up It is in this exchanger II that the fluid cools the most (about a hundred degrees), and it leaves it at a temperature of about 80 ° C., taking into account imperfections of the exchanger This temperature is still too high, and to bring it down to 500 C, before entering the pump, the fluid must pass through the last exchanger 12 o il

  transfers its calories to the ambient air, by convection.

  geur 12 can be used to heat the vehicle, for example.

  During its passage in the pump 3, the temperature of the fluid practically does not vary, because there is a compensation

  relative between the respective rise and fall in temperature

  adiabatic compression and heat loss The fluid entering at 50 C in the exchanger 11 comes out at a temperature of about 150 to 'C, after heat exchange with the hot fluid leaving the hydraulic motors 6 It then eventually passes into the heat exchanger not shown for cooling the heat engine, and in the heat exchanger 10 o it is heated by transfer of the calories provided by the exhaust gases. At the outlet, the temperature of the fluid is thus brought to the value of 180 ° C. , previously indicated,

and the cycle begins again.

  To correctly optimize the transmission efficiency

  must take into account the viscosity of the fluid which plays a determining role in the distribution of losses, in particular in hydraulic motors with axial pistons These are of two types: a mechanical loss, proportional to the viscosity and to the square of the speed of the motor 6 (and therefore also that of the vehicle), a loss by hydraulic leakage, inversely proportional to the

  viscosity, and proportional to the square of the pressure.

  It is therefore clear that at low speeds the loss

  mechanical is low Hydraulic loss should be low also

  ment, but since the pressure level cannot drop below the value of the inflation pressure of the accumulator 8, it has a value relatively greater than the mechanical loss This therefore does not encourage an increase in the temperature of the fluid , which would further increase

  loss by reduction of viscosity.

  In fact, the optimization study shows that the sum

  losses is minimal if the fluid temperature changes, depending on

  tion of the vehicle speed, and for different values of the kinematic viscosity V, expressed in centistokes, along the curves

in Figure 3.

  It can be seen that with a fluid of low viscosity (20 e St) it is excluded to raise its temperature too much, because, beyond a certain limit, the increase in the hydraulic loss by

  viscosity decrease is greater than the gain in power

  mechanical sance obtained by expansion of the fluid It is obvious that the maximum power gain is obtained, therefore the maximum efficiency, for a viscosity of 60 c St These numerical values are only given as an indication, to show the importance of the choice of fluid, and cannot be considered as valid values for all engines In fact, the optimal viscosity depends on the type of engine, and in particular on the value of the gap clearance between fixed parts

and mobile.

  Anyway, the evolution of the curves remains valid in all cases, and shows that the fluid should not be brought to the maximum temperature at low speeds The transmission efficiency can thus be increased by 5% on average, at low speeds, between 0 and 60 km / hb and 10% between 60 km / h and speed

  maximum, taking into account the relative pressure drop increase

  tive to the increase in length of the hydraulic circuit, due to the exchangers. We practically realize the evolution of the temperature

  optimization of performance as a function of speed as follows

boasts.

  A function generator 13 simulates the curve G f (V), by supplying an electrical signal proportional to the desired set temperature, function of the speed signal supplied by a sensor

  speed 14, mounted on one of the vehicle wheels (dynamo tachy-

  metric for example) The electrical signal from a thermocouple 15 mounted in the input pipe of one of the hydraulic motors 6 is representative of the temperature of the fluid at the input of the motor 6 The signal resulting from the difference of these two signals , is applied

  at the input of an amplifier 16 with threshold and hysteresis, for

  apply a solenoid valve 17 responsible for directing the exhaust gases either to the first exchanger 10 or to the atmosphere 18, according to

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  that the difference signal between the set temperature and the temperature

  actual ture at the input of the hydraulic motor 6 is greater or less than the value of the threshold of the amplifier 16 The temperature set is therefore between two levels, the difference of which is all the smaller as the threshold of amplifier 16 is weaker But

  as it is not necessary to have a great precision on the time

  temperature, we will be satisfied with a difference of + 5 C, which will allow

  the solenoid valve 17 not to be stressed too often.

  It is also possible that the role of regulation

  tion is reduced to a secondary function, or even zero, in some cases, because, for low powers, the efficiency of the exchangers must be less good, and this should result in a fluid temperature increasing with the power of the heat engine, therefore also with the speed of the vehicle It is therefore understandable that, within the precise framework of the application of the invention to a vehicle of the type known by patent application 80 15 416, the main aim is to improve the

high power output.

R E V E N D I'C A T I O N S

  1 Method for improving the efficiency of a hydraulic power transmission of the type by which energy is transmitted

  mechanics of an internal combustion engine (1) to a hydraulic pump (3)

  seeing a high pressure fluid (4) at the inlet of a hydraulic motor

  lique (6) whose output is connected to the pump (3) by a circuit (7) comprising a low pressure tank (8), characterized in that the hydraulic fluid is heated at the inlet of the hydraulic motor (6)

  using part of the calories evacuated by the engine

mique (1).

  2 Method according to claim 1, characterized in that

  that we use part of the calories evacuated in the exhaust gases

  heat engine element (1) to heat the hydraulic fluid

  at the inlet of the hydraulic motor (6).

  3 Method according to any one of claims 1 or

  2, characterized in that part of the calories from the fluid are recovered at the outlet of the hydraulic motor (6) in order to preheat the inlet fluid prior to heating by the calories of the engine

thermal (1).

  4 Method according to any one of claims 1 to

  3, characterized in that the fluid is cooled before it returns to

the pump (3).

  Process according to any one of Claims 1 to 4,

  characterized in that the temperature of the inlet fluid is optimized

  of the hydraulic motor (6) as a function of the speed of said motor (6).

  6 Method according to any one of claims 1 to

  , and in which the heat engine (1) is cooled, characterized

  in that part of the calories expelled by cooling are used

  heat engine (1) to heat the hydraulic fluid

  at the inlet of the hydraulic motor (6).

  7 Device to improve the efficiency of a trans-

  hydraulic power mission of the type comprising a thermal motor

  mique (1) mechanically connected to a hydraulic pump (3) sending a high pressure fluid (4) to the inlet of a hydraulic motor (6)

  whose output is connected to the pump (3) by a circuit (7) including

  nant a low pressure tank (8), characterized in that it comprises means (17, 10) for heating the hydraulic fluid at the inlet of the hydraulic motor (6) with part of the calories

  evacuated by the heat engine (1).

  8 Device according to claim 7, characterized in that said means comprise a heat exchanger (10) between the exhaust gases of the heat engine (1) and the fluid at the inlet

of the hydraulic motor (6).

  9 Device according to claim 8, characterized in that it comprises a heat exchanger (11) between the fluid

  hydraulic motor outlet (6) and the fluid upstream of the sample

geur (10) t

  Device according to any one of claims 7

  to 9, characterized in that it comprises a heat exchanger (12)

  between the ambient air and the fluid upstream of the pump (3).

  11 Device according to any one of claims 7

  to 10, characterized in that it comprises means (13-17) for optimizing

  the temperature of the fluid at the inlet of the hydraulic motor

  lique (6) depending on the speed of said motor (6).

  12 Device according to claims 8 and 11, character-

  in that the optimization means comprise a motor speed sensor (14) (6), a function generator (13) giving an optimum operating temperature as a function of the speed of the

  motor (6), a thermal sensor (15) measuring the actual temperature

  fluid at the inlet of the hydraulic motor (6), an amplifier

  cator (16) with threshold and hysteresis controlling, as a function of the difference between the effective temperature and the optimal temperature of the fluid, the opening of a solenoid valve (17) interposed on the exhaust duct (9) of the heat engine ( 1) and leading the exhaust gases either to the heat exchanger (10) or to

the atmosphere.

  13 Device according to any one of claims

  7 to 12 and in which the heat engine (1) is cooled,

  rised in that it includes a heat exchanger (i 9) between the re-

  cooling of the heat engine (1) and the hydraulic fluid

  the hydraulic motor inlet (6).