EP2789811A1

EP2789811A1 - System for heat recovery of a combustion engine

Info

Publication number: EP2789811A1
Application number: EP20130162727
Authority: EP
Inventors: Harald Fessler
Original assignee: FPT Motorenforschung AG
Current assignee: FPT Motorenforschung AG
Priority date: 2013-04-08
Filing date: 2013-04-08
Publication date: 2014-10-15
Anticipated expiration: 2033-04-08
Also published as: EP2789811B1; ES2635548T3

Abstract

System for heat recovery (WHR) of a combustion engine (E), the system comprising recovery means for developing a Rankine Cycle by exploiting the heat produced by the combustion engine, said recovery means comprising a condenser for condensation of a WHR medium, wherein the condenser (COND) is an exchanger of the WHR medium to air type. Heat sources are CAC, engine coolant (includes also the EGR heat) and exhaust gas.

Description

Field of the invention

The present invention relates to a System for heat recovery of a combustion engine, in particular in the field of large displacement engines.

Description of the prior art

Waste heat recovery (WHR) systems use heat sources of the engine, such as the hot exhaust gas, to convert some of this energy into mechanical power by means of an expander (Ex) exploiting Rankine Cycle.
It is well known that the thermodynamic cycle works between a high temperature source and a low temperature source.
Usually the cold source is the refreshed coolant coming out from the engine radiator to cool the condenser, and then the combustion engine. The condenser develops the phase where the gaseous WHR media, at the expander exit, is converted into liquid before entering the WHR pump (P).
A classical prior art scheme is drawn in figure 1, wherein a supercharged combustion engine is coupled to a waste heat recovery system.
A pump P pumps the WHR medium clockwise according to the dashed circuit drawn in figure 1.
The heater H1 exploits the high temperature of the exhaust gasses downstream of the SCR filter. Therefore the Heater H1 is a gas to WHR medium (independently from its nature: gas/liquid) heat exchanger.
An expander Ex generates mechanical power that could be transferred to the combustion engine or to an electric generator.
The ambient air refreshes the CAC, namely the intercooler, and the engine coolant cooler. The CAC is commonly an air/air exchanger.
The condenser C uses the engine coolant to cool the WHR media, by defining the abovementioned cold source of the cycle. Therefore, the condenser is a "WHR medium to liquid exchanger".
The disadvantage of this kind of energy recovery is the additional heat from the condenser which has to be rejected by the cooler. The limitation of the heat rejection restricts the usage of the WHR especially at higher engine loads where the engine heat rejection is at its limit.
In particular the release of the heat to the ambient is strongly constrained by the available space in the engine compartment for the "to ambient air" exchanger.
In fact the "to ambient air" exchangers, the ECC and CAC, employ a significant amount of space. The heat sources used are conventionally exhaust gas and EGR cooler power.

Summary of the invention

Therefore it is the main object of the present invention to provide a system for heat recovery of a combustion engine which overcomes the above problems/drawbacks.
The main principle of the invention is disclosed in claim 1. In particular the condenser is of the type WHR medium to air exchanger, while the engine coolant cooler is a liquid/liquid or "liquid to vapor" heat exchanger according to the functioning conditions. In the following we will use the expression "liquid to WHR media" exchanger.
This leads to a completely different thermodynamic meaning of the components. In particular, the condenser is dimensioned not only for condensing/refreshing the WHR medium, but also for refreshing the engine coolant and preferably also the compressed air at the engine intake. Therefore, the engine coolant cooler in this context develops the function of preheating/evaporating the WHR medium instead of cooperating with the condenser in refreshing the WHR medium. Hence more heat sources are used for the WHR process and the fuel consumption gain increases.
According to a preferred embodiment of the present invention the intercooler is of the liquid/liquid exchanger type, so that the space for the "to ambient air" heat release is mainly reserved to the condenser.
According to another embodiment of the present invention, bypass means are provided in order to avoid the heating of the system medium in certain critical operation conditions of the combustion engine, so that the heat rejection of the combustion engine is not compromised.
These and further objects are achieved by means of an apparatus and method as described in the attached claims, which form an integral part of the present description.

Brief description of the drawings

The invention will become fully clear from the following detailed description, given by way of a mere exemplifying and non limiting example, to be read with reference to the attached drawing figures, wherein:

Fig. 1 shows a scheme for heat recovering of a combustion engine according to the prior art,
Fig. 2 shows a scheme according to the present invention,
Fig. 3 an embodiment based on figure 2.

The same reference numerals and letters in the figures designate the same or functionally equivalent parts.

Detailed description of the preferred embodiments

A preferred embodiment of the system for heat recovering, WHR, is shown on figure 2.
The invention is particularly adapted but not limited to a combustion engine E provided with a supercharging unit, namely a turbine TB fed by the exhaust gases and a compressor COM driven by said turbine TB for compressing fresh air entering in the combustion engine E.
The engine E could be further provided with an intercooler CAC' for refreshing the fresh air compressed by the compressor COM and/or with an EGR means with an EGR cooler. According to the present invention, the condenser COND is of the type WHR medium (WHR working Media) to ambient air type. The arrow "ambient air" (only) crossing the condenser indicates that the heat is directly rejected to the ambient air instead to be released to the engine coolant.
The medium used for the Rankine Cycle, in the following WHR medium, is pumped by the pump P' through the ECC'- engine coolant exchanger and, preferably, through the intercooler CAC'. Therefore, the ECC' is a liquid to WHR medium heat exchanger, while the CAC' is an air to WHR medium heat exchanger. At the CAC' the WHR medium is in a liquid phase. This means that, in spite of the block size shown in figure 2, they, CAC' and ECC', could be really small in comparison with those implemented by the prior art.
This means also that both the CAC' and ECC' do not need to be disposed in a specific point of the engine compartment, for example in the front side or under the hood, but on the contrary they can be disposed in any place within the engine compartment, for a better optimization of the components disposition.
In addition, the evaporation of the WHR media at CAC' and ECC' reduces the problem of the prior art schemes wherein an overheating of the coolant radiator can occur since the heat source is engine water with about 105°C temperature.
This means the evaporation of the WHR media is well defined without problems of overheat.
When both the ECC' and CAC' are refreshed by the WHR medium, the WHR medium receives a preheating at the CAC' and then preheating & evaporation [namely the energy for causing the evaporation of the WHR medium] at ECC'. The EGR cooler is cooled by engine water and the energy is hence rejected with the engine water and hence to the WHR media at the ECC'. This solves the problem of state of the art systems, that a leaking EGR to WHR heat exchanger will release the WHR media into the engine (e.g. burning Ethanol). Moreover the WHR media is pumped through the heater H2 exploiting the heat contained in the exhaust gasses, where it receives the last heating contribution before crossing the expander Ex'. Therefore, the cold source for the Rankine Cycle is directly the ambient air and not the ambient air through the engine coolant. This allows more flexibility in selecting the engine water temperature for optimum performance. In particular, at the heater ECC (engine coolant), the known systems reach about 85°, while, according to the present invention the coolant temperature can be maintained at a temperature higher than 90°C and about 105°. This helps to extract more energy ad increase the WHR efficiency, without compromising the life of the combustion engine and also means that the efficiency of the Rankine Cycle is improved due to the reduced condensation temperature and the usage of more and potentially any heat vehicular source, without the problem of overheating the WHR media.
The fact that the engine coolant is refreshed by the WHR medium, instead of directly by the ambient air, enables a slightly higher coolant temperature, especially at low environment temperatures.
An increase in coolant temperature is beneficial in order to reduce the engine friction (and also reduce heat losses), mainly due to higher oil temperatures - this is well known. Therefore the CAC' develops a new function with respect to the prior art schemes.
According to a preferred embodiment of the present invention, said heater H2 (exhaust gas heat exchanger) is disposed downstream of the ATS, namely after (according to the exhaust gas flow direction) one or more devices for reducing the pollution emissions, for example the SCR' catalyst or further components, such as DOC, traps and/or ammonia clean-up filters.
According to said preferred embodiment of the present invention, the circuit of the WHR system includes means for bypassing WHRB the heater H2.
In other words a three-way valve controls the WHR medium flow crossing the heater.
According to another preferred embodiment, also the exhaust gasses can bypass the heater H2, through an exhaust gas bypass EGB, that can comprise a bypass conduct and a flap for controlling the gas flow.
The WHR system is managed by a control unit that checks the working conditions of the combustion engine E, the engine coolant temperature and commands said three-way valve and/or said flap to guarantee the required cooling of engine water at ECC' and of the compressed fresh air at the intake CAC'. When the heater H2 is bypassed, the entire heat rejecting capacity of the condenser is exploited for cooling the combustion engine E, with a more controllable behavior of the overall system WHR and combustion engine.
Due to the possibility of excluding the heater H2, only the required heating sources can be exploited. According to a preferred embodiment of the invention said heat sources are engine water (thus the ECC' itself), EGR cooler and CAC', the conditioning condenser. However this means that efficiency improvements are feasible even at (high-critical) loads where state of the art recovery systems have to be turned off due to cooling capacity limitations. While, according to the present invention, when the heater H2 is bypassed, the other heat sources permit, in any case, to perform a Rankine cycle, but without compromising the life of the combustion engine. In addition, when the Rankine Cycle is interrupted, the capacity of heat rejection of the combustion engine is strongly increased with respect to the engine not provided of WHR systems and with respect to the known engines provided with WHR systems of the prior art.
According to computer simulation the benefit in engine fuel consumption can increase from common values of 4 to 5% to 8 to 10% for a medium speed 50% load point typical load condition for a heavy duty truck with 85km/h.
By means of the present invention, a number of further advantages are achieved.
First of all, according to a preferred embodiment, only one exchanger delivers the heat "to the ambient air", instead of two as in the prior art scheme.
Secondly, the vapor pressure at the condenser is fitted to the required condensation temperature and can be strongly reduced by extending the range of suitable fluids to be employed as WHR medium. According to the present invention, the vapor pressure at the condenser can be about 1 bar or lower.
The expander can produce either mechanical, electrical, hydraulic energy to be supplied to the combustion engine. Also the pump can be driven by the combustion engine, the expander or by an electric motor.
The functions of said control unit could be integrated into an engine control unit.
The expander can be known per se.
According to a further preferred embodiment of the invention, which can be combined with the other ones, previously discussed, the combustion engine can be of the turbo-compound type. A turbo-compound scheme has a turbine TBC, driven by the exhaust gasses, and having its shaft connected with the engine crankshaft in order to provide it with mechanical energy.
Between the turbo-compound turbine shaft and the crankshaft a clutch and/or rotational reduction gears can be interposed. The control of the several architectural components described above can be implemented advantageously in a computer program comprising program code means for performing one or more steps of such method, when such program is run on a computer. For this reason the patent shall also cover such computer program and the computer-readable medium that comprises a recorded message, such computer-readable medium comprising the program code means for performing one or more steps of such method, when such program is run on a computer.
Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the scope of the invention are deemed to be covered by this invention.
Further implementations, the several embodiments disclosed can be combined between each other, also with the discussed prior art. Further well known details are not discussed as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.

Claims

System for heat recovery (WHR) of a combustion engine (E), the system comprising recovery means for developing a Rankine Cycle by exploiting the heat produced by the combustion engine, said recovery means comprising a condenser for condensation of a WHR medium, wherein the condenser (COND) is an exchanger of the WHR medium to ambient air type.
System according to claim 1, wherein the engine (E) further comprises first means for refreshing (CAC') fresh air at the engine intake, and wherein said first refreshing means comprise an exchanger of the air to WHR medium type.
System according to claim 1 or 2, wherein the engine (E) further comprises second means for refreshing (ECC') an engine coolant, and wherein said second refreshing means (ECC') comprise an exchanger of the liquid to WHR media type.
System according to claim 1 or 3, wherein said condenser (COND) is the sole "to ambient air" exchanger of both together the recovery system and the combustion engine (E).
System according to claim 4, wherein the engine (E) further comprises third means for refreshing (CAC') the fresh air at the engine intake, and wherein said third refreshing means comprise an exchanger of the air to WHR medium type.
System according to any of previous claims 1 to 5, wherein said recovery means further comprise
- air to WHR medium exchangers for heating the WHR medium, wherein said air to WHR medium exchangers comprise:
. a first heater (H2) exploiting the heat contained in the exhaust gasses of the combustion engine and/or

. a second heater exploiting the heat rejected at the condenser of the conditioning system and/or

- means for refreshing the recirculated gasses (EGR) of the air to liquid (engine coolant) type.
System according to claim 6, further comprising bypass means (WHRB) for bypassing the WHR medium from passing through one or more of said heaters and/or means (EGB) for bypassing the exhaust gasses from passing through said first heater (H2).
System according to claim 7, further comprising control means of said bypass means (WHRB, EGB) configured to reduce the heat recovery at critically high load combustion engine conditions.
System according to any of the preceding claims, wherein the engine water coolant temperature (hot source) is higher than 90°C.
System according to any of the previous claims, wherein a vapor pressure at the condenser (COND) is about 1 bar or lower.
System according to any of the previous claims, further comprising an expander (Ex') able to produce mechanical energy from side Rankine Cycle.
System according to claim 11, wherein said mechanical energy is provided to the combustion engine (E) or is exploited for producing electric energy by means of an electric generator.
System according to any of the claims from 6 to 12, wherein the engine (E) comprises an After Treatment System (ATS) having at least one catalyst and wherein the first heater (H1) is disposed downstream of said catalyst, according to an exhaust gasses flow direction.
System according to any of the previous claims, wherein the combustion engine (E) comprises supercharging means having at least one supercharging group (TB, COM).
Terrestrial vehicle provided with a combustion engine (E) and a system for heat recovery (WHR) of the combustion engine (E) according to one of the previous claims from 1 to 14.