GB2335483A

GB2335483A - Method and apparatus for cooling an engine using exhaust gas

Info

Publication number: GB2335483A
Application number: GB9805838A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1998-03-19
Filing date: 1998-03-19
Publication date: 1999-09-22
Also published as: GB9805838D0

Abstract

A cooling system for varying the engine operating temperature in dependence upon engine load in an engine comprises at least one cylinder 20 with intake 22 and exhaust 26 ports and a coolant circuit comprising a pump, flow passages 34, 36, 38 through the engine, a thermostat 40 and a radiator connected in series to form a closed loop. The coolant is additionally heated after it has traversed the engine cylinders but before it reaches the thermostat by passing the coolant through a heating passage 36, 38 deriving heat from the exhaust gases. The temperature of the coolant reaching the thermostat 40 thus exceeds the average coolant temperature in the engine by an amount that varies in direct proportion to the engine load dependent temperature of the exhaust gases.

Description

2335483 - 1 METHOD AND APPARATUS FOR COOLING AN ENGINE

Field of the invention

The invention relates to an engine cooling system comprising a coolant pump for circulating a coolant around a cooling circuit that includes the engine and a heat exchange radiator, and a valve for regulating the rate of flow of the coolant in the cooling circuit in dependence upon coolant temperature. In particular, the invention sets out to enable such an engine to operate at a temperature that varies with engine load, the engine running hotter at low load than at full load.

Background of the invention

In a conventional engine, the flow regulating valve is a so-called thermostat that opens as the engine coolant temperature rises and closes as the coolant temperature dro-Qs. The thermostat thereforef as its name implies, automatically regulates the coolant flow to maintain a constant coolant temperature.

It is not however ideal to maintain the coolant temperature constant under all operating conditions. Instead, it is advantageous to operate an engine with a higher coolant temperature at low loads and at a lower coolant temperature at high loads. At low load, a higher operating temperature acts to reduce engine friction and improve fuel economy. At high load, a lower operating temperature acts to increase the intake charge density thereby increasing engine power output and it also acts to reduce the risk of engine knock thereby improving engine durability.

It has been previously proposed to achieve a variable coolant temperature by introducing an electrical heating 2 - element within an otherwise conventional thermostat valve. The valve is calibrated to open at a higher than normal coolant temperature and the electric heating of the valve causes the valve to open below that temperature. The regulated coolant temperature will therefore vary with the heating current applied to the thermostat, thereby allowing the temperature of the coolant in circulation to be adjusted at will.

In order to achieve a variable coolant temperature, it has also previously been proposed to divert a variable proportion of the cool-ant flowing through the coolant pump and the thermostat into a bypass passage that is connected in parallel with the engine. The coolant is not heated in the bypass passage and as a result the temperature of the combined flow sensed by the thermostat is lower than the coolant temperature within the engine proper. If the thermostat is calibrated to open at a lower than normal coolant temperature, then the coolant in the engine will exceed the regulated temperature of the combined flows by an amount dependent upon the proportion of the flow that is diverted around the engine.

Both the above methods provide means for varying the engine temperature, but must rely on external control to set the engine temperature according to load. This increases the complexity of the cooling system and risks engine damage in the event of malfunction of the control system.

Object of the invention The present invention seeks to provide an engine cooling system which automatically varies the engine temperature according to load without relying on external control.

- 3 Summary of the invention

According to a first aspect of the invention, there i provided a method of varying the engine operating temperature in dependence upon engine load in an engine having at least one cylinder with intake and exhaust ports and a coolant circuit comprising a pump, flow passages through the engine, a thermostat and a radiator connected in series to form a closed loop, which method comprises additionally heating the coolant that has already traversed the engine cylinder(s) but before it reaches the thermostat by passing the coolant through a heating passage deriving heat from the exhaust gases and disposed in the coolant circuit between the engine cylinders and the thermostat, the temperature of the coolant reaching the thermostat thereby exceeding the average coolant temperature in the engine by an amount that varies in direct proportion to the engine load dependent temperature of the exhaust gases.

In the invention, the engine will always run at a lower temperature than the setting of the thermostat. In the prior art. it was possible, for example, for the electric heating element to fail and result in the engine overheating owing to the reduced circulation rate of the coolant. By contrast, in the present invention, it is not possible for the coolant not to be additionally heated while the engine is running hot, because the additional heating of the coolant is done by the exhaust gases. Furthermore, as the temperature of the exhaust gases varies strongly in direct proportion to the engine load, the need for a control system is obviated as the additional heating is automatically self regulating.

According to a second aspect of the invention, there is provided an engine having at least one cylinder with intake and exhaust ports and a coolant circuit comprising a pump, flow passages through the engine, a thermostat and a - 4 radiator connected in series to form a closed loop, and a heating passage deriving heat from the exhaust gases and arranged in the coolant circuit between the engine cylinders and the thermostat, the heating passage serving to raise the coolant temperature above that in the engine before the coolant reaches the thermostat, the temperature of the coolant reaching the thermostat thereby exceeding the average coolant temperature in the engine by an amount that varies in direct proportion to the engine load dependent 10 temperature of the exhaust gases.

Because the coolant circulation rate is set by the thermostat to maintain the temperature that it senses at a constant value, the cumulative temperature rise of the coolant during its passage from the radiator to the thermostat is effectively constant. As a consequence, if the exhaust gas temperature rises because of an increase in engine load, the temperature of the coolant in the heating passage will be increased by a set amount, and the 2 C thermostat will automatically increase the coolant circulation rate so that the engine temperature will be lowered by the same amount in order to maintain the cumulative coolant temperature at the thermostat constant. The converse will be true when the exhaust gas temperature drops because of a reduction in engine load.

The rate of dependence of engine temperature on engine load will be determined in the present invention by the efficiency of the heat transfer from the exhaust gases to the engine coolant in the heating passage, itself fixed by the design and dimensions of the heating passage Because the exhaust gas temperature can range from 2000C at idle to 10OCC at full load, while the engine operating temperature is required to vary only from 1200C to 80'C, there is ample heat available from the exhaust gases to bring about the necessary variation in coolant temperature within the engine in dependence upon engine load. Preferably at full load, the increase in temperature of the coolant along the heating passage should be between 30'C and 400C, and at least 15'C to provide a minimum variation range in the engine operating temperature between idle and full load.

In a preferred embodiment of the invention, the heating passage may be disposed within the engine cylinder head, the coolant circuit being designed to comprise a last run constituting the said heating passage and lying in thermal contact with the exhaust ports. Conventionally, exhaust ports are not cooled by the coolant in this manner as it would serve little purpose yet increases the thermal loading on the cooling system. The invention does not however set out to cool the exhaust ports but to heat the circulating coolant before it reaches the thermostat. Nevertheless it would be expedient to increase the size of the radiator to cope with the increased heat rejection at high load.

It is of course alternatively possible for the heating passage to be disposed outside the cylinder head in thermal contact with the engine exhaust manifold.

Compared with a conventional engine, the thermostat in the present invention can be set at a higher regulation temperature. At low load the engine operates at an increased temperature with lower friction and reduced exhaust emission, while at high load the engine is better cooled to avoid knock and reduce wear. Thus fuel economy and exhaust emissions during low load operation are improved without compromising safety during high load operation.

The invention also makes it easier to achieve controlled auto-ignition at low and medium loads, which has benefits in producing very low NOx emissions and very stable combustion. In this respect, it is noted that, when combined with measures such as increasing the engine compression ratio, increasing exhaust gas recirculation and increasing 6 the air charge temperature, a rise in engine cylinder temperature is effective in achieving controlled autoignition of a lean fuel/air mixture within the cylinder.

In the present invention, in addition to the cylinder temperature being hotter under low load conditions, both the intake and exhaust ports are also hotter compared with a conventional engine. A hotter exhaust port has advantage in promoting more oxidation of unburnt hydrocarbons in the exhaust gases thereby reducing hydrocarbon emissions. A hotter intake port has advantage in increasing the air charge temperature. If exhaust gases are also recirculated, internally or externally, the resulting cylinder charge will be even hotter and contain more active radicals surviving from the previous combustion cycle, thus further making it easier to achieve controlled auto-ignition.

The temperature of the through flow of exhaust gases from the exhaust ports is only a weak function of the exhaust port wall temperature. This is because the temperature gradient in the gasside thermal layer within the exhaust nort is very steep in the order of several hundred degrees, so that a small difference of a few tens of degrees in the wall temperature will not make a significant difference to the overall temperature gradient in the exhaust gases. Hence the present invention will not be disadvantaged in causing slightly higher exhaust gas temperature at high load because of the hotter exhaust ports.

On the other hand, increasing the thermal contact with the exhaust ports would significantly increase the heat transfer to the coolant and the incremental temperature rise of the coolant along the heating passage of the present invention, thereby further extending the range of operating temperature in the engine cylinders with changes in engine load. If desired, the heating passage of the present invention may additionally surround a section oil the exhaust duct in the engine exhaust manifold to increase the heat transfer to the coolant.

r, Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawing, in which the single figure is a schematic diagram representing the cylinder head of an engine embodying the invention.

Detailed description of the preferred embodiments

The drawing shows a coolant circuit in an engine cylinder head 10, the drawing being a section through a coolant chamber overlying the domes 20 of the combustion chambers, the intake ports 22 and the exhaust ports 26. Each combustion chamber also has a boss 24 for mounting a spark plug or a fuel injector that passes through the coolant chamber. The casting of the cylinder head has two side walls 12, 14 and two internal partitions 16, 18 that together constrain the direction in which coolant flows around the coolant chamber.

Coolant enters the coolant chamber from vertical passages 30, 32 that extend over the height of the engine cylinder block through the cylinder head gasket. The coolant that enters the coolant chamber through these passages has already cooled the cylinder walls and the lower half of the engine in general. The main part of the coolant chamber 34 lies between the partition 16 and the side wall 12 and serves to cool the cylinder head. Ordinarily the coolant from this part of the chamber 34 would be circulated immediately to the thermostat 40 whence it would flow to the radiator and return to the lower engine block.

8 In the present invention, the coolant from the chamber 34 is passed through a heating passage 36, 38 before reaching the thermostat 40. The heating passage 36, 38 is formed by a first run 36 defined between the partitions 16, 18 and passing above the exhaust ports 26, and a second run 38 defined between the partition 18 and the side wall 14 and passing beneath the exhaust ports 26. Hence, as represented by the arrows, the coolant flow is diverted to flow twice over the full length of the cylinder head 10 while remaining in thermal contact with the exhaust ports 26 before reaching the thermostat 40.

The operation of the engine essentially duplicates the operation of an electrically heated thermostat as is to be found in the prior art, but replaces the heating effect generated by an electrical heating element with heat extracted from the hot exhaust gases. The advantages of the substitution is that the heating is automatically regulated in dependence upon engine load and it il s not necessary to provide any form of control system. The system is also fail-safe in that it is not possible for the coolant not to be heated while the engine is opera.Ling under high load.

The thermostat setting is selected always to be higher than the temperature required within the part 34 of the coolant chamber. If the heating passage 36, 38 increases the temperature of the coolant significantly, then thermostat 40 will increase the coolant circulation rate and the temperature of the coolant in the chamber 34 will be lowered correspondingly so that the cumulative coolant temcerature reaching the thermostat would remain constant. Given that the exhaust gas temperature rises with engine load, this will automatically result in the engine cylinders running hotter at part load and cooler at full load.

- 9

Claims

CLAIMS r, 1. A method of varying the engine operating temperature in

dependence upon engine load in an engine having at least one cylinder with intake and exhaust ports and a coolant circuit comprising a pump, flow passages through the engine, a thermostat and a radiator connected in series to form a closed loop, which method comprises additionally heating the coolant that has already traversed the engine cylinders but before it reaches the thermostat by passing the coolant through a heating passage deriving heat from the exhaust gases and disposed in the coolant circuit between the engine cylinders and the thermostat, the temperature of the coolant reaching the thermostat thereby exceeding the average coolant temperature in the engine by an amount that varies in direct proportion to the engine load dependent temperature of the exhaust gases.
2. An engine having at least one cylinder with intake and exhaust ports and a coolant circuit comprising a pump, flow passages through the engine, a thermostat and a radiator connected in series to form a closed loop, and a heating passage deriving heat from the exhaust gases and arranged in the coolant circuit between the engine cylinders and the thermostat, the heating passage serving to raise the coolant temperature above that in the engine before the coolant reaches the thermostat, the temperature of the coolant reaching the thermostat thereby exceeding the average coolant temperature in the engine by an amount that varies in direct proportion to the engine load dependent temperature of the exhaust gases.
3. An engine as claimed in claim 2, wherein the heating passage is disposed within the engine cylinder head, the coolant circuit being designed to comprise a last run constituting the said heating passage and lying in thermal contact with the exhaust ports.
4. An engine as claimed in claim 2, wherein the heating passage is disposed in thermal contact with the engine exhaust manifold.
5. An engine as claimed in any one of claim 2 to 4, wherein at full engine load, the increase in temperature of the coolant along the heating passage is at least 15'C.
6. An engine having a cooling system, constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.