GB2428452A - Oil spray system for cooling pistons in i.c. engines - Google Patents

Abstract

An oil spray system for an internal combustion engine characterised by the system being having two modes of operation wherein in one mode oil is sprayed continuously, and in the other mode oil is sprayed intermittently. Mode switching may depend on engine operating parameters including any of engine load, engine speed, piston temperature and exhaust gas temperature. The frequency and duty cycle of the intermittent oil spraying may depend on engine parameters. The system may use a third mode in which oil is not sprayed. The lag in friction reduction between removal and reintroduction of oil spray enables lubrication of the cylinders using less oil to prevent overcooling but still enough to reduce rubbing friction.

Description

Oil Spray System for an Internal Combustion Engine This invention relates

to the cooling of the pistons inside an internal combustion engine, and more particularly to the use of oil squirters or sprays to dispense cooling oil.

An established method of cooling pistons in an internal combustion engine is by the use of a piston cooling jet.

These are usually situated in the block directly beneath each piston and squirt a jet of oil up underneath the piston. Heat is then transferred into the oil and in so doing cools the piston.

In most engines that use piston cooling jets, they are permanently operated, cooling the piston under all speed and load conditions. At low engine speeds, piston cooling jets are not required for piston cooling. Some engines therefore, switch the jets off at low engine speeds. This enables the engine manufacturer to minimise the oil pump size, without sacrificing oil pressure at idle speed, thus saving cost, weight and space. It is also an important to note that switching off the cooling jets substantially reduces the oil flow requirement.

Disabling the jets at low engine speeds is often achieved by providing a spring-biased valve within the spray nozzle. The valve opens when the oil pressure provides sufficient force to overcome the spring. When engine speed drops, the corresponding drop in oil pressure causes the spring to close the valve and disable the spray nozzle.

Disabling the jets in this way overcomes the disadvantages at low speed and load of high parasitic oil pump losses and more importantly, reduced combustion efficiency from over cooling.

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While this is effective in reducing the load on the engine, research has shown that the extra oil from the cooling jets gives reduced piston rubbing friction, mitigating the positive effects from improved combustion efficiency. There is therefore a need to cool the pistons only when required whilst still providing sufficient lubricating oil.

With a view to fulfilling the foregoing need, the present invention provides an oil spray system for an internal combustion engine characterised by the system having two modes of operation wherein in one mode oil is sprayed continuously, and in the other mode oil is sprayed intermittently.

Research has shown that when the cooling jets are deactivated there is a delay before the rubbing friction is seen to increase, whereas when the cooling jets are switched on the rubbing friction reduces quickly. This phenomenon is likely to be caused by the oil retained on the cylinder bore wall for a number of pistons strokes before being scraped off by the piston rings. Conversely oil is immediately restored to the cylinder walls once the cooling jets are reactivated.

By taking advantage of the lag in friction reduction between removal and reintroduction of oil spray, the present invention is able to lubricate the cylinders using substantially less oil than is used for cooling yet still enough to reduce rubbing friction significantly. This is achieved by intermittently firing of the jets at lower engine speeds and load. In the process, the total time the jets are open and thus total volume of cooling oil reaching the piston is reduced. This results in the piston not being cooled to the same degree as when the jets remain open. The pulsing of the jets in turn reduces rubbing friction. The rubbing friction steadily increases as the oil from the previous pulse is wiped away until it reaches a maximum at which point the jet pulses again providing fresh oil. Such a solution provides a halfway point between cooling and not cooling which still retains the benefits of reduced rubbing friction and thus overall increases the efficiency of combustion.

It is advantageous to switch between modes in dependence upon engine operating conditions Such operating conditions may include any of engine load, engine speed, piston temperature, and exhaust gas temperature.

It is further advantageous if the frequency at which the oil is sprayed intermittently is proportional to engine speed or a number of other operating conditions.

Likewise the duty cycle of the intermittent oil spray may be controlled as a function of engine load, engine speed, piston temperature, oil temperature or exhaust gas temperature.

Preferably, oil is sprayed intermittently by opening and closing a jet by means of a hydraulic valve which may be actuated by any of electrohydraulic means, electro-vacuum means or a solenoid.

If the piston cooling jet control is used in combination with a variable flow oil pump, the parasitic losses of the oil pump can be substantially improved.

The system may further utilise a third mode of operation in which oil is not sprayed.

According to a second aspect of the present invention, there is provided an internal combustion engine having an oil spray system as described in the appended claims.

The invention will now be described further with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an internal combustion engine showing a conventional cooling jet, Figure 2 is a graph showing the relationship between friction and cooling jet functionality for a prior art oil spray system, and Figure 3 is a similar graph to that shown in Figure 2 relating to a spray system of the present invention.

Figure 1 shows a section through a conventional engine block. In this diagram, oil spray jet 22 communicates via bore 20 with an oil gallery 18 behind the cylinder wall and water jacket. The jet 22 includes a nozzle that points the exiting oil spray at the underside of the piston 12. On touching the piston 12, heat is absorbed into the oil, cooling the piston down. The spray pattern from the nozzle will also cause droplets of oil to coat the cylinder wall as well as on the underside of the piston 12.

Oil is free to run to and from the gudgeon pin (not shown) and underside of piston. Oil also flows through the radial holes (not shown) from the underside of the piston onto the cylinder wall. The cylinder wall is wiped free from oil by the lowest ring around the circumference of the piston, the oil control ring. This ring wipes the wall of the cylinder to prevent oil passing the upper compression rings and eventually entering the combustion space.

Research has shown that once oil spray is disabled it takes several piston strokes for the oil control ring to wipe the cylinder wall, reducing the amount of surface oil to a point where the rubbing fricttori increases measurably.

In other arrangements, the oil spray jet 22 may be arranged to communicate with an oil supply gallery feeding the crankshaft main bearings and big end bearings. The effect is the same in that oil is sprayed at the underside of each piston by an oil spray jet.

Prior art oil spray systems use an internal spring- actuated check valve to close the spray jet at lower oil pressures. The preferred embodiment differs in that the opening and closing of the jet is actively controlled by a suitable valve that may be opened by a solenoid, electrohydraulically or electro-pneumatically. Such systems are already used in internal combustion engines.

The pulsing or firing of the oil spray jets is controlled by the ECU (engine control unit) (not shown) This can take into account factors such as, but not limited to, oil temperature, oil pressure, coolant temperature, exhaust gas temperature, turbo boost pressure (if applicable) , throttle position, fuel demand, engine speed and ambient air temperature. The ECU can use this information based on a reference lookup maps to determine both at which point the oil spray jets should be switched between a continuously enabled mode, and an intermittent or pulsed firing mode, and at what frequency the oil spray jets 22 should be pulsed at.

Turning now to figures 2 and 3 which show friction versus time graphs for the prior art and the present invention oil spray systems respectively.

As can be clearly seen in both diagrams, disabling the cooling jet leads to an increase in rubbing friction proportional to the time period since the jet was de- activated until it reaches a maximum.

It is clear that despite the step function of disabling the oil spray jet 22, there is a lag before the lubricating benefits of the oil spray disappear. Figure 3 shows that by re-enabling the oil spray je for a brief period a step change in the rubbing friction is produced which immediately starts to decay once the oil spray is again deactivated.

The net result is that the rubbing friction is, on average, substantially lower than its maximum value despite the oil spray jet being enabled for only a small fraction of the time. Since the cooling of the piston is directly related to the amount of oil sprayed on to it, and the amount of oil sprayed is a direct function of the time the jet is enabled, the net result is that friction can be substantially reduced without over cooling the piston. This allows cylinder temperatures to stay high, but keeps rubbing friction low, thereby improving engine efficiency. The amount of oil required by the engine and thus needing to be pumped is also reduced allowing the advantage to be taken a variable flow oil pump.

Claims (11)

1. Claims 1. An oil spray system for an internal combustion engine

   characterised by the system being having two modes of operation wherein in one mode oil is sprayed continuously, and in the other mode oil is sprayed intermittently.
2. 2. A system as claimed in claim 1, wherein switching between modes is effected in dependence upon engine operating conditions.
3. 3. A system as claimed in claim 2, wherein the conditions may include any of engine load, engine speed, piston temperature, oil temperature and exhaust gas temperature.
4. 4. A system as claimed in any preceding claim, wherein oil is sprayed intermittently by opening and closing a jet by means of a hydraulic valve.
5. 5. A system as claimed in claim 4, wherein the hydraulic valve is actuated by a solenoid, electro- hydraulically, or electro-pneurnatically.
6. 6. A system as claimed in any preceding claim wherein the frequency of the intermittent oil spray is proportional to engine speed.
7. 7. A system as claimed in any preceding claim, wherein the duty cycle of the intermittent oil spray may be controlled as a function of engine load, engine speed, piston temperature, oil temperature or exhaust gas temperature.
8. 8. A system as claimed in any preceding claim, wherein the system is supplied with oil by a variable flow oil pump.
9. 9. A system as claimed in any preceding claim, having a third mode of operation in which oil is not sprayed.
10. 10. An internal combustion engine having an oil spray system as claimed in any preceding claim.
11. 11. An oil spray system substantially as herein described with reference to and as illustrated in the accompanying drawings.