A Review of Precision Engine Cooling

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SAE TECHNICALPAPER SERIES 1999-01-0578

A Review of Precision Engine Cooling

K. Robinson, N. A. F. Campbell, J. G. Hawley and D. G. TilleyUniversity Of Bath, UK

International Congress and ExpositionDetroit, Michigan

March 1-4, 1999

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1999-01-0578

A Review of Precision Engine Cooling

K. Robinson, N. A. F. Campbell, J. G. Hawley and D. G. TilleyUniversity Of Bath, UK

Copyright © 1999 Society of Automotive Engineers, Inc.

ABSTRACT

Although successful “precision cooled” prototype engineshave been demonstrated, the design of most mainstreamcoolant jackets has evolved only cautiously, and lackedthis major change in approach.

The achievements and potential of precision cooling arereviewed, along with an extension into nucleate boilingbased heat transfer. It is demonstrated that ideas foradvanced “external” cooling systems with low flowratesare in fact extremely compatible with the “internal” preci-sion engine cooling philosophy, and in combination prom-ise even larger benefits.

INTRODUCTION

Several studies have attempted to optimise the design ofinternal engine cooling passages by greatly reducing thecoolant jacket in thermally uncritical areas, and increas-ing coolant flow in critical areas. Typically the coolantjacket of water cooled cylinder heads consisted of thespace left over once the combustion chamber, ports,spark plugs/injectors and valvetrain had been packaged.Coolant would be pumped in at one end and taken out ofthe other with little consideration over its spatial velocityprofile. With the availability of better instrumentation andsimulation tools, areas of high heatflux can now be identi-fied. Subsequently coolant velocity distribution and cool-ant flow strategies began to evolve which deliberatelyincreased the local coolant velocity in thermally criticalareas and allowed a better balance of flow velocitybetween cylinders. This can be viewed as an intermedi-ate step towards what has become known as “precisioncooling” and is typical of current practice for most enginesbeing designed today.

Precision Cooling may be defined as the “minimum cool-ing necessary to achieve an optimised temperature distri-bution”. An unoptimised temperature distribution is onewhere components temperatures

i. vary between cylinders or between equivalent loca-tions in one cylinder

ii. exceed recommended limits for material strength orlubrication requirements

iii. are unnecessarily low, causing excessive thermalstresses, thermal distortion and high heat rejection tocoolant and oil

With the precision cooling philosophy, the requirementsof the coolant jacket become an integral part of theengine design process. Thermally critical areas receiveintense cooling, whilst uncritical areas rely on heat con-duction through the head and block structures, resultingin a much more even temperature distribution, loweringthermal stresses and minimising cylinder to cylinder tem-perature variations

The potential advantages of a precision cooled gasolineengine are lower friction, faster warm-up, improved knockresistance and lower cylinder to cylinder variability – all ofwhich contribute to lowering fuel consumption and emis-sions. Other benefits include higher component durabilityand/or greater power potential, improved vehicle cabinheating, lower first cost, and lower weight. Hence thebenefits are spread over many areas of the engine’s costand function, and with the current industry practice ofseparate engineering teams focused in specific areas,the cumulative benefit has perhaps been overlooked.

PREVIOUS WORK – The majority of work has focusedon premium gasoline engines where the above benefitsare most advantageous, and the mainstream use of alu-minium heads with high thermal conductivity lend them-selves well to this purpose, and benefit most from thelowering of cyclic stresses due to poorer fatigue proper-ties than iron.

GEOMETRIC EFFECTS – One of the earliest referencesto precision cooling was by Preide and Anderton (1) in aprototype heavy-duty 3.9 litre diesel engine cylinder head(figure 1). The original coolant passages were replacedwith much smaller passages around the valve seats,injector boss and valve guides, and the remaining spaceoccupied by air voids. Full power heat rejection to thecoolant was reduced by 18%, with associated reductionsin fan power and fan noise. Brief mention is made in thispaper of a prototype precision cooled gasoline engine inwhich heat rejection to the coolant was reduced by14.5% with lower temperatures in the critical valvebridges and bore siamese area. The majority of


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published work since then has been based on gasolineengines.

Finlay et al (2) conducted a precision cooling study on thecast iron cylinder head of a small gasoline engine. Thecooling jacket was changed to include higher coolantvelocities in the thermally critical areas (figure 2), but witha bulk coolant flow rate reduced by 40%. At all measure-ment locations, the temperatures recorded in the preci-sion cooled head were lower than those in the baselinehead. Wide open throttle performance and optimumspark advance were very similar. It is also interesting tonote that the authors discovered evidence of nucleateboiling in the valve bridges of the baseline head, as

revealed by temperature increases when the cooling sys-tem pressure was raised. In other places with almoststagnant coolant flow, temperatures and heatfluxes con-sistent with film boiling were observed. With the precisioncooled head, no significant sensitivity to coolant pressurewas observed, suggesting that the higher coolant veloci-ties suppressed any form of boiling and the coolant heattransfer regime was purely convective. Further experi-ments with varying pump flow confirmed these observa-tions. Engine performance, spatial heatflux variation andoverall heat rejection to the coolant were similar for thetwo engine builds. The emissions performance of the twoengines was not studied.


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REDUCTION OF ENGINE WEIGHT – Ernest (3) con-ducted a comprehensive “unique cooling” study based ona 2 valve per cylinder Ford V8 engine with the aim of pro-ducing a 10% lighter engine with aluminium heads for thesame cost as an iron head. This work followed on fromsome earlier work by Alcoa engineers in 1963, andincluded Finite Element analysis, coolant flow rig devel-opment, engine durability testing and fleet vehicle trials.Despite the higher raw material costs of alumimium, sim-plification of the manufacturing process allowed an over-all equivalent cost to iron to be achieved, compared to atypical 33% cost increase of a conventionally cooled alu-minium head. The cylinder heads were manufacturedwithout a coolant jacket core and incorporated troughscast into the flamedeck around the edges of the cylinder,which linked, to the coolant passages of the open deckcast iron block (figure 3). Apart from these, the only headcooling was provided by a longitudunal drilling passingclose to the valve guides, and thus the cooling strategyrelied heavily on heat conduction away from critical areassuch as the valve bridge. The cast iron cylinder block wasmodified to have an open top deck and siamese bores,with longer head bolts threaded into the bottom deck toreduce bore distortion. Ernest demonstrated that aroundthe valve guides and seats, measured temperatures werebelow recommended maximum values, and lay inbetween the maximum and minimum temperatures seenin the baseline cast iron head, with an overall reduction incoolant heat rejection. Vehicle tests with iron heads, con-ventionally cooled aluminium heads and “unique cooled”aluminium heads showed similar fuel consumption andemission levels, with the unique cooled engines havingthe highest thermal efficiency and power output, and anoctane requirement increase intermediate to that of theiron heads (highest) and the conventionally cooled alu-minium heads (lowest). Durability testing was conductedat maximum power for 80 hours and maximum torque for20 hours and did not produce any failures. The engineused for this study was a 5 litre, 2 valve per cylinder V8engine producing approximately 128 kW (25.6 kW/litre).It appears doubtful whether such a minimal cooling strat-egy would be adequate for a modern 4 valve per cylinderengine, where specific ratings are typically 45-50 kW/litre.

REDUCTION OF THERMAL STRESS – Moore andMcAvoy (4) experimented with a “limited” cooling strategyon a 4 valve per cylinder heavy duty diesel engine. Theconventional water cooling jacket in the cylinder headwas replaced by four horizontal, radial drillings througheach valve bridge, meeting in a passage surrounding theinjector. Heat loss to coolant was substantially reducedand the temperature distribution was more even. Maxi-mum flamedeck temperature was increased, and sup-porting finite element modelling work predicted a 84%improvement in fatigue life as a result of reduced thermalstresses.

REDUCTION OF ENGINE WARM-UP TIME ANDWATER PUMP POWER – Clough (5) conducted a studywith a 4 valve per cylinder Jaguar gasoline engine thatincluded modification of block and head, resulting in a64% reduction in block/head coolant volume. In commonwith Finlay’s (2) approach, coolant passages were rede-signed to achieve higher coolant velocities in criticalareas (figure 4), and reduced coolant volume in uncriticalareas. Improvements were achieved in full load BMEP oftypically 0.6-0.7 bar across the speed range. The mea-sured motoring friction data suggests about 20% of thiswas due to a reduction in friction, and the rest due to anunquantified combination of an improvement in knocklimit, an improvement in volumetric efficiency, and areduction in coolant pump parasitic loss. Engine warm uptime was reduced by 18% (time to achieve 80°C top hosetemperature from a 20°C start temperature at 2000rpm,100Nm load), with a more even temperature distribution,and lower maximum temperatures in both the block andhead which provide a margin for increasing the poweroutput. A 54% reduction in water pump drive power wasalso achieved, together with a 36% reduction in full powerheat rejection. Hydrocarbon (HC) emissions wereobserved to be unchanged.


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REDUCTION OF HC EMISSIONS – Huttner and Han-cock (6) conducted a study primarily aimed at reducingthe HC emissions from a 4-valve per cylinder V6 gasolineengine. Their approach involved a large reduction in inletside cylinder head cooling to reduce fuel wall condensa-tion and equalise temperatures between the inlet andexhaust sides of the combustion chamber. They arguedthat since the inlet side normally runs cooler, this couldbe the main source of unburned HCs, and since maxi-mum temperatures would not be increased, knock char-acteristics and NOx emissions should not change mucheither. However, at higher power operation, some coolingof the intake side was considered necessary to maintainvolumetric efficiency and prevent material temperaturesexceeding critical levels. A prototype cylinder head wasbuilt with reduced coolant volume on the exhaust side ofthe head and drilled gallery under the inlet ports, con-nected by cross drillings. A system of barrel throttle, flowcontrol valves was used to manually change the coolantflowpath on the inlet side of the head (figure 5)

Tests showed an improvement in warm-up time withexhaust side only cooling, which in itself yielded animprovement in overall HC emissions due to earlier lean-ing of the air-fuel ratio. At wide open throttle without inletside cooling, inlet side temperatures did not exceed criti-cal levels, and temperatures were found to be similarbetween inlet and exhaust sides. Knock behaviour wassimilar although a reduction in power was observedresulting from the loss in volumetric efficiency caused byinlet port charge heating. This was restored by openingthe valves to allow inlet side cooling. A comparison of fullload emissions was not disclosed, but at part load condi-tions a reduction in HC emissions was observed withexhaust side only cooling, with similar CO levels and aslight increase in NOx. The authors go on to outline thedesign of a new cylinder head based on this concept.However, it seems unlikely that the advantage of full load

inlet side cooling justifies the extra complexity, cost andrisk of the barrel throttle flow valves for a large volumeproduction engine.

REDUCTION OF KNOCK – Kobayashi et al of Toyota (7)examined the possibility of reducing knock and therebyincreasing compression ratio by employing separatecooling circuits to the block and head, and using a lowercoolant temperature in the head. Relative to the baselinecompression ratio of 9:1, higher ratios up to 15:1 wereinvestigated. It was found that the effect on knock of low-ering coolant temperature was twice as large in the headcompared to the block, and the effect on volumetric effi-ciency was equal. With an estimated minimum practicalcoolant temperature of 50°C, it was concluded that themaximum achievable compression ratio was 12:1. Thisresulted in an increase in engine power of 10%, with sim-ilar low speed power output and a 5% improvement inpart load fuel consumption. HC emissions were observedto increase, and this was attributed to a lowering ofexhaust temperature and an increase in quenching layerthickness resulting from cooler combustion chamberwalls. The method used to increase compression ratiowas to skim off metal from the bottom deck of the cylinderhead. It should be noted that this also has the effect ofreducing the distance to the coolant around the edges ofthe wedge shaped chamber, and thus cause a reductionin the temperature of the cylinder head face around theend gas zones, which are known to be an important fac-tor in knock behaviour. Thus some of the benefit in knockbehaviour claimed by the authors at lowered coolant tem-peratures and higher compression ratios is likely to haveresulted from a cooler head face around the end gaszones.

Iwashita et al (8) focused on the possibility of reducingthe limitation of knock in gasoline engines by improvingthe coolant flow in the cylinder head. A prototype cylinderhead was built which had a more constant coolant flowarea along the length of the head, and the engine coolantflow strategy was changed to transfer all the coolant fromblock to head at the back of the rear cylinder. Tempera-ture reductions of 10-40°C were achieved at the combus-tion chamber surface which yielded a 4-5° increase inmid speed spark advance and a 2-3 Nm (approx 2%)improvement in torque.

MECHANISM OF PREDICTION OF HEAT TRANSFERIN ENGINE COOLING SYSTEMS – A basic understand-ing of the process of coolant heat transfer inside anengine is an essential prerequisite to the creation of aprecision cooling strategy. All of the precision coolingwork so far discussed has used the common approach oftargetting high flow velocities in areas of high heatflux,and used higher rates of convective heat transfer toreduce component temperatures. An alternative to this isto exploit the large increases in heat transfer that are pos-sible with nucleate boiling in a controlled manner, asadvocated by Campbell et al (9). This method allowsmuch lower coolant flowrates to be used, but the


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progression to film boiling needs to be carefully avoided.Further work is currently being undertaken at the Univer-sity of Bath to fully investigate the factors influencing thisprocess.

The occurrence of nucleate boiling in engines acts as anunintentional safety zone for protecting components fromexcess temperature when the coolant flow velocity is toolow to provide the required convective heat transfer. It hasalso been established that higher coolant velocities sup-press the onset of nucleate boiling (10). When nucleateboiling begins, the increased heat transfer rate cools thecomponent back down to a level where convective heattransfer can be re-established, and thus the process isself-stabilising provided the vapour produced condensesback into the bulk coolant (subcooled boiling) or is ventedto a condensor (saturated boiling). Modern CFD analy-ses reveal that the occurrence of low flow areas andassociated localised boiling is probably a lot more wide-spread than realised, since often the measured tempera-ture distribution cannot be explained by convective heattransfer alone. Finlay (2) also confirmed this by compar-ing the temperature distribution at two different coolantpressures, with the result that an increase in pressurereduces the nucleate boiling and causes metal tempera-tures to increase (figure 6). Norris (11) reached a similarconclusion based on an examination of the relationshipbetween surface temperature and heatflux.

Nucleate boiling can be used to great advantage bydesigning a cooling system that allows localised sub-cooled boiling to occur. If taken to the extreme of evapo-rative cooling, some efficiency benefits may be realised,but at the expense of greater cooling system complexity.If instead the system is designed along more conven-tional lines but with a small degree of subcooled boilingpermitted, a conventional radiator may be used without avapour separator or condensor. The above evidence sug-gests this already occurs unintentionally in manyengines.

IMPLICATIONS ON EXTERNAL COOLANT CIRCUIT –The importance of the cooling system’s role in providingheat to the passenger compartment for comfort andwindscreen demisting has risen in recent years to thepoint where it is regarded as its primary function in somecolder climate areas. The impact of a precision coolingstrategy on cabin heating must be carefully considered.Overall coolant heat rejection has been shown to reducewith precision cooled engines (1,3,4,5 & 6) but the moreimportant rate of coolant temperature rise has increased,(due to lower overall heat capacity) which in turn meansthat cabin heating will be available sooner with a preci-sion cooled engine. This was demonstrated by Clough,figure 7.

A common characteristic of all Precision Cooled engineshas been a rise in the coolant pressure drop resultingfrom the large decrease in flow area. However, bulk cool-ant flowrates have also been substantially reduced,which allows a small improvement in fuel consumption.


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Couetouse and Gentile (12) conducted a study with a lowpower electric coolant pump aimed at reducing fuel con-sumption and improving passenger thermal comfort.They observed, in common with others (2,13) that thecombustion chamber surface temperatures were fairlyinsensitive to bulk coolant flowrate, because of the effectof nucleate boiling occuring in areas of high heatfluxwhen the flowrate is lowered. Pump speed, coolant valveposition, radiator shutter position and fan speed were var-ied in response to the engine speed and load, vehiclespeed and coolant temperature. Coolant temperaturewas allowed to rise to 115°C except at higher powerwhere a 100°C limit was used to avoid excessive oil tem-perature. Fuel consumption improvements of between 3and 10% were made at steady state conditions (com-pared to a baseline 85°C coolant temperature), with thelargest gains at the lower speeds. Emissions were notgreatly affected, but showed a small reduction in HC andan increase in NOx with increasing temperature, in linewith other studies. Cabin heater performance wasimproved, especially at idle.

Pretscher and Ap (14) also experimented with a low flow,electric coolant pump cooling system which allowedeither partial nucleate boiling or fully evaporative cooling.Satisfactory engine cooling was achieved with both sys-tems, but it was felt that the partial nucleate boiling sys-tem would be easier to implement despite the higher heattransfer performance of the evaporative cooling system.

Gentile and Zidat (15) also discussed the advantages ofallowing subcooled or saturated nucleate boiling to occurand recognised that lower coolant volumes and lowercoolant flowrates were possible with such systems.

Ap and Golm (16) used a low flowrate cooling system ona diesel engine in order to improve cabin heating, reducefuel consumption and achieve a cost/weight reduction.They argued that since 95% of trips were under 120 km/h, and the coolant flowrate was normally set by maximumpower conditions, a lower coolant flowrate was justifiablefor the majority of driving conditions. At the remaining 5%of high power conditions, nucleate boiling was allowed tooccur. A 90% reduction in coolant pump power wasachieved with a 20°C increase in head temperature atmaximum speed conditions. The coolant pressure wasreduced to atmospheric pressure at maximum speedallowing a cost reduction in coolant hoses and radiator, ifthe head temperature increase could be accepted.

Willumeit et al (17) recognised the efficiency advantagesof running the coolant and oil temperatures at elevatedlevels. A cooling strategy was devised which maintainedthe cylinder wall at a constant high temperature by vary-ing coolant temperature, and yielded a reduction in fuelconsumption via lower friction. Coolant warm-up timewas also reduced by only allowing coolant circulation inthe cylinder head during warm-up.

Krause and Spies (18) replaced the traditional wax ther-mostat with an electronically controlled 4 way valve toachieve part load fuel consumption and HC emission

improvements at by running the coolant temperature atelevated levels. The warm-up period was also shortenedby preventing coolant circulation completely duringwarm-up.

The use of low flowrate cooling systems may require achange in approach in radiator design to achieve suffi-cient heat transfer. A change to a combined radiator-con-densor is one solution for cooling systems which allowsome degree of boiling.

CONSIDERATIONS FOR A PRECISION COOLED GASOLINE ENGINE

COMPONENT TEMPERATURES

Cylinder Head Casting – Normal practice for a modern 4-valve per cylinder gasoline engine is to provide a coolantpassage between the exhaust valves, but not betweenthe inlet valves. Coolant passages are normally providedabove and below both ports, surrounding the valveguides, and in the central valley around the spark plugseat.

One of the consequences of this layout is that the com-bustion chamber surface has a large temperature imbal-ance since the heat input is not uniform, leading to highthermal stresses. The maximum flameface temperatureis limited to approximately 240°C for reasons of materialstrength (for aluminium), a value which is oftenapproached in the thermally critical bridge between theexhaust valves. Reducing the temperature in this areatogether with an increase in temperature in other areaswould reduce thermal stresses, and therefore allowhigher durability and/or higher power potential, and anlower the overall heat rejection to the coolant. Carefulbalancing of the cylinder to cylinder temperature variationwould also reduce the limit imposed by the weakest cylin-der on detonation and maximum temperature limit, with agreater advantage and lower cost than individual cylinderknock sensors. Coolant volume could also be reduced toallow faster warm-up, and the passages should also beoptimised to provide a more constant flow area, avoidingstagnant flow areas and having a higher hydraulic effi-ciency.

Exhaust ports – port cooling during warm up reduces gastemperature and delays catalyst light-off, but providesheat for cabin heating and windscreen demisting. Coolingat maximum power may protect the exhaust catalyst andthe exhaust manifold mounting flange from excessivetemperature, but also increases heat rejection to the radi-ator. A compromise is necessary here.

Inlet ports – Cooling during warm-up is unnecessary, andthe heat transfer from the port to the inlet charge mayimprove fuel vapourisation – effect on warm-up is likely tobe small. At maximum power, cooling will improve volu-metric efficiency at the expense of a small increase inheat rejection to the radiator.


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Valve Guides – these are subject to a maximum tempera-ture limit imposed by lubrication requirements and areunlikely to survive without some cooling.

Valve Heads – The exhaust valve heads run at a temper-ature close to the exhaust gas temperature, and contrib-ute to in-cylinder charge heating, as evidenced by thereduced octane requirement of sleeve valved engines.Valve head temperatures are difficult to reduce, but themost promising techniques are improved valve seat cool-ing and hollow stemmed, sodium filled valves.

Spark Plug Seat – Overheating plugs can cause pre-igni-tion and plug thread distortion and seizure. It is thoughtlikely that some cooling of the plug seat area will be nec-essary on a high performance gasoline engine, even withcolder grade plugs.

Valve Seats – Valve seats inserts may be subject to dis-tortion at high temperatures, leading to gas leakage,although it is arguable that most of the distortion iscaused by expansion of the metal surrounding the seat.Cooling of the seats will also reduce the temperature ofthe valve heads.

Cylinder Block – Cylinder bore surface is subject to amaximum temperature limit of approximately 180°C setby lubrication requirements, and a minimum of approxi-mately 60°C to control corrosive wear. Temperature isusually highest at the top of bore due to more exposureto combustion. Lower down the bore the temperaturedrops rapidly, and is approximately uniform below themidstroke position. This creates an uneven bore shapethat may contribute to increasing friction. However it isalso known that increasing the bore surface temperaturegenerally reduces friction via a reduction in oil viscosity,and reduces HC emissions by reducing flame quenchingat the edge of the combustion chamber. Together thesesuggests that the optimum bore temperature distributionwould be to maintain the whole bore surface at a uniformtemperature close to the recommended maximum valueunder all operating conditions. This is currently unachiev-able with conventional engines due to the axial variationin heatflux and cooling along the bore axis, and the varia-tion in heatflux with operating speed and load. Manymodern engines employ a coolant jacket that stops shortof the bottom of the stroke, and rely on crankcase oilsplash to cool the lower bore surface. However care mustbe taken to avoid introducing structural distortion of thebore by transferring the head bolt load path to the borewall at the floor of the coolant jacket – this can result in anecking effect in the stroked area of the bore which maycause problems with oil control and an increase in fric-tion.

Piston – Since heat transfer is a product of surface areaand excess temperature it is likely that the piston contrib-utes more to charge heating during the compressionstroke than the bore wall, cylinder head or valves in isola-tion. It therefore seems reasonable that more effort

should be focused at reducing the crown temperature asa means of reducing the knock limit, and may also resultin reduced NOx emissions. This need would increase ifbore temperatures were to increase due to the heattransfer from the piston rings to the bore surface.

Reduction of piston crown temperature also presents adifficult challenge for the modern GDI piston bowls, whichhave long heatpaths to the extremities of the bowl lip.Careful management of the airflow and fuel spray mayminimise this issue by keeping the ignitable mixture awayfrom these areas.

Thus a suggested outline for a high output, precisioncooled 4-valve per cylinder gasoline engine is as follows:

Cylinder head –

• High conductivity aluminium casting

• Cooling by forced convection supplemented by low-medium level nucleate boiling

• Cooling targetted around exhaust valve seats andshort, cooled exhaust ports/valve guides. Passagebetween valves fed from block and joining into pas-sage around spark plug with coolant flow deflector toencourage flow. Low velocity coolant jet to preventvapour blockage rather than enhanced convection.

• Small inlet port cooling passages on underside andabove/around valve guides. No passage betweenvalves, minimal cooling of inlet side flamedeck.

• Near constant cross sectional flow area, avoidingstagnant coolant pockets.

• Thinnest possible flamedeck

• Minimise unnecessary coolant volume, and reducemetal volume where possible

• Sodium cooled exhaust valves

• Flat, slightly rising roofline to allow venting of vapour

Cylinder Block

• Wet liners, profiled with increased thickness belowmidstroke.

• Narrow coolant passage extending to, or near to bot-tom of stroke. Minimise metal volume

• Interbore coolant passage, with flow deflectors toencourage some interbore flow

Piston

• Increased undercrown oil jet cooling, and thin crownsection

Coolant Flow

• Longitudinal flow strategy. Lower than normal flow-rate

• Variable coolant temperature

External Circuit

• Electronic control system responding to critical metaltemperatures, coolant and oil temperatures andcabin heating demand


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• Low power electric coolant pump

• Thermostat replaced with variable flow valve

• Smaller, vertical narrow tube radiator designed tocope with lower flowrate and vapour condensation

• Oil to coolant heat exchanger with flow control valve

CONCLUSIONS

1. Much research work has been on precision cooledengines, and the potential advantages have beenshown to be faster warm-up, lower thermal stresses,improved knock behaviour, reduced coolant heatrejection, higher BMEP, lower friction, lower coolantpump parasitic losses and lower cost of material andcoolant. Precision cooling probably has greaterpotential for gasoline engines rather than diesel dueto the improvement in knock behaviour, and the wideruse of aluminium cylinder heads.

2. The majority of work with precision cooling hasfocused on convective only heat transfer, but theadvantages of combined convective and nucleateboiling based heat transfer have been outlined. Theoccurrence of nucleate boiling in existing cooling sys-tems is not widely accepted, since it is not part of thedesign intention. There is a large amount of evidencethat nucleate boiling occurs, and there exists greatpotential to further exploit this advantageous heattransfer mechanism by incorporating it into a preci-sion cooling system.

3. The availability of modern tools such as FEA andCFD are ideally suited to help exploit the advantagesof Precision Cooling by offering solutions to highlycomplex three dimensional heatflow and coolant flowproblems. However, reliance will have to be placedon empirical models for coolant heat transfer due to alack of analytical heat transfer models for two-phaseflow.

4. The lack of satisfactory heat transfer models underreal engine operating conditions has been identifiedas a weakness. Work is ongoing at the University ofBath to address this.

5. The precision cooling concept, especially with boilingenhancement, is very compatible with recent ideasfor advanced engine cooling systems with low cool-ant flowrates, including electric water pumps andelectronically controlled cooling systems.

6. The potential advantages of precision cooling arespread over a large area of engine operation, andperhaps the cumulative advantage has not been fullyrecognised.

REFERENCES

1. T Priede and D Anderton “Likely Advances in Mechanics,Cooling, Vibration and Noise of Automotive Engines” ProcInstn Mech Engrs Vol 198D No 7 1984

2. IC Finlay, GR Gallacher, TW Biddulph and RA Marshall“The Application Of Precision Cooling to the Cylinder Headof a Small Automotive Petrol Engine” SAE 880263

3. R Ernest “A Unique Cooling Approach Makes AluminiumAlloy Cylinder Heads Cost Effective” SAE Passenger CarMeeting Sept 28 1977

4. CH Moore and PC McAvoy “Effect of Cylinder Head Designon Heat Rejection and Durability for High Speed DieselEngines” Proc. Practical Limits of Efficiency of EnginesIMechE 13-11-86

5. MJ Clough “Precision Cooling of a Four Valve per CylinderEngine” SAE 931123

6. T Huttner and J Hancock “Design and Evaluation of a LowEmission Spark Ignition Engine Cylinder Head” SAE960605

7. H Kobayashi, K Yoshimura and T Hirayama “A Study onDual Circuit Cooling for Higher Compression Ratio”IMechE C427/84, SAE 841294

8. Y Iwashita, M Kanda, H Kartagiri, Y Yokoi “Improvement ofCoolant Flow for Reducing Knock” IMechE Autotech 1989

9. NAF Campbell, DG Tilley, SA MacGregor, L Wong “Incor-porating Nucleate Boiling in a Precision Cooling Strategyfor Combustion Engines” SAE 971791

10. NJ Owen, K Robinson and NS Jackson “Quality Assurancefor Combustion Chamber Thermal Boundary Conditions –A Combined Experimental and Analytical Approach” SAE931139

11. PM Norris, WJ Wepfer, KL Hoag and D Coutine-White“Experimental and Analytical Studies of Cylinder HeadCooling” SAE 931122

12. H Couetouse and D Gentile “Cooling System Control inAutomotive Engines” SAE 920788

13. IC Finlay, D Harris, DJ Boam and BI Parks “Factors influ-encing Combustion Chamber Wall Temperatures in a Liq-uid Cooled, Automotive Spark Ignition Engine” Proc InstnMech Engrs Vol 199 No D3 1985

14. M Pretscher and NS Ap “Nucleate Boiling Engine CoolingSystem – Vehicle Study” SAE 931132

15. D Gentile and S Zidat “Advanced Engine Cooling System”IMechE C389/281 (SAE 925066)

16. NS Ap and NC Golm “New Concept of Engine Cooling Sys-tem (Newcool)” SAE 971775

17. HP Willumeit, P Steinberg, H Hoetting, B Scheibner and WLee “New Temperature Control Criteria for More EfficientGasoline Engines” IMechE C433/84 (SAE 841292)

18. W Krause and KH Spies “ Dynamic Control of CoolantTemperature for a Reduction of Fuel Consumption andHydrocarbon Emission” SAE 960271


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A Review of Precision Engine Cooling