Although the coolant flow velocity is increased by as much as108.4%, the highest coolant velocity achieved is just 2.31m/s. With reference to the internal design guidelines thatrequire the coolant flow velocity not to exceed 5 m/s foraluminum alloy and 10 m/s for iron, the highest velocityderived is well below the specified limits.

POTENTIAL RISKS AND THEMITIGATION PLANThe increase in coolant flow rate to the radiator may requirethe radiator specifications to be matched accordingly toensure that the risk of radiator erosion and corrosion is low.In particular, the use of aluminum radiator may also requireextensive testing to ensure its compatibility at differentcoolant properties, flow rates and temperatures.

With reference to Table 2 simulation results, the coolant flowrate going across the cylinder block water jacket is greatlyreduced for the proposed concept. Concerns have been raisedover the possibility that the surface temperatures of the toppart of cylinder bores in particular the interbore bridges toexceed 180 C, which can lead to: -

1. Increased oil consumption

2. Piston scuffing

3. Accelerated oil degradation

4. Bore distortion

In mitigating the risks, in depth CFD simulations should beconducted during the design phase to investigate on whetherthe coolant velocity at the upper portion of the water jacket iswithin the optimum coolant flow velocity. The CFDsimulations should also include interbore bridge temperatureprediction.

In the testing phase, multiple depths thermal survey usingthermocouples should be made around the cylinder bores ofall the 4 cylinders. The survey is crucial in verifying thatnone of the cylinder bore surface reaches the 180 C limit.

If the coolant flow velocity falls below the optimum velocityor the bore surface temperature exceeds 180 C, severalcountermeasures can be applied to suit the severity of theproblem: -

1. Reduction of the cylinder block water jacket depth to aslow as 50% of the stroke. Reducing the water jacket depth isexpected to increase the average flow velocity of the topportion of the cylinder block water jacket

2. Increasing the cylinder block coolant outlet's innerdiameter until the required coolant flow velocity is achievedacross the water jacket

3. Increasing the water pump flow rate

For aluminum cylinder blocks, it is also possible to mitigatethe cylinder bore overheating risk by moving away from theuse of cast-in iron liner to linerless cylinder block. Thepresent of air pockets in between the liner and aluminumnegatively affect the heat transfer from the liner surface to thewater jacket [5].

Figure 7. Cross section diagram showing the interborecooling passages taken from granted patent US

6,776,127

To further mitigate the cylinder bore overheating risk, specialattention must be paid to the interbore bridge. As statedearlier, the cylinder block used in this study has 6mminterbore bridge. With narrow interbore bridge, the interborecooling options are very limited. In this case, the use ofinterbore cooling system as described in the granted USpatent 6,776,127 [6] as shown in Figure 7 may be the onlysolution.

In integrating this interbore cooling system, 6 short and smalldrillings together with 6 cylinder head gasket openings arerequired to enable water to flow close to the middle of theinterbore bridges to cool off the hotspots. Although, no 1D or3D simulation conducted so far to study the impact of suchintegration with the proposed concept, it is predicted thatsuch integration will not cause any problem or muchvariation to the results discussed so far.

To further mitigate the risks described in this section, it isimportant for the complete vehicle cooling system to beoptimized to avoid the coolant temperature entering theradiator from ever exceeding 115 C. This necessitates acomplete vehicle cooling development involving bothsimulation and physical testing. The development activitiesshould ensure the synergy between the: -

1. Proposed cooling strategy

2. Water pump capacity

3. Thermostat opening temperature

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4. Radiator heat rejection

5. Engine bay air flow management

These additional and extensive development activities are notcovered in this paper.

ADVANTAGESFrom the simulation results, the coolant temperature exitingthe cylinder block is about 6.6 C hotter than the coolanttemperature exiting the cylinder head. Contrary to theconventional parallel and serial cooling strategies, whichhave the cylinder block operating at few degree Celciuscooler than the cylinder head, this represents significant andrare improvement over the conventional cooling systems.

Assuming that the temperature of the coolant temperatureexiting the cylinder block water jacket is controlled at 85-95C, the coolant temperature exiting the cylinder head's waterjacket can be limited to around 78.4-88.4 C.

In comparing it with the conventional cooling strategies, bykeeping the cylinder block water jacket at around 85-95 C,the temperature at the cylinder head water jacket will be fewdegree C higher. The hotter cylinder head water jackettemperature is likely to increase the risk of engine knock andreduced air density coming into the combustion chamber asper what discussed by Kobayashi and others [2].

The temperature difference between the cylinder block andcylinder is not as big as what achieved by Kobayashi andothers using their dual cooling circuit. Nevertheless, beingable to achieve significantly cooler cylinder head without theneed for 2 sets of cooling circuits presents a significantimprovement.

FUTURE DEVELOPMENTACTIVITIESThe 1D and 3D simulations have provided reasonable designparameters for a prototype engine to be prepared and tested.As this paper is being prepared, a prototype engine is madeavailable and ready to undergo the physical testing.

Many of the simulation results discussed in this paper will becompared with the actual testing. Any variations between thesimulation results and the actual tests will be identified andscrutinized.

Once legitimate discrepancies are identified, the boundaryconditions used will be revised and iterations will be madeuntil reasonable correlations with the actual tests areachieved.

FUTURE OUTLOOKFor markets that do not require any cabin heater, the coolantoutlet as stated in Figure 2 can be eliminated by transferringthe coolant directly to the cylinder head through the cylinderhead gasket. The opening cross section area through thecylinder head gasket at the back of the cylinder block shouldbe about the same as the opening cross section area of thecoolant outlet union's inner diameter of the engine intendedfor markets with cabin heater.

In addition, the cylinder head coolant outlet union's innerdiameter must be enlarged to accommodate the increasedflow rate. This measure enables the coolant outlet union, therelated machining process, clamps and additional coolanthoses to be eliminated for cost reduction.

Figure 6. Additional improvement to the proposedcooling circuit

Considering that only about 10 percent of the total coolantflow rate goes out of the cylinder block before going to thecabin heater, further improvement to the proposed concept ispossible as shown n Figure 6.

As shown in Figure 6, the flow coming out of the cylinderblock and cabin heater is directed straight to the water pumpentry passage instead of going into the thermostat. Thisenables the T junction pipe to be eliminated and the doubleacting thermostat to be replaced with a cheaper and simplersingle acting thermostat.

In making sure that the single acting thermostat is able tocontrol the flow to the radiator in accordance to the coolanttemperature, a hole of certain diameter is required at thethermostat. This hole enables a small amount of coolant tocontinuously flow through the thermostat and into theradiator even when the thermostat valve is fully closed.

The minimum amount of coolant allowed to flow across thecylinder head is necessary to avoid coolant stagnation acrossthe cylinder head water jacket. More importantly, the coolantmovement enables the thermostat's valve opening to be variedin accordance to the coolant temperature. As the engine isbeing warmed up, the radiator fan and opening flap can also

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be adjusted accordingly to ensure minimum heat to betransferred out of the radiator.

With this arrangement, maximum coolant is circulatedthrough the cylinder block but very minimum coolantcirculation can be expected through the cylinder head duringthe engine warm up. The close to stagnation coolantmovement across the cylinder head during warming up periodwill expedite the cylinder head heat buildup which isbeneficial in minimizing the fuel wall wetting in the intakeports.

Considering that the cylinder head's oil jacket is placed righton top of the water jacket, the accelerated coolanttemperature increase inside the cylinder head will also speedup the oil temperature increase as well. This is beneficial inreducing the engine frictions during the test cycles. It is alsointeresting to point out that this accelerated engine warmingup process is achievable without having to use two separatethermostats.

In addition to that, with the overall coolant circulationinvolving mostly the circulation across the cylinder block, thetotal coolant volume involved in the circulation during thewarming up period is actually small. The relatively smallercoolant volume will also have relatively lower total heatcapacity to absorb the engine heat. This expedites thetemperature increase of the coolant. The short coolantwarming up period is not only beneficial for frictions andemissions reductions, it also speeds up the heat availabilityfrom the cabin heater during cold winter.

The proposed cooling strategy is also yet to be combined withan electric water pump where the water pump flow rate canbe mapped according to the engine speed and load rather thanjust the engine speed. Alternatively, instead of mapping it tothe engine speed and load, a temperature sensor can be usedto detect the coolant temperature either at the cylinder blockor cylinder head coolant outlet. With the temperature sensor,the electric water pump flow rate can be regulated to controlthe overall coolant temperature at various engine speed andload. The combinations described in this paragraph enablegreater energy saving and it may be possible to get rid of thethermostat and coolant bypass altogether.

Although the proposed cooling strategy in this paper isapplied to an inline 4 cylinder engine, the strategy is alsoapplicable to multi-cylinder engine in inline, v and boxerconfiguration.

CONCLUSIONThe study has shown that the proposed concept is madepossible by directing about 90% of the total flow rate to thecylinder head and the remaining 10% of the total flow rate to

the cylinder block. Such distributions can be achieved bycontrolling the outlets' cross section opening areas.

From the 1D simulation, the 3 objectives specified earlier inthe paper have been met and the results are: -

1. Temperature difference of 3 C between the inlet andoutlet of the cylinder head's water jacket

2. Temperature difference of 11 C between the inlet andoutlet of the cylinder block's water jacket

3. Temperature difference of 4.4 C between the inlet andoutlet of the coolant radiator

From the 3D simulation, the coolant flow velocities adjacentto the critical areas like exhaust valve bridges and spark plugthreads are observed to be much higher if compared to thebaseline.

In contrast, the coolant flow velocities around the cylinderbores are drastically reduced if compared to the baseline.With slower flow, there is a risk of interbore bridgeoverheating that may require the use of interbore coolingpassage and shallow water jacket.

The proposed concept enables about 6.6 C temperaturedifference between the coolant outlets of cylinder block andcylinder head. With cooler running cylinder head, the benefitsmay come in terms of better knock resistance and volumetricefficiency.

Flow rate from the engine to the radiator increases by 20%and this opens up the possibility for: -

1. The water pump capacity to be reduced to lower the powerconsumption

2. Smaller, thinner and less dense radiator for cost reductionand lower air and coolant pressure losses

The study has shown that it is not impossible to achievecooler running cylinder head without having to use twoseparate cooling circuits. In addition to that, with a smallrevision to the proposed cooling circuit's details, it is alsopossible to speed up the warming up period without the useof 2 thermostats and this is beneficial for emissions reductionand frictions reduction.

REFERENCES1. Pang, H. H., Brace, C. J., Review of Engine CoolingTechnologies for Modern Engines, Proce. Instn Mech. EngrsVol. 218, Part D: J. Automobile Engineering, 2004

2. Kobayashi, H., Yoshimura, K., and Hirayama, T., AStudy on Dual Circuit Cooling for Higher CompressionRatio, SAE Technical Paper 841294, 1984, doi:10.4271/841294.

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http://www.sae.org/technical/papers/841294http://dx.doi.org/10.4271/841294

3. Kim, K.C., Lee, J.J., Chon, M.S., Yun, J.E., A NewApproach of Cavitation Criterion Analysis for AutomotiveWater Pumps, Seoul 2000 Fisita World AutomotiveCongress, June 12-15, 2000 Seoul, Korea

4. Brace, C., Burnham-Slipper, H., Wijetunge, R., Vaughan,N. et al., Integrated Cooling Systems for PassengerVehicles, SAE Technical Paper 2001-01-1248, 2001, doi:10.4271/2001-01-1248.

5. Osman, A., Design Concept and Manufacturing Methodof a Lightweight Deep Skirt Cylinder Block, SAE TechnicalPaper 2012-01-0406, 2012, doi: 10.4271/2012-01-0406.

6. Osman, A., Interbore Cooling System, United StatesPatent Office, US 6,776,127

CONTACT INFORMATIONThe lead author can be contacted atazmiosman@hotmail.com

The Engineering Meetings Board has approved this paper for publication. It hassuccessfully completed SAE's peer review process under the supervision of the sessionorganizer. This process requires a minimum of three (3) reviews by industry experts.

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