Cooling Systems


Oil cooling of the valve seats

Recasting the crankcases, cylinder barrels and heads provided the opportunity to modify the oil galleries. This not only reduced pressure drop through the oil galleries but made machining operations simpler. Routing the oil through the cylinder barrel casting simplifies the design, eliminates the need for external oil pipes, and so reduces part count and mass.

Crankcases

Cooling oil is pumped through the crankcases as part of its distribution to each cylinder head. As part of the cooling system re-design, the oil galleries within the crankcase were simplified where possible to reduce part count and use fewer drillings. Figure 13 shows a comparison between the SMA and Ilmor oil gallery arrangements.
 
Comparison of cooling oil passage ways

Comparison of cooling oil passage ways

Figure 13 - Comparison of cooling oil passage ways

Cylinder head and barrel

After exiting the crankcases the oil is pumped up to the heads in order to deliver the cooling oil to the inlet and exhaust valve seats. The oil then returns directly to the crankcases.

The image below shows a comparison of the SMA and Ilmor oil galleries, and like the crankcases, the galleries have been designed to be more efficient (larger diameters and shorter paths yield less pressure drop around the circuit).

 Comparison of cooling oil routes through cylinder barrel and head
Figure 14 - Comparison of cooling oil routes through cylinder barrel and head

The valve seat inserts of the existing SMA engine were carried over to the prototype engine.

Section showing oil cooling passages around valve seats
Figure 15 - Section showing oil cooling passages around valve seats

The valve seats have a cavity around their outer diameter, which are complemented by profiled radial grooves machined into the cylinder head. Together these form the oil passageways that allow oil to flow around both sides of the valve seat inserts and surrounding head material.

Oil is fed to the exhaust seat first since this valve seat is the hottest and the oil has the greatest cooling capacity, it then flows through passageways formed by two drillings, around the inlet seat and back to the crankcase through an oil drilling machined though a cast feature in the cylinder barrel.

Using CFD as a guide, the geometry of the grooves was adjusted to ensure that the oil flows around the seats in the correct proportion based on the cooling demands, whilst minimising pressure losses and recirculation zones.

During testing of the engine, under full load conditions, the temperature of the cylinder head around the combustion chamber was measured to be, on average 19°C cooler in the prototype engine.

CFD output showing flow of cooling oil around valve seat inserts
Figure 16 - CFD output showing flow of cooling oil around valve seat inserts

Air cooling of the cylinder head

Many of the features of the cylinder head remain the same, for example, the inlet and exhaust ports, main stud positions, dowels, injector and glow plug. Around these constraints the goal was to introduce fins which:

• Maximise surface area
• Allow air to flow freely through the cylinder head
• Minimise recirculation zones
• Comply with manufacturing constraints
• Provide the cylinder head with adequate strength to reach target life

The use of Computational Fluid Dynamics (CFD) software was essential in visualising the behaviour of the air flow through both existing and proposed cylinder heads. The existing head was analysed to form a baseline, against which the proposed head was compared. The geometry of the fins was modified and analysed in an iterative manor until an optimum solution was found.

Figure 17 shows a section through the cylinder head with a simulated air flow entering from the left hand side. As can be seen by the colour map the velocity is relatively even throughout the section, meaning there are a minimum of recirculation zones, and equally not many areas of very high velocity.
Section through prototype cylinder head showing velocity of air flow

Figure 17 - Section through prototype cylinder head showing velocity of air flow

Under the same input conditions (constant static pressure at inlet and exit) the air mass flow rate through the prototype cylinder head was predicted to be approximately 5% higher. There was also a predicted 5% drop in mean and peak temperatures.

In addition to the modified fin geometry, and to enable a larger volume of cooling air to flow through the head, a considerable amount of material was removed from above the combustion chamber.

This resulted in achieving the objectives of reducing mass and increasing cooling capacity.

Air cooling of the cylinder barrel

The cooling area of the barrels was increased by 39% whilst still fitting within the confines of the existing engine package.

Prototype and existing Cylinder Barrels
Figure 18 - Prototype (left) and existing Cylinder Barrels

FE analysis was carried out to simulate bore distortion under operating pressures at various ambient temperatures, to verify that the size and roundness of the bore remained acceptable under all running conditions.

The analysis predicted that the bore of the prototype engine had less deformation than the bore of the existing engine. Figure 19 shows the deformation for the 0°C ambient air condition.

Predicted bore distortion at 5 positions down the cylinder
Figure 19 - Predicted bore distortion at 5 positions down the cylinder

Clamping of the cylinder barrel during engine assembly can generate some distortion of the bore. In order to achieve good roundness in the clamped state the production process involved pre-machining the cylinder bores, whilst clamped with a representative load. After plasma coating, the bores were then honed using the same clamping specifications. This ensured that the cylinder bore achieved the roundness and cylindricity tolerances necessary, and in combination with the similar thermal expansion of the piston and bore materials provided close control over the piston-bore clearance.

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This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 686533