Cylinder Heads & Barrels


The cylinder barrels and heads provided an opportunity to increase the efficiency of the engine cooling, whilst also simplifying the assembly, and reducing mass. Ensuring that the target life would be met required a thorough technical analysis using CFD and FEA techniques. The finned geometry was challenging to produce from a casting perspective, but this was solved by printing the sand cores. This process would be suitable if the cylinder head were to be produced in larger volumes.

Structural Analysis of the cylinder head

Finite Element Analysis (FEA) was used heavily to analyse the thermal, structural and fatigue response to assess whether the structural integrity of the cylinder head was sufficient to achieve the target life. Figure 4 and Figure 5 show some typical results plots from the FE analysis.
Typical FEA results plot showing equivalent stress in cylinder head

Figure 4 - Typical FEA results plot showing equivalent stress in cylinder head (IMAGE)
Typical FEA results plot showing safety factors in fatigue analysis
Figure 5 - Typical FEA results plot showing safety factors in fatigue analysis (IMAGE)

Introduction of a combustion seal

In an effort to increase the robustness of the seal between cylinder barrel and cylinder head joint faces, the traditional laminated gasket was replaced with a stainless steel, nitrogen pressurised combustion seal; a technique commonly used in racing engines
Existing and proposed cylinder barrel – Cylinder head- barrel seal

Figure 6 - Existing and proposed cylinder barrel – Cylinder head- barrel seal (IMAGE)

The combustion seal is a simple metallic ring, which is compressed to form a tight seal between the joint faces. It requires a groove to be machined in to the top face of the cylinder barrel, and is suitable for pressure ranges from ultra-high vacuum to 1000MPa, and temperature ranges between cryogenic and 800°C. Very tight control of the stack-up of tolerances between cylinder head and barrel can be achieved, and removing the gasket also provides a more efficient heat path from the hot cylinder head to the cooler barrel.
Existing gasket between cylinder barrel and cylinder head

Figure 7 - Existing gasket between cylinder barrel and cylinder head (IMAGE)
Ilmor proposed combustion seal design
Figure 8 - Ilmor proposed combustion seal design

Analysis of the joint face between the cylinder head and barrel after the engine test showed no evidence of leakage beyond the combustion seal on any of the four cylinders. Each seal was in good condition, therefore the implementation of the combustion seal concept was considered successful.

Plasma coated cylinder bore 

The existing engine uses a traditional cast iron liner inserted into the aluminium cylinder barrel. The prototype engine utilises an iron based plasma sprayed directly on to the cast aluminium barrel. The process was developed by Sulzer Metco, now Oerlikon Metco, Switzerland.  The concept is shown below:

Principle of plasma coating (Oerlikon Metco)

Figure 9 - Principle of plasma coating (Oerlikon Metco)

Schematic diagram of thermal sprayed coating (Oerlikon Metco)

Figure 10 - Schematic diagram of thermal sprayed coating (Oerlikon Metco)

The coating forms an unordered structure which contains voids throughout. These voids retain oil which in turn provides lubrication between the piston rings and the cylinder bore. As the coating wears, other voids are revealed meaning lubrication does not reduce with wear.
Comparison between the proposed plasma coated cylinder barrel, and the existing configuration.

Figure 11 - Comparison between the proposed plasma coated cylinder barrel, and the existing configuration.

As well as improved fuel efficiency and lubrication, the plasma coating offers the following additional advantages:

Increased heat transfer from the cylinder bore

Aluminium has a much higher thermal conductivity (~126 W/K.m @25°C) relative to cast iron (~50 W/K.m @25°C), so can remove heat from the cylinder bore more effectively.

It is difficult to quantify the effect that introducing the plasma coated bore has on the operating temperature of the cylinder barrel in isolation, since the geometry of the barrel itself, size and shape of fins etc. also have an effect. For instance, the prototype barrel has 39% more fin area for convecting heat away from the engine.

Reduced Mass

Cast iron has a density of 7.1g/mm3, aluminium has a density of 2.7 g/mm3. Replacing the volume of material previously taken up by iron with aluminium provides a large weight reduction.

COMPONENT MATERIAL ILMOR
CYLINDER BARREL CAST ALUMINIUM +568g
LINER CAST IRON -
     
WEIGHT SAVING/BARREL   2436g
WEIGHT SAVING/ENGINE   9744g
Table 1 – Summary of mass saving for the prototype cylinder barrel

As can be seen from the table of masses, replacing the iron liner by plasma coating the cast aluminium barrel removes almost 10Kg of mass from the engine.

Similar coefficients of expansion

The existing piston is made from an aluminium alloy, which expands at a higher rate than the iron liner (18.0x10-6M/M.°C versus 10.1x10-6M/M.°C), thus, when the engine heats up the piston grows more than the iron liner. This extra growth needs to be accounted for and means that there is a larger clearance between piston and liner at lower engine temperatures, and consequently more side-to-side piston motion and ‘blow-by’ (combustion gases escaping past the piston rings into the lower crankcase).

The barrel of the prototype engine is aluminium with a coefficient of thermal expansion (CTE) of 20.9x10-6M/M.°C; hence it grows at a more similar rate to the piston upon heating. Therefore, the clearance between piston and bore can be more tightly controlled, and will remain more constant across the temperature range.

The SR305-230E engine is air-cooled, but incorporates oil cooled inlet and exhaust valve seats.

Running the engine at the higher power output naturally increases the temperature of the components, especially around the combustion chamber. Therefore, one of the main design targets for the project was to increase the cooling performance of both the oil and air cooling systems.

The importance of maximising the cooling efficiency is revealed when studying the effect of temperature on the yield strength of aluminium.  Fig. 19 shows the response of A354 aluminium. As can be seen, the mechanical properties begin to decrease as the temperature increases beyond 200°C. Normal operating temperature for some areas of the prototype engine’s cylinder head can be in the region of 250°C.

Yield strength vs. temperature for a typical wrought aluminium alloy
Figure 12 - Yield strength vs. temperature for a typical wrought aluminium alloy

This sensitivity to temperature highlights the importance of careful design and analysis in order for the components to achieve the desired life target.

 

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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