Engine Internals

Sodium filled inlet valve

The SMA SR305 Engine has a sodium-filled exhaust valve and hollow inlet valve. SMA requested that the inlet valves of the prototype engine were sodium filled also, to investigate the cooling effect this may have.

Higher combustion pressures associated with running at the higher power output will increase the temperature in the combustion chamber, valves and valve seats. Sodium has a very high thermal conductivity (~141 W/m.K compared to ~11.2 W/m.K for nickel-alloy valve materials). The hollow cavity in the valve is partially filled with sodium, to allow for movement of the sodium during valve motion, to transfer heat away from the valve head and keep the valve temperature lower than that with no sodium. 

Comparison of SMA and Ilmor inlet valve designs
Figure 20 - Comparison of SMA and Ilmor inlet valve designs

The exact percentage of sodium fill in the valve cavity was determined by assessing the valve motion, gravity effects and cooling performance due to the turbulent motion of the sodium. Depending on application, this can range from 30-65% sodium fill. For the Ilmor-proposed design, 45-55% was determined by the valve supplier as the optimum fill for both inlet and exhaust valves.

The additional mass of 1.5g on the sodium-filled inlet valve sat within the acceptable limits of the valve train.

Redesigned Gudgeon pin

The existing SMA SR305 engine has an uncoated gudgeon pin, made from case-hardening steel, with a carburized top layer for improved load-bearing and wear properties.

For the prototype engine, the pin geometry was modified by reducing the internal taper slightly, creating a thicker wall in order to increase stiffness under the higher firing loads, as illustrated in Figure 21.

Comparison of SMA and Ilmor connecting rod and gudgeon pin designs
Figure 21 - Comparison of SMA and Ilmor connecting rod and gudgeon pin designs

Structural Finite Element Analysis at the maximum firing load case showed excellent roundness and minimal bending deflection for the new pin design.

In addition to the geometry changes, a Diamond Like Carbon coating was specified on the Ilmor gudgeon pin. Carbon-based coatings such as DLC offer a 1-4µm hard surface layer that is extremely resistant to wear and significantly reduces friction between highly loaded components.

DLC coatings require excellent initial surface finishes on the parent body, as any asperities on the surface will be highly abrasive when coated. For this reason, a ground super-finish was specified on the pin body prior to coating. Due to the elevated temperatures at which DLC coatings are applied (>300°C), a nitriding steel, GKHW was specified to ensure no loss of core hardness of the pin body after coating. The pin was nitrided prior to coating, to give a strong case-hardened layer beneath the DLC coating.

Redesigned connecting rod

The current SMA SR305 Engine has a forged steel connecting rod with a copper-beryllium small end bushing. A design study was completed by Ilmor for a new connecting rod with the following objectives:

• Reduction of connecting rod mass
• Capability to withstand higher combustion pressures 
• Capability to run at higher rpm
• Increased lubrication to the small end and big end thrust areas

Finite Element Analysis was used to assess the connecting rod’s deformation and stresses under multiple load conditions, and several design iterations were generated until a final “optimised” proposal was reached. Figure 22 shows the setup of the finite element model. In order to save computational resource, and because the model is symmetrical in two planes, only a quarter model was considered.

Quarter Connecting Rod FEA Model
Figure 22 - Quarter Connecting Rod FEA Model

The connecting rod was analysed under two load cases; firing load and inertia. Each case stressed the rod in different ways. Figure 23 and Figure 24 show the resultant Von Mises stress plots under these two conditions.

Firing Load Case – Von Mises Stress
Figure 23 - Firing Load Case – Von Mises Stress
Inertia Load Case – Von Mises Stress
Figure 24 - Inertia Load Case – Von Mises Stress

Also considered during the finite element analysis were the following contact areas;

• Joint pressure between the con-rod and the con-rod cap in the inertia case
• Big end bearing back pressure in the assembly case
• Small end bush contact pressure in the assembly case

Pressure plots of the various contact areas of the connecting rod
Figure 25 - Pressure plots of the various contact areas of the connecting rod

The final design was then subjected to a fatigue assessment to quantify material fatigue reserve factor under a fully-reversed loading condition. The reserve factor shows how ‘safe’ the design is for the intended life cycle of the part.

Fully Reversed” Fatigue Case
Figure 26 - “Fully Reversed” Fatigue Case

The final proposal included the following design highlights:

• 4340 VAR steel material, offering very high strength and excellent fatigue properties
• Machined from solid design, offering a 0.26kg mass saving per rod
• Met or exceeded fatigue life requirements for peak cylinder pressure and maximum engine speed running conditions
• Stiffened big end and cap to minimise bore deformation under firing and inertia loads
• Big end joint face relief to focus contact pressure and to reduce joint fretting.
• Small end oil feed from piston squirt jet
• Big end thrust oil grooves to aid rod-crank cheek lubrication
• Re-use of copper-beryllium small end bushing, big end bearing shells and studs/nuts from SMA current design

Revised connecting rod proposal
Figure 27 - Revised connecting rod proposal

Although the re-designed con-rod met the design objectives, the decision was made not to manufacture the rod. The reduced mass of the rod would have affected the dynamics of the propeller drive shaft sufficiently as to require further design work, which was beyond the scope of this project.

Steel Piston concept

One concept considered by Ilmor as a method of weight reduction was to replace the existing SMA aluminium piston with a steel piston of a brand new design. Although the density of steel is higher than that of aluminium (approx. 8g/cm3 vs. 2.7 g/cm3), a steel piston is able to take advantage of the improved elevated material properties which steel has over aluminium. The result of this is a piston design with far less volume of material, including a reduced compression height. Figure 28 illustrates some key differences between a steel and aluminium piston design which enable a reduction in compression height and mass.

Steel vs. aluminium piston
Figure 28 - Steel vs. aluminium piston

In order to maintain the same overall deck height, a longer connecting rod is necessary to accompany the steel piston. The longer rod would also provide additional clearance to the crankcase at BDC, aiding crankcase design and piston squirt jet packaging.

A new design of connecting rod permits further mass reduction (despite the longer centre distance) by using a higher grade of steel than the existing design, as detailed in section 4.7 for the proposed standard length con-rod. Figure 29 shows a comparison of connecting rods. An additional benefit of the longer connecting rod is the reduced piston thrust load on the liner due to the reduced rod angulation.

Comparison of standard and extended con-rod designs
Figure 29 - Comparison of standard and extended con-rod designs

A mass reduction estimate was made for the steel piston concept in conjunction with the longer connecting rod. This is summarised in the Table 2:

Variant Ilmor
Piston Assembly Mass (g) -600
Connecting Rod Assembly Mass (g) -232
Mass Reduction (g) 832
Table 2 - Potential mass savings for the proposed steel piston concept (per cylinder)

Only a short design study was carried out on the steel piston concept. There would have been knock-on effects to other parts of the engine that would have required additional design resource. This, along with the costs of manufacturing the pistons and connecting rods were beyond the budget for this project.

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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