This page documents the kinematics and dynamics analysis carried out on the PPP-106 sprint engine during its development programme. The aim was to quantify the forces acting on the piston and connecting rod assembly at maximum RPM, and to assess the stress on both OEM and uprated components. The results directly informed decisions around conrod specification and piston choice.
All values shown use the actual measured geometry of the 106 Rallye TU3 bottom end. Calculations follow the standard slider-crank model, retaining the first two terms of the Fourier series expansion - valid where R/L < 0.3.
1. Reciprocating Masses
The reciprocating mass is the total mass that the engine must accelerate and decelerate on every stroke. It comprises the piston assembly and the small-end (gudgeon pin end) fraction of the connecting rod. Reducing this mass directly reduces peak bearing loads and allows higher RPM for a given stress limit.
| Component | OEM (g) | OEM (kg) | EBD Modified (g) | EBD Modified (kg) |
|---|---|---|---|---|
| Piston assembly | 395 | 0.395 | 315 (Wossner) | 0.315 |
| Con-rod small end mass | 135.5 | 0.1355 | 135.5 | 0.1355 |
| Total reciprocating mass | 530.5 | 0.5305 | 450.5 | 0.4505 |
The Wossner forged piston saves 80 g per cylinder compared to the OEM cast item - a 15% reduction in reciprocating mass. At 8,500 rpm this has a significant effect on peak piston force and therefore on con-rod stress.
Con-rod small-end mass was measured as 135.5 g on the EBD-modified rods. The standard rod cross-sectional area was measured at 1.0754 × 10² mm² (1.0754 × 10⁻&sup4; m²).
2. Piston Acceleration
Piston acceleration varies continuously around the cycle. Using the two-term approximation, instantaneous acceleration a at crank angle θ is:
At θ = 0° (TDC) both cosine terms are +1, giving maximum positive acceleration — the piston is being pulled to a halt and reversed. This is where peak tensile load is applied to the con-rod small end.
3. Peak Piston Force
The peak inertia force on the con-rod is found from F = m × a, using the total reciprocating mass and the peak acceleration calculated above. This force acts in tension at TDC on the overrun (or on a non-firing stroke).
| Configuration | Reciprocating Mass | Peak Acceleration | Peak Force |
|---|---|---|---|
| OEM components | 530.5 g (0.5305 kg) | 39,301 m/s² | 20,849 N |
| EBD modified (Wossner piston) | 450.5 g (0.4505 kg) | 39,301 m/s² | 17,705 N (3,144 N saving) |
4. Con-Rod Stress Analysis
With the peak force known, the nominal tensile stress in the con-rod shank is calculated from the measured cross-sectional area. This gives a conservative estimate — the actual stress distribution is non-uniform, but this gives a useful baseline for material selection and factor-of-safety assessment.
| Configuration | Peak Force | Con-rod CSA | Nominal Stress | Rod UTS (material) | Factor of Safety |
|---|---|---|---|---|---|
| OEM piston | 20,849 N | 107.54 mm² | 193.9 MPa | 430 MPa | 2.22 |
| EBD modified (Wossner) | 17,705 N | 107.54 mm² | 164.6 MPa | 430 MPa | 2.61 |
Well-designed connecting rods do not fail under the compressive inertia load alone — the Euler buckling load for the rod geometry is significantly higher than the peak compressive force. Failure, when it occurs in poorly specified rods, is typically in fatigue under the combined tensile and bending cycle rather than simple compression.
5. Interactive Calculator
Adjust the inputs below to explore how changes to engine specification affect peak force and stress. All calculations use the same two-term Fourier model as the study above.
6. Conclusions
Mass reduction works
The Wossner forged piston cuts 80 g from reciprocating mass per cylinder. At 8,500 rpm this translates to a 3,144 N reduction in peak inertia force — a 15.1% improvement.
Con-rod is adequately specified
Both OEM and modified configurations return a factor of safety above 2.0 against the rod material UTS in simple tension. The OEM rod is not the weak point at 8,500 rpm for inertia loading alone.
Piston speed is the limiting factor
Mean piston speed of 21.8 m/s approaches the practical limit for cast iron bores (~20 m/s). The switch to forged pistons and bore finishing was driven partly by this consideration.
Gas loads require separate analysis
These results cover inertia loading only. Peak combustion pressure (typically 60-80 bar for a naturally aspirated 1400) creates a compressive load on the rod during the power stroke that must be assessed separately for a complete fatigue life estimate.