stable

Components/Cylinder.cs

@@ -32,7 +32,7 @@ namespace FluidSim.Components
         public float StoichiometricAFR = 14.7f;
         public float FuelLowerHeatingValue = 44e6f;
         public float EnergyVariationFraction = 0.05f;
-        public float MisfireProbability = 0.01f;
+        public float MisfireProbability = 0.0f;
         public float CylinderWallArea = 0.02f;
         public float HeatTransferCoefficient = 100f;
         public float AmbientTemperature = 300f;

Core/BoundarySystem.cs

@@ -165,7 +165,7 @@ namespace FluidSim.Core
                     continue;
                 }
 
-                // ---- Preliminary isentropic solution (used for face pressure if inertance is on) ----
+                // ---- Preliminary isentropic solution ----
                 float mdotEst, rhoFaceEst, uFaceEst, pFaceEst;
                 if (volP >= pipeP)
                 {
@@ -179,7 +179,7 @@ namespace FluidSim.Core
                     mdotEst = -mdotEst;
                 }
 
-                // ---- Compute ghost state ----
+                // ---- Compute final mass flow with limiters ----
                 float mdotFinal, rhoFace, uFace, pFace, airFracGhost;
 
                 if (d.UseInertance)
@@ -189,11 +189,10 @@ namespace FluidSim.Core
                     float dp = volP - pipeP;
                     float Rlin = d.LossCoefficient;
 
-                    // Forward Euler
                     float dmdot_dt = (dp - Rlin * d.CurrentMdot) / MathF.Max(inertance, 1e-12f);
                     float mdotNew = d.CurrentMdot + dmdot_dt * dt;
 
-                    // ---------- Symmetric flow limiters (existing) ----------
+                    // Limit outflow from volume (if volume owner is Volume0D)
                     if (d.VolumePort.Owner is Volume0D vol0)
                     {
                         float maxOut = vol0.Mass / dt;
@@ -201,15 +200,19 @@ namespace FluidSim.Core
                         if (mdotNew < -maxOut) mdotNew = -maxOut;
                     }
 
+                    // Limit inflow from pipe – pipe cell cannot be emptied in one step
+                    {
                         int adjCell = d.IsLeftEnd ? _pipeSystem.GetPipeStart(d.PipeIndex)
                                                 : _pipeSystem.GetPipeEnd(d.PipeIndex) - 1;
                         float pipeRhoAdj = _pipeSystem.GetCellDensity(adjCell);
+                        float pipeAreaCell = _pipeSystem.GetCellArea(adjCell);  // true cell area, not orifice
                         float pipeDxAdj = _pipeSystem.GetCellDx(adjCell);
-                    float pipeCellMass = pipeRhoAdj * area * pipeDxAdj;
+                        float pipeCellMass = pipeRhoAdj * pipeAreaCell * pipeDxAdj;
                         float maxFromPipe = pipeCellMass / dt;
                         if (mdotNew < -maxFromPipe) mdotNew = -maxFromPipe;
+                    }
 
-                    // ---------- Velocity clamp to Mach 0.9 ----------
+                    // Velocity clamp to Mach 0.9
                     float rhoFacePrelim = mdotNew >= 0 ? volRho : pipeRho;
                     float uFacePrelim = MathF.Abs(mdotNew) / MathF.Max(rhoFacePrelim * area, 1e-12f);
                     float cUp = mdotNew >= 0 ? MathF.Sqrt(Gamma * Rgas * volT) : MathF.Sqrt(Gamma * Rgas * pipeT);
@@ -220,46 +223,64 @@ namespace FluidSim.Core
                         mdotNew = rhoFacePrelim * uFacePrelim * area * (mdotNew >= 0 ? 1f : -1f);
                     }
 
-                    // NaN safety
                     if (float.IsNaN(mdotNew)) mdotNew = 0f;
 
                     d.CurrentMdot = mdotNew;
                     mdotFinal = mdotNew;
-
-                    // Ghost state
                     rhoFace = mdotFinal >= 0 ? volRho : pipeRho;
                     pFace = pFaceEst;
                     uFace = MathF.Abs(mdotFinal) / MathF.Max(rhoFace * area, 1e-12f);
                 }
                 else
                 {
-                    // ---- Standard quasi‑steady orifice ----
+                    // Standard quasi‑steady orifice
                     mdotFinal = mdotEst;
                     rhoFace = rhoFaceEst;
                     uFace = uFaceEst;
                     pFace = pFaceEst;
 
-                    // Limit outflow from cavity
+                    // Limit outflow from volume (if Volume0D)
                     if (d.VolumePort.Owner is Volume0D vol0)
                     {
                         float maxOut = vol0.Mass / dt;
                         if (mdotFinal > maxOut) mdotFinal = maxOut;
                     }
-                    d.CurrentMdot = mdotFinal;
+
+                    // ***** CRITICAL: Limit inflow from pipe – pipe cell cannot be drained *****
+                    if (mdotFinal < 0)
+                    {
+                        int adjCell = d.IsLeftEnd ? _pipeSystem.GetPipeStart(d.PipeIndex)
+                                                : _pipeSystem.GetPipeEnd(d.PipeIndex) - 1;
+                        float pipeRhoAdj = _pipeSystem.GetCellDensity(adjCell);
+                        float pipeAreaCell = _pipeSystem.GetCellArea(adjCell);
+                        float pipeDxAdj = _pipeSystem.GetCellDx(adjCell);
+                        float pipeCellMass = pipeRhoAdj * pipeAreaCell * pipeDxAdj;
+                        float maxFromPipe = pipeCellMass / dt;
+                        if (mdotFinal < -maxFromPipe)
+                            mdotFinal = -maxFromPipe;
                     }
 
-                // ---- Determine air fraction for ghost ----
+                    d.CurrentMdot = mdotFinal;
-                if (mdotFinal >= 0)
+
+                    // Limit outflow from cylinder into pipe (positive mdot = volume → pipe)
+                    if (mdotFinal > 0f && d.VolumePort?.Owner is Cylinder cyl)
                     {
-                    airFracGhost = volAF;
+                        float maxOut = cyl.Mass / dt;
+                        if (mdotFinal > maxOut)
+                            mdotFinal = maxOut;
                     }
+                }
+
+                // ---- Air fraction for ghost ----
+                if (mdotFinal >= 0)
+                    airFracGhost = volAF;
                 else
                 {
                     airFracGhost = pipeAF;
                     if (d.VolumePort != null) d.VolumePort.AirFraction = pipeAF;
                 }
 
-                // ---- Apply sign convention for velocity ----
+                // ---- Sign convention for velocity ----
                 if (mdotFinal >= 0 && d.IsLeftEnd) uFace = +uFace;
                 else if (mdotFinal >= 0 && !d.IsLeftEnd) uFace = -uFace;
                 else if (mdotFinal < 0 && d.IsLeftEnd) uFace = -uFace;
@@ -271,18 +292,17 @@ namespace FluidSim.Core
                 else
                     _pipeSystem.SetGhostRight(d.PipeIndex, rhoFace, uFace, pFace, airFracGhost);
 
-                // ---- Update volume port (mass flow: positive into volume) ----
+                // ---- Update volume port ----
                 if (d.VolumePort != null)
                 {
                     d.VolumePort.MassFlowRate = -mdotFinal;
 
-                    // Set enthalpy of the stream entering the volume
+                    if (-mdotFinal >= 0)   // mass entering volume (out of pipe)
-                    if (-mdotFinal >= 0)   // mass flowing into the volume (out of pipe)
                     {
                         float pipeH = GammaOverGm1 * pipeP / MathF.Max(pipeRho, 1e-12f);
                         d.VolumePort.SpecificEnthalpy = pipeH;
                     }
-                    else   // mass flowing out of the volume (into pipe)
+                    else   // mass leaving volume (into pipe)
                     {
                         d.VolumePort.SpecificEnthalpy = volH;
                     }
@@ -309,6 +329,7 @@ namespace FluidSim.Core
                 float cInt = MathF.Sqrt(gamma * pInt / MathF.Max(rhoInt, 1e-12f));
                 float pAmb = d.AmbientPressure;
 
+                // Characteristic solution (isentropic expansion to ambient)
                 float Jplus = uInt + 2f * cInt / gm1;
                 float Jminus = uInt - 2f * cInt / gm1;
                 float s = pInt / MathF.Pow(rhoInt, gamma);
@@ -318,9 +339,14 @@ namespace FluidSim.Core
                     ? (Jminus + 2f * cIso / gm1)
                     : (Jplus - 2f * cIso / gm1);
 
+                // Supersonic check
                 bool supersonic = d.IsLeftEnd ? (uInt <= -cInt) : (uInt >= cInt);
-                float rhoGhost, uGhost, pGhost, afGhost;
+                if (!supersonic)
+                {
+                    supersonic = d.IsLeftEnd ? (uIso <= -cIso) : (uIso >= cIso);
+                }
 
+                float rhoGhost, uGhost, pGhost, afGhost;
                 if (supersonic)
                 {
                     rhoGhost = rhoInt; uGhost = uInt; pGhost = pInt; afGhost = afInt;
@@ -332,15 +358,45 @@ namespace FluidSim.Core
                     afGhost = inflow ? 1f : afInt;
                 }
 
+                // ------- Mass flow limiter -------
+                int adjCell = d.IsLeftEnd
+                    ? _pipeSystem.GetPipeStart(d.PipeIndex)
+                    : _pipeSystem.GetPipeEnd(d.PipeIndex) - 1;
+                float pipeRhoAdj = _pipeSystem.GetCellDensity(adjCell);
+                float pipeAreaCell = _pipeSystem.GetCellArea(adjCell);
+                float pipeDxAdj = _pipeSystem.GetCellDx(adjCell);
+                float cellMass = pipeRhoAdj * pipeAreaCell * pipeDxAdj;
+
+                float area = d.PipeArea;
+                float mdotRaw = rhoGhost * uGhost * area;   // positive out of pipe
+                if (d.IsLeftEnd) mdotRaw = -mdotRaw;        // now positive = out of pipe
+
+                // Outflow limit
+                if (mdotRaw > 0 && mdotRaw * dt > cellMass)
+                {
+                    mdotRaw = cellMass / dt;
+                }
+                // Inflow limit (allow up to 10× cell mass to avoid starving the pipe)
+                else if (mdotRaw < 0 && -mdotRaw * dt > 10f * cellMass)
+                {
+                    mdotRaw = -10f * cellMass / dt;
+                }
+
+                // Recompute uGhost from the limited mdot, keeping rhoGhost, pGhost
+                float mdotMag = MathF.Abs(mdotRaw);
+                uGhost = mdotMag / MathF.Max(rhoGhost * area, 1e-12f);
+                if (d.IsLeftEnd)
+                    uGhost = (mdotRaw >= 0f) ? -uGhost : uGhost;
+                else
+                    uGhost = (mdotRaw >= 0f) ? uGhost : -uGhost;
+
+                // Apply ghost
                 if (d.IsLeftEnd)
                     _pipeSystem.SetGhostLeft(d.PipeIndex, rhoGhost, uGhost, pGhost, afGhost);
                 else
                     _pipeSystem.SetGhostRight(d.PipeIndex, rhoGhost, uGhost, pGhost, afGhost);
 
-                float area = d.PipeArea;
+                d.LastMassFlowRate = mdotRaw;
-                float mdot = rhoGhost * uGhost * area;
-                if (d.IsLeftEnd) mdot = -mdot;
-                d.LastMassFlowRate = mdot;
                 d.LastFacePressure = pGhost;
             }
         }

Core/Pipesystem.cs

@@ -16,23 +16,28 @@ namespace FluidSim.Core
         private readonly int _allCells;            // total allocated (padded to Vector<float>.Count)
         private readonly int _pipeCount;
 
-        // Derived state – _p is kept for visualization, _c is gone
+        // Derived state – _p is kept for visualization
         private float[] _p;
 
-        // Flux arrays (size = _allCells + 1)
+        // Flux arrays for faces INTERNAL to a single pipe (size = _allCells + 1)
+        // Only valid for faces that are NOT pipe boundaries.
         private float[] _fluxM, _fluxP, _fluxE, _fluxY;
 
-        // Damping and relaxation (computed on‑the‑fly only if used)
+        // Per‑pipe boundary flux buffers (size = _pipeCount)
+        private float[] _leftFluxM, _leftFluxP, _leftFluxE, _leftFluxY;
+        private float[] _rightFluxM, _rightFluxP, _rightFluxE, _rightFluxY;
+
+        // Damping and relaxation
         private float[] _dampingFactors;
         private float[] _relaxFactors;
         private bool _applyDamping;
         private bool _applyRelax;
 
-        // Ghost buffer
+        // Ghost buffer (per‑pipe ghost states)
         private readonly GhostBuffer _ghost;
 
-        // Wall mask – precomputed once
+        // Precomputed flag: true if a face is a pipe boundary (start or end)
-        private readonly bool[] _isWallFace;
+        private readonly bool[] _isPipeBoundaryFace;
 
         // ---------- Physical constants ----------
         private const float Gamma = 1.4f;
@@ -102,6 +107,16 @@ namespace FluidSim.Core
             _fluxE = new float[faceCount];
             _fluxY = new float[faceCount];
 
+            // Per‑pipe boundary flux buffers
+            _leftFluxM = new float[_pipeCount];
+            _leftFluxP = new float[_pipeCount];
+            _leftFluxE = new float[_pipeCount];
+            _leftFluxY = new float[_pipeCount];
+            _rightFluxM = new float[_pipeCount];
+            _rightFluxP = new float[_pipeCount];
+            _rightFluxE = new float[_pipeCount];
+            _rightFluxY = new float[_pipeCount];
+
             _dampingFactors = new float[_allCells];
             _relaxFactors = new float[_allCells];
             _applyDamping = _coeffBase != 0f;
@@ -110,18 +125,12 @@ namespace FluidSim.Core
             _ghost = new GhostBuffer(_pipeCount);
             _ambientEnergyRef = initialP * Gm1Inv;
 
-            // Pre‑compute wall face flags: each face that sits between two different pipes is a wall
+            // Mark faces that coincide with a pipe boundary (start or end)
-            _isWallFace = new bool[faceCount];
+            _isPipeBoundaryFace = new bool[faceCount];
-            for (int f = 1; f < _totalCells; f++)
-            {
             for (int p = 0; p < _pipeCount; p++)
             {
-                    if (f == _pipeEnd[p] && f < _totalCells)
+                _isPipeBoundaryFace[_pipeStart[p]] = true;
-                    {
+                _isPipeBoundaryFace[_pipeEnd[p]] = true;
-                        _isWallFace[f] = true;
-                        break;
-                    }
-                }
             }
 
             // Initialize uniform state
@@ -150,6 +159,7 @@ namespace FluidSim.Core
         public float GetCellPressure(int i) => _p[i];
         public float GetCellDensity(int i) => _rho[i];
         public float GetCellDx(int i) => _dx[i];
+        public float GetCellArea(int i) => _area[i];
         public float GetCellVelocity(int i)
         {
             float rho = _rho[i];
@@ -215,13 +225,13 @@ namespace FluidSim.Core
             }
         }
 
-        // ---------- Flux computation: fuses primitive calculation and flux evaluation ----------
+        // ---------- Flux computation ----------
         private void ComputeFluxes(float dt)
         {
             float fm, fp, fe;
             int vecSize = Vector<float>.Count;
 
-            // ---- 1. Left ghost boundaries ----
+            // ---- 1. Left ghost boundaries → per‑pipe buffers ----
             for (int p = 0; p < _pipeCount; p++)
             {
                 int idx = _pipeStart[p];
@@ -239,22 +249,18 @@ namespace FluidSim.Core
                 float cR = MathF.Sqrt(Gamma * pR * invRhoR);
                 float YR = _Y[idx];
 
-                // store pressure for cell idx
-                _p[idx] = pR;
-
                 LaxFlux(rL, uL, pL, cL, rR, uR, pR, cR, out fm, out fp, out fe);
-                _fluxM[idx] = fm; _fluxP[idx] = fp; _fluxE[idx] = fe;
+                _leftFluxM[p] = fm; _leftFluxP[p] = fp; _leftFluxE[p] = fe;
 
                 float alpha = MathF.Max(MathF.Abs(uL) + cL, MathF.Abs(uR) + cR);
                 ScalarFlux(rL, uL, YL, rR, uR, YR, alpha, out float fy);
-                _fluxY[idx] = fy;
+                _leftFluxY[p] = fy;
             }
 
-            // ---- 2. Right ghost boundaries ----
+            // ---- 2. Right ghost boundaries → per‑pipe buffers ----
             for (int p = 0; p < _pipeCount; p++)
             {
                 int idx = _pipeEnd[p] - 1;
-                int face = idx + 1;
                 int ghostIdx = p * 2 + 1;
                 float rR = _ghost.Rho[ghostIdx];
                 float uR = _ghost.U[ghostIdx];
@@ -269,45 +275,35 @@ namespace FluidSim.Core
                 float cL = MathF.Sqrt(Gamma * pL * invRhoL);
                 float YL = _Y[idx];
 
-                // store pressure for cell idx
-                _p[idx] = pL;
-
                 LaxFlux(rL, uL, pL, cL, rR, uR, pR, cR, out fm, out fp, out fe);
-                _fluxM[face] = fm; _fluxP[face] = fp; _fluxE[face] = fe;
+                _rightFluxM[p] = fm; _rightFluxP[p] = fp; _rightFluxE[p] = fe;
 
                 float alpha = MathF.Max(MathF.Abs(uL) + cL, MathF.Abs(uR) + cR);
                 ScalarFlux(rL, uL, YL, rR, uR, YR, alpha, out float fy);
-                _fluxY[face] = fy;
+                _rightFluxY[p] = fy;
             }
 
-            // ---- 3. Interior faces – vectorised SIMD ----
+            // ---- 3. Interior faces (skip pipe boundaries) → global flux arrays ----
             for (int face = 1; face < _totalCells; face++)
             {
-                // Handle walls (rare) with scalar code
+                // Skip faces that belong to a pipe boundary (they are already handled)
-                if (_isWallFace[face])
+                if (_isPipeBoundaryFace[face])
-                {
-                    int iL = face - 1;
-                    float rL = _rho[iL], rhouL = _rhou[iL];
-                    float invRhoL = MathF.ReciprocalEstimate(MathF.Max(rL, 1e-12f));
-                    float uL = rhouL * invRhoL;
-                    float pL = Gm1 * (_E[iL] - 0.5f * rhouL * uL);
-                    float cL = MathF.Sqrt(Gamma * pL * invRhoL);
-                    _p[iL] = pL;
-
-                    LaxFlux(rL, uL, pL, cL, rL, -uL, pL, cL, out fm, out fp, out fe);
-                    _fluxM[face] = fm; _fluxP[face] = fp; _fluxE[face] = fe;
-                    _fluxY[face] = 0f;
                     continue;
-                }
 
-                // If the next vecSize faces contain a wall, fall back to scalar for this block
+                // Try to vectorize a block of contiguous non‑boundary faces
                 if (face + vecSize - 1 < _totalCells)
                 {
-                    bool hasWall = false;
+                    bool canVectorize = true;
                     for (int f = face; f < face + vecSize; f++)
-                        if (_isWallFace[f]) { hasWall = true; break; }
+                    {
+                        if (_isPipeBoundaryFace[f])
+                        {
+                            canVectorize = false;
+                            break;
+                        }
+                    }
 
-                    if (!hasWall)
+                    if (canVectorize)
                     {
                         // --- Vectorised block ---
                         var rhoL = new Vector<float>(_rho, face - 1);
@@ -330,11 +326,7 @@ namespace FluidSim.Core
                         var cL = Vector.SquareRoot(Gamma * pL * invRhoL);
                         var cR = Vector.SquareRoot(Gamma * pR * invRhoR);
 
-                        // Store pressures for visualisation (left cell of each face)
+                        var ELs = pL * Gm1Inv * invRhoL + 0.5f * uL * uL;
-                        pL.CopyTo(_p, face - 1);
-
-                        // Lax‑Friedrichs fluxes
-                        var ELs = pL * Gm1Inv * invRhoL + 0.5f * uL * uL; // energy per mass
                         var ERs = pR * Gm1Inv * invRhoR + 0.5f * uR * uR;
 
                         var FmL = rhoL * uL;
@@ -362,50 +354,45 @@ namespace FluidSim.Core
                         feVec.CopyTo(_fluxE, face);
                         fyVec.CopyTo(_fluxY, face);
 
-                        face += vecSize - 1; // loop increment will add 1, so we advance vecSize faces
+                        face += vecSize - 1; // loop increment will add 1
                         continue;
                     }
                 }
 
-                // --- Scalar interior face (fallback) ---
+                // --- Scalar fallback for a single interior face ---
                 {
-                    int iLf = face - 1, iRf = face;
+                    int iL = face - 1, iR = face;
-                    float rLf = _rho[iLf], rhouLf = _rhou[iLf];
+                    float rL = _rho[iL], rhouL = _rhou[iL];
-                    float invRhoLf = MathF.ReciprocalEstimate(MathF.Max(rLf, 1e-12f));
+                    float invRhoL = MathF.ReciprocalEstimate(MathF.Max(rL, 1e-12f));
-                    float uLf = rhouLf * invRhoLf;
+                    float uL = rhouL * invRhoL;
-                    float pLf = Gm1 * (_E[iLf] - 0.5f * rhouLf * uLf);
+                    float pL = Gm1 * (_E[iL] - 0.5f * rhouL * uL);
-                    float cLf = MathF.Sqrt(Gamma * pLf * invRhoLf);
+                    float cL = MathF.Sqrt(Gamma * pL * invRhoL);
-                    float YLf = _Y[iLf];
+                    float YL = _Y[iL];
-                    _p[iLf] = pLf;
 
-                    float rRf = _rho[iRf], rhouRf = _rhou[iRf];
+                    float rR = _rho[iR], rhouR = _rhou[iR];
-                    float invRhoRf = MathF.ReciprocalEstimate(MathF.Max(rRf, 1e-12f));
+                    float invRhoR = MathF.ReciprocalEstimate(MathF.Max(rR, 1e-12f));
-                    float uRf = rhouRf * invRhoRf;
+                    float uR = rhouR * invRhoR;
-                    float pRf = Gm1 * (_E[iRf] - 0.5f * rhouRf * uRf);
+                    float pR = Gm1 * (_E[iR] - 0.5f * rhouR * uR);
-                    float cRf = MathF.Sqrt(Gamma * pRf * invRhoRf);
+                    float cR = MathF.Sqrt(Gamma * pR * invRhoR);
-                    float YRf = _Y[iRf];
+                    float YR = _Y[iR];
 
-                    LaxFlux(rLf, uLf, pLf, cLf, rRf, uRf, pRf, cRf, out fm, out fp, out fe);
+                    LaxFlux(rL, uL, pL, cL, rR, uR, pR, cR, out fm, out fp, out fe);
                     _fluxM[face] = fm; _fluxP[face] = fp; _fluxE[face] = fe;
 
-                    float alpha = MathF.Max(MathF.Abs(uLf) + cLf, MathF.Abs(uRf) + cRf);
+                    float alpha = MathF.Max(MathF.Abs(uL) + cL, MathF.Abs(uR) + cR);
-                    ScalarFlux(rLf, uLf, YLf, rRf, uRf, YRf, alpha, out float fy);
+                    ScalarFlux(rL, uL, YL, rR, uR, YR, alpha, out float fy);
                     _fluxY[face] = fy;
                 }
             }
-
-            // If damping/relaxation are active, compute the factors here (re-uses _dampingFactors/_relaxFactors arrays,
-            // but we no longer have a separate precompute pass). We compute them on demand in UpdateCells anyway?
-            // Actually UpdateCells multiplies by these factors; we can compute them there if needed.
         }
 
-        // ---------- Cell update (unchanged core, but skips relaxation/damping when not needed) ----------
+        // ---------- Cell update (per pipe, using correct boundary fluxes) ----------
         private void UpdateCells(float dt)
         {
             int vecSize = Vector<float>.Count;
             float dtRelax = -_relaxRate * dt;
 
-            // Compute damping and relaxation factors if needed
+            // Precompute damping and relaxation factors globally
             if (_applyDamping)
             {
                 for (int i = 0; i < _totalCells; i++)
@@ -418,13 +405,78 @@ namespace FluidSim.Core
             }
             if (_applyRelax)
             {
-                var relaxVal = MathF.Exp(dtRelax);
+                float relaxVal = MathF.Exp(dtRelax);
                 for (int i = 0; i < _totalCells; i++)
                     _relaxFactors[i] = relaxVal;
             }
 
-            int iCell = 0;
+            // Update each pipe separately
-            for (; iCell <= _totalCells - vecSize; iCell += vecSize)
+            for (int p = 0; p < _pipeCount; p++)
+            {
+                int start = _pipeStart[p];
+                int end = _pipeEnd[p];    // exclusive
+                int len = end - start;
+                if (len == 0) continue;
+
+                // ------- Left boundary cell (i = start) ------
+                {
+                    int i = start;
+                    float rhoOld = _rho[i], rhouOld = _rhou[i], EOld = _E[i], YOld = _Y[i];
+
+                    // left face: always the pipe's left boundary flux
+                    float fluxM_L = _leftFluxM[p];
+                    float fluxP_L = _leftFluxP[p];
+                    float fluxE_L = _leftFluxE[p];
+                    float fluxY_L = _leftFluxY[p];
+
+                    // right face: depends on pipe length
+                    float fluxM_R, fluxP_R, fluxE_R, fluxY_R;
+                    if (len == 1)
+                    {
+                        // Only one cell: right face is the pipe's right boundary flux
+                        fluxM_R = _rightFluxM[p];
+                        fluxP_R = _rightFluxP[p];
+                        fluxE_R = _rightFluxE[p];
+                        fluxY_R = _rightFluxY[p];
+                    }
+                    else
+                    {
+                        // interior face (global flux at index i+1)
+                        fluxM_R = _fluxM[i + 1];
+                        fluxP_R = _fluxP[i + 1];
+                        fluxE_R = _fluxE[i + 1];
+                        fluxY_R = _fluxY[i + 1];
+                    }
+
+                    float dtdx = dt / _dx[i];
+                    float rhoNew = rhoOld - dtdx * (fluxM_R - fluxM_L);
+                    float rhouNew = rhouOld - dtdx * (fluxP_R - fluxP_L);
+                    float ENew = EOld - dtdx * (fluxE_R - fluxE_L);
+                    float rhoYOld = rhoOld * YOld;
+                    float rhoYNew = rhoYOld - dtdx * (fluxY_R - fluxY_L);
+
+                    if (_applyDamping) rhouNew *= _dampingFactors[i];
+                    if (_applyRelax) ENew = _ambientEnergyRef + (ENew - _ambientEnergyRef) * _relaxFactors[i];
+
+                    rhoNew = MathF.Max(rhoNew, 1e-12f);
+                    float kin = 0.5f * rhouNew * rhouNew / rhoNew;
+                    float eMin = 100f * Gm1Inv + kin;
+                    ENew = MathF.Max(ENew, eMin);
+
+                    _rho[i] = rhoNew;
+                    _rhou[i] = rhouNew;
+                    _E[i] = ENew;
+                    _Y[i] = Math.Clamp(rhoYNew / rhoNew, 0f, 1f);
+                }
+
+                // ------- Interior cells (i = start+1 to end-2) ------
+                if (len > 2)
+                {
+                    int iCell = start + 1;
+                    int iEnd = end - 1; // exclusive upper bound
+
+                    // Vectorised path for interior cells (if available)
+                    for (; iCell <= iEnd - vecSize; iCell += vecSize)
                     {
                         var rhoOld = new Vector<float>(_rho, iCell);
                         var rhouOld = new Vector<float>(_rhou, iCell);
@@ -471,8 +523,8 @@ namespace FluidSim.Core
                         yNew.CopyTo(_Y, iCell);
                     }
 
-            // Scalar remainder (only a few cells)
+                    // Scalar remainder for interior cells
-            for (; iCell < _totalCells; iCell++)
+                    for (; iCell < iEnd; iCell++)
                     {
                         float rhoOld = _rho[iCell], rhouOld = _rhou[iCell], EOld = _E[iCell], YOld = _Y[iCell];
                         float fluxM_L = _fluxM[iCell], fluxP_L = _fluxP[iCell], fluxE_L = _fluxE[iCell], fluxY_L = _fluxY[iCell];
@@ -500,7 +552,70 @@ namespace FluidSim.Core
                     }
                 }
 
-        // ---------- Scalar flux helpers (used in boundaries and scalar fallback) ----------
+                // ------- Right boundary cell (i = end-1, if len > 1) ------
+                if (len > 1)
+                {
+                    int i = end - 1;
+                    float rhoOld = _rho[i], rhouOld = _rhou[i], EOld = _E[i], YOld = _Y[i];
+
+                    // left face
+                    float fluxM_L, fluxP_L, fluxE_L, fluxY_L;
+                    if (len == 2)
+                    {
+                        // Only two cells: left face is the pipe's left boundary flux
+                        fluxM_L = _leftFluxM[p];
+                        fluxP_L = _leftFluxP[p];
+                        fluxE_L = _leftFluxE[p];
+                        fluxY_L = _leftFluxY[p];
+                    }
+                    else
+                    {
+                        // interior face (global flux at i)
+                        fluxM_L = _fluxM[i];
+                        fluxP_L = _fluxP[i];
+                        fluxE_L = _fluxE[i];
+                        fluxY_L = _fluxY[i];
+                    }
+
+                    // right face: always the pipe's right boundary flux
+                    float fluxM_R = _rightFluxM[p];
+                    float fluxP_R = _rightFluxP[p];
+                    float fluxE_R = _rightFluxE[p];
+                    float fluxY_R = _rightFluxY[p];
+
+                    float dtdx = dt / _dx[i];
+                    float rhoNew = rhoOld - dtdx * (fluxM_R - fluxM_L);
+                    float rhouNew = rhouOld - dtdx * (fluxP_R - fluxP_L);
+                    float ENew = EOld - dtdx * (fluxE_R - fluxE_L);
+                    float rhoYOld = rhoOld * YOld;
+                    float rhoYNew = rhoYOld - dtdx * (fluxY_R - fluxY_L);
+
+                    if (_applyDamping) rhouNew *= _dampingFactors[i];
+                    if (_applyRelax) ENew = _ambientEnergyRef + (ENew - _ambientEnergyRef) * _relaxFactors[i];
+
+                    rhoNew = MathF.Max(rhoNew, 1e-12f);
+                    float kin = 0.5f * rhouNew * rhouNew / rhoNew;
+                    float eMin = 100f * Gm1Inv + kin;
+                    ENew = MathF.Max(ENew, eMin);
+
+                    _rho[i] = rhoNew;
+                    _rhou[i] = rhouNew;
+                    _E[i] = ENew;
+                    _Y[i] = Math.Clamp(rhoYNew / rhoNew, 0f, 1f);
+                }
+            }
+
+            // Recompute pressure for all cells (for visualization)
+            for (int i = 0; i < _totalCells; i++)
+            {
+                float rho = _rho[i];
+                float rhou = _rhou[i];
+                float u = rhou / MathF.Max(rho, 1e-12f);
+                _p[i] = Gm1 * (_E[i] - 0.5f * rhou * u);
+            }
+        }
+
+        // ---------- Scalar flux helpers ----------
         private static void LaxFlux(float rL, float uL, float pL, float cL,
                                     float rR, float uR, float pR, float cR,
                                     out float fm, out float fp, out float fe)
@@ -528,6 +643,23 @@ namespace FluidSim.Core
             fy = 0.5f * (FyL + FyR) - 0.5f * alpha * (rR * YR - rL * YL);
         }
 
+        public int GetRequiredSubSteps(float dtGlobal, float cflTarget = 0.8f)
+        {
+            float maxW = 0f;
+            for (int i = 0; i < _totalCells; i++)
+            {
+                float rho = MathF.Max(_rho[i], 1e-12f);
+                float u = MathF.Abs(_rhou[i] / rho);
+                float p = Gm1 * (_E[i] - 0.5f * _rhou[i] * _rhou[i] / rho);
+                float c = MathF.Sqrt(Gamma * p / rho);
+                float w = u + c;
+                if (w > maxW) maxW = w;
+            }
+            maxW = MathF.Max(maxW, 1e-8f);
+            float minDx = _dx.Min();  // need using System.Linq;
+            return Math.Max(1, (int)MathF.Ceiling(dtGlobal * maxW / (cflTarget * minDx)));
+        }
+
         // ---------- Profiling report ----------
         public string GetProfileReport()
         {

Core/Solver.cs

@@ -36,7 +36,8 @@ namespace FluidSim.Core
         {
             if (_pipeSystem == null || _boundarySystem == null) return;
 
-            int nSub = SubStepCount;
+            int nSub = _pipeSystem.GetRequiredSubSteps((float)_dt, 0.8f);
+            nSub = Math.Max(nSub, SubStepCount);   // never go below fixed minimum
             float dtSub = (float)(_dt / nSub);
 
             for (int sub = 0; sub < nSub; sub++)

Program.cs

@@ -50,7 +50,7 @@ public class Program
     {
         var window = CreateWindow();
         LoadFont();
-        _scenario = new HelmholtzScenario();
+        _scenario = new SingleCylScenario();
         _scenario.Initialize(SampleRate);
         _lastThrottleUpdateTime = 0.0f;
 

Scenarios/SingleCylScenario.cs

@@ -30,7 +30,7 @@ namespace FluidSim.Tests
 
         private double dt;
         private int stepCount;
-        public float MaxThrottleArea = 1e-4f;   // 1 cm²
+        public float MaxThrottleArea = 100e-4f;   // 1 cm²
 
         // pipe area for open end calculations
         private float pipeArea;
@@ -41,9 +41,9 @@ namespace FluidSim.Tests
 
             // ---- Crankshaft ----
             crankshaft = new Crankshaft(600);
-            crankshaft.Inertia = 0.2f;
+            crankshaft.Inertia = 0.05f;
             crankshaft.FrictionConstant = 2f;
-            crankshaft.FrictionViscous = 0.04f;
+            crankshaft.FrictionViscous = 0.01f;
 
             // ---- Cylinder ----
             float bore = 0.056f, stroke = 0.057f, conRod = 0.110f, compRatio = 9.2f;
@@ -77,12 +77,12 @@ namespace FluidSim.Tests
 
             pipeSystem = new PipeSystem(totalCells, pipeStart, pipeEnd, area, dx,
                                         1.225f, 0f, 101325f);
-            pipeSystem.DampingMultiplier = 0.5f;
+            pipeSystem.DampingMultiplier = 1.0f;
-            pipeSystem.EnergyRelaxationRate = 0f;
+            pipeSystem.EnergyRelaxationRate = 0.5f;
             pipeSystem.AmbientPressure = 101325f;
 
             // ---- Volumes ----
-            intakePlenum = new Volume0D(5e-6f, 101325f, 300f);  // 5 mL
+            intakePlenum = new Volume0D(100e-6f, 101325f, 300f);  // 100 mL
             plenumInlet = intakePlenum.CreatePort();
             plenumOutlet = intakePlenum.CreatePort();
             exhaustCollector = new Volume0D(10e-6f, 101325f, 800f);  // 10 mL (unused but present)
@@ -130,8 +130,8 @@ namespace FluidSim.Tests
             solver.AddComponent(exhaustCollector);
 
             // ---- Sound ----
-            exhaustSound = new SoundProcessor(sampleRate, 1f) { Gain = 1f };
+            exhaustSound = new SoundProcessor(sampleRate, 1f) { Gain = 20f };
-            intakeSound = new SoundProcessor(sampleRate, 1f) { Gain = 1f };
+            intakeSound = new SoundProcessor(sampleRate, 1f) { Gain = 20f };
             reverb = new OutdoorExhaustReverb(sampleRate);
 
             stepCount = 0;
@@ -163,11 +163,31 @@ namespace FluidSim.Tests
             if (stepCount % 1000 == 0)
             {
                 float rpm = crankshaft.AngularVelocity * 60f / (2f * MathF.PI);
-                Console.WriteLine($"Step {stepCount}, RPM={rpm:F0}, CylP={cylinder.Pressure / 1e5f:F2} bar");
+                float crankDeg = crankshaft.CrankAngle;   // public property on Cylinder
-                Console.WriteLine($"intake flow: {intakeFlow:F12}, exhaust flow: {exhaustFlow:F16}");
+                Console.WriteLine($"Step {stepCount}, CA={crankDeg:F1} deg, RPM={rpm:F0}, CylP={cylinder.Pressure / 1e5f:F2} bar");
+                Console.WriteLine($"  intake flow: {intakeFlow:F6}, exhaust flow: {exhaustFlow:F6}");
+
+                // Pipe 0 (intake before throttle)
+                var (r0L, u0L, p0L) = pipeSystem.GetInteriorStateLeft(0);
+                var (r0R, u0R, p0R) = pipeSystem.GetInteriorStateRight(0);
+                Console.WriteLine($"  Pipe0 L: rho={r0L:F4} u={u0L:F3} p={p0L/1e5:F3}bar | R: rho={r0R:F4} u={u0R:F3} p={p0R/1e5:F3}bar");
+
+                // Pipe 1 (runner)
+                var (r1L, u1L, p1L) = pipeSystem.GetInteriorStateLeft(1);
+                var (r1R, u1R, p1R) = pipeSystem.GetInteriorStateRight(1);
+                Console.WriteLine($"  Pipe1 L: rho={r1L:F4} u={u1L:F3} p={p1L/1e5:F3}bar | R: rho={r1R:F4} u={u1R:F3} p={p1R/1e5:F3}bar");
+
+                // Pipe 2 (exhaust)
+                var (r2L, u2L, p2L) = pipeSystem.GetInteriorStateLeft(2);
+                var (r2R, u2R, p2R) = pipeSystem.GetInteriorStateRight(2);
+                Console.WriteLine($"  Pipe2 L: rho={r2L:F4} u={u2L:F3} p={p2L/1e5:F3}bar | R: rho={r2R:F4} u={u2R:F3} p={p2R/1e5:F3}bar");
+
+                // Plenum and cylinder mass
+                Console.WriteLine($"  Plenum P={intakePlenum.Pressure/1e5:F3}bar, mass={intakePlenum.Mass:E4} kg");
+                Console.WriteLine($"  Cyl mass={cylinder.Mass:E4} kg");
             }
 
-            return reverb.Process(intakeDry);
+            return reverb.Process(intakeDry + exhaustDry);
         }
 
         public override void Draw(RenderWindow target)