FluidSim/Components/TwoStrokeCylinder.cs

// ============================================================
// File: TwoStrokeCylinder.cs
// ============================================================
using System;
using FluidSim.Interfaces;
using FluidSim.Components;   // for Crankcase (if in same namespace)

namespace FluidSim.Components
{
    /// <summary>
    /// Two‑stroke cylinder with forced symmetrical port timings around BDC (180°).
    /// Uses crankcase back‑pressure for accurate pumping work.
    /// </summary>
    public class TwoStrokeCylinder : EngineCylinder
    {
        // --- Port timing (computed from durations) ---
        public float IVO => 180f - transferDuration / 2f;
        public float IVC => 180f + transferDuration / 2f;
        public float EVO => 180f - exhaustDuration / 2f;
        public float EVC => 180f + exhaustDuration / 2f;

        private readonly float transferDuration;   // degrees
        private readonly float exhaustDuration;    // degrees

        // --- Crankcase reference ---
        private Crankcase? _crankcase;

        protected override float CycleLengthRad => 2f * MathF.PI;
        protected override float MaxCycleDeg => 360f;

        public override float IntakeValveArea =>
            MathF.PI * IntakeValveDiameter * ValveLift(CrankDeg, IVO, IVC, IntakeValveLift);
        public override float ExhaustValveArea =>
            MathF.PI * ExhaustValveDiameter * ValveLift(CrankDeg, EVO, EVC, ExhaustValveLift);

        public TwoStrokeCylinder(float bore, float stroke, float conRodLength,
                                 float compressionRatio,
                                 float transferDuration, float exhaustDuration,
                                 Crankshaft crankshaft)
            : base(bore, stroke, conRodLength, compressionRatio, crankshaft)
        {
            this.transferDuration = transferDuration;
            this.exhaustDuration  = exhaustDuration;

            if (EVO >= IVO)
                throw new ArgumentException("Exhaust must open before transfer port.");
        }

        public void SetCrankcase(Crankcase crankcase)
        {
            _crankcase = crankcase;
        }

        // ----- Valve lift -----
        private float ValveLift(float thetaDeg, float opens, float closes, float peakLift)
        {
            float deg = thetaDeg % 360f;
            if (deg < 0f) deg += 360f;

            float effectiveOpen = opens;
            float effectiveClose = closes;
            if (closes < opens) effectiveClose += 360f;
            float duration = effectiveClose - effectiveOpen;
            if (duration <= 0f) return 0f;

            float mapped = deg;
            if (mapped < opens) mapped += 360f;
            if (mapped < opens || mapped > effectiveClose) return 0f;

            float rampDur = duration * 0.25f;
            float holdDur = duration - 2f * rampDur;

            if (mapped >= opens && mapped < opens + rampDur)
            {
                float t = (mapped - opens) / rampDur;
                return peakLift * t * t * (3f - 2f * t);
            }
            else if (mapped >= opens + rampDur && mapped < opens + rampDur + holdDur)
            {
                return peakLift;
            }
            else if (mapped >= opens + rampDur + holdDur && mapped <= effectiveClose)
            {
                float t = (mapped - (opens + rampDur + holdDur)) / rampDur;
                return peakLift * (1f - t) * (1f - t) * (1f + 2f * t);
            }
            return 0f;
        }

        protected override void HandleCycleEvents(float prevDeg, float currDeg, float dt)
        {
            // Transfer port closing → fuel injection
            if (prevDeg >= IVO && prevDeg < IVC && currDeg >= IVC)
            {
                trappedAirMass = _airMass;
                fuelMass = trappedAirMass / StoichiometricAFR;
                fuelInjected = true;
            }

            // Spark every 360° at TDC (0°) minus advance
            float sparkAngle = (0f - SparkAdvance + 360f) % 360f;
            bool crossedSpark = false;
            if (prevDeg < sparkAngle && currDeg >= sparkAngle)
                crossedSpark = true;
            else if (prevDeg > sparkAngle && currDeg < sparkAngle)
                crossedSpark = true;

            if (crossedSpark && !combustionActive && fuelInjected)
            {
                if (_random.NextDouble() < MisfireProbability)
                {
                    combustionActive = false;
                }
                else
                {
                    combustionActive = true; burnFraction = 0f;
                    float range = EnergyVariationFraction;
                    _energyFactor = 1f + range * (2f * (float)_random.NextDouble() - 1f);
                }
            }

            if (combustionActive)
            {
                float angleSinceSpark = currDeg - sparkAngle;
                if (angleSinceSpark < 0f) angleSinceSpark += 360f;
                float newFraction = Wiebe(angleSinceSpark);
                if (newFraction >= 1f || angleSinceSpark > (WiebeDuration + WiebeStart + SparkAdvance))
                {
                    newFraction = 1f; combustionActive = false;
                    float totalMass = _airMass + _exhaustMass;
                    _airMass = 0f; _exhaustMass = totalMass;
                }
                fuelInjected = false;

                float dFraction = newFraction - burnFraction;
                if (dFraction > 0f)
                {
                    float dQ = fuelMass * FuelLowerHeatingValue * _energyFactor * dFraction;
                    cylinderEnergy += dQ;
                    _exhaustMass += fuelMass * dFraction;
                    burnFraction = newFraction;
                }
            }
        }

        // ----- Override torque calculation to use crankcase back‑pressure -----
        public new void PreStep(float dt)
        {
            // Speed‑dependent spark advance
            float rpm = Crankshaft.AngularVelocity * 60f / (2f * MathF.PI);
            SparkAdvance = Math.Clamp(10f + rpm * 0.002f, 5f, 40f);

            float prevVolume = cylinderVolume;
            float crankAngleRad = Crankshaft.CrankAngle + PhaseOffset;
            cylinderVolume = ComputeVolume(crankAngleRad);

            float dV = cylinderVolume - prevVolume;

            // Use crankcase pressure as back‑pressure, ambient if not set
            float backPressure = _crankcase?.Pressure ?? 101325f;
            float pRel = Pressure - backPressure;

            float sinTh = MathF.Sin(crankAngleRad), cosTh = MathF.Cos(crankAngleRad);
            float term = MathF.Sqrt(1f - Obliquity * Obliquity * sinTh * sinTh);
            float dxdtheta = CrankRadius * sinTh * (1f + Obliquity * cosTh / term);
            float pistonArea = MathF.PI * 0.25f * Bore * Bore;
            Crankshaft.AddTorque(pRel * pistonArea * dxdtheta);

            cylinderEnergy -= Pressure * dV;

            float cycleLenDeg = 360f;
            float prevDeg = (Crankshaft.PreviousAngle + PhaseOffset) * 180f / MathF.PI % cycleLenDeg;
            float currDeg = crankAngleRad * 180f / MathF.PI % cycleLenDeg;

            HandleCycleEvents(prevDeg, currDeg, dt);

            // Heat loss
            float dQ_loss = HeatTransferCoefficient * CylinderWallArea *
                            (Temperature - AmbientTemperature) * dt;
            cylinderEnergy -= dQ_loss;

            // Update port states
            float p = Pressure, rho = Density, T = Temperature;
            float h = Gamma / (Gamma - 1f) * p / MathF.Max(rho, 1e-12f);
            float af = AirFraction;
            IntakePort.Pressure = p; IntakePort.Density = rho;
            IntakePort.Temperature = T; IntakePort.SpecificEnthalpy = h; IntakePort.AirFraction = af;
            ExhaustPort.Pressure = p; ExhaustPort.Density = rho;
            ExhaustPort.Temperature = T; ExhaustPort.SpecificEnthalpy = h; ExhaustPort.AirFraction = af;
        }
    }
}