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Components/Crankshaft.cs

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+// Components/Crankshaft.cs
+using System;
+
+namespace FluidSim.Components
+{
+    public class Crankshaft
+    {
+        public double AngularVelocity { get; set; }   // rad/s
+        public double CrankAngle { get; set; }        // rad, 0 … 4π (four‑stroke cycle)
+        public double PreviousAngle { get; private set; }  // for TDC detection
+
+        public double Inertia { get; set; } = 0.2;
+        public double FrictionConstant { get; set; } = 2.0;      // N·m
+        public double FrictionViscous { get; set; } = 0.005;     // N·m per rad/s
+
+        private double externalTorque;
+
+        /// <param name="initialRPM">Idle speed before any combustion torque is applied.</param>
+        public Crankshaft(double initialRPM = 400.0)
+        {
+            AngularVelocity = initialRPM * 2.0 * Math.PI / 60.0;
+            CrankAngle = 0.0;
+            PreviousAngle = 0.0;
+        }
+
+        public void AddTorque(double torque) => externalTorque += torque;
+
+        public void Step(double dt)
+        {
+            // Save previous angle
+            PreviousAngle = CrankAngle;
+
+            // Friction
+            double friction = FrictionConstant * Math.Sign(AngularVelocity) + FrictionViscous * AngularVelocity;
+            double netTorque = externalTorque - friction;
+            double alpha = netTorque / Inertia;
+            AngularVelocity += alpha * dt;
+
+            if (AngularVelocity < 0) AngularVelocity = 0;  // stall
+
+            CrankAngle += AngularVelocity * dt;
+
+            // Wrap to [0, 4π)
+            if (CrankAngle >= 4.0 * Math.PI)
+                CrankAngle -= 4.0 * Math.PI;
+            else if (CrankAngle < 0)
+                CrankAngle += 4.0 * Math.PI;
+
+            externalTorque = 0.0;
+        }
+    }
+}