What is the engine starting process?

[Rylee]

When the key is turned to the ON position, the ECU powers on. Once the ECU is powered, it supplies electricity to the corresponding sensors through various pins and retrieves sensor information to control the engine's operation. At the same time, the fuel pump in the tank powers on and operates for 3-5 seconds to provide sufficient pressure to the fuel supply system. Many people often say to wait for the self-check before starting, but this is actually unnecessary. However, some cars may have issues with the fuel pump pressure relief valve or fuel injector leakage, which can cause a drop in fuel system pressure after prolonged parking. If started directly, the fuel pump may not have enough time to build sufficient pressure, leading to an extended starting time. When the key is turned to the START position, the solenoid coil on the starter is energized and engages. Through lever action, the starter drive gear is pushed outward to mesh with the flywheel ring gear. While the drive gear is pushed out, the other end of the lever closes the circuit contact of the starter motor, allowing the starter motor to power on and begin rotating, driving the flywheel. The starter drives the flywheel to rotate, which in turn drives the crankshaft. The crankshaft moves the pistons back and forth via connecting rods, while also driving the timing gear through a timing belt or chain to control the intake and exhaust valves inside the cylinder head via the camshaft. Simultaneously, the crankshaft position sensor continuously sends crankshaft angle data to the ECU, which determines which cylinder should receive fuel injection and ignition. For a specific cylinder, when the piston moves downward, the camshaft opens the intake valve and closes the exhaust valve. The ECU, based on the crankshaft position sensor signal, decides to supply fuel, so the fuel injector solenoid opens, spraying atomized gasoline toward the back of the intake valve at the end of the intake manifold under fuel system pressure. The strong intake airflow carries the gasoline into the cylinder. When the piston reaches bottom dead center, it begins the compression stroke. At this point, both intake and exhaust valves are closed, and the piston compresses the air-fuel mixture in the cylinder. The airflow inside the cylinder continues to move, further atomizing and evaporating the gasoline. When the piston nears top dead center, the ECU detects the ignition timing via the crankshaft position sensor signal, energizing the ignition coil to produce a spark plug arc, igniting the mixture for combustion. During cold starts, due to low temperatures and poor gasoline evaporation, the ECU increases fuel injection to ensure enough gasoline evaporates for a sufficiently concentrated mixture, guaranteeing a successful start. This results in some unburned gasoline, leading to a noticeable gasoline smell from the exhaust during cold starts—a normal phenomenon that disappears as fuel injection decreases after startup. Once each cylinder can achieve stable ignition, the engine generates enough torque to sustain its own operation. At this point, releasing the key returns the ignition switch to the ON position, cutting power to the starter. If the key is released too late, the flywheel may back-drive the starter, producing an unpleasant noise. After a successful cold start, the engine speed will be much higher than idle due to the fast warm-up function. At this stage, the engine is fully started.