What is Bottom Dead Center (BDC)?\n\nBottom Dead Center (BDC), referred to in Turkish driver licensing materials as Alt Ölü Nokta or simply A.Ö.N., is the lowest physical limit of a piston's vertical path inside an engine cylinder. When the piston moves downwards, it reaches a brief, instantaneous pause at this lowest boundary before reversing its direction to go back upwards. At this exact position, the piston's speed is temporarily zero, which is why it is called a dead center.\n\nUnderstanding BDC, alongside its counterpart Top Dead Center (TDC / Üst Ölü Nokta or Ü.Ö.N.), is essential for studying the internal combustion engine. These mechanical reference points define how the engine transitions through different phases of fuel combustion, compression, intake, and exhaust.\n\n## The Role of BDC in the Four-Stroke Engine Cycle\n\nMost passenger cars operate on a four-stroke engine cycle, where pistons move between BDC and TDC to complete four specific strokes:\n\n1. Intake Stroke: The piston moves downward from TDC to BDC. This downward movement creates a vacuum that draws the fresh air-fuel mixture into the cylinder through the open intake valve. The intake stroke ends exactly when the piston reaches BDC.\n2. Compression Stroke: The piston moves upward from BDC to TDC, compressing the trapped air-fuel mixture to make it highly combustible.\n3. Power Stroke: Following the spark plug ignition, the expanding gases force the piston downward back to BDC, turning the crankshaft and generating drive power.\n4. Exhaust Stroke: Finally, the piston travels upward from BDC to TDC again, pushing the burned exhaust gases out of the open exhaust valve.\n\n## Engine Displacement and Stroke Volume\n\nIn mechanical engineering and Turkish licensing questions, BDC is used directly to define engine displacement (motor hacmi). The volume of space inside a cylinder between TDC and BDC is known as the stroke volume (strok hacmi). \n\nWhen a vehicle's engine size is advertised (for example, a 1.6-liter or 1600cc engine), this number represents the combined stroke volume of all the engine's cylinders. The formula is determined by multiplying the cylinder's cross-sectional area by the distance between the top and bottom dead centers (the stroke) and multiplying that by the total number of cylinders.\n\n## Exam Tips for the Turkish Ehliyet Theory Test\n\nDuring the vehicle technique (araç tekniği) section of the Turkish ehliyet e-sınav, questions often test your fundamental knowledge of how engines function. You may be asked about the abbreviations A.Ö.N. and Ü.Ö.N. or how engine volume is calculated. Remember that engine displacement only measures the volume swept between these two dead centers, not the entire physical volume of the cylinder, as there is always a small combustion chamber left at the very top.