Question 69 QMED03 - Oiler

The Correct Answer is A. ### Why Option A is Correct A naturally aspirated diesel engine (meaning it uses no turbocharger or supercharger) draws air into the cylinders solely by the vacuum created as the pistons move down the intake stroke. To draw air through the intake filter, intake piping, and past the intake valve restriction, the pressure inside the intake manifold must be lower than the ambient atmospheric pressure outside the engine. This pressure difference (or vacuum) is usually small because the intake system is designed to minimize resistance, especially since the throttle is fixed at "full throttle" (wide open). Therefore, the intake manifold pressure will be **slightly less than atmospheric pressure** (a condition often referred to as a small pressure drop or vacuum). ### Why Other Options Are Incorrect **Option B) approximately equal to exhaust manifold pressure at all times** This is incorrect. Exhaust manifold pressure is typically significantly higher than intake manifold pressure, especially during the exhaust stroke when the hot, high-pressure gases are being forced out of the cylinder. Intake pressure is slightly below atmospheric, while exhaust pressure is usually slightly above atmospheric (or higher, depending on exhaust back pressure). **Option C) that is widely fluctuating** While there are minor pressure pulsations in the intake manifold due to the discontinuous nature of the air intake process (valves opening and closing), a naturally aspirated diesel engine at a steady state (full throttle) does not experience widely fluctuating intake manifold pressure. Furthermore, unlike a spark-ignited (gasoline) engine, a diesel engine uses the throttle primarily to control engine speed (RPM) via the fuel rack, not to restrict intake airflow, making the manifold pressure relatively stable at full load. **Option D) constantly decreasing as engine load increases** This is incorrect. If the engine is running at a constant speed and the load is increased, the required airflow will also increase slightly, potentially causing the small pressure drop (vacuum) to increase slightly (i.e., the pressure would decrease slightly, becoming even *more* below atmospheric). However, the term "constantly decreasing" implies a continuous, time-dependent drop, which is not true for a steady-state operation. At full throttle (wide open throttle), the intake resistance is already at its minimum, so the pressure will remain relatively stable, slightly below atmospheric, regardless of moderate load changes at that fixed RPM.