Question 25 MODE02 - Assistant MODU Engineer

The Correct Answer is C ### 1. Explanation for Option C (Correct) The vacuum measured between the air filter and the turbocharger blower inlet is caused by the resistance (restriction) of the air intake system (primarily the air filter) to the airflow being drawn into the turbocharger compressor. When the engine load increases from 50% to 100% of maximum continuous rated load, the engine requires a significantly greater volume of air for combustion. To meet this demand, the turbocharger must spin faster and draw a higher mass flow rate of air through the intake system. The restriction (drag) caused by the air filter and intake ducts increases exponentially or quadratically with the velocity (and thus the volume flow rate) of the air passing through it. Since the turbocharger is drawing a much larger volume of air per unit time at 100% load compared to 50% load, the pressure drop across the restriction (the air filter) will be significantly larger. A larger pressure drop means the pressure measured immediately after the filter (at the turbocharger inlet) will be lower, resulting in a **deeper (increased) suction vacuum** (a higher negative reading in inches of water column). Therefore, if the vacuum was 12" $\text{H}_2\text{O}$ at 50% load, it will be substantially more than 12" $\text{H}_2\text{O}$ at 100% load. ### 2. Explanation of Why Other Options Are Incorrect **A) A loss of vacuum will occur (now reading inches of water column positive with respect to atmospheric pressure).** This is incorrect. The turbocharger blower is an active suction device drawing air through a restrictive filter. The pressure at its inlet must always be below atmospheric pressure (a vacuum) unless the turbocharger is pushing air backward (which doesn't happen during normal operation). A positive pressure reading indicates discharge, not suction. **B) No change in the depth of vacuum will occur (reading the same inches of water column negative with respect to atmospheric pressure).** This is incorrect. The suction vacuum is directly proportional to the flow rate and the resistance of the intake system. Since increasing the engine load significantly increases the air mass flow rate, the resistance (and thus the pressure drop/vacuum) must increase substantially. **D) The depth of vacuum will decrease (reading less inches of water column negative with respect to atmospheric pressure).** This is incorrect. A decrease in vacuum (less negative pressure) would imply a decrease in the required airflow or a decrease in the resistance of the filter. Since the airflow increases dramatically, the resistance-induced vacuum must also increase, not decrease.