Question

The effectiveness of the turbocharger of a diesel engine increases when

• A. the ambient temperature increases
• B. the pressure ratio across the compressor decreases
• C. the load on the engine increases
• D. the displacement volume of the engine decreases

Hint:

Turbo "lives on" exhaust energy. More load \(\Rightarrow\) more exhaust enthalpy \(\Rightarrow\) more boost. High ambient temperature or low pressure ratio works against it.

Answer 1

The Correct Option is C

Step 1: Turbocharger basics.
A turbocharger extracts exhaust enthalpy to drive a turbine, which drives a compressor to raise intake air density. Effectiveness (boost) grows with turbine power, which depends on exhaust mass flow \(\dot m_e\) and temperature/pressure: \[ P_{\text{turbine}} \propto \dot m_e \, \Delta h_e. \]

Step 2: Link to engine load.
When load increases \(\Rightarrow\) more fuel injected \(\Rightarrow\) \(\dot m_e\) and exhaust temperature rise \(\Rightarrow\) turbine speed increases \(\Rightarrow\) compressor pressure ratio and air mass flow \(\uparrow\). Thus turbocharging becomes more effective at higher load.

Step 3: Eliminate distractors.
(A) \(\uparrow\) ambient \(T\) \(\Rightarrow\) lower air density at compressor inlet, reducing delivered oxygen per cycle; effectiveness \(\downarrow\). ✗
(B) \(\downarrow\) compressor pressure ratio means less boost (by definition), so effectiveness \(\downarrow\). ✗
(D) Smaller displacement yields less exhaust energy at a given speed/load fraction, weakening the turbo drive; effectiveness \(\downarrow\). ✗

Final Answer:
\[ \boxed{\text{C — the load on the engine increases}} \]