Engine Performance & Tuning Flashcards

What is 'boost pressure' in a turbocharged engine?

Answer: The positive pressure above atmospheric that the turbocharger adds to the intake charge

Boost pressure is the intake manifold pressure above atmospheric (measured in PSI or bar) created by the turbocharger compressor forcing more air into the engine than it could draw naturally.

A wastegate on a turbocharged engine serves to:

Answer: Bypass exhaust gases around the turbine wheel to regulate and limit boost pressure

The wastegate is a valve that diverts exhaust gas away from the turbo turbine wheel once target boost is reached, preventing overboosting and engine damage.

Why is an intercooler used in forced induction engines?

Answer: To cool compressed intake air, increasing its density and reducing knock risk

Compressing air raises its temperature and reduces density; the intercooler cools the compressed charge, resulting in denser air, more oxygen per cylinder fill, and lower knock tendency.

Turbo lag refers to:

Answer: The delay between throttle input and boost response due to turbocharger inertia

Turbo lag occurs because the turbine wheel needs exhaust gas energy to spool up to operating speed; smaller turbos reduce lag but may limit peak power.

How does a supercharger differ from a turbocharger?

Answer: A supercharger is driven mechanically by the engine; a turbocharger is driven by exhaust gases

Superchargers are belt-, chain-, or gear-driven directly by the crankshaft, providing instant boost with no lag, while turbochargers harvest energy from exhaust flow.

A blow-off valve (BOV) in a turbocharged engine is designed to:

Answer: Vent excess boost pressure into the atmosphere or back to the intake when the throttle closes

When the throttle closes suddenly, the turbo is still spinning; the BOV vents the resulting pressure spike to prevent compressor surge, which can damage the turbine and bearings.

Compression ratio in an engine is defined as:

Answer: The ratio of total cylinder volume at BDC to combustion chamber volume at TDC

Compression ratio compares the maximum cylinder volume (piston at BDC) to the minimum volume (piston at TDC); higher ratios increase thermal efficiency but require higher octane fuel to prevent knock.