Engine Efficiencies – Define Mechanical Efficiency What do we call the ratio of brake power (delivered at the shaft) to indicated power (developed in the cylinder) for a reciprocating engine?

Introduction / Context:
An engine’s performance is commonly broken down into indicated power (in-cylinder) and brake power (usable at the shaft). The ratio between them quantifies mechanical losses in bearings, valve gear, piston friction, and auxiliary drives. This ratio is fundamental when diagnosing shortfalls between combustion performance and delivered output.

Given Data / Assumptions:

• B.P. = brake power measured at the shaft (e.g., dynamometer).
• I.P. = indicated power computed from indicator diagrams or cylinder pressure data.
• Frictional power F.P. = I.P. − B.P.

Concept / Approach:

Mechanical efficiency measures how effectively mechanical components transmit the power produced by combustion to the crankshaft. It is defined as eta_m = B.P. / I.P. Values less than 1 highlight losses due to mechanical friction and pumping. Improving lubrication, reducing accessory loads, or optimizing mechanical design can raise eta_m.

Step-by-Step Solution:

Verification / Alternative check:

Experimental motoring tests (no fuel, external drive) determine F.P., confirming the relationship and enabling calculation of eta_m from measured B.P. and inferred I.P.

Why Other Options Are Wrong:

Overall efficiency includes fuel energy and is eta_o = B.P./(mf * LCV). Indicated thermal efficiency relates I.P. to fuel energy. Volumetric efficiency concerns air-handling capability. 'Relative efficiency' usually compares thermal efficiency to an ideal cycle.

Common Pitfalls:

Confusing mechanical with overall efficiency; forgetting that F.P. rises sharply with speed because friction is largely speed-dependent.

Final Answer:

mechanical efficiency