Question

Which is the correct sequence of the decreasing order of brake thermal efficiency of the three given basic types of engines?

(a) four stroke C.I. engine, four stroke S.I. engine, two stroke S.I. engine

(b) four stroke S.I. engine, four stroke C.I. engine, two stroke S.I. engine

(c) four stroke C.I. engine, two stroke S.I. engine, four stroke S.I. engine

(d) two stroke S.I. engine, four stroke S.I. engine, four stroke C.I. engine

The question was posed to me in an international level competition.

Origin of the question is Engine Efficiencies in portion Measurements and Testing/Performance Parameters and Characteristics of IC Engine

Answer 1

The correct answer is:

(c) four stroke C.I. engine, two stroke S.I. engine, four stroke S.I. engine

This sequence is based on the general efficiency characteristics of these engine types:

- Four-stroke C.I. (Compression Ignition) engines typically have the highest brake thermal efficiency due to their higher compression ratio and better fuel efficiency, especially when compared to spark-ignition engines.

  

- Two-stroke S.I. (Spark Ignition) engines generally have lower brake thermal efficiency because of incomplete combustion and greater energy loss due to the simpler design and shorter cycle.

- Four-stroke S.I. engines have the lowest thermal efficiency among the three because of their lower compression ratio compared to C.I. engines, which limits their ability to convert fuel energy into work efficiently.