Primary and Secondary Forces and Couples in an Inline Four-Cylinder Four-Stroke Engine

In an inline four-cylinder four-stroke engine with inner cranks (throws) at 180 degrees to the outer throws, the correct statements regarding primary and secondary force and couple balancing are as follows:

1. Primary Force Balanced:
The primary forces in a four-cylinder engine are generated due to the reciprocating motion of the pistons. These forces can cause vibrations and undesirable effects. To counterbalance these forces, the engine is designed in such a way that the primary forces from each cylinder cancel each other out. This is achieved by arranging the pistons and connecting rods in a specific order and with specific crankshaft throws. Therefore, the statement that the engine is primary force balanced is correct.

2. Primary Couple Balanced:
The primary couples are the torques generated by the reciprocating motion of the pistons. Similar to the primary forces, these couples can also cause vibrations and unbalanced effects. In a properly designed engine, the primary couples from each cylinder should also cancel each other out. This is achieved by arranging the crankshaft throws in a specific manner. Hence, the statement that the engine is primary couple balanced is correct.

3. Secondary Force Balanced:
The secondary forces are generated due to the rotational motion of the crankshaft. These forces can also cause vibrations and imbalance. In an engine with inner cranks at 180 degrees to the outer throws, the secondary forces are not inherently balanced. Therefore, the statement that the engine is secondary force balanced is incorrect.

4. Secondary Couple Balanced:
The secondary couples are the torques generated by the rotational motion of the crankshaft. In an inline four-cylinder engine with inner cranks at 180 degrees to the outer throws, the secondary couples are inherently balanced. This is because the arrangement of the crankshaft throws ensures that the torques from each cylinder cancel each other out. Therefore, the statement that the engine is secondary couple balanced is correct.

To summarize, the correct statement among the given options is (C) Secondary force balanced. The engine is primary force balanced, primary couple balanced, and secondary couple balanced, but not secondary force balanced.