Tie Rods of a Diesel Engine

What is a Tie Rod???

A tie Rod is a Hydraulically Tightened Long Stud which joins different components of an Engine.  The hydraulically tightened tie rods are specifically positioned to maintain a static preloading of the engine block to absorb the dynamic loads generated by the impact from the combustion process and moving masses. 

So what exactly is a Tie Rod Holding???

This rod holds the three major engine components i.e. Cylinder block or entablature, “A” frame, and crankcase in compression and transmits the firing load to the bedplate. The tie rods are fitted through the above mentioned components and are hydraulically tightened so that the whole engine can be held in compression. As per the design these tie rods are placed as close as possible to the centerline of the crankshaft in order to minimize the bending moment of the transverse girder.

We all know that above definition is for a Two Stroke Engine, Can we have a tie rod on a Four Stroke Engine???

The answer is Yes.  Surprised?  Let’s go Back to the school.

Remember Underslung Crankshaft?  Underslung Crankshaft’s are supported by means of main bearing caps i.e they are suspended in the engine frame and supported by Main bearing caps.

Always remember a two stroke engine has a Bed Plate which supports the crankshaft but on a four stroke engine with a monoblock design the crankshaft is suspended in air and supported by main bearing caps.

So if a stud is used to support the underslung crankshaft will it be called a Tie rod???

No.  The main function of a tie rod is to reduce the bending stresses and at the same time act as means to transmit all the load on the bearing caps back to the frame. Such engines may also makes use of the side tie bolts which locate the bearing cap, and prevent sideways movement.

A stud does not transmit any firing forces nor prevent bending moment, but simply holds components together.  For Example a cylinder head is bolted to Engine block by means of Studs.

For such four stroke engines where the Crankshaft is underslung, the load on the bearing caps is transferred back to the frame by the use of tie bolts. The engine also makes use of the side tie bolts which locate the bearing cap, and prevent sideways movement.