Engine components and terms

The main problem in understanding the construction of the reciprocating piston engine is being able to identify and name the various parts making up the power unit. To this end, the following briefly describes the major components and the names given to them.

Cylinder block

This is a cast structure with cylindrical holes bored to guide and support the pistons and to harness the working gases. It also provides a jacket to contain a liquid coolant.

Cylinder head This casting encloses the combustion end of the cylinder block and houses both the inlet and exhaust poppet-valves and their ports to admit air– fuel mixture and to exhaust the combustion products. Crankcase This is a cast rigid structure which supports and houses the crankshaft and bearings. It is usually cast as a mono-construction with the cylinder block. Sump This is a pressed-steel or cast-aluminium alloy container which encloses the bottom of the crankcase and provides a reservoir for the engine’s lubricant.

Piston

This is a pressure-tight cylindrical plunger which is subjected to the expanding gas pressure. Its function is to convert the gas pressure from combustion into a concentrated driving thrust along the connecting rod. It must therefore also act as a guide for the small end of the connecting-rod.

Piston rings

These are circular rings which seal the gaps made between the piston and the cylinder, their object being to prevent gas escaping and to control the amount of lubricant which is allowed to reach the top of the cylinder.

Gudgeon-pin

This pin transfers the thrust from the piston to the connecting-rod small-end while permitting the rod to rock to and fro as the crankshaft rotates.

Connecting-rod

This acts as both a strut and a tie link-rod. It transmits the linear pressure impulses acting on the piston to the crankshaft big-end journal, where they are converted into turning-effort.

Crankshaft

A simple crankshaft consists of a circular-sectioned shaft which is bent or cranked to form two perpendicular crank-arms and an offset big-end journal. The unbent part of the shaft provides the main journals. The crankshaft is indirectly linked by the connecting-rod to the piston – this enables the straight-line motion of the piston to be transformed into a rotary motion at the crankshaft about the main-journal axis.

Crankshaft journals

These are highly finished cylindrical pins machined parallel on both the centre axes and the offset axes of the crankshaft. When assembled, these journals rotate in plain bush-type bearings mounted in the crankcase (the main journals) and in one end of the connecting-rod (the big-end journal).

Small-end

This refers to the hinged joint made by the gudgeon-pin between the piston and the connecting-rod so that the connecting-rod is free to oscillate relative to the cylinder axis as it moves to and fro in the cylinder.

Main-ends

This refers to the rubbing pairs formed between the crankshaft main journals and their respective plain bearings mounted in the crankcase.

Line of stroke

The centre path the piston is forced to follow due to the constraints of the cylinder is known as the line of stroke.

Inner and outer dead centres

When the crankarm and the connecting-rod are aligned along the line of stroke, the piston will be in either one of its two extreme positions. If the piston is at its closest position to the cylinder head, the crank and piston are said to be at inner dead centre (IDC) or top dead centre (TDC). With the piston at its furthest position from the cylinder head, the crank and piston are said to be at outer dead centre (ODC) or bottom dead centre (BDC). These reference points are of considerable importance for valve-to-crankshaft timing and for either ignition or injection settings.

Clearance volume

The space between the cylinder head and the piston crown at TDC is known as the clearance volume or the combustion-chamber space.

Crank-throw

The distance from the centre of the crankshaft main journal to the centre of the big-end journal is known as the crank-throw. This radial length influences the leverage the gas pressure acting on the piston can apply in rotating the crankshaft.

Piston stroke

The piston movement from IDC to ODC is known as the piston stroke and corresponds to the crankshaft rotating half a revolution or 180. It is also equal to twice the crank-throw.

i.e. L = 2R

where

L = piston stroke and

R = crank-throw

Thus a long or short stroke will enable a large or small turning-effort to be applied to the crankshaft respectively.

Cylinder bore

The cylinder block is initially cast with sand cores occupying the cylinder spaces. After the sand cores have been removed, the rough holes are machined with a single-point cutting tool attached radially at the end of a rotating bar. The removal of the unwanted metal in the hole is commonly known as boring the cylinder to size. Thus the finished cylindrical hole is known as the cylinder bore, and its internal diameter simply as the bore or bore size.