Engine Components

Identification of major engine components makes it easier to understand its working principle. Some major engine components are, cylinder block, piston, piston rings, connecting-rod, cylinder head, crankcase, crankshaft etc. The following briefly describes the major engine components and some terms associated with 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-aluminum-alloy container which encloses the bottom of the crank-case 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.

Big-end This refers to the joint between the connecting-rod and the crankshaft big-end journal which provides the relative angular movement between the two components as the engine rotates.

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 centers When the crank arm 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.

4 stroke and 2 stroke engines

A Common List of Advantages and Disadvantages

Advantages of 2 Stroke Engines:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Disadvantages of 2 Stroke Engines:
- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

Note:
Most of what is written on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct.

Take for example the lubrication issue of 2 stroke engines, sure small chainsaw engines may have the oil mixed with the fuel but this is not a direct result of the engine being a 2 stroke, this is just a result of someone designing a very simple engine. look at any large Caterpillar, or Detroit 2 stroke they have conventional oil sumps, oil pumps and full pressure fed lubrication systems and they are 2 stroke!

Also, the argument about valves of 4 strokes versus the reeds and ports of 2 strokes is also incorrect. Sure some simple 2 strokes may use very primative systems to achieve the conrol of fuel/air mixture into the engine and exhaust out of the engine but again this is not a function of them being 2 stroke! I've worked on 2 stroke engines that feature poppet valves in the head (like a standard 4 stroke) - but they are definately 2 stroke - it's just that engines like this are not so much in the public eye - next time an ocean liner (ship) pulls into port check out its 2 stroke, turbo charged, direct injected diesel engine!

Finally, the arguments of simplicity, weight, power to weight, and cost of manufacturing are not a function as such of 2 stroke versus 4 stroke engines. The mistake of most of these commentaries is that they are comparing a simple chainsaw 2 stroke engine with a complex 4 stroke engine from a automobile - not a very fair comparision.

As far as the exhaust emmisions of 2 strokes - check out the Surrich/Orbital 2 stroke design that Mercury outboards are using - this is as clean burning as any 4 stroke.

The ONLY correct comparison of 2 strokes with 4 strokes is that a 2 stroke can (in theory) produce twice the power of a 4 stroke for the same sized engine and the same revs.