Piston-engine cycles of operation.
The internal-combustion engine
The piston engine is known as an internal-combustion heat-engine. The concept of the piston engine is that a supply of air-and-fuel mixture is fed to the inside of the cylinder where it is compressed and then burnt. This internal combustion releases heat energy which is then converted into useful mechanical work as the high gas pressures generated force the piston to move along its stroke in the cylinder. It can be said, therefore, that a heat-engine is merely an energy transformer. To enable the piston movement to be harnessed, the driving thrust on the piston is transmitted by means of a connecting-rod to a crankshaft whose function is to convert the linear piston motion in the cylinder to a rotary crankshaft movement The piston can thus be made to repeat its movement to and fro, due to the constraints of the crankshaft crankpin’s circular path and the guiding cylinder. The backward-and forward displacement of the piston is generally referred to as the reciprocating motion of the piston, so these power units are also known as reciprocating engines.
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-aluminiumalloy 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 connectingrod. It must therefore also act as a guide for the smallend 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 straightline 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 jointmade 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 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.
The four-stroke-cycle spark-ignition (petrol) engine
The first internal-combustion engine to operate successfully on the four-stroke cycle used gas as a fuel and was built in 1876 by Nicolaus August Otto, a self-taught German engineer at the Gas-motoreufabrik Deutz factory near Cologne, for many years the largest manufacturer of internal-combustion engines in the world. It was one of Otto’s associates – Gottlieb Daimler – who later developed an engine to run on petrol which was described in patent number 4315 of 1885. He also pioneered its application to the motor vehicle.
Petrol engines take in a flammable mixture of air and petrol which is ignited by a timed spark when the charge is compressed. These engines are therefore sometimes called spark-ignition (S.I.) engines. These engines require four piston strokes to complete one cycle: an air-and-fuel intake stroke moving outward from the cylinder head, an inward movement towards the cylinder head compressing the charge, an outward power stroke, and an inward exhaust stroke.
Induction stroke the inlet valve is opened and the exhaust valve is closed. The piston descends, moving away from the cylinder head. The speed of the piston moving along the cylinder creates a pressure reduction or depression which reaches a maximum of about 0.3 bar below atmospheric pressure at one-third from the beginning of the stroke. The depression actually generated will depend on the speed and load experienced by the engine, but a typical average value might be 0.12 bar below atmospheric pressure. This depression induces (sucks in) a fresh charge of air and atomised petrol in proportions ranging from 10 to 17 parts of air to one part of petrol by weight. An engine which induces fresh charge by means of a depression in the cylinder is said to be ‘normally aspirated’ or ‘naturally aspirated’.
Compression stroke. Both the inlet and the exhaust valves are closed. The piston begins to ascend towards the cylinder head. The induced air-and-petrol charge is progressively compressed to something of the order of one-eighth to one tenth of the cylinder’s original volume at the piston’s innermost position. This compression squeezes the air and atomised-petrol molecules closer together and not only increases the charge pressure in the cylinder but also raises the temperature. Typical maximum cylinder compression pressures will range between 8 and 14 bar with the throttle open and the engine running under load.
Power stroke Both the inlet and the exhaust valves are closed and, just before the piston approaches the top of its stroke during compression, a spark-plug ignites the dense combustible charge. By the time the piston reaches the innermost point of its stroke, the charge mixture begins to burn, generates heat, and rapidly raises the pressure in the cylinder until the gas forces exceed the resisting load. The burning gases then expand and so change the piston’s direction of motion and push it to its outermost position. The cylinder pressure then drops from a peak value of about 60 bar under full load down to maybe 4 bar near the outermost movement of the piston.
Exhaust stroke At the end of the power stroke the inlet valve remains closed but the exhaust valve is opened. The piston changes its direction of motion and now moves from the outermost to the innermost position. Most of the burnt gases will be expelled by the existing pressure energy of the gas, but the returning piston will push the last of the spent gases out of the cylinder through the exhaust-valve port and to the atmosphere. During the exhaust stroke, the gas pressure in the cylinder will fall from the exhaust-valve opening pressure (which may vary from 2 to 5 bar, depending on the engine speed and the throttle-opening position) to atmospheric pressure or even less as the piston nears the innermost position towards the cylinder head.
Cycle of events in a four-cylinder engine. Figure illustrates how the cycle of events – induction, compression, power, and exhaust – is phased in a four-cylinder engine. The relationship between cylinder pressure and piston stroke position over the four strokes is clearly and, by following the arrows, it can be seen that a figures of eight is repeatedly being traced.
Valve timing diagrams
In practice, the events of the four-stroke cycle do not start and finish exactly at the two ends of the strokes – to improve the breathing and exhausting, the inlet valve is arranged to open before TDC and to close after BDC and the exhaust valve opens before BDC and closes after TDC. These early and late opening and closing events can be shown on a valve timing diagram such as figure.
Valve lead This is where a valve opens so many degrees of crankshaft rotation before either TDC or BDC.
Valve lag This is where a valve closes so many degrees of crankshaft rotation after TDC or BDC.
Valve overlap This is the condition when both the inlet and the exhaust valves are open at the same time during so many degrees of crankshaft rotation.
The two-stroke-cycle petrol engine.
The first successful design of a three-port two-stroke engine was patented in 1889 by Joseph Day & Son of Bath. This employed the underside of the piston in conjunction with a sealed crank-case to form a scavenge pump (‘scavenging’ being the pushing-out of exhaust gas by the induction of fresh charge). This engine completes the cycle of events – induction, compression, power, and exhaust – in one revolution of the crankshaft or two complete piston strokes.
Crankcase-to-cylinder mixture transfer (a). The piston moves down the cylinder and initially uncovers the exhaust port (E), releasing the burnt exhaust gases to the atmosphere. Simultaneously the downward movement of the underside of the piston compresses the previously filled mixture of air and atomised petrol in the crankcase (a). Further outward movement of the piston will uncover the transfer port (T), and the compressed mixture in the crankcase will then be transferred to the combustion-chamber side of the cylinder. The situation in the cylinder will then be such that the fresh charge entering the cylinder will push out any remaining burnt products of combustion – this process is generally referred to as cross-flow scavenging.
Cylinder compression and crankcase induction (b). The crankshaft rotates, moving the piston in the direction of the cylinder head. Initially the piston seals off the transfer port, and then a short time later the exhaust port will be completely closed. Further inward movement of the piston will compress the mixture of air and atomised petrol to about one-seventh to one-eighth of its original volume (b). At the same time as the fresh charge is being compressed between the combustion chamber and the piston head, the inward movement of the piston increases the total volume in the crank-case so that a depression is created in this space. About half-way up the cylinder stroke, the lower part of the piston skirt will uncover the inlet port (I), and a fresh mixture of air and petrol prepared by the carburettor will be induced into the crankcase chamber (b). Cylinder combustion and crankcase compression (c) Just before the piston reaches the top of its stroke, a spark-plug situated in the centre of the cylinder head will be timed to spark and ignite the dense mixture. The burning rate of the charge will rapidly raise the gas pressure to a maximum of about 50 bar under full load. The burning mixture then expands, forcing the piston back along its stroke with a corresponding reduction in cylinder pressure (c) Considering the condition underneath the piston in the crankcase, with the piston initially at the top of its stroke, fresh mixture will have entered the crankcase through the inlet port. As the piston moves down its stroke, the piston skirt will cover the inlet port, and any further downward movement will compress the mixture in the crankcase in preparation for the next charge transfer into the cylinder and combustion-chamber space(c).The combined cycle of events adapted to a threecylinder engine is shown in (d) & (f) show the complete cycle in terms of opening and closing events and cylinder volume and pressure changes respectively.
Reverse-flow (Schnuerle) scavenging.
To improve scavenging efficiency, a loop-scavenging system which became known as the reverse-flow or (after its inventor, Dr E. Schnuerle) as the Schnuerle scavenging system was developed . This layout has a transfer port on each side of the exhaust port, and these direct the scavenging charge mixture in a practically tangential direction towards the opposite cylinder wall. The two separate columns of the scavenging mixture meet and merge together at this wall to form one inward rising flow which turns under the cylinder head and then flows down on the entry side, thus forming a complete loop. With this form of porting, turbulence and intermixing of fresh fuel mixture with residual burnt gases will be minimal over a wide range of piston speeds.
Note that in this particular design the charge mixture is transferred through ports formed in the piston skirt. Alternatively, extended transfer passages may be preferred so that the piston skirt plays no part in the timed transfer.
Crankcase disc-valve and reed-valve inlet charge control.
An alternative to the piston-operated crankcase inlet port is to use a disc-valve attached to and driven by the crankshaft (a). This disc-valve is timed to open and close so that the fresh charge is induced to enter the crankcase as early as possible, and only at the point when the charge is about to be transferred into the cylinder is it closed. This method of controlling crankcase induction does not depend upon the piston displacement to uncover the port – it can therefore be so phased as to extend the filling period. A further method of improving crankcase filling is the use of reed-valves (b). These valves are not timed to open and close, but operate automatically when the pressure difference between the crankcase and the air intake is sufficient to deflect the reed spring. In other words, these valves sense the requirements of the crankcase and so adjust their opening and closing frequencies to match the demands of the engine.
Comparison of two- and four stroke-cycle petrol engines.
The following remarks compare the main points regarding the effectiveness of both engine cycles.
a) The two-stroke engine completes one cycle of events for every revolution of the crankshaft, compared with the two revolutions required for the four-stroke engine cycle.
b) Theoretically, the two-stroke engine should develop twice the power compared to a four-stroke engine of the same cylinder capacity.
c) In practice, the two-stroke engine’s expelling of the exhaust gases and filling of the cylinder with fresh mixture brought in through the crankcase is far less effective than having separate exhaust and induction strokes. Thus the mean effective cylinder pressures in two-stroke units are far lower than in equivalent four-stroke engines.
d) With a power stroke every revolution instead of every second revolution, the two-stroke engine will run smoother than the four-stroke power unit for the same size of flywheel.
e) Unlike the four-stroke engine, the two-stroke engine does not have the luxury of separate exhaust and induction strokes to cool both the cylinder and the piston between power strokes. There is therefore a tendency for the piston and small-end to overheat under heavy driving conditions.
f) Due to its inferior scavenging process, the two stroke engine can suffer from the following:
i) inadequate transfer of fresh mixture into the cylinder,
ii) excessively large amounts of residual exhaust gas remaining in the cylinder,
iii) direct expulsion of fresh charge through the exhaust port.
These undesirable conditions may occur under different speed and load situations, which greatly influences both power and fuel consumption.
g) Far less maintenance is expected with the two stroke engine compared with the four-stroke engine, but there can be a problem with the products of combustion car burising at the inlet, transfer, and exhaust ports.
h) Lubrication of the two-stroke engine is achieved by mixing small quantities of oil with petrol in proportions anywhere between 1:16 and 1:24 so that, when crankcase induction takes place, the various rotating and reciprocating components will be lubricated by a petroil-mixture mist. Clearly a continuous proportion of oil will be burnt in the cylinder and expelled into the atmosphere to add to unwanted exhaust emission.
i) There are fewer working parts in a two-stroke engine than in a four-stroke engine, so two-stroke engines are generally cheaper to manufacture.
