The piston engine is known as an internal-combustion heat-engine. Internal combustion engines can be described as any group of devices in which the reactants of combustion (oxidiser and fuel) and the products of combustion serve as the working fluids of the 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..
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 movement in the cylinder to a rotary crankshaft movement (see fig 1). 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 cylinder capacity is defined as the total cylinder swept volume. Thus the engine cylinder capacity is equal to the piston displacement of each cylinder times the number of cylinders,
where = engine cylinder capacity, V = piston displacement
n=number of cylinders.
The piston engine is known as an internal-combustion heat-engine. Internal combustion engines can be described as any group of devices in which the reactants of combustion (oxidiser and fuel) and the products of combustion serve as the working fluids of the 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.
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 movement in the cylinder to a rotary crankshaft movement (see fig 1). 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 cylinder capacity is defined as the total cylinder swept volume. Thus the engine cylinder capacity is equal to the piston displacement of each cylinder times the number of cylinders,
where = engine cylinder capacity, V = piston displacement
n=number of cylinders.
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 crank-shaft 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 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 inner to outer dead centre 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 crank-shaft 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.
The four-stroke-cycle spark-ignition (petrol) engine
Petrol engines take an inflammable 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 induces (sucks in) a fresh charge of air and atomised petrol in proportions ranging ranging from 10 to 17 parts of air to one part of petrol by weight.
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 1/8 to 1/10 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.
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 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.
Swept volume: is the volume displaced when the piston moves from TDC to BDC i.e.
Mean Effective Pressure: is the average pressure inside the cylinder throughout the whole power stroke.
Engine torque: is the turning effort about the crankshaft’s axis of rotation and is equal to the product of the force acting along the connecting-rod and the perpendicular distance between this force and the centre of rotation of the crankshaft, i.e.
Engine power: is the rate of doing work and may be calculated on the basis of ‘indicated power’ (i.p), that is the power actually developed in the cylinder, or on the basis of brake power measured at the crankshaft. The brake power is always less than the indicated power, due to frictional and pumping losses in the cylinder, i.e.
Engine cylinder capacity: is the comparative size of the total cylinder swept volume and is equal to the piston displacement of each cylinder times the displacement. i.e.
The cross-sectional area of the piston crown influences the force acting on the connecting rod, since the product of the piston area and the mean effective cylinder pressure is equal to the total piston thrust, i.e.
The length of the piston stroke influences both the turning-effort and the angular speed of the crankshaft. This is because the crank-throw length determines the leverage on the crankshaft, and the piston speed divided by twice the stroke is equal to the crankshaft speed, i.e.
Compression ratio: is the ration of the maximum cylinder volume when the piston is at its outermost position to when the piston is at its innermost position, i.e.
Heat engines are generally classified by the thermal cycles involved in the energy conversion process, difference between them being the manner in which the heat is supplied to and abstracted from the working fluid at the beginning and the end of the cycle. All thermal cycles involve the four stages of air-compression to raise temperature and pressure/volume, expansion of the air to do work and reduce temperature/pressure, and finally, rejection of any surplus heat in returning the air to its condition at the start of the cycle.
Fig 7, due to Polak, shows four common thermal cycles which, from the left, are Otto or constant-volume, constant-pressure, Diesel and mixed. In a four-stroke, of course, two revolutions of the engine are involved per cycle and a typical pressure/time plot is seen in Fig 8 (left view)while the right view shows the same cycle on a base of piston travel to give the familiar indicator diagram, or work graph, relating pressure and volume. The net work done is the difference between the areas of the top and bottom of the loops, the bottom one representing the pumping work in filling the engine with air, which can be effectively removed by supercharging. In a typical two-stroke the pumping work is performed by the underside of the piston in most cases to develop crankcase pressure for scavenging.
Actual indicator diagrams differ from the theoretical ideals and the sharp corners become curved. In the plot for a slow-speed laboratory engine of Fig 9, the actual diagram (full-line) is compared with the ideal (chain-line). By deriving the plot to logarithmic scales and exponents of the compression and expansion coefficients can be readily measured. Rather than the idealised adiabatic expansion, for which r = 1.38 in this example, reflecting both the effects of heat loss and the fact that air is not a perfect gas, in thermodynamic terms.
Heating of the initial air/fuel charge by residual products of combustion can also be noted. The higher the compression ratio the smaller the volume of such exhaust products thus remain and any other means of scavenging these exhaust products improves engine performance
The mean height of the indicator diagram is the area divide by its width and corresponds to the mean effective pressure P of the engine, throughout the combustion cycle, and its use obviates the need to refer to the indicator diagram in subsequent calculations. Indicated power can then be expressed as PLAN, where A is piston area, L is stroke and N speed; brake mean effective pressure is obtained by factoring this by mechanical efficiency
During the Otto cycle, Fig 10, if the temperatures at a,b,c and d are
the efficiency can be shown to equal:
At an early stage in engine design for a particular vehicle the required shape of the torque/speed characteristic must be established, governed by such factors as valve timing, port and valve sizes, inlet and exhaust manifolding and fuel system. Fig 11 shows the relative specific performances of different engine designs based on pioneering work on detonation by Ricardo Plc. The curve indicates that the designer has a choice of 2:1 range of maximum torque-bmep available and 4:1 of maximum powers.
Direct loading on the piston due to gas pressure can be calculated from the indicator diagram, preferably using logarithmic scales. Vertical load on the connecting rod, gas pressure times piston area, can then be tabulated for a full combustion cycle on a crank case base. The gas loading is modified by the inertia loading of the reciprocating parts. Inertia reduces maximum compressive stress during combustion but causes tensile stress at the end of the exhaust stroke. Inertia force can be approximated as:
As well as being the algebraic sum of gas and inertia loads on the piston the conn-rod end load
Is also increased by a component of force due to its angularity, Fig 12, which also creates a side thrust by the piston on the cylinder wall. Force Q is a maximum when is 90° and in general equals:
Due to the partial rotary motion of the conn-rod, it is also subject to considerable inertial bending force at high speed, approximated as:
The combined effect of these forces is to produce a pattern of fluctuating stresses as indicated by Fig 13 for a slow speed laboratory engine. Their importance is also seen in complex balancing procedures required to restrict the motion of the engine on its mountings and improve vehicle refinement.
Because the piston engine is such a highly developed mechanism it is always worthwhile referring to the history of the design and development solutions as there is often a degree of surprise at how many ‘new’ solutions have been tried before.
The need for more than one cylinder: To increase the power and torque produced by an engine, the cylinder capacity has to be enlarged, but to scale up a single-cylinder engine to twice its size would involve many difficulties. By comparing two single-cylinder engines, one having exactly twice the cylinder diameter and stroke as the other: the volume of a cylinder is equal to the product of the piston’s head or crown area and its stroke;
The piston head area ; therefore doubling the diameter will increase the area fourfold. Also, doubling the cylinder’s capacity is proportional to the piston stroke, therefore making the piston stroke twice as long doubles the cylinder cubic capacity. Thus the net result will be to increase the cylinders cubic capacity by eightfold. For the same mean effective cylinder gas pressure in both engines, the piston thrust will increase in proportion to the piston head area, therefore doubling the cylinder diameter will increase the piston thrust fourfold. For a given piston speed and mean effective gas pressure, engine power will increase with the square of the cylinder diameter, therefore doubling the cylinder diameter increases the power fourfold. Conversely, the volume and hence mass of the reciprocating components will increase with the cube of their dimensions, so doubling the piston dimensions will increase the mass eightfold, so maximum speed would have to be reduced.
By doubling the piston stroke for a given crank-shaft speed, the piston speed will also be doubled; therefore to maintain the same piston speed for both engines, the crank-shaft speed of the larger engine would have to be halved.
Finally, torque is proportional to the piston thrust and the crank-throw length; therefore doubling the piston diameter and stroke increases the piston thrust fourfold and doubles the crank-throw leverage, hence the torque will increase eightfold.
The difference between spark-ignition and compression-ignition (or petrol and diesel) engines lies in the way the fuel is introduced into the engine and ignited. The compression stroke only compresses air and not fuel. On the ignition stroke (there is no need for a spark as in the petrol engine) the air is compressed to a very high pressure and this generates enormous heat which then ignites the fuel that is injected into the combustion chamber at that precise moment of maximum pressure. A diesel engine thus has a far higher ‘compression ratio’ than does a petrol engine (diesel engines usually 20 to 1, petrol engines typically 9 to 1).
Just like the four-stroke-cycle petrol engine, the compression-ignition engine completes one cycle of events in two crankshaft revolutions or four piston strokes. The four phases of these strokes are i) induction of fresh air, ii) compression and heating of this air, iii) injection of fuel and its burning and expansion, and iv) expulsion of the products of combustion.
Induction stroke (Fig 14a) With the inlet valve open and the exhaust valve closed, the piston moves away form the cylinder head. The outward movement will establish a depression in the cylinder, its magnitude depending on the cross-sectional areas of the cylinder and the inlet port and on the speed at which the piston is moving. The pressure difference established between the inside and outside of the cylinder will induce air at atmospheric pressure to enter and fill up the cylinder. A maximum depression of maybe 0.15 bar below atmospheric pressure will occur at about one third of the distance along the piston’s outward stroke, while the overall average pressure in the cylinder might be 0.1 bar or even less.
Compression stroke (Fig 14b) With both the inlet and exhaust valves closed, the piston moves towards the cylinder head. The air enclosed in the cylinder will be compressed into a much smaller space of anything from 1/12 to 1/24 of its original volume. A typical ratio of maximum to minimum air-charge volume in the cylinder would be 16:1, but this largely depends on engine size and designed speed range. During the compression stroke, the air charge initially at atmospheric pressure and temperature is reduced in volume until the cylinder pressure is raised to between 30 and 50 bar. This compression of air generates heat which will increase the charge temperature to at least 600°C under normal running conditions.
Power stroke (Fig 14c) With both the inlet and the exhaust valves closed and the piston almost at the end of the compression stroke, diesel fuel oil is injected into the dense and heated air as a high pressure spray of fine particles that will quickly vaporise and ignite rapidly converting into pressure energy. Expansion then follows, pushing away from the cylinder head forcing the piston end of the connecting rod down providing rotary movement of the crankshaft
Exhaust stroke (Fig 14d) When the burning of the charge is near completion and the piston has reached the outermost position, the exhaust valve opens. This sudden opening towards the end of the power stroke releases the still burning products of combustion to the atmosphere. The pressure energy of the gases at this point will accelerate their expulsion from the cylinder, and only towards the end of the piston’s return stroke will the piston actually catch up with the tail end of the outgoing gases.
Fuel economy: Different engines are compared by their thermal efficiencies which is the ratio of the useful work produced to the total energy supplied. Petrol engines can have thermal efficiencies ranging between 20 and 30%. The corresponding diesel engines generally have improved efficiencies, between 30 and 40%.
Power & torque: new generation of diesel car engines has different design parameters to those used for commercial vehicles in the past and are generally as powerful as their equivalent petrol driven engines.
Reliability: Due to their particular process of combustion diesel engines are built sturdier, tend to run cooler, and have only half the speed range of most petrol engines. The s factors make the diesel engine more reliable and considerably extend engine life relative to the petrol engine.
Pollution: Diesel engines tend to become noisy and vibrate on their mountings as the operating load is reduced. The combustion process is quieter in the petrol engine and it runs smoother than the diesel engine. There is no noisy injection equipment used on the petrol engine, unlike that necessary on the diesel engine. The products of combustion are more noticeable with diesel engines. It is questionable which are more harmful.
Safety: Unlike petrol, diesel fuels are non-flammable at normal operating temperature, so they are not a handling hazard and the fire risks due to accidents are minimised.
Cost: Due to their heavy construction and injection equipment diesel engines are generally more expensive than petrol,engines
The following summarises the major factors which have to be considered when comparing engines of different cubic capacity and various numbers of cylinders:
(a)The shorter the piston stroke, the higher can be the crank-shaft speed for a given maximum piston speed.
(b)The smaller the cylinder, the lighter will be the piston in proportion to the cylinder size, so that the limiting inertia forces will permit higher piston speeds to be used.
(c)For the same engine cylinder capacity and maximum piston speed, a multi-cylinder engine will produce more power than a single-cylinder engine.
(d)A single cylinder engine having the same piston area as the sum of the piston areas of a multi-cylinder engine will develop a greater torque output.
(e)The smaller the cylinder, the greater will be its surface to volume ratio, therefore higher compression-ratios are permissible due to the improved cooling of the cylinder, with a subsequent improvement in engine thermal efficiency.
(f)Acceleration response improves with the number of cylinders for a given total volume. This is due to the lighter reciprocating components and to the reduced reliance made on the flywheel so that a smaller flywheel may be used.
(g)As the cylinders and the engine length increases, torsional vibrations become a problem
(h)As the number of cylinders increases, there will be more power consumed in overcoming rotational and reciprocating drag
(i)As the number of cylinders increases, mixture distribution for carburetted engines becomes more difficult
(j)As the number of cylinders increases, the cost of duplication of components becomes increasingly higher.
(k)As the number of cylinders increases, the frequency of power impulses increases too, therefore the power output is more consistent
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