The following are some of the technical terms that are used to describe features of the engine:
Engine capacity – the total, combined, displaced volume of all engine cylinders as a single value stated in units of cubic capacity. This is generally given in cubic centimetres (cm3) or litres. In
America, engine capacity is normally stated in cubic inches (in3).
Swept volume – the volume of a cylinder bore between the TDC and BDC piston positions, excluding the volume above the piston at TDC (Fig. 2.32).
Figure 2.29 Exhaust gas recirculation (EGR)
Figure 2.30 Reduced oxygen
Figure 2.31 Phases of combustion, diesel engine:
A, start of injection; B, start of combustion;
C, controlled phase; D, bottom dead centre; E, top dead centre; F, piston travel; G, combustion chamber pressure (A–B is the delay phase)
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Clearance volume – the volume above the piston at TDC. Note that it is the volume of the combustion chamber itself (Fig. 2.32).
Bore – the diameter of the engine cylinder.
Stroke – the total linear distance travelled by the piston in the bore between TDC and BDC positions.
Note that it is twice the crankshaft throw.
Compression ratio – the total volume of the cylinder at BDC (swept + clearance volume), expressed as a ratio of the volume of the cylinder at TDC (clearance volume).
The information on cylinder dimension can generally be found in workshop or manufacturer manuals. In addition, these values can be measured directly or derived via calculations.
Swept volume can be calculated via the formula:
(lπd 2)/4
where d is cylinder bore and l is stroke. Note that units of bore and stroke must be consistent. Engine volume is mostly stated in litres by manufacturers, but remember that 1000 cc (cubic centimetres) equals 1 litre. The total engine displacement is the sum of all cylinders’ individual displacements.
The formula used to calculate the compression ratio is:
CR = (Vs + Vc)/Vc
where CR is compression ratio, Vs is swept volume, and Vc is clearance volume.
Note the correct order of preference when carrying out this calculation (remember or Google BODMAS).
Two common terms used when expressing engine performance characteristics are ‘torque’ and ‘power’.
Torque is an expression relating to work and is a
measure of the turning force provided by the engine.
Torque output can vary independently of engine speed and is a measure of the load on the engine.
The SI units of torque are newton metres (Nm) and the imperial units are pounds/foot (lb/ft). Power is a derived unit and relates to the rate of work done, or the work done per unit of time. For an engine, the power is a product or torque and speed. Power output is given in kilowatts (kW) or horsepower (HP). Engine power is normally stated as measured at the fl ywheel, via a dynamometer or brake, hence the term ‘brake horsepower’ (Fig. 2.33).
Engine manufacturers often publish performance data in a graphical form showing torque and power curves against speed. Two examples are shown in Figures 2.34 Figure 2.32 Clearance and swept volumes
Figure 2.33 Torque and power measured at the fl ywheel
Figure 2.34 Spark ignition (SI) engine
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and 2.35. Note that a petrol engine generally produces more power, at higher speed. A diesel engine produces more torque at lower speeds.
Defi nitions
Torque: The turning force produced by an engine. It is not affected by time.
Power: The rate at which energy is being converted. It is therefore related to time.
Or how about this version?: Power is how hard you hit a tree. Torque is how far you move it.
The optimum size of an individual engine cylinder is a compromise of a number of technical factors.
The optimum displacement for a cylinder is
generally between 250 and 600 cm3 for road vehicle applications. In this range, the combustion chamber size, surface area and individual components size (pistons, valves, etc.) produce an engine with optimum effi ciency with respect to fuel consumption and emissions. Typically, engines with total displacements in the range of 1–2.5 litres have four cylinders
(Fig. 2.36).
The number of power strokes per revolution can be found by dividing the number of engine cylinders by two (for a four-stroke engine). The greater the number of cylinders, the smoother the torque delivery owing to reduced peak torque fi ring pulses from each cylinder and the increased number of fi ring strokes per revolution. Over 2 litres, six-cylinder engines give smooth power delivery with optimum cylinder displacement sizes. An in-line six cylinder has a relatively long crankshaft that can be diffi cult to accommodate in a transverse engine installation layout;
therefore by using two banks of three cylinders in a
‘V’ confi guration, total length is reduced and torsional rigidity of the crankshaft is improved (Fig. 2.37).
The engine’s fl ywheel acts as an energy buffer owing to its inertia. Energy stored in the fl ywheel maintains rotation between fi ring pulses and acts as a damper to smooth torque peaks as each cylinder fi res.
There are numerous engine confi gurations with respect to the arrangement of the engine cylinders, the number of cylinders, position and fi ring order Figure 2.35 Compression ignition (CI) engine
Figure 2.36 Displacement
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(Fig. 2.38). In addition, combustion chamber designs and valve train layout all dictate the basic properties of an engine. Engine installation and orientation is another important factor to be considered in a road vehicle. There are also two-stroke and rotary engine designs with their own particular characteristics, all of which are explained in this section.