Page 117 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 101

Figure 2.33 Torque and power measured at the fl ywheel

Two common terms used when expressing engine performance characteristics
Defi nitions
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 Torque
independently of engine speed and is a measure of the load on the engine. The The turning force produced by an
SI units of torque are newton metres (Nm) and the imperial units are pounds/foot engine. It is not affected by time.
(lb/ft). Power is a derived unit and relates to the rate of work done, or the work
Power
done per unit of time. For an engine, the power is a product or torque and speed.
The rate at which energy is being
Power output is given in kilowatts (kW) or horsepower (HP). Engine power is converted. It is therefore related to
normally stated as measured at the fl ywheel, via a dynamometer or brake, hence time.
the term ‘brake horsepower’ ( Fig. 2.33 ).
Or how about this version?
Engine manufacturers often publish performance data in a graphical form Power is how hard you hit a tree.
showing torque and power curves against speed. Two examples are shown in Torque is how far you move it.
Figs 2.34 and 2.35 . Note that a petrol engine generally produces more power, at

higher speed. A diesel engine produces more torque at lower speeds.
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
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between 250 and 600 cm 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.