Variables Affecting Knock in an SI & CI Engine:
The energy flow through the engine is expressed in three distinct terms.
They are indicated power, IP, friction power FP and brake power, BP.
Indicated power can be computed from the measurement of forces in the cylinder and brake power may be computed from the measurement of forces at the crankshaft of the engine. The friction power can be estimated by motoring the engine or other methods.
It can also be calculated as the difference between the IP and BP if these two are known, then,
IP = BP +FP
FP = IP – BP
Indicated Mean Effective Pressure (Pim)
It is a mean value expressed in N/m2, which, when multiplied by the displacement volume, Vs,
Gives the same indicated network as is actually produced with the varying pressures.
Where IP = indicated power (kW)
Pim = indicated mean effective pressure (N/m2)
L= length of the stroke (m)
A = area of the piston (m2)
N = speed in revolutions per minute,
n = number of power strokes per minute= N/2 for a four-stroke engine N for a two-stroke engine
K =number of cylinders
Brake Power (BP)
This power is interchangeably referred to as brake power, Shaft power or delivered power'.
In general, only the term brake power, bp, has been used in this book to indicate the power actually delivered by the engine.
Apart from expressing engine performance in terms of power, it is also essential to express in terms of efficiency.
The air-standard efficiency is also known as thermodynamic efficiency mainly a function of compression ratio and other parameters.
INDICATED AND BRAKE THERMAL EFFICIENCIES:
The indicated and brake thermal efficiencies are based on the ip and bp of the engine respectively.
The mechanical efficiency, ⴄm of the engine can be expressed as the ratio of bmep to imep.
Mechanical efficiency takes into account the mechanical losses in an engine.
Mechanical in general, the mechanical efficiency of engines varies from 65 to 85%.
The relative efficiency or efficiency ratio as it is sometimes called is the ratio of the actual efficiency obtained from an engine to the theoretical efficiency of the engine cycle
Volumetric efficiency is defined as the ratio of the actual mass of air drawn into the engine during a given period of time to the theoretical mass which should have been drawn in during that same period of time, based upon the total piston displacement of the engine, and the temperature and pressure of the surrounding atmosphere.
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