The engine performance is indicated by the term efficiency.
Today we will discuss about diesel engine efficiency and petrol engine
efficiency. In auto industries, five engine efficiencies are used to check an
engine performance. These efficiencies are Indicated thermal efficiency, Brake
thermal efficiency, Mechanical efficiency, Volumetric efficiency and Relative
efficiency. These all are that
parameters which is used to check an engine performance and used to describe
an engine completely. Today we will learn about them.
Before learning these efficiencies, we have to learn about
energy flow in an engine. It is desirable to have a general picture of the
energy flow or energy balance of the engine so that we become familiar with the
various performance parameters.
Energy flow in engine:
We know that energy can neither be created nor be destroyed.
It can only be converted from one form to another. Therefore there must be an
energy balance between input and output of the engine. In a engine fuel is fed
into the combustion chamber where it burns and convert its chemical energy to
the heat energy. These heat energy used to driving the piston. But the liberated heat energy cannot be
totally utilized for work because of losses through engine exhaust, coolant and
due to radiation from piston. The heat energy converted into power at this
stage is known as indicated power. This power utilizes to drive the
piston. There are also other losses like
friction losses, bearing friction, pumping losses. Some of these energy used to
driving auxiliary devices like feed pump, valve mechanism etc. The sum of all
those losses are known as frictional energy and when it is expressed in power
unit, known as frictional power. The
remaining power is known as brake power available to useful work. These all
energy balance can be understood by the figure shown below.
Efficiency of engine:
These are the main efficiencies used to check engine performance.
1. Indicated thermal efficiency:
This is the ratio of energy in the indicated power to the input
fuel energy.
2. Brake thermal efficiency:
It is the ratio of energy in the brake power to the input
fuel energy.
Brake Power (KJ/s) / Energy of fuel per second ( KJ/s)
3. Mechanical efficiency:
It is the ratio of brake power to the indicate power or can
be define as the ratio of brake thermal efficiency to the indicated thermal
efficiency.
4. Volumetric efficiency:
This is one of the
very important parameter of engine. it is defined as the ratio of actual volume
flow rate of air into the intake system to the rate at which the volume is
displace by the piston or it is the ratio of actual volume flow rate of air to
the ideal volume flow rate of air according to piston displaced.
Actual volume of air drawn into cylinder / Ideal volume of air drawn into cylinder
In this parameter volumetric rate of air flow to be taken
into account and not the mixture flow. Volumetric
efficiency of diesel engine is higher than petrol engine.
5. Relative efficiency:
It is the ratio of the thermal efficiency of actual cycle to
the thermal efficiency of ideal cycle. It show the degree of development of
engine.
Actual thermal efficiency / Air Standard Efficiency
Today we have discuss about the efficiency of engine which used to check performance of petrol and diesel engine. If you have any query ask by commenting.