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Determining efficiency of a engine?2-stoke and 4-stoke!

Updated on September 28, 2011
Cylinder piston system
Cylinder piston system

It is easier to do than seems by the title. The efficiency of a engine means the efficiency of the engine thermodynamic cycle, which in turn means the efficiency to convert the heat supplied into work output. One thing should be made clear that the 2nd law of thermodynamics puts a constraint over 100% efficiency that is it is impossible to have a cycle that converts the entire heat into work output. It, however doesn't violate the energy conservation principle as some heat is rejected which balances Qin=Qout + W.

2-stoke diesel engine
2-stoke diesel engine

To determine the mechanical efficiency we need a machine or say device which we call indicator engine(see pic 2). Lets get acquainted with what it is....At the first sight it might look damn complicated but we are interested in few of its things. The very first is the cylinder piston engine (see left part of image) which performs the same cycle that is performed by the engine we are interested in.The other parts include the wheel and the drum on which a pencil connected to piston draws the indicator diagram. Now lets see how to know the efficiency, For that we first of all find the area of the indicator diagram (ad) using Planimeter . And the diameter of the wheel (ld) is measured. Knowing the spring constant ( K) of the spring attached to the cylinder-piston, we find the Mean Effective Pressure (Pm)

Pm= ad * ld / K.

Now we calculate the work done in terms of Pm. Then we have the work output in a cycle.

Before moving further lets see what 2-stroke and 4stroke engine are. The engine which completes its one cycle in 1 complete revolution of the wheel (or stroke) is termed as 2 stroke engine. And the one which takes 2 complete revolutions are 4-stroke engine.

Now if 'N' is the number of revolutions per minute of the wheel (rpm) and there are 'n' cylinders. Then the indicated power for 2-stroke is given by

I.P.= Pm* A * ld * N * n/60. The unit is in watt

for 4-stroke -

I.P.= Pm* A * ld * N * n/120.

Now we find the shaft power; the power made available for rotating the wheel.If 'T' is the Torque transmitted. Then Brake Power for 2-stroke is given by-

B.P= T * N *24 / 60

And for 4-stroke -

B.P= T * N *24 /120.

Now we can have the mechanical efficiency which is nothing but the ratio of brake power to indicated power.

It should be noted that mechanical efficiency can be increased by reducing the dissipative forces and ideally can be 100%.


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