9+ IC Engine Performance Parameters

There are a number of IC engine performance parameters. By these parameters, one means a performance criterion based on which the thermal efficiency of an IC engine can be evaluated and compared.

These parameters include power, indicated power, brake power, friction power, mechanical efficiency, mean effective pressure, indicated thermal efficiency, brake thermal efficiency, and specific fuel consumption, among many others.

Let’s discuss each performance parameter one by one:

IC Engine Performance Parameters

1. Power

The rate at which torque is produced is called the engine’s power. It is the measure of the output of the engine. It is available at the engine cylinder or crankshaft.

2. Indicated Power

The power produced in the engine’s cylinder is called its indicated power. It is the maximum achievable output of the engine.

Or, more technically, it is defined as the rate of work done by the gas on the piston of the engine’s cylinder per cycle, as demonstrated in the indicator diagram of that engine. This pressure on the piston is called the indicated mean effective pressure.

Mathematically, one can write thus:

Work\;done\;per\;cycle\;=\;Net\;area\;of\;engine\;indicator\;diagram\;=\;power\;loop\;-\;pumping\;loop
=Force\;on\;piston\;\times Stroke\;Length
=Indicated\;Mean\;effective\;pressure\times piston\;area\times stroke\;length
w_{cycle}=p_iAL

For work done per unit of time, the above expression takes the following form:

\frac{w_{cycle}}t=\;work\;done\;per\;cycle\times cycle\;unit\;time
ip=p_iALN

Where ip stands for indicated power, and N stands for the cycle per unit time or revolution per minute or the engine speed.

In the case of a four-stroke engine, four-engine processes take place in the two revolutions of the crank, and power is produced only in the second revolution during the power stroke, that is, during one of the two revolutions of a crank that make up a complete engine cycle.

Hence;

For four-stroke engine:

ip=\frac{p_iALN}2

For two-stroke engine:

ip=p_iALN

3. Brake Power

The power available at the engine’s crankshaft is called its brake power. It is always less than the indicated power chiefly due to the friction. It is the actual obtainable power of the engine.

If W is the load on the crank at the crank’s radius R, then the torque acting on the crank is given by:

T=W\times R

In angular measurements, the power is

work\;per\;cycle\;=Torque\;\times Crank\;degree\;of\;rotation\;=\;T\times2\mathrm\pi

Because during one revolution of crank, it moves by 360 degrees or 2 pi radians.

Brake\;Power\;=\;2\mathrm{πNT}

Where N stands for cycle per unit time or rpm.

4. Friction Power

The power lost in transmission from the cylinder to the crankshaft is called friction power. As obvious from its name, it is the power loss that is caused by friction.

Mathematically, it is the difference between the indicated power and brake power.

\mathrm{fp}=\mathrm{ip}-\mathrm{ip}

5. Mechanical Efficiency

The mechanical efficiency of the engine is defined as:

\eta_m=\frac{bp}{ip}

The lower the power loss due to friction caused by decreased load, the more power available at the crank (bp), and the higher the mechanical efficiency of the engine.

It lies between 80 to 90% in most of the cases. If the bp is zero, it signifies that the total power available at the engine’s piston is lost in overcoming mechanical friction or frictional resistance.

Usually, the value of friction power is constant at a given engine speed.

6. Brake Mean Effective Pressure

From the expression of the mechanical efficiency of the engine as stated above, one gets the following form by reordering it:

bp=\eta_m\times ip

For a four-stroke engine;

ip=\frac{p_iALN}2

so after substituting for ip,

bp=\frac{\eta_mp_iALN}2

Or

bp=\frac{p_bALN}2

Where

pb=\eta_mp_i

is called the brake mean effective pressure (BMEP). Let’s assume it is the pressure which, if acted on the engine’s piston to give the measured brake power if the engine were frictionless, can be compared with the brake power expression 2πNT.

\frac{p_bALN}2=2\mathrm{πNT}

Solving it gives;

p_b=\left(\frac{4\mathrm\pi}{AL}\right)T

The terms in the bracket are constant for an engine. Thus,

p_b=k\times T

It shows that brake means effective pressure, which is the function of torque only: the necessary turning force required to rotate the engine’s crank. It also underscores that bmep is independent of the engine’s speed.

7. Brake Thermal Efficiency

Every engine extracts power from the chemical energy of the fuel and converts it into mechanical energy, which is readily available as the engine’s brake power. Hence, the brake thermal efficiency of an engine is net power available as bp out of the chemical energy possessed by the fuel in the form of its net calorific value. Mathematically;

\eta_{BT}=\frac{bp}{{\displaystyle\overset.m}\times Q_{net\;v}}

Where m stands for the mass of fuel consumed per unit of time and Qnet v for the net calorific value of the fuel.

8. Indicated Thermal Efficiency

The indicated thermal efficiency of an engine is the power available at the engine’s piston out of the chemical energy possessed by the fuel in the form of its net calorific value. Mathematically,

\eta_{IT}=\frac{ip}{{\displaystyle\overset.m}\times Q_{net\;v}}

Dividing the two efficiencies, one gets the following expression,

\frac{\eta_{BT}}{\eta_{IT}}=\frac{bp}{ip}=\eta_m

Or

\eta_{BT}=\eta_m\times\eta_{IT}

9. Specific Fuel Consumption

It is defined as the mass flow rate of the fuel burned per unit of the available engine power as bp.

sfc=\frac{\displaystyle\overset.m}{bp}