In Reciprocating Internal Combustion engines, it is the measure of the capacity to do the work. The Mean effective pressure means the simply average pressure acting on the piston during the complete thermodynamic cycle in a Reciprocating internal combustion Engine.

Mean Effective Pressure

As we said it is an average pressure acting on the piston during the complete thermodynamic cycle in a Reciprocating internal combustion Engine.

There are different Mean Effective Pressures that we need to understand completely. Those are listed below.

• IMEP – Indicated Mean Effective Pressure
• BMEP –Brake Mean Effective Pressure
• FMEP – Friction Mean Effective Pressure

Indicated Mean Effective Pressure (IMEP)

The Indicated Mean Effective Pressure is the average pressure, that induced in the combustion chamber during the complete thermodynamic cycle in Internal Combustion engines. In here we will have two more different Indicated Mean effective pressures. Those are Gross Indicated Mean Effective Pressure and Net Indicated Mean Effective Pressure.

Gross Indicated Mean Effective Pressure (IMEP_g)

It is the average pressure in the cylinders during the Compression and the Expansion Strokes only. Not the complete Thermodynamic cycle.

Net Indicated Mean Effective Pressure (IMEP_n )

But where the Net Indicated Mean Effective Pressure is the average pressure in the cylinders during the complete Thermodynamic cycle.

This Indicated Mean Effective Pressure can be obtained by a direct measurement such as a cylinder pressure sensing Equipment is required to measure this.

Pumping Mean Effective Pressure (PMEP)

The difference between the Net Indicated Mean Effective Pressure and the Gross Indicated Mean Effective Pressure will give the Pumping Mean Effective Pressure (PMEP).

Pumping Mean Effective Pressure (PMEP) = IMEP_g – IMEP_n

Brake Mean Effective Pressure (BMEP)

Unlike the Indicated Mean Effective Pressure (IMEP), we can calculate the Mean Effective Pressure from the torque generated at the crankshaft.

The torque generated at the crankshaft is called the Brake Torque. So the Mean effective pressure calculated from the Brake torque is called the Brake Mean Effective Pressure.

Friction Mean Effective Pressure (FMEP)

We can understand that the Mean Effective pressure at the cylinders (IMEP_n) and the Mean effective pressure (BMEP) calculated from the brake torque are different. In fact, the Mean effective pressure (BMEP) calculated from the brake torque is less than the Mean Effective pressure at the cylinders (IMEP_n). Because of the friction losses.

So the Friction Mean Effective pressure can be defined as the Mean Effective Pressure lost due to the friction. This can be calculated from the Net Mean Effective Pressure and the Brake Mean Effective Pressure. (See the following formula)

FMEP = IMEP_n – BMEP

Well, now we have a full understanding of Mean Effective Pressure terminology.

Let’s see how we can calculate them with the different parameters.

How to calculate the Mean Effective Pressure?

Generally, Mean Effective Pressure can be calculated from work done by the engine and the engine displacement.

p_me =W/V_d

Where:

p_me– Mean Effective Pressure

W – Work performed in a complete engine cycle (Units In Joules)

V_d – Engine (cylinder) displacement (Units in m³)

We can write the above equation in terms of the power or torque also.

as we know Power (P)= Work × (Engine Speed in RPM / Revolutions per power stroke)

P = W × (N/n_r)

W = (P × n_r)/N

Where n_r = Number of revolutions per power stroke (4 Stroke engine = 2; for 2 Stroke Engine = 1)

And also we know the power in terms of Torque (P) = 2π N T

By substituting P in the above equation, we can write

W = 2π T × n_r

We can rewrite the MEP from the above equations

p_me =(2π T × n_r )/V_d

For an engine with multiple cylinders, we have to take into account the total volumetric capacity.

Here  n_c will be the number of cylinders, so the Mean effective pressure for the multiple cylinder engines is

p_me =(2π T × n_r )/n_cV_d

How to Calculate IMEP, BMEP, and FMEP?

With the above formula, we can calculate both the Indicated Mean Effective Pressure (IMEP) and the Brake Mean Effective Pressure (BMEP)

To calculate Brake mean effective Pressure we have to have the Brake Torque or the effective torque T_b

so the formula for BMEP = (2π T_b × n_r )/n_cV_d

And for the Indicated Mean Effective Pressure, we have to have the Indicated Torque T_i

so the formula for IMEP = (2π T_i × n_r )/n_cV_d

from the Friction Mean Effective Pressure, we have FMEP = IMEP – BMEP

This is how we can calculate the different mean effective pressures.

**Note: The Mean effective pressure is an Engine performance parameter to determine the engine performance and does not reflect the actual pressures inside an individual combustion chamber.

To calculate the engine efficiency we have the formula η_m= 1 – (FMEP/IMEP)

Sample problem: A 4 cylinder 4 Stroke diesel engine having 84mm of piston bore and 90mm of stroke length and the effective torque of 175 Nm and the indicated torque is 193 Nm. Please calculate the IMEP, BMEP, FMEP and mechanical efficiency?

Calculate and post the answers in the comment section below. We will send you the answers.

Conclusion

We have discussed the mean effective pressures and different types of it such as IMEP, BMEP, and FMEP. and we explained the derivation of the MEP formula. Please let us know your thoughts in the comment section below.