Mean Effective Pressure

Mean Effective Pressure, often known as MEP, is a theoretical parameter that is utilised in the process of measuring the effectiveness of an internal combustion engine (ICE). Even though it uses the word “pressure,” the readings it provides for the engine cylinder do not reflect the actual pressure that exists there.

During the course of the combustion cycle, an internal combustion engine’s (ICE) cylinder pressure is subject to ongoing shifts.

Derivation:

Let:

W = work per cycle in joule;

P = power output in watt;

P_me = mean effective pressure in pascal;

V_d = displacement volume in cubic metre;

n_c = number of revolutions per power stroke (for a 4-stroke engine, nc = 2)

N = number of revolutions per second;

T = torque in newton-metre.

The amount of work completed during one cycle of the engine’s operation is multiplied by the number of cycles completed during one second to arrive at the amount of power that is produced by the engine. If N is the number of revolutions per second and n_c is the number of revolutions that occur during each power stroke, then the number of power strokes that occur during each second may be calculated by dividing n_c by N. We are able to write:

P = W. N/n_c

Changing the order so that the work is on the left:

W = P. n_c / N

By definition, 

W = p_me V_d, 

So that

P_me = Pn_c / V_d N

Because the torque T is proportional to the angular speed, which may be expressed as N.2π , as well as the power that is generated,

P = 2πTN, 

Therefore, the equation for MEP can be expressed in terms of torque as follows:

P_me = 2π n_c T/V_d

Since speed is no longer a factor in the calculation, the only variables left are torque and the volume of displacement. We now have a measure of the torque-producing capacity of an engine design that is independent of the displacement of the engine; you could call it a specific torque. This is because the range of maximum brake mean effective pressures for good engine designs has been well established. Comparing engines with varying displacements is made easier with this information. Mean effective pressure is also useful for initial design calculations; that is, conventional MEP values can be used to predict the required engine displacement when provided with a torque value. Mean effective pressure, on the other hand, does not reflect the actual pressures present within an individual combustion chamber; despite the fact that the two are obviously related, mean effective pressure serves only as a convenient metric of performance.

The brake mean effective pressure (BMEP) is determined by using the dynamometer torque that has been measured. The net indicated mean effective pressure (IMEP_n) is computed by making use of the indicated power, which is defined as the pressure volume integral in the equation for work per cycle. The friction mean effective pressure, also known as FMEP (friction mean effective pressure), is simply the difference between IMEP_n and BMEP. This term is used as an indicator of the mean effective pressure lost to friction (or friction torque).

Types of Mean Effective Pressure:

The term “mean effective pressure,” or MEP, refers to the pressure measured at a specific location and the method that is used to calculate it. The following table provides some MEPs that are commonly used.

• Mean effective pressure of the brakes (BMEP) is determined by measuring the brake torque and then calculating the mean pressure.
• The gross indicated mean effective pressure, also known as IMEP_g, is the effective pressure that is computed from the in-cylinder pressure over the compression and expansion phase of the engine cycle (360 degrees in a four-stroke, 180 degrees in a two-stroke). Direct measuring requires cylinder pressure sensing equipment.
• The net indicated mean effective pressure, also known as IMEP_n, is the effective pressure that is computed from the in-cylinder pressure across the entirety of the engine cycle (720 degrees for a four-stroke engine and 360 degrees for a two-stroke engine). Direct measuring requires cylinder pressure sensing equipment.

The pumping mean effective pressure, also known as PMEP, is the mean effective pressure that results from the labour done to move air in and out of the cylinder, as well as across the intake and exhaust valves. Calculated based on the pressure within the cylinder during the intake and exhaust phases of the engine cycle (360° for a four-stroke engine and 0° for a two-stroke engine). Direct measuring requires cylinder pressure sensing equipment. PMEP = IMEP_g – IMEP_n.

Conclusion:

Mean effective pressure, often known as MEP, is defined as the average pressure needed to act on the piston as it moves one displacement in order to produce work W. This pressure is required in order to calculate MEP. Mean Effective Pressure, often known as MEP, is a theoretical parameter that is utilised in the process of measuring the effectiveness of an internal combustion engine (ICE).

The amount of work completed during one cycle of the engine’s operation is multiplied by the number of cycles completed during one second to arrive at the amount of power that is produced by the engine. The brake mean effective pressure (BMEP) is determined by using the dynamometer torque that has been measured. The net indicated mean effective pressure (IMEPn) is computed by making use of the indicated power, which is defined as the pressure volume integral in the equation for work per cycle. The gross indicated mean effective pressure, also known as IMEPg, is the effective pressure that is computed from the in-cylinder pressure over the compression and expansion phase of the engine cycle. The pumping mean effective pressure, also known as PMEP, is the mean effective pressure that results from the labour done to move air in and out of the cylinder, as well as across the intake and exhaust valves.