CalCon has developed the BMEP calculator for finding the Brake Mean Effective Pressure of an engine, a valuable tool for performance comparison of different engines but similar types. Calculate this value by interactive calculator on this website or with our CalCon application which is available for free download through App Store and Google Play Store.

Below, we will introduce you to the basic definition and description of Brake Mean Effective Pressure, what formula is used, how we can find the BMEP value with our calculator, and some real-life examples and problems.

Why is BMEP important? Well, it gives you a clue of how efficient your engine really is. While you are here, check some of our other calculators regarding Physics and see how they work.

What is BMEP?

Mean effective brake pressure (BMEP) is the average pressure which, if applied evenly to the pistons from top to bottom of each stroke, produces the measured performance (braking).

Mean effective pressure is used for initial calculations in engine design, with torque and MEP as input data from which the designer can calculate what capacity is required.

There are different mean effective pressures:

  • indicated mean effective pressure (IMEP)
  • mean effective brake pressure (BMEP)
  • mean effective friction pressure (FMEP)

Indicated mean effective pressure (IMEP) is the mean effective pressure calculated with the indicated force.

Mean effective brake pressure (BMEP) is the mean effective pressure calculated from the power of the dynamometer (or torque). This represents the actual power of the internal combustion engine on the crankshaft. Therefore, the mean effective brake pressure must consider the engine’s efficiency.

Mean effective friction pressure (FMEP) is an indicator of the mean effective pressure lost by friction and is the difference between the indicated mean effective pressure and the mean effective brake pressure.

Engine efficiency also depends on the carburetor CFM. A carburetor is a device that works on mixing air and fuel and converts it into an even and steady flow of gas. It is an essential part of an engine and converts the fuel-air mixture into the most efficient burning. Work with our Carburetor CFM Calculator.

Read more here.

BMEP facts

Below is an overview of some key facts when it comes to BMEP.

  • For any internal combustion engine, maximum BMEP is achieved at full load
  • engine damping reduces BMEP due to large pumping losses
  • if the displacement is fixed, if we increase the BMEP, more efficient torque is produced on the crankshaft
  • for the same BMEP value, a 2-stroke internal combustion engine has almost double the torque compared to a 4-stroke engine
  • the higher the BMEP, the higher the mechanical and thermal load on the engine components.

BMEP formula

The BMPE value is one of the important values used in comparing the performance of engines of the same type. To this end, we present the equation below:

Brake \; Mean \; Effective \; Pressure = \frac {n \cdot t \cdot 2 \cdot 3,14} {d}

Where,

n – No of Revolutions per Power Stroke

t – Torque

d – Displacement

Note:

n = 1 or

n = 2

Meaning of equation parameters

As you can see above, there are a few parameters to consider when calculating BMEP; number of revolutions, torque, and displacement. We will explain each of them.

When we talk about an internal combustion engine, the term stroke has the following meanings:

  • Engine cycle phase (e.g., compression or exhaust stroke), during which the piston travels from top to bottom or vice versa.
  • The reciprocating engine uses the type of power cycle.
  • Stroke length, the distance the piston travels during each cycle. The length – together with the bore diameter – determines the working volume of the engine.

Two-stroke engine

Engines complete the power cycle every two strokes, which means that the power cycle essentially ends with each crankshaft revolution. These engines are commonly used in marine, lawnmowers, or motorcycles.

Four-stroke engine

These engines complete the power cycle every four strokes, which means that the power cycle ends with every two crankshaft revolutions. Most car engines have a four-strokes design. Power is defined as work-per-unit time.

Torque is the rotating force produced by the engine crankshaft. The higher the torque produced by an engine, the greater its ability to do the job.

Engine displacement is calculated as the product of the cross-sectional area of the cylinder (determined by the bore) and the stroke length. Next, we multiply that number by the number of cylinders in the engine to determine the total volume.

How to calculate the BMEP?

For example, we bought a car (Mercedes Benz C Class) with a volume (displacement) of 2143 cm3 and an indicated torque of 400 Nm. We will calculate the engine’s BMEP following the next steps:

  1. The type of the engine is 4-stroke.
  2. Enter 400 Nm in torque value.
  3. Enter displacement value as 2143 cm3.
  4. The output (calculated BMEP) is 2345,6 kPa.

On the other hand, if we have a BMEP value, we can convert BMEP to torque using the same formula and rearrange it to solve the torque. Converting BMEP to torque is especially useful because torque is an important indicator of the load the engine can withstand. Try it on your own using the same example.

FAQ

What is the brake mean effective pressure?

The mean effective brake pressure is the calculation of the pressure in the engine cylinder that would give the measured horsepower of the brake. Mean effective brake pressure represents engine efficiency regardless of engine capacity or speed.

Why is BMEP important?

It is good to know what BMEP is since its value gives you a sense of how efficient your engine is. BMEP is a function of temperature of the gases in the cylinder. To increase the temperature you need to burn more fuel, thus making more heat. Or another way is to make better use of the existing fuel.

How can we improve BMEP?

The most effective way to improve it is to increase the number of cylinders. The more efficient the cylinders are, the higher the average BMEP pressure is.

What is the stroke?

It is defined as the process in which the cylinder’s piston moves down and up to the top. It is the length of the distance traveled by the piston.

What is a 2 stroke engine?

The two stroke engine needs to finish two piston movements in order to produce power. The engine creates power after one cycle.

What is a 4 stroke engine?

In a 4-stroke engine, the piston completes 2-strokes during each revolution: one compression and one exhaust stroke, each being followed by a return stroke. The spark plugs fire only once every other revolution, and power is produced every 4-strokes of the piston.

Is BMEP same as compression ratio?

The BMEP is a purely theoretical value of the pressure and it’s output is not an accurate estimate of cylinder pressures.