Brake Power, Engine Speed and bmep Relationship

In summary, the person is doing a report on Petrol Engines and is having trouble with their results not reflecting the expected increase in Break Mean Effective Pressure (bmep) as Engine Speed (rev/min) increases. They attached their results table and calculated brake power and bmep using equations they found online. However, they later discovered that BMEP is only dependent on torque and not rpm. The definition of BMEP is the average pressure that would produce the measured power output, and it is primarily affected by the filling efficiency of the cylinder during the intake stroke and fuel combustion energy, not engine speed. Superchargers and turbochargers can increase BMEP by increasing the cylinder's air charge pressure. Ignition timing and variable valve
  • #1
smr101
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0
I'm currently doing a report on Petrol Engines and I'm having a problem with my results not making sense.

Am I correct in saying Break mean effective pressure (bmep) should increase as Engine Speed (rev/min) increases? My results do not reflect this and I don't understand why.

I have attached my results table below, I calculated brake power and bmep myself using these equations:

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I believe the problem is in the Brake Power & bmep results, help is much appreciated.

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  • #3
I think the following quote from the above reference site addresses your concern:

The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.

While the equations shown are based upon torque, in fact BMEP is really a measure primary of the filling efficiency of a cylinder during the intake stroke combined with fuel combustion energy; and, neither of these factors are related to engine speed. In reality, there is a potential for a reduction in BMEP at increased engine speeds due to a flowing pressure drop in the engines inlet system as the engine demands higher and higher air cfm flow rates. Increasing BMEP by raising the cylinder(s) air charge pressure is the primary reason for and function of superchargers and turbochargers. i.e. higher cylinder charging pressure = higher compression pressures and more air to be expanded during the combustion power cycle.

Added note: BMEP can also be potentially reduced at increased engine speeds due to the fact that the piston's downward travel rate is increased during the power stroke which in turn increases the cylinder volume increase rate relative to the cylinder's reduced combustion pressure rise burn time; this is where ignition timing and variable valve timing functions can come into play to offset this effect.
 
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1. How are brake power, engine speed, and bmep related?

Brake power, engine speed, and bmep are all related because they are all key factors in determining the overall performance of an engine. Brake power is the measure of an engine's total power output, engine speed is the rate at which the engine's crankshaft rotates, and bmep (brake mean effective pressure) is the average pressure exerted on the piston during the power stroke. These three factors work together to determine the amount of power an engine can produce.

2. What is the significance of the relationship between brake power, engine speed, and bmep?

The relationship between brake power, engine speed, and bmep is significant because it helps to determine the overall efficiency of an engine. A higher brake power indicates a more powerful engine, while a higher engine speed and bmep indicate a more efficient engine. This allows engineers to optimize engine design for better performance and fuel efficiency.

3. How does changing engine speed affect brake power and bmep?

Changing engine speed can have a significant impact on both brake power and bmep. As engine speed increases, the power output and bmep also increase due to the increased number of power strokes per minute. However, if the engine speed is increased too much, it can lead to decreased efficiency and potential damage to the engine.

4. What factors can affect the relationship between brake power, engine speed, and bmep?

There are several factors that can affect the relationship between brake power, engine speed, and bmep. These include the design and condition of the engine, the type of fuel and air intake system, and the load on the engine. Other factors such as temperature and altitude can also impact the performance of an engine.

5. How can the relationship between brake power, engine speed, and bmep be measured?

The relationship between brake power, engine speed, and bmep can be measured using various instruments and techniques. Dynamometers are commonly used to measure brake power, while an engine speed sensor can be used to measure engine speed. Bmep can be calculated by measuring the pressure inside the cylinder during the power stroke. These measurements can provide valuable information for engine performance analysis and optimization.

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