Explain why the brake mean effective pressure of a naturally

Explain why the brake mean effective pressure of a naturally aspirated diesel engine is lower than that of a naturally aspirated spark-ignition engine. Explain why the bmep is lower at the maximum rated power for a given engine than the bmep at the maximum torque.

Solution

1.CI engines typically run on a leaner fuel-air mixture even at full load operation, as compared to SI engines. Thefuel-air mixture cannot be enriched beyond a limiting value in diesel engine due to the tendency of increased smoke formation.

It should also be noted that the air-fuel mixture in a CI engine is not homogenous like that in an SI engine. There will be rich and lean pockets as the liquid fuel is sprayed into the cylinder and there is no much time to mix homogenously before the combustion. In contrast, the fuel and air is mixed prior to the intake in an SI engine.
But if you compare the global air-fuel ratio (i.e. total air intake/total fuel intake of the engine), SI engine air-fuel mixtures are richer than their CI engine counterparts.

2.Mean effective pressure is a non dimensional way of showing an engines capacity to do work per cycle independent of displacement. It is effectively a measure of how well the engine is breathing (ie how well the cylinders are being filled). The engine has the most capacity for work when it is burning the most fuel, which occurs at peak torque. You may say that peak work comes at peak power, it is important to remember that this is per cycle, not per unit time. The reason why peak torque and peak bmep occur at the same rpm is because the curves are the same shape. Take the bmep value relating to power and relating to torque:

n=2 for a 4 stroke engine

Pmep = Pn/VdN

Pmap = 2pi*Tn/Vd

If we lump together the constants. n and Vd and 2*pi we can see that Pmep is proportional to torque, but power still has an engine speed term. We know friction is accounted for because it\'s a \'brake\' value. Brake values are net output or measured values. \'Indicated\' values do not take into account losses, the difference between the two values is the loss. So in this case

imep - fmep = bmep