Page 371 - Advanced thermodynamics for engineers

360 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

Pressure, p 3 3

Incremental
cycle
2 2
4 4

1 1 _
_ p b _
p
i p
f
V s Volume, V
FIGURE 16.8
Otto cycle with friction.

n cyl ¼ number of cylinders
n cyc ¼ number of cycles/s
N cyc ¼ number of cycles/min
Now, friction mep can be obtained by simple subtraction of the bmep from the imep. However,
imep which is evaluated by integrating to obtain the area of the p–V diagram presents some difficulties
in measurement accuracy because the imep values are extremely sensitive to:

• crank angle error;
• thermal shock on the pressure transducer.
Also care should be taken when considering fmep since it is difficult to separate frictional effects
from pumping work (and even auxiliaries). The pumping work of an engine is that required to draw the
gas through the engine. The pumping work of a naturally aspirated engine is almost always negative,
i.e. air has to be pumped from the inlet to the exhaust system. In a petrol engine, pumping work is used
to control the engine speed by throttling the intake. This is one of the reasons why a petrol engine is
less efficient than a diesel engine, especially at part load. In turbocharged engines, especially diesel
engines, it is possible for the pumping work to be positive: this is because the pressure in the intake
manifold can be larger than that in the exhaust.
Even taking these provisos into consideration, a better measure of the power loss due to friction is the
frictional torque or fmep. This tends to be related toengine speed but not closely related to the engine load.
Hence a correlation for fmep for four-stroke engines is of the form suggested by Chen and Flynn (1965),

N
p ¼ C 1 þ C 2 þ C 3 ^ p (16.21)
f
1000