Engine Friction and
Lubrication (Part 3)
In Previous blogs, we
have discussed about basic of Engine Friction, Lubrication, and
Grading of lubricating oil, grease lubrication and additives. Here,
in this blog, we will discuss about Effect of Engine Variables on Friction.
Click on the link below for the part I and II on Engine Friction and Lubrication.
Effect of Engine
Variables on Friction:
There are many
geometrical parameters and physical parameters which are responsible for the
frictional losses. Those parameters are considered in this blog.
a.
Stroke to Bore Ratio (L/D):
Lower
L/D ratio tends to decrease IMEP. Its lower value reduces the friction losses
as the surface area decreases with decreasing L/D ratio with the same value of
the stroke volume as shown in the figure given below:
b.
Cylinder size and number of
cylinders:
A
smaller number of larger cylinders are preferred as the fuel economy is higher.
This is because the proportion between piston area and its circumference area
is reduced. Figure given below shows the effect of engine cylinders on the
variation of friction for the same piston displacement. It is obvious from the
figure that 4 and 6 cylinder engines are more efficient than 8 cylinder engine
as fuel economy is concerned.
c.
Compression Ratio:
With
an increase in compression ratio, IMEP increases as well as MEPF by
friction also increases but with a lower rate, so the mechanical efficiency of
the engine increases with increasing compression ratio.
d.
Engine Speed:
The
friction loss increases with an increase in speed of the engine as rubbing
between cylinder and piston occurs more times as engine speed increases. The
best method to improve mechanical efficiency with higher speed engine to
increase the number of cylinders. The variation of frictional mean effective
pressure (MEPf) varies linearly with RPM and The effect of speed on
different types of frictional losses are also shown in the figure below:
e.
Engine Load:
As
the load on the engine increases, the IMEP also increases and friction loss
also increases. However this increase in friction loss is compensated by
decrease in viscosity of the lubricating oil due to higher temperature
resulting from increases load.
f.
Cooling water temperature:
The
rise in cooling water temperature reduces the frictional loss as the viscosity
of oil at higher temperature is lower which reduces the friction loss.
The
starting friction loss is higher as the water and oil will be at the same
temperature and oil viscosity is higher. This also adds rapid engine wear. The
effect of cooling temperature on friction loss is shown in figure below:
g.
Oil Viscosity:
Higher
is oil viscosity, the higher will be the pressure losses. As the oil
temperature increases, the viscosity decreases and frictional loss also upto
certain temperature of the oil as shown in the figure below:
If
the temperature goes higher than 1000C, again friction loss
increases as local film is destroys and results in metal to metal contact.
h.
Number of Piston Rings:
The
effect of the number of piston rings on the friction is not significant as the
selection depends on the size of the engine, lightness required and material
used for rings. However, the effect is shown in the figure given below and it
is obvious that generally 3 rings provide best fuel economy.
The
friction force by the rings occurs due to ring tension and due to gas pressure
force behind the ring.
Also,
the figure given below shows the cylinder gas pressure behind the top ring.
Because of ring tension, the ring pressure against the cylinder valve results
in frictional losses. In addition to the ring tension, the gas
In this blog, we
discussed about the Effect of Engine Variables on Friction. Lubricating
Systems will be discussed in next part of this blog.
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