Improving
internal combustion engine efficiency is a prime concern today. Engineers have
devised many methods like turbocharging, cam-less engines, direct fuel
injection, VIT, regenerative braking, etc. In this article we discuss the
factors limiting the efficiency of ICE and ways to increase it.
There is
a lot of concern nowadays about the efficiency of the internal combustion
engine (ICE), and a lot of research is being done to improve it. But what
exactly is the efficiency of the internal combustion engine and how do we
measure it? The efficiency of any engine is simply calculated from the energy
of the fuel supplied per unit time to do work and the output at the shaft of
the engine after subtracting all losses. The input power of the fuel can be
obtained from the mass of the fuel and its calorific value. The shaft output
can be measured from a brake dynamometer. Simply put efficiency is
Output/Input. The average ICE has an efficiency between 20 to 30%, which is
very low.
If
we see a heat balance sheet of the internal combustion engines for a spark
ignition or gasoline engine we find that the brake load efficiency is between
21 to 28%, whereas loss to cooling water is between 12 to 27%, loss to exhaust
is between 30 to 55 %, and loss due to incomplete combustion is between 0 to
45%.
Similarly
when we analyze the
heat balance sheet of a compression ignition or diesel engine we find that it
has a brake load efficiency between 29 to 42 % and loss to cooling water is
between 15 to 35 %, losses to exhaust is between 25 to 45 %, and losses due to
incomplete combustion is 0 to 5 %.
By analyzing the
two heat balance sheets we find that in Gasoline engines loss due to incomplete
combustion can be rather high. In this article we discuss the various
technologies and methods that may be employed to increase the efficiency of
Internal Combustion Engines as well as automobiles.
By analyzing the
heat balance sheet we find that the factors limiting the efficiency of an
internal combustion engine are as follows:
○ Heat losses during
cooling of engine.
○ Heat losses in
exhaust gases.
○ Friction loss
○ Transmission
efficiency losses. Losses in clutches and fluid couplings, etc.
○ Friction losses in
tires. Tire selection is a compromise between safety, stability, and
performance. A safer tire will give a minimum braking distance, good stability,
less skidding, but less fuel efficiency. A tire having less surface contact and
more of a line contact will be fuel efficient, but unsafe.
○ Losses due to
incomplete and imperfect combustion. Perfect combustion would result in the
production of carbon dioxide and water.
○ Loss due to braking
○ Losses due to
viscosity of lubricating oil.
○ Compression ratio.
The higher the compression ratio the higher the thermal efficiency. As in spark
ignition or gasoline engines the compression ratio is limited by pre-ignition
(not in compression ignition or diesel engines), the diesel engines are about
30% more efficient than gasoline engines.
○ Drag of the vehicle
○ Imperfect valve
timing
○ Losses in driving
cam shafts
○ Energy consumed by
auxiliaries like water pumps and oil pumps
The
second law of thermodynamics states it is impossible to construct an
engine which will work in a complete cycle and produce no other effect except
the raising of a weight and the cooling of a heat reservoir. Thus it there is a
limit to the thermal efficiency of heat engines.
Sadi Carnot, a French military engineer had studied the second
law and stated that, “ No heat engine working in a cycle between two constant
temperature reservoirs can be more efficient than a reversible engine working
between the same two reservoirs." Thus the maximum efficiency any heat
engine can have is by using the Carnot’s cycle (two reversible isotherms and
two reversible adiabatic). The Carnot limit is the maximum efficiency any
engine can have. To date the highest efficiency which has been obtained is 52%
in a Maritime diesel engine of 90,000 horsepower.
The
practical methods and new technology that help in increasing the efficiency of
the internal combustion engines are as follows:
○ Regenerative
braking: As braking a car or automobile wastes the kinetic energy in the form
of heat, regenerative braking is ideal method when you want to brake your
vehicle to control speed (like when going downhill). In this electromagnetic
braking is done as small motors absorb the energy and convert it into battery
energy.
○ Variable Injection
Timing: This is already used in Maritime engines. At low loads and speeds, the
injection is advanced allowing same mean effective pressure to be maintained.
This not only increases the efficiency of the engine as the scavenge pressure
is maintained, it also allows for lower quality fuel to be burnt.
○ Variable valve
timing: In this method the exhaust and inlet valves opening and closing time
can be varied, affecting the efficiency of the engine. This method can increase
the efficiency by 4 to 5%.
○ Cutting off
cylinders: In large engines in cruising or going downhill, half of the
cylinders can be cut off thus reducing fuel demand. It cannot be done on small
engines as the engine would become rough.
○ Turbochargers: A
turbocharger is an exhaust gas recovery device that increases boost air
pressure thereby optimizing combustion. It increases efficiency by 7 to 8%.
○ Direct Fuel
Injection: In previous engines, the fuel was mixed with air and injected, but
nowadays fuel is directly injected into the combustion chamber and mixing takes
place according to the profile of the combustion chamber. It increases efficiency
by 11 to 13%.
○ Twin spark plugs and
multiple injectors: As the flame front starts from the spark plug and proceeds
outward, some fuel remains unburnt as ejected before the flame front can reach
it. In a twin spark plug cylinder two flame fronts are created, causing better
combustion.
○ Using the correct
viscosity of lubricating oil, as viscous oil can result in losses due to
friction.
○ Integrated starter
and generator systems: In this system the engine is immediately stopped when
idling and started when the accelerator is pressed.