Stirling Engine (Previous)-GROUP: Engine type
The stirling engine operates by
external combustion. The air in the hermetically sealed engine cylinder is
heated by means of a heat exchanger known as a regenerator, as opposed to fuel
being burned in the cylinder. The original Stirling engine was designed by the
Revd Stirling in 1845. Stirling hot air engines were used in some industrial
applications and domestic appliances, but have dropped out of use because steam engines have proven
to be more effective. In the twentieth century, several manufacturers worked on
the development of Stirling engines for use in motor vehicles because they
should ideally produce fewer harmful emissions and have a higher thermal
efficiency compared with conventional engines.
(Automotive Science and Mathematics, Allan Bonnick, p.
178)
Stirling Engine (New)(Better)
The Stirling
engine is an external combustion, closed, cyclic heat engine which works on the
Stirling Cycle. A typical Stirling engine consists of two zones which are
maintained at different temperatures and a working fluid is shuttled between
these regions to extract work. Unlike the conventional engines and steam
engines (an external combustion engine), there are no valves and the working
fluid never leaves the engine and is used over and over again. The working
fluids commonly used are air, hydrogen or helium.
The first step in
understanding how the Stirling engine works is to know what the Stirling cycle
is. The cycle consisting of two isochoric (constant volume) and two isothermal processes
is one of only two cycles theoretically capable of achieving Carnot efficiency.
Process 1-2:
isothermal compression and heat rejection.
The working
fluid in the cold side is compressed isothermally by keeping it in contact with
the low temperature reservoir. The work required for this stroke is supplied by
the piston, utilizing the inertia of the flywheel.
Process 2-3:
Isochoric heat addition
The working
fluid comes in contact with regenerator, which transfers heat to the working
fluid and raises its temperature to Tmax. This process also raises
the pressure and entropy of the working fluid.
Process 3-4:
Isothermal expansion and heat addition
The working
fluid comes in contact with the high temperature reservoir and expands
isothermally doing work. This is the power stroke on the engine. All the heat
added is converted to work, as it is an isothermal process.
Process
4-1: Isochoric heat rejection
The working
fluid comes in contact with the regenerator which is now at a lower temperature
(Tmin) and rejects heats at constant volume before moving to the
cold cylinder. The pressure and entropy of the working fluid is reduced.
( Vineeth C S, Stirling Engines, pages
1,7,8)
EGR (Previous)-GROUP: Exhaust system
technology
Exhaust gas recirculation (EGR) used in petrol,
gasolin and diesel engine for reduce the Nitrogen Oxide emission.
EGR works by
recirculating a portion of an engine's exhaust gas back to the engine
cylinders. In a gasoline engine, this inert exhaust displaces the amount of the
combustible matter in the cylinder. In a diesel engine, the exhaust gas
replaces some of the excess oxygen in the pre-combustion mixture. Because
Nitrogen Oxide forms primarily when a mixture of nitrogen and oxyden is
subjected to high temperature, the lower combustion chamber temperatures caused
by EGR reduced the amount of NO forms of the combustion generates.
(Chrysler and Imperial
Exhaust Emissions Referance Book, Page 8)
Exhaust Gas Recirculation (EGR) (New)(Better)
Recirculating a portion
of the exhaust gas back into the incoming air/fuel mixture is frequently used
as a technique for lowering NOx. The dilution of the incoming charge reduces
peak cycle temperature by slowing flame speed and absorbing some heat of combustion.
Charge dilution of
homogeneous-charge engines by excess air and/or by exhaust gas recirculation
(EGR) has been used for many years. The use of excess air alone results in
relatively small NOx reductions, in the order of 35-40%. When EGR is incorporated,
substantially higher NOx reductions have been demonstrated. Excessive dilution,
however, can result in increased HC emissions, drivability problems or fuel
economy losses.
Fuel consumption can be
modified when EGR is utilized. Brake specific fuel consumption and exhaust
temperature decrease with increasing EGR because dilution with EGR decreases
pumping work and heat transfer, and increases the ratio of specific heats of
the burned gases. Improvements in mixture preparation, induction systems, and ignition
systems can increase dilution tolerance. The latest technique for improving
dilution tolerance is to increase the bum rate or flame speed of the air-fuel
charge. Dilution can then be increased until the bum rate again becomes
limiting. Several techniques have been used to increase bum rate including
increased "swirl" and "squish", shorter flame paths, and
multiple ignition sources.
( Asif Faiz, Automotive air pollution: issues and
options for developing countries, pages 68,69)
Surface waviness (Previous) (Surface defect type)
Most surfaces when wiewed from sufficiently far away
will appear to have a uniform texture. On close and detailed examination using
modern techniques, however all surfaces will reveal same patterning, even if
only at the atomic level.
Surface waviness is the periodic component of the
surface texture. It arises most frequently from induced vibrations of a single
point surface generator, and when the surface profiles are Fourier analyzed it
usually occupies a spatial frequency band between those of surface form and
roughness. Optical surfaces, such as metal mirrors or infrared optics,
generated by single point diamond turning, are often influenced to same degree
of waviness. The periodic nature of such a surface results in diffraction of a
reflected or transmitted beam, giving rise and multiple images rather than a
single image when the beam is brought into a focus. A surface generated by the
more common process involving area contact between the lap and the work surface
is unlikely to suffer in thids way. Polishing by the use of a flexible lap at
speed can give rise to an orange-peel effect that can exhibit dominant spatial
frequencies when Fourier analyzed. Similar patterns are sometimes seen on
painted surfaces.
( Lionel R. Baker, Metrics for high quality specular
surfaces, 2004,page 41)
Surface waviness (New) (Better)
The surface waviness is defined as a
characteristic produced by an imperfect machine tool then it should be
distinguished from roughness. There is no doubt of its relevance in this case.
Functionally, the situation is more
complicated because the relevance of the waviness depends on the nature of the
workpiece.
Contact behavior is crucial to most
industrial applications. Here, waviness is important. Furthermore, it is
different from the roughness. The key to this is the method of generation.
Figure 2.9 shows that waviness is attached to low-energy generation whilst
roughness is determined by high-energy processes. The effects of roughness
generation on material properties underneath the surface are evident at very
small depths - 0.5|jm. This is because the maximum surface stress position is
determined by the local roughness geometry e.g. the curvature. The same is true
for waviness only at much greater depths and much smaller strain because of the
longer wavelengths. The resultant effect is shown in Figure 2.10. Waviness and
roughness have different effects and so should be separated in contact situations
(Figure 2.11). It is not necessary to separate them in non-contact functions
such as in light scatter. If in doubt, measure them separately. They can always
be reconstituted.
(David J. Whitehouse, Surfaces and their measurement,
pages 24,25)
4WD (Previous)-GROUP: Vehicle Drive Technology
Four wheel drive systems use a transfer
case to send power to two differentials and all wheels. The transfer case can
be engaged and disengaged to select two or four wheel drive as desired. It is
common on off road vehicles.
(I-Car Professional Automotive Collision
Repair, James
E. Duffy, p. 53)
4WD (New)(Better)
In a 4-wheel drive system the
drive torque is transmitted to all
four wheels (as opposed, for
example, to a front wheel drive vehicle where the
torque is transmitted only to
the two
front wheels).
The advantage of a
4-wheeI drive (4WD) system is
that longitudinal tire traction
forces are generated at all
4 wheels
to help
the forward
motion of the vehicle. This is
very helpful in situations where loss of traction
is a
problem, for example in snow, off-road
terrain and in climbing slippery
hills. Four-wheel drive systems provide
no advantage,
however, in slopping on a
slippery surface. This is
determined entirely by the brakes
and not
by the
type of drive system.
The major components that enable 4-wheel
drive operation are the differentials at the front and
rear axles and die transfer
case. The differential at the front
(or the
rear) allows the left and
right wheels to spin at different
speeds. This is necessary during
a turn
where the outer wheel moves
on a
circle of larger radius
and must
turn faster. The transfer case
routes torque from the transmission to both the front
and rear
axles. Depending on the design, the transfer case may
provide equal amounts of torque
to the
front and rear axles, or it
may proportion
torque to the front and
rear axles. The transfer case routes
torque to the front and
rear using a differential called the center
differential.
In a 4-wheel drive system, when
4-wheel drive is engaged, the
front and rear drive shafts are
locked together so that the
two axles
must spin at the same speed.
Four-wheel drive systems can be
full-time or part-time systems. In a part-lime 4-wheel drive
system, the driver can select
4-wheel or 2-wheel drive operation using
a lever
or a
switch. The driver can "shift
on the
fly" (switch between 2WD
and 4WD
while driving). This allows the
use of
2 wheel drive on regular dry
roads and 4-whcel drive on
slippery surfaces where more traction is
needed.
(Rajesh Rajamani, Vehicle Dynamics and Control, pages 224,225 and Jack Erjavec,
Automotive technology: a systems approach, page 1075)
FWD (Previous)-GROUP: Vehicle Drive Technology
A front engine, front wheel drive (FWD)
vehicle has both the engine and transaxle in the front. Drive axles extend out
from the transaxle to power the front drive wheels. This is one of the most
common configurations. The heavy drivetrain adds weight to the front drive
wheels for good traction on slippery pavement.
(Auto body repair technology, James E.
Duffy,Robert Scharff, p. 25)
FWD (New)(Better)
Concentrating
the engine and transmission system in one unit and placing the assembly at the
front of the vehicle which means front-wheel drive (FWD).
Advantages:
•
Compact
vehicle construction.
•
A
flat floor within the passenger compartment - no propeller shaft tunnel or
gearbox bulge.
•
Good
traction because the majority of weight is placed above the driving wheels.
• The
engine can be mounted transversely, which reduces the length of the bonnet and
increases the size of the passenger compartment.
• Good
steering stability - the driving thrust of the wheels is aimed in the direction
that the vehicle is intended to follow. FWD vehicles rarely suffer from the over
steer characteristics.
Although
FWD vehicles need more complicated drive shafts, the
many advantages outweigh
the disadvantages. The FWD layout is very suitable for small- to
medium-sized cars and even some large cars now make use of FWD.
(Victor Hillier,Peter
Coombes, Fundamentals of motor vehicle technology, pages 267,268)
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