CHAPTER 1- Transient
Dynamometer
1.1
Introduction:
The word transient means passing with time or state of being brief and
short lived. It is also known as lasting only for a short time; impermanent.
Transient state is defined as transient state when a process variable
has been changed and the system has not yet reached steady-state. In transient
testing there are mainly two primary variables i.e. speed and torque. When
speed or torques is measured for an every sudden change of state then it is
called as transient measurement. A transient
event is a short-lived event in the system caused by a sudden change of state.
In this short lived window of time frame a measurement of parameters such as
speed and torque are performed. Transient means changing. Transient state is an
interval of time in which the system is either "warming up" or taking
its time to respond to a disturbance.
The transient condition may occur due to:
1.
The vehicle experiencing the load changes on the road even if the throttle
is constant.
2.
The change in the fuel pump rack position (throttle) resulting in change
in speed of the engine there by affecting change in vehicle speed.
3.
Cold or hot starting.
The combination of above three conditions results in
1.
Changes in speed and load.
2.
Gear shift to cater the speed torque demand of the vehicle.
3.
Transient cycles of speed and torque.
Steady is the opposite of transient. Steady state is a condition where the
system continues with an easily predictable behavior and few values of it are
changing if any.
Transient state response is a description of how the system functions
during transient state. Steady state response is a description of how the
system functions during steady state. The difference between a steady state
simulation and marching a transient solution to steady state is that the Steady
state simulation ignores many of the higher order terms dealing with time.
These terms all go to zero in steady state so they don't affect the steady
state result. The transient simulation includes all these terms. Usually this
means the steady state model has an easier convergence as there are less terms
to model and some transient non-linearities are removed, but in a few models
these non- linearities help convergence. Transient
It is important to determine the emissions, fuel economy and durability
of modern internal combustion engines which are the main prime movers of
today’s automobiles. In many countries these tests are required to get
government certifications of the engines under regulations of Environmental
Protection Agency (EPA). These tests can be performed by following three unique
methods.
1. Running the car (automobile)
on the test track or proving grounds. These tests are most cumbersome and
rarely repeatable. In this case the vehicle under test is required to be driven
by a trained driver, whose sole aim is to carry out the assigned set of tests
with precision. The test track conditions are variable in nature and are
governed by temperature pressure and humidity. Moreover the size of the car
under test limits the mounting of various sophisticated instrumentation to
perform the acquisition and logging of the important and necessary data.
2. Another popular
method is use of Chassis Dynamometer to simulate the road load in the
laboratory or test cell. This method offers repeatability and precision of
measurement compared to test track method, an entire vehicle is required to be
engaged on chassis dynamometer. The installation of chassis dynamometer
requires substantial space and may occupy larger portion of the testing
laboratory. The testing of vehicle in the laboratory has inherent problems
associated with it such as vibrations, tire noise, clamping of the vehicle.
3. This method
employs an Alternating Current machine capable of running in all the four
quadrants and Instrumentation and control system capable of simulating road
load characteristics and drive cycle simulation accurately and precisely. This
method eliminates the extraneous variables contributed by driver and vehicle
under test. This set up is called AC dynamometer or Transient dynamometer.
The main objective of the transient dynamometer is to simulate the test
system which eliminates the need to test on the chassis dynamometer or testing
tracks. A transient works by creating a load that accurately reflects the real
world in terms of the mass a motor has to accelerate rather than a fixed weight
that might be found on chassis dynamometer. The transient load is set to mimic
exactly what an engine would see if it was on the actual race track. A transient
measures directly from the engine to the and not through any transmission,
final drives or tires and eliminates the variables that can contaminate chassis
results.
The basic premise is that the transient dynamometer can simulate all the
road loads on the engine as if the car would encounter on road. It can simulate
the going downhill events by adding the energy or can simulate uphill climbing
by adding the extra load.
The modern fast microprocessors,
governed by real-time circuit and vehicle simulation, assure realistic demands
on the engine. The modern control systems assure response times of
well under half second and required by many transient test cycles. It can load
or motor the engine to follow any desired speed and acceleration profiles for
real-world applications.
1.2
Construction of transient dynamometer
The entire base plate is supported by the pneumatic
cylinders at the four corners. The engine
and dynamometer are mounted over this base plate. Since both the engine and
dynamometer are mounted on same plate, the
amplitude of vibration is same for both engine and dynamometer. This prevents the misalignment in
connecting shaft due to vibration. The whole
system is isolated from the ground level and surrounding structures. This air cushion
system is interlocked with the test cell control system.
Figure 1.1- Transient Dynamometer
1.3
Testing on Transient dynamometer:
Transient dynamometer is used to simulate vehicle
modeling on an engine test bed.
Transient
testing is done to replicate the conditions seen by the engine when located in
a vehicle. Transient dynamometer is used
to carry out following type of testing.
1.
Simulation of shift for manual transmissions
2.
Different shift styles for example racing
shift.
3.
Drive cycle simulation
The test setup on transient dynamometer requires
information about following vehicle parameters to simulate the vehicle
conditions on engine test bed.
Vehicle Parameters:
Road Load Model
Where:
n = Velocity Exponent, variable 1 to 3
W= Weight of vehicle
1.4
Relation between Wheel torque and
Engine torque
The
primary purpose of having a transmission or gear box with multiple gears is to
match the torque and speed demand of the car while traveling in different
terrain such as flat road, inclined road or slope. In first gear, there is
plenty of acceleration because of the torque multiplication, but very little
speed which is the requirement of moving the car from steady state. In second
gear, there is slightly less acceleration, but a slightly higher speed. This
trend of higher speed and lower acceleration continues through each gear in the
transmission. The following figure shows a typical powertrain arrangement.
Figure 1.2 – Power
train of automobile.
Where:
TE =
Torque at flywheel
TT=
Torque output at gear box.
TD=
Torque output at differential
TW= Torque
at wheel
The
force on the road or the force at tire surface FW is given by
following formaula:
Let:-
Figure 1.3 -
Torque at wheel
Similarly, the
speed relation is worked out
Let:
Vehicle Speed VS
is given by following relation:
Therefore,
Power at Wheel (PW)
Let:
The
road load equation determines the force required to be simulated at wheel
considering various resistances such rolling resistance, wind resistance,
gradient. This can be established by equation 1. Based on the road load the
torque to be simulated at flywheel or at transmission output can be established
considering the transmission and axle ratios while testing the engine on
transient dynamometer.
1.5
Characteristics of transient
dynamometer
The transient
dynamometer should have following inherent characteristics to achieve transient
control and measurement.
1.
Fast response and
low inertia
Response of the transient dynamometer is directly proportional to the
inertia of rotating elements of dynamometer. The response time of typically 200
ms for torque reversal from-100% to +1005 is desirable. This is achieved by
selecting hydrostatic hydraulic dynamometers, AC and eddy current dynamometers.
2.
Motoring
capability
Motoring capability of transient dynamometer is essential to simulate
the down slop road simulation or negative torque simulation. This can easily be
achieved if dynamometer under consideration is AC or DC electric machine. In
case of eddy current dynamometer an electric motor connected in tandem can to
the function of motoring. Eddy current dynamometer in conjunction with electric
motor is also a favorable configuration of transient dynamometer. This is most
popularly known as hybrid dynamometer.
3.
Feedback control:
The dynamometer should be able to be controlled in closed loop so that
various loading models can be implemented through the software control system
1.6
Power envelope of transient dynamometer.
The power envelope of transient dynamometer is power envelope of the AC
machine used as dynamometer.
Figure
1.4 AC MACHINE POWER ENVELOPE
1.7
Transient /dynamic Test Cells
Transient
dynamometer is used to simulate the vehicle road load on engine. These test
cells can be further augmented by installing emission measurement devices to
allow the running of all dynamic emission test cycles required by certification
authorities such as EPA or other customer specific transient cycles.
1.8
Closure:
The AC transient dynamometer is a most modern versatile dynamometer. It
can be used for steady state testing of the engines or dynamic simulation of
road load conditions. The transient
dynamometer can simulate all the road loads on the engine as if the car would
encounter on the road. It can simulate the going downhill events by adding the
energy or can simulate uphill climbing by adding the extra load. Transient
dynamometer is really a boon to the development engineers as they can simulate
the vehicle conditions even before the car is built. It is possible to predict
the emissions and fuel economy of the car by simulation the conditions on engine transient test stand.
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