A chassis dynamometer is a test equipment fitted with rollers that can simulate real-world driving conditions. It measures the power and torque of a vehicle by reproducing the load and inertia felt during road driving. The dynamometer loads the drive wheels to simulate forces like air drag and rolling resistance according to predefined driving cycles or modes like road load simulation, speed control, and force control. Emission testing is commonly done using a chassis dynamometer to provide repeatable and precise measurements under various conditions.

1. CHASSIS DYNAMOMETER

2.  A chassis dynamometer is a test equipment fitted with rollers for
the wheels of a vehicle, that is capable of providing
drive input and measuring output such as power and torque at the
wheels.
 The chassis dynamometer reproduces the load and inertia of the
vehicle when driven on the road.
 The chassis dynamometer measures the mechanical power of the
vehicle at the drive-wheels.
 The test is conducted on a chassis dynamometer that loads the
drive wheels to simulate real-world conditions.

3. BASIC MODES OF CHASSIS DYNAMOMETER
 Road load simulation - dynamometer simulates road according to
set parameters (according to desired simulation parameters = F0,
F1, F2 or ABC parameters, simulated inertia and gradient)
 Speed control - dynamometer holds the set speed regardless of
force or other parameters. For example, if a vehicle tries to
accelerate in this mode, dynamometer applies opposite force to
maintain set constant speed.
 Force control - in this mode the dynamometer holds set force
regardless of speed or other parameters.

4. PRINCIPLE OF ROAD LOAD SIMULATION
 Because the vehicle is secured to the chassis dynamometer, it
prevents variables such as wind resistance to alter the data set.
The chassis dynamometer is designed to add the sum of all the
forces that are applied to a vehicle when driven on an actual road
course to be simulated through the tires and calculated in the test
results. Increasing air drag with the speed on the road manifests
as increasing braking force of the vehicle wheels. The aim is to
make the vehicle on the dynamometer accelerate and decelerate
the same way as on a real road. First you need to know the
parameters of the "behavior" of the vehicle on a real road.

5.  In order to get "road parameters", vehicle must be driving on
ideal flat road with no wind from any direction, gear set to neutral
and time needed to slow down without braking is measured in
certain intervals e.g. 100–90 km/h, 90–80 km/h, 80–70 km/h 70–
60 km/h etc. Slowing down from higher speed takes shorter time
mainly due to air resistance. Those parameters are later set in
dynamometer workstation, together with vehicle inertia. Vehicle
is restrained and so called vehicle adaptation has to be performed.
During vehicle adaptation dynamometer automatically slowing
down from set speed, changing its own "dyno parameters" and
trying to get same deceleration in given intervals as on real road.
Those parameters are then valid for this vehicle type. Changing of
set simulated inertia it is possible to simulate vehicle ability to
accelerate if fully loaded, with setting gradient it is possible to
simulate force if vehicle going downhill

6. Chassis Dynamometer in the Testing of Vehicle Emission
Air pollution is a risk to health and the vehicle emission is
becoming a serious cause of the air pollution. Vehicle emission is
influenced by many factors. The measurement of the vehicle
emission can be achieved by the chassis dynamometer which can
simulate the driving on the real road. The chassis dynamometer
based testing can provide a repeatable experiment and precise
emission evaluation under various driving condition.
The purpose of the chassis dynamometer testing for vehicle
emission lies in three aspects which are analyzing the emission of a
specific vehicle, assessing the emission control performance and
testing the emission of different types of fuels, respectively

7.  Vehicle emission depends on many factors such as vehicle
characteristics, traffic situation, driving style, emission control
technology, fuel specification and ambient and operation
condition . Such emission factors decide the relation between the
driven distance and the quantity of the pollutant emitted during
the interval.
 A chassis dynamometer is a device that can simulate the
resistance imposed to the wheel of a vehicle according to
different driving cycles. During the testing, the vehicle is clamped
and running on controllable rollers keeping a stationary state.
According to the predefined time curve shown on a monitor, the
representative operates the vehicle to match the curve in different
driving cycles. Then the exhaust gas will be output to an online
exhaust gas analyzer for emission measurement. Moreover, it can
be tested in a range of different temperature according to the
testing requirement.

8. ADVANTAGES OF CHASSIS DYNAMOMETER
 The chassis dynamometer provides a repeatable way for the
emission testing, and the test is relatively fast and cost saving
compared with the on-road testing. By testing the influence of
changed driving behavior on a chassis dynamometer, engineers
can optimize the road design and traffic management for a fluent
and environmentally friendly driving style [1]. Also, the testing
on chassis dynamometer is a standard process with a high
accuracy which can benefit the comparison of performance
between different vehicles. Another advantage lies in the
experiment with chassis dynamometer can involve the factor of
climate into the experiment regardless of ambient temperature
[14].

9. DISADVANTAGES OF CHASSIS DYNAMOMETER
 However, the chassis dynamometer testing also has drawbacks.
The most important disadvantage is that such a testing cannot
reflect the real world driving conditions and real-world vehicle
emission. The driving condition varies each time while driving on
the road, and with the development of the city and improvement
of living standard, it is hard to guarantee such a circumstance can
relief the traffic congestion. Also, the driving resistance exerted to
a vehicle on the chassis dynamometer is derived from the vehicle
coast-down test under different conditions which cannot precisely
reflect the resistance on the real road.

10. MASS EMISSION
ANALYZER

11.  Mass emission analyzer is a device used for measuring emissions
precisely. It simultaneously measures CO, HC, CO2 and Nox. It
uses NDIR, FID and CLD to measure the emissions.
NDIR – non-dispersive infrared for CO & CO2
FID – Flame ionization detector for HC
CLD – Chemiluminescence detector for Nox
 The emission regulations specify the type, principle of operation
used and generic construction of the exhaust gas analyzers which
can be employed for emission certification of vehicles and
engines.

12. NON DISPERSIVE INFRARED DETECTOR
 As the absorption of IR radiations is measured only in a narrow
range of wavelengths (not the entire range of wavelength of IR
radiations) which has specifically a high absorbance for the
particular gas, the technique is called as ‘Non-dispersive infra-
red'. For example carbon monoxide has a strong absorbance in the
wavelength band of 4.5-5 µm.
 The analyzer measures differential in absorption of energy from
two columns of gas; (i) the gas to be analyzed in the ‘sample cell'
and (ii) a gas of fixed composition like N2 contained in the
reference cell which is free of the gas of interest and relatively
non-absorbing in the infrared region.
 The infrared beam from a single source is usually split into two
beams of the same intensity, one each for the sample and
reference cells.

13.  The detector is divided in two compartments separated by a
flexible diaphragm; one section receives transmitted IR energy
from the sample cell and the other from the reference cell.
 The detector is filled with the gas of interest, so that the energy
transmitted to the detector is fully absorbed.
 The flexible diaphragm of the detector senses the differential
pressure between the two sections of the detector caused by the
difference in the amount of transmitted IR energy absorbed. The
deflection in the diaphragm is used to generate an electrical signal
that determines the concentration of the gaseous species of
interest.
 A rotating interrupter in the path of IR beam is put to generate AC
signal output that can be amplified.

14. NDIR instruments are seldom used for measurement of
hydrocarbons except in the garage type analyzers as the IR
absorbance to different hydrocarbons varies substantially. The
unsaturated hydrocarbons are primarily responsible for
photochemical smog but they do not have an adequate absorption in
the IR wavelength range that is specific to the saturated
hydrocarbons and vice versa. Sensitivity and response of NDIR to
the exhaust HC is typically only half of the probable true value. NO
absorbs only weekly in the infrared region. Moreover, CO, CO2and
water vapours interfere seriously; hence NDIR analyzers are also
not used for NO measurement.

15. FLAME IONIZATION DETECTOR
 Pure hydrogen-air flames are practically ion-free but on
introduction of even little amount of hydrocarbons the flame
causes considerable ionization and becomes electrically
conducting. The ionization current is proportional to the number
of carbon atoms present in the hydrocarbon molecules. Thus, FID
is effectively a carbon atom counter e.g., one molecule of propane
generates three times the response generated by one molecule of
methane. The measurement of HC by FID is expressed as parts
per million of methane i.e. as ppmC 1 i.e., ppm of hydrocarbon
containing equivalent of one carbon atom. The HC concentration
is commonly written as ppmC. HC concentration measured as
ppm propane (C3 ) is to be multiplied by a factor of 3 to convert
it to ppmC. All classes of hydrocarbons i.e., paraffin, olefins,
aromatics, etc. show practically the same response to FID.
Oxygenates, e.g. aldehydes and alcohols however, have a
somewhat lower response.

16.  FID essentially consists of a hydrogen-air burner and an ion collector assembly
as shown. Sample gas is introduced with hydrogen in the burner assembly and
the mixture is burned in a diffusion flame. An electric potential is applied
between the collector plates that makes the ionization current to flow and
generate signal proportional to HC concentration in the sample gas. This
current is amplified and the output signal is measured.
 A well-designed burner will generate ionization current that is linearly
proportion to hydrocarbon content over a dynamic range of almost 1 to 10 6 .
The commercial FID analyzers have the most sensitive range set at about 0-50
ppmC and the maximum range reaching 0-100,000 ppmC.
 Hydrogen is mixed with helium in ratio of 40:60 to decrease flame temperature
that increases flame stability. The FID analyzer is calibrated with propane or
methane mixtures in nitrogen. For the measurement of hydrocarbons in diesel
exhaust, sampling line and FID are heated to a temperature of 191± 11°C to
minimize condensation of heavy hydrocarbons present in the diesel exhaust in
the sampling system.

17. MEASUREMENT OF NON METHANE HYDROCORBONS
Presently, the emission standards are specified in terms of non-
methane hydrocarbons. Methane content of HC emissions is
determined by one of the following methods:
 Gas chromatographic (GC) method
 Non-methane cutter (NMC) method
In the GC method, sample is injected into GC column which
separates the sample into two parts:
(i) CH 4 -air-CO
(ii) (ii) NMHC–CO2 –H2O.
A molecular sieve column separates methane from air and CO
before passing it to FID. Thus methane content is measured that is
deducted from the total hydrocarbon content.

18. In the NMC method, all hydrocarbons except CH4 are oxidized
to CO2and water on a catalyst, so that when the gas sample is
passed through NMC only CH4 is detected by HFID. The NMC
cutter is calibrated for catalytic effect on CH4 and higher
hydrocarbon (ethane) mixtures in presence of water vapour with
values typical of exhaust gas at or above 600 K. The sample can be
alternatively passed through NMC or bypasses the NMC. In this
manner, the total HC and methane alone present in the exhaust gas
sample are determined.

19. CHEMILUMINESCENCE DETECTOR
When NO and ozone (O 3 ) react a small fraction (about 10% at
26.7° C) of excited NO2* molecules is produced as per the
following reactions:
NO+O3 ↔ NO2+O2
NO2 → NO2+hv
As the excited molecules of NO2 * decay to ground state, light in
the wavelength region 0.6-3.0 µm is emitted. The quantity of
excited NO2 produced is fixed at a given reaction temperature and
the intensity of light produced during decay of excited NO2 is
proportional to the concentration of NO in the sample.

20.  The sample containing NO flows to a reactor where it reacts with
ozone produced from oxygen in ‘ozonator' .In the reactor NO is
converted to NO2 .
 A photomultiplier tube detects the light emitted by the excited
NO2 . The signal is then amplified and fed to recorder or
indicating equipment.
 For the measurement of nitrogen oxides (NOx ), NO2in the
sample is first converted to NO by heating in a NO2- to-NO
converter prior to its introduction into the reactor. At 315º C,
about 90 percent of NO2 is converted to NO2. The total
concentration of NOx in the sample is thus, measured as NO.
When the sample is introduced in the reactor bypassing the NO2 -
to- NO converter, concentration of NO alone is determined. The
difference between the two measurements provides the
concentration of NO2in the sample.

21.  The response of the instrument is linear with NO concentration.
The technique is very sensitive and can detect up to 10-3 ppm of
NOx.
 The output signal is proportional to the product of sample flow
rate and NO concentration. As the method is flow sensitive an
accurate flow control is necessary. The calibration and operation
are done at the same flow rate and reactor temperature.

22. CONSTANT VOLUME SAMPLER – CVS
When emissions are to be measured from a vehicle being run on a
driving cycle sampling of the representative gas is very critical.
Constant Volume Sampling (CVS) is used in European, US and
other tests to make it possible that a representative sample of the
exhaust gas is withdrawn for measurement of the gaseous
emissions.

23.  The entire exhaust gas from the vehicle is diluted with the filtered
room air. An air to exhaust gas dilution ratio of about 10:1 is
used. The dilution with air lowers partial pressure of unburned
hydrocarbons and water, and prevents their condensation in the
sampling line.
 The diluted exhaust gas is drawn by a constant volume pump
system employing either a positive displacement pump (PDP) or
a critical flow venturi (CFV) and a blower. A PDP capacity of
about 10 to 12 m3/min of air flow provides sufficient dilution for
most passenger cars
 The volume flow rate of the diluted exhaust (exhaust gas + air) is
maintained constant throughout the test .

24.  Before the diluted exhaust gas enters the CFV or PDP, its
temperature is controlled within the ± 5 ºC of the average gas
temperature during the test by a heat exchanger.
 From the diluted gas a small sample is continuously withdrawn
and collected in evacuated Teflon bags. This process integrates
the concentration of the pollutants over the entire driving
schedule. A small part of the dilution air is sampled
simultaneously and collected in a separate bag to correct for any
background concentration of pollutant present in the dilution air
 The sample bags are analyzed after the test is completed.
 The mass of individual pollutants is determined from its
measured concentration in the sample bag, its density and the
total volume flow rate of the diluted exhaust during the test
through CVS.

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