EDTP 75 ft
Technical Support Report for Regulatory Action
Performance and Cost Analysis of Chassis Dynamometers
by
Michael W. Leiferman
February 1976
Notice
Technical support reports for regulatory action do not necessarily
represent the final EPA decision on regulatory issues. They are inten-
ded to present a technical analysis of an issue and recommendations
resulting from the assumptions and constraints of that analysis. Agency
policy constraints or data received subsequent to the date of release
of this report may alter the recommendations reached. Readers are
cautioned to seek the latest analysis from EPA before using the infor-
mation contained herein.
Standards Development and Support Branch
Emission Control Technology Division
'Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Performance and Cost Analysis of Chassis Dynamometers
Abstract
The purpose of this report is to identify the types, makes and
models of light-duty chassis dynamometers which are presently available
for purchase. Particular attention is given to those types which can be
used for measuring exhaust emissions by the current Federal Testing
Procedure; i.e., units with adjustable inertia simulation capability and
adjustable steady-speed power absorption capability. The required
inertia range is from 1,750 to 5,500 Ibs. and the corresponding 50 mph
power range is from 7.7 to 15.8 horsepower. The dynamometers applicable
for emissions testing are then compared on the basis of performance and
cost.
Introduction
The first task in this project was to contact American marketers of
light-duty vehicle chassis dynamometers. A list of companies was made
using the "Thomas Register of American Manufacturers, 1973" and a
listing of chassis dynamometer manufacturers from the report entitled,
"Development of Specifications for a Motorcycle Dynamometer and Motor-
cycle Cooling System" by Olson Laboratories. The marketers which were
contacted in this study are listed in Table I. As shown, of the 19
companies contacted, 10 manufactured or marketed chassis dynamometers;
and of these 10 there were four whose dynamometers were capable of
simulating both speed-load relationships and vehicle inertia weight.
Consequently, four companies produce dynamometers which meet the basic
requirements of the Federal Test Procedure. These are Burke E. Porter
Machinery Company, Sun Electric Corporation, Clayton Manufacturing
Company, and Laboratory Equipment Company (Labeco).
As listed in Table I, only Clayton uses a hydrokinetic power absorp-
tion unit (PAU). The other three marketers use electric PAU's. Porter
uses regenerative direct-current (DC) absorption systems and Sun uses an
eddy current absorber. Labeco builds many different types of large roll
units. Their system which looks most attractive for emissions testing
uses an eddy current absorber. The mechanical components of this unit
are the same as the Model 100-75P mileage accumulation dynamometer. It
would require some modifications to the control system for emission
testing applications. Labeco could assembly other type systems specifi-
cally for emission testings but these would be custom machines, requiring
some basic design work. Additional information regarding the dynamometers
sold by Clayton, Porter, Sun and Labeco is listed in Table II and is
discussed below.
Discussion
A. Roll Configuration
One obvious difference among dynamometers is roll configuration and
size. These parameters have an effect on tire rolling resistance which
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is related to the amount of power loss in the tire. Ford Motor Co. has
done some recent experimentation to determine the effects of roll size
and tire type on rolling resistance (1). They conducted tests with bias
belted and radial ply tires. At 25 psi both type tires showed approxi-
mately twice the rolling resistance on the 9.5" diameter, double-roll
configuration of the Porter Model 1059 as on a flat road. When these
tires were inflated to 45 psi on the dynamometer there was a noticeable
drop in rolling resistance; but it was still substantially greater than
on a flat road at 25 psi.
Experiments have also been conducted to compare rolling resistance
on large diameter single rolls to a flat road. Ford's data of this type
indicate very little difference in rolling resistances between a 48 inch
roll and a flat road. The SAE Tire Power Consumption Task Group is pre-
sently conducting tire power consumption tests on various roll sizes.
Although testing is not complete, Calspan Corporation data showed that
tire rolling resistance on a 67-inch diameter roll was also approximately
the same as on a flat surface. Both surfaces had the same texture.
Some tests have shown that a 67" steel roll with a smooth surface gives
less rolling resistance than a flat road surface; however this could be
a surface texture effect.
Sun Electric Corporation was contacted in order to obtain data on
tire rolling resistance on their 21.6" diameter, double-roll configuration.
During the week of October 27-31, 1975, GM ran a series of tests on one
Sun dynamometer in order to compare it to a Clayton unit. One of the
parameters which was investigated was tire rolling resistance. Rolling
resistance measurements were made using a wheel torque meter with a 13
inch tire. Results of this test are shown in Figure A-l of the Appendix.
The Sun dynamometer gave approximately 15 percent less rolling resistance
than the Clayton dynamometer in the 40 to 60 mph range. At a speed of
50 mph, a rolling resistance of 10 Ib was equivalent to approximately
1.2 hp, so the observed difference in rolling resistance at 50 mph
represents a 0.4 hp difference in tire power consumption.
From the rather limited amount of testing that has been done
concerning dynamometer roll configurations effect on rolling resistance,
investigators have found that there can be a substantial difference due
to tire brands as well as basic tire construction. For example, a
certain brand bias ply tire may have very nearly the same on-road rolling
resistance as a certain brand radial ply tire; however, two other brands
of bias and radial tires may have much different on-road rolling resistances.
Dynamometer tire correction factors could be developed for different
types of tires; however, because of the variation between different
brands of tires, these correction factors should represent the mean
value for each tire type. A test program involving all brands of tires
concerned would be required to accurately predict such correction factors.
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Some data 'presented in a previous Ford Motor Co. SAE paper (2)
showed that the power consumed by a radial tire was greater than that of
a bias tire when measured on a dynamometer. The paper indicated (although
no data were given) that the reverse of this situation was observed on
the road. Personnel connected with this test work, now have some reserva-
tions about the validity of these particular data. Some of Ford's more
recent data, in reference (1), show that although a certain radial tire
did have more power consumption than a certain bias tire on dynamometer
tests, the same situation was true on a flat road. This relationship
was observed for a certain brand and size of bias and radial tire, and
it can not be generalized to the type of tire used.
B. Power Absorption Units (PAUs)
As listed in Table II there are three general types of power absorption
units used in chassis dynamometers suitable for light duty vehicle
exhaust emission testing. These are the hydrokinetic (water brake) unit
used by Clayton, eddy-current units used by Sun and Labeco and the
direct-current absorbers used by Porter. The least expensive (and least
versatile) of these units is the hydrokinetic absorber. In this unit
the mechanical energy from the rolls is used to do work on a specific
amount of water contained within the absorber. The water rises in
temperature and this heat energy is carried away by cooling water supplied
to the unit. The amount of power absorbed by a particular hydrokinetic
unit is determined by the physical configuration inside the absorber
unit. This relationship is of the form
P = AVa
where A is determined by the quantity of water in the unit and V is the
shaft velocity, "a" is dependent on construction in the unit. For
Clayton dynamometers, "a" has an average value of 2.83 and a range of
2.81 to 2.87 (3).
In addition to the power absorbed by the PAU itself there is also
an amount of power absorbed due to bearing friction and air resistance
of moving parts. This amount of power absorption is commonly called the
frictional power loss of the dynamometer, and it is the difference
between the power applied to the dynamometer rolls and the power absorbed
by the power absorption unit. In a Clayton dynamometer, friction
commonly accounts for from 2 to 4 horsepower at 50 mph. The amount of
friction is strongly dependent on the inertia setting and shows much
less dependency on the power absorber setting. This is shown in Figure
1 which presents the frictional power loss of one Clayton dynamometer as
a function of dynamometer speed. From these data it appears that the
frictional horsepower is not a linear relationship, particularly at the
higher inertia values.
Considering both the dynamometer friction and the PAU, the road-
load power absorbed by the entire Clayton chassis dynamometer is of the
form
P = KV + AV2'83
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where K will vary with dynamometer speed and vehicle inertia weight.
For the Clayton dynamometers which are now in the EPA lab, the values of
frictional horsepower at 50 mph have changed very little or none since
their installation, so the value of K at 50 mph appears to be quite
stable for a particular dynamometer. Since frictional power determin-
ation is not routinely done at other speeds, any changes which may occur
in the shape of the frictional power curve are not known.
Since the actual amount of water in the hydrokinetic units is not
measured, the unit to unit variability of A and any long-term time
change in A is not known (and is really of no practical concern since
the steady-state 50 mph hp requirement is set before each test). How-
ever, the short-term change in A is worth some comment. A vehicle's
road-load power at 50 mph is set at a steady-state condition. However,
some time is required for the water in the hydrokinetic unit to reach a
steady-state condition at any constant shaft speed. Due to this hys-
teresis effect, the load at any certain speed during a driving cycle may
be substantially different from the steady-state load, and the amount of
hysteresis is not constant from unit to unit. Data has been obtained
from an automotive manufacturer who has conducted hysteresis tests on
some of their Clayton dynamometers. These showed that some units had
almost no time lag, whereas other units had a hysteresis of up to + 1
ft-lb for accel. and decel. rates of 3.0 mph/sec (typical acceleration
in the emissions driving cycle) in the 10 to 40 mph speed range. Unit
to unit differences are believed to be mainly due.to variations in the
heat exchanger section. For a 4500 Ib vehicle at 20 mph, the torque
supplied by the dynamometer is about 17 ft-lbs. So this is a maximum
hysteresis error of + 6% in the load vs speed curve. For acceleration
rate of 1.0 mph/sec the maximum hysteresis effect dropped to about +0.7
ft-lbs torque. This is a + 4% variation in the load vs speed curve at
20 mph.
Although the hysteresis effect does account for a sizable error in
the transient load vs speed curve, it must be put in the correct per-
spective. The previous type of analysis causes the hysteresis effect to
appear more serious than it really is during actual vehicle operation.
The total dynamometer torques required (including inertia simulation)
for accel rates of 3.0 and 1.0 mph/sec are approximately .260 and 100 ft-
lbs respectively, for a 4500 Ib vehicle. Therefore, for accelerations
of 3.0 and 1.0 mph/sec, the Clayton dynamometer hysteresis effect re-
sults in a total dynamometer torque (or horsepower) error of + 0.4% and
+0.7% respectively, at 20 mph, and at higher speeds, this percentage
error decreases.
Probably the most important criteria for any dynamometer is the
accuracy to which it duplicates vehicle load as measured on the road.
The hysteresis effect for the hydrokinetic unit has already been dis-
cussed. The following discussion omits consideration of the hysteresis
affect and only steady-state values of dynamometer speed-load curve are
considered.
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Clayton states that vehicle power required to overcome wind resis-
tance is given by the relationship
P - Vb
where b varies between 2.8 and 2.9. Because of this, Clayton has
designed their PAUs so they have this same relationship between power
and speed. However, due to dynamometer and vehicle frictional losses,
this does not mean that the chassis dynamometer unit will give the
correct relationship between vehicle speed and road load power. In
fact, data supplied from GM on nine of their vehicles indicate that the
Clayton dynamometer has a noticeably different speed-power curve than
the average vehicle speed-power curve on the road. This information is
shown in Table III. When the passenger cars are set up on a dynamometer
at their true road-load hp at 50 mph, all of them are loaded lighter
than their true road-load power at speeds less than 50 mph and heavier
at speeds above 50 mph. The difference between dynamometer and true
road-load power (dyno minus road) at 20 mph ranged from -0.3 hp (-12%)
to -1.0 hp (-33%) with an average value of -0.5 hp (-18%). At 35 mph
the range was from -0.1 hp (-2%) to -1.0 hp (-14%) with an average
difference of -0.5 hp (-7%). GM also supplied data on a Chevrolet C-dO
pickup and this road-load comparison is also listed in Table III. As
shown, the dynamometer road-load power for this vehicle was 28% and 10%
higher than measured on the road at speeds of 20 mph and 35 mph, respec-
tively. These tests showed that even though a vehicle's 50 mph load may
be matched on a Clayton dynamometer, the vehicle's true speed-power
relationship may not be (and in many cases is not) accurately simulated
on the dynamometer.
Typical day to day variability of a Clayton dynamometer in regard
to steady-state road-load power at any speed is about + 3%. This includes
variability in either the manually loaded or automatically loaded 50 mph
power setting, both of which are approximately + 2%. Temperature variations
under typical stabilized conditions may account for another + 1% in day
to day operation.
Unit to unit variability in Clayton dynamometer steady-state road-
load power is due to both differences in friction and differences in
PAU's. Under constant temperature conditions, PAUs which are set up to
give the same load at 50 mph will vary by about + 1% in their loads at
any speed between 20 and 60 mph (3). Unit to unit variations in friction
makes a greater contribution than PAU variability in regards to the
variability in road-load power between dynamometers. Of the eight
light-duty dynamometers at the EPA MVEL, the 50 mph frictional power at
the 4000 Ib inertia setting ranges from 2.0 to 4.0 hp. Assuming frictional
power curves which are of similar shape to those in Figure 1, this 2 hp
difference will give variations in the total road load power of up to +
3% in the 20 to 60 mph range. Table IV summarizes the sources and
magnitudes of dynamometer variability.
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The most significant difference between hydrokinetic PAUs and
electric PAUs is the ability of the electric units to change the shape
of the load vs speed relationship. As Table II showed, the load vs
speed torque (T) equation for the DC absorbers is
T = B + DV + CV2
where B, D and C are constants which can be adjusted with potentiometers
The B, DV, and CV terms are commonly referred to as grade, friction and
windage simulation terms respectively. By adjusting the values of B, D,
and C it is possible to duplicate individual speed vs power curves
within + 3% from 15 to 60 mph. Variability among DC PAUs with similar
control systems is quite small and unit to unit variability is about +
1%.. Variations in frictional characteristics between dynamometers does
not cause a variability problem because of the ability to change shape
of the torque curve. Day to day variation in a particular DC unit
involves errors in resetting the potentiometers and the variability in
torque values for a given current input due to temperature differences.
Under typical warmed-up conditions this variability is about + 0.5%
throughout the speed-load curve.
The two largest suppliers of DC power absorption units are General
Electric and..Reliance Electric. Burke E. Porter will supply either
brand with their Model 1059 unit and the two systems are competitive in
price and performance. The type of absorber used in this dynamometer is
a DC motor which may vary in size from 30 to 60 hp. These are less
expensive than the larger torque capacity DC motor-generator sets which,
because of the slower roll speed, are necessary on the large roll dynamo-
meters. The large roll Porter (Model 1098) uses a General Electric MG
system. Regardless of which type of DC power absorber is used, the
accuracy of the system depends on the control system which is used. The
electric dynamometer accuracy information listed in Table IV is a compilation
of specifications and estimates from General Electric and data from
organizations which have purchased and/or tested such dynamometers.
Power absorbers used by the Sun and Labeco Systems are eddy current
dynamometers. The torque (T) of the Sun PAU is controlled by an equation
of the form
T = B + CV2
where B and C are adjustable values. Sun Corporation has conducted
tests to compare the road-load power curve of their unit to that of a
Clayton dynamometer. The test consisted of running a vehicle equipped
with a wheel torque meter on both a Clayton and a Sun dynamometer.
Under warmed-up conditions, wheel torque was recorded as the vehicle was
run at steady speeds of 10, 20, 30, 40, 50 and 60 mph. The data show
that the Sun unit is capable of duplicating the wheel torque measurements
obtained on the Clayton dynamometer to within + 3% throughout the speed
range. It is estimated that specific vehicle road-load curves could be
duplicated with the same degree of accuracy.
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Power absorption in the Labeco System (modified 100-75P mileage
accumulator) is done quite differently than in the other dynamometers we
have considered. The dynamometer rolls are mechanically connected to a
large centrifugal fan which directs air flow to the front of the vehicle.
Air is blown on the vehicle at the same velocity as the vehicles rear
wheels, so air flow during road driving is simulated very closely.
Because of this arrangement, most of the dynamometer load is supplied by
the fan. If some additional loading is required, the eddy current
absorber supplies it. In the cases where the fan absorbes more power
than the vehicle requires, a constant speed motor and clutch supply a motoring
torque to the rolls. The torque equation for the absorber and motor is
of the form
T = B + CVd
where B, C and D can be changed independently. This provides an added
degree of flexibility over Sun's control equation. Labeco has not
attempted to determine how closely this system can match individual
vehicles' road load vs speed curves. It is estimated that it would be
at least as accurate as the Sun eddy-current dynamometer.
In addition to the fact that electric power absorption units can be
adjusted to simulate specific load vs speed relationships, the hysteresis
effect in the road-load curve of the electric dynamometers is also less
than that in the Clayton dynamometers. For the electric units, calcul-
ations based on the system response times indicate that the maximum
hysteresis error in the emissions driving cycle is + 0.3% of the total
dynamometer power requirement.
The degree to which differences in road-load power will affect exhaust
emission and fuel economy measurements is dependent on the particular driving
cycle. Of the power transmitted to the dynamometer by a 4000 Ib inertia
class vehicle operating according to the Urban Dynamometer Driving Schedule
(UDDS), approximately 35% goes into the road-load requirement and 65% goes
into the inertia requirement. On the Highway Fuel Economy (HFE) Driving
Cycle, approximately 70% goes into the road-load requirement and 30%
into the inertia requirement. These values were calculated from the speed
vs time specifications of the driving cycles, and additional information
concerning these calculations is contained in Table A-I of the Appendix.
Some test work has been conducted to determine any effect of changes in
road-load requirement on exhaust emissions and fuel economy (4). Of the
work performed in reference (4), one vehicle was given multiple tests at
each of three different road-load power values on both the UDDS and the HFE
cycle. On both of the driving cycles there was a statistically significant
effect (at greater than the 95% confidence level) of road-load on fuel
economy. On the UDDS, the magnitude of the effect was approximately 1
percent for a 10 percent change in the 50 mph horsepower. On the HFE
cycle the effect was approximately 2.6 percent for a 10 percent change
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in the 50 mph horsepower. The only exhaust emission value which was
significantly affected (at greater than a 95% confidence level) by road-load
was NOx on the UDDS. The magnitude of this effect was about 2.4 percent
for a 10 percent change in the 50 mph horsepower requirement.
C. Inertia Simulation
As shown in Table II, there are different methods of dynamometer
inertia loading. Clayton dynamometers use a set of five flywheels which
are driven directly from the front roll shaft. Although early model
Claytons had inertia wheels which were belt driven from the drive-roll
shaft, this system is no longer available. Belt durability was a major
problem with this arrangement. Also, correlation tests have shown that
the belt-driven arrangement caused greater vehicle loading than the
direct-drive system.
The PAU in the Sun unit is a 250 hp eddy current absorber. It can
be used to simulate inertia on acceleration, but it can not motor the
vehicle a,s is required on deceleration. Therefore, for emission testing,
Sun has available a set of inertia flywheels. These flywheels are
driven at 2 1/2 times drive-roll speed by means of two cog belts from
the drive roll shaft. Since the rolls of the Sun dynamometer are about
2 1/2 times larger in diameter than those of the Clayton, the flywheel
speed of the two units is very nearly equal. Unlike other double roll
dynamometers, the front and rear rolls of the Sun unit are connected by
a cog-belt (similar to the one used to drive the flywheels). Ford has had
one of these dynamometers in operation for about two years and is
pleased with its operation. They have had no instances of belt breakage
or slippage.
The Labeco unit uses a set of seven flywheels for inertia simulation.
These flywheels are located in the drive line that connects the dyna-
mometer rolls to the centrifugal fan, and they can simulate vehicle
weight from 2,000 to 9,700 Ibs.
The standard method of inertia loading on the small roll Porter
(Model 1059) is by the DC power absorption unit. Unlike an eddy
current absorber this unit can simulate inertia both on acceleration
and deceleration. However, if mechanical inertia is desired the unit
can be supplied with a set of inertia flywheels.
The large roll Porter (Model 1098) dynamometer uses exclusively the
PAU for inertia simulation. Mechanical inertia would be rather impractical
and expensive for this unit because of the relatively low roll speed.
If inertia flywheels rotated at roll speed, the mass required would be
extremely large and/or located a large distance from the axis of rotation.
If the inertial mass were to be kept approximately the same size as
Clayton's flywheels, then the rotational speed would have to be geared
up to several times that of the rolls. This could create durability
problems.
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As the above discussion indicates, there are basically two methods,
of inertia simulation—mechanical and electrical. For dynamometers
which use a DC PAU, electrical inertia has certain advantages over
mechanical inertia. One advantage is lower cost. The added absorber
size and control system needed for the inertia simulation costs less
than the price of an inertia flywheel system. A DC dynamometer system
is also smaller in size when not equipped with flywheels; and since
electrical inertia requires no additional mechanical parts, there may be
an advantage from the standpoint of system durability.
For dynamometers which use hydrokinetic or eddy-current PAU's,
mechanical inertia has a definite cost advantage over electrical simulation.
A hydrokinetic equipped unit would require a DC absorber for electrical
inertia. An eddy current absorber could supply acceleration inertia to
the rolls, but a motor and accompanying control system would be required
to electrically simulate deceleration inertia. Both of these systems
are more expensive than a set of flywheels.
A major consideration in inertia simulation is accuracy and repeat-
ability. In these areas, mechanical inertia has a distinct advantage
over electrical simulation. The control system for electrical simulation
monitors roll speed and from this, calculates acceleration. Rate of
application of the inertia load is dependent upon the response time of
the system. Consequently, there is always some time lag between change
in roll speed and application of the correct inertia load. A typical
value of the inertia response time (to 90% of value) for a step change
in vehicle torque is 0.5 seconds. The system also has a "settling time"
which refers to the time that it takes the inertia torque to settle down
to the new steady state value. This time may vary from 1 to 5 seconds.
The greater the difference between vehicle weight and mechanical inertia
of the dynamometer, and the greater the change in vehicle torque, the
greater will be the transient inertia error. Consequently, accuracy of
transient inertia simulated by electrical means varies with vehicle size
and specific test cycle. In contrast to this, mechanical inertia simulation
involves essentially no time lag. So improvements in response of electrical
inertia simulation can only result in an accuracy which approaches the
accuracy already contained in the inertia flywheel system.
During a driving cycle there is never a true step change in vehicle
torque and most throttle position changes and acceleration rates during
the emission test cycle are moderately slow. Consequently, the transient
inertia error may have only a small effect on emission levels. Perhaps
just as important (or maybe more so) as the transient error in electrical
inertia simulation is the error in the steady-state inertia (ie., stabilized
inertia at a constant acceleration) values. As listed in Table IV, the
accuracy of steady-state electrically simulated inertia value is + 5%.
This is substantially more variability than the Clayton flywheel system
which is accurate to + 1%. Although the electrical simulation of steady-
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state inertia may be in error by as much as + 5%, the repeatability
error is only about + 1%. Therefore, only a very small increase in
emission variability would result from a change from mechanical to
electrical simulation. A somewhat larger change could occur in the
absolute level of emissions due to the + 5% range in inertia accuracy.
As in the case of an inaccurate speed-power relationship, it is difficult
to predict the effect of such inertia inaccuracies on absolute emission
levels. To do an evaluation would involve determining how the simulated
inertia differed from the true inertia throughout the driving cycle.
Perhaps a better method for determining the difference in emissions
between a flywheel system and electrical simulation would be to equip a
DC dynamometer with a set of flywheels and run a series of tests using
each type of inertia. I am not aware of any such data. Obviously, such
a test would not isolate the transient inertia error effect from the
steady-state error effect.
Another complication involved in use of electrical simulation is system
calibration and routine varification of accuracy. In the case of direct-
drive mechanical simulation, the various inertias can be calculated from
the size of the flywheels. The inertia loadings can then be verified by
simple visual inspection of which flywheels are turning. To determine
the actual inertia loading by electrical simulation would require measuring
the resistance of the dynamometer drive-roll to speed changes. To
determine the correct operation of the complete system would require
these measurements at each inertia setting (both accel and decel) and at
different speeds within each inertia setting.
In regard to system reliability, there is relatively little experience
with electrical inertia systems used for emission testing. It is not
known if this system would be more or less durable than the flywheel
system.
D. Cost
Table IV contains price information on the dynamometers. The least
expensive units are the Clayton dynamometers, which cost about $20,000.
The $8,000 site preparation and installation charge assumes that there
is already power in the room and that dewatering of the pit is not
necessary (the typical situation). It does include a service trench for
water and air, pit digging, concrete pouring and machine installation.
The complete Sun dynamometer price is about $40,000 and the site
preparation costs are somewhat higher than for a Clayton Dynamometer
because of the larger roll size.
A Burke E. Porter Model 1059 dynamometer electrical inertia
simulation would cost approximately $53,000. The DC absorber and
control system accounts for about $35,000 of the total price. This
cost varies somewhat depending on the particular power absorption unit
and control system which is used. If mechanical inertia is desired,
the total cost would be about $60,000.
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The Burke E. Porter Model 1098 and the systems built by Labeco
are generally used for mileage accumulation. However, the control
systems on these units are the same (or could be the same) as on
electric dynamometers intended for emission testing work. These large
roll units are normally equipped with large centrifugal fans. These
fans are desirable in that they simulate road air flow much better
than the fans which are presently used for emission testing. The
complete Burke E. Porter unit costs about $105,000.
The Labeco system is about $70,000 and this includes a $7,000 push
button inertia selection option. It is less expensive than the Porter
unit mainly because of the cost difference between eddy-current and DC
MG systems. The fan on the Porter unit is driven by its own DC motor.
Therefore, it would be quite easy to remove this from the system if
desired, and cost would drop to about $75,000. Since the fan in the
Labeco unit provides a part of the road load, it would be more difficult
to omit from this sytem, and the cost savings would not be as great.
The site preparation and installation cost for the two large roll units
includes $15,000 for a six foot deep pit with a sump pump and $8,000
for machine installation and wiring.
Summary and Conclusions
Tire rolling resistance is dependent on tire type, brand and size
as well as dynamometer roll configuration. Clayton dynamometer roll
configuration gives approximately twice the tire rolling resistance
as a flat road, a 67 inch diameter single roll gives about the same
rolling resistance as a flat road, and intermediate configurations
have intermediate effects.
The Clayton dynamometer gives a steeper load vs speed curve than
that exhibited by an average passenger car. This results in dynamometer
loads that are less than actual road loads at speeds under 50 mph.
At 20 mph this difference ranges from about 10% (0.3 hp) to 30% (1.0 hp).
An electric power absorption dynamometer has the ability to duplicate
load vs speed curves to within + 3%. However, to achieve this type of
accuracy would likely require on-road torque measurements on every vehicle
before each emission test. If this were done, then differences in
tire rolling resistance between the road and the dynamometer would be
automatically taken into account.
If on-road torque measurements were not made before each emissions
test, then an electrical dynamometer could be set to give a fixed load
vs speed curve shape which would best represent a majority of the
vehicles. This would give some increase in accuracy over the present
hydrokinetic unit which underloads the average passenger car at speeds
under 50 mph. This alone would not solve the problem presented by
certain types of vehicles (pickups, vans, etc.), which have considerably
more wind resistance than passenger vehicles. However, the proposed
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light duty truck regulations have made a correction for this discrepancy.
The remaining differences between true road load at 50 mph and the Federal
Register values could be minimized by taking into account individual
vehicle aerodynamics. This could be done by considering vehicle frontal
area and/or drag coefficient when determining the 50 mph horsepower
requirement.
When a predetermined power setting is used for each class of
vehicle, tire rolling resistance (power consumption) becomes of concern.
The important question is whether or not tires have the same relative
rolling resistance on the dynamometer as they do on the road. Sufficient
test data are not available to answer this question. If relative tire
behavior on the dynamometer is similar to that on the road, then there
is no need to consider differences in tire rolling resistance. The
vehicle with the "poorer" tire simply has to work harder. However, if
one tire has greater power consumption on the dyno than another tire,
and if the reverse of this is true on the road, then there are two
methods of correcting this inaccuracy. One method is to change to a
dynamometer roll configuration which simulates the road. The other is
to establish correction factors for individual brands and sizes of
tires. Experimental work is currently being done by an SAE committee
and by the EPA which should help identify the effect of roll configuration
on relative tire power consumption.
If the relative tire rolling resistance is different between the
road and the Clayton dynamometer, serious consideration should be given
to a roll configuration which accurately simulates the road. Calspan
Corporation has recently submitted to EPA an unsolicited proposal to
build and test a dynamometer with a flat tire contact surface. The
proposal includes a testing program which would compare the new unit
with a Clayton dynamometer.
In the area of inertia simulation, mechanical inertia is superior
to electrical inertia in the areas of accuracy, repeatability and ease of
calibration. Direct-drive or positive-drive flywheel system are the
recommended type.
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Recommendations
1. Direct-drive (or positive drive) mechanical inertia should
continue to be used on emission (and fuel consumption) test dyna-
mometers.
2. Present inaccuracies in the 50 mph road load horsepower settings,
as listed in the Federal Testing Procedure, could be minimized if
both vehicle weight and frontal area were taken into consideration.
3. Data should be obtained to compare relative power consumption
between the road and the Clayton dynamometer. If the current
dynamometer roll configuration behaves differently than the road in
regards to relative tire power consumption, then it could be more
accurate (and possibly less expensive on the long term basis) to
change roll configuration rather than develop tire correction
factors.
4. More data should be obtained to better define the difference
between true steady-state road-load curves and the Clayton dyna-
mometer steady-state load curve.
5. Additional data should be obtained to define the difference
between the Clayton dynamometer steady-state load curve and the
Clayton dynamometer transient load curve (i.e., define the hysteresis
effect).
-------
References
(1) W.B. Crum, "Road and Dynamometer Tire Power Dissipation," SAE
Paper 750955, October, 1975.
(2) B. Simpson, "Improving the Measurement of Chassis Dynamometer
Fuel Economy," SAE Paper 750002, February, 1975.
(3) Clayton Manufacturing Co., "Requirements and Consideration of
Test Methods and Equipment for Passenger Vehicle Fuel Economy
Measurement," August, 1975.
(4) "Variables Affecting 1975 Light Duty Truck Exhaust Emissions and
Fuel Economy", EPA contract No. 68-03-2196, Task No.l, 1976.
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Table I. Prospective Dynamometer Marketers which were Contacted
Company
Ostradyne, Inc.
Automotive Environ-
mental Systems
Go-Power Systems
AW Dynamometer, Inc.
Greening Associates,
Inc.
Hartzell Corp.
Maxwell Dynamometer
Taylor Dynamometer
and Machine Co . , Inc .
Mid-West Dynamometer
& Engineering Co.
Inductor, Inc.
Cox Instrument
Burke E. Porter
Machinery Co.
KAHN Industries
Bear Manufacturing
Corp.
Marquette
Autoscan
Sun Electric Corp.
Clayton Manufacturing
Labeco
lanufacture or Market
Light-duty vehicle
Chassis Dynamometers?
No
No
No
No
No
No
Yes
Yes
No
Yes
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Make or
Model
TAY 400
1059
1098
46-150
Bear 46-
150
8100
RAM 937
ECE-50
CTE-50
Variable
PAU (1) Speed-load
Type Simulation
DC Motor
Hydrokinetic
Disc Brake
DC Motor
DC MGU;
Prony Brake
Prony Brake
Disc Brake
Eddy Current
Hydrokinetic
Hydrokinetic
Eddy Current
Yes
Yes
No
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
Variable
Inertia
Simulation
No
No
No
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
(1) Power Absorption Unit
(2) Motor- Generator set
-------
Table II. Dynamometer Specifications on Units Capable of
Complying with Exhaust Emission Test Requirements
Marketer
Model No.
Rolls
Conf igurat ion
Diameter, In.
Centerline Dis-
tance, In.
Vehicle Tread-
width, In.
PAU
Type
Torque Control
Max Power, hp
Inertia •;
Type
Range, Lb.
Increments, Lb.
i
Clayton
ECE-50
Double
8.65
17.25
0-78
Hydrokinetic
T = AV1'83
50
Direct -Drive
Flywheels
1,750-5,500
Per FTP^ '
CTE-50
Double
8.65
20.00
20-107
Hydrokinetic
T = AV1'83
50
Direct-Drive
Flywheels
1,750-5,500
(2)
Per FTPV '
Sun Electric
RAM-937
Double (1)
21.63
21.81
40-75
Eddy Current
T = B + CV2
250
Belt-Drive
Flywheels
1,500-5,500
Per FTP^ '
Burke E. Porter
1059
Double
9.50
20.5
50-84
DC Mfctor
T = BfDV+CV2+E^
dt
30 Road load
Electrical or
Flywheels
1,500-6,000
250<3> or
per FTP '
1098 Me
Single
48
50-84
DC MG Set
T = B+DV+CV2+E^
dt
200
Electrical
1,500-6,000
Continuous
Labeco
Kiified 100-75P
Single
40.19
30-80
Eddy Current
T = B+CVd
100
Direct-Drive
Flywheels
2,000-9,750
250
(1) Front and rear roll connected by a cog belt.
(2) 1750, 2000, 2250, 2750, 3000, 3500, 4000, 4500, 5000, 5500.
(3) For electric simulation.
(4) For flywheels.
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Table III. Comparison of Clayton Dynamometer Road-Load Curve
and True Road-Load Curve when 50 mph Power is Identical
Vehicle
Corvette
Firebird
Grand Prix
Impala
Nova
Vega
Eldorado
Riviera
8-Car
Average
C-10 Pickup
Dyno Power Minus Road Power, HP
20 mph
-0.3
-0.6
-0.3
-0.4
-1.0
-0.3
-0.9
-0.5
-0.5
+0.9
35 mph
-0.1
-0.7
-0.4
-0.3
-1.0
-0.4
-0.7
-0.5
-0.5
+0.8
% Difference in Dyno and Road Power
20 mph
-12
-22
-12
-13
-33
-14
-26
-14
-18
+28
35 mph
- 2
-11
- 5
- 4
-14
- 7
- 9
- 7
- 7
+10
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Table IV. Comparison of Dynamometer Prices and Accuracies
MARKETER
MODEL
PRICES
Roller Ass. & PAU
Inertia Flywheels
Automatic Loading
Site Prep. &
Installation
Total
ACCURACY
Steady-State
Road Load
Duplication of
speed-torque
curves
Unit to Unit
Variability
Day to Day
Variability
Inertia
90% Response Time
Duplication of
Vehicle Inertia
Day to Day
Variability
CLAYTON
ECE-50
9,725
10,600
2,275
8,000
30.600
+ 30%
+ 4%
+ 3%
NegJ5
"T" -L/o
Neg.
CTE-50
11,325
10,600
2,275
8,000
32.200
+ 30%
+ 4%
+ 3%
Neg.
+ 1%
Neg.5
SUN ELECTRIC
RAM- 93 7
25,000
14,300
9,000
48.300
•+"3% —
(± 1%)
+ 0.5%
Neg.
+ 1%
Neg.
BURKE E. PORTER
1059
53,000
9,757
9,000
71,800
+3%
+ 1%
+ 0.5%
0.5 sec
+ 5%
+ 1%
1098
105, OOO1
NA
23,000
128. OOO1
+3%
+ 1%
+ 0.5%
0.5 sec
+ 5%
"• -L/o
LABECO
Modified
70, OOO2
Standard
23,000
93.000
(± 3%)4 "
(+1%)
(+ 0.5%)4
Neg.5
+ 1%
Neg.
(1) Includes large centrifugal fan which simulates on-road cooling.
fan would be reduced approximately $30,000.
(2) Includes large centrifugal fan which simulates on-road cooling.
(3) Not Available
(4) Estimated
(5) Negligible
Cost without
-------
Figure 1. Effect of Dynamometer Speed on Frictional'Power Absorption
For one Clayton Dynamometer.
Q Inertia = 4500 Ibs., 50 MPH Indicated HP = 9.0.
(TJ Inertia = 4500 Ibs., 50 MPH Indicated HP = 11.0.
A Inertia = 2000 Ibs., 50 MPH Indicated HP =9.0.
£
»-(
§ ,
o 2
.:-! i
cd
(3
A
10
20
30
40
50
Dynamometer Speed, MPH
-------
APPENDIX
-------
Table A-I. Vehicle Power Requirements for Road
Load and Inertia Simulation
Driving Cycle
Inertia Weight
Actual HP @ 50 mph
Indicated HP @ 50 mph
Fricticmal B& @' 50- mph
Average vehicle HP
into road load
Average vehicle HP
into inertia
Average vehicle HP
L
UDDS
4000
12.0
10.0
2.0
1.72
3.66
5.38
8.0
4.0.
2.00
3.66
5.66
HFE
4000
12.0
10.0
2.0
9.64
3.66
13.30
8.0
4.0
9.55
3.66
13.21
2.87
Assumption: Road Load Power = KV + AV
frictional hp.
and K = constant for a particular
-------
Figure A-l. Comparison of Tire Rolling Resistance
Between a Clayton and a Sun Dynamometer.
Rolling
Resistance,
Ib force
30 _
P155 80 D13 Tires.
Hot Inflation Pressure = 55 psi.
Dynamometer Vertical Load = 1000 Ib.
Stable Tread Temperature.
25
20
15
10
O Clayton
0 Sun RAM-937
I
_J
60
10
20 30 40
Dynamometer Speed, mph
50
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