Correlating wind tunnel tests

From Automotive Test Report, a T&MW newsletter, August 2002

by Randall R. Cook, Development Test Engineer, and Craig D. Sundlov, Vehicle Wind Tunnel Supervisor, Valeo Engine Cooling -- Test & Measurement World, 8/10/2002 2:00:00 AM

At Valeo Engine Cooling (Jamestown, NY), we design and manufacture truck cooling systems. To certify our designs, we apply a load to a running engine in our climatic wind tunnel and measure the engine’s performance characteristics. These tests establish whether our cooling systems meet specifications under worst-case test conditions and are critical in determining whether a design is ready for production.

When we first built the climatic wind tunnel in 1990, we considered a truck with a 400-hp engine to be a high-powered vehicle. Accordingly, the dynamometer we installed could handle engines with outputs as high as 500 hp. This dynamometer is a “chassis roll” dynamometer, and the drive wheels of the test vehicle ride on a pair of 6-ft-diameter steel rolls. The rolls couple through a gearbox to a DC motor/generator that serves as the power absorber.

Today, however, big rig tractor operators must haul heavier and heavier loads, and they, in turn, are demanding vehicles with even more powerful engines. When it became apparent that our 500-hp dynamometer would not be sufficient for testing these vehicles, we added a “water brake” dynamometer to our wind tunnel that can test vehicles with engines rated at up to 1000 hp. The water brake dynamometer mounts between the frame rails of the truck and couples directly to the vehicle driveshaft—eliminating the need for rolls.

Certifying Performance

Because the water brake dynamometer configuration differs radically from the older roll dynamometer, we performed a series of tests to see how closely readings from the two dynamometers correlated. We needed to be confident that the data we collect in the wind tunnel is reliable, regardless of the dynamometer we use.

In these tests, we used our Class 8 development truck, equipped with a Cummins N-14 engine. According to the engine data curve supplied by Cummins, this engine has a peak horsepower of 460 hp at 1700 rpm; the peak torque rating is 1850 ft lbs at 1200 rpm. We used these two operating points for both the rolling chassis dynamometer test and the water brake dynamometer test. In both tests, we ran the engine at full throttle while varying the dynamometer load to control the engine rpm.

We ran the first series of tests using the chassis roll dynamometer and the second set of tests using the water brake dynamometer. After the first set of tests, we removed the truck from the tunnel until we could run the second set of tests two weeks later. We did not remove any of the instrumentation between the tests.

Three Key Parameters

While we measure many different parameters during a heavy truck wind tunnel test, the three key parameters are:

• radiator top tank temperature differential (TTD),
• intake manifold temperature differential (IMTD), and
• charger air cooler system pressure drop (CAC System DP).

To obtain TTD, we measure the temperature of the coolant in the radiator’s top tank and subtract the ambient temperature. For this test, we set the ambient temperature inside the wind tunnel to 100°F. We also calculate a related parameter, the “corrected” TTD, or TTDCorr, which takes into account how many pounds of fuel an engine burns per hour. This is an important parameter, as the fuel combustion rate determines how much heat the engine will generate.

We found that it does not matter which dyno we use for our measurements. Even though the test vehicle had been idle for two weeks between test runs, the difference between the two TTD and TTDCorr measurements was less than 1°F. Even when repeating measurement on a single dynamometer, we expect variation.

The (IMTD) is the difference between the temperature inside the intake manifold and the ambient temperature. This is a key specification, because it helps to determine how well the engine is able to meet the 2002 federal emission standards. Again our tests showed that it does not matter which dyno we use to measure this parameter—the IMTD we measured using the chassis roll dyno was identical to the IMTD measured using the water brake dyno. In both tests, IMTD was exactly 28.7°F at peak horsepower and 23.5°F at peak torque.

The CAC System DP is the difference in pressure between the turbo outlet and the intake manifold. This is an indicator of how well the Valeo charger air cooler is operating. The charger air cooler cools the air, making it denser, thereby increasing the engine’s power output. As with the first two parameters, the measurements on the two dynos were virtually identical. The CAC System DP measured on the chassis roll dyno was 3.19 inHg, and the value measured on the water brake dyno was 3.20 inHg—a difference of only 0.01 inHg.

Parasitic Loss and Other Variables

While the three key test parameters were virtually unaffected by our choice of dynamometer, one parameter was fundamentally different—the horsepower output. Using the chassis roll dyno, we measured an output power of 422.2 hp at 1700 rpm; using the water brake dyno, we measured 448.6 hp—a difference of 26.4 hp. At 1200 rpm, the difference was 30.1 hp.

We determined that this apparent horsepower difference is actually a measure of parasitic losses in the rear axle and in tire slippage. Because the water brake dynamometer couples directly to the driveshaft, and the rear axle is disconnected, its horsepower readings are not influenced by such parasitic losses. In addition, speed is measured in revolutions per minute (rpm) instead of in miles per hour (mph), as on the chassis roll dynamometer.

Because the two dynamometers produce such similar test results, we are confident that no matter which dynamometer we use for future tests, the test results will be valid, and we will be able to compare the results to earlier tests.

Table 1
TRUCK NO. 1137A					COOLING COMPONENTS:
Valeo Development Veh- Class 8 Tractor w/Sleeper Cummins N-14 Engine Cal #10265 Rating: 460hp @ 1700rpm / 1850lbft Tq @ 1200rpm Trans: Eaton Man'l Test Dates: 5/24 & 6/8/2001					I = Valeo Baseline Rad & CAC w/ base Valeo condenser. (w/ recirculation shields installed around rad & Cac) 50/50 glycol, blocked thermostats, Cup anem @ 3 ft, 20% humidity F/E Ratio 1.20:1
ROLL DYNAMOMETER					WATER BRAKE DYNAMOMETER
Cooling Components Spec.			I	I	Cooling Components Spec.		I	I
Test Type		Specs	Rated 1700	PTq1200	Test Type		Specs	Rated 1700	PTq1200
Test Run #			29	30	Test / Run No.			31	32
A/C			on	on	A/C			on	on
Engine RPM	RPM	1700/1200	1699	1201	Engine RPM	RPM	1700/1200	1702	1226
Cup Wind	MPH	30	30.5	30.3	Cup Wind	MPH	30	31.1	30.7
Ambient	DegF	100	100.1	100.2	Ambient	DegF	100	100	100.4
ATB	Deg F		119.1	107	ATB	Deg F		118.8	107.1
TTD	Deg F		92.9	105	TTD	Deg F		93.2	104.9
TTDCorr	Deg F	114/123	92	102.8	TTDCorr	Deg F	114/123	93	101.6
IMTD	Deg F	43	28.7	23.2	IMTD	Deg F	43	28.7	23.5
CAC System ?P	InHg	3.5	3.19	1.74	CAC System ?P	InHg	3.5	3.2	1.8
Roll HP	HP		-422.2	-386.9	WaterBrake DynoHP	HP		448.6	417

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