Chapter 3 Lecture.ppt

Chapter 3: Engine and Vehicle Testing

BAE 517 - Lecture 3

Dynamometers

Four essential features Means of controlling torque Means of measuring torque Means for measuring speed Means for dissipating power

Fig. 3.1: Prony Brake Dynamometer

Eddy-Current Dynamometers

Eddy-Current Dyno Theory

Eddy-current dynamometers are comprised of a notched disc (rotor) and magnetic poles (stators) around the periphery at a specified gap.

The coil which excites the magnetic pole is wound in a circumferential direction. When a current runs through exciting coil, a magnetic flux loop is formed

around the exciting coil through stators and rotor. The rotation of rotor produces density difference causing eddy-currents to flow

to stator. The electromagnetic force is opposite the direction of rotation creating a brake.

Fuel Consumption Measurement

Volume-Based Flow Measurement Mass-Based Flow Measurement Important Note: CI engines have a return

line from the injectors to the tank to handle leakage – must account for the return flow. Return fuel is hot, and may cause problems

with supply fuel temperature control. Return fuel mass flow measurement is easy –

add a second container to the mass balance for return fuel.

Fig. 3.2: Volumetric Flow Measurement

Rotameter – variable area flow meters with “float.”

Rotameters must be calibrated for fuel viscosity.

Corrections for temperature are possible.

Mass Flow Measurement

Mass balance with beaker and feed pump – a bit cumbersome.

Diesel supply of sufficient quantity for test run is suspended on load cells. Load cells signals are sampled and digitized periodically to track fuel ues.

Air Consumption Measurement

Air-consumption limits ability of engine to produce power – important measurement!

Orifice-style flow meters are used to assess air flow rates.

Pressure drops across a calibrated orifice is used to specify air-flow rate.

Caution -- pressure drop at orifice reduces air flow to engine!

Combustion Data Acquisition

HDC – head dead center (same as Top Dead Center)

Crank rotation should be measured at a resolution of 0.25o or better.

Piezoelectric pressure transducers are installed in the cylinder – require water cooling, and high impedance.

Pressure reading must be logged at 57.6 kHz (0.25o increments) for an engine operating at 2400 rpm.

P-V Diagrams

From the recorded crank position readings, cylinder

volume is calculated as,

2

sin11cos1 L

R

R

L

RA

V

RA

V

p

c

p

g

Where Vg is the gas volume (cm3), Vc is the clearnce volume (cm3), R is the crank throw radius (cm), L is the connecting rod length (cm), Ap is the area of the piston (cm2), and is the crankshaft angle measured from HDC.

Fig 3.3: Typical Data Acquisition System

Rate of Energy Release from Fuel

Instantaneous energy release from fuel can be

approximated using the following relationship,

d

dQddV

pddp

V

d

dQ w

1

Where dQ/d is the rate of energy release (J/o), is the crankshaft angle measured from HDC, is the ratio of specific heats (1.4 for air), and dQw/dis the rate of heat transfer to the cylinder walls.

This relationship has been found useful for reducing NOx emission from diesel engines by timing the injection rates.

Power Correction for Atmospheric Conditions

Power output varies with local atmospheric conditions. SAE Standard J1349 provides an method for correcting engine power to standard conditions. This approach begins with the ideal gas law. Because , mass density, is mass per unit volume,

RT

BP

V

M

Where is the air mass density (kg/m3), M is mass (kg), BP is the barometric pressure (kPa), V is volume (m3), R is the universal gas constant, and T is absolute temperature (K).

Power Correction for Atmospheric Conditions

Rearranging the previous equation, the ratio of

densities becomes,

so

os

o

s

TBP

TBP

Where s is the air mass density at standard conditions (kg/m3), o is the air mass density at observed conditions (kg/m3), BPs is the barometric pressure at standard conditions (kPa), BPo is the barometric pressure at observed conditions (kPa), Ts is absolute temperature at standard conditions (K), and To is absolute temperature at observed conditions (K).

Power Correction Continued

Increased ambient air temperature reduces air density. The engine also transfers heat to the air as it enters thereby increasing the density, and therefore the density change is not directly proportional to T-1. The power correction factor for SI

engines is,

n

s

o

o

sa T

T

BP

BPf

Where fa is the power correction factor, and n is an exponent that takes on a value of 0.5 for naturally aspirated SI engines.


Theoretically, the correction factor is applied to

indicated power, and therefore,

foboafsbs PPfPP

However, because Pfs is equal to Pfo, the previous equation can be rewritten as,

1 afboabs fPPfP


Because Pf is often much smaller than Pb, and because fa is often close to 1, the last term of the previous

equation is dropped, and the correction is applied as,

boabs PfP

The choice of standard conditions is typically BPs=99 kPa and Ts = 298 K.


The power correction for CI engines is somewhat

more complicated – for example,

boffm

bs PffPa

Where fa is defined as,

n

o

m

oa

T

BPf

298

99


From the previous slide,

ff is defined as,

f

f SG

SGf

142.0

945.0850.0

7.01

Where SG is the specific gravity of the fuel, and f is the kinematic viscosity (mm2/s) of the fuel.

Special Note: For unit injectors, ff reduces to,

SG

SGf f

850.07.01

Table 3.1: Values for m and n.


The fm exponent is as follows,

652.1

652.37036.0

2.372.0

r

qfor

r

qfor

r

qr

qfor

fm

Where q is proportional to ISFC,

eeND

FXq

670,16


From the previous slide, F is fuel consumption (kg/h), X is the stroke factor (1 for 2-cycle, 2 for 4-cycle).

The value of r is,

where pb is the turbocharger boost pressure.

turboforBP

pnaturalfor

r

o

boost1

0.1

Homework Set No. 2

Do problems 3.2, 3.4, 3.6, 3.10, 3.12, 3.14 and 3.15 at the end of Chapter 3 for next Tuesday.