Improvement of Wankel engine performance at high altitudes

L. Tartakovsky, V. Baibikov, M. Veinblat

3rd Conference on UAV Propulsion Technologies

Technion, Haifa

30 January 2014

Technion – Israel Institute of Technology Faculty of Mechanical Engineering

Internal Combustion Engine Laboratory

The problem

UAV engine's power drop under high-altitude flight

conditions

One of Possible Solutions

Engine turbocharging

Main objectives

Keeping a Wankel engine’s rated brake power

as constant as possible in the altitudes range

between 0 and 15,000 feet

Possible improvement of engine’s efficiency

Selection of a turbocharger currently available

on the market for Wankel engine supercharging

Simulation approach

Application of the GT-POWER software initially

intended for modeling reciprocating piston

engines for simulating a rotary Wankel engine

Main differences between Wankel and piston

engines affecting their performance

Difference in patterns of working chamber volume and

surface change with the shaft angle

Differences in the heat transfer conditions

‘Hot’ and ‘cold’ stator zones of the Wankel working chamber

surfaces are separated contrary to a piston engine

Charge rotational movement of Wankel working chamber

Differences in combustion patterns

Unfavorable shape of Wankel working chamber - higher

surface/volume ratio

Larger relative value of crevice volumes

Development of a piston-to-Wankel

engine geometric similarity criteria

Displacement equality

Compression ratio equality

Identical behavior of working chamber volume dependence

on angle of shaft rotation

Identical behavior of working chamber surface-to-volume

ratio change vs. angle of shaft rotation

Identical behavior of intake and exhaust ports discharge

coefficients vs. angle of shaft rotation

The method

Compilation of the virtual reciprocating piston engine,

geometrically similar to the considered Wankel engine

Modifying the Wiebe equation used for simulation of the fuel

combustion by taking into account the peculiarities of the

combustion process in a Wankel engine

Application of the modified relationship between Nusselt,

Prandtl and Reynolds numbers for calculation of the heat

transfer coefficient;

Virtual blowing of the intake and exhaust ports for calculation

of their discharge coefficients.

Simulated Wankel engine

Naturally aspirated, spark ignition, single-rotor

Rated shaft speed – 8,100 rpm

Rated brake power – 70 HP

Displacement of each working chamber – 344cc

Designation Parameter Value

z Number of cylinders 3

Compression ratio 7.6

B Bore, mm 118.6

S Stroke, mm 31.2

R Crank radius, mm 15.6

L Conrod length, mm 220

Vd Displacement, cm3 1032

Parameters of the virtual piston engine

Geometric similarity

0

50

100

150

200

250

300

350

400

450

0 30 60 90 120 150 180 210 240 270 300 330 360

Rotor or 0.5*crankshaft angle, degrees

Wo

rkin

g c

ha

mb

er

vo

lum

e,

cm

3

Wankel

Piston

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 30 60 90 120 150 180 210 240 270 300 330 360

Rotor or 0.5*crankshaft angle, degreesW

ork

ing c

ham

be

r surf

ace/v

olu

me r

atio,c

m2/c

m3

Wankel

Piston

Working chamber volume Working chamber surface/volume ratio

Altitude performance of the naturally

aspirated Wankel engine

At flight altitude of 15,000 feet the engine's power drops by a factor of 1.7

70

.1

61

.6

53

.9

46

.9

40

.5

20

0.9

19

9.7

20

1.5

20

3.6

20

9.2

0

25

50

75

100

125

150

175

200

225

250

275

300

325

0 3750 7500 11250 15000

Altitude, feet

Break power, hp

SFC, g/(hp*h)

Air pressure , bar x100

Air temperature, K

Air density x 100kg/m3

Turbocharger selection

Garret GT12 and GT15 turbochargers were selected for further consideration

Turbocharger selection

The required air flow rate is marked by red line

Turbocharger Garret® GT1548 was finally

selected

Simulation model of the

turbocharged engine in the GT-Power

1 – cylinders; 2 – turbine wheel; 3 – compressor wheel; 4 – throttle; 5 – wastegate controller

Altitude performance prediction throttling the intake manifold

Sea level 7,500 feet 15,000 feet

At altitude 15,000 feet and 4500 rpm the compressor enters into the surge zone

At altitude 15,000 feet and 8100 rpm the compressor works very close to its

maximal operation speed – 180,000 rpm

Altitude performance throttling the intake manifold

192.0

190.8

190.3

69.6

69.4

71.6

118.6

134.6

180.9

31 3

8 43

18.624.6

29.7

0.0

25.0

50.0

75.0

100.0

125.0

150.0

175.0

200.0

225.0

0 3750 7500 11250 15000

Altitude, feet

BSFC, g/(hp*h)

Break power, hp

Turbocharger speed, rpm / 1000

Throttle angle, deg

Throttle discharge coefficient x100

Altitude performance prediction Damping volume in the intake manifold

1 – cylinders; 2 – turbine wheel; 3 – compressor wheel; 4 – throttle; 5 –

wastegate controller; 6 - damping volume

Dependence of the air flow swing on

the value of the damping volume

Altitude performance prediction Damping volume in the intake manifold

Sea level 7,500 feet 15,000 feet

No surge danger

Turbocharger operates far from its maximal operation speed

Altitude performance Damping volume in the intake manifold

Plenum

184 185.5 187.3

70 71.1 70.7

115.607

131.496

151.004

3645

90

0 3750 7500 11250 15000

Altitude, feet

BSFC, g/(hp*h)

Break power, hp

Turbocharger speed,rpm / 1000

Throttle angle, deg

Comparison of the engine performance with

different methods of pressure waves

suppression - Altitude 15,000 feet

Better specific fuel consumption by about 1.5%

Turbocharger operates far from its maximal operation speed

Engine power under partial loads is up to 27% higher

1 – the damping volume

2 – the throttle screening

Conclusions A modeling was carried out by using the GT-POWER software. The

method of compiling a virtual piston engine was applied

At the flight altitude of 15,000 feet Wankel engine’s power drops by a

factor 1.7

Performance predictions of the turbocharged engine confirmed a

possibility of maintaining its power at all altitudes up to 15,000 feet, but

revealed presence of a strong pressure wave process in the engine's intake

manifold

Two different methods of the compressor wheel protection from impact

of the intake air pressure waves were studied: throttling and dumping

volume use

Very important advantage of the damping volume method is a reduction

of the turbocharger's speed by about 17% under maximal power and higher

engine efficiency by about 1.5% practically under entire engine speed range

Expected increase of the supercharged engine weight may be assessed

as 6 kg.

Thank you!

More info:

tartak@technion.ac.il

Technion – Israel Institute of Technology Faculty of Mechanical Engineering

Internal Combustion Engine Laboratory