The Role of Cylinder Geometry on Thermo-mechanical Process in I.C. Engines-2

P M V SubbaraoProfessor

Mechanical Engineering Department

Geometry is an Universal Solution of All the Issues….

The Geometrical Description of Engine Cylinder

31

2

4,

BS

d

R

VLStroke

31

4,

BSd RV

BBore

Define Rod ratio 31

422

BSd RV

lL

l

a

lR

22 sincos12

11

RR

r

V

V

c

Instantaneous Displacement Volume

Instantaneous Engine Cylinder Volume

Identification of Events

Instantaneous compression ratio during compression

22 sincos12

11

RR

r

V

V

c

For a general thermodynamic compression process:

ConstantnpV

nV

Cp

Kinetics of Engine Assembly & Generation of Primary Dynamic Forces

dt

d

d

dp

dt

dp

60

2 N

d

dp

dt

dp

60

21 NVd

dC

dt

dpn

60

2)1(

N

d

dV

V

nC

dt

dpn

22 sincos11

2

11 RRrVV c

22 sincos11

2

1RR

d

drV

d

dVc

60

2)1(

N

d

dV

V

nC

dt

dpn

Cylinder Geometry Vs Frictional Losses

• Engine friction is affected by the bore-to-stroke ratio because of two competing effects:

• Crankshaft bearing friction and power-cylinder friction.

• As the bore-to-stroke ratio increases, the bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings.

• However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface.

The Loss of Valuable Earnings …..

Non

-dim

ensi

onal

Fric

tiona

l Los

s

Geometry Vs Engine Heat Transfer

Energy Audit of Conventional S.I. Engine Indicative Cycle at Design Conditions

• Net Indicative work per cycle : 373.2 J

• Fuel Energy Input (J): 943.0 J & Total cooling loss -187.3 J

• Heat transfer density (W/cm²) at...

• ...cylinder head: -45.168

• ... piston upper face: -42.378

• ...cylinder wall: -14.228

• Effective torque (Nm): 110.0

Instantaneous Heat Transfer (loss) form Cylinder

cg

coolantgas

hkx

h

TT

A

Qq

11

Gas to Surface Heat Transfer

• Heat transfer to walls is cyclic.

• Gas temperature Tg in the combustion chamber varies greatly over and engine cycle.

• Coolant temperature is fairly constant.

• Heat transfer from gas to walls occurs due to convection & radiation.

• Convection Heat transfer: wallgasgcconv

conv TThA

Qq

w

w

g

g

wallgasradrad

F

TT

A

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111

21

44

Radiation heat transfer between cylinder gas and combustion chamber walls is

Cylinder Surface Area at any Crank Angle

salBAAA pch

22 sincos12

RRBL

AAA pch

31

2

4,

BS

d

R

VLStroke

31

4,

BSd RV

BBore

223

12

sincos1

RR

R

VAAA

BS

dpch

Shapes of Piston Heads

Shapes of Cylinder Heads

Instantaneous Cylinder Surface Area

223

12

2 sincos14

RR

R

VKKBA

BS

dpch

223

123

2

sincos14

4

RR

R

VKK

RVA

BS

dpch

BSd

31

4,

BSd RV

BBore

The Heat Transfer Dictates !!!!!!

B/L= 1.4B/L= 1.0

B/L= 0.7

Cooling of Piston

Ability to Transfer Heat : A Signature of Survival

• Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively.

• These characteristics favor higher engine speeds, over-square engines are often tuned to develop peak torque at a relatively high speed.

• Due to the decreased piston- and head surface area, the heat loss decreases as the bore/stroke-ratio is decreased.

• These characteristics favor lower engine speeds and use of poor quality fuels with low running cost.

Geometry Vs Engine Breathing

Improper Breathing in an Engine Creates A Pumping Cycle

Cylinder Geometry Vs Breathing Issues

• The smaller bore reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number.

• These factors favor lower engine speeds, under-square engines are most often tuned to develop peak torque at relatively low speeds.

• An under-square engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel.

• An over-square engine allows for more and larger valves in the head of the cylinder.

Instantaneous Piston Speed

Effect of Rod Ratio on Piston Speed

Instantaneous Piston Acceleration

Extreme Limits of RBS

• The extremes to bore-to-stroke ratio is the inertial forces origination from the piston motion.

• To achieve high power density, the engine must operate at a high engine speed (up to 18,000 rpm for the Formula 1 engine), which leads to high inertial forces that must be limited by using a large bore-to-stroke ratio.

• For applications that demand high efficiency, a small bore-to-stroke ratio is necessary and, again because of the inertial forces of the piston, requires a slower engine speed and lower power density.

• For the marine application that has a 2.5 m stroke, the engine speed is limited to 102 rpm.

The World Largest I C Engine

Despite the green hype, internal-combustion engines will keep powering vehicles for the foreseeable future.

The Latest News

• The world’s biggest engine is the Wärtsilä-Sulzer RTA96.

• It’s the largest internal combustion engine ever built by man.

• Wärtsilä-Sulzer RTA96-C is a 14-cylinder, turbocharged diesel engine that was specially designed to power the Emma Maersk which is owned by the Danish Maersk.

• Wärtsilä-Sulzer RTA96, the world’s biggest engine, has a weight of 2.3 million kilogrammes.

• If the weight of the average adult person is 70 kgs, this world’s biggest engine has a weight equivalent to the weight of 33,000 people.

Wärtsilä-Sulzer RTA96-C

Economies of Scale in Sea Transportation

• Maersk Lines have done the world proud by providing cheap sea transportation that is costing cents instead of a dollar per every kg weight.

• They are able to do this by using economies of scale in sea transportation.

• It is getting cheaper to ship goods from USA to China and from China to USA.

• It has now become cheaper to transport goods from China to a US port than to transport the same goods from a US port to the final destination inland of US by a truck.

Cylinder Geometry Vs Thermo-chemistry

• Otto developed a cycle assuming that the fuel burns at rates which result in constant volume top dead center combustion.

• Diesel developed a cycle assuming that the fuel burns at rates which result in constant pressure combustion.

• Later it was realized that in an actual engine pressure and temperature profile data do not match these simple models.

• A more realistic modeling namely, Finite Rate Heat Release Model was innovated.

• Finite Heat Release Model is a Geometric representation of rate of combustion suitable for SI and CI engines.

• This geometric model can generate more information about the role of engine kinematics.

Real Cumulative Mass Fraction Burning Curve in an Engine

The Ultimate Effect…….

43.1LB

0.1LB

77.0LB

Need for Flexibility in Engine Kinematics

• Field level Requirements of Otto’s Cycle:

Kinematics of Unconventional Piston Movement

Optimum Cylinder Geometry• Identification of the optimum engine geometry that

provides the best opportunity to have a highly efficient internal combustion engine is the first step in designing an engine.

• In-cylinder simulations have shown that the heat transfer increases rapidly above a bore-to-stroke ratio of about 0.5.

• Engine systems simulations have shown that the pumping work increases rapidly above a bore-to-stroke ratio of about 0.45.

• Engine friction models have shown that the crankshaft bearing and power-cylinder friction values, for the most part, cancel each other out for our opposed-piston, two-stroke engine.

The Dilemma of the Classical Engine Kinematics

1. High efficiency requires high expansion ratio1. High efficiency requires high expansion ratio

2. High power density requires low compression ratio2. High power density requires low compression ratio

BUT UNFORTUNATELYBUT UNFORTUNATELY

Expansion ratioExpansion ratio == Compression ratioCompression ratio

How can we change a

“BUT UNFORTUNATELY “

into an

“AND FORTUNATELY >” ?

In 2000, a new engine is born ...

The software design...

The hardware manufacturing...

The New p-v Diagram

Six Stroke Engine

• Velozeta Six-stroke engine• German Charge pump• Crower six stroke engine • Griffin six stroke engine • Velozeta six-stroke engine• Bajulaz six stroke engine

Macro Geometrical Parameters to be selected

• Engine Cylinder Volume: V

• Bore & Stroke of the cylinder: (B/L).

• Connecting Rod length Vs Crank radius (l/a).

• Engine Compression Ratio : (Vd/Vc+1).

Cylinder Geometry Vs Frictional Losses

• Engine friction is affected by the stroke-to-bore ratio because of two competing effects:

• Crankshaft bearing friction and power-cylinder friction.

• As the bore-to-stroke ratio increases, the bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings.

• However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface.

Cylinder Geometry Vs Thermo-chemistry

• Otto developed a cycle assuming that the fuel burns at rates which result in constant volume top dead center combustion.

• Diesel developed a cycle assuming that the fuel burns at rates which result in constant pressure combustion.

• Later it was realized that in an actual engine pressure and temperature profile data do not match these simple models.

• A more realistic modeling namely, Finite Rate Heat Release Model was innovated.

• Finite Heat Release Model is a Geometric representation of rate of combustion suitable for SI and CI engines.

• This geometric model can generate more information about the role of engine kinematics.

Need for Flexibility in Engine Kinematics

• Field level Requirements of Otto’s Cycle:

The Ultimate Effect…….

Optimum Cylinder Geometry

• Identification of the optimum engine geometry that provides the best opportunity to have a highly efficient internal combustion engine is the first step in designing an engine.

• In-cylinder simulations have shown that the heat transfer increases rapidly above a bore-to-stroke ratio of about 0.5.

• Engine systems simulations have shown that the pumping work increases rapidly above a bore=to-stroke ratio of about 0.45.

• Engine friction models have shown that the crankshaft bearing and power-cylinder friction values, for the most part, cancel each other out for our opposed-piston, two-stroke engine.

Development of Comprehensive Mathematical Templates for I.C. Engines

• Modeling of IC engine process can be carried out in many ways.

• Multidimensional, Transient Flow and heat transfer Model.

• Thermodynamic Transient Model USUF.

• Fuel-air Thermodynamic Model