Operating Characteristics Internal Combustion Engines

For an engine with bore B, crank offset a, engine speed N,

Stroke length is:

Engine Parameters – Avg piston speed

aS 2

Average piston speed is:

SNU p 2

Maximum average piston speed: 5-20 m/s

Typically inversely proportional to engine

size

Reason for this range:

1. Safe limit of material strength

(piston, connecting rod)

2. Limitation of gas flow in and out

Cf) Table 2-1: Typical engine parameters

S

r

Vd

Instantaneous Piston Speed

Distance bet’n crank axis and wrist pin

222 sincos aras

arR

RU

U

dtdsU

p

p

p

/

sin

cos1sin

2

/

22

Bore to stroke ratio B/S Square: B=S

Under square B<S

(large engine, ~4)

Over square: B>S

Instantaneous piston speed

S

r

Vd

Piston Velocity vs. Crank Angle

R: Ratio of connecting rod to crank offset For small engines: 3-4

For large engines: 5-10

Displacement

Displacement (volume):

SBV

VVV

d

TDCBDCd

24/

Its range: 0.1 cm3 – 8 L For a given displacement,

Longer stroke → smaller bore: less hear loss, more friction loss

Shorter stroke → larger bore:

More heat loss, less friction loss Modern engines: Near square

Clearance volume:

dcBDC

TDCc

VVV

VV

S

r

Vd

(caution in text)

Compression Ratio

Compression ratio

SI engines : 8-11

CI engines : 12-24

Engines w/super- or turbochargers have lower compression ratio than naturally aspirated engines.

Fig. 2-5

Variable compression ratio (see text)

cdc

TDCBDCTDCBDCc

VVV

vvVVr

/

//

S

r

Vd

Cylinder Volume and Areas

22

2

sincos112

11

4

RRrV

V

sarB

VV

c

c

c

Cylinder volume at any crank angle

Cross-sectional area (=surface area of a flat-topped piston)

2

4BAp

Combustion chamber surface area

22 sincos12

RRBS

AA

sarBAAA

pch

pch

: Cylinder head surface area

Prob. 2-1

chA

S

r

Vd

Work

Work can be calculated using (Work per cycle)

Work can also be calculated

as an area under the process

lines in the indicator diagram

Pdvw

PdVdxPAFdxW p

Volume, V

Pre

ssu

r

e, P

Indicator diagram

Indicated Work

Indicated work : Work inside the combustion

chamber

Brake work : Actual work available at the crankshaft

Work lost due to friction and parasitic loads

Parasitic loads include oil pump, supercharger, air conditioner

compressor, alternator, etc.

Relations between three types of work

fib www

iw

bw

fw

Net Indicated Work

Indicated work is classified into the following:

Gross indicated work: Compression and power strokes

Pump work: Intake and exhaust strokes

(Net) Indicated work: pumpigrossineti www

grossiw

pumpiw

Super- or Turbocharger

Without supercharges, pump work is negative.

With super- or turbochargers, pump work is positive since intake pressure is greater than exhaust pressure.

B AreaA Area netiw

B AreaA Area netiw

Supercharge increases net indicated work but add to friction work.

Mechanical Efficiency

Mechanical efficiency: Ratio between brake work at a

crankshaft to indicated work in a combustion chamber

Excluding parasitic loads, the mechanical efficiency of

an engine is:

55-60% at a high speed (friction loss)

85-95% at a low speed (heat loss)

~0 at idle

ibibm WWww //

Mean Effective Pressure

Mean effective pressure:

A good parameter for comparing engines with regard to design

or output because of independency of both engine size and

speed

Brake mean effective pressure

Indicated mean effective pressure

Pump mean effective pressure

Friction mean effective pressure

Relations of all mep’s

nmep = gmep + pmep

bmep = mimep

bmep = imep – fmep

(No difference bet’n nmep and imep?)

dVWvw //mep

vwb /bmep

vwi /imep

vwpumpi /pmep

vw f /fmep

: # revolution per cycle

Torque

A good indicator of an engine’s ability to do work

: Brake work per revolution (caution in text)

For two-stroke engines:

For four stroke engines:

Maximum torque/displacement 80-110 N-m/L

Maximum brake torque (MBT) CI > SI, generally

nVW d

R

b /(bmep)2

n

R

bW

2/(bmep) dV

4/(bmep) dV

Power

Rate of work

Brake power, net indicated power, gross indicated power, pump power can be defined depending on which work or mep used

Power relations

fib

pumpigrossineti

imb

WWW

WWW

WW

(caution in text)

ppUAn

N

nWNW

(mep))2/1(

2

/

ppUAn

N

nWNW

(mep))2/1(

2

/

Torque and Power: Discussion

Torque and power Both increases with speed at low speed, reaches a maximum, and decreases at high speed

Indicated power increases with speed

Brake power increases to a maximum and then decreases at higher speeds

Friction losses increase with speed and become dominant at very high speeds

Speed at maximum brake power ~ 1 ½ speed at maximum torque

To increase power, increase displacement, mep, speed Faster and higher mep w/super- and turbochargers

Read 2.5, Prob. 2-2, 3

Air-fuel and Fuel-air Ratios

Their definitions:

Gasoline SI engines

AF = (too rich) 6 - 15(ideal, stoich) - 25 (too lean)

Rich mixtures: when accelerating, starting cold

Lean mixtures: when cruising at light load (local rich zone)

Diesel CI engines

AF = 18-70 (nonhomogeneous air-fuel mixture)

Equivalence ratio:

Lambda value:

ff

aa

afaf

fafa

mm

mm

mmmm

mmmm

,

,

AF/1//FA

//AF

: mass (flow rate) of air

: mass (flow rate) of fuel

actstoichstoichact AF/AFFA/FA

stoichactactstoich AF/AFFA/FA/1

Specific Fuel Consumption

Definition:

Brake specific fuel consumption

Indicated specific fuel consumption

fsfc: friction specific fuel consumption

igsfc: indicated gross specific fuel consumption

insfc: indicated net specific fuel consumption

psfc: pump specific fuel consumption

Useful relations

Wm f /sfc

bf Wm /bsfc

if Wm /isfc

bsfc/isfc//// bfifibm WmWmWW

Specific Fuel Consumption

Brake specific fuel consumption Increases at high speed due to friction loss

Increases at low speed due to heat loss

Decreases at high compression ratio

Minimum near stoichiometry (Fig. 2-18)

Decreases with engine size

Fuel economy: distance traveled per unit fuel See text

Engine Efficiencies

Combustion efficiency: To account for incomplete

combustion

Thermal efficiency (enthalpy efficiency)

can be given as indicated or brake, etc:

Fuel conversion efficiency

itbtm /

HVfinHVfinc QmQQmQ //

ct

HV

HVfHVff

Qsfc

QmWQmW

/

)/(1

//

ct

HVQ

fmW ,

cHVfinint QmWQWQW ///

: Work and mass of fuel for one cycle

: Heating value of fuel

Volumetric Efficiency

Air enters the chamber less than ideal (short cycle time, flow restrictions, intake manifolds, etc)

Definition:

: air mass per cycle, mass flow rate

Standard air conditions

Air conditions in the intake manifold used sometimes

K-kJ/kg 287.0

kg/m 181.1/

C25K 298

kPa 101

3

00

0

0

R

RTP

T

P

a

Prob. 2-4

NVmnVm daadaav //

aa mm ,

Emissions

Specific emissions (SE): Units of g/kW-hr

Emission index (EI): Units of g/kg

Read 2.11-2.13

Prob. 2-5, 6

b

b

b

b

Wm

Wm

Wm

Wm

/SE

/SE

/SE

/SE

partpart

HCHC

COCO

NOxNOx

f

f

f

f

mm

mm

mm

mm

/EI

/EI

/EI

/EI

partpart

HCHC

COCO

NOxNOx

Please look at Heywood 2.1-2.5, 2.7-2.15