auto

Basic of

Engine Operating Characteristics

Background on IC Engines

• “An internal combustion is defined as a

heat engine in which the chemical energy

of the fuel is released inside the engine

and converted directly into mechanical

work on a rotating output shaft, as

opposed to an external combustion engine

in which a separate combustor is used to

burn the fuel.”

Internal combustion engines are so called

because the heat required to drive them is

released by oxidizing a fuel inside the engine

itself

.

This approach has advantages and

disadvantages, but is still the most popular for

transport and small power generation plant.

We will be looking at some common types of

engine, examining some ways of analysing their

performance parameters, and some of the

problems encountered in improving efficiency

and output.

Internal combustion engines include systems

which function like

"closed" systems (e.g. petrol

engines) or as "open" systems (e.g. gas turbines).

All the engines we will examine contain the

same basic activities:

• invest some work to compress a working fluid,

• inject heat into the fluid,

• recover a greater amount of work

,

• return to initial conditions by removal of some

heat

.

Typical Processes

Background on the Otto Cycle

• The Otto Cycle has four basic steps or strokes:

– F-A : An intake stroke that draws a combustible mixture of fuel and air into the cylinder – A-B : A compression stroke

with the valves closed which raises the temperature of the mixture. A spark ignites the mixture towards the end of this stroke.

– C-D : An expansion or power stroke. Resulting from

combustion.

– E-F : An Exhaust stroke the pushes the burned contents out of the cylinder.

Figure idealized representation of the Otto cycle on a PV diagram.

Crank shaft 90o TC 0o 180o BC 270o θ

Otto (SI Engine) Operating Cycle

Spark plug for SI engine Fuel injector for CI engine

Top Center (TC) Bottom Center (BC) Valves Clearance volume Cylinder wall Piston Stroke

Pressure-Volume digram of a 4-stroke SI engine One power stroke for every two crank shaft revolutions

1 atm Spark TC Cylinder volume BC Pressure Exhaust valve opens Intake valve closes Exhaust valve closes

BC L TC l V_C s a θ B

a

s

=

2

An average piston speed is:

LN U_p = 2

Compression ratio:

For an engine with bore B; crank offset a, stroke length L, turning at an engine speed of N:

Average piston speed for all engines will

normally be in the range of 5 to 15 m/sec with large diesel engines on the low end and high-performance automobile engines on the high end.

BC L TC l V_C s a θ B

(

)

The cylinder volume at any crank angle is:

) ( 4 2 s a l B V V = _c +π + −

Cylinder volume when piston at TC (s=l+a) defined as the clearance volume V_c

Maximum displacement, or swept, volume:

L B V_d 4 2

π

Engine Geometric Parameters

The distance s between crank axis and wrist pin axis is given by:

Compression ratio: c d c TC BC c V V V V V r = = +

BC L TC l V_C s a θ B

The combustion chamber surface area at any crank angle is:

) (l a s B A A A = _ch + _p +

π

The combustion chamber surface area is: The cross-sectional area of a cylinder and the surface area of a flat-topped piston are given by:

4

2

B A_p = π

For most engines B ~ L (square engine)

Engine Geometric Parameters

Cylinder volume at any crank angle can also be written in a non-dimensional form as:

( ) ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − − + − + = θ 2θ 2 sin cos 1 1 2 1 1 a l a l r V V c c ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − − + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + = π θ 2θ 2 sin cos 1 2 a l a l BS A A A _{ch p}

BC L TC l V_C s a θ B

( )

(

)

Average and instantaneous piston velocity are:

dt ds U LN U p p = = 2

Where N is the rotational speed of the crank shaft in units revolutions per second

(

)

cosθ l a θ

a

s = + −

Average piston speed for standard high performance auto engine is about 15 m/s. Ultimately limited by material strength.

Therefore engines with large strokes run at lower speeds those with small strokes run at higher speeds.

R = l/a Piston Velocity vs Crank Angle

R is the ratio of connecting rod length to crank offset and usually has values of 3 to 4 for small engines, increasing to 5 to 10 for the largest engine.

Torque is measured off the output shaft using a dynamometer. Load cell Force F Stator Rotor b N

The torque exerted by the engine is T:

J Nm b

F

T = ⋅ units: =

Torque is measured off the output shaft using a dynamometer. Load cell Force F Stator Rotor b N

The torque exerted by the engine is T:

W& Watt J s rev rev rad T N T W ⎟ = ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⋅ ⋅ = ⋅ =ω (2π ) units: ( ) & J b F T = ⋅ units:

The power delivered by the engine turning at a speed N and absorbed by the dynamometer is:

Note: ω is the shaft angular velocity in units rad/s

Torque is a measure of an engine’s ability to do work and power is the rate at which work is done

The term brake power, , is used to specify that the power is

measured at the output shaft, this is the usable power delivered by the engine to the load.

The brake power is less than the power generated by the gas in the cylinders due to mechanical friction and parasitic loads (oil pump, air conditioner compressor, etc…

The power produced in the cylinder is termed the indicated power, .

b

W&

i

W&

Indicated Work per Cycle

Given the cylinder pressure data over the operating cycle of the engine one can calculate the work done by the gas on the piston.

This data is typically given as P vs V diagram.

The indicated work per cycle is given by W_i = PdV_∫

Compression W<0 Power W>0 Intake W>0 Exhaust W<0 W_A > 0 W_B < 0

Gross indicated work per cycle – net work delivered to the piston over

the compression and expansion strokes only:

W_i,g =area A + area C (>0)

Pump work – net work delivered to the gas over the intake and exhaust

strokes:

W_p =area B + area C (<0)

Net indicated work per cycle – work delivered over all strokes:

W_i,n = W_i,g – W_p = (area A + area C) – (area B – area C) = area A – area B

Indicated power:

where N – crankshaft speed in rev/s

n_R – number of crank revolutions per cycle = 2 for 4-stroke

= 1 for 2-stroke

Power can be increased by increasing: • the engine size, V_d

• compression ratio, r_c • engine speed, N cycle rev s rev cycle kJ n N W W R i i ) )( ( = & Indicated Power

Indicated Work at Part Throttle

At WOT the pressure at the intake valve is just below atmospheric pressure, However at part throttle the pressure is much lower than atmospheric

Therefore at part throttle the pump work (area B+C) can be significant compared to gross indicated work (area A+C)

Indicated Work with Supercharging

Engines with superchargers or turbochargers can have intake pressures greater than the exhaust pressure, giving a positive pump work

W_i,n = area A + area B

Supercharges increase the net indicated work but is a parasitic load since they are driven by the crankshaft

Mechanical Efficiency

Some of the power generated in the cylinder is used to overcome engine friction and to pump gas into and out of the engine.

The term friction power, , is used to collectively describe these power losses, such that:

g i f g i b m W W W W , , 1 & & & & − = =

η

Friction power can be measured by motoring the engine.

The mechanical efficiency is defined as:

b g

i f

• Mechanical efficiency depends on throttle position, engine design and engine speed.

• Typical values for car engines at WOT are:

90% @2000 RPM and 75% @ max speed.

• Throttling increases pumping work and thus decreases the brake power so the mechanical efficiency drops and approaches zero at idle.

• Power varies with speed but torque is “independent” of engine speed

cycle cycle W N T T W W N W ∝ ⋅ and ∝ ⋅ so ∝

recall & &

There is a maximum in the brake power versus engine speed called the rated

brake power (RBP).

At higher speeds brake power decreases as friction power becomes significant compared to the indicated power

There is a maximum in the torque versus speed called maximum brake torque (MBT). Brake torque drops off:

• at lower speeds do to heat losses

• at higher speeds it becomes more difficult to ingest a full charge of air.

cycle cycle T W W N W ∝ ⋅ ∝ & f g i b W W

W& = & _, − & Max brake torque

1 kW = 1.341 hp

Rated brake power

Indicated Mean Effective Pressure (IMEP)

imep is a fictitious constant pressure that would produce the same work per cycle if it acted on the piston during the power stroke.

R p p R d i d R i d i n U A imep n N V imep W N V n W V W imep ⋅ ⋅ ⋅ = ⋅ ⋅ = → ⋅ ⋅ = = 2 & & T W T ∝ _cycle so imep ∝ recall

imep does not depend on engine speed, just like torque

imep is a better parameter than torque to compare engines for design and output because it is independent of engine speed, N, and engine size, V_d.

Brake mean effective pressure (bmep) is defined as:

R d d R d b n V bmep T V n T V W bmep ⋅ ⋅ = → ⋅ ⋅ = =

π

π

The maximum bmep of good engine designs is well established:

Four stroke engines:

SI engines: 800-1000 kPa* CI engines: 500 -900 kPa

Turbocharged SI engines: 1200 -1700 kPa Turbocharged CI engines: 1000 - 1400 kPa

Two stroke engines:

Standard CI engines comparable bmep to four stroke

Large slow CI engines: 500 - 1600 kPa (with supercharging)

*Values are at maximum brake torque at WOT

Note, at the rated (maximum) brake power the bmep is 10 - 15% less

Can use above maximum bmep in design calculations to estimate engine displacement required to provide a given torque or power at a specified speed.

Maximum BMEP

• The maximum bmep is obtained at WOT at a particular engine speed

• Closing the throttle decreases the bmep

• For a given displacement, a higher maximum bmep means more torque

• For a given torque, a higher maximum bmep means smaller engine

• Higher maximum bmep means higher stresses and temperatures in the engine hence shorter engine life, or bulkier engine.

• For the same bmep 2-strokes have almost twice the power of 4-stroke

2 d R d b V n T V W bmep = = π ⋅ ⋅

Vehicle Engine type Displ. (L) Max Power (HP@rpm) Max Torque (lb-ft@rpm) BMEP at Max BT (bar) BMEP at Rated BP (bar) Mazda Protégé LX L4 1.839 122@6000 117@4000 10.8 9.9 Honda Accord EX L4 2.254 150@5700 152@4900 11.4 10.4 Mazda Millenia S L4 Turbo 2.255 210@5300 210@3500 15.9 15.7 BMW 328i L6 2.793 190@5300 206@3950 12.6 11.5 Ferrari F355 GTS V8 3.496 375@8250 268@6000 13.1 11.6 Ferrari 456 GT V12 5.474 436@6250 398@4500 12.4 11.4 Lamborghini Diablo VT V12 5.707 492@7000 427@5200 12.7 11.0

Road-Load Power

• A part-load power level useful for testing car engines is the power required to drive a vehicle on a level road at a steady speed.

• The road-load power, P_r, is the engine power needed to overcome rolling resistance and the aerodynamic drag of the vehicle.

v v v D a v R r C M g C A S S P = ( + 1₂

ρ

Where C_R = coefficient of rolling resistance (0.012 - 0.015) M_v = mass of vehicle

g = gravitational acceleration

ρ_a = ambient air density

C_D = drag coefficient (for cars: 0.3 - 0.5) A_v = frontal area of the vehicle

Specific Fuel Consumption

• For transportation vehicles fuel economy is generally given as mpg, or L/100 km.

• In engine testing the fuel consumption is measured in terms of the fuel mass flow rate .

• The specific fuel consumption, sfc, is a measure of how efficiently the fuel supplied to the engine is used to produce power,

f m& b f W m bsfc & & = hr kW g W m isfc i f ⋅ = units: & &

• Clearly a low value for sfc is desirable since for a given power level less fuel is consumed

Brake Specific Fuel Consumption vs Engine Size

•Bsfc decreases with engine size due to reduced heat losses from gas to cylinder wall. r L r rL volume cylinder area surface cylinder 2 1 2 ∝ =

π

π

Brake Specific Fuel Consumption vs Engine Speed

• At high speeds the bsfc increases due to increased friction i.e. smaller

• At lower speeds the bsfc increases due to increased time for heat losses from the gas to the cylinder and piston wall, and thus a smaller

• Bsfc increases with compression ratio due to higher thermal efficiency

b W&

i W&

Performance Maps

Performance map is used to display the bsfc over the engines full load and speed range. Using a dynamometer to measure the torque and fuel mass flow rate you can calculate:

d R V n T bmep = 2

π

π

Constant bsfc contours from a two-liter four cylinder SI engine

Engine Thermodynamic Efficiencies

While bsfc is commonly used because it is a fairly direct

measurement, it is also possible to work out the

engine's thermodynamic efficiency if you know the

heating value of the fuel.

Typical hydrocarbon fuel heating values are:

Fuel Heating

Value

(lower heating value, fuel is liquid if that is its normal state at STP)

Methane 50

MJ/kg

LPG 46

MJ/kg

Gasoline 44.5

MJ/kg

Diesel 43

MJ/kg

Engine Efficiencies

• The time for combustion in the cylinder is very short so not all the fuel may be consumed or local temperatures may no favour combustion

• A small fraction of the fuel may not react and exits with the exhaust gas

• The combustion efficiency is defined as:

HV f in HV f in c Q m Q Q m Q ut l heat inp theoretica t input actual hea ⋅ = ⋅ = = & &

η

Where Q_in = heat added by combustion per cycle m_f = mass of fuel added to cylinder per cycle

Engine Efficiencies (2)

• The thermal efficiency is defined as:

HV f c in th Q m W Q W ⋅ ⋅ = = =

η

η

η

η

• Thermal efficiencies can be given in terms of brake or indicated values

• Indicated thermal efficiencies are typically 50% to 60% and brake thermal efficiencies are usually about 30%

Engine Efficiencies (3)

• Fuel conversion efficiency is defined as:

HV f HV f f Q m W Q m W ⋅ = ⋅ = & &

η

Note:

η

η

η

Recall:

Therefore, the fuel conversion efficiency can also be obtained from: W m sfc f & & = HV f Q sfc ⋅ = ) ( 1

η

Volumetric Efficiency

• Due to the short cycle time and flow restrictions less than ideal amount of air enters the cylinder.

• The effectiveness of an engine to induct air into the cylinders is measured by the volumetric efficiency:

N V m n V m d a a R d a a v _{⋅ ⋅} ⋅ = ⋅ = =

ρ

ρ

η

where ρ_a is the density of air at atmospheric conditions P_o, T_o and for an ideal gas

ρ

ρ_a= 1.181 kg/m3₎

• Typical values for WOT are in the range 75%-90%, and lower when the throttle is closed.

• If an engine is throttled, the volumetric efficiency will be much less than 1, (eg 25-30%), and

• If it is running at full torque, volumetric efficiency can be about 1.

Volumetric Efficiency

Volumetric efficiency is used in two ways.

•

Some engineers want to measure the tuning effectiveness of the

intake manifold and valve system . They use volumetric efficiency as

their indicator. For this purpose, the "i" conditions would refer to the

density at intake manifold temperature and pressure. The ideal

volumetric efficiency would be around 1 (ie 100%). Actual ηV would

be reduced by flow losses at the valve but could also be increased

by pulsation tuning.

•

The more common use of volumetric efficiency is to indicate how

much mixture is flowing through the engine, (without worrying

whether it ought to be 100%). For this purpose, the calculation is

usually done including fuel/air mixture and with the reference density

set at ambient atmospheric conditions.

Air-Fuel Ratio

• For combustion to take place the proper relative amounts of air and fuel must be present in the cylinder.

The air-fuel ratio is defined as

f a f a m m m m AF & & = =

• The ideal AF is about 15:1, with combustion possible in the range of 6 to 19.

• For a SI engine the AF is in the range of 12 to 18 depending on the operating conditions.

• For a CI engine, where the mixture is highly non-homogeneous, the AF is in the range of 18 to 70.

Engines Comparison

Engine performance can be compared by the following

parameters:

• Mean effective pressure

• Brake specific fuel consumption

• Engine efficiency

• Volumetric efficiency

• First law analysis – energy conservation

Engines Comparison

• mep= work done per unit displacement volume

– Or average pressure that results in the same amount

of indicated or brake work produced by the engine

– Scales out effect of engine size

– Two useful types: imep and bmep

• imep: indicated mean effective pressure

– the net work per unit displacement volume done by the gas during compression and expansion

• bmep: brake mean effective pressure

– the external shaft work per unit volume done by the engine

BMEP

• Based on torque:

V

bmep

=

4

⋅

π

⋅

τ

V

bmep

=

2

⋅

π

⋅

τ

• Brake specific fuel consumption (bsfc)

– Measure of engine efficiency

– They are in fact inversely related, so a lower bsfc

means a better engine

– Often used over thermal efficiency because an

accepted universal definition of thermal efficiency

does not exist

N

f

m

b

W

f

m

bsfc

⋅

⋅

⋅

=

=

τ

π

2

&

&

&

Engines Comparison

bsfc

• bsfc is the fuel flow rate divided by the brake power

• We can also derive the brake thermal efficiency if we give

an energy to the fuel called heat of combustion or, q

N

f

m

b

W

f

m

bsfc

⋅

⋅

⋅

=

=

τ

π

2

&

&

&

qc

bsfc

qc

f

m

b

W

⋅

=

⋅

=

1

&

&

η

• Volumetric Efficiency, e

– The mass of fuel and air inducted into the cylinder

divided by the mass that would occupy the displaced

volume at the density ρ

in the intake manifold

– Note it’s a mass ratio and for a 4 stroke engine

– For a direct injection engine

N

V

m

m

e

ρ

)

(

2

_&

_&

=

0

=

f

m

&

Engines Comparison

• First law analysis- energy conservation

– For a system open to the transfer of enthalpy, mass,

work, and heat, the net energy crossing the control

surface is stored into or depleted from the control

volume

• Second Law Analysis – entropy conservation

– This approach takes into account the irreversibility

that occurs in each process

– Another outcome of this analysis is the development

of the usefulness of each type of energy (exergy)