1

Supplement 1Engine Timing and Engine Mapping

The next four slides review the 4-stroke SI cycle. Note that Slide 3 provides a PV diagram that shows when valves operate and the spark occurs. It is valuable to show these actions on a diagram that uses crank position and angle as the x-axis. Such charts are shown in Slides 4 and 5. Students should be able to duplicate the diagrams given in Slides 4 and 5 and provide the following labels:

IVO Valve OverlapEVC IntakeIVC CompressionSpark ExpansionEVO Combustion

2

Four-stroke Spark Ignition (SI) Engine

Stroke 1: Fuel-air mixture introduced into cylinder through intake valve

Stroke 2: Fuel-air mixture compressedStroke 3: Combustion (roughly constant volume) occurs and

product gases expand doing workStroke 4: Product gases pushed out of the cylinder through the

exhaust valve

CompressionStroke

PowerStroke

ExhaustStroke

A I R

CombustionProducts

Ignition

IntakeStroke

FUEL

Fuel/AirMixture

3

Pressure-Volume Graph 4-stroke SI engine

One power stroke for every two crank shaft revolutions

1 atm

Spark

TC

Cylinder volume

BC

Pressure

Exhaust valveopens

Intake valvecloses

Exhaust valvecloses

Intake valveopens

4

IVO - intake valve opens, IVC – intake valve closesEVO – exhaust valve opens, EVC – exhaust valve opensXb – burned gas mole fraction

Motored Four-Stroke Engine

10

Pressure (bar)100

Intake Exhaust

TC

BC

5

IVO - intake valve opens, IVC – intake valve closesEVO – exhaust valve opens, EVC – exhaust valve opensXb – burned gas mole fraction

Four-Stroke SI Engine

Valve overlap 10

Pressure (bar)100

Intake Exhaust

6

Timing Charts for CI Engines

Slide 8 provides charts showing cylinder volume, fuel mass flow rate, cylinder pressure and fuel mass burn rate as a function of crank position and angle. Students should be able to label such a diagram with the following:

IVC EVOSOI EOISOC EOC

7

CompressionStroke

PowerStroke

ExhaustStroke

A I R

CombustionProducts

IntakeStroke

Air

Fuel Injector

Four stroke Compression Ignition (CI) Engine

Stroke 1: Air is introduced into cylinder through intake valve Stroke 2: Air is compressedStroke 3: Combustion (roughly constant pressure) occurs and

product gases expand doing workStroke 4: Product gases pushed out of the cylinder through the

exhaust valve

8

SOI – start of injection EOI – end of injectionSOC – start of combustionEOC – end of combustion

Four-Stroke CI Engine

Fuel mass flow rate

Fuel mass burn rate

Cylindervolume

Cylinderpressure

9

Two-Stroke Diagrams

• Students should review the following diagrams for two-stroke cycle.

• Students should understand the concepts, but they will not be required to duplicate these diagrams in a quiz or exam.

10

Modern Two-Stroke Spark Ignition Engine

Stroke 1: Fuel-air mixture is introduced into the cylinder and is then compressed, combustion initiated at the end of the stroke

Stroke 2: Combustion products expand doing work and then exhausted

* Power delivered to the crankshaft on every revolution

11

Two Stroke Spark Ignition Engine

Intake (“Scavenging”)

Compression Ignition

ExhaustExpansion

Fuel-air-oilmixture

Fuel-air-oilmixture

Crankshaft

Reedvalve

ExhaustPort*

TransferPort*

*No valves and thus no camshaft

12

EPO – exhaust port open EPC – exhaust port closedIPO – intake port openIPC – intake port closed

Two-Stroke CI Engine

scavenging

Ai

Ae

Intake area (Ai)

Exhaust area (Ae)

PiPe

Exhaust Press (Pe)

Intake Press (Pi)

Cylinder Press (P)

110 CA

13

Cross Loop Uniflow

Scavenging in Two-Stroke Engine

14

Advantages of the two stroke engine:

• Power to weight ratio is higher than the four stroke engine since there is one power stroke per crank shaft revolution. • No valves or camshaft, just ports

Most often used for low cost, small engine applications such as lawn mowers, marine outboard engines, motorcycles….

Disadvantages of the two-stroke engine:

• Incomplete scavenging – limits power• Fuel-air short circuiting – low fuel efficiency, high HC emission• Burns oil mixed in with the fuel – high HC emission

15

Single Cylinder Engine

Single-cylinder engine gives one power stroke per crank revolution (360 CA) for 2 stroke, or every two revolutions for 4 stroke.

The torque pulses on the crank shaft are widely spaced, and engine vibration and smoothness are significant problems.

Single cylinder engine used in applications where engine size is more important

180 CA0 CA(TC)

720 CA (TC)

540 CA360 CA (TC)

180 CA

4-stroke

2-stroke

16

Engine Mapping and Applications

• Slides 17-24 review how engines are mapped with the use of a dynamometer. To create a map of torque, power and SFC as a function of speed, the engine is operated at various throttle settings. At each throttle setting, the load is varied to obtain a desired speed. Dynamometer force and fuel flow rate are recorded.

17

Measuring Engine Performance

Engine Load

Speed, Torque

Throttle

Fuel

Power

Torque

Speed

WOT

BMEPOrTorque

Speed

ConstantEfficiencyLines

18

Torque and PowerTorque is measured off the output shaft using a dynamometer.

Load cell

Force FStatorRotor

b

N

The torque exerted by the engine is T:

JNmbFT :units

19

Torque and PowerTorque is measured off the output shaft using a dynamometer.

Load cell

Force FStator

Rotor

b

N

The torque exerted by the engine is T:

W

) 341.1k 1( )( :units )2( hpWWJs

rev

rev

radTNTW

JbFT :units

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

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

20

Mean Effective Pressure (MEP)

Brake mean effective pressure (bmep) is defined as:

R

d

d

R

d

b

n

VbmepT

V

nT

V

Wbmep

2

2

21

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

nT

V

Wbmep

22

Brake Specific Fuel Consumption Versus 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 heatlosses from the gas to the cylinder and piston wall, and thus a smaller

• Bsfc decreases with compression ratio due to higher thermal efficiency

bW

iW

• There is a minimum in the bsfc versus engine speed curve

23

Performance MapsPerformance 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 for different throttle positions you can calculate:

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

bmep@WOT

d

R

V

nTbmep

2

b

f

W

mbsfc

)2( TNWb

24

Performance MapsPerformance 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 for different throttle positions you can calculate:

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

bmep@WOT

d

R

V

nTbmep

2

b

f

W

mbsfc

)2( TNWb

25

Application

Requirements

Design

Models

ENGINECONCEPT

Testing

Validation

Specifications

Matching

Product

R&D• Concepts• Systems• Components• Fuels• Lubricants

How IC Engine Activities Are RelatedOverall Model

The activities of engine design, modeling, engine design testing, and engine validation testing are related as shown in the chart above. Note that the goal of an engine design is for the resulting engine product specification to match the engine requirements based on the application.

26

Designing an Engine Given the Map of an Individual Cylinder and the

Requirements for a Specific Application

• This example is taken from a book on designing large diesel engines for ship propulsion applications.

• Large ship engines are made up of a given number of standard cylinders. Slide 28 is a map for a standard cylinder.

• The designer must decide how many cylinders to use in the specific engine to best meet the requirements of a specific ship.

27

Explanation of Slide 28

• The map has engine speed on the x-axis.• Torque is on the left y-axis. BMEP is on the right y-axis.

Remember BMEP is a function of torque and displacement volume.

• The heavy lines are islands of constant SFC.• The dotted lines show power as a function of engine

speed. The power in kW is given as one of the y-axes on the right.

• The green line is the WOT torque vs speed line. • The student should think through how this chart was

made from engine testing data.

28

Example Engine Map

Source: Mechanical Prime Movers, Macmillan Press, 1971, pg 56-59

• Torque vs. Speed for constant bsfc lines• Includes engine driven lube pump• Add 0.0020 kg/kWh for driven water pump• Heating Value is 42.9 MJ/kg or 18,400 Btu/lbm• XX line is limiting performance

• Turbocharger• Fuel Injection Equipment• Governor• Combustion and Scaven- ging Characteristics

• SS line limits light loading• Avoid for long periods• Engine Deposits

29

Explanation of Slide 30

• Slide 30 shows how the engine’s performance at 6 kNm of torque and 475 rpm engine speed. This shows that the power is 300 kW. What is the SFC?

• Ans. SFC = 0.208 kg of fuel per brake kWh

EXAMPLE T = 6 kNm, N = 475 rev/minN = 475 rev/minPwr = 2πNT= 2π(475 rev/min)(min/60sec)(6kNm)= 300 kW

30

Example Engine Map (performance limited by WOT torque curve)

Source: Mechanical Prime Movers, Macmillan Press, 1971, pg 56-59

EXAMPLE T = 6 kNm, N = 475 rev/minN = 475 rev/minPwr = 2πNT= 2π(475 rev/min)(min/60sec)(6kNm)= 300 kW

Torque at WOT

31

Engine Rating Example

• Slide 32 explains how the engine map can be used to develop an engine rating that takes into account – Light load limits– Maximum continuous operations rating– Overload rating– Power decrease as a function of engine

operating time since last overhaul.

32

Engine Rating Example

Source: Mechanical Prime Movers, Macmillan Press, 1971, pg 56-59

Torque at WOT

• Avoid light load running• Region below line SS• Deposits

• Max Continuous Rating (MCR)

• 10% Overload Rating• ex. One hour in twelve

• Match engine for power just before overhaul.• ex: use 90% MCR

ring grooves stuck ringspistons gas blow-byexhaust turbine manifolds fouling

33

Constant Speed Operation

Engine AC Gen

ElectricalLoad

• Control system adjusts throttle to maintain constant speed as load varies• Selection depends on

• Max power requirement• Duty cycle• Life cycle• Equipment costs

Percent Full Power

ThermalEfficiency

Typical Diesel

Typical Steam

34

Load Line P1P2 for Constant SpeedOperation

35

Example: • Assume 110% MCR is WOT limit• 3330 kW @ 100% MCR• Duty Cycle

• 50% time at 90% MCR• 50% time at 65% MCR

• CASE 1• At 450 rpm• MCR = 333 kW/cyl• Cylinders = 3330 kW = 10 333 kW

• CASE 2• At 400 rpm• 110% MCR = 320 kW/cyl• 100% MCR = 291 kW/cyl• Cylinders = 3330 kW = 11.4 => 12 291 kW

Why Not 400 RPM?Case 2

Answer depends on situation. It requires a systems approach.

36

Variable Speed Operation

Engine

• Full power at M2• Idle power at M1• Use CP to get best bsfc load line

• Little time at M2: Use dotted line for better overall bsfc

• Crash reversal• High torque at all speeds • Ex: M2M3 line

Example: Controllable Pitch (CP) Marine Propeller

CP

37

Variable Speed Operation

Engine

• Power proportional to square of speed as shown in line F1F2.• If ship drag increases, line shifts to left--L1L2.• If ship runs light, line shifts to right—R1R2.

Example: Fixed Pitch Marine Propeller

38

Homework

1)Use the map for one cylinder given previously. Assume each cylinder has the following mission profile:291 kW for 3 hrs200 kW for 3 hrs100 KW for 3 hrsAssume the engine drives a generator at 400 rpm engine shaft speed.Determine the fuel required for each cylinder for the mission in kgs.2) Be able to label the timing map for a SI engine for the Quiz 3 next Wednesday.3) Read Supplement 2.