Xin Q. Diesel Engine System Design

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1.23 Continued

I3 I4

I5 I6

V6 V8

V12 V16

I3 I4

I5 I6

V6 V8

V12 V16

I3 I4

I5 I6

V6 V8

V12 V16

I3 I4

I5 I6

V6 V8

V12 V16

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Engine displacement (liter)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Engine displacement (liter)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Engine displacement (liter)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Engine displacement (liter)

Engine weight (dry mass, kg)

Engine speciﬁc weight (dry mass, kg/kW)

8000

7000

6000

5000

4000

3000

2000

1000

Engine rated BMEP (bar)

Engine compression ratio

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77The analytical design process and diesel engine system design

Figure 1.24 illustrates the analysis for the on-highway heavy-duty diesel

engines in North America from 5 to 16 liters. There is a clear trend of increasing

engine rated power (or torque) with increasing engine displacement. There

is also a clear trend of decreasing engine specic power with increasing

engine specic weight. Engine dry weight increases almost linearly with

bore diameter. Figure 1.25 presents the analysis for the light-duty diesel

engines and some heavy-duty diesel engines used in pickup trucks.

1.5.5 Competitive benchmarking analysis in mechanical

design

Engine block and crankcase designs

The design issues concerning the number of cylinders, engine balance, Vee

bank angle, and uneven ring order were discussed by Hoag (2006). Most

diesel engines used for automotive cars and trucks are inline 4-cylinder

(I4), inline 6-cylinder (I6), V6, and V8. The reciprocating mass is roughly

proportional to the cube of the cylinder diameter. The I4 engine has a shorter

block length. Although the secondary reciprocating inertia force of an I4

engine is not balanced, its magnitude is relatively small, being only a fraction

of the primary inertia force. For the I4 engines having a small cylinder

diameter, the engine balance is still satisfactory. Therefore, I4 engines are

popular in passenger cars and light trucks that have stringent requirements

on engine size. I6 engines have the best engine balance and strong exhaust

pulse energy for turbocharging. I6 is the most popular conguration for

commercial truck and bus engines. V8 engines with 90° bank angle are also

popular because of good engine balance and low height. V6 engines are the

most compact in length and height, and are used in some passenger cars and

light trucks. But the V6 engines with 90° bank angle have an unbalanced

secondary reciprocating inertia force, and the V6 engines with 120° bank

angle have an unbalanced primary inertia force.

High stiffness and strength, low NVH, and weight are important for engine

block design. The potential competitive benchmarking parameters include the

following: engine width; the geometric clearances between rotating moving

parts and the inner wall of the engine block; engine length; and the ‘C

’

ratio, which is the ratio of the cylinder-to-cylinder centerline distance to the

cylinder bore diameter. Note that the C

ratio reects the compactness of

the engine length and is affected by crankshaft length, cylinder head design,

and liner design. Plotting the C

ratio or typical thicknesses of the engine

block or head as a function of cylinder bore diameter for various engines

is an example of conducting competitive design analysis. Connecting rod

length and piston compression height are the two main parameters that affect

engine height.

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I4 I6

V6 V8

I4 I6

V6 V8

I4 I6

V6 V8

I4 I6

V6 V8

0 5 10 15 20

Engine displacement (liter)

0 5 10 15 20

Engine displacement (liter)

0 5 10 15 20

Engine displacement (liter)

0 5 10 15 20

Engine displacement (liter)

Engine rated power (kW)

Engine peak torque (N.m)

Engine rated speed (rpm)

Engine peak torque speed (rpm)

600

500

400

300

200

100

3000

2500

2000

1500

1000

500

3500

3000

2500

2000

1500

1000

2500

2000

1500

1000

1.24 Competitive benchmarking performance analysis of 2008 North America on-highway HD diesel engines.

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1.24 Continued

I4 I6

V6 V8

I4 I6

V6 V8

I4 I6

V6 V8

I4 I6

V6 V8

0 5 10 15 20

Engine displacement (liter)

70 80 90 100 110 120 130 140 150 160

Engine cylinder bore diameter (mm)

0 1 2 3 4 5 6 7 8 9

Engine speciﬁc weight (kg/kW)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Rated mean piston speed (m/sec)

Engine speciﬁc power (kW/liter)

Engine rated speed (rpm)

3500

3000

2500

2000

1500

1000

500

Engine weight (dry mass, kg)

Engine stroke-to-bore ratio

2000

1500

1000

500

1.4

1.3

1.2

1.1

1.0

0.9

0.8

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1.25 Competitive benchmarking performance analysis of North America light vehicle diesel engines.

I4 I6

V6 V8

V10

I4 I6

V6 V8

V10

I4 I6

V6 V8

V10

I4 I6

V6 V8

V10

Engine rated power (kW)

Engine rated speed (rpm)

300

250

200

150

100

5000

4500

4000

3500

3000

2500

2000

Engine speciﬁc power (kW/liter)

Coefﬁcient of torque backup

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0 1 2 3 4 5 6 7 8

Engine displacement (liter)

0 1 2 3 4 5 6 7 8

Engine displacement (liter)

0 1 2 3 4 5 6 7 8

Engine displacement (liter)

0 1 2 3 4 5 6 7 8

Engine displacement (liter)

Notes:

(1) The engines of 2-3

liters in displacement are

2011 model year light-duty

diesel engines in the US

market.

(2) The engines of larger

than 6 liters are heavy-duty

diesel engines used in

pickups.

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81The analytical design process and diesel engine system design

Cylinder liner design

The following are important design considerations for cylinder liner: low

friction and wear occurring on the liner inner surface between the liner and

the piston ring or piston skirt; minimum cavitation occurring on the coolant

side of the liner caused by piston slap and liner vibration; sufcient cooling

at the liner top; proper metal surface temperature across the circumferential

and axial directions of the liner; minimum bore distortion; and high stiffness

for NVH reduction.

Cylinder head design

The subjects of cylinder head design usually include the following: number

of valves; valve diameter and seat angle; port ow discharge coefcient

and swirl ratio; intake and exhaust port orientation (to avoid intake port

heating by exhaust gas); wall thickness; exhaust port length and heat

rejection losses; cooling; re deck temperature control in the area between

the valves in the cylinder head (to prevent thermo-mechanical fatigue);

cylinder head gasket design, etc. The basic competitive benchmarking

parameters for cylinder head design include:

∑ the ratio of port diameter to port length;

∑ the ratio of port length to cylinder bore;

∑ the characteristic curve of swirl ratio vs. port ow coefcient, which is

used for the evaluation of the trade-off between in-cylinder turbulence

and volumetric efciency;

∑ the number of cylinder head bolts and the head height, which are used

for stiffness and sealing evaluation

∑ the ratio of cylinder head bottom wall thickness to bore diameter which

is used for the evaluation of the trade-off between mechanical load and

thermal load.

Connecting rod, crankshaft and bearing designs

The design guidelines for power conversion components include the

following: sufcient mechanical strength and stiffness in compression,

stretch, torsional and bending motions under cylinder gas pressure and

inertia loading; light weight and light balancing weight; compact geometry;

smooth chamfers and low local stress; proper structural design of the

lubricant oil feed hole angle; proper settings of bearing clearances; low

bearing wear, friction, impact noise, etc. The basic competitive benchmarking

parameters include:

∑ the connecting rod length, which affects engine height, width, and piston

assembly dynamics;

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82 Diesel engine system design

∑ the ratio of connecting rod length to crank radius, which affects secondary

reciprocating force and piston side thrust;

∑ the crank pin diameter, which affects friction and rotating mass;

∑ the main bearing diameter, which needs to satisfy strength and lubrication

requirements and helps adjust the designs of crank web thickness, crank

pin diameter, and cylinder centerline distance;

∑ the overlap between the main bearing journal and the connecting rod

crank pin journal, which affects crankshaft strength; and

∑ the bearing aspect ratio (i.e., length divided by diameter).

1.6 Subsystem interaction and analytical engine

system design process

1.6.1 Engine subsystem interaction

The design of automotive diesel engines has been driven by emissions

regulations, fuel economy, NVH, drivability, durability and product cost.

Diesel engines are designed specically for their applications. In on-highway

applications, heavy-duty engines are used for heavy weight trucks and

buses under highly loaded duty cycles with much time spent at full load.

In contrast, automobiles and light trucks usually operate at very light loads

and low engine speeds, with high speed and load encountered only at brief

transients. The duty cycle difference determines the design features related

to durability and fuel economy.

Engine design is one of the most sophisticated industrial design processes

due to its complex nature that the functions of many components affect

each other. Inside the engine, air is mixed with fuel to combust to produce

power and generate emissions. The function and design of the fuel system

are closely related to those of the combustion system. Engine system design

is largely related to air handling. An engine has several major subsystems

related to air delivery and pumping loss: intake manifold, exhaust manifold,

cylinder head, valvetrain, turbocharger, EGR circuit, coolers, and exhaust

aftertreatment. Their performance is affected by certain key parameters

such as air and EGR ow rate, exhaust temperature, peak cylinder pressure,

boost and back pressures. The optimization among those subsystems needs

to be carried out carefully. Modern electronically controlled engines require

an on-target precise design to deliver air–fuel ratio, EGR rate and intake

manifold temperature for meeting the nominal design/calibration target at

critical operating speed/load modes. Meanwhile, the durability design limits or

constraints should not be violated. Neither an over-design nor under-design is

acceptable. The following guideline can be used to determine whether a given

design parameter should be optimized at a system level or at a subsystem

or component level. If the parameter is shared among several subsystems,

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83The analytical design process and diesel engine system design

it is a system level parameter because its change impacts all the related

subsystems. For instance, turbocharger matching is a system design issue

because it has a direct impact on air–fuel ratio and EGR circuit restriction

design. In contrast, an exhaust manifold runner design is more like a local

component design issue. As system optimization propagates more widely

and deeply throughout the whole engine to address the interactions among

different components, the work scope and job boundary between ‘system’

and ‘subsystem/component’ could become blurred. The purpose of system

engineering is to ensure excellent functional interaction with neighboring

subsystems.

Engine subsystem interaction occurs at the interfaces between the

individual subsystems, and between the major system and each subsystem.

Interactions of engine–vehicle, engine–aftertreatment, engine–turbocharger and

combustion–fuel system are just a few examples. The interaction manifests

itself at both steady-state and transient operations (i.e., static and dynamic)

and at the interface between hardware characteristics and software controls.

Therefore, a systematic analysis is necessary in order to understand engine

subsystem interactions. The analysis includes engine thermodynamic rst

law, second law, heat transfer, uid mechanics, dynamics and electronic

controls.

The tasks in engine system integration include the following for each

design attribute:

∑ identify the system design objective parameters that are affected by each

subsystem;

∑ analyze how the system design objective parameters are affected by

each subsystem;

∑ optimize the subsystems and the system;

∑ optimize among the four design attributes (performance, durability,

packaging, and cost).

1.6.2 Empirical engine design process

Figure 1.26 shows an example of the empirical ‘trial-and-error’ engine design

process which lacks systems engineering and concurrent engineering. An

empirical design process is characterized by the following:

1. Lack of advanced analysis.

2. Lack of a systems engineering approach to guide the design work. As

a result, a precisely dened system design specication is not available

at the beginning of the design process.

3. Sequential design. Conicting requirements or assumptions are used by

different subsystem teams.

4. Low efciency and low quality in design.

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84 Diesel engine system design

The diesel engine is a very complex machine and it consists of many

subsystems or components (as shown earlier in Fig. 1.5). In the empirical

design process the detailed designs of different subsystems start without a

formalized requirements analysis and engine system specications. This

leads to duplicated work and waste of resources. As many design problems

are discovered during testing and integration at a later development stage,

great effort has to be spent to reconcile the conicting issues at the system

level. Some component design work may have to start over again.

1.6.3 Advanced analytical engine system design process

Simultaneous engineering process

The advanced analytical engine system design process is characterized by

systems engineering, simultaneous (or concurrent) engineering and advanced

product quality planning (APQP). Simultaneous engineering is a design and/

or manufacturing process where cross-functional teams strive for a common

goal. It reduces development cycle time by replacing the sequential series

of phases by a simultaneous engineering process. In the sequential process,

the results are relayed from one area to the next area for execution. In the

simultaneous process, all the subsystem/component areas carry out their

design work concurrently with a unied system design specication.

Conﬂicting

or slow

sequential

design

assumptions

used by

different

subsystems

without

system

analysis

Reconcile

assumptions

and revise

design

specs

Many

engine

problems

discovered

Conﬂicting

designs

Integration, testing,

and calibration

– Combustion

– Performance

– Emissions

– Aftertreatment

– Vehicle integration

– Mechanical

durability

– NVH

Mechanical design (hardware)

– Cylinder head (valves, ports)

– Power cylinder and piston assembly

– Connecting rod

– Crankshaft

– Valvetrain and camshaft

– Intake manifold

– Exhaust manifold

– Turbocharger

– EGR system (cooler, valve)

– Intake throttle valve

– Cooling system

– Water pump

– Charge air cooler

– Oil cooler and oil pump

– Aftertreatment

– Fuel system

– Engine brake and exhaust brake

– Starter and alternator

Engine electronic control design

(software)

– Air-path control strategy

– Fuel-path control strategy

– Controller design

– Sensors

– Actuators

– ECU

1.26 Empirical engine design process.

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85The analytical design process and diesel engine system design

APQP is a structured process of dening and establishing the steps

necessary to assure a product satises customer expectations (APQP reference

manual, 1995). In the initial phase of the program great effort is spent in

engine system design by using advanced simulation tools to analyze the

functional requirements. Then, the system design specications are issued

to the subsystem design teams. Because the primary risk factors have been

analyzed and resolved, and the critical interface and functional analysis

have been completed by the system design team, the subsystem engineers

can start their designs with a much lower risk of failure. APQP uses process

planning to ensure the design changes or the corresponding development

efforts continuously decrease from the beginning of the engine program to

the end for a smooth transition to the start of production.

The engineering product development process has evolved from the

traditional slow process to today’s accelerated process where the research

and advanced engineering functions are combined. Organizational processes

and project management for production engineering were introduced by

Menne and Rechs (2002) in detail, including a summary of key checkpoints

for a program. A simultaneous engineering process of detailed subsystem

packaging and structural designs was provided by Dubensky (1993). Design

and process tools were elaborated by Carey (1992) and Gale et al. (1995).

Diesel engine design details were introduced by Merrion (1994).

Engineering functions in engine development

For each engine component, the engineering job functions can be classied

into three types, namely analysis, design, and testing. The analysis function

can be further divided into two main subareas: thermal/uid performance

and mechanical structure. It is usually impossible for one person or one

department to carry out all the three job functions for all the components.

In large engineering organizations usually the work functions of analysis,

design, and testing are carried out in different departments. Cross-functional

cooperation is the key to success.

The job function of analysis can be dened as an activity to generate design

specications and analyze problems for the four engine design attributes

(performance, packaging, durability, and cost) with advanced calculation

tools. The traditional job function of design can be dened as an activity to

realize the design specications with computer aided design (CAD) tools,

drawings, and prototypes. The job function of testing can be dened as an

activity to validate the design specications with experimental means.

In a product development process, design stays as a central role receiving

support from analysis and testing. Simulation is a part of analysis. It supplies

detailed information about many parameters which are difcult or impossible

to measure, for example, heat transfer in all components, composition of gas

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