Xin Q. Diesel Engine System Design

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892 Diesel engine system design

13.7 Turbocompounding

Turbocompounding is an important waste heat recovery technology.

It uses a compound turbine at the downstream of the turbocharger turbine

to extract mechanical work from the exhaust gas and put this work back

to the engine crankshaft in order to increase the thermal efciency of the

engine. It should be noted that the effectiveness of turbocompounding depends

on the availability of the exhaust gas, which is low for modern high-EGR

engines, and the negative impact of the increased exhaust restriction on the

turbocharger turbine performance. Diesel turbocompounding analysis is

reported by Assanis and Heywood (1986), Tennant and Walsham (1989),

Hopmann and Algrain (2003), Stobart and Weerasinghe (2006), and Hountalas

et al. (2007).

13.8 Thermodynamic second law analysis of engine

system

While the rst law of thermodynamics (i.e., energy balance in ‘quantity’)

has been widely used in diesel engine system design and analysis, the

second law of thermodynamics (i.e., the ‘quality’ of the usable energy) is

in fact equally or even more important in system design. During the course

of engine system design, it is important to understand the ultimate potential

of design improvement in system efciency and to compare the quality of

system design from an energy quality point of view. Modern diesel engines

have the following areas that need guidance from the second law analysis:

1. The potential of waste heat recovery from the exhaust and cooling

systems (e.g., turbine outlet exhaust gas, EGR heat, charge air cooler

heat).

2. The ultimate potential of design improvement in engine air system.

3. The root causes of high losses in the engine system.

4. The comparison of system/component losses between different hardware

congurations (e.g., different turbocharger systems, EGR systems, air

system control valves, aftertreatment systems that have different exhaust

restriction levels).

Advanced engine system analysis should be conducted using the second law

of thermodynamics to address these issues. The second law analysis is able to

quantify various losses occurring in different subsystems (e.g., combustion,

air, cooling, friction) and point out the root causes of the system deciency.

For cooling system components, the losses include the destructions from heat

transfer, ow restriction, etc. For air system components, the losses include

the destructions from component efciency losses (e.g., low turbocharger

efciency), ow losses (e.g., exhaust and intake restrictions), and throttle

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893Diesel engine air system design

losses at all the air system control valves (e.g., EGR valve, turbine wastegate

valve, EBP valve, intake throttle valve, etc.).

Figures 13.15–13.17 present a comprehensive analysis of the comparison

between the thermodynamic rst law and second law analyses for a modern

heavy-duty diesel engine based on engine cycle simulation data. The second

13.15 Thermodynamic ﬁrst law and second law analyses at peak

torque and rated power.

First law, peak

torque

Percentage of

miscellaneous

heat losses in

fuel energy

(method 1),

4.00%

Percentage of

brake power in fuel

energy, 40.48%

Percentage of base

engine coolant heat

rejection (excluding

EGR cooler and

ISC) in fuel energy,

14.14%

Percentage of

EGR cooler

heat rejection

in fuel energy,

11.73%

Percentage

of charge air

cooler and ISC

heat rejection

in fuel energy,

7.75%

Percentage of

sensible exhaust

enthalpy at

turbine outlet

in fuel energy,

21.90%

Second law,

peak torque

Destruction

from base

engine heat

transfer,

7.34%

Destruction

from

combustion,

heat transfer

and ﬂow

restriction

in exhaust

manifold,

intake

manifold/port

and all pipes,

28.51%

AvaiIabiIity

from LP-

turbine outlet

exhaust gas,

8.04%

Availability

from EGR

coolers,

2.68%

Availability

from base

engine heat

rejection

(excluding

friction), 5.47%

Engine brake

power, 38.60%

Destruction from

EGR coolers, 2.20%

Destruction

from exhaust

restriction,

1.44%

Destruction from

turbocharger system,

3.11%

Availability from CAC

and ISC

0.66%

Destruction from

engine friction

0.85%

Destruction from intake

throttle

0.00%

Destruction from EGR

valves

0.20%

Destruction from air

and EGR mixing process

0.00%

Destruction from turbine

wastegate

0.11%

Destruction from ISC

and CAC

0.80%

Destruction from intake

restriction

0.02%

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894 Diesel engine system design

law analysis is conducted with the ‘dead state’ datum dened at 1 atm (1.013

bar) and 298.15 K (77°F). Figure 13.15 illustrates the breakdown percentages

of the energy and availability/destruction distributions at the peak torque

and rated power conditions. It is observed that the second law analysis is

able to quantify more realistically the potential of each waste heat source

than the rst law analysis. For example, the rst law analysis shows that at

13.15 Continued

Percentage of

brake power

in fuel energy,

37.64%

Percentage of

EGR cooler

heat rejection

in fuel energy,

13.28%

Percentage

of charge

air cooler

and ISC heat

rejection in

fuel energy,

9.10%

First law,

rated power

Percentage of

miscellaneous

heat losses in

fuel energy

(method 1),

3.20%

Percentage of base

engine coolant heat

rejection (excluding

EGR cooler and

ISC) in fuel energy,

13.31%

Percentage of sensible

exhaust enthalpy at

turbine outlet in fuel

energy, 23.48%

Second law,

rated power

Destruction

from base

engine heat

transfer,

6.18%

Destruction

from

combustion,

heat transfer

and ﬂow

restriction

in exhaust

manifold, intake

manifold/port

and all pipes,

26.86%

Availability

from EGR

coolers,

2.89%

Availability

from base

engine heat

rejection

(excluding

friction), 5.47%

Engine brake

power, 35.77%

Destruction

from EGR

coolers, 3.62%

Destruction

from exhaust

restriction,

1.70%

Availability

from LP-

turbine

outlet

exhaust gas,

9.61%

Destruction from

turbocharger

system, 3.96%

Availability from CAC and ISC

1.04%

Destruction from engine friction

1.15%

Destruction from intake throttle

0.00%

Destruction from EGR valves

0.39%

Destruction from air

and EGR mixing process

0.07%

Destruction from turbine

wastegate

0.23%

Destruction from ISC and CAC

1.01%

Destruction from intake

restriction

0.04%

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895Diesel engine air system design

rated power the EGR cooler heat rejection, the charge air cooler plus inter-

stage cooler heat rejection, and the turbine-outlet exhaust gas energy occupy

13.28%, 9.10%, and 23.48% of the total fuel energy, respectively. The second

law analysis points out that the availabilities of these three terms are 2.89%,

1.04%, and 9.61% in the total availability of the fuel, respectively.

Moreover, it is observed that the second law analysis is able to reveal the

source of system losses, compare the destructions of various subsystems/

components and processes, and indicate the potential of design improvement

Percentage of brake power in fuel energy (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of base engine coolant heat rejection

excluding EGR cooler and ISC (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of EGR cooler heat rejection in fuel

energy (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of sensible exhaust enthalpy at

turbine outlet in fuel energy (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of charge air cooler plus ISC heat

rejection in fuel energy (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of miscellaneous heat losses in

fuel energy, method 1 (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

13.16 Thermodynamic ﬁrst law analysis in engine speed–load

domain.

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896 Diesel engine system design

for those losses. In contrast, the rst law analysis is not able to achieve

these. For example, the second law analysis indicates that at rated power

the destruction in the turbocharger system, exhaust restriction, engine

mechanical friction, EGR valve throttling, and turbine wastegate throttling

are 3.96%, 1.70%, 1.15%, 0.39%, and 0.23% of the total availability of the

fuel, respectively. This shows that the damage to the system caused by the

high exhaust restriction in modern diesel engines’ aftertreament devices is

about the same as the total losses of engine friction, EGR throttling loss, and

Percentage of engine brake power in total

availability of the fuel into the system (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of availability from base engine

coolant heat rejection (excluding friction, %)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

8.5

7.5

6.5

5.5

Engine BMEP (bar)

Percentage of availability from EGR coolers (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

2.5

1.5

0.5

Engine BMEP (bar)

Percentage of availability from LP-turbine

outlet exhaust gas (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of availability from CAC and ISC (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

0.8

0.6

0.4

0.2

Engine BMEP (bar)

13.17 Thermodynamic second law analysis in engine speed–load

domain.

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897Diesel engine air system design

Percentage of destruction from combustion, heat

transfer and ﬂow restriction losses in exhaust

manifold, intake manifold/port and all pipes (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of destruction from base engine

heat transfer (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of destruction from turbocharger system (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

5.5

4.5

3.5

2.5

1.5

Engine BMEP (bar)

Percentage of destruction from EGR coolers (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of destruction from engine friction (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

Engine BMEP (bar)

Percentage of destruction from ISC and CAC (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1.2

1.0

0.8

0.6

0.4

Engine BMEP (bar)

13.17 Continued

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898 Diesel engine system design

turbine wastegate throttling loss. And this destruction caused by the exhaust

restriction is as high as 43% of the destruction of the turbocharger system.

This second law analysis indicates that large potential to improve diesel

engine efciency resides in the waste heat recovery from the turbine outlet

exhaust gas and EGR gas, followed by improvement in turbocharger efciency

and reduction in exhaust restriction, cooler restriction, mechanical friction,

and the throttle losses at the EGR valve and the turbine wastegate. If an intake

throttle is used, it may create large or non-negligible destruction in the system.

It should be noted that for a poorly designed air system the ow losses and

throttle losses in the system can be much higher than the data presented in

13.17 Continued

Percentage of destruction from exhaust restriction (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1.8

1.6

1.4

1.2

Engine BMEP (bar)

Percentage of destruction from intake restriction (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

Engine BMEP (bar)

Percentage of destruction from EGR valves (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1.2

0.8

0.6

0.4

0.2

Engine BMEP (bar)

Percentage of destruction from intake throttle (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

0.12

0.1

0.08

0.06

0.04

0.02

Engine BMEP (bar)

Percentage of destruction from turbine wastegate (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

0.5

0.4

0.3

0.2

0.1

Engine BMEP (bar)

Percentage of destruction from air and EGR mixing

process (%)

800 1000 1200 1400 1600 1800 2000

Engine speed (rpm)

0.1

0.05

Engine BMEP (bar)

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899Diesel engine air system design

this case. Figures 13.16 and 13.17 present the complete analysis of the rst

law and second law breakdown maps in the engine speed–load domain. It

illustrates that the second law analysis provides an in-depth understanding

of the engine system behavior of the energy conversion processes, and is a

powerful approach to guide the system and component design in the right

direction to maximize the engine system efciency.

Good reviews of the fundamentals of the second law of thermodynamics

and engine analysis are provided by Flynn (2001) and Rakopoulos and

Giakoumis (2006, 2009). A complete list of good references on the engine

applications of the second law of thermodynamics are provided in Section

13.9.1.

13.9 References and bibliography

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900 Diesel engine system design

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901Diesel engine air system design

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