Xin Q. Diesel Engine System Design

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448 Diesel engine system design

Motoring

Bleeder brake

Conventional

compression brake

Motoring

Bleeder brake

Conventional

compression brake

Motoring

Bleeder brake

Conventional

compression brake

–90 0 90 180 270 360 450 540 630 720

Crank angle (degree)

–90 0 90 180 270 360 450 540 630 720

Crank angle (degree)

Intake valve ﬂow rate (one

valve, kg/sec)

Cylinder pressure (bar, absolute)Cylinder heat transfer rate (kW)

–10

0.30

0.25

0.20

0.15

0.10

0.05

0.00

–0.05

–0.10

Crank angle (degree)

6.21 Performance comparison between conventional compression

brake and bleeder brake at 2000 rpm (instantaneous processes).

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449Engine brake performance in diesel engine system design

Motoring

Bleeder brake

Conventional

compression brake

Motoring

Bleeder brake

Conventional

compression brake

Motoring

Bleeder brake

Conventional

compression brake

–90 0 90 180 270 360 450 540 630 720

Crank angle (degree)

–90 0 90 180 270 360 450 540 630 720

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Instantaneous in-cylinder volume

Cylinder gas temperature (k)

Braking exhaust valve ﬂow rate

(one valve, kg/sec)

Cylinder pressure (bar, absolute)

1100

1000

900

800

700

600

500

400

300

1.4

1.2

0.8

0.6

0.4

0.2

–0.2

–0.4

Crank angle (degree)

6.21 Continued

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450 Diesel engine system design

6.4.5 Ambient effect on compression brake performance

The air system theory of turbocharged engine performance is discussed in

Chapter 4. Different ambient performance can be explained by the four ‘core

equations’ in Chapter 4. For example, at high altitude, there are basically

two effects that can increase the turbine pressure ratio: a lower turbine outlet

pressure and a higher turbine inlet temperature. The higher temperature results

from the lower air mass ow rate caused by a lower ambient air density.

The increased turbine pressure ratio makes the turbine run at a higher shaft

speed than at sea-level altitude. This partly compensates for the reduced

compressor outlet boost pressure. At a given altitude, hotter ambient air

temperature will reduce the engine air mass ow rate, turbine pressure ratio,

compressor pressure ratio, and hence intake manifold boost pressure.

Engine retarding power depends largely on the intake manifold boost

pressure in the compression-release process. The boost pressure is dependent

on the particular turbocharger used. The effects of ambient temperature,

altitude, and humidity were analyzed by Israel and Hu (1993). Their results

showed that the retarding power decreased slightly as the relative humidity

increased. For example, when the relative humidity changed from 0% to 100%,

the retarding power only changed by 5 kW from 240 kW to 235 kW at 2100

rpm engine speed. This effect is minor compared to the effects caused by the

changes in the ambient temperature and pressure. A formula was proposed

Table 6.1 Retarding performance comparison between conventional compression

brake and bleeder brake

Motoring Conventional

compression

brake

Bleeder

brake

Engine speed (rpm) 2000 2000 2000

Engine retarding power (hp) 79 385 387

LP-stage turbine outlet pressure (bar,

absolute)

1.026 1.111 1.101

Exhaust manifold pressure (bar, absolute) 2.355 5.367 5.020

Intake manifold boost pressure (bar, absolute) 1.199 2.363 2.720

Engine delta P (bar) 1.157 3.004 2.300

Engine air ﬂow rate (lb/min) 29.77 58.81 55.45

Exhaust manifold gas temperature (°F) 338 934 936

Peak cylinder temperature (°F) 1279 1138 1359

Cycle-average cylinder temperature (°F) 334 480 528

one cylinder cycle-average heat transfer rate

(kW)

2.65 5.77 7.41

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451Engine brake performance in diesel engine system design

by Israel and Hu (1993), ignoring the retarding system compliance resulting

from the cylinder pressure, as follows:

IMEp,retarding

= – 2.9105T

AMB

+ 11.7884p

AMB

+ C 6.13

where ϖ

IMEp,retarding

is the retarding indicated mean effective pressure (kPa),

AMB

is the ambient air temperature (∞C), p

AMB

is the ambient absolute pressure

(kPa), and C is a constant (kPa) dening the absolute level of the retarding

power at a baseline ambient condition. The retarding power decreases with

increasing ambient temperature and altitude. Strictly speaking, this formula is

limited to the particular turbocharger that was used to generate their simulation

results. Israel and Hu (1993) also concluded that the system compliance in

the compression brake, which was affected by the cylinder pressure and

ambient conditions, had a signicant adverse impact on the braking valve

timing and the retarding power, especially at high engine speeds.

6.4.6 Two-stroke braking with compression brakes

As the four-stroke braking power is limited by various design constraints,

innovative two-stroke braking may be considered to increase the power

without violating the design constraints such as the braking assembly

loading. By removing the normal exhaust valve event, the exhaust stroke

in the four-stroke engine cycle becomes a second compression stroke from

which more retarding power can be obtained via the strong compression-

release effect, compared to the low pumping loss during that stroke in the

four-stroke braking. Yang (2002) reported that more than 40% braking power

increase was achieved by using a two-stroke compression brake compared

with the four-stroke brake. More analysis on two-stroke braking is presented

in the next section on the VVA compression brake since VVA is a practical

mechanism to generate two-stroke valve events.

6.4.7 VVA and camless compression brakes

Usually the performance of the conventional compression brake can only

be optimized at a certain engine speed within a narrow range due to the

limitation of the mechanical driving mechanisms involved in the braking

valve motion control. The VVA compression brake with solenoid controls

provides great exibility to optimize the retarding performance at each engine

speed by adjusting the braking valve motion and opening/closing timing

so that the retarding process efciency can be greatly increased across the

entire engine speed range. For example, at higher speeds, an advance of the

braking valve opening timing is required to achieve optimum blow-down

of the cylinder charge and the maximum retarding power. As analyzed by

Hu et al. (1997a), the xed braking valve timing results in a sharp drop in

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452 Diesel engine system design

retarding efciency as the engine speed decreases, and an optimized valve

timing achieved with a VVA at each speed results in a high and constant

retarding efciency of 84% across the entire engine speed range. The VVA

brake can also easily achieve variable retarding power levels to satisfy the

different braking power requirements of the vehicle during driving.

The conventional compression brake usually uses hydraulics added on

top of the engine valvetrain and uses the motion of the injector cam or the

exhaust cam to open one exhaust valve near the compression (braking) TDC.

It introduces extra height and weight. The VVA brake can be much smaller,

lighter (e.g., weight reduced by half) and quieter than the conventional brake.

VVA brake can be less noisy because it can work like a bleeder brake to

exibly adjust the multiple pieces of the braking valve motion during the

engine cycle in order to alter the gas ows through the exhaust valves and

the exhaust pressure wave characteristics.

An integrated lost motion variable-valve-timing (VVT) diesel engine

retarder was introduced by Hu et al. (1997b). A comprehensive simulation

of VVA compression brake systems on performance, hydraulics, CFD, and

nite element structural analysis was elaborated by Schwoerer et al. (2002).

Diesel VVA compression brake performance was also investigated by Israel

(1998) and Fessler and Genova (2004).

A comprehensive simulation analysis of various VVA or camless

compression brakes is presented in Figs 6.22–6.25. Figure 6.22 shows

the engine valve lift proles used in seven different compression brakes,

including both four-stroke and two-stroke brakes, as well as their retarding

performance summary. Figure 6.23 shows the simulated instantaneous

engine intake and exhaust valve ow rates of these brakes. Figure 6.24

shows the engine operating points located on the compressor map for these

brakes. Figure 6.25 presents a comparison between the two-stroke and the

four-stroke compression brakes. It is observed that turbine area and braking

valve opening timing have a signicant impact on the retarding power, and

the two-stroke brake produces much higher retarding power than the four-

stroke brake.

6.4.8 Engine brake noise

The US federal noise regulations effective since 1978 require that all vehicles

meet noise requirements. According to US EPA’s Noise Regulation 40 CFR

Part 205, new trucks made in the US are required to emit less than 80 dBA

of noise at 50-feet-away full-pedal drive-by acceleration. The noise must not

exceed 83 dBA in any operating mode under 35 mph, and it cannot exceed

87 dBA over 35 mph. Some trucks with standard OEM exhaust systems may

produce higher noise levels (e.g., 83 dBA) when certain types of compression

brakes are turned on, compared to their 80 dBA noise during acceleration. A

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A: 4-stroke compression brake

E: 4-stroke compression brake-2 F: 4-stroke bleeder brake G: 2-stroke bleeder brake

B: 2-stroke compression brake 1 C: 2-stroke compression brake 2

D: 2-stroke compression brake 3

(only for naturally aspirated

engine)

Retarding power: 214 hp

engine ﬂow rate: 864 kg/hr

Compressor surge happens!

Retarding power: 237 hp

engine ﬂow rate: 254 kg/hr

Retarding power: 133 hp

engine ﬂow rate: 547 kg/hr

Retarding power: 222 hp

engine ﬂow rate: 1004 kg/hr

Retarding power: 404 hp

engine ﬂow rate: 1517 kg/hr

Retarding power: 405 hp

engine ﬂow rate: 1134 kg/hr

Retarding power: 143 hp

Intake valve is kept closed.

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake Exhaust Intake Exhaust

Intake

Exhaust

Valve lift (mm)

–90 0 90 180 270 360 450 540

Crank angle (degree)

–90 0 90 180 270 360 450 540

Crank angle (degree)

–90 0 90 180 270 360 450 540

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

Note: The retarding power

data here were obtained

by using a 9.3L I6 camless

engine with a ﬁxed-geometry

wastegated turbine (except

for the Brake D).

6.22 HD diesel engine camless engine compression brake concept analysis – valve lift and summary.

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–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

A: 4-stroke compression brake

E: 4-stroke compression brake-2 F: 4-stroke bleeder brake G: 2-stroke bleeder brake

B: 2-stroke compression brake 1 C: 2-stroke compression brake 2

D: 2-stroke compression brake 3

(only for naturally aspirated

engine)

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Intake

Exhaust

Severe compressor

surge happens!

Valve ﬂow rate (kg/sec)

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

0.9

0.6

0.3

–0.3

6.23 HD diesel engine camless engine compression brake concept analysis – valve ﬂow illustration.

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455Engine brake performance in diesel engine system design

truck without an exhaust mufer can produce a much higher noise level such

as 100 dBA noise when the conventional compression brake is turned on.

In the conventional compression brake, the braking valve opens near the

end of the compression stroke where the air is compressed to a very high

pressure. The sudden, almost instantaneous, release of the air creates explosive

pressure pulses in the exhaust manifold, causing a distinctive staccato (loud

popping) braking noise. This effect can be seen from the simulation result

in Fig. 6.26 on the exhaust port pressure pulse waves. It is observed from

Fig. 6.26 that the sharp high-frequency pulse at 10∞ crank angle before the

braking TDC (i.e., –10°) is the reason for the loud noise in the conventional

compression brake. Note that the other sharp pulse wave at 480∞ crank angle

comes from the braking event of another cylinder.

Many truck noise problems on the road are in fact due to defective

mufers or illegal exhaust systems (e.g., an unmufed straight stack). The

braking noise problem is most severe with illegally modied or defective

exhaust systems. The braking noise causes many towns to ban the use of

the conventional compression brake due to serious concerns for the quality

of life in the community. Road signs such as ‘Unmufed engine brake use

prohibited except emergency’ or simply ‘No Engine Brake’ are seen, for

example, near a downhill section of a fast road. In fact, the compression

brake is a valuable legitimate safety device for the vehicle.

Widespread concerns about compression brake noise faced by many local

(Surge!)

0.0000 0.0175 0.0350 0.0525 0.0699 0.0874

Reduced mass ﬂow rate, (kg/s)k

0.5

/kPa

Pressure ratio

3.720

3.176

2.632

2.088

1.544

1.000

Compressor efﬁciency map and engine brake operating points

MAx: 78.0

MIN: 20.0

EFFIC.

[%]

20.0

30.0

32.5

35.0

37.5

40.0

42.5

45.0

47.5

50.0

52.5

55.0

57.5

60.0

62.5

65.0

67.5

70.0

72.5

75.0

77.5

80.0

82.5

100.0

6.24 HD diesel camless engine compression brake concept analysis –

compressor operating points.

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456 Diesel engine system design

2-stroke compression brake simulation (2000 rpm)

Power (VNT vane open) Power (VNT vane closed)

Power (wastegated turbo) Cylinder P (VNT vane open)

Cylinder P (VNT vane closed) Cylinder P (wastegated turbo)

Retarding power (hp)

550

500

450

400

350

300

250

200

150

100

550

500

450

400

350

300

250

200

150

100

–90 –80 –70 –60 –50 –40 –30 –20 –10 0 10 20

EVo timing (degree crank angle)

–90 –80 –70 –60 –50 –40 –30 –20 –10 0 10 20

EVo timing (degree crank angle)

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

Cylinder pressure at EVo (psia)

VNT vane fully open

VNT vane fully closed

Wastegated turbine

4-stroke compression brake simulation (2000 rpm)

6.25 Comparison between two-stroke and four-stroke engine

compression brake performance of an I6 engine.

legislation bodies for noise abatement indicate that a quiet compression-release

engine brake design is highly desirable and will offer market competitiveness.

The noise problem can be addressed by more advanced braking mechanism

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457Engine brake performance in diesel engine system design

and/or effective exhaust mufing. The advanced braking mechanism is

preferred since it controls the noise problem at the source by utilizing better

braking valve events that favorably affect the exhaust gas wave dynamics

in the exhaust manifold and the tailpipe.

Different types of compression brakes produce different levels of noise.

For example, a bleeder brake is quieter than a conventional compression

brake without BGR (e.g., lower by 15 dBA at high engine speeds and 10

dBA at lower speeds). Bleeder or BGR may signicantly reduce the braking

noise due to its different exhaust ow and pressure pulse characteristics

(Fig. 6.26). The noise of exhaust-pulse compression brake stays between

the conventional brake and the bleeder brake, depending on its particular

braking valve lift proles. The noise of the conventional compression brake

decreases as the engine speed decreases. The speed-dependency of the noise

for the bleeder brake is much weaker (i.e., basically equally quiet at all

speeds). Progressive braking using fewer cylinders also reduces the noise.

It should be noted that the exhaust brake and the driveline brakes are nearly

silent, and their noise levels are usually no louder than the plain engine

noise. Schmitz et al. (1992) conrmed that their bleeder brake was more

silent than the conventional brake. Schmitz et al. (1994) also reported that

the use of a small decompression valve and throttling by a narrow overow

duct connected to the exhaust manifold reduced the compression brake

noise, compared to using the larger exhaust valve as the braking valve in

the compression brake.

Motoring

Bleeder brake

Conventional compression brake

–90 0 90 180 270 360 450 540 630 720

Crank angle (degree)

Cylinder-1 exhaust port pressure (bar, absolute)

6.26 Root cause of engine brake noise – simulation of exhaust

pressure pulses of different engine brakes (2000 rpm).

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