Xin Q. Diesel Engine System Design

Подождите немного. Документ загружается.

398 Diesel engine system design

normalized weight basis. A passenger car may have 1 ton of weight, while

a typical heavy vehicle may weigh 36 tons. The natural retarding power per

vehicle weight of the heavy vehicle is in the order of 10% of the passenger

car. The service brake area or braking force per weight of the heavy vehicle

is in the order of one half of the passenger car (Habib, 1992). The insuf cient

service braking capability per vehicle weight in heavy vehicles is the primary

reason for using retarders. Retarders can minimize the speed difference

between cars and trucks on downgrades.

Theoretical analysis on required retarding power

The parametric dependency of the required vehicle retarding power is analyzed

as follows. For level ground driving, the kinetic energy of a vehicle with

the travelling speed N

is given by

kV,kV

6.2

The total braking power required to slow down the vehicle is given by



evel

rl,rl

kV,kV

6.3

Assuming a constant deceleration during the braking, i.e., N

= N

+ a

· Dt or a

= (N

– N

)/Dt, where N

is the initial vehicle speed, the total

braking power can be derived as



evel

rl,rl

Wm =Wm

(

Wm (Wm

NN – NN

(

Wm (Wm

Na (Na

(

Na (Na

Na +Na

)

Na )Na

ta )ta

)

ta )ta

◊D

)

)◊D )

)

◊D

)

DDD

6.4

The total braking time is Dt = – N

. The total braking distance is given

rV,rV

lN = – lN

()

a()a

()

a()a

VV0VV

()2()

()

. The required braking power reaches a maximum,

, at the beginning of the deceleration (i.e., at t = 0). It is observed

that the maximum braking power is linearly proportional to the vehicle’s total

equivalent mass xm

, the initial vehicle speed N

and the deceleration a

The amount of braking power required to stop a vehicle can be enormous.

For example, if accelerating a vehicle from standstill to 60 mph within

one minute requires 100 hp engine power, to bring the same vehicle from

60 mph to a complete stop within six seconds would require 1,000 hp!

Another observation is that while the deceleration may be constant during

the braking event, the distance that the vehicle travels each second under

braking varies greatly as the speed decreases. The distance being travelled

each second is always greater at the beginning of the braking. Therefore, in

order to minimize the stopping distance, it is important to make the brake

fully effective immediately after it is engaged.

Diesel-Xin-06.indd 398 5/5/11 11:50:25 AM



399Engine brake performance in diesel engine system design

A more important application of the retarders is to maintain a constant

descending vehicle speed along a downgrade. In this case, the deceleration

is zero, and the potential energy of the vehicle is given by

p,V

= m

a,V

6.5

where l

a,V

is the altitude level for the vehicle, which can be expressed as

a,V

= N

t · sinq ≈ N

t · tanq = N

6.6

where G

is the road grade and q is the road slope angle. The total braking

power required to maintain the vehicle control speed is given by



ade

rader

rg,rg

mg mg

N N

pV,pV

aV,aV

◊ª

si◊ªsi

n◊ªn

◊ª

6.7

It is observed from equation 6.7 that the braking power is linearly proportional

to the vehicle weight, descending control speed and the road grade. If the

aerodynamic and tire–road rolling friction resistances are neglected, the amount

of power required to maintain a constant-speed downgrade travel would be

basically the same as the power required to propel the vehicle going uphill

at the same speed. Since the engine rated power is usually reached for heavy

vehicles travelling uphill, a similar power requirement is often speci ed for

the engine brake retarding power used for travelling downhill. For a heavy

vehicle running on a steep long downgrade at a high speed, a large amount

of retarding power must be continuously supplied for a long time. Usually,

only using the service brakes cannot meet such a requirement due to brake

pad overheat or even fade. In mountainous regions the road grade can often

be 5–8% for a length of several miles. For example, a typical European legal

requirement is to maintain 30 km/hour (19 mph) on a 7% downhill grade

for 6 km (3.73 miles). Long and steep mountainous grades with switchback

curves are very common and therefore require downhill braking. Long

descending ramps near bridges are also very common.

The required retarding powers for downhill cruising and level-ground

deceleration have similar magnitude. Meistrick (1992) mentioned that to

decelerate a Class-8 truck from 55 mph to 25 mph over 0.4 km (1/4 mile)

distance, the retarding power required is equal to the retarding power that

is required to maintain the vehicle speed at 62.5 mph on a 4% downgrade.

The downhill retarding capability of the retarders can be expressed by a

combination of vehicle weight, vehicle speed, and road grade (equation 6.7).

The level-ground retarding capability can be expressed by a combination of

vehicle weight, vehicle speed, and deceleration (equation 6.3).

Methods of retarding

Different levels of vehicle braking can be achieved by the following

means.

Diesel-Xin-06.indd 399 5/5/11 11:50:26 AM



400 Diesel engine system design

∑ Disengage the engine from the drivetrain by placing the transmission

in neutral gear and use the aerodynamic drag, tire–road rolling friction

resistance, and uphill gravity resistance (if on a upgrade) to retard.

∑ Engage the engine with the drivetrain by selecting an appropriate

transmission gear (and the resulting engine speed) and use the engine

motoring power to retard.

∑ Activate an engine brake or a drivetrain retarder to retard.

∑ Apply the service brakes to retard.

Usually the aerodynamic and tire–road friction resistances are small compared

to the retarding forces from the service brakes or retarders. Although

downshifting the transmission can increase the engine motoring power via

the increased engine speed to sufciently maintain or slow down passenger

cars, downshifting is usually not sufcient for heavy vehicles due to the

substantial weight difference. The details of predicting retarder downhill

performance and matching a retarder with the vehicle can be found in SAE

J1489 (2000), which includes many typical values for the related parameters.

SAE J1489 also illustrates the maps of retarding power and the maximum

grades allowable vs. vehicle speed for each gear ratio without using the

vehicle’s service brakes.

6.1.2 The beneﬁts and challenges of retarders

The service brakes are powerful for short application periods but they cannot

be used continuously. Friction-type service brakes are ill-suited for prolonged

braking in heavy vehicles due to the physical limitations of convective cooling

for the brake pads. When the vehicle speed increases on a downhill grade,

the frictional heat absorbed by the service brakes increases. This results in

an increase in the brake temperature, regardless of using continuous braking

or periodic repeated (‘snub’) braking. When the brake drum temperature is

over 200∞C, the brake lining average friction coefcient decreases rapidly

to reach a point of fading. Avoiding brake overheating is important to

maintain the nominal braking capacity of the service brakes. In downhill

driving, brake overheating is related to vehicle weight, road grade, vehicle

speed, and braking time (or travel distance). Less fade can only be achieved

through lower values of braking energy such as less vehicle weight or speed,

or through increased cooling capacity of the brake pad (Limpert, 1975).

However, increased convective cooling coefcient or cooling area is difcult

to achieve in current service brake designs without increased cost.

The service brake lining wear is nearly proportional to braking energy and

increases rapidly with brake temperature. Reducing brake lining temperature

is important for extended lining life. To stop or slow down an 80,000 lbs

heavy vehicle or maintain a high vehicle speed on a long downgrade by only

Diesel-Xin-06.indd 400 5/5/11 11:50:26 AM



401Engine brake performance in diesel engine system design

using the service brakes results in a large amount of wear on the brake pads.

The pads have to be replaced frequently or may even fail due to overheat and

fade. Vehicle retarders supplement and save the use of the service brakes,

reduce service brake fade/failure and wear, and increase driving safety. The

use of retarders has the following benets:

∑ Control vehicle downhill speed for driving safety. Service brakes can

remain cool and have sufcient reserve for safe stopping in an emergency.

The primary use of vehicle retarders is on long downgrades where the

service brakes would otherwise have to be frequently used to prevent

the vehicle from over-speeding. O’Day and Bunch (1981) summarized

the truck runaway events by severity, from low to high as follows:

smoking brakes, loss of brakes – rode it out; loss of brakes – used runoff

ramp (no damage); loss of brakes – used runoff ramp (damage); loss of

brakes – crash; loss of brakes – injury; and loss of brakes – fatality. The

probability of a runaway accident due to brake failure can be reduced

by using the retarders. It should be noted that owing to their limited

capabilities, some retarders cannot absolutely prevent runaways. In fact,

retarders are so important that they are a necessary part of the continuous

braking system for heavy vehicles subject to meeting the legislative

requirements of effective and safe braking in many countries (e.g., in

European Union countries and Switzerland).

∑ Reduce the thermal load and wear of the service brakes on brake linings

and drums, and extend the brake life by reducing the use of the service

brakes. Brake overhaul intervals of heavy vehicles can be increased.

This is especially important for frequent downhill driving or in frequent

start–stop driving cycles (e.g., transit buses).

∑ Extend tire casing life for more retreads by keeping the brake and tire

casing cooler.

∑ Enhance the ease of vehicle control and reduce driver fatigue due to

repeated use of the service brakes.

∑ Increase vehicle productivity and longer transportation mileages

accumulated due to reduced trip time by maintaining a high vehicle

control speed on long downgrades. Other road users will not be impeded

when the vehicle speed is sufciently high.

∑ Save overall cost for vehicle operation. The cost savings from the

increased brake lining life, reduced long-term maintenance cost, less

down-time, shorter travel time and the elimination of damage and injury

due to a brake-failure accident usually exceed the cost of the retarder.

The economic benets have been conrmed by many studies including

certain federally sponsored investigations. A detailed analysis of the

economic benets of the heavy truck retarders was conducted by O’Day

and Bunch (1981). They used a return-on-investment model and a model

Diesel-Xin-06.indd 401 5/5/11 11:50:26 AM



402 Diesel engine system design

of the probability of a runaway incident. Their study provided a quantied

example of cost–benet analysis for retarding system designs. Other

economic analysis on retarders can be found in Schreck et al. (1992).

They concluded that the retarder has the advantage of a quick pay-off

for itself and brings nancial advantages to the transportation companies

by increased mileages of retarder-equipped vehicles.

∑ Bring performance benets to transient ring operation. Some retarders

such as engine brakes can maintain engine component temperatures (e.g.,

in exhaust braking) and turbocharger speeds (e.g., in compression-release

braking) at their operating levels during long braking, thus assuring rapid

engine response and good transient performance in terms of emissions

and noise when the engine is switched back from the no-fuel braking

mode to the ring mode.

The challenges in retarder design include the following:

∑ Resolve the design complexity and compromise (e.g., turbocharger

selection) from the powertrain system to the component when designing

a highly capable retarder that has a high torque at low speeds and a high

retarding power at high speeds.

∑ Achieve progressive or variable (continuously, if possible) levels of

braking power for smooth and controllable braking. Progressive braking

is a desirable feature to control the vehicle speed and reduce driver

stress, just like the progressive fueling controlled by the accelerator pedal

position in engine ring operation. Progressive braking power levels

may also reduce or eliminate wheel lockup caused by over-braking in

order to keep the vehicle stable.

∑ Resolve the dynamic braking coordination and system integration between

the retarder and the service brakes under various operating conditions

through powertrain controls on its interfaces with other systems (e.g.,

anti-lock brake, speed limiters).

∑ Accommodate additional vehicle cooling requirement resulting from the

maximum retarder braking performance, especially for the drivetrain

retarders, which operate by friction absorption and heat dissipation.

∑ Endure additional or alternating stress loads on the components.

∑ Accommodate additional weight and packaging space requirements of

the retarder.

∑ Minimize the higher engine noise produced from certain compression-

release engine brakes.

∑ Minimize the power dissipation (loss) during the shut-off conditions

(e.g., for the hydrodynamic retarder).

∑ Achieve short response and cut-off times for fast engaging and disengaging

of the retarder, respectively.

∑ Achieve a low-cost design of the retarder.

Diesel-Xin-06.indd 402 5/5/11 11:50:26 AM



403Engine brake performance in diesel engine system design

∑ Minimize additional wear in drivetrain. The potential higher wear on the

tires and the braking axles to which the retarder is connected should not

be neglected in retarder design and operation. Higher retarding torques

at lower engine speeds can usually cause more wear on the driving tires.

Moreover, non-smooth retardation may cause tire hop and lockup and

hence increase tire wear. Although these effects can be offset by other

non-braking axles, a balanced design and the minimum wear are always

preferred.

6.1.3 Vehicle braking force distribution and retarder force

target

Vehicle wheel lockup, braking efciency, and vehicle instability

The braking effectiveness of the vehicle is not only related to the braking

power but also to how the total available braking force is distributed among

all the wheel axles and whether over-braking and under-braking occur.

These topics are related to optimal braking, braking efciency, and wheel

lockup. In fact, they affect the realistic maximum design target of the

retarder torque. The braking force of a retarder may alter the braking force

distribution of the vehicle and hence affect the vehicle’s directional stability.

When the vehicle weight is low or the road adhesion coefcient is low,

over-designing the retarding power capability may cause wheel lockup and

vehicle instability.

According to the force and moment balances of the vehicle body and each

axle, the load acting on the front axle decreases and the load on the rear axle

increases during vehicle acceleration or hill climbing, when compared to

their steady-state or level-ground driving. Similar force and moment balance

analysis applied to vehicle braking operation reveals that the braking force

distribution reaches an ideal situation when the tire–road adhesion limits

of the front and rear axles are equal. Consequently, the minimum braking

distance of the vehicle is achieved. However, when the braking force

changes, the ideal distribution can be maintained only when the braking

forces on the front and rear axles are properly adjusted. The concept of

braking efciency has been used to measure how close the braking is to the

optimum. The braking efciency refers to the ability of the braking system

to utilize available tire–road friction for the vehicle to achieve a relatively

high deceleration before wheel lockup and the corresponding loss of control

occur (Radlinski, 1989). A 100% braking efciency indicates a perfect

braking force balance where all the wheels would lock up simultaneously.

Braking force distribution directly affects braking efciency. Braking force

distribution also affects brake lining temperatures.

As in the vehicle tractive situation (engine ring), the maximum vehicle

Diesel-Xin-06.indd 403 5/5/11 11:50:26 AM



404 Diesel engine system design

retarding (braking) force that the tire–ground contact can support is determined

by the tire–road adhesion force. Above the adhesion limit, the tire will slide

or lock up. Locking up rear tires or semitrailer tires in a trailer–semitrailer

combination makes the vehicle completely lose directional stability in yaw

motion. It creates the dangerous tractor ‘jackkning’ and semitrailer swing,

or makes a two-axle vehicle rotate by 180∞. The lockup of front tires causes

a loss of steering directional control but not yawing instability. For rear-

wheel-driven vehicles on wet or icy roads with very low tire–road friction

coefcient, over-braking on the rear wheels should be avoided in order to

prevent the rear-wheel lockup and the associated vehicle instability.

Optimum braking force distribution between the front and rear tires ensures

the maximum braking forces on the front and rear axles to be developed

simultaneously to achieve the maximum deceleration rate and the minimum

stopping distance. But the optimum braking force distribution varies with

vehicle load, vehicle design, and road surface condition. With a non-optimum

distribution, one of the axles will lock rst. To prevent locking, less braking

force must be applied, resulting in a reduced deceleration capability. The

braking force distribution depends on the weight and the dimensions of the

vehicle. Highway commercial motor vehicles have a wide range of vehicle

weight, ranging from the single-unit trucks having three axles and 50,000–

65,000 lbs to the turnpike double length combination vehicles having nine

axles and 105,000–147,000 lbs. A detailed description of worldwide highway

commercial motor vehicle weights, dimensions, and brake-related standards

is provided by Freund (2007). SAE J2627 (2009) and J257 (1997) provide

fundamental information on vehicle braking systems.

Service brake force distribution

Two comprehensive reviews on the braking force distribution on axles and

heavy-vehicle braking performance have been provided by Radlinski (1987,

1989). He pointed out that it is difcult to optimize the braking performance

for all operating conditions without adding complexity to the design. He also

compared different braking design philosophies between the US and Europe

as a result of their different brake regulations and design practices. For

example, braking force distribution was regulated in European requirements,

while the US regulations do not specify braking force distribution directly.

As a result, European heavy vehicles must use relatively large brakes on the

steering (front) axles to match the high front axle loads occurring during

braking, and use load-sensing proportioning valves on the driven (rear) axles

to reduce the braking force under unloaded or partially loaded conditions or

on slippery road surfaces.

In the US design philosophy, balancing axle brake loads according to

gross axle weight ratings (GAWR) is considered to be the general practice.

Diesel-Xin-06.indd 404 5/5/11 11:50:26 AM



405Engine brake performance in diesel engine system design

The dynamic braking force distributions among the various axles in the US

and European designs are very different. Radlinski’s studies may provide

valuable insights for selecting the appropriate power target of vehicle

retarders when they are integrated into the braking system. The retarding

force will change the braking force distribution. Detailed descriptions of

the vehicle force balance between axles are provided by Wong (1993).

Vehicle service brake design and braking dynamics are elaborated by

Limpert (1992).

Retarder force distribution

The required retarder power and force distribution should be analyzed by

conducting a vehicle dynamic analysis and then cascaded to the engine

system design team as a design target. The role of retarders is to supplement

the service brakes to achieve sufcient retarding power and the optimum

braking force distribution among all the axles under various vehicle operating

conditions such as unloaded and loaded, on dry, wet, or icy road surfaces.

The optimum distribution again refers to the condition that the tire–road

adhesion limits of the axles are equal (i.e., fully utilizing the maximum

braking potential of each axle). The retarding force acting on the driven

axle should not exceed the road adhesion limit in order to prevent wheel

lockup. This retarder characteristic needs to be considered in the service

brake–retarder system design.

It should be noted that unlike the service brakes which are usually

effective for both driven and non-driven wheels, the engine brake or

vehicle retarder is only effective for the driven wheels. Another important

issue when analyzing the wheel lockup with a retarder is the effect of the

retarder’s ‘torque vs. speed’ characteristics on longitudinal slip and wheel

lockup. Because the retarder has different characteristics compared to the

service brakes, the rotational and lockup characteristics of the wheels during

retarder braking may be different from the characteristics of the braking

using the service brakes. This topic is discussed in detail by Fancher and

Radlinski (1983).

For lightly loaded vehicles or driving on slippery roads, the use of full

powerful retarding may make the vehicle lose its directional stability. Such

an improper use of the retarder was analyzed by Fancher and Radlinski

(1983). The technique in their study can be used to analyze the maximum

retarding power requirement under various vehicle operating conditions by

considering the practical effect of wheel lockup. Vehicle directional stability

with the operation of an engine brake and a hydrodynamic retarder was also

analyzed by Göhring et al. (1992).

Diesel-Xin-06.indd 405 5/5/11 11:50:26 AM



406 Diesel engine system design

6.1.4 Classiﬁcation of drivetrain retarders and engine

brakes

In vehicle brake system design, all the retarders belong to the category of

auxiliary brakes, which supplement the service brakes. By denition (Limpert,

1992), an auxiliary brake is a continuous brake in which the retarding torque

is not generated by the friction between two sliding surfaces such as linings

and a drum. Therefore, essentially they are wear-free brakes. All retarders

can be classied into two categories: the drivetrain retarders (also called

transmission or propeller shaft brakes, including the trailer axle retarders),

and the engine brakes. In general, the drivetrain here may include the

transmission and the driveline.

Some authors (e.g., Haiss, 1992) used the term ‘primary or input retarders’

to refer to the engine brakes or the drivetrain retarders that are installed

in the powertrain (i.e., either in the engine or between the engine and

the transmission). Their braking torque varies with the transmission gear

selected. On the other hand, some authors used the term ‘secondary or output

retarders’ to refer to the drivetrain retarders that are either bolted directly to

the transmission output ange or located between the transmission and the

driven axles. Their braking torque is irrelevant to which gear is engaged.

Sometimes retarders are also installed on multi-axle towed vehicles. Such

a classication is useful when evaluating the impact of transmission gear

change on retarder performance. The primary retarders may increase the

retarder speed by gear shifts (e.g., on a steep downgrade with a low vehicle

speed), but they may lose the braking effect during the gear shift process

in the case of manual transmissions. Among all the retarders/brakes, the

compression-release engine brake is the most common type used in heavy

vehicles equipped with diesel engines.

The drivetrain retarders include any retarding device that is located in the

transmission or the driveline, essentially between the transmission and the rear

axle, such as the friction retarder (the friction brake), the hydrodynamic and

electric (electrodynamic and electromagnetic) retarders. Drivetrain retarders

are usually used with mid-range-power diesel engines, for example in the

city buses and the refuse trucks running at low speeds and with stop-and-start

type of driving cycles. Drivetrain retarders are popular in Europe in a wide

range of vehicles from the light-weight classes to 7.5 ton vehicles (Göhring

et al., 1992).

The engine brakes, strictly speaking, include all the retarding devices

whose retarding power is generated from the engine or from the directly

attached devices on the engine crankshaft before the transmission. They

include the following ve types:

1. The crankshaft/ywheel attachment brake. A typical example of this

type of brake is Caterpillar’s BrakeSaver (Darragh, 1974), which was

Diesel-Xin-06.indd 406 5/5/11 11:50:26 AM



407Engine brake performance in diesel engine system design

a ywheel hydraulic retarder. Other examples may include the power

absorption devices through mechanical, hydraulic, pneumatic, or electrical

means directly attached to the engine crankshaft or ywheel.

2. Engine motoring as a brake (i.e., by simply shutting off fueling).

3. The intake-throttle brake.

4. The exhaust brake.

5. The compression-release brake (or simply called compression brake or

decompression brake).

Among the above ve types, the exhaust brake and the compression brake

are most popular and they are the most common ‘engine brakes’ people refer

to. In the exhaust brake, there are three types: (1) the conventional exhaust

brake by using a ap or buttery valve installed at the turbine outlet; (2)

the variable-geometry turbine (VGT) exhaust brake by modulating the ow

area of the VGT; and (3) the variable-valve-actuation (VVA) exhaust

brake.

There are generally four types of compression brakes, namely (1) the

conventional compression brake; (2) the bleeder brake (i.e., the braking

valve constantly held open during the engine cycle); (3) the exhaust-pulse-

induced compression brake (also called the EPI or exhaust pulse brake; the

braking valve actuated by exhaust manifold pressure pulses); and (4) the

VVA compression brake. Compression brakes are usually used on Class-7

and Class-8 heavy vehicles (e.g., big semi-trailers, 18-wheel trucks, buses)

and off-highway equipment. More than 90% of the Class-8 trucks in North

America and a majority of the big bore diesel engines (larger than 10 liters

in displacement) are equipped with compression brakes.

Very often the terms ‘engine brake’ or ‘engine retarder’ are mistakenly

used to only refer to the compression-release brake, or even only to the ‘Jake

Brake’. For example, there are road trafc restriction signs of ‘No Engine

Braking’ which result from the noise control ordinance in some towns.

The ‘Jake

’, ‘Jake Brake

’ and ‘Jacobs Engine Brake

’ are registered

trademarks of the Jacobs Vehicle Systems

(JVS, formerly known as Jacobs

Manufacturing Company and Jacobs Vehicle Equipment Company). The

idea of the compression-release brake was conceived by Clessie Cummins,

the founder of the Cummins Engine Company, in 1934 after a hair-raising

and nearly fatal truck ride down a mountain pass with faded service brakes.

Later, the Jacobs Manufacturing Company applied Cummins’ idea (Cummins,

1959) and successfully implemented the compression brake in production,

producing the rst Jake Brake in 1961. JVS has been a leading manufacturer

of engine brakes in the US and has a long history of implementing braking

technologies. The compression brakes made by JVS were so famous that

the term ‘Jake brake’ is sometimes incorrectly used to refer to compression-

release engine brakes in general. In fact, Jake Brake refers to all of the JVS

Diesel-Xin-06.indd 407 5/5/11 11:50:26 AM

