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Torque Curves

PRODUCT INFORMATION

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CONTENTSIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

How a Torque Curve is Created . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

What the Torque Curve Means . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Changing the Torque Curve Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8

Transient Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Additional Information and Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

P R O D U C T I N F O R M A T I O N T O R Q U E C U R V E S

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Torque curves are used by performance

engineers to graphically express a dieselengines ability to maintain torque output at a

reduced speed. In simple terms, a torque curve

(also called lug curve) is a performance map

created for a given diesel engine.

This manual will present the following

information to help the user/operator understand

how torque curves are created, and their

relationship to engine application.

How Torque Curves are Created

What the Torque Curve Means

Changing the Torque Curve Shape

Operating Factors

Transient Performance

Summary


4

TORQUE CURVES

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Diesel engines have two performance limit

controls high idle and rack settings. These

settings are made on the engine governor. The

high idle setting establishes the top operating

speed of the engine, while the rack setting limits

the amount of fuel that can be injected per stroke.

To build the performance map or torque curve

these two limits must be considered separately.

High Idle Setting

High idle is the maximum speed the engine will

run with the governor wide open. In other

words, through the governor, this settingestablishes a speed at which the engine operates

when the speed control is wide open. As load, or

torque requirement, is added to the engine, the

governor opens the rack to maintain the engine

speed which is set by the high idle stop and the

governor position. Under isochronous governor

control, the torque speed relationship looks like

a straight vertical line.

When mechanical governors are used, the engine

slows down slightly as load is added. This speed

change activates the governor. It is a relatively

small change and can be varied by governor

design to suit the need.

The various high idle settings result in a whole

group or family of torque speed lines with

slants determined by governor characteristics. If

the high idle stop is set at a given position, and

load is added to the engine, the engine speed

drops slightly according to the characteristic

slant (or droop) of the line for that setting.

Rack Settings

For each constant rack position (such as mightbe used by the rack stop) a certain torque speed

is obtained. The shape of this curve is

determined by several engine characteristics

including: injection pump efficiency, combustion

efficiency, breathing (or volumetric) efficiency,

friction. The shape of the curve is characteristic

of the given engine and can be changed.

For every rack setting there is a torque speed

curve. As with high idle settings, this means that

each engine actually has a whole family of

torque speed curves. This group of curves is

subject to various engine efficiencies and

operating conditions just like a single curve.

The operating map is created by superimposing

the two families of lines. The result is a grid of

the entire operating torque speed range of the

engines.

There are limits which dictate where an engine

can operate successfully. These include: speed,

smoke, temperature, etc.

Turbocharged Engines

If the engine is turbocharged, limit lines (a

fence) may be drawn around that part of the

torque speed graph which is usable by engineers.

This is the part of the performance data which

can be appropriately used for selling. There are

engine high and low speed limits, turbo speed,

exhaust temperature and smoke which are all

factors.


HOWA TORQUE CURVE IS CREATED

HIGH IDLE

SETTING

ENGINE RPM

TORQUE

TORQUE CURVE MAP

TORQUE

ENGINE RPM

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Any chosen high idle setting and rack settingwill result in a single torque curve. The curve

will be made up of two parts the overrun

portion, where the engine speed and rack

positions are controlled by the governor, and the

lug part of the curve, where the rack is against

the stop and additional load causes the engine to

slow down.

The point at which the overrun and the lug meet

on the torque curve is called the balance point.

This point is used as the engines rating, as thisis the point of maximum horsepower (where the

engine is doing the most work.)

High idle, the maximum speed the engine will

run with the governor wide open, is limited by

the high idle setting. At the balance point, the

rack hits the rack stop and cannot be opened

further. When more load is added, more fuel

cannot be injected unless a torque spring is used.

Several factors come into play to add additional

torque as the engine lugs down. Any

combination of these can contribute to torque

rise:

Fuel Injection Equipment may be more

efficient at lower speeds and the engine may

get more fuel per injection pump stroke even

though the rack stays constant.

Combustion Efficiency may increase at

lower speeds as the injected fuel may burn

more thoroughly.

Air Supply per Piston Stroke may be

greater at lower speeds, due to an increase in

volumetric efficiency. This air increase can

assist in burning fuel more efficiently and

result in an increase in torque.

Friction Decrease also contributes to

torque rise

The difference in speed (rpm) between the

balance point and high idle is the overrun, or

speed regulation. With vehicular governors this

may be 7-10 percent of rated speed, and may be

as low as 3 percent for generator sets where

close regulation is required.

Overrun is zero on isochronous governors.

In the lug portion of the curve, two factors areimportant:

Torque rise

The speed at which the peak torque

occurs

Torque rise is generally expressed in percent

increase over rated torque at the balance point.

This torque rise is related to the diesels ability

to hang on under overload and is, therefore,

an important feature.


6

WHAT THE TORQUE CURVE MEANS ANDHOW IT IS USED

TORQUE

ENGINE RPM

LUG CURVE LIMITS

PEAK TORQUE

POINT

BALANCE

POINT

TORQUE

RISE

HIGH

IDLE

LOW

IDLE

OVERRUN

ENGINE RPM

TORQUE

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The ideal development target is to have the

torque peak between 60 percent and 70 percent

of rated speed. A typical diesel engine of today

might have 1000 lb-ft of torque at 1800 rpm and

lug to 1200 rpm. This is a 20 percent torque rise

at 67 percent of rated speed. High idle for this

engine might then be 2150 rpm, or 17 percent

overrun.

Another way to change the torque curve shape is

by using a mechanical device such as a flexible

rack stop or torque spring. The engine can be

given more fuel during lug operation by

allowing the rack to open an additionalcontrolled amount. Such devices simply allow

the torque curve to be modified by a more open

rack position as the engine lugs down. This

results in a high torque curve.

To avoid mechanical failures or complaints

about excess smoke, upper operating limits (ie:

turbo speed, smoke, and exhaust temperatures)

are not exceeded.

A torque curve with no torque rise is unable to

carry additional torque beyond what is rated.

Operating Factors Affecting Torque

Rise/Engine Performance

Torque rise cannot be measured with instruments

as in a laboratory. Rather, the operator has to

feel the change. This means he or she must feel

acceleration or changes in speed; the load being

moved or the engine stalling.

Because many of the overload torques are

required only for a short period of time, the

operator may feel the amount of speed drop

when the engine hits the overload. In some

cases, even the sound of the engine may indicate

either extra power or sluggishness.

Factors which will affect torque rise and

subsequent engine performance include:

Load

Combustion efficiency

Governor action

Turbocharger

Load

Load has a very definite effect on the action and

feel of the engine. Acceleration, for example,

will vary considerably depending upon how

much load it is carrying. An engine with no load

will accelerate to full speed within seconds, as itmust speed up only its own mass.

Acceleration is inversely proportional to mass. If

a heavier flywheel or load is added to the

engine, it will accelerate more slowly. There is

enough power in the typical shovel engine to

spin the shovel like a top once it is up to speed.

But because the shovel swing is less than 180,

the engine barely gets the swing started when

the shovel must be braked and the engine must

start the reverse swing. The same problem holdstrue when the engine has to start up a rock

crusher or other large, high mass equipment.

In vehicles, a transmission is used to get high

torque multiplication for acceleration. This

approach is also useful in other applications.

A torque converter provides torque

multiplication at low output shaft speeds. The

shaft output torque of the torque converter takes

the place of the flywheel output of the engine,

so far as the load speed characteristics are

concerned. Engine speed varies through a

small range and the engine does not stall

with a converter.

CHANGING THE TORQUE CURVE SHAPE

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8

The torque multiplication advantage of the

converter is exchanged for some power loss due

to converter inefficiency.

Combustion Efficiency

The speed with which an engine responds to

load demands is directly proportional to the

size of fire that can be built in the

combustion chamber and how quickly it can

be built. Cold engines just started would not

respond as rapidly as hot engines because

the cold combustion chamber walls tend to

inhibit good combustion and because the

oil drag would be too high.

Governor Action

When the engine is given more load, it slows

down momentarily. That slowing action

causes the governor to open the rack to get

more fuel for the engine. The time this takes

is lost time and can be a significant part of

response time.

Turbocharger

The turbocharger is also responsible for

some lag time in engine response. However,

two factors must be considered. First there is

generally excess air in the cylinder, more

combustion begins at once even before

the turbo speeds up. Full power will be

reached only after the turbocharger speed has

moved substantially toward its rated speed.

Second, the turbocharged engine generally

has less inertia or mass within its own

rotating parts than does a similarly rated,

naturally aspirated engine.

If these factors are considered, turbocharger

lag is not a problem on an engine which has

been properly matched by the turbo.

CHANGING THE TORQUE CURVE SHAPE

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Transient performance is of interest in engine

application because, in rapidly changing

conditions, the engine does not exactly follow

the steady-state torque curve.

In theory, if the engine governor is wide open

and the engine is running at high idle as load is

added, it will slow down slightly with increased

torque until it reaches the balance point. Then

the engine will follow the lug part of the curve,

slowing down more as additional load is added

until the torque peak is reached. Here, of course,

any further load will stall the engine.

If the load is applied rapidly, something different

can occur. The engine output torque may

actually be less than the torque curve indicates.

At the same time, the high rotating energy of the

engine flywheel drive system is transferred to

the load in such as way that the load feels an

entirely different torque (at least momentarily).

This illustrates the advantage of the engine and

drive inertia in picking up loads when the engine

is operating at high idle. In this instance,

flywheel mass is good, but the operator mustalso remember that, when the engine lugs down,

it must also speed back up before it can pick up

another load cycle. The engine must also carry

that same flywheel mass back up with it. If

response is critical, the excessive flywheel mass

may be a source of trouble.

This brings us to the fact that the torque curve is

a two-way street. In some applications, the

engine must pick up both load and speed. The

engine may start at the low idle point and work

up the back side of the torque curve.

Transient data taken in this manner shows that

there is also a possibility of variation from the

torque speed curve if that load change or load

application is rapid enough.

This is characterized in such uses as drill rigs.

The direct drive drawworks engine must pick up

the load, a string of pipe, at a very low speed inorder to avoid shock damage to the drawworks.

It must then accelerate that pipe to perform the

operation in a reasonable time.

Low speed torque is very important in such an

application. High horsepower alone at rated

engine speed may mean nothing. The

drawworks engine is loafing when it reaches

rated speed.

Here it becomes important to know which way

the engine will pass through the torque curve.

Will it lug down from high idle and thenrecover with no load?

Will it start from low idle and pick up bothload and speed?

Will it operate in some intermediate manner?

TRANSIENT PERFORMANCE

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In general, when an engine lugs down from highidle, it is good to stay above the peak torque

point. This is not, however, absolutely necessary.

Some operators lug their engines down until it

is almost possible to count the firing strokes

without stalling the engine. In other applications,

the same model engine stalled as if it had no

power at all. The load demand of the application

is probably the reason. By controlling the load,

the operator was able to actually work on the

back side of the lug curve where steady

operation would not be stable.

The torque speed characteristic of the load

demand can be plotted in the same manner used

to plot the torque speed curve of the engine. One

application may require nearly constant torque

for steady operation. Other applications may

require increased torque as speed goes up. Still

other applications may require a combination of

characteristics.

If, for example, an engine is attached to a

variable load, the possible torque speed demandsfor the different load conditions would form a

family of torque curves. Over this, one can

superimpose the engine torque curve. Now the

engine will operate at the condition determined

by the intersection of the engine torque curve

and the load curve.

If loads A, B, or C are used, the engine will

operate on the overrun portion of the curve. If

the load is adjusted to condition D, the engine

will lug down to a lower speed.

If the load is adjusted to condition E, the enginewill lug down still further. Now, by adjusting the

load, the engine can be made to operate between

these two points on the lug curve. This condition

is stable as long as the load curve intersects the

engine curve and the load torque requirement is

less at the next lower speed than what the engine

will put out. This holds true all the way to load

F and shows how an engine can operate on the

back side of the lug curve if the load is under the

operators control.

By describing unstable operation we have

inferred that where the two torque curves

crossed, the operation would be stable. This

condition is also one that can also be felt by the

operator. Further, the greater the angle of the

intersection between the load demand curve and

the engine torque curve, the more stable the

operation will be. By controlling the load, the

operator can move the torque requirement and

manually operate the engine in the speed range

in which he desires to work.

ADDITIONAL INFORMATION ANDCHARACTERISTICS

TORQUE

ENGINE RPM

LOAD MAP

G FE

D

C

B

A

LOAD CONDITIONS

TORQUE

ENGINE RPM

STABLE

UNSTABLE

VERY STABLE

TORQUE

REQUIREMENT

SPEED RPM

TORQUE REQUIREMENT TYPES

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Torque curves are useful tools by which the

engine can evaluate diesel engine performance.

They are influenced by design variables

which can be manipulated to obtain specific

characteristics in the engine. They can be

changed by governor setting and by special

torque spring devices in order to obtain special

characteristics.

The torque characteristics can be used in the

application of engines if the applications are

known. Poor torque characteristics may be the

cause of poor diesel engine performance and

unsatisfactory results in their application.

SUMMARY

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