Hydra 2.7 Idle Speed Control
The purpose of idle speed control is to adjust the position of the ISC (Idle Speed Control) valve
or throttle that feeds air into the intake manifold when there is no throttle input. In the simplest
terms, idle control prevents the engine from either stalling or from revving overly fast when there
is no throttle input. In practice, idle control requires sophisticated features to handle all the
special conditions that arise.
Overview
The idle control mechanism is engaged on the Hydra Nemesis when the throttle is closed (the
TPS (%) gauge indicates 0%) and the engine speed is below the decel fuel cut lower limit (even
if decel fuel cut is disabled). When idle control is active, the Hydra Nemesis will adjust the
position of the idle control valve to raise or lower the engine speed to closely match the current
idle speed target. The current idle speed target is determined by the Base Idle Speed Target map
plus several other trim maps and settings that may apply under special conditions.
Idle Valves
There are many different types of idle valves, but they generally fall into two different groups:
linear and stepper. A linear valve has an infinite operating range between being fully closed (0%
duty cycle) to fully open (100% duty cycle). The Hydra Nemesis splits the operating range of a
linear valve into 256 discrete steps. The minimal movement of a linear valve is (100/256)%,
which is approximately 0.4%. Thus, the valve can be 0% open or 0.4% open, but nothing inbetween. A stepper valve physically has a fixed number of steps between fully closed and fully
open and its position is limited to these steps. The minimal movement of a stepper valve is
(100/steps)%. A stepper valve with 50 steps can be 0% open or 2% open, but nothing inbetween. The Hydra Nemesis will still calculate the desired location of the stepper valve based
on its internal (100/256)% steps, but physically the stepper valve will be placed at the
(100/steps)% value closest to the desired internal steps.
Vehicles with drive by wire (DBW) throttles will often not have an idle control valve but will
use the main throttle to perform idle control. In such cases, the Hydra Nemesis DBW system
defines an internal range over which is will control the DBW throttle such that 100% on the idle
control system will only be a slight opening of the physical DBW throttle.
Stepper Valve Setup
If your vehicle has a stepper valve, you will need a Hydra Nemesis unit equipped with a stepper
motor control chip to control it. If your unit was specifically ordered with a harness for a vehicle
with a stepper valve, this should be the case. If not, you may need to contact your Hydra
Nemesis dealer and provide the serial number of your system to determine if you have the proper
hardware. If not, you will have to have your system reconfigured.
If the stepper motor control chip is added to the system, auxiliary inputs AUX7 through AUX10
become unavailable since these pins become the outputs used to control the stepper valve.
Tuning Notes for the New Hydra2.7 Idle Strategies
Perfect idle behavior under all conditions is one of the hallmarks of excellent tuning. This
excellence is difficult to reach without the appropriate set of maps and settings and the
knowledge and patience to put them to use. Expect to spend a significant amount of time
achieving the perfect idle behavior, particularly if the vehicle has been significantly modified
from the configuration used to develop the original base map. Expect to need to adjust the
vehicle both during the warmest and coldest parts of the year with varying levels of electrical and
mechanical loads.
The ISC (%) gauge on the dashboard window is your best friend at all times while you are
setting up and adjusting the idle control system. It should always be making slow, deliberate
steps to adjust the ISC valve in the direction required to have the actual engine speed match the
target idle speed. The datalogger can also be indispensable when things get confusing.
PID Parameters
At the heart of the idle control system is a PID controller. Idle control is best achieved with slow,
deliberate movements of the idle valve and for this reason the idle speed PID controller is a
slower acting system than most of the other PID controllers on the Hydra Nemesis. There are
few cases where quick valve position changes are needed, and these will be controlled through
appropriate settings.
The proportional, integral and derivative values of the idle PID controller can be adjusted from
the idle control settings panel. To reach this panel, select Tuning Maps->Base Idle Speed Target
and press the Setting button. In most cases, these values should be properly set in the base map
for your vehicle.
As with all PID controllers, start with low I term and increase the P term value until the ISC (%)
gauge indicates that the system is moving at a reasonable rate to match the idle target value.
After this is achieved, use a moderate amount of integral to allow the valve to move towards the
target when the difference between the idle target and actual engine speed are fairly small. Too
much integral, particularly when the proportional value is small, will cause a lot of instability
and hunting. If you keep seeing the valve close well after the engine speed has gone below the
idle target, chances are you are using too much integral and not enough proportional to achieve
your target.
Cranking ISC Valve Duty Cycle Map
Idle control begins before the engine is started. The Cranking ISC Valve Duty Cycle 2D map
indicates the desired position of the idle control valve depending on the coolant temperature of
the engine. Cold engines generally need more air to start and reach their desired starting idle
speed than warm engines.
To reach this map, set your experience level to Apprentice or higher and select Tuning Maps>Idle Speed Control->Cranking ISC Valve Duty Cycle.
The value indicated in the Cranking ISC Valve Duty Cycle 2D map determines the position of
the valve during cranking and for the first eight crank revolutions of the engine after it starts.
This value is not constrained by the settings of the Minimum and Maximum ISC Duty Cycle
maps.
You should adjust this map so that the engine starts and quickly reaches the desired post start
idle target speed. If the value is set too high, the engine will flare to a much higher RPM than
desired on start and you will see the idle control value rapidly drop after start to bring the engine
speed to the desired target range. If the value is set too low the engine might not start or might
take several attempts to start and after start it will struggle at a fairly slow speed as the idle
control system works to open the idle valve to reach the desired post start idle target.
Base Idle Speed Target Map
The Base Idle Speed Target 2D map indicates the desired engine speed depending on the coolant
temperature of the engine. This allows the idle speed to be raised above the normal warm idle
speed when the engine is cold. A higher cold idle will bring the engine to operating temperature
more quickly.
To reach this map, select Tuning Maps->Base Idle Speed Target. This map is available at all
experience levels. Selecting this map also provides access to the Idle Speed Target Settings
panel.
What idle speed is best for your engine depends on the engine itself, the stiffness of the engine
mounts to the vehicle chassis, the requirements of the charging system and on your personal
preference. Modern vehicles idle at fairly low speeds in their stock form once they have reached
operating temperature and do not have extra loads (AC, thermal fans, power steering, etc.) placed
on them.
Minimum and Maximum ISC Duty Cycle Maps
In rare instances, there may be a need to constrain the range of the idle control valve. Except
during initial cranking or in cases where the throttle is open, the idle control system will stay
within the indicated minimum and maximum ISC valve duty cycle values.
To reach these map, set your experience level to Apprentice or higher and select Tuning Maps>Idle Speed Control->Maximum ISC Valve Duty Cycle or Tuning Maps->Idle Speed Control>Minimum ISC Valve Duty Cycle.
In all cases, the tuning of the idle control system should start with the minimum values set to
zero and the maximum values set to 100% across all coolant temperatures. If the range of the
ISC valve must be constrained, then it should be done for a very deliberate purpose that cannot
be achieved using other methods. One reasonable case might be for motors that require an
unusually large amount of air to crank but will then flare to an engine speed that is much too
high unless the ISC range is immediately constrained after engine start. Another reason might be
if a much larger valve than required is fitted on the engine to feed it extra air during anti-lag
operation. In such a case the maximum opening allowed during normal operation may be
curtained to prevent the idle control system from ever “accidentally” running the engine at too
high a speed while the idle control system is being tuned.
Post Start Idle Speed Target Trim Map
The Post Start Idle Speed Target Trim 2D map allows the idle speed to be raised above the base
idle speed target for the first 3840 engine revolutions after engine start. This allows the engine to
idle higher for the first couple of minutes after engine start. Note that the internal idle speed
target limit is 2040 RPM, so if the combination of the current idle target plus the post start idle
target trim is greater than 2040, the idle target will be 2040.
To reach this map, set your experience level to Apprentice or higher and select Tuning Maps>Idle Speed Control->Post Start Idle Speed Target Trim.
How much trim the base idle speed target needs right after engine start is dependant on many
factors. The extra idle speed helps the charging system restore the battery back to a full charge
more quickly and to circulate the oil around the engine more quickly, thus reducing wear.
Beyond that, the post start idle speed target trim is a matter of personal taste.
ISC Return Anticipate Setting
The ISC return anticipate setting affects how much the ISC valve position will open from the
point at which the engine was in a steady state idle when the throttle is opened. The ISC return
anticipate setting must be properly adjusted to make the transition back to a closed throttle idle
smooth.
To reach this setting, set your experience level to Expert or higher and select Tuning Maps>Base Idle Speed Target and press the Setting button. The ISC return anticipate setting can be
adjusted using the slider provided or by entering a value directly into the box provided.
Let’s say that the engine is comfortably idling with the ISC value at X% duty cycle. When the
throttle opens, the idle speed valve will be gradually moved to (X+ISC return anticipate) and
parked there while the throttle stays opened (the ISC position might change under high load and
anti-lag, but it will return back to this parked location when these conditions stop). When the
throttle returns to the closed position, the idle control system will have the ISC valve at the
parked position and when engine speed drops below the decel fuel cut lower limit it will then
allow the PID controller to adjust the ISC valve back to its normal idling point.
If the ISC return anticipate setting is set too low, engine speed may drop below the idle speed
target when the engine tries to return back to idle. If the ISC return anticipate setting is too high,
the engine will hang and take longer than expected to drop back to its normal idle speed after the
throttle closes. Adjust the ISC return anticipate setting with the engine at normal operating
temperature and the vehicle stopped. Turn off all loads and then hold the throttle slightly open
(so that the engine speed goes at least 500RPM above the decel fuel cut lower limit) for five
seconds and let it return quickly to the closed position. If the engine speed drops and picks up
again before settling, try a larger return value. If the engine speed hangs too high before settling,
try a smaller value.
AC On Idle Speed Target Trim Setting
Most modern engines can idle very well at low engine speeds while air conditioner compressors
still require a slightly higher rotational speed to produce suitable cooling results. The AC On Idle
Speed Target Trim 2D map allows the idle speed to be raised while the AC compressor is
operating. One of the auxiliary inputs must be configured with an AC request to activate this idle
target trim and the AC compressor control capability must be enabled. Note that the internal idle
speed target limit is 2040 RPM, so if the combination of the current idle target plus the AC on
idle target trim is greater than 2040, the idle target will be 2040.
To reach this map, set your experience level to Apprentice or higher and select Tuning Maps>Idle Speed Control->AC On Idle Speed Target Trim.
How much trim the base idle speed target needs when the AC is on depends greatly on the base
idle speed of the engine and the AC system itself. Some AC systems require very few extra
engine revolutions above normal idle to maintain a reasonable cabin temperature while idling.
Other systems may need a few hundred RPM more.
ISC AC Clutch Anticipate Setting
The instant that the AC clutch engages the AC compressor to the engine, the extra load will
cause the engine speed to drop. If the base idle speed target of the engine is low, this load may
bring the engine very close to its stall point. The idle control PID is intentionally designed to
make slow, deliberate valve adjustments, but in the case of an instant, heavy load the transition
into and out of AC mode will be much smoother if the idle control system makes a very fast
change to the ISC duty cycle. This is the purpose of the ISC AC clutch anticipate setting.
To reach this setting, set your experience level to Expert or higher and select Tuning Maps>Base Idle Speed Target and press the Setting button. The ISC AC Clutch anticipate setting can
be adjusted using the slider provided or by entering a value directly into the box provided.
The percentage value in the ISC AC clutch anticipate setting indicates how many percent will be
added to the current valve duty cycle the instant that the AC clutch is engaged. If the engine is
currently idling with the AC off and the ISC valve position is X%, then if the instant that the AC
clutch engages the ISC valve will be moved to (X+ISC AC clutch anticipate)%. Likewise, when
the AC clutch disengages, the ISC valve will be instantly moved to (X-(ISC AC clutch
anticipate/2))%.
The recommended method for setting the ISC AC clutch anticipate value is to do so after the fuel
and timing advance and base idle speed target tuning have been properly done. Start by setting
the value to zero. Now bring the engine to normal operating temperature and allow it to idle with
all the loads off. Note the average position of the ISC valve (it may naturally move up and down
a percent or two, but it shouldn’t be more than that unless something is not right). Now, switch
on the AC and allow the ISC to recover from the extra load and bring the engine up to the
desired AC on idle speed target. Now note the average position of the ISC valve with the AC
running. The difference between the two ISC positions is the best starting point for the ISC AC
clutch anticipate value. Now allow the AC to cycle on and off several times and make minor
adjustments up or down as needed.
PS Load Idle Speed Target Trim Setting
The PS Load Idle Target Trim 2D map provides for an extra idle target trim when external
engine loads such as power steering are indicated. Any idle target increase indicated in this map
will be engaged when an external power steering load is indicated. One of the auxiliary inputs
must be configured with a power steering load request to activate this idle target trim and the
steering load feature capability must be enabled. Note that the internal idle speed target limit is
2040 RPM, so if the combination of the current idle target plus the PS load idle target trim is
greater than 2040, the idle target will be 2040.
To reach this map, set your experience level to Expert or higher and select Tuning Maps->Idle
Speed Control->PS Load Idle Speed Target Trim.
To adjust this map, switch the required load on and off and adjust the map to affect the change is
engine speed required to properly deal to the load. Note that there is no anticipate setting for the
PS load unlike the AC and thermal fans because the PS load is expected to have a much smaller
instantaneous effect of the engine speed that the heavier loads do.
Some vehicles provide two or more idle up load request signals to the ECU. For example, there
may be a PS load, a defroster load and a brake load signal all sent to the ECU on separate wires.
All these can be used to produce a single PS load signal either externally with diodes or
internally by using one of the internal feedback channels as the PS load request and a logical
expression is to suitably mix the multiple input signals (usually with logical ORs) to produce the
correct the internal feedback channel expression.
Moving Vehicle ISC Target Trim Setting
The moving vehicle ISC target trim setting can be adjusted from the idle control settings panel.
This setting indicates an additional idle speed trim to add to the idle speed target when the
vehicle is moving. To use this trim, a vehicle speed sensor must be used and the vehicle speed
sensor capability must be enabled. This trim allows the idle target to be set higher in anticipation
that the brakes are likely to be applied and a slightly higher engine speed will provide more
vacuum force for the brake power booster. Note that the internal idle speed target limit is 2040
RPM, so if the combination of the current idle target plus the moving vehicle idle target trim is
greater than 2040, the idle target will be 2040.
To reach the moving vehicle ISC target trim setting, select Tuning Maps->Base Idle Target 2D
map and press the Setting button. You will need to be in apprentice mode or higher to access the
moving vehicle ISC target trim setting.
How much moving vehicle ISC target trim your vehicle needs depends on the demands of the
brake booster and on personal preference. Too high a value will make the idle seem to stick too
high while the vehicle is coming to a stop while too low a value may cause engine speed to
easily drop below the desired target idle speed while braking to a stop.
Thermofan On ISC Target Trim Setting
The thermofan on idle target trim setting can be adjusted from the idle control settings panel. To
reach this panel, select the Base Idle Target 2D map and press the settings button. You will need
to be in expert mode to see and adjust these values. This setting indicates an additional trim to
add to the idle speed target when the thermofan is on. This trim allows the idle to be set higher to
compensate for the extra load that an electric fan will place on the engine. To use this trim the
thermofan control capability must be enabled. Note that the internal idle speed target limit is
2040 RPM, so if the combination of the current idle target plus the thermofan on idle target trim
is greater than 2040, the idle target will be 2040.
To reach the thermofan on ISC target trim setting, select Tuning Maps->Base Idle Target 2D
map and press the Setting button. You will need to be in apprentice mode or higher to access the
thermofan on ISC target trim setting.
How much thermofan on ISC target idle speed trim your vehicle needs depends on many factors.
Generally, adjust the engine speed so that the extra electrical load of the fan will not cause the
charging voltage to drop below the desired amount.
ISC Thermofan Anticipate Setting
The instant that the thermofan switches on, the extra load will cause the engine speed to drop. If
the base idle speed target of the engine is low, this load may bring the engine very close to its
stall point. The idle control PID is intentionally designed to make slow, deliberate valve
adjustments, but in the case of an instant, heavy load the transition into and out of thermofan
mode will be much smoother if the idle control system makes a very fast change to the ISC duty
cycle. This is the purpose of the ISC thermofan anticipate setting.
To reach this setting, set your experience level to Expert or higher and select Tuning Maps>Base Idle Speed Target and press the Setting button. The ISC thermofan anticipate setting can
be adjusted using the slider provided or by entering a value directly into the box provided.
The percentage value in the ISC thermonfan anticipate setting indicates how many percent will
be added to the current valve duty cycle the instant that the thermofan is switched on. If the
engine is currently idling with the fan off and the ISC valve position is X%, then if the instant
that the fan switches on the ISC valve will be moved to (X+ISC thermofan anticipate)%.
Likewise, when the fan switches off, the ISC valve will be instantly moved to (X-(ISC thermofan
anticipate/2))%.
The recommended method for setting the ISC thermofan anticipate value is to do so after the fuel
and timing advance and base idle speed target tuning have been properly done. Start by setting
the value to zero. Now bring the engine to normal operating temperature and allow it to idle with
all the loads off. Note the average position of the ISC valve (it may naturally move up and down
a percent or two, but it shouldn’t be more than that unless something is not right). Now, switch
on the fan by adjusting the temperature at which the fan switches on and allow the ISC to recover
from the extra load and bring the engine up to the desired thermofan on idle speed target. Now
note the average position of the ISC valve with the fan running. The difference between the two
ISC positions is the best starting point for the ISC thermofan anticipate value. Now allow the fan
to cycle on and off several times and make minor adjustments up or down as needed. Do not
forget to set the temperature at which the termofan switches on back to its normal point after you
are finished.
ISC Maximum Duty Cycle Under Load Setting
The ISC maximum duty cycle under load setting allows the ISC to be closed under load on
engines that leak boost if the ISC valve is not fully closed.
To reach this setting, set your experience level to Master or higher and select Tuning Maps>Base Idle Speed Target and press the Setting button. The ISC maximum duty cycles under load
setting can be adjusted using the slider provided or by entering a value directly into the box
provided.
On vehicles with DBW or idle control valves where the inlet of the valve is just before the
throttle body, the ISC maximum duty cycles under load setting should be set to 100%. On
vehicles where the inlet to the valve is directed to the atmosphere or to the intake pipe before the
turbocharger, the value should be set to 0%.
ISC Stepper Steps Setting
The ISC stepper steps setting indicates how many steps the stepper ISC valve has. This value
needs to be set only if the vehicle uses a stepper ISC valve.
To reach this setting, set your experience level to Master or higher and select Tuning Maps>Base Idle Speed Target and press the Setting button. The ISC stepper steps setting can be
adjusted using the slider provided or by entering a value directly into the box provided.
It is imperative that this value reflect the true number of steps that the ISC stepper valve has
between ts fully open and its fully closed position. If this setting is not properly set, the idle
control system will not properly maintain idle speed.
ISC Error Ignition Trim Map
The ISC error ignition trim 2D map allows the current ignition advance to be adjusted to help the
engine more closely track the idle target. In cases where a vehicle has not ISC valve, the ISC
error ignition trim in conjunction with the coolant temperature trim 2D map may be the primary
method of achieving any form of idle speed control. To use this trim the ISC error ignition trim
capability must be enabled.
To reach this map, set your experience level to Expert or higher and select Tuning Maps->Idle
Speed Control->ISC Error Ignition Trim.
The X axis of the 2D map is the idle target error calculated as (idle target - actual engine speed).
When the engine is running faster than the idle target, the left (negative) side values of the map
will be used. Generally, you want these values to have increasingly negative timing advance
trims to retard the timing more as the engine speed moves farther above the desired idle target
speed. When the engine speed drops below the idle speed target, the right (positive) side of the
map will be used. Generally, you want these values to have increasingly positive timing advance
trims to advance the timing and help the engine produce more torque as engine speed drops
farther below the desired target speed.
Tune this map after fuel and timing advance have been properly adjusted and after the other idle
control maps have been properly tuned. Start with a modest slope going from negative trim
values on the left up and right towards positive trim values on the right with a zero trim in the
middle of the map (when engine speed matches the idle target speed). Too much ignition trim
may make the engine overly-sensitive to idle speed errors and produce undesired hunting.
If the engine has no ISC valve and engine idle speed is to be maintained solely through ignition
advance control, use the Coolant Temp Ignition Trim map in conjunction with the base timing
map so that ignition advance suring cold start is around 15-20 degrees BTDC and drops to
around 0-5 BTDC degrees when the engine reaches operating temperature. In some engines, you
may even need to idle at 5-10 degree ATDC during warm idle in order to provide a suitably high
cold start idle speed while still maintaining a reasonable warm idle speed. Once the engine is
within a 100-150 RPMs of the desired idle speed with just the coolant temperature ignition trim,
adjust the ISC error ignition trim to keep the engine within 50 RPMs of the desired idle speed. If
the engine has no ISC valve and an AC compressor, the AC on ignition trim and AC clutch
enrichment 2D maps will need to be used to help the engine produce extra torque when the AC
compressor engages.