ISBe/ISB Series Engines – 101-028   Unique Operating Characteristics of an Engine with EGR

Variable Geometry Turbocharger

The variable geometry turbocharger is used to create back pressure in the exhaust system to aid in the exhaust gas recirculation (EGR) flow. Non-EGR engines typically utilize wastegated turbochargers that minimize the amount of exhaust manifold back pressure in the exhaust manifold, but do not use all of the exhaust gas flowing through the turbocharger. The design of the variable geometry turbocharger helps create the back pressure needed for EGR flow and still control boost pressure since there is no exhaust gas being wastegated around the turbine wheel.

What to expect with Variable Geometry Turbocharged Engines:

1. Turbocharger boost can fluctuate depending on speed, load, and desired EGR flow.
   • Boost will peak briefly at a snap throttle and return to a lower boost setting at the same throttle position and engine load. The control system aids in throttle response and by design, will overshoot it’s boost needs rather than undershoot.
   • Boost can also increase when you let off the throttle, as the control system closes down the variable geometry in anticipation of the next on throttle event.
   • When the engine is at a constant speed and experiences no load change, boost can still fluctuate as EGR flow demand is changed by the control system.
2. Boost pressure can vary day to day.
   • Boost pressures can vary depending on power requirements, EGR flow requirements, and ambient conditions. Because the control system can closely control the turbocharger the engine only gets the boost pressure it needs. For example: The driver can notice boost pressures of around 172 kPa [25 psi] while pulling a hill at 45 mph during the summer with a certain EGR flow. However, in winter conditions the boost pressure can be 138 kPa [20 psi] as EGR demands can change and boost pressure needs not be as great.
3. Turbocharger may whistle loudly in certain normal engine operating conditions.
   • The turbocharger may whistle loudly periodically when the engine control module (ECM) request more exhaust heat for aftertreatment regeneration purposes. This turbo whistle has been described as sounding like a boost leak. Refer to the Owners Manual, Unique Operating Characteristics of an Engine with Aftertreatment Procedure 101-047 for further information regarding variable geometry turbocharger and Aftertreatment system interactions.

Turbocharger Control Valve

The turbocharger control valve uses a signal from the ECM to control the amount of air pressure to the variable geometry actuator. Higher air pressures to the variable geometry actuator create higher back pressures and higher boost pressures. Conversely, lower air pressures to the variable geometry actuator create lower exhaust back pressure and lower boost pressures.

1. The turbocharger control valve receives its air supply from the OEM dry air tanks.
   • The valve can be heard exhausting at lower engine rpm (600-1000). The noise is produced by the valve as it relieves air pressure from the variable geometry actuator, as the ECM changes boost commands. This can be heard when the engine is idling or running at low road speeds when docking. The sound is similar to that of the fan clutch solenoid releasing air or air system exhaust valve relieving pressure.
   • The shutoff valve is designed to positively stop compressed air flow to the turbocharger control valve while the key is in the off position in order to keep the vehicle’s air tanks from bleeding down overnight. The filter is attached to this assembly to filter out any debris that can be in the vehicle’s dry tanks.

Engine Cooling

Because of changes to the cooling system as required by the addition of the EGR system, the engine cooling system characteristics will be different from that of a non-EGR engine.

1. Engine can run cooler at cold ambient air temperatures.
   • Due to more coolant flow from the water pump and more capable cooling packages provided by the OEM, there will be more coolant flow going through the engine. Due to the design changes, when operating in colder ambient temperatures, the engine coolant can average 83°C [180°F]. The engine is designed to run efficiently at lower coolant temperatures. Thermostats must not be replaced because the engine will not run constantly at 82°C [180°F]. Additional control of coolant temperatures can still be accomplished by using winter fronts, same as today’s engines.
   • Engine can run hotter at hot ambient air temperatures. Due to added heat rejection from cooling the EGR valve, EGR cooler, and variable geometry turbocharger, the engine has been designed to operate at higher coolant temperatures. It is not abnormal to see coolant temperatures between 82°C to 107°C [180°F to 225°F] while operating in hotter ambient temperatures. Coolant fans will engage when the coolant temperature reaches 99°C [210°F]. Engine protection for coolant temperature will begin at 107°C [225°F].
   • Thermostat opening is not noticeable when monitoring coolant temperatures. Due to decreasing the number of coolant thermostats from two to one on the EGR engine, it is no longer noticeable when the thermostat opens. A gradual climb in coolant temperature occurs as the thermostat opens, compared to the slight decrease and eventual stabilization of temperature seen on engines when the dual thermostats are utilized.
   • With the addition of the cooled EGR system, the under hood temperature have increased by approximately 7°C [ 20°F] compared to an non-EGR engine.

Performance/Driveability

With the addition of the EGR system to the engine, some of the characteristics of the engine are different from that of a non-EGR engine.

• It is normal to see boost pressure fluctuate while the engine brakes are activated. To take full advantage of the increased braking power provided by the addition of the variable geometry turbocharger the engine brake selector switch level chosen will not necessarily activate the same number of engine brakes as a non-EGR engine. The ECM controls the number of engine cylinders that are activated for braking and the variable geometry turbocharger will be positioned to meet the desired braking horsepower. The variable geometry turbocharger will adjust in order to meet the power requirements depending on the engine brake position switch. More braking power correlates to a more closed variable geometry turbocharger position, more cylinders braking, or both.

Shifting Techniques

Drivers have experienced a change in shifting techniques with EGR engines. With the variable geometry turbocharger improving boost response, the engine speed will decelerate and accelerate quicker than a non-EGR. The additional back pressure on the engine during out of gear, no throttle conditions causes the engine speed to drop quickly. The ability of the variable geometry turbocharger to spool up quickly causes quicker engine speed acceleration during out of gear, not throttle conditions. Drivers have characterized this as a more sensitive throttle pedal, meaning small pedal movements create large engine speed changes.