Cummins ISX11.9 CM2250 – Service Manual 010-999   Air Intake System – Overview

General Information

Variable Geometry

To maximize the performance of the engine and also to help decrease emissions levels, a variable geometry turbocharger is used on the ISX11.9 CM2250 engine.

Variable geometry turbochargers have quicker response time, and quicker engine deceleration for quicker shifting than fixed geometry turbochargers.

Because of the active control of the variable geometry turbocharger, the intake manifold pressure and turbocharger noise can often change. There is not a loss of power associated with the change in intake manifold pressure or turbocharger noise; however, customer perception of engine power can be affected. Typically, when the intake manifold pressure and turbocharger noise are changing during steady state operation, the electronic control module (ECM) is adjusting the flow of exhaust gas recirculation (EGR) into the engine and the engine power is not affected.

A turbocharger whistle may be observed at idle conditions. The engine control system is controlling the position of the variable geometry turbocharger to generate heat in the SCR aftertreatment system. This is normal engine operation and will return to a more traditional turbocharger sound if the engine idles for a period of time. The turbocharger whistle may be described as sounding like a boost leak.

When the throttle is released, perhaps for a gear change, the variable geometry actuator closes. This prepares the turbocharger to be ready to build intake manifold pressure quickly to provide improved turbocharger response when the throttle is depressed after the gear change. Because of this design for improved turbocharger response, after releasing the throttle, the engine speed of the engine with a variable geometry turbocharger can decrease more quickly than an engine without a variable geometry turbocharger. Fast deceleration in engine speed can cause operators to adjust their shifting styles, until they become accustomed to the different deceleration speeds.

Similar to all of Cummins® electronically controlled heavy duty engines, an engine with a variable geometry turbocharger incorporates a power derate to protect the turbocharger from damage while operating in high altitudes. An engine with a variable geometry turbocharger meets, or exceeds, the power output of an engine with a fixed geometry turbocharger at most altitudes. At about 2.438 km [8000 ft] elevation, however, a slight power decrease can be noticeable when operating an engine with a variable geometry turbocharger, when comparing its performance to an engine with a fixed geometry turbocharger.

The variable geometry turbocharger is electronically actuated. The ECM sends a command directly to the variable geometry actuator mounted on the turbocharger. Closing the variable geometry mechanism increases the exhaust gas pressure, facilitating EGR flow through the engine. Turbocharger speed and intake manifold pressure will also increase when the variable geometry mechanism closes under certain engine operating conditions. Closing the variable geometry turbocharger will also increase exhaust gas temperature under certain normal engine operating conditions and during aftertreatment regeneration events. This is used to improve aftertreatment component efficiency. Use the following procedure for further information regarding variable geometry turbocharger and aftertreatment system interactions. Refer to Procedure 011-999 in Section F. Opening the variable geometry mechanism decreases exhaust gas pressure, turbocharger speed, and intake manifold pressure under certain engine operating conditions.		SMALL | MEDIUM | LARGE
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The combustion air system consists of intake air piping, turbocharger, charge-air cooler piping, charge-air cooler, and exhaust piping.		SMALL | MEDIUM | LARGE
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The turbocharger uses exhaust gas energy to turn the turbine wheel. The turbine wheel drives the compressor impeller that provides pressurized air to the engine for combustion. The additional air provided by the turbocharger allows more fuel to be injected, which increases the power output from the engine. The correct turbocharger must be used. Providing too much additional air will increase the cylinder pressures and shorten the life of the engine.		SMALL | MEDIUM | LARGE
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The turbine and compressor wheels and the shaft are supported by two rotating bearings in the bearing housing. Passages in the bearing housing direct filtered, pressurized engine oil to the shaft bearings and thrust bearings. The oil is used to lubricate and cool the rotating components to provide for smooth operation. The oil then drains from the bearing housing to the engine sump through the oil drain line. An adequate supply of good, filtered oil is very important to the life of the turbocharger.		SMALL | MEDIUM | LARGE
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As the intake air is compressed by the turbocharger, the air temperature increases. This heated air is then passed through the charge-air cooler, which cools the air. Cool air is more dense, which allows more air to be compressed into the cylinder, yielding much greater combustion efficiency.		SMALL | MEDIUM | LARGE
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