To maximize the performance of the engine and also to help decrease emissions levels,a variable geometry turbocharger is used on the Cummins ISX engine, referred to as automotive with Cummins CM870 and automotive with CM871
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The 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, 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 Cummins ECM is adjusting the flow of EGR into the engine and the engine power is not affected.
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 ISX 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 drivers to adjust their shifting styles until they become accustomed to the different deceleration speeds.
Similar to all of Cummins Inc. electronically controlled heavy duty engines, the ISX engine with a variable geometry turbocharger incorporates a power derate to protect the turbocharger from damage while operating in high altitudes.
The ISX engine with a variable geometry turbocharger meets or exceeds the power output of the ISX engine with afixed geometry turbocharger at most altitudes. At and around 2.438 km [8000 ftl elevation, however,a slight power decrease can be noticeable when operating the ISX engine with a variable geometry turbocharger, when comparing its performance to the ISX engine with a fixed geometry turbocharger.
Automotive with CM870
The Cummins ISX with CM870 variable geometry turbocharger is pneumatically actuated with air from the OEM air tanks.
High air pressure from the turbocharger control valve closes the variable geometry mechanism, which increases the exhaust gas pressure and facilitates EGR flow through the engine.A closing variable geometry mechanism also increases turbocharger speed and intake manifold pressure under certain engine operating conditions. Lower air pressure from the turbocharger control valve opens the variable geometry mechanism, which decreases exhaust gas pressure, turbocharger speed, and manifold pressure under certain engine operating conditions.
Depending on the build date, the ISX with CM870 uses either a low mount turbocharger control valve or a high mount turbocharger control valve.
The early Cummins ISX with CM870 use the high mount turbocharger control valve, which is located on top of the front gear housing.
