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Deep inside the Chrysler Pacifica hybrid system

by Mike Volkmann

People on the Allpar forums have been asking many questions about the technical aspects of the Chrysler Pacifica Hybrid. The best place to start is with the electronically variable transmission, known as the “e-flite” or “SI-EVT.”

hybrid cutaway view

Allpar sources have informed us that FCA worked closely with technical consulting firm Ricardo to develop the transmission for the Pacifica Hybrid; and that the design is based on previous patented units by Timken with Ricardo. Electronically variable transmissions are not new; Toyota pioneered the electronically variable transmission some time ago, and licensed the system to Ford, so the basic premise of the design is well proven.

hybrid automatic transmission for chrysler pacifica

The SI-EVT transmission is a fully electronically controlled, variable speed transmission that includes:

The motors run on three-phase alternating current (AC), and are rated at 102 kW and 65 kW (a kilowatt is a thousand watts; an incandescent light bulb used 60-100 watts). The frequency of the alternating current determines the motors’ speed; the amount of current controls the torque.

The gasoline engine is also different; it uses the Atkinson cycle (like Toyota hybrids), which is more efficient but produces lower peak power. There is no exhaust gas circulation (EGR) or alternator; and the air conditioner is not driven directly by the engine (so it can be powered when running on full electric). Internally, changes including different camshafts, pistons, and spark plugs; the oil pan and intake are also different from the non-hybrid version.

Brains of the system

An integrated Power Inverter Module (PIM) mounted to the transmission case converts energy from the high voltage DC battery to AC to drive the electric motors, and also converts the AC power back to DC to charge the battery, when the motors are used as generators (“regenerative braking,” though the motors also generate some electricity while coasting). The PIM can be serviced separately if needed.

PIM - Power Inverter Module

The PIM is the brain of the entire system; it controls all electrified powertrain components, connects to both the high and low voltage wiring systems, and communicates directly with all of the primary vehicle controllers. The PIM itself has two major computer processors, the Auxiliary Hybrid Control Processor (AHCP) and the Hybrid Control Processor (HCP). The HCP controls and monitors Motor 1 (102 kW, mechanically connected to the output); the AHCP controls and monitors Motor 2 (65 kW, mechanically connected to the input). To simplify, Motor 1 is the main drive motor and Motor 2 handles auxiliary functions.

The electric motors are both inside the transmission, connected to the PIM with shielded wire. Motor direction and rpm signals are transmitted to the AHCP from Resolver A and the HCP from Resolver B. If either resolver signals are lost, the vehicle will not be allowed to operate.

The SI-EVT transmission is filled with transmission fluid for life, and has no fill tube or fluid level indicator — like many recent automatics.

The motors: how gasoline and electric power is pushed to the wheels

Let’s talk a little more about the motors themselves. The electric motors inside the si-evt transmission can do the following:

The driver uses an electronic shifter in the center of the dashboard; when the ignition is on and the brake pedal is applied, the driver can rotate the shift knob to select the conventional Park, Reverse, Neutral, or Drive modes. The Electronic Shift Module (ESM) sends the shifter position and neutral block release to the PIM (via the CAN-C bus and EPT-CAN system). The park pawl — the device that locks the transmission in place when in Park — is electronically controlled by the EPT/HEV CAN bus, which releases it only if the brake pedal is depressed.

PHEV with controller

Mechanically, the transmission is quite simple. On the input side, a single planetary system has four pinions and a clutch-operated sun gear, directly coupled with Motor 2. The planetary carrier gear is connected to the output ring gear on the differential, feeding mechanical power from the gasoline engine or Motor 2 to the front wheels (the park pawl is located on the planetary as well). To provide fully electric power to the mechanical system to drive front wheels, a transfer pinion gear on Motor 1 is also directly connected to the output ring gear on the differential. (I have not seen the clutch in person yet, but I have seen the sun gear; based on that, my understanding is that is simply engages and disengages the sun gear within the planetary by sliding it forward and back on the input shaft.)

The SI-EVT transmission has three basic modes of operation:

Electric Only Forward Motion

In electric-only mode, both motors apply torque to the wheels. As noted earlier, the first motor (the larger one), which generates around 142 horsepower, is directly coupled to the differential through the transfer gear; it applies direct torque to the wheels. Most of the time, even a minivan pushing two and a half tons can run on this power.

The second motor, generating around 88 horsepower, pushes its torque through the power-split planetary gear set — applying torque to the sun gear. The one-way clutch keeps the pinion carrier from rotating, and the ring gear is forced to rotate. That rotates the transfer gear, which rotates the differential, pushing torque to the wheels.

Electric Reverse

The gasoline engine is not used to move the vehicle in reverse at all, simplifying the gearing. Motor 1 simply spins the differential, through the transfer gear, in the opposite direction. (Because Motor 1 is directly coupled to the differential, it can drive the vehicle in forward or in reverse, depending on which way it is rotated; that is determined by the Hybrid Control Processor.)

General Hybrid Operation

In hybrid mode, gasoline engine torque is applied to the wheels through the planetary gear set. Motor 1 can also apply torque to the wheels, at the same time; Motor 2 and the sun gear apply reaction torque to the planetary gear set, while engine torque is applied to the carrier. The ring gear applies engine torque to the differential through the transfer gear.

hybrid engine

Motor 2 is used as a generator for recharging the battery pack during Hybrid operation; much of the current generated by Motor 2 is used to power Motor 1. Motor 2 controls the sun gear in the planetary gear set, allowing it control gasoline engine speed and torque being transferred to the wheels with optimum efficiency.

Control strategies

There are two types of range for this system: all-electric range (the total distance driven with the engine completely off, before the engine turns on for the first time) and charge-depleting range (the total distance driven before reaching charge-sustaining mode).

The PIM has two basic strategies:

charging cap

To quote FCA directly, “the Chrysler Pacifica Hybrid’s control strategy leverages the power of the onboard battery pack to optimize fuel efficiency and deliver strong vehicle performance characteristics, whether in electric or hybrid mode. In fact, under normal driving conditions, the top speed in all-electric mode is approximately 75mph (120 km/h).”

Who makes it, and what’s next?

The SI-EVT transmission is assembled at the Tipton Transmission plant; the gears and case are machined at Kokomo Transmission.

Magnetti Marelli has made several announcements to the Italian press about supplying electronics and possibly motors for the SI-EVT transmission. Allpar believes that Magnetti was (and is) involved in the supplying some of the electronic components, but we believe some of the Motor 1 internal components are supplied by a Chinese vendor. The high voltage battery, which is liquid cooled and weighs 368 pounds, is supplied by the South Korean company LG Chem. The battery has an On-Board Charging Module (OBCM) and the (yes, really) Electric Coolant Heater (ECH).

Allpar predicts that this transmission and hybrid system will be used in multiple products, with the next being the Chinese-market Jeep K8 for the 2019 model year, followed by an upcoming Chrysler crossover similar to the K8. Our sources claim that the AWD system will be set up by tapping the same basic system, but with a single additional motor for the rear wheels, allowing AWD vans to have Stow ’n’ Go.

Sources: FCA, SAE, and the EPA


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