Changing Dynamics of Hybrids and Plug-ins, Plus Service How-Tos - Tire Review Magazine

Changing Dynamics of Hybrids and Plug-ins, Plus Service How-Tos

When the term "hybrid vehicle" is used, it most often refers to a hybrid electric vehicle. As you will see, when discussing hybrids, it's important to remember that hybrid technology is a work in progress and that there's no such thing as a "generic" hybrid design.

At its introduction, the Chev­rolet Volt not only grabbed headlines on the business pages, but also those of the automotive publications. Some say this was a “feel good” story of a company on the mend, but it was actually the next chapter of the “electrification” of the automobile that started with the introduction of the EV1 in 1996 and the Prius in 2002. 

Last year, the Volt was named the “2011 North American Car of the Year,” a title garnered from votes by a jury of automotive journalists from the U.S. and Canada, representing newspapers, magazines, and websites, as well as television and radio shows.

Since the industry is all abuzz on this new technology, it’s a safe bet that your customers are, too.

If you’re not keeping up with these vehicle technologies, you could find yourself missing out on service opp­ortunities, as the number of hybrid vehicles on the road is expected to grow despite initial costs.

At a recent Society of Automotive Engineers (SAE) Congress, some industry experts were predicting that by 2020 some 25% of all passenger cars and light trucks in the U.S. will be hybrids, plug-in hybrids or full electrics.

With a little more than one million hybrid vehicles on the roads today, it has given some of you an opportunity to view them up close in your shop.

Whole New Language
When the term “hybrid vehicle” is used, it most often refers to a hybrid electric vehicle. These encompass such vehicles as GM’s AHS2 platform (Chevrolet Tahoe, GMC Yukon, Chev­rolet Silverado, Cadillac Escalade and the Saturn Vue), Toyota Prius, Toyota Camry Hybrid, Ford Escape Hybrid, Toyota Highlander Hybrid, Honda Insight, Honda Civic Hybrid, Lexus RX 400h and 450h, and others.

And, as you will see, when discussing hybrids, it’s important to remember that hybrid technology is a work in progress and that there’s no such thing as a “generic” hybrid design.

Today, there are two classifications and two acronyms: Battery Electric Vehicle (BEV) and Hybrid Electric Vehicle (HEV). SAE defines an HEV as a vehicle with two power sources – electricity and the internal combustion engine.

Charging the Batteries
There are two variations of the HEV theme. The most common is the conventional hybrid that uses an internal combustion engine as the source for charging the batteries.

The second type – and next logical step – are hybrids that not only turn fuel from the tank to charging power, but also are able to plug into the po­wer grid for a charge.

The hybrid uses electrons like a combustion chamber burns hydrocarbons. But, like most gas pumps, the energy can come in many forms, such as gasoline and diesel. For electrified vehicles, it comes in alternating current (AC) and direct current (DC). The real advancement is in how the hybrid converts and manages the two types of current.

Most hybrids on the roads use a high voltage AC motor/generator and internal combustion engine to drive the vehicle. The engine also drives a second motor/generator and inverter to supply DC to the battery pack. The DC from the battery is converted back into three-phase AC by the inverter diode bridge to drive the motor/generator. 

For a plug-in HEV, a separate single-phase inverter can be used to convert AC from the power grid to DC to charge the batteries. Some of these charge stations are mounted in the home or garage.

A BEV and Extended Ran­ge Electric Ve­hicle (ER­EV), like the Volt, will use only the power grid to charge the batteries. A BEV and EREV will use a three-phase inverter to drive the high voltage motor and a separate single-phase inverter to charge the batteries. The EREV uses an onboard generator driven by the gas engine to supply three-phase high voltage AC to the motor.

Two Drive Systems
Power split: The Toyota Prius Hybrid Synergy Drive, Chevrolet Sil­verado two-mode and Ford Fusion hybrid are considered power split drive systems. The system is made up of a high voltage battery pack, a mot­or/generator (MG1 in the illustration below) that has a primary function of charging the battery pack. 

A planetary gear set transmits the power from a motor/generator (MG2 in the illustration) to the drive wheels. The sun gear of the planetary gear set is connected to the engine, planetary gears to MG1 and the ring gear to MG2. The power split system has two modes of operation: 

1. Electric vehicle mode uses MG2 only to drive the vehicle at low speeds. 

2. The Prius ICE has a clutch that is used as a damper for continuous engagement to the planetary gear set.

The engine, MG2 and MG1 are en­gaged to drive the vehicle and charge the batteries during normal operation. The Prius Synergy Hybrid Drive system programming controls the ICE, MG1 and MG2 to maintain vehicle speed and battery pack state of charge. The Silverado two-mode drive has a high voltage battery pack, an MG1, MG2 and planetary gear set in the same manner as the Prius, plus a four-speed transmission with reverse.

Parallel: A parallel system does not use the planetary gear set. The mot­or/generator MG1 is di­rectly connected to the engine and transmission by a clutch or torque converter. The motor/generator can be switched to drive the vehicle or charge the high voltage batteries. This system is used in the Honda Insight.

Technically speaking, the Volt EREV system is not a hybrid. Powered by GM’s Voltec propulsion system, it consists of a 16-kWh lithium-ion battery pack and electric drive unit that provides pure electric range between 25 and 50 miles, depending on terrain, driving techniques and temperature.

A 1.4L gasoline-powered engine extends the range up to an additional 344 miles on a full tank of fuel by operating the vehicle’s electric drive system until the car can be plugged in and recharged or refueled. This distinguishes the Volt from electric-only vehicles, which cannot be operated when recharging is not immediately available – such as during a power interruption or on a long-distance trip.

The generator and battery pack are connected electrically in parallel to the AC motor, and the motor has regenerative braking capability to charge the high voltage battery pack. Note: The AC motor also contains a two-speed transmission with reverse.

Regenerative Braking
Electric-powered vehicles also incorporate regenerative braking technologies into their designs. Basically, regenerative braking uses the inertial energy of the vehicle to charge the high-voltage battery pack by switching the motor to a generator and converting mechanical energy to electrical energy. Every regenerative stop is an ABS stop.

The ABS is used to isolate the master cylinder and reduce the hydraulic force to the calipers. The charge rate of regenerative braking is dependent on the deceleration mode of the vehicle.

In coast-down mode, the regenerative rate is constrained to allow for rolling resistance to simulate the engine braking against cylinder compression. In braking mode, the hydraulic application is restricted to the drive wheels and motor.

The MG2, seen in the Silverado illustration, becomes a generator and braking is done by the conversion of mech­anical energy to electrical energy. In ABS mode, both regenerative and hy­draulic braking can be applied. Unless it is altered, the brake pedal will res­pond in the same manner as an ABS stop.

To simulate the operation of a traditional vacuum booster hydraulic brake application, the Toyota Prius uses a stroke simulator and brake actuator for normal and ABS operation.

The stroke simulator is a solenoid-activated valve that diverts hydraulic pressure from the master cylinder to an accumulator. It is located between the master cylinder and the brake actuator. It uses two coil springs to simulate a normal pedal stroke under normal braking.

The brake actuator houses the pump, accumulator, relief valve and pressure sensor to apply hydraulic pressure to the calipers in a regenerative stop. To control the pressure, there are two master cylinder valves, four pressure apply valves, four pressure release valves, two master cylinder pressure sensors and four wheel cylinder pressure sensors.

The brake pedal stroke sensor works in conjunction with the stroke simulator and provides input to the skid control ECU that controls the stroke simulator. The brake pedal stroke sensor is a non-contact rotary sensor with a two-axis Hall sensor element.

A normal controlled stop signal from the brake pedal stroke sensor would activate the stroke simulator to allow maximum regenerative braking. A panic stop signal from the brake pedal stroke sensor would apply a full ABS application. However, brake pedal stroke sensor can affect the operation of the stroke simulator. Verifying the operation of the stroke simulator can eliminate it as the source of the problem.  

Service Notes
Pad replacement on a hybrid vehicle is the same as a conventional ABS-equip­ped vehicle. Caliper pistons can be reset in their bores and not trigger a trouble code with the vehicle shut down.

This can be a concern for vehicles equipped with a transponder keyless push-button start system, such as the Toyota Smart Key. If the transponder is within range of the vehicle, the ABS and other systems may be active. With the ABS system activated, resetting the cal­iper pistons in their bores could trip an ABS code. To make sure the transponder is out of range of the vehicle, press the “Start” button.

Prius Smart Key: The disable switch for the ‘smart key’ is located on the bottom of the instrument panel to the left of the steering wheel. Pressing the button will disable the smart key. When disabled, the button is extended. Press­ing the button again will latch in and enable the smart key system. Check the owner’s manual for the exact location of the switch.

When installing a brake pedal stroke sensor, a new sensor lever is locked in the “0” stroke position by a small pin. Do not detach the pin until the installation has been completed, then firmly press the brake pedal once to break off the pin. Remove the broken pin from the sensor lever.

If a brake job includes an alignment, Toyota service bulletin T-SB0020-08 recommends that the 12-volt battery be disconnected and a zero point calibration be performed. You must also reset all of the accessories memory positions that the 12-volt battery powers. This bulletin covers all Toyota vehicles with vehicle stability control.

If the second generation Prius parking brake warning light remains on with the brake released, service bulletin BR002-07 provides the information to correct the condition.

The two-mode Silverado master cylinder contains a travel sensor and the hydraulic control unit contains a stroke simulator that operates in the same manner as the Prius. The Ford Escape master cylinder contains a stroke simulator and pedal force sensor.

System Integration
NHTSA mandates for vehicle skid and rollover safety systems has brought about the integration of the ABS with the electronic throttle control (ETC).

This integration requires the input of sensors and software programming. The sensor input is shared by the controllers that operate the ABS and ETC. The wheel-speed sensors, brake pedal position/force sensor, hydraulic pressure sensors and throttle position sensor are sensors that can be directly involved in the operation of the regenerative braking operation.

But, when there is a problem with these units, a scan tool and service information are key to “fixing it right the first time,” and remain the best tools for success. 

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