1. Increased fuel efficiency under US EPA 5 cycle fuel economy test conditions.

2. Ability to package in a range of front-wheel drive vehicles, and reduction in overall packaging volume and mass.

3. Ability to support a range of engine displacements depending on vehicle requirements.

4. A largely common drive unit between EREV and strong hybrid vehicles, with the drive unit kinematically capable of high speed EV operation and transitions between EV and engine modes. For the HEV, this allows the envelope of EV power and duration to be set by the battery capability rather than limited by the drive unit.

Malibu Hybrid System components in vehicle. The Malibu hybrid system comprises a 1.8-liter 4-cylinder engine; 60 kW, 1.5 kWh Li-ion battery pack; accessory power module (APM); Voltec transaxle drive unit—including transmission, motors, and power inverter—and associated thermal systems to provide cooling. Conlon et al. Click to enlarge.

Engine subsystem. The engine subsystem includes associated thermal and emissions hardware. The Malibu Hybrid has a larger 1.8L displacement engine than its predecessor. The new engine is fitted with an exhaust gas heat recovery (EGHR) system. GM chose the displacement as a result of system balancing of engine vs. battery size and power to meet packaging constraints and the performance and fuel economy targets for the vehicle.

The engine—dedicated to the hybrid system—features cooled Exhaust Gas Recirculation (EGR)to reduce pumping losses, and incorporates a hybrid dedicated combustion system with relatively high compression ratio of 11.5:1. An integrated exhaust manifold improves packaging, and, in conjunction with the EGHR system and SVCC catalyst, helps to reduce engine warmup time and fuel use during warmup.

Features of the new 1.8-liter engine. From a presentation by Tim Grewe, General Director – Electrification, presented at the 2016 SAE Hybrid and Electric Vehicle Technology Symposium earlier this year. The Malibu Hybrid engine shares many of the technologies of the 1.5L engine in the MY 16 Chevrolet Volt EREV (earlier post) but is operated differently in the HEV. Click to enlarge.

The exhaust gas heat recovery (EGHR) unit recycles waste exhaust heat that exits the engine and uses it to warm the engine coolant, improving engine efficiency, and also increasing the heat available for warming the passenger cabin.

The EGHR improves the overall vehicle efficiency under cold ambient conditions by recapturing about a third of the wasted exhaust power and using it to warm the engine coolant. The system allows the engine to warm up more quickly and enables a higher coolant temperature during cold operation.

The exhaust gas heat recovery unit is attached at the outlet side of the catalytic converter. Exhaust gas exiting the converter enters the EGHR and is diverted either through the heat exchanger in order to warm up coolant, or through the bypass tube which bypasses the heat exchanger. When the coolant temp is below a required value the diverter position is commanded to enable heat recovery. Once the engine coolant reaches a specified temperature the diverter is switched to the bypass position. Conlon et al. Click to enlarge.

The powertrain cooling system includes three loops:

• The low-temperature coolant loop to cool the power electronics.

• The transmission oil loop for cooling of the transmission and motors.

• he engine coolant loop which contains the engine, EGHR heat exchanger, and cabin heater core. The engine includes a mechanically driven coolant pump to circulate engine coolant.

Voltec drive unit. The drive unit is a transaxle configuration comprising transmission gearing and clutches; hydraulics; electric machines; and integrated power electronics. The basic layout is an on-axis design which is similar to GM’s 6T40/45 family of small-car front-wheel drive automatic transmissions. The center distance and vertical drop between the engine and output axes were designed to be common; this design approach enables the use of a common chain transfer design and final drive gearing, which provided a wide range of final drive sprocket and planetary gear sets allowing an effective final drive ranging from 2.64 to 3.50.

The transaxle includes two electric machines coaxial with the input shaft as well as an integrated Power Inverter Module which connects via busbars to the motors. The Power Inverter Module and motor stators are common between the EREV (Volt) and HEV (Malibu) versions. However, the transaxles were designed with unique rotors specific to the application. The stators utilize a bar winding design, while the rotors are two barrier interior permanent magnet designs using NdFeB magnets.

While the Volt application uses a Ferrite rotor in Motor A, the Malibu HEV application switches to NdFeB, due to the difference in Motor A utilization. In the hybrid, when operating as an EV, unless axle torque is very high, all propulsion power is provided by motor B, and motor A is unloaded but spinning. When the hybrid is operating with engine on, motor A torque is commanded to react engine torque in either low or high range. In the SAE paper, the GM team explains:

NdFeB magnets have higher strength than Ferrite magnets, which allows a reduction in copper losses, since less current is required to produce torque. However, iron losses at no load increase with stronger magnets. In an EREV vehicle, the system will operate as an EV most of the time. With this usage schedule, reduction of motor A iron loss is of primary importance in order to improve EV efficiency and maximize range. However, in the HEV vehicle, due to more frequent engine operation, it was important to balance reduction in copper and iron losses in order to maximize fuel economy.

… the motor operates at higher torque and lower speed in HEV mode than in EV mode, predominantly in the area of NdFeB motor advantage. Simulation studies showed that the NdFeB magnet motor provided a fuel economy benefit, and due to this, the NdFeB Magnet motor was selected for the HEV configuration in order to reduce cycle average losses for HEV vehicle usage.

—Conlon et al.

Source: Tim Grewe, 2016 SAE HEVT Symposium. Click to enlarge.

The Malibu hybrid drive unit uses Low and High modes. Low mode is capable of high output torque and acceleration; High mode allows the system to operate efficiently at lower engine and motor speeds and with less power transmitted through the motors and inverter. GM developed a transmission shift control strategy allowing a shift between Low and High modes to occur without transitioning through the fixed speed ratio.

Drive unit powerflow lever diagram. The powerflow allows transitions between Low Mode and High Mode to occur either by synchronizing the engine speed and then transitioning through the fixed gear state, or by shifting the speed of PG2 and motor B independently while the engine speed is controlled separately via engine torque and motor A. Click to enlarge.

Battery pack. The air-cooled battery pack is mounted in the vehicle trunk. The accessory power module (APM) is located on top of the battery pack cover and shares the same thermal system.

Operation. Lowest fuel consumption—representing the highest efficiency of the engine—covers a power band of 12 to about 40 kW. The system thus tends to select engine operating points within this power band, using battery power as necessary to adjust engine power demand.

As examples, in operation on the EPA FETS cycle, driving intensity and vehicle speeds are relatively low; on this cycle, the system operates with the engine on only 22% of the time. During accelerations below 30 to 40 mph, the system tends to operate in low range or fixed gear, depending on the vehicle speed and the power required.

The battery is usually charged at these points to provide for electric driving at low loads and/or speeds. When driving at constant speeds, if the engine is on, the system operates in high range at lower engine speeds and powers.

On the US06 cycle—representative of higher driving intensities—the engine is on 76% of the time. Low and high range operating points tend to shift to higher engine powers, and there is less low range operation due to the higher average speed of the schedule.

Left. System engine operation and mode selection, EPA FETS. Right. System Engine Operation and Mode Selection, US06. Conlon et al. Click to enlarge.

Resources

• Conlon, B., Barth, M., Hua, C., Lyons, C. et al. (2016) “Development of Hybrid-Electric Propulsion System for 2016 Chevrolet Malibu,” SAE Int. J. Alt. Power. 5(2) doi: 10.4271/2016-01-1169