S. S. VERMA

ProjectsSplit hybrid vehicle

Built 2013–14 Auto Expo 2014 · B.Tech capstone

A split hybrid electric vehicle

We turned a compact petrol hatchback into a hybrid by giving each axle its own drivetrain. The stock petrol engine still drives the front wheels, and a battery-powered electric motor with regenerative braking drives a new live rear axle. In town the car runs on electricity alone; on the highway the engine takes over.

Role
Team lead; B.Tech capstone, Lovely Professional University, mentored by Varun Goyal
Funding
Lovely Professional University, for Auto Expo 2014, New Delhi
Base car
Compact hatchback, 796 cc petrol engine, front-wheel drive
Electric drive
10 kW brushless DC motor, 36 V lead-acid pack, 12:1 reduction

The idea

In most hybrids the engine and the electric motor share a transmission. Ours share nothing but the road: the engine drives the front axle, the motor drives the rear axle, and either one can move the car on its own. This layout is also called a through-the-road hybrid. Keeping the two drivetrains apart meant we could add the electric side to an existing car without touching its engine or gearbox.

Each drivetrain does the job it is best at. A petrol engine is least efficient in slow, stop-and-go traffic, and that is exactly where the electric drive runs, with no tailpipe emissions, and where frequent braking gives back the most energy. On the highway the engine runs near full load, where it is most efficient, and the motor is decoupled.

ENGINE 796 cc 4-speed batteries 3 × 12 V DC-DC CTRL MOTOR 12:1 gears differential FRONT AXLE · ENGINE REAR AXLE · ELECTRIC front
Fig. 1. The layout seen from above, front at left, in its three modes. The dashed line marks where the two drivetrains would meet in a conventional hybrid; here they have no mechanical link.

Electric drivetrain

The rear drivetrain was designed around regenerative braking, since recovering braking energy is the main benefit of hybridizing a city car. A 10 kW permanent-magnet brushless DC motor drives the rear differential through a 12:1 gear reduction. I sized it with a MATLAB vehicle-dynamics model that accounts for the car's weight, road grade, rolling resistance, aerodynamic drag and acceleration, and checked the model against a commercial electric vehicle's published specifications. Three 12 V, 200 Ah lead-acid batteries supply it. Lead-acid stores little energy per kilogram, but it delivers high surge currents, is cheap, and is rechargeable, which regeneration needs.

The accelerator pedal is linked to a pair of variable resistors, which tell a DC controller how much power to pass from the batteries to the motor, anywhere from zero to full. A DC-DC converter holds the link voltage while the motor drives the car and sends current back to the batteries while it generates. An onboard charger runs from a 220 V outlet, with current control and a voltage limit against overcharging. The CAD layout also places supercapacitors beside the batteries.

CAD model of the rear electric drivetrain on a blue base plate: the differential on the rear axle, a gear pair, a large red motor, an orange controller box, a dark battery box and small cylindrical supercapacitors, with dimension lines.
CAD side view of the drivetrain: differential, gears and motor in a line, with the battery behind the motor and dimension lines below. CAD top view: the differential on the rear axle at left, a shaft to the motor, the controller above and the battery below it.
Fig. 2. CAD model of the electric drivetrain: isometric view (top), side view (left) and top view (right). The motor (red) drives the differential through the 12:1 gears; the controller is orange and the battery pack dark grey.

Regenerative braking

When the driver brakes or lifts off, the rear wheels keep turning the motor, which now works as a generator. Its current flows back through the DC-DC converter into the batteries, and the drag of generating slows the car (Fig. 1, Braking). Friction brakes stay in the loop. They take over for hard stops, when the motor cannot supply enough braking torque, and when returning the charge all at once would overload the batteries.

  • Stability

    Regeneration brakes only the driven axle, not each wheel separately, so braking force is uneven, most of all in a corner. A brake controller has to decide when to hand over to the friction brakes before the rear wheels lock.

  • Battery limit

    Lead-acid cells accept charge slowly, which caps how much braking energy they can take in. The planned upgrades were lithium-ion cells, with higher cell voltage and easier state-of-charge sensing, and ultracapacitors to absorb braking peaks.

Converting the car

The base car is front-wheel drive. Its rear wheels sit on a dead axle, a beam that carries load but transmits no power. We took it out and fitted a live axle with a differential, so the motor had something to drive, and added an extra leaf to the rear springs to carry the weight of the batteries.

Underside of the car at the rear, showing the original dead axle beam between the rear wheels. The removed dead axle lying on a workshop floor beside three wheels. A live rear axle with a central differential housing, resting on a steel frame. Rear leaf spring under the car with an extra leaf added.
Fig. 3. The rear end, step by step: the original dead axle in place; the axle removed; the live axle with its differential; and the extra spring leaf for the battery load.

To switch between the two drivetrains, the motor has to be coupled to the axle in town and freed on the highway. We adapted a drum brake as a clutch for this, worked by a lever and cable from the cabin.

The new live axle mounted under the car: fuel tank at left, the differential at the center of the axle, and the double-layered leaf spring near the right rear wheel.
A drum brake backing plate with its actuating lever and cable, shoes removed. A brake drum held in one hand, seen from the open side.
Fig. 4. Top: the live axle installed, with the fuel tank at left, the differential at center and the double-layered leaf spring at right. Bottom: the drum brake parts adapted as the clutch between the motor and the axle.

The finished car

Repainted blue with white racing stripes and a hood scoop, the car went on to represent the university at Auto Expo 2014.

The finished car, a blue hatchback with two white stripes over the hood and a hood scoop, with eight members of the team gathered around it outside a brick campus building.
Fig. 5. The finished car with the team.

Specifications

Table 1. The two drivetrains
Front axle (engine)Rear axle (electric)
Source796 cc three-cylinder petrol, 28 L tankThree 12 V, 200 Ah lead-acid batteries
Machine37 hp at 5000 rpm; 57 N·m at 2500 rpm10 kW brushless DC motor, 1440 rpm
Transmission4-speed manual12:1 gears, differential on a new live axle
ControlStock clutch and throttlePedal-driven DC controller; DC-DC converter; 220 V charger
Used forHighwayCity, reverse and regenerative braking

The car is 3.34 m long and 1.44 m wide on a 2.175 m wheelbase. With the batteries in the back it seats two, and the design curb weight rose to 950 kg, with a gross weight of 1300 kg.

My role

I led the student team that designed and built the car, coordinating its mechanical and electrical sub-teams, and the design of the split drive pattern, the regenerative braking system and the controller circuit is the subject of my B.Tech capstone report. I also designed and tested the liquid cooling loop for the batteries, including hose fittings and pressure testing, used Ansys to optimize the transmission and the coolant flow rate, and ran the FMEA. I sized the motor with the MATLAB vehicle-dynamics model, wrote the fabrication drawings, vendor specifications, bills of materials and assembly procedures, and bought the components within the cost budget and build schedule.