Mechanical, electrical and hybrid powertrains

Follow the conversion chain

A combustion powertrain converts fuel energy to mechanical shaft work, then transmits some through gears and a final drive to the wheels. Not all fuel energy reaches the shaft; heat leaves through exhaust and cooling, and auxiliaries also consume energy. Performance depends on the engine’s operating point rather than one efficiency valid everywhere.

An electric path sends battery energy through power electronics to a motor, reduction gear and wheels. The inverter controls electrical conversion and motor operation; a separate low-voltage system supplies controls and accessories. Torque, electrical current, speed, voltage and temperature constrain different parts of the chain. A large stored energy capacity does not automatically provide sufficient peak power.

Connect gearing with force and speed

Define gear speed ratio i=ω_motor/ω_wheel. With effective wheel radius r and negligible slip, vehicle speed v=rω_wheel. At supplied mechanical efficiencyη, wheel torque isη i T_motor and wheel force isη i T_motor/r. Motor mechanical power is T_motorω_motor; wheel power isη times that input under the model.

A constant torque ceiling gives a force ceiling at fixed ratio. A constant power ceiling gives force proportional to 1/v once power limits dominate. The figure uses an invented envelope to show the change. At zero speed the force can be nonzero while mechanical power Fv is zero; electrical heating can still occur. Continuous operation and short peaks need different thermal evidence.

Hybrid paths add choices and dependencies

In a series hybrid an engine can drive a generator while a motor supplies wheel torque. A parallel architecture allows mechanical engine contribution to the wheels alongside electric contribution; other configurations combine paths. A plug-in hybrid also accepts external charging, whereas a conventional non-plug-in hybrid obtains energy from fuel and recovery of previously supplied vehicle motion.

Regenerative braking converts some mechanical energy into stored electrical energy. Motor, inverter, battery acceptance and tyre limits restrict it; friction brakes remain necessary for braking conditions beyond that path. Compare a hybrid mission with consistent initial and final battery energy. Finishing with a lower charge and calling the fuel decrease an efficiency gain would omit an energy source.

Apply the two motor limits

Supply motor torque limit 80 N·m, mechanical power limit 48 kW, i=9, η=.90 and r=.30 m. Wheel-force ceiling from torque is .90×9×80/.30=2160 N. At v=20 m/s, wheel speed 66.667 rad/s gives motor speed 600 rad/s; 80×600=48000 W. Thus 20 m/s is the transition in this ideal envelope.

Above that speed, wheel power is 43200 W, so at 40 m/s force is 1080 N. The gear has increased torque while reducing speed, with 4800 W lost at the 48 kW input point. If the battery or thermal controller supplies less power, actual force falls below the plotted envelope. None of these invented limits describes a named vehicle or qualifies a gear, motor or braking system.

Try the reasoning

1. Can gearing double both output torque and output speed at unchanged input power?

   Show the answer and reasoning

   No. Their product is power, and losses reduce output power further.

2. Why can stationary motor torque still require cooling?

   Show the answer and reasoning

   Electrical losses can generate heat even when mechanical shaft power is zero.

3. Why compare hybrid tests at equal battery state?

   Show the answer and reasoning

   Otherwise a change in stored electrical energy can hide part of the energy consumed.

What to carry forward

Trace every energy path and apply torque, speed, power and thermal limits together.