Engines · Electric10 min read

The EV drivetrain: no clutch, one gear, and a pedal that refuels

Strip a petrol car of its clutch, its gearbox, its exhaust and its idle, and you have not broken it — you have described an electric car. All of those parts exist to babysit one weakness of the piston engine: it makes no torque until it is already spinning. An electric motor has no such weakness, and the whole drivetrain collapses around that fact.

The torque curve that changes everything

A piston engine makes torque by breathing: it needs airflow, and airflow needs revs. Below about 1 000 rpm it cannot even keep itself turning; its torque then climbs to a peak somewhere in the middle of the rev range and falls away again. An electric motor makes torque with a magnetic field and a current — and both are at full strength while the shaft is still bolted-down stationary. So its torque curve starts at maximum, holds a flat plateau, and only begins to fall when the electronics run out of voltage.

A representative 150 kW EV drive unit, computed from its two limits: below base speed the inverter's current limit fixes the torque; above it the voltage limit fixes the power. Nothing about this curve is tuned — it is the physics of the two ceilings.

The knee in the middle is called the base speed. Below it the motor is current-limited: torque is proportional to current, the inverter feeds the maximum current it and the windings can stand, and torque sits at its ceiling — a constant-torque region. As the motor spins faster, though, it also acts as a generator: it pushes a voltage back at the inverter (the back-EMF), growing in proportion to speed. At base speed that back-EMF has risen to meet the battery voltage and the inverter can no longer force full current in. From here torque must fall as 1/ω, which means torque × speed — power — stays constant: the constant-power region. You can watch both regions being born in the rotating-field demo by sliding the supply frequency past its base value.

Back-EMF, field weakening, and the two ceilingsfor engineers

A spinning permanent-magnet rotor induces a voltage in the stator proportional to speed, with the same constant that converts current to torque (in SI units, numerically equal):

e = k_e\,\omega, \qquad T = k_t\, i, \qquad k_t = k_e \;\; \text{(SI)}

Neglecting resistance, the inverter can only push current while $V_{dc} > e$ . Below base speed $\omega_b = V_{dc}/k_e$ the binding limit is current: $T = k_t I_{max}$ , constant. Above it the drive field-weakens: it injects current along the magnet axis to partially cancel the rotor flux, holding the terminal voltage at the ceiling while speed keeps rising. The usable envelope becomes

T(\omega) \approx \min\!\left(k_t I_{max},\; \frac{P_{max}}{\omega}\right), \qquad P_{max} \approx k_t I_{max}\,\omega_b

Field weakening is not free: the flux-cancelling current produces no torque but still heats the windings, so efficiency sags at high speed, and if the electronics ever cut out up there the un-weakened back-EMF can exceed what the inverter survives — one of the real design trade-offs in choosing $k_e$ . Induction motors field-weaken too (flux ∝ V/f), which is why their envelope has the same two regions with a slightly droopier top end.

Why there is no clutch

A clutch exists because a piston engine must keep spinningeven when the car is stopped. Something has to let a shaft turning at 800 rpm meet wheels turning at zero — so we slip friction plates (manual) or stir a fluid (torque converter) and accept the wear and waste. An electric motor is perfectly content at 0 rpm: energised and stationary, it simply holds torque like a spring, indefinitely and without overheating anything. Wheels at zero, motor at zero — nothing to decouple, ever. Pulling away is not a delicate biting-point negotiation; it is the inverter raising current from zero as smoothly as a dimmer switch.

Why one gear is enough

A gearbox is a torque-and-speed matching machine. A petrol engine only works well between roughly 1 000 and 6 500 rpm — barely a 6:1 span — while the car needs wheel speeds from 0 to over 1 500 rpm. Six ratios chop the road-speed range into slices, each one holding the engine inside its narrow happy band. The EV motor above works from 0 to 16 000 rpm and delivers full power across more than two-thirds of that span. One fixed reduction maps its range onto the whole road-speed range with room to spare:

Wheel torque available at each road speed, computed for both drivetrains at the same ~150 kW peak. The petrol car's six humps each trace the engine's torque curve through one ratio — and every gearchange is a fall off one hump onto the next. The EV's single curve is the motor envelope through one 9.7:1 reduction; it starts higher than first gear and never lets go of the wheels.

Notice what the sawtooth costs: below each hump’s peak the petrol car is off its best, and during every shift the torque drops to zero. The EV curve is not just smoother — it is the upper envelope the gearbox is desperately trying to approximate. A gearbox is a mechanical impression of a motor curve.

Why the magic number is about 10:1for engineers

The single ratio is pinned from two ends. The top end sets a ceiling: at maximum motor speed the car must reach its intended top speed,

G \;\le\; \frac{\omega_{motor,max}\, r_{wheel}}{v_{max}} \;=\; \frac{16\,000 \cdot \tfrac{2\pi}{60} \cdot 0.334}{208/3.6} \;\approx\; 9.7

The bottom end sets a floor: launch torque at the wheels is $T_{wheel} = G\,T_{motor}$ (here 9.7 × 310 ≈ 3 000 N·m — enough to spin the tyres, which is the real launch limit anyway) and hill-start gradeability scales the same way. A bigger G would also let a smaller, faster motor do the same job — motor mass falls roughly as 1/G for the same wheel torque — which is why designers push G as high as the top-speed ceiling and gear-noise allow. Around 9–11:1 is where those constraints meet for most passenger EVs. The exceptions prove the rule: the Porsche Taycan adds a second ratio precisely because it wants both a 260 km/h top speed and a violent launch, and one ratio cannot serve two masters that far apart.

Battery → inverter → motor

The battery stores energy as DC. The motor, as the rotating-field demo shows, wants three-phase AC whose frequency sets its speed and whose current sets its torque. Sitting between them is the inverter: banks of transistors switching the battery voltage thousands of times a second to synthesise three smooth sine waves of any frequency and amplitude it likes. It is the throttle, the gearbox logic and the engine-management computer rolled into one silicon box — when you press the accelerator you are not opening anything, you are asking the inverter for a torque, and it computes the currents that deliver it.

The whole drivetrain. Note the dashed return path: the same four boxes run right-to-left when braking. No separate machinery is added to regenerate — it is a property the chain has for free.

Braking that refuels

Every electric motor is also a generator; which one it is at any instant is just the sign of the torque. Ask the inverter for torque againstthe direction of rotation and the machine converts the car’s kinetic energy back into electrical energy, pumping it into the battery instead of boiling it off as brake-disc heat. That is regenerative braking: no extra hardware, only a sign change in the control software. It is why one-pedal driving exists, why EV brake discs famously rust from disuse, and why city driving — a petrol car’s worst case — is an EV’s best.

Where regen runs outfor engineers

Regen is capped by whichever link says no first: $P_{regen} = \min(P_{motor},\, P_{inverter},\, P_{battery,charge})$ . The battery is usually the weak link — a cold or nearly-full pack may accept only a fraction of the motor’s braking capability, which is why regen feels weaker on a winter morning and the friction brakes quietly blend in (the pedal is a decelerator request, not a hydraulic lever). Hard stops exceed the motor’s rating outright: a 150 kW drive unit cannot absorb the ~500 kW a full emergency stop from motorway speed releases. And because regen acts only on the driven axle, stability control limits it on slippery roads. Net result: around 60–70% of braking energy comes back in town driving — a rounding error for a piston car, a double-digit range gain for an EV.