KERS Efficiency Chain — Energy Losses Through the Conversion Stages (2D)

Energy Recovery System

Harvest power limit 120 kW

Deploy power limit 120 kW

Energy store capacity 4.0 MJ

Conversion-Chain Efficiency

Motor/generator efficiency 93%

Battery round-trip efficiency 90%

Energy Store

Stored energy

2.00 MJ

State of charge

50%

Speed

0 km/h

Lap time

0.0 s

Harvested / lap

0.00 MJ

Delivered / lap

0.00 MJ

Heat lost / lap (harvest + deploy)

0.00 MJ

Measured round-trip efficiency (last lap)

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How it works (2D model)

The 3D KERS sim tracks stored energy as a single lossless number: every joule tapped off the brakes lands in the store, and every joule drawn from the store reaches the wheels. Real regenerative-braking systems are not lossless — energy passes through four real conversion stages, each with its own efficiency, and this 2D companion models all four explicitly.

Braking (harvest):  P_mech → [generator, η_mg] → P_elec → [battery charge, η_c] → store
Accelerating (deploy): store → [battery discharge, η_d] → P_elec → [motor, η_mg] → P_mech

η_c = η_d = √(battery round-trip efficiency)
Round-trip (wheel → store → wheel) = η_mg² · η_battRT

Cornering speed and braking deceleration are still set purely by tyre grip (v = √(μgR), same as the 3D model) — KERS never changes how hard the car can brake or corner, only how much of the unavoidable braking energy is recaptured versus lost as heat at each conversion stage.

Motor/generator efficiency — the same electrical machine runs as a generator under braking and a motor under deployment; its loss appears twice in a full round trip, so it enters the round-trip formula squared.
Battery round-trip efficiency — the fraction of electrical energy returned by one full charge+discharge cycle; split evenly between the charge and discharge legs as √η here.
The top strip chart plots speed against distance around the lap, colour-coded green where the car is currently harvesting and orange where it is deploying.
The energy-flow diagram below it shows real-time power flowing between wheel, generator/motor and store, with heat puffs sized by the instantaneous loss at each stage.

Real-world relevance: F1's MGU-K/ERS, hybrid regenerative braking and flywheel KERS buses all report round-trip efficiencies well under 100% for exactly this reason — motor/generator losses and battery internal resistance both take a real bite out of the energy a lossless model would promise.

Harvesting (braking) Deploying (accelerating) Heat loss