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256*Typical petrol engines are about 25% efficient.* Encarta [6by8x] says “The

efficiencies of good modern Otto-cycle engines range between 20 and 25%.”

The petrol engine of a Toyota Prius, famously one of the most efficient car

engines, uses the Atkinson cycle instead of the Otto cycle; it has a peak

power output of 52 kW and has an efficiency of 34% when delivering 10 kW

[348whs]. The most efficient diesel engine in the world is 52%-efficient, but

it’s not suitable for cars as it weighs 2300 tons: the Wartsila–Sulzer RTA96-C

turbocharged diesel engine (figure A.15) is intended for container ships and

has a power output of 80 MW.

–*Regenerative brakes roughly halve the energy lost in braking*. Source: E4tech

(2007).

257*Electric engines can be about 8 times lighter than petrol engines*.

A 4-stroke petrol engine has a power-to-mass ratio of roughly 0.75 kW/kg.

The best electric motors have an efficiency of 90% and a power-to-mass ratio

of 6 kW/kg. So replacing a 75 kW petrol engine with a 75 kW electric motor

saves 85 kg in weight. Sadly, the power to weight ratio of batteries is about

1 kW per kg, so what the electric vehicle gained on the motor, it loses on the

batteries.

259*The bike’s engine uses energy with an efficiency of 0.25*. This and the other

assumptions about cycling are confirmed by di Prampero et al. (1979). The

drag-area of a cyclist in racing posture is *c*_{d}*A* = 0.3 m^{2}. The rolling resistance

of a cyclist on a high-quality racing cycle (total weight 73 kg) is 3.2 N.

260*Figure A.12*.

Prius data from B. Z. Wilson [home.hiwaay.net/~bzwilson/prius/]. BMW

data from Phil C. Stuart [www.randomuseless.info/318ti/economy.html].

Further reading: Gabrielli and von Kármán (1950).