256Typical petrol engines are about 25% efficient. Encarta [ ] 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
[ ]. 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
257Electric 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
259The 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 cdA = 0.3 m2. The rolling resistance
of a cyclist on a high-quality racing cycle (total weight 73 kg) is 3.2 N.
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).