Readers letters
Brian Daniels asks why electric cars are being built when their efficiency is lower than petrol engine cars (Letters, PE September). I am not sure where he obtained his engine efficiency figures.
My copy of Engineering Thermodynamics by Rogers and Mayhew suggests full load efficiencies of 25-35% for petrol engines and 30-40% for diesel engines. A more recent paper, “Approach to high efficiency diesel and gas engines” from Mitsubishi, suggests similar efficiencies for car and truck engines and quotes efficiencies approaching 50% only for very large stationary and marine diesel engines.
Taking account of actual operating conditions, the actual efficiency is likely to be less than 25%, probably close to 20%. In fact, the US EPA suggests that the average efficiency of car engines operating under real conditions may be as low as 17%. The CO2 released by burning petrol or diesel is about 210g/kWh of fuel consumed.
For the electric car the efficiency can be divided between the electricity supply 39% efficient (34% in France), the battery charge and discharge cycle (98% efficient) and the vehicle control and drivetrain (80% efficient). The overall efficiency is therefore about 30% (27% in France). About 78.5% of UK electricity is generated by fossil fuels (10% in France) with CO2 emissions of 570g/kWh (120 in France).
Assuming the traction energy for both cars is the same, the primary energy consumption, fossil fuel consumption and CO2 emissions of the two systems can be estimated (all units are per kWh of traction energy).
For a conventional (25% efficient) car the fuel consumption will be 4kWh and the CO2 emission rate is 835g.
If E85 fuel is used the fossil fuel consumption reduces to 3.4kWh and the CO2 emission rate reduces to 710g.
For the electric car, under UK conditions, the figures are 3.3kWh fuel energy, 2.6kWh fossil fuel and 742g CO2.
For the electric car, under French conditions, the figures are 3.8kWh fuel energy, 0.4kWh fossil fuel and 158g CO2.
Under UK operating conditions, the electric car provides some benefits when compared with a conventional car; the additional benefit of zero emissions at the point of use is desirable for town and city driving. Under French conditions, the electric car has clear benefits.
If the operating efficiency of the conventional engine is closer to 20%, then the benefits of electric cars, even under UK conditions, are more marked.
Robin Trow, Snodland, Kent
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