Some serious folks look at the subject
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The pneumatic hybrid engine, which follows the downsizing and supercharging paradigm, offers a fuel-saving potential that is almost equal to that of hybrid electric powertrains while inducing much lower additional mass and cost penalties, according to the ETH Zürich researchers.
When required, e.g. when starting from rest or after changing gear, compressed air flows into the engine through an electronically controlled valve. If fuel is also injected, the engine responds quickly.
The compressed air supply also allows extreme downsizing. Conventional car engines can have peak powers of 150 hp or more, but usually need no more than 30 hp for everyday driving. Downsizing the engine in this instance halves the number of cylinders from four to two. This also halves frictional losses and increases the engine’s average efficiency. To keep the maximum power and thus satisfy the consumer’s drivability demands, the engine is highly supercharged by a turbocharger—which exploits the exhaust gas enthalpy as an energy source, and which boosts the to the desired levels.
Initial results from a prototype on the test stand in the ETH Zürich Machinery Laboratory show an increase in the engine’s average efficiency in the European Test Cycle from 18 to 24%. This corresponds to a fuel saving of one-third. Energy savings of up to 50% are achievable in purely urban traffic, because the engine can pump air into the compressed air tank during braking, thus recovering the kinetic energy.
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