The prototype bus incorporated a small car engine whose only task was to keep the flywheel spinning. The engine ran at the constant speed at which its fuel efficiency was optimal.

Equipped with the steel combo, the small-block produced 535 hp at 6,400 rpm and 467 lb-ft of torque at 5,200 rpm. The engine was perfectly repeatable, making any change in power immediately ...

A digital readout shows it spinning at a sky-high 11,502 RPM and honestly, ... This is another small-scale engine from JohnnyQ90 on YouTube. ... and there's a new flywheel at the back.

Volvo claims that the flywheel’s accelerative boost not only aids fuel economy, but performance as well, with up to 80 additional horsepower available when the flywheel and engine join forces.