Air-Injection Surpercharging -- A Page from History

Improving the fuel economy of a passenger car by installing a small-displacement low-power engine is a methodology long recognized, but the accompanying loss in vehicle performance is a tradeoff unacceptable to the customer. Recovering the power deficiency by boosting the engine with a turbocharger or an engine-driven supercharger has often been suggested as a remedy. Turning back the pages of history, about thirty years ago an unusual supercharging scheme was evaluated that involved injection of high-pressure air from a storage reservoir directly into the cylinders of a downsized engine. Makeup air was provided by a pair of 21-MPa (3000 psi) engine-driven compressors. Large gains in fuel economy were measured when the compressors were not required to recharge the storage reservoir, as might be expected, but in simulated city and highway driving, those gains were greatly diminished by the need to replace stored supercharging air. That fact, together with concerns about safety, cost, and loss of trunk space, caused rejection of the idea.