Talk:GM New Look (Fishbowl) Bus

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[edit] SUPERCHARGER

I edited the statement that referred to the Detroit Diesel (DDA) engine as being supercharged. Since this engine is a two-stroke design, it cannot aspirate without some means of positive induction. This is the purpose of the Roots blower.

The blower plays a dual role in this engine: it scavenges the exhaust and supplies combustion air. The blower does this by pressurizing the "air box" (a cored out passage in the engine block), which delivers air to the cylinders as the pistons are passing through bottom dead center. The air box pressure is slightly above atmospheric pressure to prevent residual combustion gas backflow into the air box and blower at the point where the piston initially exposes the air intake ports located near the bottom of the cylinder. The blower's capacity is matched to the engine displacement to deliver a suitable volume of air.

Referring to the blower as a supercharger is a common error amongst those who are not familiar with DDA engines and how they operate. There were some cases where a turbocharger was fitted to DDA engines -- the turbo pressurized the intake side of the blower, thus producing a form of supercharging. Since the blower is a positive displacement device that moves a (more or less) fixed volume of air per revolution, resulting in a fixed pressure increase, the only effect of turbocharging was to increase air box pressure above atmospheric in order to improve fuel economy and reduce exhaust emissions. The turbo could not actually increase the volume of air flow due to the behavior of the blower.

Bigdumbdinosaur 21:49, 30 October 2007 (UTC)