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Old 11-08-2011, 11:00 PM   #10 (permalink)
t vago
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Originally Posted by Frank Lee View Post
IF it did partially bypass the throttle body, would you not simply let up on the pedal to compensate, leaving the engine with exactly the same pumping losses as before?
The driver would likely let up on the gas pedal to compensate, but that does not mean the engine would revert to the exact same pumping losses as before.

A throttle valve gives throttle control at the expense of a pressure loss induced across the throttle valve itself. There's no way around it. At the actual opening point in the throttle valve itself, the sonic velocity of the fluid (air in the case of a butterfly throttle for a gasoline engine) limits the flow through the throttle valve. Small openings will only admit small amounts of the fluid to be passed, and the passed fluid will expand in volume once it gets past the actual throttle valve opening. That incurrs a throttling loss by itself.

Now, along with this unavoidable loss, there is an additional loss incurred with butterfly valves because the butterfly valve is itself an inherently high-drag device at low throttle settings. The air past a butterfly valve forms eddies and regions of higher vacuum right past the butterfly plate. It's exactly the same principle as with the rear ends of cars as they pass through the air at speed.

Poppet valves are more efficient at throttling than butterfly valves, but they do not give as wide a range of throttling control that butterfly valves give. In order to maintain a fine amount of control using only poppet valves, many different-sized poppets are needed, and a cam mechanism is needed to schedule opening each poppet valve in sequence to give fine throttling control. This is a common form of control on steam turbine-powered ships.

Now, if you combine a butterfly valve with a small electrically operated poppet valve (such as this nifty Dyno-whatsit gadget), the small poppet valve can be cycled open at light loading, allowing less air to pass through the butterfly valve, reducing the drag losses through the butterfly valve while retaining desired part-throttle control. Keep in mind there'd still be losses due to maintaining a high intake vacuum, and there'd be losses because of the actual throttling action taking place, but the aerodynamic drag losses through the butterfly valve would be minimized.

However, that's a rather complicated arrangement, and it would take a fair amount of genius to incorporate that into a gasoline engine without having its engine computer throw a fit of some sort.

Hm... I might experiment with this. I have 6 vacuum valves laying around my garage.
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