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Old 01-02-2008, 05:31 AM   #1 (permalink)
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Join Date: Jan 2008
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Fix Me - '71 Datsun 240Z

BlueStreak - '02 Bianchi Bianchi Eros
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Engine braking without using fuel idea

I have read that some hypermilers will shut off their engines and coast in neutral to save gas. While this is definitely fuel efficient as long as you don't lose too much speed and end up having to accelerate can be dangerous because you lose brake vacuum, power steering assist, electrical charging, etc.

In order to maintain these other functions don't need to burn just need to keep your engine spinning. That way the cylinders are still producing vacuum for your brakes and the alternator and power steering pump are still working.

So my idea is to add a fuel cut-off selenoid in my fuel system both upstream of the carburetors (dual carbs) and between the float bowls and needle valves (SU carburetors, the float bowls and venturi bodies are connected by an external fuel line). The selenoids will be electrical, so I can just flip a switch to cut off fuel to the engine...and as long as the clutch is engaged and the tranny is in gear with the car moving, the engine will still spin and everything will still work (I don't have power steering though, so no worries about that). If I need engine power again, I just flip the switch, fuel will flow to the carbs, and as long as I'm still moving the engine should make power again.

Obviously since this is engine braking instead of coasting, I'll slow down more quickly. This would probably be most usefull going downhill or slowing down to a stoplight.

I haven't actually built this yet since I just had the idea and money's a bit tight right now, but I plan to sometime in the future.

Actually this would be much easier and cheaper to do with a fuel injected car. Since the entire fuel system is controlled electrically, you could probably just put a switch inline with the ECU or something like that and not have to screw around with selenoid valves like I would with my carburetors (the fuel injectors are the selenoid valves).

Also, I'm not sure if this would work with an automatic transmission since it uses a torque converter instead of a clutch. Anyways, just wanted to see what you guys would think of my idea.

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