Quote:
Originally Posted by Ecky
Reading "How do you get your car to warm up quickly?" I was struck with an idea. I haven't fully thought it through yet, and would like some feedback. It might be completely idiotic for reasons I haven't considered yet.
My premise:
Having too large an exhaust pipe can rob an engine of low-end torque because it disrupts scavenging which is carefully tuned by the manufacture to assist the engine in removing exhaust gases, right?
If that's correct, I'd surmise that it increases engine efficiency. What happens then, if there is decreased air pressure on the exhaust side? Would it function something like a reverse-turbo, giving "free" power, so long as you don't actually suck any unburnt fuel out of the cylinder during the period of valve overlap that many engines have?
Of course, something would have to create the vacuum, but you don't necessarily need a belt-driven turbine, which would almost certainly defeat the purpose. What about using something like the venturi effect? Use restriction someplace like behind the radiator (where you can't help but have turbulent airflow anyway) to create vacuum, and run a small hose to somewhere on the tailpipe.
If the added vacuum-assistance is non-negligible, would this do anything for engine efficiency?
Thoughts please! I may go out tomorrow with a shop-vac and some duct tape and experiment with different positive and pressures on my exhaust pipe. If I pull the IACV, I should be able to tell whether I'm generating power by the idle RPMs.
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Basically what you are saying here is; during the exhaust stroke the engine is doing work pushing exhaust gas out.
Work that could be contributing to turning the wheels (fuel economy) if it weren't needed in this department.
ie: With enough vacuum the exhaust stroke would become an extra mild power stroke.
Well there is in fact a
free vacuum in the wake behind cars. (at speed)
It's not as strong as you/we'd like for this purpose, but IT IS FREE..!
Not only that but the engine is doing work overcoming that vacuum that's pulling the car backward, using fuel.
So by exhausting into the area behind the car where there is the highest vacuum, on average (any/all speeds used), would 'kill these 2 birds with one stone'.
So the question becomes:
Where exactly, on a car's vertical rear, would be the optimal place for the exhaust to exit?
That is why I linked the only research paper on the subject here:
https://ecomodder.com/forum/showthre...tml#post695293
While the paper seems to have relied on a very early version of google translator, translating Vogon, or some human translator, way better versed in English than physics,
I NB that the optimal position for the exhaust to exit seems to just below dead center in the back of a car's vertical trunk area.
There are legal issues with that and condensation/water buildup was said to be an issue elsewhere in the forum.
Both of these issues 'go away' if one of those valved (quiet/loud) exhaust systems is modded to achieve this.
The car's rear bumper may have to suffer some modding however and I would guess that a flattened trumpet shape is likely to work best by slowing the exhaust down so that it isn't all shot out through/past that low pressure wake.
Conversely:
One can relieve the high pressure area in front of a car and decrease the work done by the engine during the intake stroke by putting the (Bell Mouth) intake at the position of maximum pressure in front of the car.
For gasoline engines where the intake is purposefully restricted to control engine power/rpm this only works at or around WOT, so not as much incentive for this mod.
On Diesels it's a completely different story and a Bell Mouthed ram air intake located at the point of maximum pressure in the car's nose is worth considering.
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I'd love to hear that Beetle!
