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Old 07-02-2022, 12:16 AM   #1 (permalink)
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Question Warm Ram Air Intake?

Howdy all!

BACKGROUND
I've just spent the last few days reading a kabillion threads, articles, and papers on Air Intakes (AI): CAIs, WAIs, Ram Air Intakes (RAI), superchargers, turbochargers, and the like. Several of those were here from ecomodder.

What I found was a miasma of special cases where one type of intake would give better HP / FE in one regime, but could have the opposite effect in another. So I tried to distill down the things we can definitively say:
1) Reducing pumping loss through the engine improves fuel economy. Viewing the engine as a big air pump, that has the immediate corollary
2) Increasing AI pressure could improve fuel economy under certain conditions, such as WOT on a fuel injected vehicle, vehicles with carburetors, turbos, etc. But under low load with fuel injection, the higher pressure is wasted, because the throttle just restricts to keep stoichiometric.

The essential problem is that, while a crapton of modifications make the oxygen at the AI more dense (cold, pressure), that delivers more power, not necessarily better fuel economy. It helps pulse and gliders, but does little for someone like me who sets the cruise control.

I'm currently designing the front to Champrius 4.0 and a big part of that is how to properly route the radiator outlet flow. The stock design is to dump a huge portion right under the engine, and about 40 sq in through the cat channel. I'm going to enclose the entire engine bay as best I can, but am still concerned about air near the top of the radiator having to take several twists and turns to exit through a rough compartment.

My bright? idea was to put an (Warm Ram Air Intake) WRAI behind the radiator near the top and connect it to the stock air box. Even if this does nothing for the engine itself, I believe it will give me minor gains just providing suction near the top of the radiator. More specifically, it will make the radiator more efficient enabling a smaller air inlet, and it will reduce outlet flow across a rough engine bay. This is maybe 0.5% stuff, but it's low hanging fruit since I'll be modding that whole area anyway.

Champrius is a 2009 Toyota Prius with normal coolant temperatures 185-190F and stock air intake around 5-10F above ambient. The inlet duct is located on the passenger side above the wheel, which goes through a resonator, then a hose, and into the air box. I've already done tests where using engine bay air directly makes FE worse by around 2%, even though the temp was +5F higher. The only explanation I had at the time was that the higher air pressure near the front of the wheel was significant (I didn't know about RAIs back then), or that my margin of error was larger on those runs for some reason.

QUESTIONS

Would the WRAI described above (behind the radiator) be better than the stock intake for FE at 65 mph cruise control? That question really boils down to:

Does anyone know the difference in pressures at the wheel well top vs. behind the radiator? (Julian Edgar, where are you when we need you? I know the MAF measures mass air flow and temperature, so this could be the same as...

Does the car measure AI pressure? If so, I could just mock up what I'm thinking and measure all the stats directly.

Can anyone think of issues with doing the WRAI? Right off the bat, it seems to me that we'd be trapping heat in a loop, which should help with cold starts / weather BUT might be dangerous in a heavy load / hot day scenario. Subtleties like ignition timing etc. need to change with the air characteristics, but I'm assuming the fuel injected ECU magically takes care of all that.

And for the bonus question: has anyone already built and measured such a device? I came across a ton of DIY mods for CRAIs (Cold Ram Air Intakes) but couldn't find any WRAIs.

Apologies in advance for ye readers that have already seen variants of AI questions a dozen times. If this is a standard question that I simply haven't found yet, please direct me to the appropriate sources and I'll do more research myself.

Thanks!

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Old 07-03-2022, 10:27 AM   #2 (permalink)
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I doubt you're going to get the ram air effect behind the radiator. Maybe in front of it. But then, you won't get the heating.

That said, the only way to know for sure is to try it.
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Old 07-03-2022, 04:55 PM   #3 (permalink)
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Quote:
Originally Posted by Talos Woten View Post
QUESTIONS

Would the WRAI described above (behind the radiator) be better than the stock intake for FE at 65 mph cruise control? That question really boils down to:
I do not believe so.

Quote:
Does anyone know the difference in pressures at the wheel well top vs. behind the radiator? (Julian Edgar, where are you when we need you? I know the MAF measures mass air flow and temperature, so this could be the same as...
I doubt there is much difference at all.

Quote:
Does the car measure AI pressure? If so, I could just mock up what I'm thinking and measure all the stats directly.
I do not believe so. It just has a MAF, not a MAP sensor.

Quote:
Can anyone think of issues with doing the WRAI? Right off the bat, it seems to me that we'd be trapping heat in a loop, which should help with cold starts / weather BUT might be dangerous in a heavy load / hot day scenario. Subtleties like ignition timing etc. need to change with the air characteristics, but I'm assuming the fuel injected ECU magically takes care of all that.
It is possible that warm air may be somewhat beneficial for your fuel economy on a Prius, but I am skeptical. On a non-hybrid I would certainly recommend it (and I run one on my Mirage), but the Prius is a bit different. As soon as the Prius' rpm are above 1200, the engine is pretty optimally loaded. RPM is the only differing factor now, and the RPM is changed by vehicle speed and engine load. Lets take a look at a 1nz-fxe brake specific fuel consumption chart to explain this.



This map shows that the Prius' engine is most efficient when operated between 2000-4000 rpm. If you're already in this range on while cruising you're not going to get a ton better. Hotter air should theoretically move the sweet spot on the map to lower rpms if that is what you want while colder air should move it to higher rpms.

Ignition timing is pulled at higher rpms on most engines to protect it. This is all done automatically via the ECU. I'm not sure if that is the case on the Prius' engine because of its atkinson-ish cycle it uses. Its likely the intake charge never really gets hot enough to require this, which would be another perk of the design albeit at the expense of maximum power output.

Quote:
And for the bonus question: has anyone already built and measured such a device? I came across a ton of DIY mods for CRAIs (Cold Ram Air Intakes) but couldn't find any WRAIs.
I don't believe a ram air intake will do anything for fuel economy. There isn't enough pressure build up at the speeds we travel at, and combine that with the fact it may only be useful during acceleration and you have such a limited case scenario that its just not worth putting time into versus other mods.
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Old 07-04-2022, 07:41 AM   #4 (permalink)
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I tend to agree with the above. Like he said, the 2nd gen Prius uses mass airflow rather than speed-density. I also doubt ramming air into the intake will do anything for fuel economy. It could potentially help with power at high speed / full throttle. But if you're trying to hypermile at WOT, you're doing something wrong.

Personally, I would not want to modify the drivetrain in the Prius because of how it's configured. There is a balance of power, so to speak, between the gas engine and the electric motor. If you upset that balance with engine mods, it could have unforeseen consequences.

Actually, that could explain why you lost fuel economy with a hot air intake. The reduced power of the gas engine probably reduced the overall efficiency of the hybrid system.

Anyway, it seems like your main goal is to find somewhere to send the hot air exiting the radiator. How about a hood vent? I'm not talking about a louvered race car hood, but rather a smallish vent placed strategically in a low-pressure zone. IIRC the best place to put a vent is right behind that sharp curve at the front of the hood.
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Old 07-05-2022, 10:03 AM   #5 (permalink)
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Quote:
Originally Posted by Blacktree View Post
I doubt you're going to get the ram air effect behind the radiator. Maybe in front of it. But then, you won't get the heating.

That said, the only way to know for sure is to try it.
Heya Blacktree!

It turns out that while there is a bunch of pressure loss on the fluids inside the radiator, the loss on airflow is much smaller:
https://www.researchgate.net/figure/...fig3_286352432

If I want to increase pressure, I can always play games with the cross section of the intake.

However, I don't even know if that's what I should be doing. I had a chat with an automotive friend of mine that suggested that I don't want to be increasing air pressure but decreasing it (to get better fuel economy). In his view, if we don't have direct control over the fuel injection (say via altering fuel maps), everything boils down to how much oxygen enters the piston chambers. Cold, high pressure air has more oxygen per unit volume, while warm, low pressure air has less. So he said the only conceivable way to get better fuel economy is rarified air at WOT; otherwise, the ECU will simply adjust the throttle to ignore all the modifications upstream.

That view jibes with what I've read about the Prius' air intake. It's ludicrously restrictive, far more than is necessary to simply filter air. Both the resonator and airbox cause significant pressure drops... which seems like what's it's trying to do is decrease pressure. But then that makes the placement (in a high pressure zone near the wheels) somewhat puzzling. It seems they are gathering high pressure air just so they can make it low pressure air. Maybe if they put it in a low pressure zone it would cause too much pumping loss on the engine?

Man, I understood the model of the engine as an air pump, and why turbochargers were so brilliant. But this stoichiometry stuff makes my head hurt.
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Old 07-05-2022, 10:25 AM   #6 (permalink)
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Thumbs up Decision made... procrastinate! :)

Quote:
Originally Posted by Daox View Post
I doubt there is much difference at all.
There is a huge difference in air pressure around the wheels induced by the rotation:
https://www.researchgate.net/figure/...fig1_309416293
In a free flow wheel, the highest pressure zone is near the top while the lowest is near the bottom (right behind the surface contact). As we cowl the wheel more (with wells, skirts, etc.) the highest pressure zone migrates toward the front of the wheel (right in front of surface contact). Basically, the friction of the wheel is "lifting" air from behind the wheel and slinging it around to the front in the direction of rotation.

From an airflow perspective, the wheels are the dirtiest part of the car. Not only does the rotation of the wheels directly induce vortices, but the top of the wheel is traveling twice the cruising speed against the airflow. It's a big fat mess that only gets worse the faster we go.

Quote:
Originally Posted by Daox View Post
I do not believe so. It just has a MAF, not a MAP sensor.
(sigh) I was hoping it might be a derived quantity or somesuch. That would certainly make my life a lot easier.

Quote:
Originally Posted by Daox View Post
It is possible that warm air may be somewhat beneficial for your fuel economy on a Prius, but I am skeptical. On a non-hybrid I would certainly recommend it (and I run one on my Mirage), but the Prius is a bit different. As soon as the Prius' rpm are above 1200, the engine is pretty optimally loaded. RPM is the only differing factor now, and the RPM is changed by vehicle speed and engine load. Lets take a look at a 1nz-fxe brake specific fuel consumption chart to explain this.



This map shows that the Prius' engine is most efficient when operated between 2000-4000 rpm. If you're already in this range on while cruising you're not going to get a ton better. Hotter air should theoretically move the sweet spot on the map to lower rpms if that is what you want while colder air should move it to higher rpms.
Ahhh... this is very educational. Thank you. Thus far what I've noticed is that my cruising RPMs at 65 mph is somewhere in the 1600-1800 range, on reasonably flat roads.

Quote:
Originally Posted by Daox View Post
Ignition timing is pulled at higher rpms on most engines to protect it. This is all done automatically via the ECU. I'm not sure if that is the case on the Prius' engine because of its atkinson-ish cycle it uses. Its likely the intake charge never really gets hot enough to require this, which would be another perk of the design albeit at the expense of maximum power output.

I don't believe a ram air intake will do anything for fuel economy. There isn't enough pressure build up at the speeds we travel at, and combine that with the fact it may only be useful during acceleration and you have such a limited case scenario that its just not worth putting time into versus other mods.
Righto. Actually, that helps clinch the decision for me that this is a low priority project, so I'll put it on the back burner.

By the way, something else that I looked into was trying to change the fuel map somehow, either by getting a performance chip or fooling the ECU somehow. That was to see whether I could make the car going into lean burn. It turns out... Toyota already uses lean burn a lot, just not at cruising. When the load falls below a certain threshhold (say a slight incline), the engine will typically measure 16:1. Then at no load (coasting down an incline), the engine revs at 900-1000ish and goes into an 18.1:1 burn mode. Then at even less load, the engine shuts off and it uses electricity.

I've also noticed that the car only gets less than 1/5th the aero gain uphill. So when version 3.0 was fully aeroed and got 50%+ mpg, I would get less than a +10% increase going up a serious hill. The lion's share of the gain was going down inclines, and in particular, how quickly it would move between the three regimes above.
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Old 07-05-2022, 10:38 AM   #7 (permalink)
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Quote:
Originally Posted by Blacktree View Post
I tend to agree with the above. Like he said, the 2nd gen Prius uses mass airflow rather than speed-density. I also doubt ramming air into the intake will do anything for fuel economy. It could potentially help with power at high speed / full throttle. But if you're trying to hypermile at WOT, you're doing something wrong.
I am convinced... it's probably better to trust the Toyota engineers know way more about engine efficiency than I do. Also, I'm a master at doing nothing.

Quote:
Originally Posted by Blacktree View Post
Anyway, it seems like your main goal is to find somewhere to send the hot air exiting the radiator. How about a hood vent? I'm not talking about a louvered race car hood, but rather a smallish vent placed strategically in a low-pressure zone. IIRC the best place to put a vent is right behind that sharp curve at the front of the hood.
Hahaha! I was actually about to put in a bonnet exhaust a few weeks ago. It's how I accidentally discovered that hood insulation increases mpg. After doing several tests with results I could scarcely believe, I was forced to conclude that it was more efficient to trap heat in the engine compartment, rather than vent it.
https://www.instagram.com/p/CewAQ08LN2l/

And my main concern was less about exhaust and more about utilizing the top end of the radiator. Once I get done with plastering over the underside of the engine, there will be a 40 sq in exit through the cat channel and two 20 sq in exits near the rear of each wheel (in the low pressure zone). Since I'm targetting a 50 sq in-ish inlet, that's more than enough exhaust venting. The problem in my mind (which may or may not exist in reality) is that it seems like the airflow will favor the bottom half of the radiator, as the path of least resistance.

It also seems like the Toyota engineers felt the same way. The main radiator intake is below the impact bar at the bottom of the radiator, but they deliberately put a small intake above the bar at the top. I assume that's to get airflow saturation near the top. Since I plan on ducting the inlet, I thought that suction near the top would help airflow move that way, and be more efficient use of the radiator.

It looks like I'll just have to do some asymmetric aero on the inlet duct to achieve the same effect. While is a shame... I do so love symmetry.
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Old 07-05-2022, 12:43 PM   #8 (permalink)
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Quote:
Originally Posted by Talos Woten View Post
Hahaha! I was actually about to put in a bonnet exhaust a few weeks ago. It's how I accidentally discovered that hood insulation increases mpg. After doing several tests with results I could scarcely believe, I was forced to conclude that it was more efficient to trap heat in the engine compartment, rather than vent it.
https://www.instagram.com/p/CewAQ08LN2l/
That's interesting. I never would have guessed.

Just out of curiosity, were you testing no insulation vs stock insulation? Or stock vs extra insulation? Or some other configuration?
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Old 07-05-2022, 08:08 PM   #9 (permalink)
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Quote:
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That's interesting. I never would have guessed.

Just out of curiosity, were you testing no insulation vs stock insulation? Or stock vs extra insulation? Or some other configuration?
Nor I. I tested all three configurations (no, stock, extra) and my mpg increased with each, by 8% and 2% (on a short 15 mi trip). Said differently, removing the stock insulation caused an 8% drop in mpg.

Here's another post about putting the extra insulation in:
https://www.instagram.com/p/Ceym7-lrTVx/
and a how-to I whipped up especially for this site:
https://ecomodder.com/forum/showthre...tml#post669246

Enjoy!
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Old 07-21-2022, 04:03 PM   #10 (permalink)
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a couple of values

These are from the 2013 Volkswagen Jetta Hybrid land speed record at Bonneville:
* Local barometric pressure, 25.90" Hg, 12.7249 psi.
* Ram-air pressure @ 70-mph = 0.075 psi ( 517.4 (Pa) )
* Ram-air pressure @ 187.6-mph = 0.575 psi ( 3,963.9 (Pa) )

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