Hood vents... Maybe not...

[DennisG01]

I hope this is OK to post this here - it's not engine related, but it does deal with a topic that appears here once in a while...

Every so often someone brings up the idea of hood vents - primarily to evacuate hot air - usually not in regards to bringing in cold air. I've always been a proponent of using wind streamers to figure out where they could be placed, if at all. In keeping with that theory, here's something I thought was quite interesting...

First, I have a bug deflector and one of those Lund "sun visors" sticking out from the roof, over the top of the windshield.

A few weeks ago we had about 1" of snow, so I didn't clear the hood - figuring it would blow away in the first mile or so. It took until I was up to about 45MPH to start working it's way off the hood, but here's the "hmmmm" part: The snow closest to the windshield started blowing off first, BUT, it blew forward, towards the front of the vehicle! Amazing, I thought! OK, log that into the old nogin for later use.

Fast forward to today... there was still a little bit of ice on the cowling (the black thing at the base of the windshield) from a recent snow. The ice was thin and not all in one piece. So, I get on the highway and the ice starts rattling and breaking up more - it eventually gets loose enough that it starts to fly away. BUT... most of the pieces went forward again! And, not just under the pull of gravity - they were forcefully going forward to the bug deflector, then would very quickly fly to the left or right.

At first, all I could think of is that the bug deflector causes air to flow further away from the hood than normal. Then the air must be catching the sun visor and swirling down. BUT, here's another twist... I also know that rain on the windshield will be pushed upwards, not down.

Hmmmmm....

 

[Burning oil]

Cowl induction

 

[635]

As mentioned, cowl induction. I'm not sure but didn't Smoky Unick figure this out years ago to force air into the carbs? I put hood vents on my pickup but on the far rear corners. I'm not sure but I'm thinking this will take some of the engine heat out of the engine compartment. Maybe it helps, maybe not but put your hand over the vent after a few miles of driving, then stop, and your hand will get burned from the engine fan pusing air!

 

[schiker]

I think there are some boundry layer issues going on. Laminar flow at glass surface then turbulence and eddy's near cowl. Cars and trucks are badly shaped wings and air scoots underneath much easier than over top just like an air plane wing this causes lower pressure on top of vehicle which I think may contribute to water droplets rising too.

Goodmark industries use to offer fenders with vents near upper corner behind tire. But not sure that is a low or high pressure area either.

I have often pondered how to open some holes in the inner fender to allow air to escape but to avoid water and dirt from tires to enter the engine compartment due to slinging off tires and splash up etc.

I think the other best thing to do is put the air damn below the front bumper. This increases the pressure differential across the radiator by moving the air stream closer to the ground. It also reduces the turbulence over the lower A arms and front suspension drivetrain etc.

Look to seal any holes in the sheetmetal around the radiator too. The front grill area catches air like a bucket and it will take the path of least resistance into the engine compartment and down to the ground to get back into the air stream underneath the truck.

 

[96 OILBURNER]

There was someone that installed hood vents with small ribbons to demonstrate the amount of airflow they achieved with electric fans. I believe it may have been Franko914.

When a vehicle is at highway speeds, there is a high pressure area at the lower windshield area. This is why cowl induction works so well. If you could figure out a way to create a low pressure area there, it would suck the hot air out if the engine bay. The 2nd generation Trans Am had heat extractors in the front fenders. Air flowing over the vents would pull the heat out of the engine compartment.

 

[DennisG01]

Interesting - that all makes sense - thank you. I guess I just never thought about it that way. I'll tell 'ya, though, it was pretty cool to see that snow and those ice pieces running down the hood - it was like they were racing each other! I was very surprised at how fast they went. They slide down right into the gap between the deflector and the hood.

Vents on the side of the fender? This one I have thought about. I've imagined the air flowing past the vertical surface of the fender sort-of like a boat's bow wake. I mean, air is a fluid and it should act similar to water. Except for the compressability part. The question is, how far away from the fender would the vents need to be in order to catch enough air to make it worth while.

I suppose one could bend some coat hangers to hold some streamers at various distances from the fender - then just duct tape them onto the fender.

On the other hand, like mentioned, flush mounted vents on the vertical surface of the fender would probably do a decent job of removing heat - if there is enough hot air gathered there (on the inside) to start with. I wonder how much help a small, electric fan (mounted inside the engine bay) would provide to help evacuate the air through those fender vents if it was placed directly on the inside of the fender?

 

[Grendahl]

As mentioned, the back of the hood and base of the winshield area is a high pressure zone at highway speeds. The front to middle of the hood is a low pressure zone. Many cars that have low profile forward facing hood scoops in the middle of the hood are actually tring to take air from one of the worst places as far as air pressure goes. Forward facing hood scoops need to be up pretty high off the surface of the hood to get a good ram air effect, but then the scoops get to be hard to see around.

 

[96 OILBURNER]

Many cars that have low profile forward facing hood scoops in the middle of the hood are actually tring to take air from one of the worst places as far as air pressure goes. Forward facing hood scoops need to be up pretty high off the surface of the hood to get a good ram air effect, but then the scoops get to be hard to see around.

This is even worse when a bug deflector is involved.

 

[DennisG01]

Found the Franko914 video that Dave mentioned - post #21. (I PM'd Franko)

http://www.thetruckstop.us/forum/showthread.php?t=17864&highlight=vent&page=2

Now I have a question about that:smile5:

Franko:

I see that the vents work quite effectively at a standstill. Any idea what's going on at higher speeds? Or, was your only concern at slow speeds?

If I assume for a minute that air enters the vents at high speed - like a cowl induction as mentioned above, do you think that disrupts the normal air flow through the engine bay from front to back? Could that create an elevated pressure area at the back of the engine bay, which would then, in turn, actually cause less air to enter through the grill?

Teach me! I'm very curious! :smile5:

 

[Pruittx2]

under the hood, along the front edge of the cowl cover,, I removed the rubber seal that goes from fender to fender. To help with air flow & heat removal in the engine compartment. Don't have any numbers, just thinking that removing any restriction to air flow could help in the summer. Have also thought of shimming the rear of the hood up, for the same reason, or Bowing the middle of the hood at rear edge, so at least the hood still lines up with fenders. What I really want to do is box in the air intake inside of the fender, then putting a air intake on the right fender to make a Real Fresh air intake!

 

[SnowDrift]

I had a glass cowl inducted hood on my half ton years back and had a bug deflector on it for about two days. Going down the road, the hood would "throb" for a lack of better term right up near the cowl. It probably moved 1/2" at the most, up and down. I was concerned about it cracking, so I removed the bug guard and the problem went away.

 

[Anubis]

under the hood, along the front edge of the cowl cover,, I removed the rubber seal that goes from fender to fender. To help with air flow & heat removal in the engine compartment. Don't have any numbers, just thinking that removing any restriction to air flow could help in the summer. Have also thought of shimming the rear of the hood up, for the same reason, or Bowing the middle of the hood at rear edge, so at least the hood still lines up with fenders. What I really want to do is box in the air intake inside of the fender, then putting a air intake on the right fender to make a Real Fresh air intake!

you can use a snorkel and that will get you a true cold fresh air intake. I was thinking Cowl inducted hood and some fender vents. A cowl inducted hood should work at low speed (less than 35 mph say pulling heavy on a grade) with the fan locked up to evacuate the heat. At cruising speed with no restriction in the cooling system and a properly functioning thermostat there should not be any cooling issues.

I have never seen a big rig with a properly functioning cooling system over heat at speed, only when climbing or moving slow ..... and rigs like the 385 pete, 387 pete, Century class, T600, and VN series Volvos all have low speed cooling problems due to poor air flow when moving less than 50 mph. I went from Kansas City Kansas to Southbend IN in a Freightliner Classic with no fan....... only started getting hot when moving less than 50 mph or idling..... when it was moving it stayed cool.

 

[SmithvilleD]

There are some reports that pulling the inner fender splash guard/flaps results in additional airflow thru the radiator/cooling stack. If this is so, would expect the mechanism is allowing air to escape from underhood easier/faster.

 

[Franko914]

Each vehicle model has its own unique airflow characteristics. Sleek, low, aerodynamic sports cars have a lower coefficient of friction than flat, tall, brick-like SUVs. Not even considering the front/grille area of the vehicle, but looking just at the top-of-the-hood to the top-of-the-windshield area, positive pressure and negative pressure zones vary considerably.

In the attached .jpg [AirFlow Over Hood.jpg], airflow over a sports car (top illustration) varies from that of an SUV (bottom illustration). The airflow closest to the vehicle's surfaces are smoother and more horizontal with the sports car when compared to the SUV's lines. A vehicle's aerodynamic design has much to do with airflow over its surfaces: tall/high windshields act like "sails" (in fact, ocean tankers with vertical front surface bridges are referred to as having a large sail areas).

The airflow begins to move further from the vehicle's hood more forward of the windshield base in the lower illustration, and moves closer to the windshield higher up from the base. The area between the airflow line and vehicle's surfaces is under "negative" pressure (red minus sign). This area (actually, volume) increases dramatically as the vehicle's speed increases.

This explains why snow flakes or raindrops fly towards the windshield but do not impact onto the windshield at the last moment and flows over the top of the vehicle when you travel above a certain threshold (determined by the weight of the snowflakes/raindrops). This also explains why loose ice sheets slide forward under their own weight acted upon by gravity on the sloped hood -- the airflow line is high enough above the hood to, counterintuitively, not move it backwards. Then, when it reaches the front of the hood, the airflow picks it up and blows it over the top of the vehicle without impacting onto the windshield (usually followed by a loud "OH, S**T from the operator and/or front passenger...).

By placing a hood vent or hood scoop strategically on the vehicle's hood, you can draw in more air into the engine compartment or draw out more air from the engine compartment. Note that installing high profile hood scoops change the airflow characteristics of the vehicle in the vicinity of the scoop.

In my 98 Tahoe, the hood vents are situated as close as I could cut into the hood supports nearer to the windshield because I wanted to "draw out" hot engine compartment air by utilizing the "negative pressure" zone in that area where the hood meets the windshield. It works VERY well when the vehicle is moving forward (my flow strips blew off above 30 mph and the temperature of the stainless steel louvers were too hot to touch when I stopped to check convection out of them). Thus, airflow through the radiator is also improved by strategic placement of hood vents.

Regards,
Franko

 

[Franko914]

Creating openings in the front fender walls from inside the engine compartment behind the wheel wells will work better to draw air out of the engine compartment if the louvers are angled towards the rear of the vehicle (note Range Rover designs).

Because the airflow along the sides of a vehicle create a "negative pressure," air will still be drawn out of the engine compartment even if only a screen or grille is placed over the opening (did that to my 81 CJ-7, not because it needed help with cooling, but because it looked pretty cool). This would explain why cigarette smoke/ash fly out of an open window that is lowered to a certain point (depending if other windows are open, i.e., one front and one rear window open creates a crosff-flow where air is blown into and through the vehicle), or the buffeting sound when only one rear window is lowered partially or fully, etc.

Regards,
Franko

 

[DennisG01]

Thank you Franko! I was beginning to understand what was going on from the other guys who graciously answered my question. But, it just wasn't "clicking'. That was a great explanation - it really helped me understand what is going on. 'Course, the picture didn't hurt, either! :smile5:

Let's see if I'm getting this... Cowl induction... there is a low pressure area at the cowl opening. Which, at first thought, would seem to be counter-intuitive to getting more air in. But, because the engine (air pump) is drawing/pulling air in, it works?

Plus, as I witnessed with the snow and ice particles, the pressure differential (layered on the hood) causes the air to move forward, as well.

Am I on track? Or, am I still missing something?

Oh, since I have a bug deflector and a sun visor, is it safe to assume that the area of low pressure is even larger with my truck?

 

[Grendahl]

Let's see if I'm getting this... Cowl induction... there is a low pressure area at the cowl opening. Which, at first thought, would seem to be counter-intuitive to getting more air in. But, because the engine (air pump) is drawing/pulling air in, it works?

The base of the windshield on most vehicles is definitely high pressure, not low pressure. How high the pressure is proportional to how sharp the angle between the hood and windshield is and how fast you are going. You can easily verify this for yourself. At highway speeds you can close all your windows and turn off your heater fan entirely and hold your hand in front of the heater outlets in your truck. You will still be able to feel the air blowing through the heater because the heater air intake is right at the base of the windshield. The faster you drive the more air will blow though the system. Lots of different types of race cars use cowl induction in order to take advantage of this high pressure zone.

 

[Chevylover]

But they didn't have a Bug Visor ! Remove that thing and you have high pressure at the base of the windshield. Had exactly the same symptom, Dennis mentioned, but after removing the bug visor, the symptons have been gone. AND I also have a lunar visor.

Cu,
Sven

 

[Franko914]

The base of the windshield on most vehicles is definitely high pressure, not low pressure.

I agree but the "high pressure" is due to eddy currents that form, get caught and swirl between the airflow lines and the vehicle's surface.

How high the pressure is proportional to how sharp the angle between the hood and windshield is and how fast you are going.

I agree. The "volume" between the airflow lines and the vehicle's surfaces increases with speed (I think I state that in a previous reply) and allows more and larger eddy currents to be formed.

You can easily verify this for yourself. At highway speeds you can close all your windows and turn off your heater fan entirely and hold your hand in front of the heater outlets in your truck. You will still be able to feel the air blowing through the heater because the heater air intake is right at the base of the windshield. The faster you drive the more air will blow though the system.

I agree, and, again, it is the eddy currents that produce the "high pressure" that forces outside air through the heater/ventilation system. This high pressure has to work against the internal pressure in the passenger compartment -- crack open a little a rear window and the flow of air through the heater increases but nowhere near the velocity of the outside air relative to the vehicle.

Lots of different types of race cars use cowl induction in order to take advantage of this high pressure zone.

Yes. You can introduce air into the engine compartment by inducting it or ramming it in.

Cowl induction scoops face rearward and are closer to the windshield to take better advantage of the eddy currents that form there naturally, and even more so with the addition of the induction scoop which changes the vehicle's airflow characteristics.

F1 race cars (no hoods to speak of in this context...) have scoops high up in order to "scoop" into the high pressure airflow zone that rams air all the way into the vehicle's rear engine compartment.

Hood scoops are best placed forward rather than rearward because as the vehicle's speed increases, the airflow lines move much higher above the vehicle's surface at the windshield base (need taller scoop) than the front of the hood (size it right and it doesn't matter how fast the vehicle travels, the airflow lines remain about the same distance from the vehicle's surface).

In the attached illustration, [AirFlow Over Hood2.jpg], if the vehicle's speed is low, very little, if any, eddy currents form, hence the zone at the bottom of the windshield remains at negative pressure (top illustration).

As the vehicle's speed increases, so does the volume of air in which the eddy currents can now form (lower illustration). As more and more eddy currents form creating a higher pressure in that zone, the airflow lines are, literally, pushed further away from the vehicle's surface.

So, imagine placing an induction cowl strategically so that its rear-facing opening is smack in the middle of the high pressure zone where the eddy currents are. Also, imagine placing a hood scoop strategically so that its forward-facing opening is right in the path of the airflow lines above the vehicle's surface -- forward of the hood is best, towards the rear would require a taller scoop.

To stay along the subject of this thread... strategically placing a hood vent/louver so that it is directly beneath the negative pressure zone would result in engine compartment air being drawn out.

Regards,
Franko

 

[94K30]

besides, cowl hoods look cool....