An unanticipated side effect of Cd reduction

[basjoos]

One unanticipated side effect of reducing my car's Cd is that I have reduced the pressure of the high pressure zone at the base of the windshield low enough that the remaining air pressure there isn't enough to move air through the air vents without having to run the fan. So which is the more energy efficient way to ventilate the interior? Run the fan on low with its slight load of the alternator, or install a small air scoop somewhere around the windshield to catch and direct air into the car's interior, resulting in a slight increase in Cd. Of course with the latter, I'd have to design it to separate out any rainwater that the scoop might catch, so it doesn't rain inside the car when driving through a heavy downpour.

[davidgrey50]

On my 1984 Dodge Rampage the cabin air intake is directly under a grill at the trailing edge if the hood. The intake hole itself is surrounded by a sheet metal 'wall', and the rain is kept out by a plastic shield which sits an inch above the opening and extends an inch over the sides and stops an inch from the bottom of the 'wall' -a sort of umbrella. Simple and effective.

PS You're an inspiration!

[Frank Lee]

You need one of these:



I'm referring to the cowl vent, not the windshield. Although that might work for you too.

[Allch Chcar]

I've considered this too, but this was actually a couple months ago. The fan on low is still better than adding drag IMHO the drag from the alternator is already there and it functions on-demand. The alternator's drag on the engine can be fixed with a deep cycle battery too. But I understand that won't work too well for you since you need your headlights for night driving. The scoop is the worst way of venting air into your cab, it slows you down and the cutting/filtering would have to be very well thought out and applied to keep water out. The water alone isn't the only consideration.

This may explain why the air vents in my Ranger always spit air into the cab despite everything being turned off. Messing with the direction doesn't stop it. But the air blows the fastest through the panel mounted vents like the fan is stuck on. I've actually had leaves and twigs come shooting through the vents when I turned the fans to max on a hot day (no A/C). I knew it was a bad thing too.

Personally I'd prefer to pull the air from UNDER the engine bay than from the base of the windshield. Although it would probably be more efficient to just wire up an intake from the front of the car or hook it up to the intake airbox.

[basjoos]

Originally Posted by davidgrey50

On my 1984 Dodge Rampage the cabin air intake is directly under a grill at the trailing edge if the hood. The intake hole itself is surrounded by a sheet metal 'wall', and the rain is kept out by a plastic shield which sits an inch above the opening and extends an inch over the sides and stops an inch from the bottom of the 'wall' -a sort of umbrella. Simple and effective.

PS You're an inspiration!

That's where the air inlet on my car is too, but my mods have dropped the high pressure that normally drives the air through the vents to where the air won't move without fan assist.

[basjoos]

Originally Posted by Allch Chcar

I've considered this too, but this was actually a couple months ago. The fan on low is still better than adding drag IMHO the drag from the alternator is already there and it functions on-demand. The alternator's drag on the engine can be fixed with a deep cycle battery too. But I understand that won't work too well for you since you need your headlights for night driving. The scoop is the worst way of venting air into your cab, it slows you down and the cutting/filtering would have to be very well thought out and applied to keep water out. The water alone isn't the only consideration.

This may explain why the air vents in my Ranger always spit air into the cab despite everything being turned off. Messing with the direction doesn't stop it. But the air blows the fastest through the panel mounted vents like the fan is stuck on. I've actually had leaves and twigs come shooting through the vents when I turned the fans to max on a hot day (no A/C). I knew it was a bad thing too.

Personally I'd prefer to pull the air from UNDER the engine bay than from the base of the windshield. Although it would probably be more efficient to just wire up an intake from the front of the car or hook it up to the intake airbox.

Right now I'm thinking about some sort of 2" diameter flexible duct running through the engine bay from the grill inlet to the vent air inlet plenum. That would be taking air from the one high pressure region remaining in my car's aerodynamics and any water sucked in would be dumped into the inlet plenum that is already designed to separate water from the incoming.air flow. There would be a slight drag increase, but it should be very minimal.

[basjoos]

Originally Posted by Frank Lee

You need one of these:



I'm referring to the cowl vent, not the windshield. Although that might work for you too.

Cars and trucks designed in the days before A/C had all sorts of ingenius ways to create a gale of wind through the interior to handle the summer heat. My favorite were the kick panels in the foot wells in the pickup trucks.

[RobertSmalls]

A very interesting problem. I suspect that a wind-powered intake *ought* to be more efficient than an electric one, especially since you have a port for air to exit at the rear of the cabin. Since your alternator and blower motor are less efficient than your transmission, driveline, and tires, the latter should take less energy than the former.

[davidgrey50]

Originally Posted by basjoos

That's where the air inlet on my car is too, but my mods have dropped the high pressure that normally drives the air through the vents to where the air won't move without fan assist.

That was just regarding keeping the rain out. If it's hard to get air coming in from this low pressure area, your other option is to relocate your exit vent to an even lower pressure area i.e. if you can't push it in, maybe you can pull it out. Perhaps a rear-facing scoop or two in an area of smooth, fast airflow - on the roof or on the boat-tail.

[evolutionmovement]

The Lamborghini Espada used two NACA scoops on the hood for cabin air (no they weren't for the 6 carbs). Don't know how they kept out the water, but probably an air filter set up like an engine air intake. Don't know how efficient it was aerodynamically, but it's another option.