Minimum grill opening requirement

[BrianAbington]

My air dam spans from the upper part of the bumper to about 4" above the ground. Only air inlet is Twenty four 3/4" holes in the air dam right in front of the bumper inlet. The drivers side is completely blocked inside the bumper and the AC condenser and the gap between the two is completely blocked. So all the air that goes through those holes goes right into the radiator.

The grill block is held on by only 2 zip ties so if the gauge does start to climb I cut them off and add the upper grill to the air flow.

[aerohead]

Originally Posted by BrianAbington

My air dam spans from the upper part of the bumper to about 4" above the ground. Only air inlet is Twenty four 3/4" holes in the air dam right in front of the bumper inlet. The drivers side is completely blocked inside the bumper and the AC condenser and the gap between the two is completely blocked. So all the air that goes through those holes goes right into the radiator.

The grill block is held on by only 2 zip ties so if the gauge does start to climb I cut them off and add the upper grill to the air flow.

You might do a GOOGLE search for 'vena contracta.' Wiokipedia has some good images of it and you might think about the holes you're using.Just a thought.

[gone-ot]

...what aerohead was alluding to (from Wiki):

It is the ratio between the area of the jet at the vena contracta to the area of the orifice.

Cc = area at vena contracta/ area of orifice

The typical value may be taken as 0.64 for a sharp orifice (concentric with the flow channel). The smaller the value, the more effect the vena contracta has.

[christofoo]

I was thinking about my calculation a few weeks ago and it occurred to me that heat is lost to the exhaust as well as the radiator. Air passing through the radiator is (probably) only heated to 180-200F or so, but the exhaust is at about 1000F when it exits the engine compartment, so it's significant. I worked the exhaust heat out to about 22% of the ICE thermal load, and I updated my spreadsheet to reflect that. I edited my first post with the new attachment.

Also, I'm pretty sure the number I used for ICE thermodynamic efficiency, 22%, is lower than I'd expect under climbing conditions. I think 28% is more likely, at least for economy cars, considering low throttle losses during climbing.

Net result; most of my suggested grille openings have gone down by almost 50%. I edited my original table accordingly.

Originally Posted by christofoo

...

• Sustained 5% grade climb at 65MPH, 3,000 lbs gross weight, 120F ambient, 15MPH tailwind, 5,000ft elev., 2kW AC: 167in^2 (4.5in^2/hp) 89in^2
• Sustained 5% grade climb at 65MPH, 3,000 lbs gross weight, 120F ambient, 0 tailwind, 5,000ft elev., 2kW AC: 141in^2 (3.5in^2/hp) 81in^2
• Sustained 5% grade climb at 65MPH, 3,000 lbs gross weight, 100F ambient, 0 tailwind, 0ft elev., 1.5kW AC: 96in^2 (2.2in^2/hp) 56in^2
• Sustained 3% grade climb at 65MPH, 2,500 lbs gross weight, 100F ambient, 0 tailwind, 0ft elev., 1.5kW AC: 66in^2 (2.3in^2/hp) 38in^2
• 0% grade, "loaded sedan" at 75MPH, 100F ambient, 0 tailwind, 5,000ft elev., 0kW AC: 48in^2 (2.2in^2/hp) 32in^2
• 0% grade, "loaded sedan" at 75MPH, 40F ambient, 0 tailwind, 5,000ft elev., 0kW AC: 30in^2 (1.3in^2/hp) 20in^2
• 0% grade, "aerocivic" at 70MPH, 40F ambient, 0 tailwind, 0ft elev., 0kW AC: 15in^2 (1.1in^2/hp) 10in^2

...

[Tesla]

Originally Posted by NeilBlanchard

The one mod I made was to block up the escape vent holes in the chin area, after I had blocked up most of the grill. Because the stock grill opening was way too big, they had to let air escape; but after the grill blocks are in, all the air needs to go through the radiator. So, in my case it is not so much a duct, but rather a plenum.

An exhaust louver in the center of the hood would greatly improved the flow, but that is a lot of work.

That's basically what I have done, sealed all holes to front of engine bay and forcing all air through the radiator and in my case also the engine intake air draws from this plenum, it's a turbo diesel so there was an immediate improvement in performance there.

I am now working on a design to shroud the outlet side of the radiator and use this shroud to control airflow through radiator, so if the radiator outlet is controlled, how relevant is the blocking of the actual grill because the flow is already restricted by the shroud?

Edit: Just to add I have a top mounted intercooler with bonnet scoop, which also performs better with all excess holes sealed and this is where the engine bay area gets the bulk of it's ventilation from in a downdraft fashion, I have a very narrow gap on the exhaust side which allows a small amount of air past the intercooler and down past the exhaust to keep this area at a reasonable temp.

[aerohead]

At Bonneville I blocked the trash can lid opening down to 40 square inches for the last couple of runs.The 150 bhp engine was capable of 124 bhp at the track and at WOT for a mile the temp gauge never really moved.
I left the block in place and drove on to Carson City, Nevada and then back home here to the Dallas/Ft.Worth area and never had a cooling problem.
On one 10,600 pass in Colorado I did run the heater to full heat and max blower setting to augment the cooling system.
The block is still in place today and I've had no trouble in daily commuting or 70 mph travel.

[christofoo]

Originally Posted by aerohead

At Bonneville I blocked the trash can lid opening down to 40 square inches for the last couple of runs.The 150 bhp engine was capable of 124 bhp at the track and at WOT for a mile the temp gauge never really moved.
I left the block in place and drove on to Carson City, Nevada and then back home here to the Dallas/Ft.Worth area and never had a cooling problem.
On one 10,600 pass in Colorado I did run the heater to full heat and max blower setting to augment the cooling system.
The block is still in place today and I've had no trouble in daily commuting or 70 mph travel.

It's 10-20F here this week and my temp gauge barely gets going with a full grille block and a block heater over my 10 mile commute with defroster half on. I've also seen the heater core provide most of the engine cooling under heavy climbing loads, but that isn't an option I would exercise in hot weather. I think my calc is mostly useful for summertime pass climbing at this point... If you want to set a fixed inlet size and leave it alone forever, I think it's a good guideline. ...EDIT... except actually I think my numbers might still be a bit high. I'm sure if I keep refining it I'll get closer.

[Vekke]

I have measured my lupo 3l with 1.2 tdi engine fully blocked grill:


When outside temperature is +5 celsius it works up to 105 km/h. after that speed fan kicks in and you can drive up 125 km/h and after that speeds the water temperature starts to be over 100 celsius and rises.

If outside temperature goes to -10 celsius or colder you can drive +15 km/h faster without the fan kicking in and +25 km/h faster to reach over 100 celsius.

[aerohead]

Originally Posted by christofoo

It's 10-20F here this week and my temp gauge barely gets going with a full grille block and a block heater over my 10 mile commute with defroster half on. I've also seen the heater core provide most of the engine cooling under heavy climbing loads, but that isn't an option I would exercise in hot weather. I think my calc is mostly useful for summertime pass climbing at this point... If you want to set a fixed inlet size and leave it alone forever, I think it's a good guideline. ...EDIT... except actually I think my numbers might still be a bit high. I'm sure if I keep refining it I'll get closer.

When doing straight,steady-velocity highway travel,modern gasoline engines are achieving up to 38% thermal efficiency.
As you add drag reduction your heat flux is going to go down and down as your road load falls lower and lower.And of course your cooling requirements will fall as a consequence as your numbers-crunching is showing.
As long as you can 'open' your grille for worst-case scenario driving loads,or impart a higher static pressure across the heat exchanger (electric fans) you ought to be able to address the full spectrum of loads.
The fixed,concentric bullet valve behind my inlet, is intended as a 'future' active element to optimize cooling air flow (complete shutoff during winter parking,etc.) but since it is not within the KISS design framework must languish until that system is idiot-proofed.