Minimum grill opening requirement
 

I worked up a spreadsheet that (supposedly) calculates a grill opening requirement. ...I couldn't find this calculation elsewhere so I'm assuming this is helpful...

EDIT 1/6/13: I added a term for the loss of heat through the exhaust gas. This matters because the exhaust gas is much much hotter than the radiator, so although it is a smaller volume it represents about 20% of the total thermal load. Therefore my new area calcs are about 20% lower than previously.

My purpose is to help guide grill block design (since I'm doing one soon). I.e. how far does your grill need to open and under what conditions can you expect it to be required?

I think it's particularly crude since I don't know how much the radiator actually heats the air that passes through it, but I'm making an assumption so I can use the specific heat of air to determine heat dissipation for a given flow rate. Most likely the openings I calculate are low for that reason, but they are probably not so terribly off to be a bad reference point.

It'd probably be good if someone can check my work. I checked my drag force against the one on the wiki, and most of what else I did seems straight forward, but who knows? :rolleyes:

Assuming it's all correct, here is some tabulation of typical results: EDIT 1/6/13 I corrected the values to include heat lost through the exhaust gas. I also thought that 22% for ICE thermodynamic efficiency is low and used 28% instead, an important rationale is that cooling requirements are most stringent while climbing, but in that condition throttle loss will be low and thermodynamic efficiency will be higher than normal. For the 0% grades I backed off to 25%, which may be reasonable for economy cars but not sportier cars. The net result of both changes is almost a 50% reduction in my grille area suggestions below:

• 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

(I think this spreadsheet is also less accurate when you're not climbing, because it assumes that the air through the grill is the only means of cooling, which is more true when the cooling demand is very high (lots of horsepower in use), and it's particularly untrue at low ambient temperature and of course the whole premise is false if your trip is short.)

(Note that where I write in^2/hp I'm referring to wheel output hp. I get the feeling that I must be typically climbing at least 2X below my Civic's "specified" engine power output for some reason, maybe because I don't like the sound of high RPMs and like to keep it in 5th mostly, and use 4th for pulses when I drop below 60MPH.)

Conclusions (not surprising):

• Need it wide open for absolute worst case, climbing north towards Flagstaff in summer heatwave with a tailwind (hint: avoid that scenario).
• Need a good size opening for climbing passes in inland summers.
• Need a minimal-to-modest opening for flat ground in the summer (also requires a decent sized opening for AC condenser if used, not calculated here, note AC compressor efficiency will suffer if the opening for the condenser is too small, so engine load penalty may be higher than aero gain for all you AC users).
• Leave it closed for flat ground below freezing (probably).
• Cd reduction will also have the effect of reducing grill opening requirement on flat ground, since it (obviously) reduces engine output. Not as true during sustained steep climbing.
• Weight reduction will have the effect of reducing grill opening requirement while climbing. Not as true on flat ground.

BTW, this is my first time posting here, but I have really enjoyed this site since I found it earlier this year. I've been hypermiling since 2007 and I really wish I'd found this site earlier. I'm now nearly finished with phase I of my aero mods on my Civic (wheel slicks and passenger mirror delete). Phase II will be front end and wheel skirts. For phase III I'm thinking I will make a partial-boattail-type shell for my hitch-mounted luggage rack that my family of 4 uses on long hauls. (Sooner or later I'll put my stuff in the ecomodder garage too.)

It all depends on the drag through the cooling system. If it is fully ducted and has an efficient intake opening and well placed exhaust vent (see Hucho on this) then the sizing I remember reading about (maybe in reference to NASCAR?) is 16in^2 per 100HP.

Hi Neil,

I disagree with the idea of using racing rules in application to passenger cars.

1. Racers generally have a much larger ratio of HP available to HP in continuous use (due to desire for highest possible acceleration), so if the racing rule references HP available than it will bias you strongly towards smaller openings. Please note again that my numbers above are in reference to HP in use rather than HP available - sorry, that does make them more difficult to apply. You can probably cut your HP spec by 2-4X, but I argue it makes more sense to look at the application rather than the engine. I included in^2/hp to make it easier to separate effects of power demand vs ambient conditions.
2. Race tracks generally don't include sustained climbing. It seems quite clear to me that extended climbs present the most strenuous need for grill opening in passenger cars. [Edit - this next bit was irrelevant] In addition to my calcs, I remember that AndrewJ ran into his first serious issue with a fully closed grill while climbing a pass. (See http://ecomodder.com/forum/showthrea...cs-312-10.html) (This is also an argument in favor of some kind of remote or automated variable grill opening, unless you want to get out of your car in the baking sun and adjust your grill before and after you climb, or if you can avoid the mountains.)
3. I'm pretty sure NASCAR doesn't run their Phoenix circuit in July or August(?). Even if you yourself never climb passes in life-threatening heat, it's worth knowing how strong the effect of ambient temperature can be, and when to expect your grill block design to be limiting.

I'm also not convinced that the drag through the cooling system affects the cooling system efficiency... as opposed to Cd.

That sounds about right to me.:thumbup:

I'm doing a grill block for Baa at the moment, working out opening size by trial and error.
The first afternoon, coolant temp started climbing (fast) on the motorway at 110kph. I stopped and took off the whole grill block.
Last night, with a much larger opening, temp was fine on the motorway and only started to rise (slowly) on the climb through the Blue Mountains at 80kph.

With the opening a little larger again, I'm hoping for no increase in temp on the way home tonight.:)

Oops, AndrewJ concluded it was a problem with altitude adjustment, not overheating. :o

The drag of the cooling system contributes up to about 10% of the overall drag of the vehicle. This is one of the advantages of an electric drivetrain, by the way.

I'm cooling my ~105HP Scion xA with a ~15" x 4" opening. And that is with minimal changes to the overall system. With an efficient exhaust vent, this could be reduced a lot more. Vekke has a car cooled with just a 4" diameter intake.

You should look at the Hucho section on cooling system drag. And several folks here on EM have added exhaust vents in the center of the hood that greatly improves cooling by increasing flow through by lowering drag inside the system.

I like your
 

approach to predict engine coolant temp with your listed factors. Might add humidity level, in-town/stop and go traffic, electric fan vs. full time mechanical fan vs. temp. activated mech. fan. "Hot" ambient temps equals my a/c on (around 15 hp loss) and activation of the temp. triggered mechanical fan(around 15 hp loss). I have to turn off air just to get away from a stop light. Total hp for the 2.5 L Mazda engine is 107 hp no acc.'s. Both fan and A/C 107-30hp='s 77hp for a 3400 lb(me and daily gear). Ouch.

Solution for me: full block radiator block and transmission heaters.Variable air intake block. I have a 2.5 foot "Super Bird" nose that is fully sealed to both the radiator and A/C "radiator" so that any admitted air is ducted directly to the radiators. I'm using an ABS cover plate that I move manually to admit none, some, or more air.

Goal: Regulate incoming air in summer temp's to keep mech. fan off and be able to use A/C in a pulse and glide manner. This includes stop and go and
highway speeds. Trial and error for the factors had allowed me to "figure" the opening needed. Don't have to change this much. Only takes 2-3 minutes to change the opening( four screws, loosen and move ).

Working well. As Leon Russell sang..." Up on the tightwire...on side is ice and the others fire..."

Future: Manual control from the driver's seat.

The only air intake in my Canyon is an opening under and behind the license plate. It's been enough, even in the summer with the AC running.

So I assume you have no ducting from the grill to the radiator, correct?

Running into issues with this, on my car the section under the bumper has a factory airdam, that significantly contributes to the cooling system. In order to run a full undertray, that has to be removed, but removing it significantly increases cooling system temps at speed.

In the summer, I can block 90% of the front grill area, as long as that factory airdam behind and under the bumper is still on. But with half of the factory grill blocked, and the airdam removed, the car overheats during the hottest summer months. I'll get pics in an hour or so to illustrate the area.

sticky
 

The mod-data sticky should have Walter Korff's low-drag cooling inlet diagram.You'll want to see it as it is germane to your project.

Now I understand the advantage of the ducted, low drag cooling design. Are there any DIY build threads / pages on that approach? I want to think in terms of cost-benefit, it's easier to assess difficulty if it's been done already - bonus points if the benefits are A-B-A tested or CFD'd. This seems like a frontier of ecomodding.

But my calcs may still be useful, as a guideline it probably applies somewhat to both ducted and non-ducted designs. A good ducted design would have the advantage that the Cd is less affected by the opening, so variable blocking would be less of a concern.

Higher humidity increases the specific heat of air which makes it cool more effectively, lessening the required opening (which seems backwards, but that's because humans rely on evaporative cooling, which decreases in effectiveness with increasing humidity). Instead I just used a number for dry air. So you can reduce your grill opening in coastal areas, but only by a percentage.

Stop-and-go traffic is too complicated for me. At low speeds your fan may turn on to compensate for low air flow. Other than that, if you're staying off the brakes and getting good gas mileage, grill opening demand should be no more than a little higher than at cruising.

My grill block experience with '96 Geo Metro 3cyl 5spd: For what it's worth, I made a roadside, ad hoc grill blocker over a year ago on my '96 Metro. I used cardboard to fashion my behind the grill ducting, made such that the little air that gets through, MUST go threw the tiny radiator. I completely covered the factory openings in the bumper, thick plastic from a 5qt Castrol oil jug, held on with Gorrilla tape. The only air the radiator gets is from the slit that exists between the bumper cover and the lower lip of the hood, about 10" wide x 1-1/2" tall.

I monitor coolant temp and incoming air temp via my scan gauge. I've never had the coolant temp go up, but IAT is slightly higher, maybe 15degF. The only noticeable difference on this vehicle, blocked versus unblocked, is the electric fan comes on more often in summer, at lower speeds, idling, and in stop and go traffic. The fan never triggers to come on while driving over 35mph, and it always keeps up with the additional heat, even with restricted air flow. I did not do back to back testing, but I think the ducting is helping mange what little air does get in.

Just checked this out. This is exactly what I was looking at doing, just didnt realize this was what it was called. I remember seeing something about this in a racing mag a few years back about Fomula One cars using it in the side pods for the radiators.


I'm going to try some ducting experiments this weekend, and post back on here. I'm also going to close off that bottom area where the airdam is supposed to be and see if it gets hot with a duct to the radiator.

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.

I was looking at this, including drawing C in the page that Aerohead mentioned, but after tuft testing the hood, there isnt an area that doesnt show attached flow with the upper grill block in place. So there isnt a way to suck that air on on my car without putting some sort of lip in front the vent, which seems it would be detrimental to overall aero. At this point it seems the rear radiator idea is still the best option for me, depending on this weekends tests.

...have you seen Ford's Patent for their "air-deflecting" grill, maybe it can provide some useful info(?): http://www.google.com/patents?id=zyM...ed=0CDQQ6AEwAA

grille-deja vu
 

Is this unlike their Ford of Cologne,'Lamillar' grille of 1978?

Aerohead, to be honest, I don't know the answer to your question (FoC'78), only that the current Ford grills, with their multiple, broad, wide, slats are *supposed* to be 'dynamically' aerodynamic with increased speed, ie: air flow 'thru' the grill is reduced as the speed is increased...similar to *how* hydrodynamic water flow becomes 'blocked' through wire-mesh at higher speeds but not at lower speeds.

similar
 

I'll guess that its a boundary layer effect.The slats are so small,they'll have a low Reynolds number at lower speed and air will have free reign into the opening,then at some higher velocity,it will transition to a turbulent boundary layer,air will stall over the sections because of their relative angle of attack and the turbulence will choke off some of the flow.

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.

vena contracta
 

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.

...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.

Adjustments: inclusion of heat lost to exhaust, fiddling with ICE efficiency
 

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.

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.

40 sq in. and 150 bhp
 

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.

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.

thermal efficiency
 

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.