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Old 05-15-2012, 06:20 PM   #1 (permalink)
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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?

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.)

Attached Files
File Type: xls GrillArea.xls (29.0 KB, 38 views)
File Type: xls GrillArea_wExhaust.xls (13.0 KB, 28 views)

Last edited by christofoo; 01-06-2013 at 04:28 AM.. Reason: inclusion of exhaust gas heat loss rate
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Old 05-15-2012, 10:57 PM   #2 (permalink)
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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.
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Old 05-16-2012, 02:48 AM   #3 (permalink)
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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.

Last edited by christofoo; 05-18-2012 at 07:46 PM.. Reason: correction
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Old 05-16-2012, 04:49 AM   #4 (permalink)
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Quote:
Originally Posted by christofoo View Post
It seems quite clear to me that extended climbs present the most strenuous need for grill opening in passenger cars.
That sounds about right to me.

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.
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Old 05-16-2012, 10:01 AM   #5 (permalink)
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Quote:
Originally Posted by christofoo View Post
...
I remember that AndrewJ ran into his first serious issue with a fully closed grill while climbing a pass.
...
Oops, AndrewJ concluded it was a problem with altitude adjustment, not overheating.
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Old 05-16-2012, 10:16 AM   #6 (permalink)
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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.
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Old 05-16-2012, 01:08 PM   #7 (permalink)
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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.
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Old 05-16-2012, 02:30 PM   #8 (permalink)
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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.
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Old 05-16-2012, 06:24 PM   #9 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
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.
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.
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Old 05-16-2012, 07:23 PM   #10 (permalink)
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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.

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