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Old 07-23-2008, 10:40 PM   #1 (permalink)
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Moving air intake into the engine compartment?

On most unmodified cars air enters from the front both through the engine air intake located above the radiator, and through the radiator itself. At highway speeds a lot of air blows through the grills, through the radiator, and then exits into the wheel wheels or under the car, and of course the air from the engine intake eventually goes out the exhaust. Once we start blocking the front grills less air will be available to be pushed through the radiator, and the engine air intake location may no longer be in or next to the primary air flow into the car.

Hypothetically, once the grills are blocked, would there be an advantage to moving the air intake into the engine compartment behind the radiator somewhere?

We can estimate how much air the motor moves. For a four stroke 2 liter motor at 3000 rpm, there will be one intake stroke per cylinder every other rotation, so total engine intake volume is 1500 * 2 = 3000 liters/minute (or 106 cubic feet per minute). That's an upper limit, since the EGR valve will feed exhaust back in, some percentage of this will not be from the air intake. (It's remarkable how little air passes through a small car motor, typical pedestal fans like one finds around the house move thousands of CFM.)

I have no idea what the air flow through a radiator is typically on the highway. Radiator fans tend to be rated at 1000-3000 CFM, but that is most likely a free air rating, so it doesn't really tell us how much air these actually drag through a radiator.

Anyway, if the air intake is placed in the engine compartment it will tend to drop the pressure in there, which will tend to draw more air through the radiator from whatever grill openings remain. Better still, the radiator air consumed by the motor doesn't have to exit the engine compartment in the normal manner, instead it will be blown out the exhaust.

There are some problems that could arise. The intake air will be considerably warmer than the car was designed for. Some "cold air intakes" are inside the engine compartment, but these are usually employed with a normal front grill. The motor isn't going to be damaged by the hotter intake air, but the MAF sensor might have issues. The inside of the engine compartment tends to be oily, which would foul the air filter more quickly. The pressure drop near the relocated air intake, in conjunction with the blocked front grills, might cause unexpected air circulation problems, which could result in even less flow through the radiator.

Thoughts?

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Old 07-23-2008, 10:53 PM   #2 (permalink)
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The amount of air the engine consumes when FE driving, at light loads, is only a fraction of its full open throttle air, so any underhood pressure reduction would be really trivial. Tons more air is available from under the sides than is needed to supply the engine.

One issue with a warm air intake is that the computer may retard spark timing if the engine is knock limited, reducing efficiency and FE. If it's not knock limited you MAY pick up a bit of FE from the increased volumetric efficiency. YMMV.
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Old 07-24-2008, 03:37 PM   #3 (permalink)
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The convention has been to keep the inlet tract opening ahead of the radiator bulkhead to take advantage of the high static pressure which can be harvested at this forward stagnation point.------------------------------------- The cooling system also shares the forward stagnation point, and above 40 mph(65 km/h),the cooling fan is superfluous,with all cooling achieved by ram- air.------------------------------------- EFI CPU proms are calibrated for and dependent on original OEM body architecture.It's not to say that things can't be massaged around,but you'd want to be careful.----------------------------------- Engine power is a direct function of charge density,and by moving the intake inlet behind the heat exchanger and exposing it to the heat-flux of the powerplant, would certainly affect incoming air density, with no provision for compensation or modulation.I wouldn't recommend it. The air mass moving "through" the engine itself, would be of such low volume, as compared to that flowing through the cooling sytem,as to render it" invisible" and of no consequence to the cooling system airflow.On a Messeschmitt Me 109 I'd say it's an issue,but not for a passenger car.
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Old 07-25-2008, 12:21 PM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
The convention has been to keep the inlet tract opening ahead of the radiator bulkhead to take advantage of the high static pressure which can be harvested at this forward stagnation point.
Sure, on an unmodified car that makes sense. However, once the grill blocks go in, is that position necessarily still optimal, assuming it is still possible. On my Protege5 the engine intake is through a space under the leading edge of the hood. It's hard to describe this, but it is sort of on the upslope of the front end of the car. If the upper grill was blocked aerodynamically the intake would still be possible but probably the air pressure there would be greatly reduced by better flow over the front. For our Subaru Legacy the engine air intake is 1/4 of the upper grill (which is split into 4 parts by a sort of "+" shaped member). There at least one could block 3 of the 4 quarters and I think the engine intake would be little affected as the ram effect at speed would be unchanged. If the entire upper grill was blocked the intake would have to move, else it would be literally sealed off, at which point the question becomes, where to move it to. In this regard, brucey avoided the whole issue by blocking the lower intake:

http://ecomodder.com/forum/showthrea...html#post46349

Quote:
Originally Posted by aerohead View Post
The air mass moving "through" the engine itself, would be of such low volume, as compared to that flowing through the cooling sytem,as to render it" invisible" and of no consequence to the cooling system airflow.
Ok
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Old 07-25-2008, 02:29 PM   #5 (permalink)
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optimal

Quote:
Originally Posted by pasadena_commut View Post
Sure, on an unmodified car that makes sense. However, once the grill blocks go in, is that position necessarily still optimal, assuming it is still possible. On my Protege5 the engine intake is through a space under the leading edge of the hood. It's hard to describe this, but it is sort of on the upslope of the front end of the car. If the upper grill was blocked aerodynamically the intake would still be possible but probably the air pressure there would be greatly reduced by better flow over the front. For our Subaru Legacy the engine air intake is 1/4 of the upper grill (which is split into 4 parts by a sort of "+" shaped member). There at least one could block 3 of the 4 quarters and I think the engine intake would be little affected as the ram effect at speed would be unchanged. If the entire upper grill was blocked the intake would have to move, else it would be literally sealed off, at which point the question becomes, where to move it to. In this regard, brucey avoided the whole issue by blocking the lower intake:

http://ecomodder.com/forum/showthrea...html#post46349



Ok
The upper grille-block essentially creates an airtight duct common to the lower grille opening.Stagnation pressure is still communicated through the duct to the inlet above.Since this air is harvested at the point of maximum static pressure,by moving the inlet elsewhere,you potentially re-locate it to an area of reduced pressure as demanded by the Bernoulli Theorem.
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Old 07-25-2008, 03:24 PM   #6 (permalink)
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Try sucking air out of the fender, I've completely blocked my grill btw. The upper grill is blocked with clear tape.

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