Aircooled Engine - Extraction of hot air via low pressure

[Mase007]

Hi there,

I have a project that I am working on and I am trying to reduce load and thus energy expenditure on a crank powered cooling fan by utilising fast moving cool air.

Basically the engine is split into two sides; the cool side (top) and the hot side (bottom). The fan blows cool air over the top and down through the engine and it is then directed rearwards and then exits the vehicle.

I have attached an image of the standard setup and wandered if you guys could help me come up with a system of using the large body of fast moving cool air below the engine to "suck" the hot air from the lower engine area, thus reducing the workload on the fan...

Any ideas are welcome, and I will post my own as and when I have drawn them out.

Cheers, Mase.

 

[IRstuff]

Based on what you've given, there's nothing you can do to change the load on the fan, since it's the mass flow and temperature rise of the air that dictates how much cooling can be achieved. However, if there were a way to directly transfer heat from the source to the cold air, then there would be less heat that the fan air is required to carry, and this would reduce the mass flow required, and lessen the load on the fan.

TTFN
faq731-376

 

[patprimmer]

A thermostatically controlled variable gear drive to the cooling fan is the way I see to save wasting power over driving the fan when not required.

The air cooled VW already uses an airflow system as shown in your diagram and did so since the 1930s.

Regards
Pat
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[Mase007]

Hi Pat,

This is actually for an aircooled Type 4 VW motor.

The way I figure it, the fan, driven directly from the crank is pressurizing the top half of the engine with air flowing through to the underside and then out the back. The underside will still be at a positive pressure (I would like to actually test and confirm this...) so to aid the fan and therefore reduce it's power requirements I want to create negative pressure in the area under the cylinders/heads by using the fast moving air underneath. Think of a rear diffuser, it creates negative pressure by "expanding" the air flowing over/through it, and that is what I would like to achieve here.

Cheers, Mase.

 

[patprimmer]

The bottom cylinder covers and lower duct that is between the exhaust and the crankcase cause a low pressure at the exit of the system already. That is why it is important that they are installed as tardy mechanics who think they are unnecessary and then leave them off learn when the engine overheats.

Regards
Pat
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[tbuelna]

Mase007,

First, we should remember that a fluid will normally flow from areas of higher pressure to areas with lower pressure. So obviously you want to create just such a condition in your engine cooling air heat exchange circuit. With a compressible fluid like air, there are lots of methods at your disposal for accomplishing this. You can use forced air with electrically or mechanically powered fans, you can use ram air intake ducts, you can use an engine exhaust gas flow assisted ejector, or some combination of these.

Second, we should also remember that air cooling of engine cylinders is greatly influenced by air mass flow. If a forced air fan system is desired, optimizing it for proper mass flow over its operating conditions will make it efficient. And to minimize the cooling air mass flow rate provided by the fan, the cooling air must be used efficiently. This means creating as large a temperature rise as possible in the cooling airflow as it passes over the cylinders/heads. You can reduce the work required to drive your cooling fan by improving the heat transfer mechanism of your cylinders/heads, through denser fin pitch, using fins made from high thermal conductivity metals like aluminum, and making sure your fin surface temps are kept as high as practical.

Lastly, you want to make sure that the air baffles around the cylinders/heads fit snug and minimize leakage. It might also be worthwhile to add some internal deflectors/vanes/restrictors to improve the distribution of airflow. Some areas, like around the exhaust port, likely require more airflow.

Good luck with your project.
Terry

 

[patprimmer]

Terry

On air cooled VWs, the lower cylinder covers are specifically designed to direct air around the corner across the bottom of the cylinder.

I got away with turning my fan about 25% slower I think from memory and with using an old model fan that was somewhat smaller. This was on a Beetle with a belt driven fan and lots of different models with the same layout.

I think on the OPs engine the fan is direct crank mounted and there will be few if any stock fans of different capacity available. Maybe he can remove some vanes to reduce capacity?

I found that by very carefully looking at airflow and carefully assembling the ducts and sealing all joints with silicone rubber and relocating the OEM oil cooler and fitting a big remote mounted oil cooler I could rum an engine increased from 1200cc to 2200cc and compression increased from about 6:1 up to 10.3:1 with comparable cam, carby, valve and port and exhaust changes, it still stayed cool.

I did similar to a Type 3 which had a crank mounted fan. Left the fan stock but heavily reworked the top covers and some baffles to even it out after removing the OEM oil cooler. I could not get the very noticeable power increase from pumping less air through the much improved cooling system, but I could increase cylinder pressures a lot without detonation or overheating.

Oh, I also did quite a bit inside the engine to reduce heat transfer into the head by smoothing and polishing and deburing. Of course the less heat transferred into the head, the more goes to drive or out the exhaust rather than the cooling system.

Also re casting flash on the fins and cooling air passages. I removed flas that was to restrictive of air flow through critical areas, but left enough to create turbulence at the surfaces and to maximise surface area without being to obstructive. That was a real judgement call, but verified as somewhat effective by thermal sensitive crayon tests.

Regards
Pat
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[Mase007]

Hi Terry,

Thanks for your response, you have raised some interesting points.

Firstly, I will start by saying that the engine I am working on is as it left the factory with every piece of cooling tinware in place and totally sealed with the correct stock rubber seals and also Loctite flexible silicon between adjoining pieces of tinware. Even included are the stock heat exchangers that may or may not pull a little extra heat away from the exhaust port/valve.

Secondly, I want to build a high horsepower motor and what I am trying to achieve here is additional high speed cooling and lower "pumping" losses from the fan, but crucially low speed cooling needs to be unaffected. Air-cooled VW's weren't designed for today's high ethanol/low lead fuels and are not known for their aerodynamic shapes. Pushing any of these classic VW's above 70mph or so creates too much heat that the cooling system has trouble dealing with for any length of time. To counter this heat build-up, many tuners will run an overly rich mixture setting that will help with cooling and prevent detonation to some extent, but I also want this motor to be as fuel efficient as possible. The build of the engine is taken care of but I would love to hear from Pat about how you reduced heat transfer into the heads. Ultimately I want to take this motor to the track where it will be run at high rev's/high speed for long periods of time, and so utilising the fast moving cool air running under the car seems like a good, free way of increasing cooling efficiency.

The engine already has a mechanically powered fan that unlike the type 1 engine, cannot be varied in speed vs engine rpm. With regards to ram effect cooling, I think this may be difficult to implement as the type 4 fan puts out a huge amount of pressure, and adding ducting to the topside would likely result in pressurised air leaving the engine compartment. The exhaust gas flow assisted ejector sounds interesting though so I would love to hear a little more about this sort of system.

I am trying to think of simpler ways of extracting the hot air and reducing pressure and so I though about adding louvers to the bottom tinware. Would this likely reduce pressure enough to make a difference? I think at 80mph+ the "suction" here should be significant (see attachment).

Also, would the addition of a small "spoiler" protruding down from the rear of the tinware into the airflow under the car create enough of a low pressure zone behind it to extract more hot air?

Thanks for your input guys, very interesting and very much appreciated.

Cheers, Mase.

 

[patprimmer]

I did mine from the late 60s to late 70s and we did not have many of the tools we have today.

To reduce heat transfer, I simply smoothed, rounded off and polished things to reduce surface area. With todays materials I would also ceramic coat the chambers and piston tops with a thermal barrier and maybe coat the fins with heat rejection coatings. I know thermal barrier works. I am not sure if heat rejection coatings work though.

The main extra cooling came from the oil system. I used the biggest OEM oil pump that fitted and hard anodised the body to give I think 0.0015" rotor to body clearance and the thin paper gasket end clearance.

I used a Mazda RX2 oil cooler mounted in the spare wheel well behind where the front bumper would normally be. I ran 1/2" copper lines from the engine to and from the cooler. They where under the running board in probably turbulent airflow. On the type 3 I ran them through the cabin beside the tunnel.

I used tubular exhaust which eliminated the heaters. OK I was young and Sydney is never really cold. I always planned to run ducts from behind the oil cooler but never got around to it.

I used very long duration cams and more compression than anyone thought possible.

I also used very large carbies with one choke per cylinder on an IR manifold. a:f was set according to plug readings as wide band O2 sensors where not available to mere mortals like me, but it was set to normal OEM tune level. Accelerator pumps where set to smallish volume with small nozzles and light springs to give a low volume delivery over a longer than normal time. Ignition was exactly OEM recommendation.

I always attributed the good drive ability despite over caming and over carburation to the boxer layout and head and port design so that each carby choke had direct downhill line of sight to the inlet valve. Of course direct port fuel injection does this even better. Oh. Long cam duration reduces heat during compression and allows heat out early via the exhaust.








Regards
Pat
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[rb1957]

how about accounting for ram air ? does your inlet allow ram air to add to the fan ?

 

[thruthefence]

Design some sort of augmenter exhaust system to use the velocity of the exhaust gases to extract air from the low pressure side of the engine.

http://tinyurl.com/c9m6s48

 

[Mase007]

Thanks Pat, interesting stuff. I have emailed a couple of companies regarding ceramic coating the pistons/combustion chambers so that's definitely something I'd be interested in.

Going back to the heat extraction, what do you think of the attached image? Do you see any benefits?

I have a trial copy of Solidworks but am having a hard time with it at the moment, I just want a simple model to be able to test this thing without having to go buy some sheet metal to cut up (yet)

Cheers, Mase.

 

[rb1957]

i don't think that venting the exhaust more directly to the outside airflow will increase the heat flow significantly.

how about ... (see attached pic) ... entraining some of the outside air into your hot exhaust ?

 

[patprimmer]

I csan't see either doing much.

I would think a fully enclosed duct that discharges into the area at the rear beaver tail, preferably designed as a diffuser.

Regards
Pat
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[patprimmer]

If the car has the side vent grill in the top of the rear guards (fenders) then an air dam behind the vent should help.

Regards
Pat
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[nobog]

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