Date: Mon, 21 Nov 2005 03:06:06 EST
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Re: Comments on the Oil Cooler Upgrade Issue
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In a message dated 11/19/05 5:55:28 AM, Libbybapa@wmconnect.com writes:
> I would still like to hear your comments on my original idea. I don't know
> if you read my e-mail to you a few days ago, but here is the idea again.
> Drill and tap the waterpump housing under the alternator bracket on the high
> pressure side just before the coolant enters the block. Add a fitting there
> and run that to the oil heat exchanger. Remove and block the line to the hose
> that comes from the radiator. That would run the flow from waterpump through
> heat exchanger. Actually it would heat up the oil considerably sooner than
> the stock diesel vanagon setup which requires the opening of the thermostat
> before coolant flows through the heat exchanger. Prior to thermostat opening
> the coolant through the cooler would be a mix of that from the bypass hose
> and exp. tank. After the opening of the thermostat the flow through the cooler
> would be thermostatically controlled by the thermostat and the flow from the
> cooler would go to the radiator rather than being recycled. The only
> downside I can come up with would be slightly decreased flow through the block.
> Certainly a restrictor might well be needed in the line from waterpump to
> exchanger. I plan on using a valve to be able to adjust the flow. The slight
> decrease in flow through the block would at least partially be offset by the
> reduced coolant temps at high heat situations due to the fact that the coolant
> through the heat exchanger would no longer be recirculated .
> Sounds like you have a very workable system devised.
>
Andrew,
I've now looked at your suggestion. Didn't see your earlier email because of
an overly active delete key! I believe I understand your suggested flow
circuit revision and would offer some serious concerns. Many of the relevant issues
are covered in a reply to a post by Ken Lewis that I just posted and added a
copy of below. As you know, I feel the stock system is well engineered and has
far more cooling capacity than needed for any engine conversion application
contemplated.
As far as cooling the oil with coolant, there will be no measurable change in
reversing the direction of coolant through the heat exchanger. The key issues
are the surface area of the exchanger and the lack of air in the oil. If the
oil has been aerated in the crankcase and any significant amount of this
aerosol is in the oil liquid mix, then the efficiency of heat transfer in the
exchanger will be drastically reduced. Often the surface area of the oil filter is
adequate to coalesce the aerosol/liquid mix to straight liquid, so less damage
is done to the bearing surfaces than could be the case, but this happens
after the heat exchanger.
Adding an additional inlet port to the back of the water pump could be
catastrophic. The point is that in all water pump designs, the inlet, bypass and
outlet passages are a careful compromise with impeller dimensions, body volume
and impeller shaft speed. The pressure difference between the inlet and outlet
must be carefully controlled to minimize the possibility of pump cavitation.
Should pump cavitation occur by changing this pressure balance, the engine can
readily overheat, despite proper coolant source temperatures. This is a common
failure in racing engines with the switch to improperly sized electric motor
driven water pumps as well as power pulley size changes.
That said, If you proceed, I would rig a test engine with either a boroscope
or an acoustic pickup to carefully monitor the onset of cavitation at the
water pump. If no cavitation over the operating rpm range then no harm done.
Hope this helps,
Frank Grunthaner
Ken,
Possible effect but very limited. Typically, if the coolant system is
functioning correctly, the coolant temperature going out of the block to the radiator
is typically 15 F hotter than the temperature at the entry point in the block
(after the outlet of the water pump). If the thermostat is calibrated for 190
F (typical for US/Canadian 1Z engines) for those conditions in which the
thermostat is controlling (not fully open) but the engine is warm, the block inlet
coolant temperature is 190 F +/- 5 degrees. So the outlet temperature is 205
F, still under the oil temperature of 220 you target.
Now for reference, the temperature difference between the coolant inlet to a
fully functioning vanagon radiator to the exit is typically 45 F when the
inlet temp is 200 F or above. With the 15 psi pressurization of the cooling
system, a 50% glycol/water coolant will boil at temperatures above 260 F. For the
other side of the ledger, Rust is generated rapidly in a glycol/water cooling
system when the coolant temperature is below 130 F; and below 110 F, water
rapidly accumulates in the crankcase oil. Cylinder wall wear rates are nearly 10 x
faster whenever the coolant temperature is below 150 F.
So the cooling system is designed to get the coolant, oil and block
temperatures up to nominal operating parameters (160 F to 200 F) as quickly as
possible. Hence the bypass of radiator circuit, the head bypass circuit and the
complex design of internal block/head coolant flow with head gasket design.
Now as to the oil temperature problem. In general, VW has clearly designed
the oil cooling system for typical loads incurred during harsh operating
conditions. Their emission controlled engines dump the peak thermal load to the
coolant at extended idle because of retarded ignition timing maps. While the load
would appear to be more at full throttle, these conditions are accompanied by
high coolant flow rates (higher rpm) and correspondingly higher thermal
capacity. The heat exchange or transfer efficiency at the coolant to radiator fin to
air interfaces far exceed the ability or efficiency of the oil to water
transfer per unit area. The cooling system of the vanagon could efficiently support
a carburated 350+ hp 427 cu. in . GM V8 with A/C as I have discussed in the
Vanagon list archives.
So the problem with rising oil temperatures under load with turbo charged
diesels at load is the result of two probable issues: 1) elevated coolant
temperatures (around 205 - 220 degrees) thereby minimizing the thermal gradient to
cool the oil in the water to oil heat exchanger or 2) the inability to dump the
excess heat to the coolant (or equilibrate oil and coolant temperatures) over
the common surface area of the heat exchanger. The real problem is the second
of these ... the thermal transfer efficiency of the oil at any surface in the
liquid phase is a complex function of thermal transfer coefficient, viscosity,
wetting angle and flow type (turbulent or laminar). Suffice it to say that
oil is about 5 times poorer a thermal transfer agent than is either water or the
water glycol mixture. The answer chosen by VW for higher output engines is
more heat exchanger surface area. In the case of their more powerful Euro TDI
engines, they made the oil to water heat exchangers with larger surface area
(and consequently larger volume). For their high performance gasoline engines
with high rpm capability, they chose thermostatically controlled external oil
coolers (VW Euro GTi. Audi 5000T, and others). For the high rpm gas engines, oil
film strength at high shear loads was most important. For the high performance
TDi engine, the same conditions apply with possibly more issues of film
compressive stress.
So to summarize, your solution will drop oil temperatures as much as 10 to 20
degrees with some complexity. It will have no effect on the cooling system
thermal capacity which is already overkill. However, as oil temperatures climb
above 230 F, they will go rapidly to a runaway condition not impacted by your
solution. The cheapest solution to go with your dual oil filter system (Audi or
Amsoil or ...) and add the standard VW oil cooler to each filter element.
This will immediately double your thermal capacity, giving you the equivalent of
the largest OEM VW oil to water heat exchanger.
Frank Grunthaner
