Date: Mon, 21 Nov 2005 02:41:37 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
Content-Type: text/plain; charset="US-ASCII"
In a message dated 11/19/05 6:09:01 AM, kdlewis_wating_time@allvantage.com
writes:
> So you think the heat exchanger is maxed out?
>
> I was toying with the idea of adding a radiator, like the heater under the
> back
> seat, between the output of the water pump and the input to the
> oil-to-coolant
> heat exchanger. The fan would be controlled by the oil temperature. When the
> (oil) temp goes above 220 the fans would kick in and reduce the temperature
> of
> the coolant going to the heat exchanger, hence cooling the oil going to the
> bearings. I did not think this setup would delay the initial heat up time of
> the oil.
>
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
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