THERMOSTATS AND CABIN HEAT
RANS S-7
2010/06/26
Topics:
Thermostasis
better than a Permacool
The problem
While capturing muffler heat is
an option on the S7, a more effective approach is to use engine coolant through
a small rad in the cabin. Some people have packaged a 5x7” Earl's oil cooler
with a muffin fan hooked in parallel with the main rad but more than one person
has said it is inadequate. Perhaps by looking at the factors which affect heat
transfer we could make it work better.
The amount of heat we can get
from the rad is proportional to the difference between the coolant temp and
ambient temp so it is important to keep coolant temp high. One approach is to
block off the cold air flow to the rad with duct tape
or a moveable shutter; another is by installing a thermostat.
There is no doubt in my mind
that coolant and oil thermostats are a necessity. They work flawlessly and consistently
with no monitoring and maintain temps right where we want them.
Oil Thermostats and
keeping the oil hot:
An oil thermostat like the Permacool unit that Lockwood sells does a fine job of
keeping oil temp constant IN COOL air temps.
With no cover over the inlet to the oil cooler it will peg the temp at
the thermostat rating in all but the coldest weather.
The Permacool
thermostats work very well for (both water and oil) during the cooler months
and do an excellent job of keeping the oil temp constant in cold weather but
appear to restrict cooling when we really need it in very high ambient temps
due to not fully closing the bypass circuit allowing some hot oil to avoid the
cooler.
These thermostats operate a
little differently from the way an automobile coolant thermostat works. In a
car, when the system is cold, the thermostat restricts the flow of coolant and
only allows full flow when the coolant warms up. In the Permacool
thermostat, when the oil is cold, a valve is open which allows the full oil
flow to avoid going through the oil cooler; as the oil heats up this valve
closes forcing the oil to go through the cooler.
If you open one up you can
see notches in the circumference where the large washer closes the flow of oil
across the unit (the valve) which it does when hot to force the oil to go via
the cooler. These notches allow about 10% of the oil flow to avoid or bypass
the cooler which is not what we want. The unit can be machined to remove most
of the notches thereby making ALL the oil go through
the cooler. This was confirmed in a test
by a guy in BC.
Further down are pictures of
this unit and details on the modification.
Some data:
The unit is shipped with a 77
C (171F)pellet (or “waxstat) so unless you change it
that is the highest temp you will see IN COLD weather when the thermostat is
limiting temp and this is too low for oil.
Most (if not all) automobile
thermostats have a pellet that fits the Permcool so
you can select whatever temp you want for any car at an auto parts store. You
will have to carefully remove a ring pressed on to the pellet which is not used
when in the Permacool. The highest temp you can find
is maybe 195 F which is fine.
Lockwood ships units with ½”
pipe thread openings and fittings to fit. The racing shops sell both ½ and 3/8”
pipe thread. 3/8” is more than enough for the oil lines since the ID is about
7/16” like existing lines. Smaller fittings are lighter and take less space or
you can go to aluminum fittings from the racing shops. The ½” pipe models work
fine to control coolant flow in the winter. You would order 3/8”MIP x ½” hose
fittings which are readily available. Three straight and one 90 work well on an
S7.
Permacool Internals
Below is a picture of a pellet (wax stat) from
the Permacool.
The pellet on the left is
from the Permacool; the one on the right from an automotive thermostat. When
installed the one on the left has a large stainless washer similar to the
formed ring around the one on the right. The size of the ring is almost the
same as the washer and could probably be used but to be certain, the ring on
the right can be removed and the same washer used. I have replaced the 171
degree pellet with the one on the right which is 195 for oil (for coolant I’m
using 180).
Changing the thermostat
“pellet” is quite straight forward. Removing the retaining ring allows the
piston to slide out followed by the pellet.
NOTE re oil temps
when using the Permacool thermostat.
This unit does an excellent job of maintaining
a constant oil temperature and thus is very slick for winter operation. There
is a problem during operation at 30 deg C OAT however in that the design of the
bypass is such that even when the bypass is closed when we want all oil to go
through the cooler it is not a perfect seal and allows some oil to bypass the
cooler raising temps by maybe 10 C or so. This can be fixed with some machining
of the seat. Below is a view of the seat which has been machined. You can see
that two of the four notches have been removed; parts of the other two are
still visible.
Since about .1” has been removed, I inserted a
similar length into the pellet pin socket to move the pellet that much closer
to the seat. I also cut one coil off the spring.
Thermostasis thermostat
This unit is fairly new on
the market and appears to have removed or reduced a couple of the issues with
the Permacool. Here is a picture of one with some
aluminum fittings added (it does not come with the hose barbs).
Physically it is a little
smaller than the Permacool and in a more elegant
package. Functionally it is the same as the Permacool
except it is available in a variety of temperature settings up to 205 F so no
exchanging of the wax stat is needed and it allows a much smaller amount (2%)
of the oil to bypass the cooler when hot so internal modification should not be
required.
The price is over double the Permacool but it is worth it.
Keeping Coolant at
the right temperature:
In one experiment, I
installed a Permacool thermostat (normally used in the oil system) in the
coolant lines to speed warm-up and maintain the right temp. Here is a picture
of the lines going to and from the rad on the floor
between front and rear seats and the thermostat (rad
on the left, engine on the right):
Caution. While the Permacool
works very well in this application in the winter, because its fittings are max
½” pipe, it will restrict the flow of coolant which will
likely cause overheating during hot summer operation. There may be enough
material to allow boring it out to take 1” tubing but I have not tried this.
Also for summer use the seat would have to be machined as mentioned below.
See below for a better solution using a BMW thermostat
and a Motorad (both full bypass thermostats)
Bypass Thermostats
There are several methods that
incorporate a thermostat to control temperatures. The oil thermostats units mentioned
above, bypass the radiator by simply offering a shorter path through the
movement of one valve. The alternative path through the cooler is still open so
some of the liquid may still take it but most will take the easier route.
Another method when flow
restriction is an option (it is not an option with oil) is to just close off
the flow leaving only a small aperture for minimal circulation, or close it off
totally and include a small diameter bypass route. Both of these methods cause greatly reduced
circulation through the engine when cold which could result in hot spots and
the second with the permanent bypass results in not all the coolant going through
the radiator when hot which reduces cooling capacity.
A better alternative is a
full bypass thermostat which is a dual valve system which results in both full
flow through the engine when cold or hot and zero flow through the rad when cold for fastest warm up time.
There are two of these units
from the automotive area which are worth looking at.
BMW
This unit available from NAPA
stores in the US works well in the coolant system without any modifications The
one down side is that it is made for 1 ¼ hose so some adaptation is needed.
Here is one about to be
installed under the seat in an S7.
The
markings on the cardboard show the two tubes which carry the coolant to and
from the rad. The cross hatching is the rad with the inlet and outlet shown as
two circles. The fact that the housing is for 1
1/4 hose actually works out fine at the rad since the rad fittings
are 1 ¼” (Rans has welded a 90 degree fitting on the 1” aluminum tubes to
increase size to the 1 ¼ rad fittings. These are no
longer needed with the BMW but reduction is need further along).
I bought a preformed hose at the auto
parts store to get the two 90's that connect the thermostat to the radiator
(not shown here) and the 45. The aluminum lines have to be shortened and a bead
put on them (or, better, get some new pipe and keep the originals in tact;
you’ll need two 2’ lengths if you take hose out through the firewall as I
did). (You could also put ½” Ts in these lines to hook up to a cabin
heater rad). Eventually I'll get the T and reducing
fittings welded up in aluminum.
The
housing has two valves in it which move together near the "H" and
"C" marked on the housing. When cold they are to the left so the
coolant coming in on the top left pipe is routed to the T on the right and back
to the engine bypassing the rad. When hot they are to the right forcing the
coolant out the left side, into the rad and back out to the engine.
Here
it is tucked under the pilot seat on a 94 S7 (rad to
the left; engine to the right):
And
here is a pic of the hoses exiting the firewall with a baffle to insulate them
from the exhaust stack heat:
This setup worked very well
winter and summer.
Motorad bypass thermostat.
This
unit works internally exactly like the BMW but has a more useful geometry.
Here
is one version of the Motorad thermostat and
copper/brass elbow/T
to complete the bypass circuit:
This
version is made out of brass so goes well with copper pipe fittings (some of
the same type are made out of aluminum). The two on
the right are ¾ copper x ¾ female pipe and had to be
machined to fit the 1 ¼ thermostat pipes (as did the bypass fitting which uses
¾ hose).
There
is no need to retain the full 1” diameter on the bypass circuit because we only
need enough flow to prevent hot spots while the engine warms up. Here it uses
¾” hose; likely you could get by with ½” (when hot there is no circulation
through this bypass).
This
fits nicely within the motor mount on this 2000 S-7 short tail. Below is prior
to the installation:
Here
it is installed:
You
can make out the uncovered ¾” pipe on the hot side of the thermostat. This will
be convenient as a source of coolant for the cabin rad
along with another T in the return side.
With
some slightly different fittings it should also work well above the rad in an S7S model. Watch for an update on this later.
On
this installation, prior to installing the thermostat, it took 5 minutes for
the coolant temp to come up to the 140 degree mark. After installation it took
about a minute and was up to normal operating in 3 minutes (on a 26 C day)
Back to Cabin Heat
Another factor which will
affect cabin heat is the amount of air moved over the cabin heater rad. Because the muffin fan doesn’t move a lot of air and
re-circulates existing air, I decided to use a supply of external air. I think
this will also tend to keep the moisture level down and be a safety factor. The
slipstream air enters a 2” central vac elbow and is routed to the rad through
an automotive 2” flexible aluminum hose to a shroud around the rad. Probably
not everyone would want to add such un-aerodynamic protrusions to their works
of art. I tend to like function ahead of form.
This is the rad with shroud:
If this unit
works well I may make a more elegant shroud out of fiberglass as well as experiment
with a shroud on the bottom (outlet) to route some heated air to the passenger
but the way it is seems to push some heated air under the seat anyway.
The coolant shut off valve is
at the bottom right. Shutting off the flow at the valve does not stop heat
transfer into the rad from the coolant so you are always getting some heat off
the rad. This is of some concern in
summer when you don’t need it.
First flight was on a -10 C (about 15F) and
cabin temp was quite comfortable. I’ll take some temp measurements next time.
Another short tail installation:
Here is another heater
installation which taps into the cabin floor pipes to/from the rad in a 94
short tail:
At top right is the valve to shut off the flow and on
the hex fitting at the rad is an air valve to help with bleeding the air out
when filling the system and also to put some air in there when the heat isn’t
needed in the summer. Here is another view from the side.
And this is the central vac elbow that feeds it:
For oil cooler mods click
here