Swapping in a higher temp thermostat
 

So, the Paseo look like it needs a new thermostat. I went online and browsed over the selection I have. It seems the stock thermostat is set to 180°F (82°C). Now, I've heard of guys using higher temp thermostats and getting better mileage. So, I continued digging and found I can get a 192°F (89°C) thermostat for the car. Hurray. Now, why does this help? What does it hurt? OEMs must not use 192°F stock for a reason. I'm thinking it probably hurts power output as ignition timing may need to be retarded due to insufficient cooling and hotspot developing. I'm not sure why hotter would be better except that you loose less energy through your coolant system.

192-195 is stock on most cars Ive come across.

The thermostat on my 2008 starts to open at 177°F and is fully open at 201°F.

I also do not understand the logic behind running a higher temp. thermostat. Theoretically, wouldn't you be lowering efficiency by increasing the temp. of the cold reservoir? Then again, I'm wondering what the true cold reservoir is...the coolant or the air.

The equation I'm thinking of is the Carnot Theorem: 1 - Tc/Th

I can't really think of any benefits...:(

- LostCause

Hello -

I am not an authority, but I have read on forums that the higher temp means that the engine is running more efficiently. The analogy would be a cold engine in the morning that does not become "efficient" until it is all warmed up.

If you are an auto manufacturer, I think that you are engineering with two goals in mind. Efficiency *and* reliability. The "balanced" response is to get the engine to be hot enough to run efficiently, but cool enough to be as far away from the "overheat zone" as possible. If you want reliability, you choose the T-Stat that gives you that compromise.

Also, you are designing the cooling system for the broad majority spectrum of drivers. Most drivers are not the "feather foots" that we are, so the cooling system needs to serve their driving habits, not ours.

As an Ecomodder, we are doing mods that "push" the engine for more fuel efficiency. As a result, we have to monitor the engine's health to a greater degree. We have to be vigilant, just like someone who puts a high-performance mod onto their car.

I have the hotter T-Stat. I used to run at about 180 degrees F. Now I run at 190 degrees F.

CarloSW2

You had to see this comming. :p

Did it make a noticable difference? :D

IIRC the reason for higher engine temps for economy are:

1) the higher the engine temp the lower the friction (mainly due to design clearances and pumping of fluids),
2) also the better the vapourisation of fuel prior to combustion leads to more complete combustion (a more homogenious mixture)
3) and the lower the differential between combustion flame temperature and piston/cylinder/combustion chamber temperature.

The chamber temperature is an interesting one as the heat released to the metal by the flame (and hence the loss of energy to the coolant) is actually less the higher the engine temperature. This means that more energy is available to expand the gasses in the cylinder and propel the vehicle.

Ideally the engine would run at the same temperature as the combustion flame, so there would be no heat rejection to the coolant and therefore no energy loss & perfect efficiency :)

Thermal losses accounting for roughly 65% of all the energy spent in city driving, there's a lot to gain in reducing those losses.

Daox -

Good question. I made the switch in my early less-accurate data recording days. ... dig dig dig ... Ok, here are my gaslogs :

1999 Saturn SW2 with switch to hotter T-Stat
39.45 / 37.04 = 1.0548 => 5.5% increase in MPG.

Soooooo, it appears that I got a +5% increase in MPG. Here are my only modifications during this time :

(*) Now, here's the rub. On October 19, 2006, I discovered the P&G (Pulse and Glide) driving strategy. This was "mild" engine-on P&G, but my daily runs improved.

Therefore, after October 19th, I cannot say if it was the T-Stat or the modified driving style, :( .

CarloSW2

Efficiency improvement simple, you are wasting less heat from combustion in to the cylinder walls/ head.

+ other things.

I didn't believe it at first, but it is true.

I read about a racecar inventer one time that created an engine that would produce 2-3 times the horsepower as his competors with the same displacement engine because it ran at 400-500 degrees F. Problem was it wouldn't reliably make it through a single race. I'll try to find that article, but it was several years ago that I read it.

The carnot cycle, like the stirling cycle, is more efficient when run a higher temperature and higher differential. In the stirling cycle I know it's because it provides greater expansion, but I'm not sure if it's the same in the carnot cycle.

Edit: It's crazy how hard it is to find an old article about a hot engine. I'm usually pretty good with google, but all I'm coming up with a bunch of people whining about their car overheating. Does anybody know what I'm talking about or remember any more?

I'd treat a hotter thermostat as a slight, unquantifiable gain. The theory's all there, but it's just not something that I'd expect to actually be able to see in my gas logs.

Sure you may see a difference if you bump up the temps a few hundred degrees, but at only +10º you aren't likely to see much.

I put a 203 degree stat on my 7.3 and got a consistent 0.5 MPG increase. Not bad for such a cheap mod.

I also use Evans NPG coolant. This is a nonaqueous mixture of 10% ethylene glycol and 90% propylene glycol. It boils at a temperature over 400 degrees. I use it to defeat the cavitation problem common to 7.3s, but I have always wondered: "What if I could get a 300 degree stat?" One thing for sure: My heater would burn my fingers off. Even on the coldest days I have to temper the heater supply air.

Bypass the rad and find out :)

That would be great. You've got the coolant problem covered. What about rubber seals at 300*F?

Thats pretty much what I was thinking.

Still didn't stop me from putting in a 195º thermostat :thumbup:

Every little bit helps.

If that is the case, have any of you considered ceramic coating the cylinder walls/pistons? I know ceramic engines have been developed by Nissan, Toyota, and Isuzu during the 80's that showed extreme efficiencies.

Ceramic Engine
http://www.youtube.com/watch?v=7Q0PcqqfA90

They never took off due to the brittle nature of ceramics and the fact it was like machining diamonds, but the concept is there. It seems the theory behind bumping up the thermostat is present, why not take it a step further...:D

- LostCause

I'll answer that one. Money. :)

My thermostat needs to be replaced anyway, so I might as well use a more efficient one.

Going from 160 to the 190 (stock) was a big improvement in the tdi.

Ceramic coatings could improve efficiency . . .

Best credit I can give to ceramic coatings is

A friend of mine had Ceramic coated piston tops (7mgte heavily modded) he was running lean (before the days of cheap wideband o2's) and he melted his cylinder head. (yes that big beefy chunk of aluminum melted holes clean through) and the pistons live on in another motor. So yes, ceramic coatings can do amazing things, if I was to build a motor they would go on everything.

Touche...:p

That got me thinking, so I priced out what it would cost to get piston, cylinder heads, and cylinders coated.

I couldn't find cylinders, but for everything else:

pistons = 4 x $24.50
4 cylinder inline head = $200.00
========================
Total: ~$298.00

I'd assume cylinder walls would cost $100-150 as they are less complex then cylinder heads, but more surface area then piston domes.

So total: ~$398.00-448.00...

Not cheap...and it requires the engine to be rebuilt, but it's worth it for that 0.5% MPG gain...right? :D

- LostCause

I dont think you can coat cylinder walls. You would want an anti-friction coating on those. The rings will wipe out the finish after I dont know how many miles. Something that I have always wondered. Doing the exhaust port would be good on a turbo car too.

LostCause -

As I read your post, that's what I was thinking. Soooooo, if you need a rebuild, maybe go all out and see what happens!

CarloSW2

Some of the hot-rod techniques are about getting more air and fuel into an engine. Some are about running higher rpm's. And some are about running the engine more efficiently. I've got an unbuilt 350 SBC in my shop, and I planned to use some of their efficiency techniques.

I bought hypereutectic pistons, which transfer less heat, keeping it in the cylinder where we want it. They are stock on Corvette's. I also bought a coating kit for the engine. Slick stuff for the piston skirts and cylinder walls, and ceramic coating for the pistons and head. The ceramic also has aluminum in it. After you bake the ceramic coating on in the oven, you buff the top to remove some of the ceramic and leave an aluminum veneer. So you end up with a reflective surface with ceramic insulation underneath. The kit wasn't very expensive.

I don't think I would tear my engine apart just to apply the coatings. But if I had to rebuild my engine anyway...