Engine temps: what is considered "too hot" for efficiency?

OK, none of my hot rod stuff here. I'm actually working for efficiency and mpgs.
Car in question is an 03 Cavalier. 2.2 Ecotec w/ 4 speed auto. With a commute and large gas prices in my face, I'm trying to do some simple mods to this car to increase my MPGs. One area I'm currently playing with is aerodynamics. I've made some block off covers for my front grills, and with the decreased air flow I've seen increased engine temps. Max I've seen is 225 degrees. Red zone on factory temp gauge is around 260 degrees. I run Amsoil 0-20 synthetic oil, and don't beat on the car. The grill covers are simply cardboard and duct tape, so I can easily open them up for air flow if I'm running too hot. But my question is, just what is "too hot?" I'm not over heating, performance is good, and the temp never stays as high as 225. I'm also somewhat under the impression that a "hotter" engine is better for efficiency. So, what do you gurus think?

Bob: good topic. Sorry that I don't have any answer. I do have the same general impression - that it's more efficeint when it is hot. But much more than that I can't say except that 225 is not "too hot". Water jacket temps really mean very little anyway. Sorry I can't really help with your question.

Rich

Bob,
The answer to your question will be very engine specific. Although
there are benefits to additional engine heat, there are also limits.

Reduced component life because of the elevated temperatures is
secondary. The primary concern is auto-ignition. As the CHT goes up
so does the end gas temperatures. During combustion the unburned air
fuel mixture ahead if the flame is highly compressed, this equates to
very high temperatures in the region. When elevated CHT puts this end
gas temperature above the limits of the fuel in use, auto-ignition will result.

As coolant and cylinder wall temps increase the efficiency of heat transfered from the combustion space will decrease. The efficiency of the heat transfer is basicly related to the temperature difference and the surface area available for transfer at a given time. Reducing heat lost to coolant will increase the thermal efficiency of your engine and thus increase your gas mileage. Thermal efficiency by definition is net work out divided by heat addition. Heat added is calculated by multiplying the lower heating value of the fuel and the fuel mass. Net work would be brake mean effective pressure times swept volume.

If you want better mpg, slow down when you see a red light ahead, do whatever you can to avoid a total stop. its better to just lose enough momentum from 60-40 rather than lose the momentum from 60-0 just because you want to hurry up and stop like so many do. The other thing is use the cruise control as much as possible and keep your speed to a minimum, change in power from aero drag is related to velocity by the velocity cubed so small percentage changes in speed lead to huge changes in power to overcome drag. I know on my fbody drag doesnt really become significant until after about 45mph. Graphing airflow vs mph at cruise states is a pretty eye opening thing if you can find a flat road to test on during a still day and have a data logger. The last thing is dont lug your motor if its a manual keep the rpms up a bit, friction on that small an engine is not going to be a big deal, but lug the engine and its going to take more air and fuel to move the thing.

I think 225 is a little on the too hot side. Modern EFI engines normally operate at 195. Engine fans don't usually turn on until 205. Your hot engine may be good for emissions but performance will be lower.

The key to engine efficiency is to reduce burn time (increased engine
temperature will do this). An engine modification that allows a reduction
in ignition advance yet develop peak cylinder pressures while the piston
in a favorable position to generate power (~ 15 – 20 degrees ATDC). The
result is a reduction in overall pressure rise BTDC reducing negative work.

http://members.cox.net/dwynne7/pressure%20trace%20r1.jpg

OK, none of my hot rod stuff here. I'm actually working for efficiency and mpgs.
Car in question is an 03 Cavalier. 2.2 Ecotec w/ 4 speed auto. With a commute and large gas prices in my face...
So I take it you're not dring the Riviera or the Lincoln to work any more :D :D :D

If they could have made it more fuel friendly from the factory but making it run hotter, then wouldn't they?

I think you are riding around with card board and duct tape on your car for no reason, and probly about to pop a head gasket, b/c its not even summer yet and its running 225.

Bob, I know you have plenty of money.......I don't even know what your worried about. I don't even own a vehicle that gets double digit mileage. And I'm poor

maybe redirect air from under the car to the radiator to help?

So I take it you're not dring the Riviera or the Lincoln to work any more :D :D :D

Heh, those boats are LOOOOOONG gone! :D

If they could have made it more fuel friendly from the factory but making it run hotter, then wouldn't they?

I think you are riding around with card board and duct tape on your car for no reason, and probly about to pop a head gasket, b/c its not even summer yet and its running 225.

Bob, I know you have plenty of money.......I don't even know what your worried about. I don't even own a vehicle that gets double digit mileage. And I'm poor

I don't take "what the factory does" as anything set in stone. If the factory gets everything so perfect, why do we even mess with cars...and why do we see improvements?
And since you know I have plenty of money, please show me where it is! :D If I had plenty o' bux I wouldn't be driving a 5 year old "Gramma" car, and I wouldn't be looking for a part time job! Yup, ubg might be flipping that burger you are ordering at McDs! ;)

The easiest enemies of fuel efficiency to defeat are weight and rolling resistance of the vehcle, and drving style of the driver. I liked what WS6T3RROR said about keeping up momentum. I have heard it said as drive farther ahead of the car than the hood ornament.

On 8 Mile Road the lights are about 1/2 mile apart in some areas and most everybody accelerates, cruises, slows down and stops when going from light to light. I have been trying to stay in a lane where I can maintain a fairly constant speed without getting run over and never have to get below 30. It doesn't make too many other drivers happy and they give me a lot of one finger salutes, but it does save gas.

If you can stand the ride, inflate your tires to the maximum pressure shown on the sidewall. That is probably 44 psi. Don't hit any potholes with that pressure. Take all of the junk out of the car. Every pound you have to accelerate costs more gas. Spare tires and jacks are dead weight. That's why you have AAA. I don't know what you look like, Bob, but if you are 'stout', consider losing weight. I don't take my own advice on this but I have my pizza delivered which saves my gas.

From what I have read, you probably don't want to run your production car much above 200-210 degrees.



Joe Urban

Bob:

Check the Evans Coolant site... I think he is a proponent of elevating the engine operating temp to improve efficiency, and uses the properties of pure propylene glycol to achieve that, without increasing the tendancy for detonation.

http://www.evanscooling.com/catalog/C_npg1.htm

http://www.evanscooling.com/html/tech1.htm

True... also, ceramic coating the combution chamber & piston head also increases the heat retained in the cylinder gas... and hense more power/heat used increase efficency. Higher coolant temps reduce the heat taken out of the combustion chamber... and likewise increase efficiency.

Probably not a great thing for hp, and too hot is obviously a problem for rubber under the hood... but for MPG it may help a bit.

Interesting stuff; I never knew the extra heat will help mileage... I took a more indirect approach and looked at things other than my vehicle that I drive to work. I ride a motorcycle everyday it doesn't rain, and I only go about 10 miles to work and back, so its not a big deal to me to increase my efficiency. But since I lead a boring life at the North Carolina Dept of Transportation every weekday, I looked into the Traffic Control section and found out how the lights work on the route I take to my job. I can either take the Beltline around Raleigh (15 miles to work) or go straight through the city down the road I live off of, (10 miles) The beltline is usually quite busy at 7:00 in the morning, so I opt for the city streets near NC State Univ, because every college kid but myself gets to sleep in.

Anyways, I found out that for the route I take, which is one road all the way down to the NC-DOT, the lights are set to be green if you keep a constant speed of 35 mph at the time that I am going to work. Knowing that I keep that speed up and get through about 80-90% of the 27 lights I go through each day to work and the 27 I go through on the way back home :) (Obviously, I'm the only one that knows the secret, so I still have to stop at some lights for the traffic that keeps up with the 45 mph speed limit through the stretch, therefore hitting quite a few of the lights...)

Maybe you and anyone else interested in better city mpg would be able to look into your routes (if they are fairly direct and don't require many turns) and your DOT will be able to help out. :) I got 60+ mpg last tank; 200 miles on 3.3 gallons

Another way to achieve higher engine efficiencies with out the power
robbing elevated temperatures is to raise the compression ratio with
in the limitations of auto-ignition. Assuming the manufacturer allowed
a safety factor in the initial design.

The extra heat from the higher cylinder pressure will help to vaporize
the fuel to provide faster more complete combustion and improved
thermal efficiency.

I'm not going to do the derivation but you can relate efficiency to compression ratio by the equation (at least in theory.)

efficiency = 1 - 1/(comp^.4)

In the real world, the value is going to shake out to be about half what is calculated here. Note .4 is the value for air only assuming constant Cp and Cv and air following ideal gas law.

PS: Yes thats correct, your car only puts about 30% of the power you supply it in the form of gas, and thats under some of the best cases.

The graph below shows indicated thermal efficiency for the range of
compression ratios. WS6T3RROR, can you tell us why indicated values
are so much higher than "real word" or brake specific values?




http://members.cox.net/dwynne7/thermal%20efficiency.jpg

im not sure what kind of compression ecotech's run but i would imagine with 87 octane along with higher than normal temps could cause you to run into some detonation and timing retard which will reduce power and of course fuel consumption, The OP might be shooting himself in the foot, if it had made sense to run an engine at 220-240 on a regular basis im sure the next big thing would be 235 degree thermostats

Well theres really alot of reasons why the efficiency just doesnt add up. Part of it is in the way we handle thermodynics with regaurds to combustion engines. Most of the time in our analysis we assume some things just to make analysis easier but it introduces errors. What we assume is that all heat addition occurs after compression is complete at a constant volume, as we all know combustion takes place over several degrees (really its time related but i'd rather avoid talking about that). We also assume that heat loss occurs after expansion is complete. Obviously we know that this is not the case in the real world. We also know that exhaust gas is not really back to ambient conditions so there is another error. Also heat transfer occurs as expansion takes place etc etc. The other problem is that our analysis is based on air, well we have to deal with humidity polutants etc etc that are not air and do not follow our model. We account for water formed during combustion and vaporization of the water formed by using the lower heating value for a fuel so that isnt really a concern. There are also tons of places where energy leaves the system and makes it hard to model but still drops efficiency.

Here is a pv diagram of how we handle the process thermodynamicly.
http://i94.photobucket.com/albums/l97/jdavis_ws6/pvdiag.jpg
1 is the initial condition with only the clearance volume at atmospheric conditions for t and p and we use the ideal gas law to figure volume if its not given to us (although in the real world we always have this). Then we use another form of the ideal gas law to find our way to point 2 which is after compression. Then we use the addition of internal energy from the burning of fuel to get to point 3. After that we merely expand the volume back to the original and drop a line back to point 1 for heat loss at a constant volume. We ignore work done by the intake and exhaust strokes, which adds more error. Usually we do the calculations assuming 1kg of air, and adjust according to engine size and ve because it makes the book keeping easier.

Now, here we have a graph from a project I had in my class on internal combustion engines.
http://i94.photobucket.com/albums/l97/jdavis_ws6/pvactual.jpg
This graph is alot more accurate, its show for a 1000cc cylinder with a 10:1 compression ratio. It has about a 16:1 air fuel its operating at 3000 rpm and has a 100% ve just for the sake of calculations. the burn duration for the fuel is 60 degrees starting at 10* btdc.

Interesting things of note from analysis of these calculations. Taking equivalence ratio from 0.7-1.2 will increase power as expected but will drop thermal efficiency almost 10% (this is assuming you still get a complete burn). Also assuming you get a complete burn varying the compression ratio from 8-12 you only pick up about 2%. Also of note my testing shows that max cyl temp drops with increased compression because its higher before heat addition and heat transfer is based on the temp difference, so higher temps longer and sooner means more transfer away from it. Think of it like an ice cube in cold water vs one in a cup of coffee.

For all my analysis of this model the thermal efficiency was between 24-30% which is what alot of the dyno data from the actual engine backed up. In order to get there you have to do calculations for every degree, and use the weibe equation with different values for various processes to get the heat transfer from the cylinder worked out. Again though we still ignore the intake and exhaust processes.

Sorry for the book, but thats about as short an answer as I can give. Internal combustion is one hell of a hard process to model and get anywhere near close. Its never the same even for the same cylinder in consecutive cycles at steady state.

Bob:

Try some of this oil..... 0W-2 (cSt100 = 2.9). No pumping losses there.

No, that is not a misprint. Its one of three new ultra low viscosity racing oils made by Lubeatech Advanced Technologies..... 0W-2, 0W-5 and 0W-10. Don't know anything about them, but they have an ad in the latest issue of National Dragster. They don't appear to have any info on their website yet.

http://www.lubeatech.com/

Sounds like WD40!

Another way to achieve higher engine efficiencies with out the power
robbing elevated temperatures is to raise the compression ratio with
in the limitations of auto-ignition. Assuming the manufacturer allowed
a safety factor in the initial design.

The extra heat from the higher cylinder pressure will help to vaporize
the fuel to provide faster more complete combustion and improved
thermal efficiency.

How would you raise compression without changing the head or the pistons?

These 0W20 type oils....aren't they too low? 20 weight with 225°F coolant temp/hot engine sounds like something is mismatched.

I wonder how much power the coolant fan(s) would take away from the engine vs mpg gain from higher running temp?

I know using the block heater saves a bunch of gas. But at this time of the year it's a moot issue. I use it whenever the temp drops below freezing.

Buy a header!

I'd suggest getting the fuel injectors cleaned. When I sprayed a can of that stuff in to my Sunbird, I wound up getting a 2.7 mpg improvement. I was AMAZED at the amount of crap that came out the tail pipe. Bunch of blue, followed by a bunch of James Bond white smoke. Stunk like a BBQ. But after that, it ran much smoother and as mentioned, got the improved mileage. It was a 1987 and in 2005, I could still get 33 mpg on the highway with it (rated at 36 from the factory). Mileage was over 200K.

Would adjusting the timing above or below stock change the mpg?

Try some of this oil..... 0W-2 (cSt100 = 2.9). No pumping losses there. No, that is not a misprint. Its one of three new ultra low viscosity racing oils made by Lubeatech Advanced Technologies..... 0W-2, 0W-5 and 0W-10. Don't know anything about them, but they have an ad in the latest issue of National Dragster. They don't appear to have any info on their website yet.

http://www.lubeatech.com/

Wow... that's crazy. Here's the datasheet: http://www.lubeatech.com/images/LAT-Race.pdf

0w2 0w5 0w10 0w20 5w30 20w50
Blue Red Red Blue Blue Blue <-- Appearance
3.1 4.2 4.4 7.96 11.6 20.8 <-- Viscosity @ 100°C
11.4 17.6 18.5 49.1 66.8 168.35 <-- Viscosity @ 40°C
365 392 401 446 455 471 <-- Flash Point (°F)
139 146 153 160 165 169 <-- Viscosity Index

So the question is... Who wants to try it out and give us some before/after oil analysis for wear indicators. :)

Any volunteers?

0w2 0w5 0w10 0w20 / 5w30 20w50
Blue Red Red Blue / Blue Blue <-- Appearance
3.1 4.2 4.4 7.96 / 11.6 20.8 <-- Viscosity @ 100°C
11.4 17.6 18.5 49.1 / 66.8 168.35 <-- Viscosity @ 40°C
139 146 153 160 / 165 169 <-- Viscosity Index
365 392 401 446 / 455 471 <-- Flash Point (°F)

Mobil1:
0W20 0W30 0W40 / 5W30 10W30 15W50
8.6 11.0 14.0 / 11.3 10.0 18.1 <-- Viscosity @ 100°C
45.5 63.1 78.3 / 64.8 62.0 131.2 <-- Viscosity @ 40°C
??? ??? 186 / 169 147 154 <-- Viscosity Index
226 228 230 / 230 224 235 <-- Flash Point (°C)

Redline:
5W20 5W30 10W30 15W50
9.1 10.6 10.7 19.6 <-- Viscosity @ 100°C
55 62 70 138 <-- Viscosity @ 40°C
145 162 142 162 <-- Viscosity Index
484 486 480 486 <-- Flash Point (°F)

How would you raise compression without changing the head or the
pistons?

First I would consider reducing piston to head clearance within reason. If
I still need more compression, I mill the head. Last resort is changeling
piston volume unless I'm changing pistons anyway.

Bob:

Try some of this oil..... 0W-2 (cSt100 = 2.9). No pumping losses there.

No, that is not a misprint. Its one of three new ultra low viscosity racing oils made by Lubeatech Advanced Technologies..... 0W-2, 0W-5 and 0W-10. Don't know anything about them, but they have an ad in the latest issue of National Dragster. They don't appear to have any info on their website yet.

http://www.lubeatech.com/

Heh, and I was nervous about dropping to 0W-20! :eek:

A trick I learned and used on my heavy SUV, is run the tire pressures well beyond the max listed. I have done this and actually gotten more life as well as 1 MPG more. Granted it reduces your footprint and therefore safety but that is up to you to decide.

44 PSI max on my tires and I run 57 PSI.

This is interesting. So then there may be such a thing as not hot enough for efficiency? I was once told by a mechanic that the 160* thermostat I ran in my v6 camaro that I had at the time wasn't hot enough the majority of the time and caused me to run rich and get worse gas mileage and performance. This has always been in the back of my mind as a possibility. The only time it seemed to go above 180-190 was when I wasn't moving.

160 is hot enough for decent thermal efficiency. While going from 160-220 sounds like a huge jump in coolant temperature its not a huge difference to the engine because it compares that to its combustion temperature. The thing with gas mileage is that its no one big thing that gives you a huge bump, its a lot of little changes that add up.

What you're trying to do is keep internal energy of the air in the cylinder as high as possible so it can do work. Any heat lost to the coolant is heat that could have been used to push the piston down. As a gas cools at a fixed volume it loses pressure and thus the potential to do work.

Think of it like trying to cool off two cups of coffee, put one in the freezer and one in an oven at the temp the coffee is already at. Its obvious which one cools off faster, its also obvious which one has more potential to do work (burn your lip) when you take them out too. You're trying to do the same thing with the engine by keeping the temps up.

This is all at a cost to performance, the hotter coolant temps will heat the air up more because of the greater temp difference. An interesting thing is that under wot tests I have seen on the dyno, if you maintain high airflow long enough (high rpm/power) enough air will travel through the manifold to cool it off. When it stabilizes its usually well below the coolant temp depending on how the manifold passes coolant through it, if it even does. Results are even better if coolant doesnt pass through the manifold. Needless to say if you stay on the throttle that long on a public road you will see cherries and blueberries behind you.

LMAO @ cheeries and blueberries! :lol:

Bob,

I know you specifically asked about engine temperature. Engine operating temp/efficiency aside, however, keep this in mind:

Blocking front openings for improved Cd provides a small reduction in energy spent to overcome aero drag. However, whenever the reduced opening area causes the fans to turn on instead of using 'free' ram air to do the job, the couple of hundred fan watts required will generally use more energy than you saved by aero improvement. So, you would get an improvement on sufficiently cool days/high speeds/low loads for the fan to stay off, but on warmer days/lower speeds/higher loads this would hurt fuel economy. Since you are now operating above the fan on temp, I suspect the fans are always on, so this mod is likely hurting your vehicle efficiency. Unless you raised the fan on temp accordingly.

I was driving through a blizzard in my wrx when I noticed my oil temp was 245+. This was way hotter than normal. The stock coolant temp showed just a little hotter/warmer than usual. It turned out my grill was clogged up with ice/snow. That car had a water to oil cooler.

Do you have an oil pressure gauge or light?

Also what mpgs are you getting? Did you actually see an improvement with the aero mods?

UPDATE: I didn't see any real earth shattering improvements w/ my grill blocks, so I removed my upper grill block to allow the car to run somewhat cooler than with the full upper and lower grill blocks. I think with my daily commute of speeds less that 60 MPH that the aerodynamic efficiency I was looking to achieve is kinda a moot point. I've gotten 34 MPG outa my last 2 tanks w/ mixed driving, so I really can't complain, considering that the car is rated for 25 MPG combined driving.