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?

 

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.

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.

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!

 

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?

 

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?

 

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)

 

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.

 

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

 

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.