Thermal Efficiency-does engine heat mean fuel milage?

[squirrl22]

Interesting topic.   I've switched to a "waterless" coolant, NPG.  Very expensive stuff,($30/gal) but it has eliminated any risk of overheating, or blowing hoses or heater cores.

Personally, I'd be willing to bump my coolant temps to over 300F if I could figure out a way to do it, and a way to keep my oil temps normal.

[Mark(The Miser)UK]

I wouldn't think that a hotter engine runs better... Where does that heat come from to raise the temp? Larger temp gradient  means greater losses to environment. Engines that run hot are only running at their best when hot because the boffin decided to machine the mechanical parts to be the correct size at that temp. My father's 1990 Citroen 1.5 will not get 'hot' but gets over 85mpg [uk]...

Isn't the power from an engine determined by the difference between the air temp prior to combustion and the air temp during/after combustion ie P1 V1/T1 = P2 V2/T2 kind of stuff where V1 =V2 etc etc
Doesn't a TDi run cooler? If not in the chamber then certainly losses to the head and the coolant are less...
Does a Lister/Petter type engine run hot?

[Turbinepowered]

Isn't the power from an engine determined by the difference between the air temp prior to combustion and the air temp during/after combustion ie P1 V1/T1 = P2 V2/T2 kind of stuff where V1 =V2 etc etc
Doesn't a TDi run cooler? If not in the chamber then certainly losses to the head and the coolant are less...
Does a Lister/Petter type engine run hot?

Power in an engine is determined by the difference between ambient temperature and combustion temperatures. Ambient air is the "cool" reservoir, and the combustion chamber is the "hot" reservoir.

TDIs run lower coolant temperatures, somewhat. They also take longer to heat up, due to increased efficiency. Longer warmup times and lower coolant temperatures mean less heat is migrating from the chamber to the head/block and then to coolant. Less heat loss means more heat is retained in the chamber, to add to the combustion heat to produce a hotter "hot" reservoir.

[Mark(The Miser)UK]

Isn't the power from an engine determined by the difference between the air temp prior to combustion and the air temp during/after combustion ie P1 V1/T1 = P2 V2/T2 kind of stuff where V1 =V2 etc etc
Doesn't a TDi run cooler? If not in the chamber then certainly losses to the head and the coolant are less...
Does a Lister/Petter type engine run hot?

Power in an engine is determined by the difference between ambient temperature and combustion temperatures. Ambient air is the "cool" reservoir, and the combustion chamber is the "hot" reservoir.

TDIs run lower coolant temperatures, somewhat. They also take longer to heat up, due to increased efficiency. Longer warmup times and lower coolant temperatures mean less heat is migrating from the chamber to the head/block and then to coolant. Less heat loss means more heat is retained in the chamber, to add to the combustion heat to produce a hotter "hot" reservoir.

So that's what I said then... So I guess you'd have to agree that a hotter engine should not be more efficient, if it were not for possible friction issues :?:
What benefits are there from capilliary tube sized factory issue exhaust systems I see on many cars, or is it all a conspiracy to burn our money :evil:

[Mark(The Miser)UK]

Power in an engine is determined by the difference between ambient temperature and combustion temperatures. Ambient air is the "cool" reservoir, and the combustion chamber is the "hot" reservoir.

TDIs run lower coolant temperatures, somewhat. They also take longer to heat up, due to increased efficiency. Longer warmup times and lower coolant temperatures mean less heat is migrating from the chamber to the head/block and then to coolant. Less heat loss means more heat is retained in the chamber, to add to the combustion heat to produce a hotter "hot" reservoir


Now I've reread this there's something wrong...
TDI's are more efficient because less heat is lost to surrounds ...correct...
 However compressing the air in both engines add nothing to  power [other than improved combustion] but just add another source of inefficiency. It's borrowed energy stored  in the flywheel. Ideally returned on the expansion stroke but not so due to losses [less so for the TDi]
Introduced power can only come from the point of injection of chemicals with their stored energy So its combustion temperatures less compression temperature less the difference between theoretical compression temperature and actual compression termperature  :shock:  :?

Does that read correctly?? :wink:

[Turbinepowered]

Power in an engine is determined by the difference between ambient temperature and combustion temperatures. Ambient air is the "cool" reservoir, and the combustion chamber is the "hot" reservoir.

TDIs run lower coolant temperatures, somewhat. They also take longer to heat up, due to increased efficiency. Longer warmup times and lower coolant temperatures mean less heat is migrating from the chamber to the head/block and then to coolant. Less heat loss means more heat is retained in the chamber, to add to the combustion heat to produce a hotter "hot" reservoir


Now I've reread this there's something wrong...
TDI's are more efficient because less heat is lost to surrounds ...correct...
 However compressing the air in both engines add nothing to  power [other than improved combustion] but just add another source of ineffi

ciency. It's borrowed energy stored  in the flywheel. Ideally returned on the expansion stroke but not so due to losses [less so for the TDi]
Introduced power can only come from the point of injection of chemicals with their stored energy So its combustion temperatures less compression temperature less the difference between theoretical compression temperature and actual compression termperature  :shock:  :?

Does that read correctly?? :wink:

Compressing the air does add to power. Cylinder pressure is the driving force in any piston-driven combustion engine. Higher temperatures = higher pressures, from the same formula you gave earlier, PV=nRT; for a given volume and molar quantity of air, higher temperatures will produce higher pressures, and vice versa.

Given that cylinder pressure is important, an engine with a higher compression ratio will produce more absolute cylinder pressure for a given volume of combusted fuel than an engine of lower compression.

For instance, you look at two direct injection diesels (to remove all extraneous differences, such as piston-chamber versus prechamber combustion, that might affect output) and assume that they are for the purposes of this exercise 100% adiabatic during compression stroke. Because they have adiabatic compression, they do not lose heat to their surroundings, or gain heat from their surroundings, during compression.

Now, one of these theoretical engines has a CR of ~16:1, while the other's ratio is 20:1. For a given amount of fuel injected into each cylinder, the first engine with it's 16:1 compression ratio will produce less power than the latter engine. The heat of the burning fuel will elevate both pressures an approximately equal amount, based on the amount of potential energy in the arbitrary volume of fuel, so the cylinder with the higher base pressure will have a higher post-combustion pressure as well.

This is also why high-performance naturally aspirated gas engines (and some forced induction engines, with the right fuels) are much higher compression than their lower performance counterparts. High compression engines are more efficient at converting heat energy into usable work, using less fuel to produce more power than they would have needed a lower ratios.

You do reach a point of diminishing returns, where a small change in compression ratio no longer gives you the same immense increases in efficiency, and in spark ignition engines you are largely limited by the compression pressures and temperatures your fuel can withstand before it self-ignites, but neither of these changes that fundamental connection between compression ratio and efficiency.

Turbocharging, as I stated in a previous post, can be used to effectively increase a compression ratio, an effect known sometimes as "precompression." This increase in effective compression can also add to the efficiency and power of an engine, thus why turbocharged diesel that does not have boost compensation in its fueling (IE an "ECO" diesel in VW terms) will return more power than a naturally aspirated engine of the same displacement and fueling.

[Mark(The Miser)UK]

Turbinepowered wrote:
"Now, one of these theoretical engines has a CR of ~16:1, while the other's ratio is 20:1. For a given amount of fuel injected into each cylinder, the first engine with it's 16:1 compression ratio will produce less power than the latter engine."

Is that correct? Or is it that higher compressions allow you to burn more fuel faster and get more power that way?

Turbinepowered wrote:
"The heat of the burning fuel will elevate both pressures an approximately equal amount, based on the amount of potential energy in the arbitrary volume of fuel, so the cylinder with the higher base pressure will have a higher post-combustion pressure as well."
 
If as you say [as I also think is so] the +'P's are the same then energy output attribruted to the set amount of fuel must be the same surely? The extra energy out in the higher compression cylinder on the power stroke has come at the expense of removing more energy out of the flywheel.[Hasn't it?] I don't know but does this mean that higher compression engines have bigger [inertia wise]flywheels to stop them going slower than a low compression engine on the compression stroke and faster on the expansion stroke ... which make sense and is the penalty of those 'flywheel trimmers'
Would a 23:1 TDi perform more efficiently :?:
Anecdotally...At the moment I'm slowly tuning my current Quantum TD for efficiency. It's getting better but still @47.5mpg [uk] does not match my temporarily retired Q that could get 62mpg uk even though the retiree has [still unmeasured :roll: ] lower compression than this 'low mileage' engine

[Turbinepowered]

Turbinepowered wrote:
"Now, one of these theoretical engines has a CR of ~16:1, while the other's ratio is 20:1. For a given amount of fuel injected into each cylinder, the first engine with it's 16:1 compression ratio will produce less power than the latter engine."

Is that correct? Or is it that higher compressions allow you to burn more fuel faster and get more power that way?

Why would higher compression change the burn speed of a fuel? The flame front speeds are determined by your air/fuel mixture and the chemical properties of the fuel itself. Higher compression IDI engines tend to have more capacity for higher revolutions than TDIs because the swirl chamber's inlet/outlet hole produces more intense swirl, improving the air/fuel mixing upon injection.

"The heat of the burning fuel will elevate both pressures an approximately equal amount, based on the amount of potential energy in the arbitrary volume of fuel, so the cylinder with the higher base pressure will have a higher post-combustion pressure as well."
 
If as you say [as I also think is so] the +'P's are the same then energy output attribruted to the set amount of fuel must be the same surely? The extra energy out in the higher compression cylinder on the power stroke has come at the expense of removing more energy out of the flywheel.[Hasn't it?] I don't know but does this mean that higher compression engines have bigger [inertia wise]flywheels to stop them going slower than a low compression engine on the compression stroke and faster on the expansion stroke ... which make sense and is the penalty of those 'flywheel trimmers'
Would a 23:1 TDi perform more efficiently :?:

Marginally so, but the sheer volume of engineering and research that would be required to redesign the chamber for the smaller combustion space volume, coupled with the diminishing returns for increases in compression past 17:1 or so (I think that's the "cutoff" for such diminishing returns) would mean that the marginal increase in efficiency wouldn't be worth the effort, or particularly noticeable.

Keep in mind that most diesels are already past that "cutoff" point. Increases and decreases in compression aren't going to produce massive changes in efficiency. The IDIs, and TDIs to a certain extent, need compression past the cutoff ratio to help with starting, to compensate for heat loss through the cylinder walls, into the piston, head, and the swirl chamber walls in the case of the IDI.

Energy consumed during the compression stroke pulls from the stored mechanical energy of the flywheel and the thermochemical energy produced by whichever cylinder is in its power stroke. The "extra energy" output by the higher compression engine does not equal the energy consumed in the extra compression, but exceeds it. The extra heat and pressure that are the difference between a high-compression and a low compression engine just before the moment of combustion represent the difference in energy consumed from the system itself.

To a certain extent, too, a higher compression diesel will have decreased ignition delay upon injection of fuel, again due to this increase in heat and pressure. Reduced ignition delay leads to better management of fuel injection and power production, improving fuel economy, emissions, and overall system efficiency (you aren't injecting as much fuel that you don't need just to get ignition when you want it). TDIs are down at 18:1 for engineering expediency, materials limitations, boost capacity, and a host of other reasons that do more to improve fuel economy than increasing compression alone would have.

Am I actually making sense? I seem to see a lot of the same question coming up, hoping I'm explaining things well. It's the primary reason I never intend to become a teacher, my explanations aren't the greatest.

Anecdotally...At the moment I'm slowly tuning my current Quantum TD for efficiency. It's getting better but still @47.5mpg [uk] does not match my temporarily retired Q that could get 62mpg uk even though the retiree has [still unmeasured :roll: ] lower compression than this 'low mileage' engine[/color]

This could be due to a vast array of possibilities. Compression is by no means the "all powerful overriding force" in efficiency, especially in the real world conditions that most folks drive. Real World efficiency depends on thousands, even hundreds of thousands, of tiny interactions within the machine, between the machine and the environment, and between man and the machine.