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Posted: 4/15/2008 12:27:26 PM EDT
[Last Edit: 4/15/2008 12:59:55 PM EDT by TexasSmooth]
My question is in regards to efficiency of the modern gasoline/diesel engines.  I'll stick with gasoline to keep it a little simple.

So we know 1 gallon of gasoline has roughly 121MJ.

My first question:  if you were to set the entire gallon of gasoline on fire, how efficient is the reaction?  I'm sure there's probably some gasoline that won't burned, but it's pretty flammable.  Would the reaction be along the lines of 90% efficient?

Now with a gasoline engine, when fuel is injected into the cylinder and the spark plug gets the juice to make it go boom, is that reaction itself not also of similar efficiency?  

I realize that internal combustion engines themselves are very inefficient, along the 15-20% lines because there are so many other factors in play to actually propel a vehicle forward.  But to keep it simple, is not the combustion of gasoline inside the cylinder of similar high efficiency?
Link Posted: 4/15/2008 12:37:07 PM EDT
I'm no expert, but I believe that one of the reasons that a modern internal combustion engine is inefficient is not because of unburned fuel, but that most of the energy produced is wasted as heat.  
Link Posted: 4/15/2008 12:40:31 PM EDT
You can run a much higher compression ratio in a diesel, allowing the expanding gases to do more work (pushing the piston down the cylinder) before being exhausted.

Link Posted: 4/15/2008 12:43:52 PM EDT
[Last Edit: 4/15/2008 12:48:09 PM EDT by VBC]

Originally Posted By pearl1982:
I'm no expert, but I believe that one of the reasons that a modern internal combustion engine is inefficient is not because of unburned fuel, but that most of the energy produced is wasted as heat.  



Agreed.  Majority of gasoline's energy lost to heat.  Heat not converted to mechanical energy from combustion (dissipated by radiator, engine block and out through exhaust) and heat from friction (cylinder rings sliding along walls, bearing friction, camshaft friction, valve friction, etc.).  Also, some of the mechanical energy produced is used (lost) to compress the air/fuel mixture in the cylinders, which also produces heat.


Friction is also the reason there will never be a perpetual motion machine.


Link Posted: 4/15/2008 12:51:10 PM EDT
Ok, let me add a couple more questions to the mix then....

-When exploding gasoline in the combustion chambers of an engine, effectively, how much of that is translated into heat and how much into usable mechanical energy?  Again, I'm not referring to the overall low efficiency of the engine itself.  I'm mainly interested in understanding the breakdown of the combustion reaction.

-How much compression would normal gasoline have to undergo in order to combust on its own the way a diesel engine does?
Link Posted: 4/15/2008 12:52:51 PM EDT
[Last Edit: 4/15/2008 12:54:46 PM EDT by mw365]
Gas is too volatile to use like diesel.

And, as far as your question about the combustion breakdown, you're thinking backwards. You wouldn't measure the heat to find the energy, you'd know the energy of the fuel, calculate the engine output, and assess the rest to waste in the form of heat/friction.
Link Posted: 4/15/2008 12:54:21 PM EDT
Depending on the octane rating of the gas, detonation (self-ignition) starts anywhere from 10 to 12:1. Diesels runa anywhere from 14:1 to ~20:1

As that gas is expanding (and doing work by pushing the piston) it is also cooling.
Link Posted: 4/15/2008 12:55:44 PM EDT
It's all controlled by this toy.

The maximum efficiency of any engine is directly controlled by the DIFFERENTIAL between the heat portion of the cycle and cold portion of the cycle.  In order to increase the efficiency of any such system, the limiting factor becomes not how much heat you can produce, but how cold you can get the cold cycle.

Now some people are going to make a fuss that the carnot engine is not an internal combustion engine, but the principles remain the same.  In order to produce a more efficient internal combustion engine, you must reduce all the energy losses, this means heat, sound, and vibration must all be reduced since each of these represents a reduction in useful energy.

Just think about all the moving parts in an internal combustion engine, now think of all the contact points between those parts.  Each of those contact points represents a loss of useful energy due to friction.
Link Posted: 4/15/2008 12:55:47 PM EDT
From what I remember from thermodynamics, the best efficiency you can get out of a Otto cycle engine (aka the standard internal combustion gasoline engine) is about 37%.

Link Posted: 4/15/2008 12:57:36 PM EDT

Originally Posted By mw365:
Gas is too volatile to use like diesel.

And, as far as your question about the combustion breakdown, you're thinking backwards. You wouldn't measure the heat to find the energy, you'd know the energy of the fuel, calculate the engine output, and assess the rest to waste in the form of heat/friction.


You're absolutely right.  I've corrected my original post to indicate the proper units.
Link Posted: 4/15/2008 1:09:28 PM EDT
You might find this Wiki article interesting.
Link Posted: 4/15/2008 1:09:45 PM EDT

Originally Posted By Mr_Dictionary:
In order to increase the efficiency of any such system, the limiting factor becomes not how much heat you can produce, but how cold you can get the cold cycle.


So, when the radiator in your car helps to dissipate the heat, does this simply help remove heat or does it also help to get the cold cycle a little colder?  If the latter, when why not design a vehicle that also has radiator fins on the sides as well or something similar?  Hell with all of the computer tech kids around these days that design some of the craziest damn cooling sinks just so they can overclock their CPU another 1 MHz, you'd think things like that might be useful.
Link Posted: 4/15/2008 1:21:00 PM EDT
[Last Edit: 4/15/2008 1:29:38 PM EDT by dablues]

Originally Posted By TexasSmooth:

Originally Posted By Mr_Dictionary:
In order to increase the efficiency of any such system, the limiting factor becomes not how much heat you can produce, but how cold you can get the cold cycle.


So, when the radiator in your car helps to dissipate the heat, does this simply help remove heat or does it also help to get the cold cycle a little colder?  


All the radiator does is to keep the engine from melting -- it's a really a monument to the inefficiency of engine design -- after all,  the heat that travels into the walls of the cylinders is heat that could have been used to push the piston.  

There's been some work on ceramic engines....ceramic materials don't conduct heat as well as metal...ergo, better efficiency.

"Kyocera has made turbo rotors and other components to run at high temperatures. The big plus with ceramic is said to be the ability to run at higher temperatures where no cooling system is required and greater efficiency can be achieved. http://www.wikimirror.com/Ceramic "A couple of decades ago, Toyota Toyota Motor Corporation...researched production of an adiabatic...ceramic engine which can run at a temperature of over 6000 �F (3300 �C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts."  

www.ultrahardmaterials.co.uk/ceramic_rotary_engine.htm

kind of interesting idea for a ceramic Wankel engine...

" In addition, very little heat is transmitted from the combustion chamber through the engine casing to the surroundings. A normal metal engine (reciprocating or rotary) may throw away 30% of the heat energy extracted from the fuel during combustion as waste heat. Normally this finishes up heating the surrounding atmosphere via the radiator. The ceramic rotary engine has no radiator and is designed to cut this loss by at least 90%."
Link Posted: 4/15/2008 1:31:03 PM EDT
Reciprocating internl combustion engines get the maximum efficiency at compression ratios between 14:1 and 16:1.  Any higher and the returns in efficiency are offset by the back work to compress.

That being said, yes, gas engines CAN run at that compression ratio IF they are direct injected engines.  These engines operate with a pilot injection into plain, compressed air.  The injection cycle starts before the piston reaches TDC and is ignited by a conventional spark plug.  Injection happens like a diesel, that being over a process of constant pressure and because of intense stratification, NOx formation is reduced.

Full power requires an additional injector of more conventional type.


The BEST Otto (gasoline) cycle efficiency was in the large radial aircraft engines, generating about .37 Hp/lb*hr.  Only because they operated at higher altitudes where the temps are -40  F or C, they are the same there.  They used recuperative turbines to harness a bit of energy off the exhaust and compound supercharging.
Link Posted: 4/15/2008 1:43:45 PM EDT
Link Posted: 4/15/2008 1:48:44 PM EDT
For the record, the cold source of the IC engine cycle is the temperature of the intake charge.  The hot source is limited by the engine's thermal strength.
Link Posted: 4/15/2008 1:51:26 PM EDT
[Last Edit: 4/15/2008 1:52:23 PM EDT by VBC]
Thus is part of the reason your car engine produces more power when it's cold out.

(Other reason is cold air is more dense thus more oxygen per volume is sucked into the engine).


Pair that with the cold, hard tires and cold pavement = FUN FUN FUN  
Link Posted: 4/15/2008 3:28:29 PM EDT
[Last Edit: 4/15/2008 3:30:39 PM EDT by TexasSmooth]
I guess my whole point in making this post is understanding what got us to this point in the first place.

In my opinion, I cannot understand why all standard passenger cars and trucks simply don't use a diesel engine?  For the small cars, a 4 cylinder diesel is more than enough for the typical commuter.  I imagine many arfcommers have wondered the same thing.  

I can understand that gasoline engines might be more suitable for some colder climates in some circumstances, but the majority of the U.S. public I can't see why a diesel engine could not work.

I know that there are a couple of major factors for why the cost of diesel is so high these days, first of course our dollar is dropping in value and second the EPA mandated some new chemical or whatever be included in diesel fuel to make it burn cleaner.  If the majority of our country, say 70-80% used diesel fuel, then couldn't the oil companies' refineries realize an additional cost savings for not having to refine as much gasoline?  Heck plenty of truckers and trains use diesel now.

Edit:  But since diesel has a bad reputation and the treehuggers don't like it, I know my above scenario likely won't happen anytime soon, thus why I'm interested in seeing more development in batteries for electric vehicles.
Link Posted: 4/15/2008 5:42:15 PM EDT

Originally Posted By TexasSmooth:
I guess my whole point in making this post is understanding what got us to this point in the first place.

In my opinion, I cannot understand why all standard passenger cars and trucks simply don't use a diesel engine?  For the small cars, a 4 cylinder diesel is more than enough for the typical commuter.  I imagine many arfcommers have wondered the same thing.  

I can understand that gasoline engines might be more suitable for some colder climates in some circumstances, but the majority of the U.S. public I can't see why a diesel engine could not work.

I know that there are a couple of major factors for why the cost of diesel is so high these days, first of course our dollar is dropping in value and second the EPA mandated some new chemical or whatever be included in diesel fuel to make it burn cleaner.  If the majority of our country, say 70-80% used diesel fuel, then couldn't the oil companies' refineries realize an additional cost savings for not having to refine as much gasoline?  Heck plenty of truckers and trains use diesel now.

Edit:  But since diesel has a bad reputation and the treehuggers don't like it, I know my above scenario likely won't happen anytime soon, thus why I'm interested in seeing more development in batteries for electric vehicles.


Because diesel engines cost more, have a negative reputation, are more complex, have no advantage over a direct injection gasoline engine...these are becoming more common each year.  Even in 2-stroke outboard engines where it offers the added benefit of making a 2-stroke as clean as a 4-stroke!  

Times are changing.
Link Posted: 4/15/2008 6:06:25 PM EDT
I't been a while but basically, the factors limiting efficiency of an internal combustion engine fall into two classes.

1)  Engineering considerations:  Frictional losses in the drive train, pumping losses in the induction and exhaust sytems, etc.

2)  Fundamental Thermodynamic limitations:  Basically, an ideal heat engine has an efficiency that depends only on the difference in temperature between the hot and cold reservoirs.  Internal combusion engines are a kind of heat engine, with the cold "reservoir" effectively the exhaust gas temperature.  The efficiency is also dependant on the exact path that the combusting mixture takes through P, V, T space.  One of the reasons that a diesel engine is more efficient is that the diesel combustion cycle follows a different thermodynamic path.


The takeaway is that when an engine is "ineffiecient" it isn't because it is not combusting the fuel completely, it is because the engine is limited in how much of the heat from the fuel can be converted into work (the work of running your transmission, heating up your tires, accelerating the vehicle, and pushing air out of the way of your vehicle)
Link Posted: 4/19/2008 6:59:48 PM EDT
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Perfect!

...and add to that all the crap that makes "Clean Air"...detune the engine, burn ethanol which already has extra oxygen in its molecule (you don't burn O2,  O2 burns other stuff), exhaust gas recovery systems that basically feed already burnt gases back into the the combustion chamber...kind of like a rare steak with a hunk of shit on top...(hmmm..didn't I already eat that ? )  
Link Posted: 4/19/2008 7:11:05 PM EDT


My first question:  if you were to set the entire gallon of gasoline on fire, how efficient is the reaction?  I'm sure there's probably some gasoline that won't burned, but it's pretty flammable.  Would the reaction be along the lines of 90% efficient?

Eventually, it would all react due to evaporation and oxidation.

Now with a gasoline engine, when fuel is injected into the cylinder and the spark plug gets the juice to make it go boom, is that reaction itself not also of similar efficiency?  

No, you put energy into the reaction which makes it more rapid, so you will get more energy out faster.  Secondly, it is under pressure, so that changes the thermodynamics and kinetics as well.  The efficiency is calculated based on a variation of the carnot cycle equation.


I realize that internal combustion engines themselves are very inefficient, along the 15-20% lines because there are so many other factors in play to actually propel a vehicle forward.  But to keep it simple, is not the combustion of gasoline inside the cylinder of similar high efficiency?

The factors that play a role are heat loss and friction (uncontrollable) and kinetics (% oxygen/air).  If you injected pure oxygen, you would get higher performance.  This is why some people use nitrox blends.  However, it is not that economical to do so on a full time basis.  If they found an economic way to make oval cylinders, it would increase the efficiency by at least 10%.  Car engines are far more efficient than you think.
Link Posted: 4/19/2008 8:20:36 PM EDT
a lot of energy is wasted as heat. If someone would hook up a seconday system to convert the heat into electricity (AKA sterling, etc) it would gain a lot.


like say take a prius, and have a second engine.. a sterling... running an extra generator.
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