Ever wonder about Specific heat (Heat of Combustion)????

[Bob Zambelli]

On numerous occasions, we speak of fuel and what kind of power can be obtained from it. 

It's pretty well known that certain liquids burn at different rates and give off more heat (energy). Therefore, choosing the proper one can well increase the power output of an engine - yes, there are many other factors involved but here is some general information, courtesy of the Marks Mechanical Engineering Handbook.

  The heats of combustion in BTUs per Pound mass of some very commons fuels are give as:

                              Gasoline (automotive)      = 20,750
                              Gasoline (Aviation)          = 18,750
                              Methanol (wood alcohol)  = 9,758
                              Ethanol (grain alcohol)     = 12,770

Of course, in our fuels, especially glow fuel, there are many additives which can increase or diminish the potential energy but let's see some discussion on the topic. 

Bob Z.

[captcurt]

Absolutely!!

Now, show Diesel or Kerosene...

Also, hot engines are more efficient.   

Curt

[Igor Burger]

NIce try, but instead of heat per mass you must compare heat per volume of air combustable with some amount of fuel, that will change your table :-)

But it is still not enough, then you must figure up the volume of cold gases after combustion - that will answer why medium like a nitrogen work in combustion   

[Bob Zambelli]

"NIce try, but instead of heat per mass you must compare heat per volume of air combustable with some amount of fuel, that will change your table"

Wrong - the table contains the ABSOLUTE heats of combustion - that is, the amount of energy in a given mass.

How the energy is extracted DOES NOT CHANGE THE TABLE.

Bob Z.

[dave siegler]

ugh pound mass...... and btu's ..... I am having a bad flashback to engineering thermo. ( I studied physics)

I like SI units a lot better.


it would be interesting to see heat/ volume comparisons.

[Mark Scarborough]

Very valid points Igor, I know from racing cars, you put approximatly twice the alchohol into the motor versus gasoline, so you get close to the same BTU potential per cylindar charge, the other benefit was that the motor ran much cooler with alchohol than the gas,, more timing advance as I recall was possible,, ie then more power,, (my memory might be some what faded its been a long long time,,)

[Jim Thomerson]

Here is an experiment I did and what I think it taught me.  Alternative explanations welcome.

I took a K&B 4011 I had been flying and ran it on the bench with Sig Champion 10% nitro, 20% 50-50 oil.  It turned an APC 11 x 5 at 9,700.  I filled the tank with Davis 1/2A diesel fuel.  Hooked up the glow plug and fired the engine up.  After about 20 seconds, the engine would keep running along nicely without the glow plug hooked up.  Turned the ST NV in 1.5 turns.. RPM 9,700.  Tried another K&B 40, maybe with a thicker head gasket.  It would undercompress without the glow plug lit.  9,700 RPM with the glow plug lit.

Given the same venturi, same prop, change fuel, change needle setting, get same RPM.  I conclude that if the fuel mix is right, and the combustion chamber temperature is right, then RPM is directly proportional to the airflow through the engine. 

 

[Igor Burger]

Wrong - the table contains the ABSOLUTE heats of combustion - that is, the amount of energy in a given mass.

no, it is not wrog, you cannot burn the same mass of different fuels per time (that is what power means) because burning in engine is limited by oxygen in volume of air used for combustion. That is why you must compare heat of mass of fuel which can be used per volume of air.

And second, you need PRESSURE not heat and presuure depends on volume AND heat ... every fuel releas differen amount of gases - it is different if you burn simple gases like H2 or complex gasoline - gasoline will release more simple gases then H2 and if you add niitrogen in some form, then you will see far better result

I just wanted to say that heat of combustion is only fraction of answer.

[Igor Burger]

RPM is directly proportional to the airflow through the engine. 

I do not thing so ... we use nitro for power boost 

[Jim Thomerson]

As I understand it, nitromethane contributes oxygen, and is therefore a part of the airflow.  More nitro, more airflow, more RPM's. I just read an account in a 1952 Aeromodeller of running a Dooling 61 with magneto and 100% oxygen and achieving 192 MPH.   That is why you can't run 100% oxygen in AMA CL speed. 

[Igor Burger]

As I understand it, nitromethane contributes oxygen

Definitelly, but it is still only part of equation, it contributes also released N2 as free gas, means it bring also some "cold" pressure. For example if you add some volume of N2O (it is power boost agent for gasiline engines) it dissociate and make 1.5x more volume of cold gases, it adds some heat and still gives free oxygene. The same happens with nitrogen in nithromethane.  It comes out of burning like a N2.

example:

if we use H2 as a fuel:
2H2 + O2 -> 2 H2O
it means that from some volume of burning gases we can get 2/3 = 0.666 of residual gases ... menas LESS after burning then before burning.

if we burn methylalcohol:
4 CH3OH + 6 O2 -> 4 CO2 + 8 H2O
will give 12/10 = 1.2 ... means onli slightly more then before burning

and if we burn nitromethane:
4 CH3NO2 + 3 O2 -> 4 CO2 + 6 H2O + 2 N2
we will get 12/7 = 1.7 ... means much MORE after burning


it all means that it really makes difference WHAT is burning ... but I fully agree, you can see that the same volume of nitromethane does not really contribute oxygen, but anyway needs for burning only half of oxygene (compared to methylalcohol - two atoms nitromethane, instead of one methylalcohol) ... means that burning process can use more fuel

[Keith Spriggs]

Hmmmmm, I think that is exactly right

[Alan Hahn]

I think we need to convert a nanogram of material to energy!

[Scott Hartford]

But what if we used a heavier piston???

[Dick Fowler]

I'm simply not going to go there!

[Jim Thomerson]

OK I revise my statement.  If you do a principle components analysis, you will find that airflow through the engine is the  major component of engine power.  Airflow is, I think, the major part of the story, but there are modifiers as Igor documented.

[Phil Bare]

Very interesting................