Fuel Combustion Characteristics

[maineSS]

This is my first thread on the forum, though I've visited here occasionally.
I thought I'd throw out some things for comment that aren't usually considered when people modify engines for more efficient performance. Some of these will contradict what people generally accept as "fact".

What prompted me to comment was some of the ideas brought up in Nery's thread on removing Ethanol from gasoline. I want to stay out of the politics and stick to technology. He brought up some very good observations which got sidelined by other things- I don't want that happening here.

I'll cover fuel preparation and ignition as well as combustion starting tomorrow, as I have to get to work. Cheers!

[gone-ot]

gasoline -- a hydrocarbon mixture typically varying between C8H16 and C8H18 (iso-octane) and requiring only about 30 milli-joules (watt-seconds) of electrical discharge energy for ignition under compression.

[mwebb]

Originally Posted by Old Tele man

gasoline -- a hydrocarbon mixture typically varying between C8H16 and C8H18 (iso-octane) and requiring only about 30 milli-joules (watt-seconds) of electrical discharge energy for ignition under compression.

and the above spark should be timed so that
peak combustion pressure always occurs at 15 degrees ATDC
no matter the load
or rpm
or conditions at the time

[user removed]

Green Car Congress: Transonic Combustion injection system shows 5-21% fuel consumption reduction compared to PFI engine; simultaneous reduction in NOx and PM at high EGR rates

Interesting read.

regards
Mech

[user removed]

Green Car Congress: Mazda Launches Next-Generation SKYACTIV Technologies

When these two technologies merge into one the efficiency of a liquid fueled ICE will be moving forward dramatically.

Add capacitive energy storage and 80+% efficient regeneration and you have a game changer.

Then eliminate the reciprocation penalty for even further improvement.

regards
Mec

[maineSS]

I going to try and get started here- been defeated 4 times so far as my post disappears into the Void after hitting "Submit Reply". I'm still digging for some data in my files, but for now I'll start at one end of this thing and work towards the other.

Nerys made a very good observation in post #163 of the "Removing Ethanol From Gasoline" thread when he said "this is not about energy content, this is about the Wrong fuel for the engine".

Now some "progressively" minded people might argue that he's got the wrong engine for the fuel, but the point is that energy content of a fuel does NOT always predict what is delivered in useful work from the engine.

Let's look at two highly tuned engines. One is a NASCAR level V8 producing 767 Hp at 14:1 compression on race gas. The specs are at tfxengine.com/NaturallyAspiratedCombustionData.html- you'll have to add www, as my post count is insufficient to link. I chose this engine because cylinder pressure, temp, A/F ratios, etc. are displayed by Engine Cycle Analysis equipment in real time on a real engine.

The other is a 105mm Tank Cannon- a highly developed example of the original IC engine- the gun. It has a similar overall thermal efficiency (~30%), and loses heat in similar ways. The energy distribution of a gun, (as taken from Hatcher's Notebook) is:

Projectile Energy- 30.9%
Heat & Kinetic Energy of Gas-40.7%
Heat to Barrel & Shell Case-28%

Projectile Friction is 7%, but was part of "Heat to Barrel" in the test. The energy distribution for a "typical" engine is:

Shaft Output- 24%
Exhaust-23%
Coolant-25%
Internal Friction-7%
Thermal Radiation-11%

Both of these engines have a 4" dia bore, but the fuel energy and overall performance are quite different. The ECA data show the NASCAR engine is developing 96 Hp in cyl #4 at 1812 Psi (11.4 tons on the piston). It's interesting to note that peak pressure is developed at 5-10 deg ATDC, not the theoretical "ideal" of 14-15 deg ATDC. Also note 35% of the A/F mixture is burned by TDC- doing negative work.

When you tap the firing pedal on the 105, chamber pressure rises to 60,000 Psi. Force on the projectile is 400 tons, and by the time it exits the muzzle, it's travelling over 4900 ft/sec. Projectile acceleration is 50,000 G's, and the energy release rate is 30 MegaJoules in .01 sec, which is over 4 million Hp.

Note I said "rate of energy release". Gun propellant is NOT a Super Fuel- it averages 8-10 times LESS energy per lb than gasoline. The energy in the 105mm charge is about 28,455 Btu- a little more than 1 lb of gasoline at the High Heat Value of 20,000 Btu/lb. Maybe this is what Albert Einstein had in mind when he remarked that we had harnessed only a tiny fraction of the energy present in gasoline. Really, when you compare the car to the cannon, you have to wonder- after 120 years of development, is this the BEST we can do with gasoline?

If you suspect there's more to internal combustion than heat values- your right. Nerys and Olympiades are absolutely right when they point out that HOW fuel burns in an engine is much more important. To be continued....

[UFO]

Don't get too carried away with your extreme examples of fuel. We are really only talking about a normal ICE with 8:1 to 10:1 static compression and very similar ignition and cam timings. Very little in common with both of your examples. If you want to make a point about combustion characteristics, to be credible you really need to stick to the context.

[cr45]

Originally Posted by maineSS

The energy distribution for a "typical" engine is:

Shaft Output- 24%
Exhaust-23%
Coolant-25%
Internal Friction-7%

There would appear to be 21% missing for the energy distribution of a typical engine!

24+23+25+7=79

[rmay635703]

Originally Posted by maineSS

Note I said "rate of energy release". Gun propellant is NOT a Super Fuel- it averages 8-10 times LESS energy per lb than gasoline. The energy in the 105mm charge is about 28,455 Btu- a little more than 1 lb of gasoline at the High Heat Value of 20,000 Btu/lb. Maybe this is what Albert Einstein had in mind when he remarked that we had harnessed only a tiny fraction of the energy present in gasoline.

If you suspect there's more to internal combustion than heat values- your right. Nerys and Olympiades are absolutely right when they point out that HOW fuel burns in an engine is much more important. To be continued....

Try fitting 1lb of gasoline into a single cylinder once.

That said Burning is very complex and even the octane values are comprised of multiple sets of averaged data remember RON? He aint exactly pump Octane.

Also I don't think anyone here actually believes that heat values have any direct relationship to fuel economy. There are thousands of factors that will cause fuel economy to vary and BTU content is only one.

[rmay635703]

Originally Posted by Old Mechanic

[url=http://www.greencarcongress.com/2010/10/tsci-20101028.html#more]Green Car Congress: Transonic Combustion injection system shows 5-21% fuel consumption reduction compared to PFI engine; simultaneous reduction in NOx and PM at high E
regards
Mech

"Additionally, the stratified nature of the charge under part load conditions reduces heat loss to the surrounding surfaces"

Hmm, detonation is more efficient than controlled burn, wouldn't it be rather hard to keep that stratified charge away from metals causing blow holes, erm I mean heat loss?