SpudFiles

Just having some thoughts.....

OK, in a closed chamber a fire starts. As the freshly burned gases try to expand they compress the cold (unburned) gases (assuming a sub-sonic flame propogation rate). As the cold gases are compressed, they heat up (adiabatic compression).

Eventually, if compressed enough, these cold gases will reach the autoignition temperature and the flame propogation rate will experience in essense a step function increase to the local sonic velocity as the pressure gradient being emmited by the burning gases puts the cold gases "over the top" and they "spontaneously" ignite.

This is not a detonation, but rather, a true deflagration.

Whether or not that deflagration transitions to a detonation is another story.

Elsewhere, somebody mentioned that the autoignition temperature for hydrocarbons is on the order of 873 K (600 C).

If we start at 300 K and assume ideal gas laws....


(oops, gotta go.)

Summary (will come back later to complete)....

Doesn't seem like MUCH compression is required to achieve deflagration. Seems like this could have some serious impact on assumptions of internal ballistics.

This idea could easily be done with acetylene,as it is self ignited at a relatively low pressure.

I'll wait till you finish this post to further go into anything.

Acetylene easily detonates (that was a big part of why I used it for my patent ).

But I don't WANT a detonation. What I was wondering about is whether or not propane routinely deflagrates via spontaneous ignition (ie, dieseling) within spudgun designs.

If it DID do that, what it would tell me is that detonations aren't a realistic threat in systems such as those discussed. Why? Because such a deflagration would be EASY to transition to a detonation compared to any other combustion mode.

However.... Since my first post I've done the math.

Short version? Propane can not drive itself to spontaneous ignition of unburned air/fuel. To do so would require a 1X mix to burn at about 650 psi. Funny thing: That's about a 15:1 compression ratio... Huh, diesel engines tend to operate at 15-20:1 compression ratios. Funny how that works, eh?

In any event, 'twere just something I was contemplating but hadn't had the chance to run the numbers. Now I have and I see it is not a valid mechanism.

Thanks for that information Mr Hall. It certainly seems like DDT as we discuss it is a myth.
I'll be making more Hybrids i suspect.

This gives me an idea on a hybrid design(i think). Watch this space( )

Following up on the diesel engine analogy....
Propane, hexane, octane etc. will autoignite (diesel) if they are compressed much beyond ~9:1. Hence, gasoline engines are rarely built with compressions ratios of more than about 9:1. Pinging, knocking, dieseling (and a couple other words for the same basic process) occurs when fuel autoignites in small pockets in the cylinder. Curriously, the detonations really are in small pockets and do not envolve the bulk of the unburned fuel. A single combustion stroke can have multiple "ping" (autoignition) events.

D_Hall wrote:propane can not drive itself to spontaneous ignition of unburned air/fuel. To do so would require a 1X mix to burn at about 650 psi. Funny thing: That's about a 15:1 compression ratio... Huh, diesel engines tend to operate at 15-20:1 compression ratios. Funny how that works, eh?

How did you calculate that? Is it just the compression ratio required to get the gas temperature above the autoignition point of propane (assuming 600C)?

Using GasEq for propane + air, "adiabatic/compression expansion" with "frozen chemistry" (so GasEq doesn't burn the fuel) I get that a compression ratio of just 7.5:1 (half of what D_Hall calc'd) will get the mix up to 600C. That is 15ATM (220 PSIA). The peak combustion pressure of propane+air is only a bit over 9 ATM so it still won't autoignite.

A 4.3:1 compression will get the mix to 500C, 7.2ATM (106PSIA). That is just within the capabilities of propane+air (assuming a perfectly adiabatic system and no work done). Of course, to get to that temperature you would have burned about 80% of the fuel. 500C is a bit above the minimum autoigntion temperature for propane (see table below).

Autoignition temps for some common fuels (from here) These are lower limits, in practice it often takes higher temperatures to get consistent autoignition.

hydrogen 500C
acetylene 305C

dietheyl ether 160C

methanol 385C
ethanol 365C

methane 580C (CH₄)
ethane 515C (C₂H₆)
propane 480C (C₃H₈)
butane 420C (C₄H₁₀)
iso-butane 462C (C₄H₁₀)
pentane 260C (C₅H₁₂)
iso-pentane 420C (C₅H₁₂)
hexane 225C (C₆H₁₄)
heptane 215C (C₇H₁₆)
octane 220C (C₈H₁₈)
iso-octane 447C (C₈H₁₈)

Octane has an amazingly low autoignition temperature of just 220C.

Notice the very nice relationship between longer straight chain hydrocarbons having lower autoigntion temperatures. Also notice that branching of the chain (the ones marked in red) significantly raises the autoignition temperature. So, if you want to avoid autoignition you could use methane. If you wanted to increase the chances of autoignition you could switch to octane.

I wonder how race engines that burn pure methanol (autoignition temp 385C) keep it from autoigniting, dieseling, pinging etc?

I wonder if some of the high frequency "noise" I get in piezo recordings of propane+air combustion in a closed chamber is actually pinging?

EDIT: The "octane" referred to in the "octane rating" of gasoline is iso-octane (autoignition temp=447) and not (n-)octane (autoignition temp=220)

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There are modern engines that take compression ratios over 11:1,which run on pump fuel 98 octane,the Honda B18C engine has a compression Ratio of 11.1:1 and I have seen cars that run compression ratios of 12:1 on 98 octane fuel that have forced induction and they never miss a beat,although the life of the engine would be decreased dramatically.Not sure how relevant this is to the topic,but some food for thought.

Also some food for thought: According to your chart above, Ethanol has an autoignition temp of only 365C, and yet, engines running on E-85 are consistently used with compression ratios of 12:1 or even higher, with added boost, definitely out of the range of regular pump gas. E85's octane rating is somewhere around 105; how can it be this high, if ethanol has such a much lower autoignition temo compared to other fuels?

Ethanol has a lot of cooling properties as a fuel,and the octane rating has a big factor in the pinging problem.There isn't even a need for intercooling with forced induction when ethanol is used because of it's cooling properties in engines,but the same can't be said for this use.

jimmy101 wrote:

D_Hall wrote:propane can not drive itself to spontaneous ignition of unburned air/fuel. To do so would require a 1X mix to burn at about 650 psi. Funny thing: That's about a 15:1 compression ratio... Huh, diesel engines tend to operate at 15-20:1 compression ratios. Funny how that works, eh?

How did you calculate that? Is it just the compression ratio required to get the gas temperature above the autoignition point of propane (assuming 600C)?

During adiabatic heating...

P*V^gam = constant.

Where: P = pressure, V = volume, gam = specific heat ratio

Let's keep it simple....

P = 100 kPa
V = 1 m^3
gam = 1.4

=> constant = 100 kPa

So... P = 100 kPa / V^1.4

Using ideal gas relationships....

P / (rho * R) = T

R = gas constant for air = 287
rho = density
T = 540 C = 813 K (got 540 as auto ignition off some website)

We're now at....

100k / V^1.4 / rho / 287 = 813

.4286 = rho * V^1.4

Now, looking at different compression ratios....

rho = rho*ratio = 1.29 * ratio
V = V/ratio = 1 / ratio

.4286 = 1.29 * ratio * (1/ratio)^1.4

.3322 = ratio * (1/ratio)^1.4 = ratio^(-0.4)

ratio = .3322^(-2.5)

ratio = 15.7

Different auto-ignition temperatures will yield different compression ratios, obviously.

Now, back substituting....

P = 100k / (1/15.7)^1.4 = 4.731 MPa

= 687 psia = 672 psig

Ooops, I screwed up the GasEq calc big time. The 500C I got for a 4.3:1 compression ratio is actually 500K. So, make that ~230C final temperature.

To get propane to a 480C autoignition temp. GasEq says it would take a 15.7:1 compression ratio. Essentailly the same as the 15:1 D_Hall got for a 540C autoignition temp.

Using 540C, GasEq says it is a 20.4:1 compression ratio.

I wonder where I can get some n-octane. For an autoigntion temp of 220C it takes just a ~4:1 compression ratio for autoignition. That is a pressure of 6.8ATM (85 PSIG).

EDIT: BTW, The "octane" in gasoline ratings is "iso-octane" and not (n-)octane. Iso-octane has a substantially higher autoignition temp. than does octane.

Spitfire wrote:This gives me an idea on a hybrid design(i think). Watch this space( )

Uh... Spitfire, I'm not sure you want to go down this road.

I've been doing some more reading on flame propogation within piping systems.

The mechanism I describe wherein the flame propogation rate is sonic and due to spontaneous combustion? As I read the literature, it's real.

It's also the triggering mechanism in what's refered to as an "overdriven detonation." 'Tis a very nasty albeith short-lived phenomenon that *decays* into a "mere" stable detonation.

I don't think you really want to go there.

I did a little reading on autoignition temperatures (AIT). Some highlights;

1. AITs are pretty fuzzy numbers. The values for a particular fuel + oxidizer + pressure + temperature bounce around a lot. Propane is 490C, or 600C or, ...

2. Part of the problem is that AIT is measured without regard to how long the fuel has been held at the particular temperature. Some fuels will autoignite almost instantly at the AIT, others have to sit for a couple minutes before they'll autoignite. This makes the use of AIT in predicting dieseling/pinging in a spudgun difficult. The gases may not be hot long enough to autoignite consistently.

3. AIT drops as pressure rises. So, an AIT of 500C at STP might be only 400C at 5 ATM.

4. For short duration temperture excursions the AIT is considerably higher than the numbers you usually see. Propane's AIT may be over 1000C if the temperature only persists for a short time.

5. The AIT depends on the material in contact with the gas. I believe most AITs are measured in glass containers. Steel/iron is probably similar. Copper may be slightly catalytic and give lower AITs. More exotic metals (nickel, platimum, palladium...) may give AITs much lower than glass or steel.

I don't want to intrude on a topic that I'm not too familiar with, but doesn't it also matter somewhat the rate at which the pressure is risen? Or did I miss something here?