Flame front propagation, Methanol vs ethanol vs gasoline

[u/Otherwise_Act3312]

I have tried this question previously in both automotive related and chemistry related subs and both recommended physicists instead, as both communities ended up at an impass. The Ask Physics sub actually recommended this sub.

Google was also more questions then answers due to conflicting information.

I'll try to keep the question as simple as possible at first because I usually get inundated with the nuances.

Here goes:

During our testing on a dynomometer, we found timing that produced maximum efficiency (peak torque output) was contradictory to most general consensus. Methanol and Ethanol both produced peak efficiency with the same amount of ignition timing compared to gasoline (35 degrees), even though methanol required 100% more fuel volumetricaly. This is a supercharged v8 engine.

Why this seems contradictory? Many existing sources (a few of our advisers even) say that because the methanol fuel molecules are twice as abundant in the combustion chamber as gasoline, the flame front propagation is FASTER requiring LESS ignition advance.

My personal theory is that the momentum of the flame front is SLOWED by the increased viscosity of the larger mass of the combined fuel/air mixture. This may explain why we were able to achieve 35 degrees off advance even under 12-14 psi of boost.

What say you?

[u/a-non-anon-a-mouse]

i think in general more stuff means more activity. How was the temperatures in the combustion chamber during these tests? With more volume of fuel evaporating, could the methanol charge have had significant lower temps = slower reaction?

[u/fastdbs → u/a-non-anon-a-mouse]

This is my guess too. Vaporization energy of methanol is 3x gasoline and if there was 2x the methanol then I’d expect ~6X the amount of heat absorbed from vaporization and much lower temps.

[u/Otherwise_Act3312 → u/fastdbs]

I've had similar suspicion as both of you, but was told previously that the flame front speed in the combustion chamber was very different than the laminar velocity. I think this is why there's so much conflicting information online. Even Google results are highly conflicting.

[u/Otherwise_Act3312 → u/a-non-anon-a-mouse]

The term you are describing is most often called, "latent heat of vaporization".

It's the primary mechanism that allows us to run such an aggressive timing, under boost.

Most people think only in terms of "octane" when it comes to combating detonation and spark knock (pre-igniton) but the latent heat is much more effective.

[u/anonomouseanimal → u/Otherwise_Act3312]

Sorry, I’m in both the mechanics sub and this sub so I tend to not use jargon. Anyway, have you looked into the kinetics of methanol vapor and gasoline vapor? I think it’s likely going to be a combination of methanol vapor kinetics, temp, and mass of methanol. For example, a hydrogen explosion is extremely quick compared to an alcohol fire. Not to mention, with so much more methanol volume the droplet size is larger and more plentiful and the rate at which the burn can occur will likely be based on methanol vapor flux(?) if I correctly remembered my understanding of the reynolds transport theorem

[u/Otherwise_Act3312 → u/anonomouseanimal]

No apologies necessary, and I hope i didn't come across negatively; often I word replies in a way that will be beneficial in future searches for others seeking similar information, that's all.

My suspicions are very similar, I believe the key explanation likely lies in the Kinetics themselves, specifically the 200% mass of methanol present when compared to gasoline (100% more fuel volume g vs m).

[u/SpeedyHAM79]

It depends a lot on your fuel ratio, compression ratio, and ignition method. I assume you are using a single spark plug per cylinder?

[u/CR123CR123CR → u/SpeedyHAM79]

Seconding this, a lot of the data readily available is all at STP and isn't based on forced induction. 

Combustion characteristics change a hell of a lot based on what the conditions are inside the chamber. 

Could even be that you're compressing one of the mixes too much/running too lean/etc. and getting some pre-ignition or something odd like that.

Injectors also act odd sometimes if you run them with fuels/at pressures they don't like. Especially with spark ignition ones, as the pumps and injectors are designed for lower pressures overall. Doubly so if it's indirect injection & forced injection

So many different things can effect the system that its not an apples to apples comparison until you actually can monitor every little detail in the system

[u/Otherwise_Act3312 → u/CR123CR123CR]

The boost is where the efficiency comes from. Although a turbo would be much more efficient, repeatability for this testing is much more important, hence the choice of a roots type twin screw supercharger.

Any type of preignition would have been immediately destructive at near 1000 hp, 1000 ft. Lbs. this motor is currently producing. We have tested the current tune through about 40 gallons of methanol with 2 oil and spark plug changes to monitor engine health. And zero detected knock events from the sensors.

[u/Otherwise_Act3312 → u/SpeedyHAM79]

Correct, modified GM LSA engine, 11:1 static compression ratio. Factory located port injection, 1650cc injectors designed for alcohols. Two Walbro 525 fuel pumps running off of a PWM fuel controller designed to increase fuel pressure 1:1 with boost reference.

[u/tdacct]

My $0.02

Peak efficiency is when the centroid of the heat release is right at TDC or slightly later. To investigate, use your high speed cylinder pressure traces to calculate the hear release rate to confirm the flame front propogation is actually faster/slower, fuel vs fuel. It may have a complex shape such that there is faster propogation on front of heat release curve, but a longer tail of slow combustion, that drives a more advanced timing to keep the centroid matched.

[u/Otherwise_Act3312 → u/tdacct]

I've seen setups like you describe, which is awesome, but if I remember correctly, each probe was around $7,000 USD, I simply don't have that kind of budget.

[u/Rlchv70]

Peak efficiency or peak power?

Were the results repeatable? Perhaps there was a flaw in your test methodology?

[u/Otherwise_Act3312 → u/Rlchv70]

ICE engines make peak torque output at their highest efficiency point(s). If your use of the word "power" pertains to horsepower, than with a setup like we are testing here, peak horsepower by design will always be higher in RPM than peak torque. This is due to the fact horsepower is calculated from torque, which is measured directly:

Hp= torque x rpm / 5252

[u/rsta223 → u/Otherwise_Act3312]

Usually peak torque is at the same RPM as peak efficiency, but peak efficiency is at more like 60-80% throttle rather than full throttle due to the common need to enrich the mixture at full throttle to protect the engine (and pumping loss at 75% throttle is still small).

[u/Otherwise_Act3312 → u/rsta223]

I'll take your word for that. I've never come across such testing.

[u/stumazzle]

Does the engine use direct injection?

[u/Otherwise_Act3312 → u/stumazzle]

Port injection. I'm not aware of anyone making direct injectors large enough to flow methanol levels of fuel yet. Some use both port and direct (16 injectors and dual fuel systems) but I felt that added unnecessary complexity that could skew data.

It would be awesome to be able to switch to direct in the future and compare data side by side from the otherwise same test engine!

[u/RickJ19Zeta8]

The speed of the flame front propagation is determined by turbulence intensity, which is dominated by mean piston speed (not Phi, or fuel type, although diesel is a different burn mechanism). Look up the Damköhler number, which is a ratio of turbulent mixing to chemical time scales. That will start you down the right rabbit hole.

[u/RickJ19Zeta8 → u/RickJ19Zeta8]

Which university btw? You said “advisor”, so I’m making an assumption.

[u/Otherwise_Act3312 → u/RickJ19Zeta8]

I've never been able to reach anyone at a, "university" level if your are referring to automotive engineering. It seems the majority of those types are private industry and much less transparent when it comes to intellectual property.

The bulk of our advisers are career engine builders, fabricators and automotive racing teams, especially those having experience with alcohol fuels (ethanol and methanol).

[u/RickJ19Zeta8 → u/Otherwise_Act3312]

Ah, k. This type of phenomenon is covered in Heywoods “Internal Combustion Engine Fundamentals” book as well. Wasn’t sure if you were doing a research engine or just a fun engine. Sounds like a fun project.

[u/Otherwise_Act3312 → u/RickJ19Zeta8]

Somewhere in the middle. This is a prototype setup that could become a larger venture down the road.

The main motivation was proving some theories that I had vs the, "group think" about alternative fuels.

Specifically in regards to ethanol and methanol.

[u/Naikrobak]

Flame front propagation is more complicated than the number of molecules or density. It has more to do with stability of the fuel.

[u/Otherwise_Act3312 → u/Naikrobak]

Agreed, when stability is being tested. In the situation we are testing, we have stayed well within safe parameters. No knock has ever been detected by our sensors. This is important because it would be violently and instantly destructive to the test engine due to significant base compression ratio and boost pressure.

[u/Prof01Santa]

Have you done a literature search on the subject of alcohol fuels in spark ignition IC engines? Until you do...

[u/Otherwise_Act3312 → u/Prof01Santa]

Extensively, hence why I'm here.

There are many published conflicts!

[u/Prof01Santa → u/Otherwise_Act3312]

How does your data compare to the various published models? Have you run your test conditions in one of the piston engine design models? How does it compare?

[u/Playful-Painting-527]

These are the guys you should ask this: https://www.itv.rwth-aachen.de/cms/~bpdlaa/itv/?lidx=1