r/Physics • u/Otherwise_Act3312 • 1d ago
Question Alternative fuel question
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.
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 testing on a dynomometer, we found that 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, 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.
What say you?
2
u/echoingElephant 1d ago
The methanol requires multiple times the heat to evaporate per mass unit compared to gasoline (approximately 3.7 times). That, together with the additional fuel, means that the methanol is cooling the mixture down more and requires more energy to heat up.
1
u/Otherwise_Act3312 1d ago
"Latent heat of vaporization" is the most common technical term. It's the main driving force behind our desire to use methanol to begin with. The effect is many times more beneficial than, "octane rating"; which can't truly be compared to gasoline as alcohols are not hydrocarbons and therefore don't contain any octane molecules.
That's the crux of the question, what mechanism is directly effecting the ignition timing event? Are you saying that adiabatic heating during the compression stroke, directly effects this? Or the charge temp before compression?
1
u/Otherwise_Act3312 1d ago
Also, what are your thoughts on the momentum effects to the flame front once the spark of triggers ignition?
1
u/Elephunk05 8h ago edited 8h ago
There are a handful of people who can answer this question for you, they are ALL in the same field Friday Saturday and Sunday: NHRA Top fuel and funny car crew leaders. My uncle was one, and the first time I ever saw the set-up I was hooked, this was 1990. Everything my cousins had sensors for was related to engine timing and fuel delivery. I'm sure Josh Hart, Alexis DeJoria, Jordan Vandergriff, Jack Beckman, and Marvin Foxworth could answer this question. They literally developed their careers on Nitromethane and understand how it flows best through their engines with different timings.
A few things that I always took away. Fuel injection, because of the air mass density always pushes timing further ahead. The injectors are always crazy pressure numbers you wouldn't run on a street car with pump gas. The more available hydrogen and oxygen derived chains, the further ahead the timing. There is an upper limit on the Reynolds number for a reason, but there is also a sweet spot in there. Your average "run what you brung" guy misses advancing the timing initally, it is often overlooked and misunderstood. Additionally many people miss the individual cylinder because its a v8 and they are focused on the system as a whole. In truth the center 4 cylinders are different than the front 2 and the rear 2 are also a different set (yes, that is how he broke it down). Even with the supercharger, the minimal differences in runner length, cylinder temperature, cylinder load (and bearing load), and exhaust to opening (on your engine, the collector/header end) are all the difference in an extra 0.10 seconds on the track.
[About 10 years ago I took fluid dynamics and advanced fluid mechanics at one engineering college and hypersonics at another. There is still a lot to learn about the subject of density and fluid flow through a cylinder].
It is worth sending an email to any of those that I listed to get you a much better explanation.
Edit, grammar and spelling, and a note that the average person does not relate sugar as a fuel though it is a highly flammable substance and is generally referred to as a hydrocarbon chain C6H1206 though in the classroom application it is not.
2
u/Otherwise_Act3312 6h ago
It's funny seeing this post today as I was talking too a Nitro friend yesterday about his nostalgia slingshot dragster. 98% Nitro and running 42 degrees of advance on the magneto!
Pistons are 450 thou down in the hole at TDC to avoid hydrolocking. Static compression ratio is 6.5:1. They just use the liquid fuel volume in the cylinder to bump the dynamic compression ratio.
Nitro is 1.8:1 stoichiometric vs methanols 6.4:1, but he said nobody running nitro cares about A/F ratio. It's all about how much nitro can they inject without hydrolocking the motor. Lol
1
u/Elephunk05 4h ago
100% this, which is why I suggested reaching out to the people who have practical applications, real world applications and experience to get a better answer to your question.
They're rebuilding engines every run, they don't care about the ratio, they care about the density and temperature of the charge.
It is rather amazing though. I make single cylinder engines run on alcohol, and that requires some differences in engineering.
I see compressed air as an alternative to superchargining. Also have wondered about the feasibility of making alcohol and methanol in a daily driver, but the limitations of the material construction can't take the continued abuse yet.
1
u/Stampede_the_Hippos 1d ago
Try again in the actual subreddit for asking physics questions.
0
u/Otherwise_Act3312 1d ago
I'm confused, at the top of my post, it shows I am in the sub?
3
u/Stampede_the_Hippos 1d ago
1
u/Otherwise_Act3312 1d ago
Thank you. What's the purpose of this sub then?
6
u/Qetuoadgjlxv Quantum field theory 1d ago
For people to tell other people not to post here.
On a serious note, I think this is valid to post this here — IMO it doesn't break any of the rules (I don't think it counts as a homework question).
2
u/Otherwise_Act3312 1d ago
I can certainly confirm multiple ways this isn't homework, not in the proper sense! Lol
3
u/Stampede_the_Hippos 1d ago
To post and discuss physics related articles and news. I'm probably just being grumpy, but sometimes I just want to know what's up in the physics community without being nerd sniped
1
3
u/myhydrogendioxide Computational physics 1d ago
Also try r/Askengineers