Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Flame Velocity vs. Ignition Timing

Flame Velocity vs. Ignition Timing

Flame Velocity vs. Ignition Timing

I am trying to do some calculations to estimate/replicate the timing table for an LS1. Here are my assumptions. The spark plug is offset approximately 1/3 of the way across a 99mm bore, so max flame travel distance is 66mm (I got this by estimating from a picture. Stock timing for 600rpm is 11 BTDC, and at 6400rpm it is 26. This is from a table I found online and is at .72g/cyl.

So, assuming flame termination is at 15 degrees ATDC I determined the flame velocity to be about 9.1 m/s at 600 rpm. Then I used a graph from Pulkabrek's ICE book and got a slope of about .02375 m/s/rpm to estimate how much flame velocity increases from 600 to 6400. This gave me a flame front velocity of 146.75 m/s. This puts timing at only 2 degrees BTDC, obviously not right and far off of the table's 26. I tried another method from the book, which is to scale the ratio of the rpm's with a factor of 0.85. This gives a velocity of 82.9 m/s and flame travel time of .000796s and leads to a timing of 15 BTDC, which is better but still way off of 26, which seemed like a low number to begin with.

What am I doing wrong? Am I way off on the flame travel distance of 66mm because of the pent roof combustion chamber? I would assume that the table would have been generated more or less empirically, but it seems like there should be a way to get a lot closer than I got.

RE: Flame Velocity vs. Ignition Timing

Your assumption of flame termination 15* ATDC is one issue. Perhaps around 80% of mixture mass will be burned by peak cylinder pressure which is often assumed to be optimally located at 15* ATDC. Google images for "Mass Fraction Burned". Link
Difficult to infer the position or velocity of the flame front from MFB though. The last mixture to burn is very slow due to chamber wall quenching. Perhaps the velocity for the first 80% burned is a good starting point.

je suis charlie

RE: Flame Velocity vs. Ignition Timing

Keep in mind that your original flame kernel has a very small front and burns in three dimensions. It will burn the bulk fuel charge much slower than a later large flame front. On the other hand there is also likely squish and/or tumble in the fuel charge that changes the flame travel from conduction to convection, i.e. maybe a 10X to 15X increase in flame travel rate.

RE: Flame Velocity vs. Ignition Timing

Ok that makes sense. What sort of algorithm can be used to arrive at more realistic timing numbers?

RE: Flame Velocity vs. Ignition Timing

Lots of dyno testing.

RE: Flame Velocity vs. Ignition Timing

There's got to be some model that can get you in the ballpark, right?

RE: Flame Velocity vs. Ignition Timing

Highest pressure ( at your estimated 15 degrees ) does not mean combustion is over at that point, so I wonder if the flame ain't still travellin' .

I think combustion in a gasoline engine requires a few "stages."
- Spark
- ionization
- initial slow kernel growth sufficient to create any pressure at all
- usefully fast flame "propagation" if swirl, tumble and turbulence to wrinkle and fold the flame front sufficiently to compensate for the turtle-like basic "flame speed" of an air gasoline air vapor. (Only a foot or so per second, according to Kevin Cameron).

RE: Flame Velocity vs. Ignition Timing

I am sure that the engineers who are in this business have an extremely complex computational fluid dynamics model that includes combustion simulation which might let them get close to an estimate of what the ignition timing should be under various conditions.

The rest of us make do with a starting point based on previous experience with this type of engine, guess on the slightly late side to avoid melting a piston before even getting started, and then do lots of dyno testing to refine it.

The computer simulation will not get you out of doing the dyno testing.

There is absolutely no way you will ever just plug in some simple rule of thumb formula and expect results that are anywhere close to being accurate. It doesn't work that way. There are too many variables and uncertainties. Squish clearance. Squish band area. Initial tumble or swirl in the incoming charge. Fuel distribution in the incoming charge. Spark plug location. Operating temperature of all of the surfaces in the chamber.

RE: Flame Velocity vs. Ignition Timing

The general rule of thumb here is that there are no rules of thumb in your calculations. They use pressure transducers in the cylinder head and combustion imaging cameras. Hard data on the crank angle domain. That's actually what I'm doing right now.

Educated guessing will get you as close as the calculations will, because you're going to guess on a lot of your variables.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


White Paper: Industrial Control Basics: Contactors
A contactor is an electrical device used for switching an electrical circuit on or off. Considered to be a special type of relay, contactors are used in applications with higher current carrying capacity, while relays are used for lower current applications. Download Now
Research Report: State of IoT Adoption in Product Development 2019
This research report, based on a survey of 234 product development professionals, examines the current state of Internet of Things (IoT) adoption by product design teams, its perceived importance, and what features and capabilities teams consider important when making decision about adding IoT functionality to their products. Download Now
Research Report: Augmented Reality for Maintenance, Repair and Overhaul (MRO)
The term Industry 4.0 denotes a cluster of technologies that’s poised to fundamentally reshape manufacturing and bring about a new industrial revolution. These include 3D printing (AM), the Industrial Internet of Things (IIoT), artificial intelligence (AI) and mixed reality technologies, more commonly known as virtual reality (VR) and augmented reality (AR). Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close