Ignition timing advancement/retardation

[Mikkel]

I've heard some random tid-bits about this concept. Nothing substantial though.

Has anyone got some insights into what benefits and drawback such alterations might produce. Quantitative performance gains?

[Malcolm]

virtually all 4 stroke petrol engines will produce more power the more the ignition is advanced - until they start to detonate. This is especially true at higher RPMs, and it occurs because it allows a greater conversion of heat to mechanical force on the piston. In some cases ignition can become so advanced that the expansion of the combustion gases occurs too soon and it exerts a force on the piston during the compression stroke, which reduces power - but this seldom a problem on 4 stroke automotive engines. You'll also typically get better fuel economy with more ignition advance, due to an increase in thermal efficiency.

The downsides of running more ignition advance are the increased likelihood of detonation - which is sometimes hard to hear (especially if you don't know what you're listening for). Retarding ignition makes things safer, but typically increases exhaust gas temperatures, which can cause all sorts of fun problems like burning exhaust valves, or creating hot spots in the head which can also cause detonation

Depending on how shit the engine is currently set up, running 98 and advancing the timing might get you up to 5 percent increase in power? Can't tell you too much about the practicalities of playing with timing of a bike as I'm still pretty new to them, but that at least covers the theoretical side

[Mikkel]

Thanks, that was very informative

What if you have an otherwise well tuned engine? What gains could be achieved by mucking around with the timing?

I suppose you'd want to tune it for 96 octane since you can get to areas where the 98 octane isn't available...

[mujambee]

... as I'm still pretty new to them, but that at least covers the theoretical side

Sorry mate but I think you don't have it quite right.

I'll stick to 4 stroke, but the theory applies also to 2S engines.

first things first: Ignition advance is the position of the crankshaft at which the spark ignites, measured in degrees from the top dead center. Simple engine diagrams usually show the spark igniting when piston is in its TDC, but it usually happens before that.

We'll start with an engine with no advancement. At TDC you have the mixture at is higher compression, spark it and it starts to burn. But that doesn't happen instantly; it takes time. So pressure starts to build up over some time (really short, but the piston moves fast). There is some point at which the most of the gas has burned and that gives you the maximum pressure over the piston, but with our setup that happens too late to get all power from the engine.

So you advance the spark in order to have your maximum pressure before it's too late. You start burning gas before the piston has reached its TDC so when the flame reaches it's maximum you'll be near your best efficiency.

That has to be done at the right point, and that changes with revs. The higher the revs the sooner it must be done. Flames take almost the same time to reach all the chamber but the piston is moving much faster, so you have to compensate.

So there is not that the sooner the better. Is more that you'll have to find the right point.

And you should bear in mind that advance also plays against you, since it adds some resistance at the end of the compression stroke (gas is starting to burn). It pays back at the expansion stroke, so it is a balance thing.

Regarding detonation. During the compression stroke, the piston is running up pressing the air/gas mixture against the cylinder head. Physics say that if you compress a gas it's temperature raises; as pressure is so high inside the chamber the temperature also raises enough to ignite the gas spontaneously, thus giving what we call detonation. So that's a limit: you always have to advance the ignition before detonation exists (and not later as Malcom said). Higher octane number gives you higher resistance to detonation, giving you more freedom to play with compression and advance.


Edit: Hope someone understands it, it's hard to explain this kind of stuff in a foreign language.

[Mikkel]

You're doing well mate.

So what kind of performance gain could you expect from fine tuning the ignition advance?

[Malcolm]

There is some point at which the most of the gas has burned and that gives you the maximum pressure over the piston, but with our setup that happens too late to get all power from the engine.

Yes, that point is called MBT (Maximum ignition advance for Best Torque), however as I eluded to in my post - on virtually all engines I've seen, the engine will start to detonate at advance values lower than the MBT value. In fact the only case I've seen where this doesn't happen is in an SAE paper I was reading while doing some research on hydrogen as a fuel - co-injecting hydrogen with gasoline delayed the onset of knock until higher advance values than MBT.

Regarding detonation. During the compression stroke, the piston is running up pressing the air/gas mixture against the cylinder head. Physics say that if you compress a gas it's temperature raises; as pressure is so high inside the chamber the temperature also raises enough to ignite the gas spontaneously, thus giving what we call detonation. So that's a limit: you always have to advance the ignition before detonation exists (and not later as Malcom said). Higher octane number gives you higher resistance to detonation, giving you more freedom to play with compression and advance.

I'm sorry but I most definitely disagree with that. Too much advance will cause detonation. This is a well known fact of spark ignition 4 stroke engines. I don't necessarily understand the exact mechanism that causes it, but I know that too much timing advance will cause detonation.

I feel like for the most part we're saying the same thing, but explaining it in different ways, terms like 'before' and 'after' can be somewhat misleading when you're talking in terms of advance and are therefore working with a negative time-scale

[mujambee]

So what kind of performance gain could you expect from fine tuning the ignition advance?

Not much, unless you also change admision and exahust systems. Engines are usually quite optimised in that area nowadays.

I'm sorry but I most definitely disagree with that. Too much advance will cause detonation. This is a well known fact of spark ignition 4 stroke engines. I don't necessarily understand the exact mechanism that causes it, but I know that too much timing advance will cause detonation.

I feel like for the most part we're saying the same thing, but explaining it in different ways, terms like 'before' and 'after' can be somewhat misleading when you're talking in terms of advance and are therefore working with a negative time-scale

I believe you are mistaking detonation with knocking.

Detonation is the process of self-ignition of the gas produced by pressure.

Knocking is the fact that an ignition ocurred inside the chamber long before the TDC, so the piston gets pressure against moving before the end of the compression stroke.

Detonation is the most usual cause of knocking; but too much advance may also produce knocking.

Think about it. The sooner you spark, the less pressure there is inside the chamber when you spark, so the less chance to get detonation. So it makes no sense thinking that there is a limit there.

[Malcolm]

I think you'll find you're arguing about semantics. Detonation/knocking both usually refer to sudden and uncontrolled ignition of the entire air/fuel mixture, rather than the controlled burn that should occur.

Pre-ignition is when the mixture spontaneously ignites, either due to excessive pressure/temperature or hot spots, before the spark fires - the problem here isn't that the timing is too retarded, the problem is that you have hot spots (possibly due to increased temperatures due to retarded ignition), too lean of a mixture, too much boost pressure, too high a compression ratio for the octane rating of your fuel etc.

[mujambee]

I think you'll find you're arguing about semantics.

Maybe. Don't really want to argue.

[Motu]

Detonation is post ignition - it occurs after the sparkplug fires.It is caused by a pressure increase - from the flame front forcing fuel into a quench area.

Pre ignition happens before the sparkplug fires,mainly from a hot spot,but there are other causes.

[Malcolm]

Maybe. Don't really want to argue.

Neither, have a beer

[Malcolm]

shit - double post. Is there no delete option? Or am I blind?

[svr]

Interesting - I assumed that detonation and pre-ignition were the same thing. My sv currently suffers serious detonation (I think) on 98 octane, but is fine on avgas. With hi-comp pistons the squish area on the perimeter of the combustion chambers looks minimal and flat - I was thinking of reshaping those areas as I assume there is excessive pressure buildup and resultant hotspots there causing detonation. Winters comming on...
Also, I've played with advancing the ignition and now theorize that its dependant on whatever advance is required at peak power rpm of the engine i.e. if you raise max efficient rpm range with more cam lift or bigger valves / carbs etc there is benefit to advancing the ignition - otherwise it may be minimal.
Thanks for the info.

[98tls]

Interesting - I assumed that detonation and pre-ignition were the same thing.

Somewhat related but vastly different as can be the end result.

[warewolf]

Nice. In the last month, I've discussed this on advrider.com and another Triumph list. Some good & concise info here.