Ignition advance - more is better right?

Along the lines of useless 'performance parts' are one-size-fits-all ignition advancers. The logic behind these contraptions is that if some is good, then more is better. Half an extra ounce of advance will get you a long way. ...or will it?

But what is ignition advance anyway? Why do we need it? Can't we just light up the plug when the piston is at Top Dead Centre (TDC) and get on with our lives?

The answer is that we can - and the engines would run fine and reliably too, only they wouldn't be making anywhere near as much power as they do now. Try to imagine the piston during the power stroke. It's at TDC, inertia moving it fast towards BDC. The power stroke is the phase where the ignited mixture burns, creating pressure inside the chamber and pushing the piston downwards with even more force - there are 3 'dead' cycles ahead and without this extra force the engine would stop pretty quickly!

Let's say that the force pushing down the piston is (more or less) fixed. In order to get more value from this 'push' (i.e. more engine torque) we want the force to be pushing the piston for as long as possible. Experiments have shown that we want it in full swing from 15degrees ATDC until BDC (most of the work is done by 90deg ATDC). We don't want it earlier nor later.

Now comes the complication: the mixture doesn't burn instantaneously. Oh no, that would be too easy. If all goes well, the flame front moves at 15-16m/sec. Let's focus on cylinder #1 for a moment. Suppose the engine is idling happily at 1200rpm. Our cylinder experiences 600 power strokes per minute, or 10 per second. In other words within 100msec we have cyl#1 experiencing a Power Stroke (PS) and an Exhaust Stroke (ES). So the PS lasts 50msec and that relates to 180 crank degrees.

Put another way, at idle the piston needs 50ms to move from TDC to BDC.

If the mixture needs 10msec to burn fully, then if we ignite the plug at TDC, the flamefront will be pushing our piston 10msecs later, that's 36 degrees ATDC. We are power-hungry though, and we want it to start pushing earlier, in fact at the theoretical optimum - 15degrees ATDC. The only way to do that (we can't speed up the burn!) is to introduce the spark 21degrees BTDC. That way, the flamefront will be up and kicking at 15degrees ATDC. What we've done, is introduce ignition advance of 21 degrees at idle.

Now let's check if that's all we had to do. Revving the engine at 2400rpm we realise that the piston now takes only 25msecs to move from TDC to BDC. Double the revs, half the time needed - no major science here. But our mixture doesn't burn any faster - even if it does (because it's now more homogenous and ignitable because of the swirl) it's nowhere near as twice as fast. So if we still want to keep the flamefront pushing at 15 degrees ATDC we now need to ignite even earlier, say 40 degrees BTDC. Therefore advance values vary with engine rpm.

The amount of optimum advance also varies with the ignitability of the mixture - if the AFR, temperatures and pressures are all within the right ranges, then the speed of burn will be quick. Less ignition advance will be needed, compared to situations where AFR is wrong (too rich or too lean) or temperatures are off the scale - or the mixture is just not homogenous enough.

Q & A

I want more advance - a mate of mine works in a garage and swears that more advance always makes cars go faster

This is sometimes true for older stock cars. Manufacturers tended to settle for conservative advance settings, trying to be on the safe side and avoid fixing melted engines under warranty. So a grease-monkey fiddling with the distributor could add another few degrees and the car would feel nippier with no ill effects - until the weather gets hot and someone floors it at low revs in 5th gear and it sounds like a hammer drill.

Modern engine management systems don't allow easy access to these operations, and they tend to operate in the most efficient range anyway - so fiddling blindly can only damage things.

 

I have read somewhere that the spark hangs around waiting for the most desirable time to jump

The spark doesn't hang around at all.
The ignition fires, and if the environment is right there is a spark. If not, there isn't a spark.
Now IF there is a spark, this doesn't necessarily mean that it will ignite the mixture.
Even if it DOES ignite the mixture, it doesn't mean that it's the only rooster in the henhouse (other hotspots may have also ignited the mixture at different spots - preignition)
Even if it IS the only spark in the room it could be timed too early or too late, creating an explosive flamefront while the piston is still moving upwards:
Then we have the crank pushing piston (through the rod) UPWARDS and the flamefront pushing DOWNWARDS.
Something expensive will break.

 

But I've actually felt this on a dyno. OK, me mate's a moron, but this I felt for myself while playing on the dyno. The spark waits around, I tell you.

It doesn't hang round... but it may *appear* to do so.

These events last only fractions of a millisecond - so there is no way a human being could ever sense timeshifts of this magnitude.

What you DO feel is that it's down on power.
While the car is on the dyno, you have more than a thousand ignition events per second. These are not all identical. Some will find the cylinder full of burnt crap and will be far too lean to ignite.
Others will be far too rich to ignite.
Others will have way too much pressure for the spark to even form.
Others will be within range and *will* fire, producing power to keep the crank turning (what the dyno measures).
It's a statistical game, no engine ignites properly everything, all the time. It would be an extremely clean, powerful and efficient engine, the one that could achieve that noble goal.

The 'dead' cycles will not contribute to the turning of the crank - instead stored energy (from the flywheel, pistons etc.) will keep it from stopping immediately. This increased 'lack of co-operation' will manifest itself as 'down on power'. The 4-stroke engine has already 3 'dead-weight' strokes for every power one, losing those as well is not a good idea!

 

I've tried running high boost and big shots of NOS.
I was initially down on power, because the ignition couldn't cope (my spark would 'blow out'). But using resistorless plugs and this rotor arm mod it regained its power, the car was missing but only enough to decrease performance, you cannot hear this miss with the ear or feel it in the car!!. Isn't that the spark waiting for Godoh?

A stronger 'spark' will result in a greater percentage of cycles actually igniting, because some instances where the mixture was previously too lean (or too rich) are now ignitable.
So you get less 'dead weight' ignition cycles, so more power at the wheels.

The spark waits for no mortals...

 

 

Nitrous cools the charge - we all know that. So how come my rod ended up in the other side of the motorway, when I went full throttle it wasn't even boosting! I just do not understand why the mixture with n20 would detonate -, its way too cold to detonate by itself, even with a big spark it still wont fire so why should it ignite with no spark?

It may NOT ignite even with a good spark - that's the problem. But where will the unburnt mixture go? Some will be pushed through the exhaust, some will remain for the next cycle, some will be pushed back through the intake again. So the next cycle will be even more contaminated. It goes on, until the cycle after that has an ignitable AFR and the thing DOES ignite.
Only now it's well outside the manufacturer's design limits, and the flamefront moves at a different speed. Unpredictable things might happen.

The overall aim of ignition timing is to have the whole flamefront ignited and pushing the piston downwards JUST AFTER the top dead centre (TDC).

The ignition is ALWAYS advanced in real terms, because as we've said it takes a certain time for the flamefront to propagate (a couple of milliseconds usually). Even when we say it's retarded what we actually mean is that it's less advanced (say 10 degrees instead of 15)
If the flamefront is fully on swing a bit after TDC, no big deal, you're just down on power a bit (because the 'push' doesn't last as long as it could)
But if it happens just BEFORE the TDC then suddenly you're in the dark side: the crank pushes up, the gases push down! The forces are so powerful that the rod can shatter or bend. That's many tons of force, suddenly working against you.

Back to the blow-ups, suppose that the ignition and the n20 were all at full working potential. Is there any scenario where it can it blow up with only a small amount of gas?

Even a (relatively) tiny amount of gas could be demanding a timing retard of quite a few degrees. The LET ECU doesn't know that, and fires when it's stock map tells it to.
That could send you *suddenly* to the catastrophic scenario described earlier.
Without expert verification/tweaking of the ignition timing on a dyno (and the gas bottle full) I wouldn't risk full throttle, especially at low revs.

The cooling effect of nitrous is not an advantage at low revs, because the mixture is nowhere near hot by itself (the turbo is off-boost and is not adding any heat to speak of)

In fact, I can think of a couple of reasons that NOS-cooling might make ignition *worse*. In-cylinder temps being well below the optimum. So just to be on the safe side, avoid injecting nitrous at very low revs.

 

Can an engine be blown with the spark triggering too late?

If the spark triggers too late, there is the risk that the mixture in the meantime will self-ignite (like diesels do), but that would be uncontrollable and could end in tears. As a general rule, if your ignition timing is going to be miles off, its better to be late rather than early - best of a bad bunch.

 

But I thought that ignition retard is what the car does when the knock sensor starts erm...knocking. Is that a bad thing too?

The knock sensor is just a microphone. It picks up the reverberations throughout the block, created by the flamefronts colliding. The damage is already happening.
The ECU will then try to reduce boost and/or retard the ignition by up to 10 degrees. It's not a miracle cure, it's only meant to minimise damage in certain circumstances.

There are intricate events happening in the combustion chambers and very few people really know what's going on in there (F1 designers and the like). The causes and behaviour of detonation are not 100% fully understood even today.

In general, running high boost, and/or NOS without fiddling with the compression ratio and ignition timing is like playing Russian Roulette with the engine. Some changes will result in accelerating the flamefront, others will slow it down. If you're running stock ignition timing, you're betting that these two sets of changes cancel each other out. Maybe they do, maybe they don't...

Can you be more specific please? What factors ask for more advance and which for retard? I'm just curious now.

Factors slowing down the flame (resulting in need for more advance)

• mixture leaner than 12:1. For example at stoich gasoline burns at 330mm/sec, while at 16.3:1 at 270mm/sec and 18.4:1 at 250mm/sec
• very rich mixture - richer than 12:1 (11.3:1 burns at 320mm/sec, 10.5:1 at 250mm/sec and 9.2:1 at 220mm/sec). No wonder cars running pig-rich lose power, these 3 extra AFR points cause the burn speed to drop by more than 30%! This is equivalent to heavily retarding the ignition.
• low throttle openings
• wild cams outside their power range
• butchered squish band (i.e. result of a head gasket thick as a brick)
• excessively high exhaust backpressure (melted cat, stock turbine pushed hard, etc.)
• anything else that further reduces the volumetric efficiency
• alternative fuels such as alcohol or nitro

Factors accelerating the flame (need for retard). Strictly speaking they don't accelerate the speed of burn, more like bring it up closer to it's optimum. The net effect is the same though...

• correct air/fuel mixture helps. In fact, according to the latest data, a charge of a fixed density, will experience the quickest burn rate at about AFR 12:1 (typical speed of burn for a n/a engine mapped properly is 360mm/sec. Turbocharged/nitrous oxide charges burn faster.)
• full throttle
• cams (and turbo) inside their 'sweet range'
• gasflowed cylinder head
• nitrous injection
• anything else that increases the volumetric efficiency

Special note has to be made for high-boost situations and large nitrous shots. These increase the charge density which in itself accelerates the speed of burn. A denser mixture will always burn quicker (for the same AFR). This is the main reason that high-boost and substantial NOS installations come with various forms of ignition retard.

And that's the easy bit. The hard one is to quantify each one and calculate which way the final figure will swing. This in practice is futile, and the best way is to stick the engine on a dyno and take it through the revs under various loads. Armed with a knock sensor (or even better det cans) you find out the edges of detonation (experimentally) and reduce advance by a couple of degrees from that point. This has to be repeated in the summer temps, or if fuel of different octane will be used (higher octane won't detonate as easily)

Also note that the knock signature of nitrous-induced detonation is different from the 'normal' one. In fact it's so different that it is a commercial secret for the few who even know about it. I was fortunate enough to listen to it and I can attest that it sounds nothing like the run-of-the-mill 'ping' that traditional knock sensors are geared to listen for. As for the secrecy involved, it is understandable since it takes the sacrifice of at least one engine on the dyno for someone to have a chance to hear it. The one I experienced was not the first, because that went bang without us hearing anything (in fact we realised later that we had heard it, but didn't relate it to detonation. Goes to show how different it is.)

As a result, nitrous installations that solely depend on the 'donor' car's knock sensor for ignition retard, may be in for a nasty surprise or two.

 

But I know of people running large Nitrous shots or quite high turbo boost without retarding their ignition. They haven't even got a knock sensor, so the ECU doesn't do it either. How's that possible?

They are probably running too rich, either because their nitrous pressure is low, or they're injecting too much fuel out of fear of losing their pistons. This costs them a lot of power of course, but it also slows down the speed of burn, reducing the need for ignition retard. In fact some may be running so rich that the stock ignition curve appears adequate.

Of course running so rich will cost a lot of power, making a 150bhp nitrous shot feel more like a 50. These people are not sophisticated enough to tell the difference, so they live in eternal bliss believing that it's safe to run large nitrous shots (or boost) on the stock ignition curve. At some point they'll realise that they're running so rich so they'll lean it down a bit, hoping to get even more power. Bang.

Two wrongs can make one right --- but not for long

.

So what's the worst that can happen if I have too much advance? It will start pinging right? Preignition, detonation, autoignition, whatever...

These are not all the same thing, although the terms knocking or pinging generally cover all situations where hammering noises are heard.

Pre-ignition is when the mixture self-ignites, not necessarily at the right moment! That's roughly how diesels work, but that's part of their design and their heavyweight engine structure reflects these loads. Carbon deposits in the chambers can become hotspots, sort of 'spark-plug wannabes', starting pockets of ignitable mixture. Exhaust valves that get too hot because of too much reversion or blocked coolant passages. Even a spark plug that is too hot for the conditions can have its tip glowing and messing up the ignition timing. These rogue flamefronts collide with each other and all together with the piston as it tries to move upwards. Nothing good can come out of this situation.

Detonation is even more deadly. The piston crown may become pitted like someone attacked it with a chisel. In extreme cases holes may be opened through the pistons. That's when the flamefronts reinforce each other in much a similar fashion as explosive charges are positioned in armour-piercing ammunition. These flamefronts can start even after the spark has ignited. They do their own thing, go their own way, and when the fronts collide, the forces can be so powerful that rods bend or shatter.

See below how violent the knock 'pressure spikes' become as ignition is advanced by just a few degrees:

Excessive intake temperatures can typically lead turbos to detonation - hence the need for adequate charge cooling.

Nitrous is very effective at in-cylinder cooling as it expands from liquid into gaseous state. This works in a similar fashion as water injection, only a nitrous system will also release loads of spare oxygen, which will be mated (hopefully) with extra fuel. This will result to extra heat produced, and hopefully extra power, too! But the cooling effect will pretty much counterbalance the extra heat it's burning introduces - so under some conditions NOS may not dictate ignition amendments.

There is even the view that there is such a thing as detonation-induced-preignition. If anything, it shows that things are not always nice and clear-cut. There are grey areas too. (local copy here)

LET piston that suffered detonation damage (pic courtesy of Savage, from MIGWeb)

Here is another one, claimed to be from the 'notorious' cyl#3 If you believe the myth, there is a design flaw that leads to the inevitable demise of piston#3. In reality bad fuelling or excessive cylinder pressures are usually to blame. #3 is simply a bit more likely to buy the farm first - being closest to the turbine wheel it shows slightly higher EGTs.

Another Salvatore Dali creation: Severe detonation - no engine can survive, no matter how trick pistons and rods are used. Avoid it at any cost

This is an 'oliver' aftermarket rod from an EVO. They are routinely used as upgrades. Note how it shattered with the bolts still tight in place. A careful X-ray inspection would have prevented this.

 

Crankcase ventilation, the hard way

Diagram from an old manual showing detonation a bit more extensively:

 

 

If you're further interested in spark phase angles and spark cavitation, then click on this image. Local copy here

 

Good description of the ignition and combustion process. (local copy here)

Here's another internet article on detonation. (local copy here)

Yet another advanced discussion on knock-sensing, too much is never enough (local copy here)

An interesting dissertation on closed-loop ignition control (local copy here)

An interesting article on ion-sensing, using the spark plugs as knock sensors

 

Back to ignition...

 

AFR	18.4	16.3	14.7	13.35	12.25	11.3	10.5	9.2
mm/sec	250	270	330	350	360	320	250	220

 

Knock sensor sitting on the cyl head

Spark Timing Myths Debunked

By Klaus Allmendinger, VP of Engineering for Innovate Motorsports

A widely-held myth is that maximum advance always means maximum power. Here’s what’s wrong with this thinking:

The spark plug ignites the mixture and the fire starts burning. The speed of this flame front depends on the mixture, this means how many air and fuel molecules are packed together in the combustion chamber. The closer they are packed together in the same volume, the easier it is for the fire to jump from one set of molecules to the other. The burning speed is also dependent on the air-fuel-ratio. At about 12.5 to 13 air-fuel-ratio the mixture burns fastest. A leaner mixture than that burns slower. A richer mixture also burns slower. That's why the maximum power mixture is at the fastest burn speed. It takes some time for this flame front to consume all the fuel in the combustion chamber. As it burns, the pressure and temperature in the cylinder increases. This pressure peaks at some point after TDC. Many experiments have shown that the optimum position for this pressure peak is about 15 to 20 degrees after TDC. The exact location of the optimum pressure peak is actually independent of engine load or RPM, but dependent on engine geometry.

Typically all the mixture is burned before about 70 deg ATDC. But because the mixture density and AFR in the engine change all the time, the fire has to be ignited just at the right time to get the peak pressure at the optimal point. As the engine speed increases, you need to ignite the mixture in the combustion chamber earlier because there is less time between spark and optimum peak pressure angle. If the mixture density is changed due to for example boost or higher compression ratio, the spark has to be ignited later to hit the same optimal point.

If the mixture is ignited to early, the piston is still moving up towards TDC as the pressure from the burning mixture builds. This has several effects:

The pressure buildup before TDC tries to turn the engine backward, costing power.
The point where the pressure in the cylinder peaks is much closer to TDC, with the result of less mechanical leverage on the crankshaft (less power) and also causes MUCH higher pressure peaks and temperatures, leading to knock.
Many people with aftermarket turbos don't change the spark advance very much, believing that earlier spark creates more power. To combat knock they make the mixture richer. All that happens really then is that the mixture burns slower and therefore hits the peak pressure closer to the right point. This of course reaffirms the belief that the richer mixture creates more power. In reality the flame front speed was adjusted to get the right peak pressure point. The same result (with more power, less emissions and less fuel consumption) could be achieved by leaving the mixture at the leaner optimum and retarding the ignition more instead.

Turbo charging or increasing the compression ratio changes the mixture density (more air and fuel molecules are packed together). This increases the peak pressure and temperature. The pressure and temperature can get so high that the remaining unburned mixture ignites by itself at the hottest part in the combustion chamber. This self-ignition happens explosively and is called 'knock'. All engines knock somewhat. If there is very little unburned mixture remaining when it self-ignites, the explosion of that small amount does not cause any problems because it can't create a large, sharp pressure peak. Igniting the mixture later (retarding) causes the peak pressure to be much lower and cures the knock.

The advances in power of modern engines, despite the lower quality of gasoline today, comes partially from improvements in combustion chamber and spark plug location. Modern engines are optimized so that the flame front has the least distance to travel and consumes the mixture as fast as possible. An already burned mixture can no longer explode and therefore higher compression ratios are possible with lower octane fuel. Some race or high performance engines actually have 2 or three spark plugs to ignite the mixture from multiple points. This is done so that the actual burn time is faster with multiple flame fronts. Again, this is to consume the mixture faster without giving it a chance to self-ignite.

Higher octane fuel is more resistant to self-ignition. It takes a higher temperature and pressure to cause it to burn by itself. That's why race fuels are used for engines with high compression or boost. Lead additives have been used, and are still used to raise the self-ignition threshhold of gasoline, but lead is toxic and therefore no longer used for pump-gas. Of course a blown engine is toxic to your wallet.

KnockLink Bosch sensor 0-261-231-006