4 Valve Head: Single Central Spark Plug vs Dual Spark Plugs

[RDY4WAR]

Ditch the 2nd plug, put the one in the center, and put in bigger valves.

[hoffman900]

The Cosworth CA and other F1 engines of the late 2000s era were reaching 20,000rpm with 98mm bores with a single plug. A big part of that was port injection pressures (rules limited to 100 bar), but in testing they were using 200bar. The classic pentroof design with a single spark plug was more than capable of good combustion (Cosworth's words) with that bore size, at that rpm. I am inclined to believe it also worked well at part throttle openings, something which drag races don't pay attention to, which is where things get really hard.

“Nevertheless, when we first tried bigger bores in the V10 days, we didn’t manage to make them work successfully, because we couldn’t get the combustion right. The necessary mixture preparation was enabled by running higher and higher fuel pressures. By regulation we run at 100 bar now, whereas for a while the CA was running at 200 bar on the dyno. We never raced it at that level (due to the 100 bar rule), but that was what we were developing. We did find performance from it, because the mixture preparation was enhanced, from the higher pressure.” - Bruce Wood, Cosworth

This is covered in Race Engine Technology Issue 073

Honda has a good white paper on developing this system for their V8 at the same time. That's free to read here: https://www.hondarandd.jp/point.php?pid=624&lang=en

Important factors in boosting the performance of today’s Formula One engines include: the realization of the formation of ideal air-fuel mixtures and the achievement of greater combustion efficiency, through the use of shorter fuel injection periods and increased spray atomization resulting from higher fuel pressures; and, in addition to this, the achievement of stable combustion in the low-load operating range.
A comprehensive analysis of injector spray characteristics was conducted, leading to the development of a Honda-made high-efficiency, high-pressure fuel supply system. This enabled the achievement of a 15 kW increase in engine power

also if you want to read about combustion measuring of those engines: https://www.hondarandd.jp/point.php?pid=616&lang=en

Combustion diagnosis of a Formula One engine during wide open throttle (WOT) acceleration and deceleration operations was performed using a micro-Cassegrain system.
The air/fuel mixture (A/F) in each cylinder was measured, which helped in the development of controls to minimize the torque loss due to unstable combustion.
In transient conditions where acceleration and deceleration are performed repeatedly, the air/fuel mixture around the spark plugs becomes too rich or too lean, and results in unstable combustion. In addition, the air/fuel mixture formation is different inside each cylinder.
The fuel distribution to each cylinder needed to be controlled to a high degree of accuracy, and to do this, a highly responsive air/fuel ratio sensor (LAF sensor) is required.

Audi's DTM 2L engine uses a 88mm bore, 4 valve head with canted valves, direct injection (rules stipulated 35MPa fuel pressure, but their design can handle 50MPa) and a single centrally mounted spark plug. With the utilization of a good amount of tumble, they are able to run 11:4:1 geometric compression with a rule stipulated 3.5 bar boost and 90kg/hr flow rate, on commonly available 102 octane gasoline. They're running these engines at 1.15-1.38 lambda and 8500rpm and have 42% brake thermal efficiency. The dossier on this engine is in Race Engine Technology Issue 136

Modern F1 engines are as follows, using 4 valve pentroof designs with a combination of TJI and HCCI:
Thermal Efficiency of 52%
* BSFC 167gms/kw-hr (0.27lbs/hp-hr), occurs at peak power
* 800bhp from 1.6L (then another ~200hp or so from the hybrid unit)
* Lambda 1.3-1.4. Compare that to the DI / spark plug Audi DTM engine which runs at Lambda 1.15-1.38
* Rules limited 18:1 geometric compression ratio. All are at max
* Spark assisted HCCI, which Honda pretty much showed (earlier in the thread)
* They are Miller Cycle engines, with IVC before BDC
* Separate oil circuit for piston oil jets that runs cooler than the other circuits
* Miller Cycle requires very aggressive intake valve opening / closing designs and a lot of boost
* Valve angles low (5-7*), obviously done for squish geometry
* "Omega" piston bowls. Illustration in video helps visualize that
* Modeling from the FIA shows around 5.5bar boost (~80psi or so) and 50% mass fraction burn by 8* ATDC. Lose 1-2% of MFB in the prechamber. Quick combustion from 2-80% MFB and a slowing combustion beyond that. This final 20% with slowing combustion speed is where knock can occur.
* 102 octane gasoline (specially formulated)

This is all covered here: https://www.youtube.com/watch?v=mUq-K9jcaB8

So from a performance standpoint, I have a hard time looking past a 4 valve pentroof design with a single plug or TJI. Anything else is just done for marketing, packaging, or cost.

[David Redszus]

Ditch the 2nd plug, put the one in the center, and put in bigger valves.

Yes. And pay attention to squish velocity as a function of rpm.

[nicholastanguma]

The Cosworth CA and other F1 engines of the late 2000s era were reaching 20,000rpm with 98mm bores with a single plug. A big part of that was port injection pressures (rules limited to 100 bar), but in testing they were using 200bar. The classic pentroof design with a single spark plug was more than capable of good combustion (Cosworth's words) with that bore size, at that rpm. I am inclined to believe it also worked well at part throttle openings, something which drag races don't pay attention to, which is where things get really hard.

“Nevertheless, when we first tried bigger bores in the V10 days, we didn’t manage to make them work successfully, because we couldn’t get the combustion right. The necessary mixture preparation was enabled by running higher and higher fuel pressures. By regulation we run at 100 bar now, whereas for a while the CA was running at 200 bar on the dyno. We never raced it at that level (due to the 100 bar rule), but that was what we were developing. We did find performance from it, because the mixture preparation was enhanced, from the higher pressure.” - Bruce Wood, Cosworth

This is covered in Race Engine Technology Issue 073

Honda has a good white paper on developing this system for their V8 at the same time. That's free to read here: https://www.hondarandd.jp/point.php?pid=624&lang=en

Important factors in boosting the performance of today’s Formula One engines include: the realization of the formation of ideal air-fuel mixtures and the achievement of greater combustion efficiency, through the use of shorter fuel injection periods and increased spray atomization resulting from higher fuel pressures; and, in addition to this, the achievement of stable combustion in the low-load operating range.
A comprehensive analysis of injector spray characteristics was conducted, leading to the development of a Honda-made high-efficiency, high-pressure fuel supply system. This enabled the achievement of a 15 kW increase in engine power

also if you want to read about combustion measuring of those engines: https://www.hondarandd.jp/point.php?pid=616&lang=en

Combustion diagnosis of a Formula One engine during wide open throttle (WOT) acceleration and deceleration operations was performed using a micro-Cassegrain system.
The air/fuel mixture (A/F) in each cylinder was measured, which helped in the development of controls to minimize the torque loss due to unstable combustion.
In transient conditions where acceleration and deceleration are performed repeatedly, the air/fuel mixture around the spark plugs becomes too rich or too lean, and results in unstable combustion. In addition, the air/fuel mixture formation is different inside each cylinder.
The fuel distribution to each cylinder needed to be controlled to a high degree of accuracy, and to do this, a highly responsive air/fuel ratio sensor (LAF sensor) is required.

Audi's DTM 2L engine uses a 88mm bore, 4 valve head with canted valves, direct injection (rules stipulated 35MPa fuel pressure, but their design can handle 50MPa) and a single centrally mounted spark plug. With the utilization of a good amount of tumble, they are able to run 11:4:1 geometric compression with a rule stipulated 3.5 bar boost and 90kg/hr flow rate, on commonly available 102 octane gasoline. They're running these engines at 1.15-1.38 lambda and 8500rpm and have 42% brake thermal efficiency. The dossier on this engine is in Race Engine Technology Issue 136

Modern F1 engines are as follows, using 4 valve pentroof designs with a combination of TJI and HCCI:
Thermal Efficiency of 52%
* BSFC 167gms/kw-hr (0.27lbs/hp-hr), occurs at peak power
* 800bhp from 1.6L (then another ~200hp or so from the hybrid unit)
* Lambda 1.3-1.4. Compare that to the DI / spark plug Audi DTM engine which runs at Lambda 1.15-1.38
* Rules limited 18:1 geometric compression ratio. All are at max
* Spark assisted HCCI, which Honda pretty much showed (earlier in the thread)
* They are Miller Cycle engines, with IVC before BDC
* Separate oil circuit for piston oil jets that runs cooler than the other circuits
* Miller Cycle requires very aggressive intake valve opening / closing designs and a lot of boost
* Valve angles low (5-7*), obviously done for squish geometry
* "Omega" piston bowls. Illustration in video helps visualize that
* Modeling from the FIA shows around 5.5bar boost (~80psi or so) and 50% mass fraction burn by 8* ATDC. Lose 1-2% of MFB in the prechamber. Quick combustion from 2-80% MFB and a slowing combustion beyond that. This final 20% with slowing combustion speed is where knock can occur.
* 102 octane gasoline (specially formulated)

This is all covered here: https://www.youtube.com/watch?v=mUq-K9jcaB8

So from a performance standpoint, I have a hard time looking past a 4 valve pentroof design with a single plug or TJI. Anything else is just done for marketing, packaging, or cost.

Terrific post, thanks for taking the time!

[nicholastanguma]

Basically, after all this, here's my takeaway: if, like me, you're an afficionado of ol' skool air cooled and carbed machines, especially motos of the kickstart variety, then dual plugs have some very real benefits. However, the more modern an engine becomes, especially once electronically controlled high pressure fuel injection is introduced, the less and less necessary dual plugs are for anything except emissions mitigation.

[Momus]

Single central plugs worked well on the carbed 20000 rpm plus capable 250 and 400 Jap. sports bikes of the 90's.

[ptuomov]

Single central plugs worked well on the carbed 20000 rpm plus capable 250 and 400 Jap. sports bikes of the 90's.

That makes sense.

I would add that I believe the second plug will hurt everything in a pent roof four valve head engine, including emissions, at the power band rpm. The second spark plug may help at low rpms when there is not enough tumble velocity, since the best high performance four valve heads have no squish at all, just tumble. But in the power band, I’d just fire the second plug to prevent it from fouling and fire it late enough to not make any difference to combustion.

[MetricMuscle]

Basically, after all this, here's my takeaway: if, like me, you're an afficionado of ol' skool air cooled and carbed machines, especially motos of the kickstart variety, then dual plugs have some very real benefits. However, the more modern an engine becomes, especially once electronically controlled high pressure fuel injection is introduced, the less and less necessary dual plugs are for anything except emissions mitigation.

My understanding is that the purpose of a dual spark plug set up in a 2-valve engine is to decrease the time it takes for the intake charge to complete it's burn. This allows for less timing advance, which creates less residual heat, which will allow for more compression without detonation. A single centrally located spark plug already does this.

You would only want more timing (ignition advance) for more power, if the heat release rate (dQ/dt) is too low for your specific application. Generally speaking: the higher the heat release rate is, the higher thermal efficiancy will be. Which is exactly what everyone wants, more mechanical work out of chemical heat.

Aluminum as a material is not used to absorb heat. It is used because of its specific weight and thermal conductivity.

Best power happens when peak cylinder pressure is optimal. Timing is what controls this and if peak cylinder pressure can be timed for best power without detonation then no additional spark plug is needed.

[David Redszus]

My understanding is that the purpose of a dual spark plug set up in a 2-valve engine is to decrease the time it takes for the intake charge to complete it's burn. This allows for less timing advance, which creates less residual heat, which will allow for more compression without detonation. A single centrally located spark plug already does this.

A second spark plug adds very little to performance, but was used as far back as 1956 to compensate for unreliable ignition systems.

Aluminum as a material is not used to absorb heat. It is used because of its specific weight and thermal conductivity.

Thermal conductivity and heat absorbtion are two side of the same coin; they always come together.

Best power happens when peak cylinder pressure is optimal.

Not exactly. What we want is maximum cylinder pressure under the curve, located at the proper crank angle. Not peak pressure.

Timing is what controls this and if peak cylinder pressure can be timed for best power without detonation then no additional spark plug is needed.

Actually, ignition timing does not control the combustion pressure curve. The pressure curve is determined by
combustion timing, which must include ignition delay and chamber turbulence.

[nicholastanguma]

My understanding is that the purpose of a dual spark plug set up in a 2-valve engine is to decrease the time it takes for the intake charge to complete it's burn. This allows for less timing advance, which creates less residual heat, which will allow for more compression without detonation. A single centrally located spark plug already does this.

Yes, my understanding as well. Hence the beauty of the Fueling 3 valve head, I'd say.

[nicholastanguma]

Single central plugs worked well on the carbed 20000 rpm plus capable 250 and 400 Jap. sports bikes of the 90's.

My concern with using those motos as examples is the tiny size of their bores. I'm primarily concerned with carbed, larger bore engines. I have absolutely no trouble believing carburetion and small bore sizes work beautifully with single plugs. What I'm looking for is the same confidence in carburetion and big bore sizes with single plugs.

I WANT single, centrally located plugs to be the answer, I'm definitely not looking for reasons to claim dual plugs are the best and only way carbs and air cooling and big bores can work all across the rpm range.

[MetricMuscle]

A second spark plug adds very little to performance, but was used as far back as 1956 to compensate for unreliable ignition systems.

What is the purpose on a modern engine like the ones mentioned above?

Best power happens when peak cylinder pressure is optimal. Not exactly. What we want is maximum cylinder pressure under the curve, located at the proper crank angle. Not peak pressure.

Right, I meant to say "when peak cylinder pressure timing is optimal" which can be manipulated with ignition timing since chamber turbulence is not as easy to manipulate.

[MetricMuscle]

My concern with using those motos as examples is the tiny size of their bores. I'm primarily concerned with carbed, larger bore engines. I have absolutely no trouble believing carburetion and small bore sizes work beautifully with single plugs.

I agree, bore size is what increases the time the charge needs to burn completely, smaller bore size can more easily run higher compression ratios.

[David Redszus]

My concern with using those motos as examples is the tiny size of their bores. I'm primarily concerned with carbed, larger bore engines. I have absolutely no trouble believing carburetion and small bore sizes work beautifully with single plugs.

I agree, bore size is what increases the time the charge needs to burn completely, smaller bore size can more easily run higher compression ratios.

While it is true that plug position determines the length of the flame path, it assumes a quiescent chamber.
The charge is not stationary, waiting for the flame front, as often visualized.

Actually, the squish velocity and direction will produce the largest component of turbulence and thus control
flame speed and combustion angle (or time).

Consider a central single plug, with high squish velocity compared to twin plugs with reduced squish velocity.
Which will perform better? Who knows? It depends on specific chamber design, and several other contributing
factors. Years ago, the solution was to build protypes of each and dyno test them. Slow and expensive process
with a shallow learning curve.

Today, combustion simulation prograams exist that will simulate and visualize the combustion process.
But they are not for the faint of heart or weak of wallet.

[MetricMuscle]

While it is true that plug position determines the length of the flame path, it assumes a quiescent chamber.
The charge is not stationary, waiting for the flame front, as often visualized.

Actually, the squish velocity and direction will produce the largest component of turbulence and thus control
flame speed and combustion angle (or time).

Consider a central single plug, with high squish velocity compared to twin plugs with reduced squish velocity.
Which will perform better? Who knows? It depends on specific chamber design, and several other contributing
factors. Years ago, the solution was to build protypes of each and dyno test them. Slow and expensive process
with a shallow learning curve.

Today, combustion simulation prograams exist that will simulate and visualize the combustion process.
But they are not for the faint of heart or weak of wallet.

A few years ago I did lots of research into squish/quench so I could modify my Honda CRF230F dirtbike head. Even though it is a 2006 model, the engine uses 2nd generation Honda SOHC air cooled 2-valve hemispherical technology from the '70s ish. I completely agree that a properly located single spark plug is the best way to go but not really possible with a 2-valve.

In my quest for information I had to find forums dedicated to designs with the same issues. American V8s, Harleys, Triumphs, Yamaha SR500s and I'm sure Speed-Talk.com. Every once in a while I would stumble into a 2-stroke forum and learn lots of other intricate details before realizing how much more them folks do with squish/quench that we don't have to worry as much about with a 4-stroke. One thing I do remember is how having a slightly OFF-CENTER spark plug location was supposed to be advantageous.

I had my Honda head welded and I machined it but my priorities were to make sure the valves were unshrouded and a target combustion chamber volume, I did not consider squish direction or velocity. Might you have any good articles discussing Squish design and direction?

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