Ignition on engines with two plugs per cylinder

Free and Open Source Engine Management discussions excluding more specific things on this board.
User avatar
johu
QFP80 - Contributor
Posts: 57
Joined: Sat Aug 16, 2008 4:39 pm
Location: Se oli ainakin näiiiiiin iso!

Ignition on engines with two plugs per cylinder

Post by johu »

There's various engines on market with two spark plugs, 2004 and up Chrysler Hemi engines for example. Chrysler says they use two plugs to keep emissions low, but I've been wondering if there might be power or mileage benefits as well? I'd be also interested on comments regarding any gains or losses depending on if both plugs are fired concurrently vs. creating longer spark duration by adding small delay between. Any potential benefits probably also depend on shape of combustion chamber and how flame front travels inside. Some cylinder heads intended for race use for same engine blocks have only one spark plug hole machined which might suggest that at least for power there's no gains from two plugs vs single.

There appears to be patent (6814065) on idea how to implement delay for second plug without doubling ignition outputs on ECU with some comments that dual plug benefits could be optimized with precise timing between two sparks.
User avatar
Fred
Moderator
Posts: 15431
Joined: Tue Jan 15, 2008 2:31 pm
Location: Home sweet home!
Contact:

Re: Ignition on engines with two plugs per cylinder

Post by Fred »

One reason for having twin plugs is with a large bore to get a decent flame propagation speed. I think that's why some old jap 4 cylinders used it before they tightened up on quench and used swirl and ports to get the speed up. The faster the flame front fills the chamber the less advance you need to get correct PCP and the less backward work is done by the flame and the more torque you get with less chance of detonation. It would depend on the geometry as to which timing was best, but most would probably be best with the same timing I would imagine. That's an unfounded statement though :-)

Fred.
DIYEFI.org - where Open Source means Open Source, and Free means Freedom
FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!
User avatar
BenFenner
LQFP144 - On Top Of The Game
Posts: 360
Joined: Wed Jul 09, 2008 3:15 pm

Re: Ignition on engines with two plugs per cylinder

Post by BenFenner »

A similarly unfounded statement:

Misfires at lower rpms happen under normal engine operation and increase harmful emissions. It's a fact of life. Multiple sparks from a single spark plug can be used to give twice the chance that the mixture is ignited. Similarly, two spark plugs provide this same x2 chance of the mixture being ignited and have the added advantage of being able to be fired at the same time to retain ignition timing integrity. Assuming a single coil for each plug, firing at the same time should have no ill effects. Firing the second plug with a delay doesn't immediately present an upside to me. =/
User avatar
Fred
Moderator
Posts: 15431
Joined: Tue Jan 15, 2008 2:31 pm
Location: Home sweet home!
Contact:

Re: Ignition on engines with two plugs per cylinder

Post by Fred »

The bits about misfire aren't unfounded! They are right on the money. Good thinking :-)

The bit about "no upside seen" is a nice way of putting what I was thinking too. I also can't see a benefit to them being staggered, but someone else might :-)

Fred.
DIYEFI.org - where Open Source means Open Source, and Free means Freedom
FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!
User avatar
jharvey
1N4001 - Signed up
Posts: 1607
Joined: Tue Jun 10, 2008 5:17 pm

Re: Ignition on engines with two plugs per cylinder

Post by jharvey »

Also note, air planes use 2 plugs to increase reliability.
shameem
LQFP112 - Up with the play
Posts: 135
Joined: Thu May 01, 2008 6:30 pm
Location: Ann Arbor, MI
Contact:

Re: Ignition on engines with two plugs per cylinder

Post by shameem »

If you have two plugs per cylinder then you have the unique opportunity to either mess with ion sensing or connect a pressure sensor to the second port.....
GrowlingandBiffo
QFP80 - Contributor
Posts: 56
Joined: Sat Apr 26, 2008 7:42 am

Re: Ignition on engines with two plugs per cylinder

Post by GrowlingandBiffo »

jharvey wrote:Also note, air planes use 2 plugs to increase reliability.
They use two plugs because they have two independent ignition systems.
User avatar
longracing
LQFP112 - Up with the play
Posts: 140
Joined: Wed Jul 16, 2008 9:21 am
Location: NSW, Australia

Re: Ignition on engines with two plugs per cylinder

Post by longracing »

Aircraft use a dual-magneto ignition system for redundancy. There is a 2-300rpm drop when each one is switched off at alternate times during the pre-takeoff checks.
Aircraft engines also have large diameter bores and operate a relatively low rpm, <3000rpm. The twin sparks also increase flame propogation, which is useful at low-rpms with a large bore.
User avatar
Fred
Moderator
Posts: 15431
Joined: Tue Jan 15, 2008 2:31 pm
Location: Home sweet home!
Contact:

Re: Ignition on engines with two plugs per cylinder

Post by Fred »

longracing wrote:The twin sparks also increase flame propogation, which is useful at ALL-rpms with a large bore.
Fixed or broken? That is always a good thing isn't it? I think so because it means less negative work done during the compression stroke and therefore more power.
DIYEFI.org - where Open Source means Open Source, and Free means Freedom
FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!
GrowlingandBiffo
QFP80 - Contributor
Posts: 56
Joined: Sat Apr 26, 2008 7:42 am

Re: Ignition on engines with two plugs per cylinder

Post by GrowlingandBiffo »

Fred wrote:The bits about misfire aren't unfounded! They are right on the money. Good thinking :-)

The bit about "no upside seen" is a nice way of putting what I was thinking too. I also can't see a benefit to them being staggered, but someone else might :-)

Fred.
They used to stagger them on the Aston-Martin Zagato, 12 Plug Head. Lord Smail of Milk 'n Beans (Jelly Mould) said that he would be a yard or two faster out of a corner with the 12 plug head.

However two plugs are often only really useful to cover up combustion chamber inadequacies. Cosworth built an engine with 3 plugs per cyl, but then dropped the idea when they could find no more benefit, after much development and reverted back to one.
Post Reply