why only 2?
Outstanding HDF Member
Joined: Apr 2006
Posts: 2,395
Likes: 25
From: St George, UT
My buddy's Nomad has 4 plugs too. It does make a difference. In airplanes with piston engines, there are two ignition systems. Turn one off, and you get about a 150 rpm drop. Most light aircraft engines are governed to about 2750 rpm so a good flame front is essential to efficient fuel burn. It would be a good improvement in gas mileage if HD engineered two plugs per cylinder.
Cruiser
Joined: May 2009
Posts: 175
Likes: 1
From: Redding, CA
BMW went back to the twin-plug design. My 2004 R1150RT and my 2006 R1200GSA both had twin plugs. Even with higher compression than a Harley, it only needed mid-grade gas, possibly related to the twin-plug?
Outstanding HDF Member
Joined: Jul 2006
Posts: 2,380
Likes: 9
From: Texas
My buddy's Nomad has 4 plugs too. It does make a difference. In airplanes with piston engines, there are two ignition systems. Turn one off, and you get about a 150 rpm drop. Most light aircraft engines are governed to about 2750 rpm so a good flame front is essential to efficient fuel burn. It would be a good improvement in gas mileage if HD engineered two plugs per cylinder.
I would assume in an airplane engine that you refer to, there are two issues in play. First is the safety aspect of having two spark sources in the case of one failing in flight. Not being familiar with the engine design, I would also think the "subcyclinders" are designed to act as two independent burn areas within the cylinder which would explain the reduced HP if one spark source was cut off.
In the late 70's, I was on a team of engineers who worked years in developing various stratified charge enhancements in large industrial reciprocating engines. We were seeking a more efficient engine as just a 1% improvement can mean $ millions over the life of one of these machines.
Road Captain
Joined: Mar 2009
Posts: 702
Likes: 1
From: U.S.S.A.
Actually gas mileage is improved in today's engine by using only one spark park per cylinder and another benefit is the reduction of the NOx pollutants produced during combustion. If you are interested in why, there are many papers available online concerning "stratified charge combustion" which during the 80's and 90's radically changed how engineers approached the most efficient, and less harmful, burn during combustion. It was once thought that having a unified air/fuel mixture equally distributed in the combustion area would allow for the best "burn" but that has since changed. Today, engineers strive to provide a fuel rich mixture in a subcylinder area (usually around the perimeter of the cylinder) and then a center area which is very lean, or almost all air. As the burn flame enters the lean area, the cooling effect creates more CO and CO2 than any of the NOx so the pollutants are less. With this gas rich area, ignition is very easy since this mixture will burn very fast and smoothly so the need for additional spark plugs or exotic spark devices are no longer needed.
I would assume in an airplane engine that you refer to, there are two issues in play. First is the safety aspect of having two spark sources in the case of one failing in flight. Not being familiar with the engine design, I would also think the "subcyclinders" are designed to act as two independent burn areas within the cylinder which would explain the reduced HP if one spark source was cut off.
In the late 70's, I was on a team of engineers who worked years in developing various stratified charge enhancements in large industrial reciprocating engines. We were seeking a more efficient engine as just a 1% improvement can mean $ millions over the life of one of these machines.
I would assume in an airplane engine that you refer to, there are two issues in play. First is the safety aspect of having two spark sources in the case of one failing in flight. Not being familiar with the engine design, I would also think the "subcyclinders" are designed to act as two independent burn areas within the cylinder which would explain the reduced HP if one spark source was cut off.
In the late 70's, I was on a team of engineers who worked years in developing various stratified charge enhancements in large industrial reciprocating engines. We were seeking a more efficient engine as just a 1% improvement can mean $ millions over the life of one of these machines.
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