The Tow Rat
Junior Member
- Joined
- Oct 27, 2023
- Posts
- 14
- Reaction score
- 0
- Location
- Los Angeles
- Ram Year
- 1991, 1995
- Engine
- 5.9 12V VE Pump, 5.9 12V P Pump
Is it true that the purpose of the 24V is entirely to reduce emissions?
Traditionally four valve cylinder heads are used to achieve:
1) Lower reciprocating valvetrain mass per cam lobe.
2) Straight-shot port geometry.
3) Tumble turbulence during the intake stroke.
Until recently I'd never looked under the valve cover of a 24V Cummins, so I'd not realized that the 24 valve heads use a single cam lobe to operate each pair of valves.
Each cylinder on the Cummins 24V has only two cam lobes and two rockers, operating four valves. Each pair of valves uses a rocker arm bridge to transfer the valvetrain drive from the rocker tip to the two valves.
Wow. That seems like a lot of valvetrain mass to move around. And using one cam lobe for two valves? That doubles the load on each cam lobe. Plus the chambers in the 24V head have the intakes and exhausts oriented diagonally. So the intake port pairs are not symmetrical.
Summarizing, this design does not successfully meet objectives #1 -OR- #2, as listed above for four valve cylinder heads!
Forgive me in advance, 24V owners, but this feels like a compromise on the coattails of an afterthought. Maybe that's just because of my own viewpoint. I'm used to thinking about 4 valve engines from the standpoint of volumetric efficiency, not emissions.
I'm more familiar with 4 valve engines that are designed for high volumetric efficiency at high RPM... Like over 3x the RPM of our diesels. Valvetrain forces go as the square of RPM, so (given equivalent reciprocating masses) at 9000 RPM, valvetrain forces are 10 times what they are at 2800 RPM.
In an engine that operates at 9000 RPM, nobody would suggest using rocker arm bridges.* Every valve needs to have its own cam lobe, there are no pushrods, the valves have really narrow stems for minimum weight, the rockers are highly geometry-optimized for minimum inertia, etc.
Back to our diesels, why is everyone (not just Cummins) using 4 valve heads now?
Maybe the diagonal port geometry is a clue.
Normal 4 valve heads use pent-roof chambers with axial valve and port geometry, and symmetrical port pairs on both intake side and exhaust side. The geometry of the ports and chambers is highly conducive to tumble turbulence.
Is the purpose of the four valve chambers with diagonal port geometry, and unsymmetrical ports, to induce swirl turbulence in addition to the usual four valve tumble turbulence? And to thereby achieve lower emissions through better mixture uniformity?
Competitive sled pullers and drag racers almost always use the 12-valve cylinder head, usually with extensive porting and huge valves and an intake shelf that has been milled off. They are not probably worrying about emissions. ;-)
Regards,
The Tow Rat
* Keith Duckworth would be turning over in his grave, and Shoichiro Irimajiri would be laughing out loud.
Traditionally four valve cylinder heads are used to achieve:
1) Lower reciprocating valvetrain mass per cam lobe.
2) Straight-shot port geometry.
3) Tumble turbulence during the intake stroke.
Until recently I'd never looked under the valve cover of a 24V Cummins, so I'd not realized that the 24 valve heads use a single cam lobe to operate each pair of valves.
Each cylinder on the Cummins 24V has only two cam lobes and two rockers, operating four valves. Each pair of valves uses a rocker arm bridge to transfer the valvetrain drive from the rocker tip to the two valves.
Wow. That seems like a lot of valvetrain mass to move around. And using one cam lobe for two valves? That doubles the load on each cam lobe. Plus the chambers in the 24V head have the intakes and exhausts oriented diagonally. So the intake port pairs are not symmetrical.
Summarizing, this design does not successfully meet objectives #1 -OR- #2, as listed above for four valve cylinder heads!
Forgive me in advance, 24V owners, but this feels like a compromise on the coattails of an afterthought. Maybe that's just because of my own viewpoint. I'm used to thinking about 4 valve engines from the standpoint of volumetric efficiency, not emissions.
I'm more familiar with 4 valve engines that are designed for high volumetric efficiency at high RPM... Like over 3x the RPM of our diesels. Valvetrain forces go as the square of RPM, so (given equivalent reciprocating masses) at 9000 RPM, valvetrain forces are 10 times what they are at 2800 RPM.
In an engine that operates at 9000 RPM, nobody would suggest using rocker arm bridges.* Every valve needs to have its own cam lobe, there are no pushrods, the valves have really narrow stems for minimum weight, the rockers are highly geometry-optimized for minimum inertia, etc.
Back to our diesels, why is everyone (not just Cummins) using 4 valve heads now?
Maybe the diagonal port geometry is a clue.
Normal 4 valve heads use pent-roof chambers with axial valve and port geometry, and symmetrical port pairs on both intake side and exhaust side. The geometry of the ports and chambers is highly conducive to tumble turbulence.
Is the purpose of the four valve chambers with diagonal port geometry, and unsymmetrical ports, to induce swirl turbulence in addition to the usual four valve tumble turbulence? And to thereby achieve lower emissions through better mixture uniformity?
Competitive sled pullers and drag racers almost always use the 12-valve cylinder head, usually with extensive porting and huge valves and an intake shelf that has been milled off. They are not probably worrying about emissions. ;-)
Regards,
The Tow Rat
* Keith Duckworth would be turning over in his grave, and Shoichiro Irimajiri would be laughing out loud.
