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The Tow Rat

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1991, 1995
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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.
 

rzr6-4

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I am far from an expert on diesels so feel free to call me out if I'm out of line here:

Tumble turbulence on the intake is beneficial for gas motors to aid in a better, more consistent air-fuel mixture, helping increase both power and efficiency. That is only possible because the fuel is injected during the intake stroke. Diesels, if I understand correctly, inject fuel the exact moment combustion is desired, completely nullifying any turbulence during the intake stroke.

The small engines you typically work with would have a lot to gain here from that turbulence, while the 24v Cummins, I would think is doing it for some other reason.
 

NETim

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I had an '02 HO 24 valve. Before that a '97 12V.

The 24V wants to rev and it will! The '97 was governed, so 75-80MPH was the best it would do... stock. (auto tranny on the '97)

The injector pump on the 12V is pretty much bulletproof. It is cooled by both engine oil and fuel. The 24V pump is cooled by fuel ONLY. Losing the lift pump on the 24V can be catastrophic for the injector pump.

Both pumps can be tweaked all kinds of ways.

I liked my 24V more. Pulled like the proverbial freight train. I had a mild box on it. Nothing wild.
 
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The Tow Rat

The Tow Rat

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1991, 1995
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5.9 12V VE Pump, 5.9 12V P Pump
I am far from an expert on diesels so feel free to call me out if I'm out of line here:

Tumble turbulence on the intake is beneficial for gas motors to aid in a better, more consistent air-fuel mixture, helping increase both power and efficiency. That is only possible because the fuel is injected during the intake stroke. Diesels, if I understand correctly, inject fuel the exact moment combustion is desired, completely nullifying any turbulence during the intake stroke.

The small engines you typically work with would have a lot to gain here from that turbulence, while the 24v Cummins, I would think is doing it for some other reason.
Yes, rzr6-4, that is a really good point. I'm not a diesel expert either, but that is diesel 101 and I should have realized that.

But OK then, how does the 24V improve emissions?? Because I know from reading various forum posts, that was at least part of the reason they switched to the 24V?

Regards,
The Tow Rat
 

NETim

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Yes, rzr6-4, that is a really good point. I'm not a diesel expert either, but that is diesel 101 and I should have realized that.

But OK then, how does the 24V improve emissions?? Because I know from reading various forum posts, that was at least part of the reason they switched to the 24V?

Regards,
The Tow Rat
The injector pump on a 24V is an electronic device whereas on the 12V it's mechanical. Better fuel control with the 24v pump system. And I imagine the 24V head breathes better which probably reduces emissions.

My 24V was a racehorse alongside the 12V though. (But it made about 120 horses.)
 

chri5k

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The high pressure fuel pump on both engines are mechanical. The injectors on the 24v are electronically actuated. This allows more precise fuel injection timing as well a multi-shot capability. Both improve combustion efficiency. This produces more HP and torque, improves fuel efficiency and lowers emissions. This electronic mechanism is also necessary to provide the "late injections" required for the DPF regeneration. So it is not just the valves but the whole system that is improved in the 24v engine.
 
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crash68

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how does the 24V improve emissions?? Because I know from reading various forum posts, that was at least part of the reason they switched to the 24V?
It's all about more airflow, most of the principles for building power in a gasser goes right out the window with a diesel. Basically a diesel engine runs on air(lots of it), the fuel controls how fast it spins. Shoving a high density/pressurized and cooled air into the cylinder make it capable of lower the combination temperatures which reduce things like NOx.
Probably why the pullers like 12V heads is simplification, less parts to break and the ones there can be bigger/stronger. The 12V were mechanically injected which makes it easier to up the power, there's even some people who have mechanically injected 24V engines.
 

Gr8bawana

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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.
Someone has too much time on their hands. :smokin:
 
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The Tow Rat

The Tow Rat

Junior Member
Joined
Oct 27, 2023
Posts
14
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Location
Los Angeles
Ram Year
1991, 1995
Engine
5.9 12V VE Pump, 5.9 12V P Pump
The injectors on the 24v are electronically actuated. This allows more precise fuel injection timing as well a multi-shot capability. Both improve combustion efficiency. This produces more HP and torque, improves fuel efficiency and lowers emissions.
Thanks chri5k,
Sure that makes a lot of sense. I would imagine injection timing, multi-shot, and also improved injectors with finer atomization would all potentially benefit emissions. Maybe the higher injection pressure on the common rail setup even helps too.

Maybe even the central injector location helps, which is the only thing I've heard that is directly connected to the 24 Valve chambers?

crash68,​

Right, agree (sure, gassers run on air too) and theoretically adding more valves allows more curtain area within the same bore size, benefiting airflow. That and reduced reciprocating mass per cam lobe are good reasons for using extra valves on gassers.

But I am reading claims on the forums that the 12V head flows more CFM. Not that that would significantly affect emissions one way or the other, but is this true? Or is this just folklore? The pullers seem to believe it...

Anyway forgive me if I have more misconceptions than understanding about diesels, but how is emissions about airflow? Isn't it about injection timing and atomization and EGR and cats and uniform burn and EGT?

Bottom line, so far I don't get why cummins didn't just build a common rail 12V to reduce emissions. Other than the central injector location...

Regards,
The Tow Rat
 
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The Tow Rat

The Tow Rat

Junior Member
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Ram Year
1991, 1995
Engine
5.9 12V VE Pump, 5.9 12V P Pump
Oh sorry crash68,
I didn't read your post carefully enough. Hmm your point about excess air reducing EGT and thereby reducing NOx add up for sure, and makes a lot of sense.
 
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