Cummins "Low flow" system, in a nutshell

[Geoff Weeks]

Coupla people remarked about this on Freightrain thread, rather than muck it up I thought I would start anew.

I had a Lowflow truck in the fleet until retirement and got to know and understand the system well.

The 1st thing to understand is there are basically 2 loops in the system that run in parallel. one is the coolant that is in the block and one is the coolant that runs through the after cooler radiator and oil cooler. The all come together at the water pump and mix there.

The term "low flow" is kinda misleading. The total flow is the same or higher than a normal system. The term refers to only the amount that is passing through the radiator. The radiator is a two pass core, where coolant enters at the BOTTOM goes up to the top tank and across to the other side and back to the outlet again at the bottom. There is a divider in the bottom tank to keep the sections separate. 

 

That divider must be there and seal or the engine will quickly over-heat as coolant will bypass the tubes, and just flow across the bottom tank and back to the engine. As heat moves quicker from a higher temp to a much lower temp, the side where the coolant enters and rises to the top tank is most often smaller then the side where it descends to the bottom tank and out. This allows for much of the heat to be lost when the temp difference is greatest on the "hot side" and then a larger area on the cool side to continue to bring the temp down.

 An engine at full load only has to loose 10 deg of coolant temp to not over heat. That is why in conventional systems, the shutter thermostat when in the lower tank is the same as the engine thermostat. So when the engine (180 deg) 'stat is full open, (190 deg) the shutters will be full open when the return temp is 180 deg. A conventional cooling system can do this if designed for the load. So a working engine, the return temp of the coolant will be 180 deg. IF you try and use this to cool the air charge from the turbo, it can never get very cool. If, however, you could reduce the returning coolant lower, say 140 deg or 160 deg, you can remove more heat from the air charge. 

 Before low flow or air to air, often 170 deg 'stats were used to try and reduce the returning coolant temp, but this made the engine really too cool for best combustion. These early coolant based systems always took the aftercooler supply from the lowest point in the block to get the coolest water.

 In the Cummins system, the two loops are in parallel, that means any restriction in one loop, causes more coolant to circulate in the other. There are restrictor plates in the outlet of each head into the upper water rail. The upper water rail goes straight back to the inlet of the water pump, much like a conventional system with a weirstat, where when "closed" the coolant is returned to the water pump. This keeps the coolant flowing at all times and prevents hot spots and promotes better temp control by the thermostat as it has flow over the bulb at all times.

Where the low flow system start to get complicated, is the coolant from the block to the radiator and from the radiator back to the block. The "outlet" coolant come from the oil cooler on the side of the block. It then goes to the thermostat housing where there are two thermostats, one that is full open when cool, and closes on temp rise, and one that is full closed when cool and opens on temp rise. The return flow to the block (inlet of water pump) can either be through the bypass thermostat and/or the charge cooler. When block temps are low, the bypass 'stat is fully open and the all the coolant circulates in the block, as temps start to rise, the bypass starts to close . once it reaches about 160 deg the bypass 'stat is fully closed and all returning coolant to the block must go though the aftercooler. Once the coolant from the block/oil cooler reaches 160 the main 'stat starts to open and send some of the coolant to the radiator. The return from the radiator must pass through the aftercooler before going to the waterpump, where it mixes with the hot coolant from the upper water rail, and gets sent back to the bottom of the block. Mixing of the hot upper water rail coolant and the coolant from the radiator via the after cooler reduces the temp of the returning coolant to the block enough so it can keep the engine cool.

 There are some "trouble points" with the system. 1st any restriction increase on one of the loops diverts more of the coolant to circulate in the other loop. So radiator tubes that a partially plugged reduce the flow in that circuit and more will stay in the block and not be cooled. 2nd the radiator tubes must be smaller (dimpled) to slow the flow through the radiator enough to loose the maximum heat before returning, but still flow enough to provide enough return coolant to lower the block coolant temps (this is where "low-flow term comes from).  3rd, the water pump must be able to move a lot of coolant without aeration. 4th the block pressure in the coolant jacket in a low flow system at 2100 rpm is 46 psi! Chew on that for a moment! The radiator never sees 46 psi because of the restrictions in the system.

The 1st Big Cam 4's (low flow) look very much like the Big Cam 3's (high flow) and many didn't see the difference and mixed parts with big problems as the result. The upper water rail gaskets and water pump are different. The water pump looks the same and takes the same housing, but the pulley is smaller and the top of the housing is vented to move any air out of the system. Put a Big Cam 3 water pump on, and it will work, but not cool well and over heat when working hard.

 The upper water rail O rings have restrictor plates that control how much coolant returns to the inlet of the water pump. Some when seeing this, think it will cool better without the plates or with a bigger hole, when the opposite is true. Opening up the hole allow more hot coolant to return to the inlet of the waterpump and less to make it to the radiator.

With the NT88, the upper water rail and water pump (as well as thermostats) changed design to where it is obvious that you can not use the Big Cam 3 and down stuff on the block. To be able to have 46 psi block pressure the water pump belt had to carry a high load and not slip, multi groove belts were used (no change from early big cam) but were less tolerant of being run while loose and would slip at higher rpm/load. Multi rib belts can not suffer slipping long before they shred.  

Does it all work? Yes! With a large enough radiator and a good, clean system, the charge temp is reduced, the block temp is increased and the fuel mileage is between .5-1 mpg better with the same load.

Likely more than you wanted to know about Low flow, much of what I know about cooling came from Glenn Akers, the rest from hard knocks owning a low flow for 30 years and working it hard.

 None of it is as good as an air to air charge cooler, and when trucks were re-designed for enough room for a charge cooler in front of the radiator, the N-14 got an air to air and the low-flow era ended.  

Edited 3 hours ago by Geoff Weeks

[Geoff Weeks]

By comparison, Mack's tip turbine is a more "elegant" solution to the problem, but I suspect more expensive to produce.

I could be wrong about that, the last re-core I did on my Low Flow radiator cost me $2100, the out of chassie rebiuld kit cost $1500 for the engine. 

The need for each went away when space in front of the radiator was provided by the truck Mfg.