Question Why the need for two oil filters in DCT transmissions?

[GLC]

I've been trying to think of the reason why Honda decided a second oil filter is necessary for DCT transmissions. Clearly I am not technically advanced enough to answer this question myself, but if the engine and gearbox are using the same oil with only one filling point, then clearly the oil that is going around the crankshaft, up the cylinder walls and around the valve gear, is the same oil that is going into the gearbox, just like all other Honda motorcycle engines. So if it is the same oil from the same sump, why does it need two oil filters? Enlighten me someone please.

 

[lootzyan]

The primary task of the oil filter is to retain fine metal and other hardened particles from getting on the surface of engine cylinders, valves and bearings. Perhaps it has been found that DCT is more susceptible to wear than normal transmission and hence the extra protection.

 

[Rabbit]

Honda found after intense study that people who ride motorcycles with 2 oil filters buy twice as many oil filters as those who don’t and this increases their profit margin. . Honestly though lootzyan is likely right. The dct may be more susceptible to wear because of its dual clutch mechanism.

 

[lootzyan]

...The dct may be more susceptible to wear because of its dual clutch mechanism.

I think this is more due to the use of a shift motor rather than a "gentle human foot touch" to engage the gears. The speed and force of the engagement. In addition, the frequency of gear changes is greater. This is due to the fact that the PCM tries to predict from the way you ride which gear should be engaged.

 

[GLC]

Honda found after intense study that people who ride motorcycles with 2 oil filters buy twice as many oil filters as those who don’t and this increases their profit margin. . Honestly though lootzyan is likely right. The dct may be more susceptible to wear because of its dual clutch mechanism.

Hi Rabbit, Lootzyan, Hmmm, I'm not convinced of either the increased filter sales stratagem, or the greater wear hypothesis. I'll leave the former for the cynics, but on the latter, if you have two filters filtering the same oil and, in this case, the DCT filter is claimed to be a finer filter to protect the gears from smaller metal particles, then by definition it makes the primary oil filter obsolete since the finer DCT filter is filtering everything to a greater level than the primary oil filter. If there were only one clutch, that clutch mechanism working the same 6 gears as the DCT (with the DCT being simply two clutches, one working gears 1,3 and 5 while the other works 2,4 and 6), then why would there be increased wear in a DCT working half the time of a single clutch system? No, there has to be something more to it than that. I think we have to go back to the drawing board on this one and in particular address the question of two filters for one common volume of oil. Of course, if the DCT worked in a closed oil system unconnected to the rest of the engine, then of course you would have two oil filters, but you would also have two drain plugs and two filler caps, just like you have separate systems for secondary gear systems like Harley. Perhaps I need to call Honda technical support but hoping you smart guys in the Forum can come up with the answer before I have to revert to searching the bowels of Honda HQ. Now, about the double filter sales theory.... Could you be right? Let's hope not.

 

[lootzyan]

...No, there has to be something more to it than that. I think we have to go back to the drawing board on this one and in particular address the question of two filters for one common volume of oil.

You are well on your way to exploring how the original NC DCT concept came about. There are certainly still technical bulletins from the designers of the first generation of DCT like this fragment:

 

[GLC]

Hi Guys, we are on our own to figure this out. I called Honda Powersports HQ in California and all they could do was refer me to my local dealer. They have no technical or design department in all of North America apparently and have no contact with Honda in Japan, if you can believe that (which I find hard), so couldn't give me any details of any engineering department to email in Japan.
Meantime I got this explanation off Bing: "Honda DCT motorcycle engines require a separate oil filter for the DCT mechanism because the DCT system has its own oil circuit that is separate from the engine oil circuit. The oil filter for the DCT system is located in the oil pump cover and is separate from the engine oil filter. The DCT system has its own clutch pack and hydraulic actuators that require clean oil to function properly. The DCT oil filter is designed to remove any contaminants that may be present in the DCT oil circuit and keep it clean." Apparently this came from the Africa Twin forum.

So, then I got hold of the service department at Delmarva Powersports in Salisbury MD and they confirmed to me that not only are there two filters but also two separate oil pumps, one dedicated to the DCT circuit and one for the engine circuit. So, though there is only one source of oil, one drain point and one filler point, internally there are two separated oil circuits, each with their own pumps and filters, but drawing off the same source of oil in the sump, to which both circuits return their oil. I guess that leaves only one question left, namely, why did the Honda engineers feel it necessary to have two dedicated oil delivery systems? Above the pay grade at the dealership I think and certainly above mine, but I guess someone at Honda knows, just no one in Honda North America that can tell me.

 

[lootzyan]

...I called Honda Powersports HQ...They have no technical or design department in all of North America apparently and have no contact with Honda in Japan, ...

They don't have access to the Service Manual because all of these have been sold out and are no longer available...

 

[GLC]

Thanks very much for that diagram Lootzyan, which confirms everything in our previous thread. It would also seem to imply that the DCT filter is a finer filter than the regular oil filter otherwise, what would be the point? That makes me think about oil filter change intervals. The regular filter is every 8000 miles but the DCT every 16,000 miles. One might think that if it is a finer filter it would clog quicker than the regular filter and so need changing more frequently, but apparently that is not the case as evidenced by the aforementioned schedule. As always in life, an answered question leads to another question yet to be answered.

 

[670cc]

Thanks very much for that diagram Lootzyan, which confirms everything in our previous thread. It would also seem to imply that the DCT filter is a finer filter than the regular oil filter otherwise, what would be the point? That makes me think about oil filter change intervals. The regular filter is every 8000 miles but the DCT every 16,000 miles. One might think that if it is a finer filter it would clog quicker than the regular filter and so need changing more frequently, but apparently that is not the case as evidenced by the aforementioned schedule. As always in life, an answered question leads to another question yet to be answered.

The regular oil filter change interval is every 16,000 miles on later models NC700/NC750X. I don’t remember what year Honda changed the engine oil filter interval to every other oil change. The engine filter interval nowadays is the same as the the clutch filter. See attachment, from a 2020 owner’s manual.

 

[dduelin]

I'm just guessing the reason for a DCT filter is for packaging and commonality of manufacturing a MT and DCT version off of one engine/transmission platform. (reducing production cost by sharing the same power unit between models).

It appears the oil supply is common at the intake strainer then splits before simultaneously entering the engine oil pump and the DCT oil pump. The DCT filter is placed just upstream of the linear solenoid valves that control which clutch set is engaged or disengaged. These valves control oil pressure to the two hydraulic pistons which drive the clutch plates together, one piston to each clutch pack. One piston is pressurized at a time. The shift control system is "playing with fire" by having one gear engaged and driving, clutch engaged, and the next gear engaged but not driving, clutch disengaged, and as such is dependent on the control valves operating with precise timing to disengage and engage the two clutches. 500 milliseconds is needed to complete the diversion of oil pressure from one clutch to the other. Bad timing and the shock loads to the gears and transmission shaft bearings are destructive at worse and extremely noisy at least. My guess is that Honda wanted the cleanest oil possible entering the linear solenoid valves and Honda's design allows both MT and DCT to share a common power unit with all the DCT hydraulic control system components under the right hand engine cover outside of the common engine case. It presents a plumbing problem to use a passage from the engine oil filter for DCT control without manufacturing a separate crankcase or a different location for the engine oil filter. Honda designed a separate oil pump for the MT and DCT models but both are drven off a common balancer shaft within the crankcase. After passing through the DCT clutch system the oil lubricates the transmission mainshaft and countershaft and the gears riding on them before mixing again with engine oil in the sump.

 

[T.S.Zarathustra]

ChatGPT says this.

"The Honda NC750 series, particularly the models equipped with the Dual Clutch Transmission (DCT), features two oil filters for specific reasons. The DCT system consists of two separate clutches that engage and disengage gears in an automated fashion, allowing for seamless gear shifts.

The primary reason for having two oil filters in the DCT transmission is to ensure optimal filtration and to maintain the cleanliness of the transmission oil. The DCT system operates under high loads and requires a clean and properly lubricated environment to function efficiently. The oil filters work to remove contaminants, such as metal particles or debris, from the transmission oil, preventing them from circulating and potentially causing damage to the transmission components.

Having two filters allows for enhanced filtration capacity and efficiency. The filters may have different specifications, with one filter designed to capture larger particles and the other focusing on smaller particles. This dual-filter setup ensures a comprehensive filtration process, providing effective protection for the DCT system.

By maintaining clean oil, the two oil filters contribute to the longevity and reliability of the DCT transmission, helping to optimize its performance and durability. It is recommended to follow the manufacturer's guidelines for oil filter replacement and maintenance intervals to ensure proper functioning of the DCT system."

Not really answering the question so I asked it to regenerate the answer.

"
The Honda NC750 models equipped with a Dual Clutch Transmission (DCT) feature two oil filters for specific reasons related to the transmission system. Here's an overview of why there are two oil filters in DCT transmissions:

1. Main Transmission Oil Filter: The primary oil filter is responsible for filtering the transmission fluid used in the main gearbox. It removes contaminants such as dirt, metal particles, and debris that can accumulate over time and affect the transmission's performance and longevity. This filter helps maintain the cleanliness of the transmission fluid and promotes smooth gear shifting.
2. Clutch Circuit Oil Filter: The secondary oil filter is dedicated to filtering the hydraulic fluid that operates the clutches in the DCT system. The DCT employs two clutches—one for odd gears and another for even gears—to facilitate seamless shifting. The clutch circuit filter helps keep the hydraulic fluid clean, ensuring proper clutch engagement and disengagement.

By having separate filters for the main transmission and the clutch circuit, each filter can focus on filtering the specific fluids they encounter, optimizing filtration efficiency. This helps maintain the overall performance and reliability of the DCT system.

Regular maintenance, including timely oil and filter changes, is crucial to ensure the DCT system operates smoothly and remains in good condition. It's recommended to consult the owner's manual or reach out to a Honda dealership for specific maintenance guidelines and intervals for your NC750's DCT transmission."

A little better but I don't think AI is going to take over the world anytime soon.

 

[FIRE UP]

"A little better but I don't think AI is going to take over the world anytime soon."
Ha!
If you (or anyone reading this) didn't watch it, you should find a copy of tonights *60 minutes*. The first half of it is all about "A.I." It's almost down right scary as to what kind of level it has advanced to in our life time. And it's RAPIDLY advancing by the second! Machines teaching themselves!
Scott

 

[670cc]

That first ChatGPT explanation reads like something made up by someone who doesn’t know the real answer. The second answer is closer but contains at least two errors and a potentially misleading term ( hydraulic fluid). It talks about the two filters filtering the specific fluids they filters encounter - uh, they both filter the same fluid, just at different times.

dduelin’s human generated answer is far superior to the AI’s answer.

 

[potter0o]

The AI stuff is interesting. My understanding is that it cannot be relied on to be factual. The second response is a perfect example as it talks about filtering hydraulic fluid.

 

[lootzyan]

They already have a name for it:
AI hallucinations - "a confident response by an AI that does not seem to be reliable by its training data". But a lot of people will fall for it.

 

[GLC]

ChatGPT says this.

"The Honda NC750 series, particularly the models equipped with the Dual Clutch Transmission (DCT), features two oil filters for specific reasons. The DCT system consists of two separate clutches that engage and disengage gears in an automated fashion, allowing for seamless gear shifts.

The primary reason for having two oil filters in the DCT transmission is to ensure optimal filtration and to maintain the cleanliness of the transmission oil. The DCT system operates under high loads and requires a clean and properly lubricated environment to function efficiently. The oil filters work to remove contaminants, such as metal particles or debris, from the transmission oil, preventing them from circulating and potentially causing damage to the transmission components.

Having two filters allows for enhanced filtration capacity and efficiency. The filters may have different specifications, with one filter designed to capture larger particles and the other focusing on smaller particles. This dual-filter setup ensures a comprehensive filtration process, providing effective protection for the DCT system.

By maintaining clean oil, the two oil filters contribute to the longevity and reliability of the DCT transmission, helping to optimize its performance and durability. It is recommended to follow the manufacturer's guidelines for oil filter replacement and maintenance intervals to ensure proper functioning of the DCT system."

Not really answering the question so I asked it to regenerate the answer.

"
The Honda NC750 models equipped with a Dual Clutch Transmission (DCT) feature two oil filters for specific reasons related to the transmission system. Here's an overview of why there are two oil filters in DCT transmissions:

1. Main Transmission Oil Filter: The primary oil filter is responsible for filtering the transmission fluid used in the main gearbox. It removes contaminants such as dirt, metal particles, and debris that can accumulate over time and affect the transmission's performance and longevity. This filter helps maintain the cleanliness of the transmission fluid and promotes smooth gear shifting.
2. Clutch Circuit Oil Filter: The secondary oil filter is dedicated to filtering the hydraulic fluid that operates the clutches in the DCT system. The DCT employs two clutches—one for odd gears and another for even gears—to facilitate seamless shifting. The clutch circuit filter helps keep the hydraulic fluid clean, ensuring proper clutch engagement and disengagement.

By having separate filters for the main transmission and the clutch circuit, each filter can focus on filtering the specific fluids they encounter, optimizing filtration efficiency. This helps maintain the overall performance and reliability of the DCT system.

Regular maintenance, including timely oil and filter changes, is crucial to ensure the DCT system operates smoothly and remains in good condition. It's recommended to consult the owner's manual or reach out to a Honda dealership for specific maintenance guidelines and intervals for your NC750's DCT transmission."

A little better but I don't think AI is going to take over the world anytime soon.

Thanks for the ChatGPT comments. I have to agree with your appraisal. It's very good at telling me what the DCT system actually does but doesn't actually answer the question as to why the need for two filters and two oil pumps and two different oil circuits using a single source of oil returning to the same sump. Putting my Sherlock Holmes hat on I have to conclude that as there is a second oil pump designated for the DCT with a separate oil circuit, the only reason for having a second pump and separate circuit in such a small engine must be that the pumps are pumping oil at different pressures, otherwise a single pump would suffice. It would appear that the DCT requires oil at probably a higher pressure than the rest of the engine and perhaps therefore the filter associated with it has a higher spec to cope with the higher pressure than the standard filter. Then it would all make sense. So perhaps the DCT actuators require a high pressure oil feed. I guess I'll have to leave it at that unless someone can take the subject further and explain how the DCT actuators physically work.

 

[GLC]

Just a P.S. here. Looking again at Lootzyan's schematic it does appear that the DCT oil pump is physically much bigger than the engine oil pump, hence my proposition that the pressure requirement for the DCT oil system is much higher than for the rest of the engine. QED??

 

[lootzyan]

...the pressure requirement for the DCT oil system is much higher than for the rest of the engine. QED??

The DCT has two hydraulically operated clutches, so it is obvious that it needs more oil pressure. The engine oil pump is only used to lubricate the cylinder surfaces and bearings.

 

[T S N]

Hydraulic systems require finer filtering, due to smaller clearances in the systems than in an engine or transmission.
Same fluid in both. Fluid used in the hydraulic system is referred to as hydraulic fluid. Particle sizes that would be ok in an engine, or transmission, could stop a hydraulic system.