I was reading through some R&D papers that Honda releases on their website and thought people here would be interested in it. FYI, this is probably gonna be a very long post.
TLDR: Fuel sticks to cylinder walls and ends up in the engine oil.
Since you can't post PDFs on BITOG, here is the link to the site where you can download the paper yourself if you want. The paper is about the development and validation process of internal combustion engines and how different variables affect different outputs. It dives into multiple different area of engine/combustion development, and one of the areas it talks about is oil dilution from fuel. For the paper they create a scatter plot of multiple different engines and their associated oil dilution. Based on the way the paper is written I believe they measured fuel dilution after running the engine on a dyno. Then they correlate back to some 3D-CFD analysis they performed.
There were two listed validation conditions which were as follows, 1st: 2500 rpm, 16 bar Indicated Mean Effective (IMEP) (Turbocharged engine only) and 2nd: 2500 rpm, 10 bar IMEP (Turbocharged & Naturally Aspirated). For the oil dilution validation conditions, the cylinder walls were held at 50C and the engine controls (valve timing, spark timing fuel injection timing) were held constant/similarly for each engine. Then for some context on IMEP, IMEP is BMEP (Brake Mean Effective Pressure i.e. torque per liter) and FMEP (Friction Mean Effective Pressure) added together, so basically the gross torque output of the engine before internal engine friction is subtracted out. For some additional context on Honda's 1.5L turbo engine, it is rated at a BMEP in the range of 18.2 bar (162ft-lbs of torque) to 21.8 bar (192ft-lbs of torque) depending on application. Based on the IMEP, these engines were being run pretty hard, i.e. not just running unloaded.
The engines from the table below that are of most interest to the people on this site would be engine #6 (1.5 Turbo) and engine #7 (2.0 Turbo). The next two to keep in mind are engine #5 (1.5 N/A with DI) and engine #8 (2.4 N/A with DI). For engine #6, the quantity of fuel dilution they found was a little bit over 5% fuel or about 165-170 grams of fuel in the oil. That percent fuel seems to align quite well with some of the UOA that have been posted to this site. Engine #7 has a similar amount of fuel getting in the oil, but since it has a larger rated oil capacity, almost 5.7qt vs 3.7qt, the dilution percent is much lower at about 3%. That all correlates quite well with the CFD analysis they ran where they measured the amount of fuel adhesion to the cylinder wall. Both engines #6 and #7 had a similar amount of fuel adhere to the cylinder wall in the analysis, so it makes sense a similar amount of fuel was getting in the engine oil.
Not much discussion happens about the mechanism of fuel dilution of the engine oil but it does have quite a nice chart showing it. The main way of fuel getting into the engine oil is through it adhering to the cylinder wall. The factor that contribute to fuel adhering to the cylinder wall is the cylinder wall temperature and the spray pattern of the injector. While some other metrics come into play like tumble ratio/tumble kinetic energy, thermal efficiency, and combustion chamber design. It seems the main factors can be boilied down cylinder wall temperature and injector spray pattern/control.
Some surprises for me in this paper was that the 2.4L engine (Engine #8) and the 1.5L (Engine #4 and #5). Engine #8 had the highest amount of fuel getting in the oil per this paper. But I don't remember too much chatter about oil dilution on that engine, maybe I'm just not remember it. Then when you compare engine #4 and #5, you can also see how adding direct injection increase fuel in the oil. But it also shows that, there is more than just cylinder wall fuel adhesion that goes into engine oil dilution even though it is the main contributor. When you look at the fuel adhesion to the cylinder wall between engine #4 and #5, #5, the direct injected engine, actually has less fuel adhereing to the cylinder wall but still has more engine oil dilution from fuel. So there must be more going on there.
Towards the end of the paper, it discusses a little about design and controls options to reduce engine oil dilution on the 2.0T. For that discussion, they changed the injection pressure from 10MPa to 30MPa while also increasing the number of injection events from 1 to 3. According to the chart, they were able to reduce the oil dilution by about 42% with those changes. That seems like might have been a good trade off to reduce engine oil dilution from fuel. So I wonder why they wouldn't have gone that route. Maybe injector wouldn't have last as long as needed doing 3 times the number of injection pulses.
Hopefully we can all learn something from this.
TLDR: Fuel sticks to cylinder walls and ends up in the engine oil.
Since you can't post PDFs on BITOG, here is the link to the site where you can download the paper yourself if you want. The paper is about the development and validation process of internal combustion engines and how different variables affect different outputs. It dives into multiple different area of engine/combustion development, and one of the areas it talks about is oil dilution from fuel. For the paper they create a scatter plot of multiple different engines and their associated oil dilution. Based on the way the paper is written I believe they measured fuel dilution after running the engine on a dyno. Then they correlate back to some 3D-CFD analysis they performed.
There were two listed validation conditions which were as follows, 1st: 2500 rpm, 16 bar Indicated Mean Effective (IMEP) (Turbocharged engine only) and 2nd: 2500 rpm, 10 bar IMEP (Turbocharged & Naturally Aspirated). For the oil dilution validation conditions, the cylinder walls were held at 50C and the engine controls (valve timing, spark timing fuel injection timing) were held constant/similarly for each engine. Then for some context on IMEP, IMEP is BMEP (Brake Mean Effective Pressure i.e. torque per liter) and FMEP (Friction Mean Effective Pressure) added together, so basically the gross torque output of the engine before internal engine friction is subtracted out. For some additional context on Honda's 1.5L turbo engine, it is rated at a BMEP in the range of 18.2 bar (162ft-lbs of torque) to 21.8 bar (192ft-lbs of torque) depending on application. Based on the IMEP, these engines were being run pretty hard, i.e. not just running unloaded.
The engines from the table below that are of most interest to the people on this site would be engine #6 (1.5 Turbo) and engine #7 (2.0 Turbo). The next two to keep in mind are engine #5 (1.5 N/A with DI) and engine #8 (2.4 N/A with DI). For engine #6, the quantity of fuel dilution they found was a little bit over 5% fuel or about 165-170 grams of fuel in the oil. That percent fuel seems to align quite well with some of the UOA that have been posted to this site. Engine #7 has a similar amount of fuel getting in the oil, but since it has a larger rated oil capacity, almost 5.7qt vs 3.7qt, the dilution percent is much lower at about 3%. That all correlates quite well with the CFD analysis they ran where they measured the amount of fuel adhesion to the cylinder wall. Both engines #6 and #7 had a similar amount of fuel adhere to the cylinder wall in the analysis, so it makes sense a similar amount of fuel was getting in the engine oil.
Not much discussion happens about the mechanism of fuel dilution of the engine oil but it does have quite a nice chart showing it. The main way of fuel getting into the engine oil is through it adhering to the cylinder wall. The factor that contribute to fuel adhering to the cylinder wall is the cylinder wall temperature and the spray pattern of the injector. While some other metrics come into play like tumble ratio/tumble kinetic energy, thermal efficiency, and combustion chamber design. It seems the main factors can be boilied down cylinder wall temperature and injector spray pattern/control.
Some surprises for me in this paper was that the 2.4L engine (Engine #8) and the 1.5L (Engine #4 and #5). Engine #8 had the highest amount of fuel getting in the oil per this paper. But I don't remember too much chatter about oil dilution on that engine, maybe I'm just not remember it. Then when you compare engine #4 and #5, you can also see how adding direct injection increase fuel in the oil. But it also shows that, there is more than just cylinder wall fuel adhesion that goes into engine oil dilution even though it is the main contributor. When you look at the fuel adhesion to the cylinder wall between engine #4 and #5, #5, the direct injected engine, actually has less fuel adhereing to the cylinder wall but still has more engine oil dilution from fuel. So there must be more going on there.
Towards the end of the paper, it discusses a little about design and controls options to reduce engine oil dilution on the 2.0T. For that discussion, they changed the injection pressure from 10MPa to 30MPa while also increasing the number of injection events from 1 to 3. According to the chart, they were able to reduce the oil dilution by about 42% with those changes. That seems like might have been a good trade off to reduce engine oil dilution from fuel. So I wonder why they wouldn't have gone that route. Maybe injector wouldn't have last as long as needed doing 3 times the number of injection pulses.
Hopefully we can all learn something from this.
