We have been working on the simulation of oil distribution in transmission gears for more than 10 years now.
Until today our simulation kit M&P OilSim® was missing an essential component, which is on top of our customers’ wish list: The consideration of the temperature.
Frictional heat is generated in the gearbox and the oil has the task to transport this heat to the housing in addition to the lubrication. The heat has a significant influence on the viscosity of the oil and thus on the flow behavior.
I had to learn this painfully during my practical training at Dornier in Oberpfaffenhofen in 1983. My old Mercedes 200 D had involuntarily given up the ghost or was forced to be shut down by the TÜV. To be more precise, the bodywork was so dilapidated that even the foot wells filled with concrete did not hold the vehicle together anymore, especially since the TÜV was not amused about this kind of my repair.
A business colleague at Dornier knew a friend who sold a Mercedes 220 D, allegedly in very good condition. When I inspected the vehicle in winter, I was wondering why the engine was already running, but I did not think anything of it. So, I signed the sales contract and drove home with the car. The next winter morning I was surprised that the engine did not start despite the new battery with all the tricks. Unfortunately, I had to find out that the seller had filled in very viscous oil to hide the fact that the compression was at an end. With the viscous oil the compression was better, but the starter was never able to start the cold engine.
As a student I had to write off the paid 4.000 DM, bought it as seen, after all I should have noticed that as a prospective engineer…
And today? Even today oil is damn tough at -20°C.
Following the discussions, we had togehther with BMW, Audi, VW, Porsche, Daimler and at ZF, respectivly, the topic of temperature calculation is on top of the wish list.
So, the most important thing is to optimize the temperature distribution in the components and the cooling of the transmission. The resulting temperature distributions are to be determined on the basis of the power loss in the tooth engagement (which is known) and the cooling on the outside (which can also be determined). And the dependence of the oil viscosity? Must be taken into account, matter of honor.
Although we are not badly positioned in this regard, the problem has been the enormous computing time and the required storage space. But who wants to wait a few months?
Christian, who likes to call himself an old fox, has now rummaged through his drawer and revived an ancient concept from Audi times.
The idea is simple and worked well back then with AURA in calculating the temperature in the car body:
Let us simply assume that the gear wheel rotates faster than the heat spreads and that the flow field is faster than a quasi-stationary temperature distribution.
Is it not sufficient then to calculate the flow at selected points in time and keep it constant in between?
In engineering terms, we are also moving on different time scales. The pure flow in the gearbox moves in the range of milliseconds to seconds, whereas the temperature is sluggish and ranges from minutes to even hours.
Now, if you run a co-simulation with different time scales, it saves a lot of computing time. And memory. And nerves. Says Oli.
Kai recently run a large transient autoclave simulation by means of a co-simulation in Star CCM+ with top results. So why shouldn’t the whole thing work for a transmission gear? At least for “smaller” gears. We still have our doubts about an 8-speed dual-clutch transmission, but I am standing with the whip in the background.
We show an interim solution in video 1, which we have implemented with PreonLab. Since it is impossible, as always, to show customer projects that do not have 10 years on the job, Oli “raped” a tutorial example with the kind permission of Fifty2.
The gearbox and the oil have an assumed ambient temperature of -20 °C. The oil is high viscous and heats up during the simulation. This makes the oil more and more thin fluid. The temperature can be recognized by the color.
In this example, a constant, high temperature is simply applied to the gears.
The procedure with other software (Star CCM+) is shown in figure 2.
Completely without using the whip. ????
Did we whet your appetite for more?
Please contact us.
We look forward to helping you.
Your Stefan Merkle