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Author Topic: Overhauled physics model  (Read 18282 times)

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Offline jamespetts

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Re: Overhauled physics model
« Reply #35 on: January 11, 2010, 08:10:01 PM »
Hajo,

interesting thoughts. What are useful formulae for calculating those factors?

Offline Spike

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Re: Overhauled physics model
« Reply #36 on: January 11, 2010, 08:46:25 PM »
In old Simutrans I used something very simple, power-based accelleration and a hard cap for top speed. Prissi then refined this into a more detailed model. I still don't know what would be "right", but I think school physics f=m*a (force is mass multiplied by acceleration) doesn't really cover the problem.

Offline ӔO

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Re: Overhauled physics model
« Reply #37 on: January 21, 2011, 06:25:45 PM »
I know it has been a while since the current version of the physics model, but could the gearing/transmission be looked at again?

I think they are necessary, because when I looked at the specs of the GWR railcar, I saw that it uses an 8.85L AEC engine with small power output, about 97kW, which is smaller than in game and it is still underpowered.


Diesel hydraulic locomotives and automobiles use a transmission gearing system, and it is necessary, because the power/torque curve output is not the same at all speeds. This results in differing rates of acceleration when in different gears.

For trains, a diesel-electric has no gearing, but instead uses the diesel engine to power an electric generator, which powers the electrical motors. In this case, the power/torque/rpm of the engine doesn't need to change much to move the locomotive at various speeds.

But that's not the only type of transmission used.
Diesel-hydraulic uses a hydraulic system with transmission gears to drive the wheels, and this means the power/torque and rpm of the diesel engine matters greatly. If the gearing was poorly selected, then the locomotive could have trouble accelerating, as the gear is too tall and puts the engine in a low power/torque rpm range.

We don't normally see mechanical gear transmissions used in locomotives, because they would break prematurely under the amount of weight involved.



On cars and small trucks, there may be 3 to 7 gears so that they can operate in a variety of speed ranges. On full sized trucks, there can be anywhere up to 18 gears

Taking a look at this 500hp scania engine, we see that there is a lot of torque in the low range, and a lot of power in the high range. Torque is needed to get the vehicle moving and accelerating, while power is needed to maintain the speed.
http://www.scania.com/products-services/trucks/truck-range/v8/Engine/500-hp.aspx

Tractive effort is not needed as much with road vehicles, on dry pavement at least, because the factor of adhesion for rubber to pavement is much greater than steel to steel. In this case, torque becomes a more important factor for determining the rate of acceleration and total amount that the vehicle can tow. I'm not sure how to convert torque to tractive effort, but it is most likely a fixed number.



Gear ratio and power/torque curves can be complicated, because it can depend on many factors. To keep it simpler, I think, the wheel size and final ratio should be fixed and either of the two ways to use gears should be used.


first method, more complex, but detailed for pak maintainers.
"gear1", "gear2", etc. will specify the gear ratio, usually something like 4.16 (to 1) -or- % ratio between gears
"rpm" will specify maximum rpm range of the engine
"rpm_torque" will specify the rpm range where there is maximum torque
"rpm_power" will specify the rpm range where there is maximum power
"rpm_band" will specify the power/torque band, something like narrow, normal, wide or very wide.



Or, the more simpler to implement
"gear1", "gear2" & "gear3" will specify the amount the torque is divided by, but also multiply power by the gear.
This means that
gear1=0.50 would multiply torque by two, and divide power by half. (Good acceleration and climbing)
gear2=1.00 would be direct ratio
gear3=2.00 would divide torque in half and multiply power by two. (poor acceleration, but able to maintain high speed)

The game will assume or calculate when to change gears. 5 or 6 gears should be able to cover the full range of all the vehicles. It should also be possible to implement flaws in the vehicle with the gears, because if the ratio between gears is too high when the vehicle doesn't have enough power, it won't be able to accelerate in that gear. The burden will still be on the pak maintainer to find a ratio that works as desired.

http://en.wikipedia.org/wiki/Gear_ratio