911Time
Suzuka
- Joined
- 25 Sep 2018
- Messages
- 1,172
Hi Guys,
Sorry I've not been around much lately, just the usual life and health/treatment stuff :sad:
Anyway, I've spent a bit of time recently, immersing myself in the engineering of our cars and thought I'd start posting some of the 'technical nuggets', in case anyone else is interested.
The following information/pics are taken from a variety of sources, including Porsche technical manuals but in my own words and re-stating basics, where it helps with context. I'll concentrate on the 991.1 and 991.2.
Both of the following come under the area of Thermal Management.
Dynamic Rear Spoiler 991.2 - Not Just About Speed
Hot air, being less dense than cold air, isn't ideal for 'filling' the cylinders of a combustion engine, which is why turbo-charged engines tend to use air-to-air intercoolers (the opposite of a radiator), to reduce the temperature of the incoming charge air, before it enters the cylinders.
In the case of the 991.2, when circumstances demand additional cooling, it's got a further ace up its sleeve, by using aerodynamics to help increase the flow of air over the two intercoolers (one mounted each side, behind the rear wheel arches).
Under normal circumstances, air for the intercoolers, is drawn in at the back of the rear grille however, if sensors within the engine bay detect the temperature has risen above a pre-defined level, the vehicle can deploy the rear spoiler, to act as an air dam - re-directing the air.
This change in aerodynamics, encourages more air to flow into the low pressure zone behind the rear bumper, which draws additional air through the intercoolers - reducing the temperature of the charge air, helping to increase volumetric efficiency (how well packed the cylinders are), for better power.
Intelligent Heat Transfer - 991.1 Carrera 4/S
Operation of the 4 wheel drive system in the Carrera 4/S, engages the front axle via an electronically controlled clutch unit, which is connected directly to the front axle housing.
The casing of the front axle housing, uses cast external fins to help transfer heat away to air however, there's also an additional method of thermal management, taking place inside, hidden from view.
Whilst the gears of the front differential (left of picture) run in a hypoid gear oil, the PTM clutch pack (right of picture) uses special ATF fluid and so, the two must sit in separate housings.
Two heat sinks (fins) are cast into the wall between the two housing areas (item 2) and help manage heat generated by the activity of the two components and transfer heat between the hypoid oil and the ATF fluid, as follows:
1) In circumstances where the multi-plate clutch pack has to engage, to accommodate differences in front/rear wheel speeds - for instance on loose surfaces, or in slippery conditions. The ATF in the clutch chamber will tend to become hotter than the oil in the differential housing and so heat is transferred from the ATF oil, through the fins on the casting, into the cooler hypoid oil.
2) When running at high vehicle speeds (where differences in front/rear axle speeds are negligible and the PTM clutch pack experiences limited use), the hypoid oil in the front differential will tend to become hotter than the ATF and so, this heat can be directed (via the fins) into the ATF fluid.
Hope some of you found that interesting - if so, I'll post some more as time allows. To those who didn't, you can wake up now!!
Sorry I've not been around much lately, just the usual life and health/treatment stuff :sad:
Anyway, I've spent a bit of time recently, immersing myself in the engineering of our cars and thought I'd start posting some of the 'technical nuggets', in case anyone else is interested.
The following information/pics are taken from a variety of sources, including Porsche technical manuals but in my own words and re-stating basics, where it helps with context. I'll concentrate on the 991.1 and 991.2.
Both of the following come under the area of Thermal Management.
Dynamic Rear Spoiler 991.2 - Not Just About Speed
Hot air, being less dense than cold air, isn't ideal for 'filling' the cylinders of a combustion engine, which is why turbo-charged engines tend to use air-to-air intercoolers (the opposite of a radiator), to reduce the temperature of the incoming charge air, before it enters the cylinders.
In the case of the 991.2, when circumstances demand additional cooling, it's got a further ace up its sleeve, by using aerodynamics to help increase the flow of air over the two intercoolers (one mounted each side, behind the rear wheel arches).
Under normal circumstances, air for the intercoolers, is drawn in at the back of the rear grille however, if sensors within the engine bay detect the temperature has risen above a pre-defined level, the vehicle can deploy the rear spoiler, to act as an air dam - re-directing the air.
This change in aerodynamics, encourages more air to flow into the low pressure zone behind the rear bumper, which draws additional air through the intercoolers - reducing the temperature of the charge air, helping to increase volumetric efficiency (how well packed the cylinders are), for better power.
Intelligent Heat Transfer - 991.1 Carrera 4/S
Operation of the 4 wheel drive system in the Carrera 4/S, engages the front axle via an electronically controlled clutch unit, which is connected directly to the front axle housing.
The casing of the front axle housing, uses cast external fins to help transfer heat away to air however, there's also an additional method of thermal management, taking place inside, hidden from view.
Whilst the gears of the front differential (left of picture) run in a hypoid gear oil, the PTM clutch pack (right of picture) uses special ATF fluid and so, the two must sit in separate housings.
Two heat sinks (fins) are cast into the wall between the two housing areas (item 2) and help manage heat generated by the activity of the two components and transfer heat between the hypoid oil and the ATF fluid, as follows:
1) In circumstances where the multi-plate clutch pack has to engage, to accommodate differences in front/rear wheel speeds - for instance on loose surfaces, or in slippery conditions. The ATF in the clutch chamber will tend to become hotter than the oil in the differential housing and so heat is transferred from the ATF oil, through the fins on the casting, into the cooler hypoid oil.
2) When running at high vehicle speeds (where differences in front/rear axle speeds are negligible and the PTM clutch pack experiences limited use), the hypoid oil in the front differential will tend to become hotter than the ATF and so, this heat can be directed (via the fins) into the ATF fluid.
Hope some of you found that interesting - if so, I'll post some more as time allows. To those who didn't, you can wake up now!!
