VarioCam Plus
VarioCam Plus
The new 911 Turbo (997) is provided with the latest development version of the variable valve control VarioCam Plus with continuous adjustment of the intake camshafts and valve lift switching of the intake valves. This system permits optimization of engine output and torque on the one hand and, on the other, also makes it possible to reduce fuel consumption and exhaust emissions while improving running smoothness.
Both individual systems of the VarioCam Plus (camshaft adjustment and valve lift switchover) are controlled by the Motronic control unit ME7.8.1. This control unit has been designed specifically for these requirements with a high processor capacity. This is necessary because the input values for "engine speed", "accelerator pedal position", "engine oil and water temperature" as well as gear speed detection are required to control VarioCam Plus. The demand for torque or power is compared with the stored program maps. A decision on how VarioCam Plus must react is made in milliseconds.
Camshaft Adjustment Of The Intake Camshafts
The 911 Turbo (997) features continuous adjustment of the intake camshafts by vane adjusters. The load and speed dependent adjustment range of the intake camshafts is 0 to 40° crankshaft angle.
Valve Lift Switching Of The Intake Camshafts
The small valve lift was increased from 3.0 mm (996) to 3.6 mm in order to make more efficient use of the advantages of VarioCam Plus with continuous camshaft adjustment and a larger adjustment range with respect to consumption, output and exhaust emissions. The valve lift adjustment system consists of switchable flat-base tappets on the intake side of the engine which are operated by means of an electrohydraulic 3/2-way switching valve.
Since there are two different cam forms on the intake camshaft, the corresponding valve lift curves act on the engine when the respective cams are switched. The flat-based tappets consist of two nested tappets which can be locked against each other by way of a pin. The inner tappet is in contact with the small cam and the outer tappet with the large cam. A hydraulic compensating element for the valve clearance is always integrated in the power flow of the tappet.
Cold Start
VarioCam Plus already significantly improves cold starting of the engine, and also allows emissions to be reduced during the warming-up phase.
Idle Speed
The engine is operated with the small intake cam (3.6 mm) at idle speed. Optimum timing is guaranteed thanks to fully variable camshaft adjustment. The small valve lift permits a reduction in fictional loss, a significantly increased charge movement thanks to the extremely short opening times, as well as lower emissions from previous combustions in the combustion chamber. This results in consumption and emission reductions of up to 10 % at the same time as significantly improved idling quality.
Partial Load
Operation with internal exhaust gas recirculation is optimum under partial load conditions for the purpose of dethrottling and in order to reduce the engine consumption. For this purpose, the camshaft phasing for the small valve lift is adjusted in order to achieve a large overlap, therefore allowing a large proportion of time for exhaust recirculation.
Full Load
In full-load operation, a high torque and high maximum output are achieved on the one hand through a low-loss charge cycle and, on the other, by an uncompromising cam contour design with a maximum valve lift of ten millimeters and correspondingly adapted opening and closing times of the valve strokes.
Positive Crankcase Ventilation
The positive crankcase ventilation and turbocharger ventilation are routed into the oil tank. In idling operation, the oil tank ventilation is routed to the intake manifold. Under full-load condition (with boost pressure), the oil tank ventilation is routed to the intake side of the left turbocharger.
Vacuum Pump
Like the current 911 Carrera (997) generation, the new 911 Turbo also has a mechanically driven vacuum pump that uses rotary vane technology. This replaces the conventional vacuum amplifier and provides the vacuum for the brake booster as well as various switching valves for the exhaust turbocharger (pop-off control) and the coolant. The vacuum pump is located on the left cylinder head, and is driven by the corresponding exhaust camshaft.
Electric Auxiliary Water Pump
The new Turbo is equipped with an electric auxiliary water pump which is installed on the left under the intake manifold. This pump can be activated as required by the DME control unit (also when the ignition is switched off) for the purpose of coolant circulation and in order to cool the watercooled turbochargers.
There are three conditions for auxiliary water pump activation:
1. Defined pump operation after engine start. 10 seconds after the engine is started, the pump runs for 10 seconds. This guarantees that the pump starts after every engine start, and therefore allows final stage diagnosis to be performed.
2. Support of cooling in an applied range during engine operation. The pump is switched on when the engine speed is less than 2,000 rpm and the water temperature is greater than 208 °F (98 °C). It is switched off as soon as the engine speed exceeds 2,500 rpm or if the water temperature falls to below 203 °F (95 °C). Since both switch-on conditions are linked, it is sufficient if one condition is not satisfied for the pump to be switched off.
3. Cooling during control unit after running after the engine is switched off or stalled. The higher of the two exhaust temperatures is sent through a low pass filter with a filter time constant of 256 seconds. This filtered exhaust temperature value and the water temperature are the input variables which are evaluated in the DME control unit. On the basis of this information, the DME control unit calculates as the output variable the pump after-run time (function of engine temperature and filtered exhaust temperature) in seconds. The pump after-run time may be between 0 to 600 seconds, depending on the engine temperature (185 °F/84.8 °C to 248 °F/120 °C) and exhaust temperature (1,260 °F/700 °C to 1,922 °F/1,050 °C).
The engine oil temperature is routed through a different low-pass filter with a filter time constant of 64 seconds. This filtered engine oil temperature value and the water temperature represent the input variables for a different map. Output variable: Pump after-run time (function of engine water temperature and filtered engine oil temperature) in seconds.
The pump after-run time may be between 0 to 600 seconds, depending on the engine temperature (194 °F/90 °C to 248 °F/120 °C) and engine oil temperature (194 °F/90 °C to 302 °F/150 °C).
Example: The after-run time is 300 seconds for an oil temperature of 248 °F (120 °C) and a water temperature of 194 °F (90 °C). The engine compartment temperature and ambient temperature do not have any effect on these functions.
Electric Fans
The 911 Turbo (997) is provided with a infinite fan control. The electric fans behind the radiators are controlled by the front-end control unit. The front-end control unit receives the information about the desired fan output (in %) via CAN from the DME control unit. The desired fan output is determined in the DME control unit on the basis of a program map as a function of the refrigerant pressure (CAN message from air-conditioning control unit) and engine water temperature. The requested output in % can be read out from the actual values with the PIWIS Tester (e.g. refrigerant pressure = 15 bar, tmot = 194 °F (90 °C) = greater than 50 %).
Engine Compartment Fan
The DME control unit switches the 2 relays for the engine compartment fan.
Stage 1 (with low fan speed) is activated from an engine compartment temperature of greater than 77 °F (24.8 °C).
Stage 2 (with full fan speed) is activated additionally from an engine compartment temperature of greater than 158 °F (69.8 °C) or a coolant temperature of greater than 221 °F (105 °C).
When the engine is stopped:
Stage 2 is activated for 180 seconds from an engine temperature of greater than 163 °F (72.8 °C). The fan runs again for 180 seconds if the temperature is exceeded again. This continues until the temperature is no longer exceeded again or for a maximum of 40 minutes.