Variable valve timing

Variable valve timing sets the most advantageous valve timing for the engine in each operating mode: idle, maximum power, and torque; as well as for exhaust gas recirculation.

Idle

At idle, the intake camshaft is set so that the intake valves open and close late. The exhaust camshaft is set so that exhaust valves close well before top dead center (TDC). Because there are only small amounts of residual gases from combustion, engine idle is smooth.

Power

To achieve good power at high engine speeds, the exhaust valves are opened late. This allows the expanding burning gases to act against the pistons longer. The intake valves open after top dead center and close well before bottom dead center (BDC). This enables the dynamic self-charging effect of the entering air, and increases power output.

Torque

To achieve maximum torque, good volumetric efficiency is needed. This requires that the intake valves be opened early. Because the intake valves open early, they close early as well. This keeps the fresh gases from being pushed back out the intake valve port. The exhaust camshaft closes the exhaust valves just before top dead center.

Exhaust Gas Recirculation

Internal exhaust gas recirculation can be achieved by adjusting the intake and exhaust camshafts for a period of valve overlap, when both the intake and exhaust valves are open. The amount of overlap determines the amount of recirculated exhaust gas. The intake camshaft is set so that it opens the intake valves well before top dead
center. The exhaust camshaft does not close the exhaust valves until just before top dead center. As a result, both valves are open and some of the exhaust gas is recirculated in the cylinder for reburning during the next power stroke. The advantage of internal exhaust gas recirculation over external exhaust gas recirculation is the fast reaction of the system and very even distribution of the recirculated exhaust gases.

Operation of Variable Valve Timing

The Motronic Engine Control Module J220 controls the variable valve timing. To do this, it requires information from various sensors about engine speed, engine load and temperature, and the positions of the crankshaft and the camshafts. To adjust the camshafts on the cylinder bank shown, the Motronic Engine Control Module J220 actuates Valve 1 for Camshaft Adjustment N205 and Camshaft Adjustment Valve 1 (Exhaust) N318. They in turn open oil galleries in the control housing. Engine oil flows through the control housing and camshafts into the camshaft adjusters. The inner rotors (pivot motor rotors) rotate and adjust the intake and exhaust camshaft positions in accordance with the specifications programmed into the Motronic Engine Control Module J220.

Intake Camshaft

Intake Camshaft Adjustment

Each intake camshaft is regulated by the Motronic Engine Control Module J220 over the entire speed range of the engine. The maximum amount of adjustment possible is through 52 degrees of crankshaft angle. Adjustment is determined by a control map stored in the Motronic Engine Control Module J220.

Intake Camshaft Adjuster Design

The adjusting mechanism of each intake camshaft adjuster consists of:

• A combined housing and outer rotor
  connected directly to the timing chain
  or belt.

• An inner rotor (pivot motor rotor)
  attached to the end of the camshaft.

The adjusters are locked mechanically until the necessary engine oil pressure has built up. Using a mechanical detent device, a springloaded differential pressure pin prevents the camshaft from being adjusted during the engine start cycle. The adjuster is designed to move to the retard position and remain locked there whenever the engine is turned off. The rising engine oil pressure unlocks the spring-loaded differential pressure pins.

How the Intake Camshaft is Advanced

For exhaust gas recirculation and increasing torque, each intake camshaft must be set so that the intake valves open well before top dead center. To advance valve timing on the cylinder bank shown, the Motronic Engine control Module J220 actuates Valve 1 for Camshaft Adjustment N205 which adjusts the position of the control piston in the control housing. The control housing oil gallery for timing advance is opened up in accordance with the new position of the control piston. With the oil gallery opened, engine oil under pressure flows through the control housing and into the forward ring channel in the intake camshaft. From there the pressurized oil flows through five passages drilled through to the front face of the camshaft and into the five advance chambers of the camshaft adjuster, where it presses against the vanes of the inner and outer rotors. This causes the inner rotor (pivot motor rotor) to rotate to an advanced position within the outer rotor. Since the camshaft is fixed to the inner rotor (pivot motor rotor) and the crankshaft is mechanically linked to the outer rotor, this effectively rotates the intake camshaft in the direction of crankshaft rotation and the intake valves open sooner.

How the Intake Camshaft is Retarded

When the engine is idling or when a lot of power is required from the engine, each intake camshaft is rotated so that the intake valves open late — after top dead center. To retard the intake camshaft on the cylinder bank shown, the Motronic Engine Control Module J220 actuates Valve 1 for Camshaft Adjustment N205, which opens the gallery for timing retardation by moving the control piston. Engine oil under pressure flows through the control housing into the rearward ring channel of the camshaft. From there the pressurized oil flows through drilled passages in the camshaft to the pocket hole of the securing bolt for the camshaft adjuster. The pressurized oil then flows through five drilled passages in the camshaft adjuster and into the oil chamber for timing retardation where it presses against the vanes of the inner and outer rotors. This causes the inner rotor (pivot motor rotor) to rotate to a retarded position within the outer rotor. This movement effectively rotates the intake camshaft in the opposite direction from crankshaft rotation and the intake valves open later. At the same time that the oil gallery for timing retardation is opened, the control piston opens the oil return for the gallery for timing advance, relieving the pressure there. The rotation of the inner rotor (pivot motor rotor) in the retard direction pushes the oil out of the timing advance chamber through the timing advance oil gallery.

How Regulation Works

Regulation enables continuous variation of the position of each intake camshaft between advanced and retarded through a maximum adjustment range of 52 degrees of crankshaft angle. On the cylinder bank shown, the Camshaft Position (CMP) Sensor G40 provides a signal to the Motronic Engine Control Module J220 which allows it to monitor the exact position of the intake camshaft at any given moment. The control map in the Motronic Engine Control Module J220 determines the intake camshaft adjustment using this camshaft position information as well as engine load, speed, and coolant temperature readings. When the control map calls for advanced timing, the Motronic Engine Control Module J220 activates the Valve 1 for Camshaft Adjustment N205, which moves the control piston in the advance timing direction. Pressurized oil is diverted through the control housing and drilled passages in the camshaft into the camshaft adjuster and moves the camshaft into an advanced position. Moving the control piston in the advanced direction simultaneously opens the oil return through the oil channel for retarding timing. When the desired angle of adjustment is attained, the control map initiates movement of the control piston to a position that maintains equal pressure in both camshaft adjuster chambers to maintain the adjustment angle. To move the camshaft in the retard timing direction, the regulation process is similar but pressurized oil flow is reversed.

Exhaust Camshaft

Exhaust Camshaft Adjustment

In contrast to the intake camshaft adjustment which is continuously variable over the entire range of 52 degrees of crankshaft angle, the adjustment of each exhaust camshaft is essentially “on” or “off” to advance the exhaust camshaft timing or return it to normal. The adjustment variation between these two positions is 22 degrees of crankshaft angle. Exhaust Camshaft Adjuster Design The adjusting mechanism of the exhaust camshaft adjuster is nearly identical to the design of the intake camshaft adjuster:

• A combined housing and outer rotor
  connected directly to the timing chain
  or belt.

• An inner rotor (pivot motor rotor)
  attached to the end of the camshaft.

• The vanes of the inner rotor (pivot motor
  rotor) are wider to limit adjustment travel
  to the smaller 22 degrees of crankshaft
  angle required of the exhaust camshaft.

Exhaust Camshaft Normal Position

Each exhaust camshaft is in its normal position when the engine is being started and at engine speeds above idle. In the normal position, the exhaust valves close just before top dead center. Each exhaust camshaft is in the normal position in the engine operating modes for maximum power and torque; as well as for exhaust gas recirculation. Under these conditions, on the cylinder bank shown the Camshaft Adjustment Valve 1 (Exhaust) N318 is not actuated.

How the Normal Position Works

In the normal position, the exhaust camshaft is positioned so that the exhaust valves close shortly before top dead center. The Camshaft Adjustment Valve 1 (Exhaust) N318 on the cylinder bank shown is not actuated by the Motronic Engine Control Module J220. In the normal position, the oil gallery for timing retardation is open. Through this oil gallery, engine oil under pressure reaches the rearward ring channel in the exhaust camshaft. From there the pressurized oil flows through drilled passages in the camshaft to the pocket hole of the securing bolt for the camshaft adjuster. The pressurized oil then flows through five drilled passages in the camshaft adjuster and into the oil chamber for normal position where it presses against the vanes of the inner and outer rotors. This causes the inner rotor (pivot motor rotor) to rotate to the stops at the normal position within the outer rotor, rotating the camshaft along with it. The exhaust camshaft on the cylinder bank shown remains in this position as long as the Camshaft Adjustment Valve 1 (Exhaust) N318 solenoid is not actuated. At the same time that the oil gallery for timing retardation pressure is open, the oil return for the gallery for timing advance is open, relieving the pressure there. The rotation of the inner rotor (pivot motor rotor) in the retard direction pushes the oil out of the timing advance chamber through the oil gallery for timing advance.

Exhaust Camshaft Advanced Position

Each exhaust camshaft is set to the advanced position at engine speeds from idle to about 1,200 rpm. How the Exhaust Camshaft is Advanced To advance exhaust valve timing on the cylinder bank shown, the Motronic Engine Control Module J220 actuates the Camshaft Adjustment Valve 1 (Exhaust) N318, which adjusts the position of the control piston in the control housing. The control housing oil gallery for timing advance is opened up in accordance with the new position of the control piston. With the oil gallery opened, engine oil under pressure flows through the control housing and into the forward ring channel in the exhaust camshaft. From there the pressurized oil flows through five passages drilled through to the front face of the camshaft and into the five advance chambers of the camshaft adjuster, where it presses against the vanes of the inner and outer rotors. This causes the inner rotor (pivot motor rotor) to rotate to an advanced position within the outer rotor. Since the camshaft is fixed to the inner rotor (pivot motor rotor) and the crankshaft is mechanically linked to the outer rotor, this effectively rotates the intake camshaft in the direction of crankshaft rotation and the exhaust valves open and close earlier. At the same time that the oil gallery for timing advance pressure is open, the oil return for the gallery for timing retardation is open, relieving the pressure there. The rotation of the inner rotor (pivot motor rotor) in the advance direction pushes the oil out of the timing retard chamber through the oil gallery for timing retardation.

Oil System

The variable valve timing system operates at an oil pressure of 10.2 psi (70 kPa) and above. Oil flow through the exhaust and intake camshafts is virtually identical.

Learning Ability of the System

The entire variable valve timing system is adaptive. This adaptability compensates for component and assembly tolerances and wear. The Motronic Engine Control Module J220 initiates adaptation when the engine is idling and the coolant temperature is greater than 140°F (60°C). During adaptation, the Motronic Engine Control Module J220 uses signals from the Engine Speed (RPM) Sensor G28, the Camshaft Position (CMP) Sensor G40, the Camshaft Position (CMP) Sensor 2 G163, the Camshaft Position (CMP) Sensor 3 G300, and the Camshaft Position (CMP) Sensor 4 G301, to check the idle settings for the intake and exhaust camshafts. If the actual value does not agree with the control map specification stored in the Motronic Engine Control Module J220, the camshaft positions are adjusted to match the specification.

 

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