Variable Valve Timing

22-01 Classify VVT systems.

Until this point, camshaft operation and theory referred to traditional designs of camshafts and conventional fixed cam timing. In a conventional design, the camshaft sprocket bolts directly to the camshaft, and they rotate as a single assembly. A belt or timing chain mechanically determines the relationship between the camshaft and the crankshaft. The belt or timing chain drives the camshaft from the crankshaft sprocket that permanently attaches to the crankshaft. During engine assembly, aligning the timing marks on the camshaft and crankshaft gears sets the valve timing, and it does not change. In a conventional engine without VVT camshaft rotation is “fixed” to crankshaft rotation. Fixed timing means both the intake and exhaust valves always open and close at the same point during the combustion process unless a timing chain or belt stretches or jumps a tooth. Base cam timing is a compromise between optimal performance, emissions reduction, and fuel economy in normal automotive use. A problem arises, however, during normal operation, because the most efficient cam-to-crank timing relationship varies with operating conditions, such as load and rpm. This compromise forces engineers to pick the “best” compromise based on intended vehicle use. While outstanding high-rpm performance may the ultimate goal for some, the majority of automotive vehicles spend their time operating in a low- to mid-rpm range, where efficiency is the primary objective. Optimal valve timing for a light-load condition typically is not ideal for high-torque operation.

The ability to control the advance and retard of the cam timing is a huge benefit to engine performance. VVT allows the PCM to change the fixed camshaft profile in relation to the crankshaft based on operating conditions (FIGURE 22-1). This removes the inhibiting mechanical timing compromises that exist during all operating conditions on non-VVT engines. Improvements include idle quality, reduced emissions, better fuel economy, and enhanced torque curves and power bands.

FIGURE 22-1 The key to VVT operation is the ability to move the camshaft to advance or retard the opening of the valves at the proper time. Infinite VVT allows the camshaft to match the situation that the engine is operating in, which increases the capability of the vehicle.

Retarded cam timing provides greater engine efficiency and increases airflow for power, making it best for mid- and high-rpm operation. Advancing the cam timing increases intake air velocity for higher torque, making it useful for idle during low-rpm operations. In past applications, an engine designer would have to decide which valve timing best served the operating range of the engine and then design the camshaft for that purpose. VVT allows the relationship between one or more of the camshaft(s) and the crankshaft to be changed. Altering valve timing allows torque and power to be available at different times. Do not confuse cam timing with ignition timing; the effects and operating characteristics are completely different.

TECHNICIAN TIP

Intake pumping losses occur during engine operation at part load because the throttle restricts the airflow to the engine, reducing volumetric efficiency. A closed throttle plate results in air pressure in the intake manifold falling significantly below atmospheric pressure. To pull air from the intake manifold into the cylinder, the piston needs to perform work against the manifold vacuum, known as “pumping work.” The term “pumping losses” refers to the work done by the piston because of the pressure differential between the manifold and the crankcase.

TABLE 22-1 Reasons for Varying Valve Timing

•Emissions: exhaust cam phasing enables manufacturers to use fewer components to meet more stringent emission standards

•Performance increased low-end torque: intake camshaft phasing varies based on changing operating conditions and loads

•Increased high-speed power: intake camshaft phasing also varies based on operating conditions and loads

•Increased fuel economy: exhaust camshaft phasing

•Engine efficiency: pumping losses are reduced during low-load to part-load operation

•Elimination of the exhaust gas recirculation (EGR) valve: exhaust camshaft phasing; this system uses one or more camshaft position (CMP) actuators that can change the camshaft timing up to 50 degrees in relation to the crankshaft position (CKP). Some engine designs used by other manufacturers may still include an EGR valve

Today’s automobiles ensure optimum engine performance in a vehicle used both for street and race applications by using an electronic control module (ECM) and hydraulically assisted VVT. This setup manages optimum valve timing for such widely differing engine demands while the engine is operating. The system, in theory, is very simple. Camshaft phaser systems are generally one of two types: spline phaser or vane phaser. Overhead camshaft engines incorporate the spline phaser system. The vane phaser system is used in two designs: overhead cams and cam-in-block engines. Cam phasing is controlled hydraulically with engine oil pressure by an oil control valve (OCV), either directly controlled electronically or controlled electromagnetically.

Most systems use a variable sprocket or gear, known as a cam phaser/actuator, attached to the front end of the camshaft that advances or retards the cam timing based on oil flow and direction. The phaser is made up of two parts: the stator housing (which contains the vaned rotor) and the sprocket. The stator housing attaches to the camshaft and rotates with it. The sprocket typically connects to the stator housing by bolts and is driven by a timing chain. A simple two-wire solenoid containing a spool valve assembly controls oil flow based on a duty cycle input from the PCM. The phaser typically takes the place of the standard cam gear or pulley, and the engine’s oil pump develops the oil pressure. A small number of helical-spline systems exist. Toyota and Lexus used helical-spline systems on timing belt–equipped engines. These systems had limited range (movement). Toyota designed the system so that if a leak were to develop, oil would return to the engine, keeping the timing belt dry. The final system used on a very limited number of modern imports only is the magnetic system. Nissan used this system on VQ25HR and VQ35HR engines.

TECHNICIAN TIP

VVT alters the camshaft-to-crankshaft relationship. Both single-camshaft and dual camshaft engines use VVT. VVT varies the timing of the valve opening and closing events to increase or decrease valve overlap.

Almost all VVT systems use an electromechanical solenoid (spool valve) to control oil supply pressure to the camshaft actuator. Most manufacturers label these solenoids OCVs. These valves are commanded by a PCM and moved using a fixed-frequency variable duty-cycle (pulse-width–modulated) command (FIGURE 22-2). Some manufacturers may use on/off solenoids (e.g., Honda’s VTEC) and some magnetic systems. The use of magnetic systems has been limited to a small number of modern import vehicles, such as the Nissan VC25HR and VQ35HR engines, to this point in time. These two variants are used in a range of Nissan vehicles worldwide.

FIGURE 22-2 Diagnosing the vehicle’s VVT should start with understanding the inputs and outputs of the vehicle. By looking at the whole VVT system, the technician should be able to determine the cause of the failure and thus repair the issue.

Cam phaser usage can appear on any camshaft arrangement, such as cam-in-block or Overhead cam (OHC) engines. If the cam is a single cam with exhaust and intake lobes, rotating the camshaft to the advanced or retarded position affects both the intake and exhaust valves. A dual overhead cam (DOHC) engine may use two or four variable-controlled camshafts. OHC engines use one CMP actuator for each camshaft. Each CMP actuator has a separate OCV and CMP sensor. What controls multiple camshafts varies by manufacturer. Control may be intake only, intake and exhaust independent control, and in some cases, exhaust only. Control of the intake camshafts, to create more power is the most frequently used on modern engines. VVT use on a DOHC engine alters valve overlap. Regardless of what system variation of control is used, valve overlap is always low at idle and low rpm and increases at higher rpm. This is true despite control by either advancing intake overlap with rpm or retarding exhaust overlap.

The following summarizes VVT operation:

The Operation of an Electronic Control Module–Controlled Oil Pressure–Actuated Vane Phaser

VVT Cam Phasing Principles Based on Engine Design

All VVT systems, regardless of their manufacturer, operate under the following principles:

Basic Inputs of Variable Valve Timing

Camshaft timing control by the PCM is based on enabling some or all of the following conditions:

Variable Valve Timing Feedback

The PCM has to be able to monitor and verify operation. This feedback from the CMP sensors forms a feedback loop. CMP sensors serve a dual purpose: cylinder identification and the actual camshaft-to-crankshaft relationship. One CMP sensor is present for each camshaft controlled by the PCM.

Variable Valve Timing Component Overview

While differences across manufacturer designs exist, there are many similarities between the components used. Coverage of the basic mechanical function of these components will follow here. As with any system always, refer to manufacturer’s service information for in-depth information on a component’s design and operation and how to diagnose it.

The Two Core Components of Vane-Style Actuators
  1. Actuator: the hydraulic device in the cam gear that reacts to oil pressure and volume (FIGURE 22-3).
  2. Solenoid and spool valve: the device that routes oil to, flows oil into and out of, or blocks oil from going into the actuator, to advance, retard, or hold the timing, as commanded by the PCM (FIGURE 22-4).

FIGURE 22-3 The VVT solenoid or spool valve controls the camshaft actuator/phaser in the operation of changing the orientation of the camshaft in the engine.

FIGURE 22-4 The phaser/actuator is attached to the camshaft and is the component that physically rotates the camshaft to the proper position. The movement is from oil pressure that is directed toward one side of the phaser or the other to create movement.

Each VVT-equipped camshaft uses one solenoid and one actuator. When performing diagnosis, begin with the solenoid because it is typically easier to access and because actuators are difficult, if not impossible, to test conclusively for a fault. In other words, condemning an actuator is more of a system of elimination, similar to module diagnosis. The process of elimination includes verification of proper oil level, viscosity, and pressure; correct mechanical timing; and correct solenoid and spool valve operation. If a system is inoperative, the causes can be limited to either an actuator or a solenoid-to–actuator port blockage.