From Mechanical Engineering into the Car
Gear Motors for Electro-Mechanical Cam Phasing in Combustion Engines
Siegbert Path
The development of economical and low-emission combustion engines requires significant
improvements in the control, adjustment speed and operating temperature range of cam phasing
systems. Hydraulic cam phase adjusters have reached their limits in this respect. Newly developed
electro-mechanical cam phase adjusters (see Fig. 1) achieve and exceed the required specifications and
give the engine developer improved application flexibility.
Dipl.-Ing. Siegbert Path, Project Manager Sales
OVALO GmbH, Limburg.
OVALO (a subsidiary of Harmonic Drive AG) provides electro-mechanical cam phase adjusters based on the wave gear principle.
The phase shift between the camshaft and the crankshaft is realised by means of a combination of a wave gear and a specially
developed electric motor fitted as a unit directly into the camshaft drive. The basic components of a wave gear are a radially
deformable thin walled ball bearing and an equally deformable ring or cup-shaped gearwheel.
The operation of the OVALO wave gear is depicted in Figure 2. The cylindrical outer ring gear is provided with internal gearing.
A thin-walled flexible gear with external gearing is inserted into this ring gear. The flexible gear has two teeth less than the ring
gear, but with the same module. The pitch diameter of the flexible gear with the external gearing is therefore smaller than the
pitch diameter of the ring gear with the internal gearing and so the teeth are not meshed. An elliptical steel plug with a pressed-on
thin walled ball bearing (also known as a wave generator) is inserted into this flexible gear and ensures the two gear contact areas
as seen in fig. 2.
Appeared in antriebstechnik 7/2010
2: Method of Operation of the OVALO Gear.
The vertical axis of the ellipse is sized to provide the exact required teeth engagement.
If the ellipse is then turned, the external gearing of the flexible gear meshes with the hollow ring gear. The ball bearing separates
the fast speed of the wave generator from the slow speed of the flexible gear. Both the flexible gear and the ball bearing are
continuously elastically deformed during each rotation. Because of the differing number of teeth, a relative movement occurs
between the flexible gear and the hollow ring gear due to the rotation of the elliptical plug.
A high gear ratio between 50:1 and 160:1 is therefore possible in a compact size depending on the number of teeth selected.
Two versions of the OVALO wave gear are available (Fig. 3):
Cup-Type gear:
The elastic thin-walled gearwheel is in the shape of a cup (Flexspline). The ring gear is used as a flange for connection to the
machine housing. High speed and low torque at the wave generator results in high torque and low speed at the flange built onto
the flexible cup.
Ring-Type gear:
The elastic thin-walled gearwheel is in the shape of a ring or cylinder with external teeth (Flexspline). Two further ring gears with
internal teeth are used: ring gear 1 has the same number of teeth as the inner cylindrical gearwheel. Ring gear 2 has two additional
teeth. If the wave generator rotates quickly with low torque, a relative rotation occurs between the two ring gears. The low speed
and high torque between ring gear 1 and ring gear 2 can then be used as the output.
3: The two types of Ovalo gearboxes: Ring-type and cup-type.
Design of the Cam Phase Adjuster
The cam phase adjuster consists of a mechanical unit, a coupling and an electric motor. The ring-type gear is selected for use in
the camshaft drive due to the design advantages it offers: Different basic gear sizes can be fitted within the outer diameter of the
camshaft wheel and the gearbox length of around 15 mm allows an optimum structural integration in the space available,
including an integral bearing for absorbing the lateral forces present.
Appeared in antriebstechnik 7/2010
The camshaft is connected to ring gear 1 and the camshaft drive is connected to ring gear 2 via the cam drive system (e.g. chain,
belt, wheelsets) (Fig.1). When the wave generator is rotated relative to ring gear 2 using an electric motor, a relative movement
occurs between the camshaft and its drive.
This results in a shift of the valve opening and closing times relative to the crank and piston position.
A special coupling is used to compensate for installation and component tolerances and to connect the shaft of the electric motor
to the wave generator. The coupling makes it extremely easy to install the unit on the assembly line.
Geometrical restrictions and optimisation of the power consumption must be taken into consideration as far as the electric motor
is concerned. The design of the electro-mechanical cam phase adjuster is finalised by optimising the performance characteristics
of the electric motor, the necessary drive torque of the camshaft and the gear ratio.
The stator of the electric motor is fixed to the combustion engine. When no phase adjustment is required the motor operates
continuously at the camshaft speed. In order to make the system durable and maintenance-free, brushless motors are used (BLDC).
By increasing or decreasing the motor speed, the wave generator provides the differential speed between the two ring gears, and
the camshaft is advanced or retarded. The controller for the adjuster unit receives the target position for the camshaft that has been
calculated from the crankshaft and camshaft angles, and the angle difference required depending on the engine characteristics.
The position of the rotor is measured via the integrated sensors. Four Hall sensors combined in an integrated circuit attached to
the housing are excited by a two-pole permanent magnet on the rotor shaft. Internal interpolation of the difference of the
sinusoidal signals results in the output of high-resolution incremental speed and position values. The Power electronics for the
cam phase adjuster can be integration directly into the engine control unit or installed as a separate control unit, preferably in the
immediate proximity of the actuator.
Performance Potential of an Electro-Mechanical Adjuster
Wave gears have been successfully used in mechanical engineering for decades. A service life of more than 30000 operating
hours and an adjusting accuracy in the arcsecond range clearly exceed the requirements car engine manufacturers expect from an
electro-mechanical cam phase adjuster. With regard to the specific OEM requirements for the cam adjuster, the system described
here can be customised as required. This concept has been developed as a modular system and so these units available in various
sizes and with various gear ratios. The size and ratio of the unit is selected depending on customer requirements such as adjusting
torque and speed and a suitable electric motor is designed to fit.
The required overall adjusting range is covered with approximately 3 to 15 rotations of the electric motor (depending on the
version) so that adjusting speeds > 300°/s at the camshaft can be achieved with a precision that is significantly better than that
specified. The adjusting range is limited via integrated mechanical stops to mechanically protect the combustion engine in the
event of a malfunction. As well as good performance with regard to control quality and adjusting speed, the electro-mechanical
system has other advantages. The system requires power only for the phase adjustment, except from the minimal amount of power
required for the synchronous operation of the motor. This is an advantage compared to hydraulic systems which must be
continuously supplied with hydraulic pressure and oil via an appropriately large dimensioned oil pump. The electro-mechanical
system is also ready for immediate operation at all expected temperatures, regardless of the oil viscosity, and therefore has
emission advantages under cold start conditions and improved control reliability at higher temperatures.
Another advantage is that the adjustment of the phase angle is possible with the engine switched off meaning the optimum
camshaft position can be preset, for example for pulse starting when switching to combustion engine operation in hybrid vehicles
and for start / stop systems.
The electro-mechanical cam phase adjuster is also extremely compact. Since the wave gears always have approximately 30% of
the teeth in engagement at any one time, high torque levels can be transmitted in spite of the small gear module and the compact
overall dimensions. It is usual to integrate the can phase adjustment system directly into the drive wheel of the camshaft.
State of Development
Development projects with various customers are currently in progress at OVALO. Initial series production is planned. The oil
pump for engines currently under development will be significantly smaller compared to current motors with hydraulic phase
adjusters because the pressurised oil supply to the camshaft adjuster is no longer required. OVALO will development the entire
adjuster unit, including electric motor and sensor development. Quality-critical and cost-critical are simplifies by means of
consistent use of our development and manufacturing know-how. The delivery and programming of the required control units
should be carried out by the OEM’s system partner responsible for the engine control unit.
Appeared in antriebstechnik 7/2010