Principle of operation:
A rack gear is secured on a piston.
The piston rack gear is meshed with some gearwheels.
The gearwheels are meshed, at their opposite side, with a control rack gear.
Through the connecting rod, the rotating crankshaft forces the gearwheels to reciprocate.
Due to their meshing with the control rack gear, the reciprocating gearwheels rotate.
And due to its meshing with the reciprocating - rotating gearwheels, the piston rack gear performs, together with the piston, a reciprocating motion along the cylinder.
Rollers, of diameter equal to the pitch circle diameter of the gearwheels, are secured on the gearwheels and abut on rolling surfaces to take the thrust loads.
The displacement of the control rack changes the compression ratio.
In the carriage-gear arrangement, the piston has two projections/legs, each having two "back to back" racks
and rolling-surfaces. The "back to back" piston-rack and the symmetrical design eliminate the bending loads.
The above animation shows the operation at a specific compression ratio. Click to enlarge.
The animation below shows how the compression ratio is varied.
The symmetrical distribution of the piston mass around the cylinder axis keeps piston's center of gravity on the
cylinder axis. The asymmetrical piston of the one-sided arrangement of the state-of-the-art, besides being heavier,
has also center of gravity offset to the cylinder axis, resulting in additional inertia moment that generates additional
thrust force on the cylinder and therefore additional friction.
Oppositely to the piston-racks there are control-racks having rolling-surfaces beside them.
A carriage is pivotally mounted, by the wrist pin, on the small end of the connecting rod.
Two double gear-wheels are pivotally mounted on the carriage, each having rollers that abut on the rolling-surfaces
to take the thrust loads.
Click on any image to download the relevant animation or file (first check the size).
With the animation running:
Press . key a few times to move the piston at TDC
Press (or keep pressed) the SpaceBar key to change compression ratio
Press , key to return to animation mode
Move mouse to the lower right side to slow-down motion
Press SpaceBar to change the compression ratio
Finally double click or press ESC to quit animation.
The four control-racks together with their rolling-surfaces are integrated on a robust rack-frame.
The arrangement of the parts makes the carriage to perform a parallel to itself motion.
The rotation of the crankshaft causes the reciprocation of the carriage with the gear-wheels.
The gear-wheels, meshed with the control-racks, force the piston-racks and the piston to reciprocate along a
stroke twice as long as the stroke of the carriage.
The carriage-gear arrangement reduces the maximum force on the roller several times as compared to the maximum force on the
roller of the one-sided arrangement of the state-of-the-art (as explained in the "wrist-pin-gear" arrangement).
The light and robust piston structure,
the small inertia loads of the carriage with the gear-wheels (half stroke than the piston stroke),
the simple and robust rack-frame structure and rack-frame support on the casing,
the short height,
the equal distribution of the gas pressure force and of the inertia force on the four piston-racks and on the teeth of
the four gear-wheels,
the way smaller maximum force on the roller,
the reduced friction (because the thrust forces pass to the casing by pure rolling and not by sliding of the
piston skirt on the cylinder wall),
are some of the advantages of this arrangement.
The RackGear-VCR combined with the PatAir VVA (or the
HyDesmo VVA) enable instant digital microalignment of the actual compression ratio independently on each cylinder.
In the wrist-pin-gear arrangement, below, the piston comprises four offset racks, the two of them having teeth at one
direction, the other two having teeth at the opposite direction.
Oppositely to each piston-rack there is a control-rack and a rolling-surface beside it. The four control-racks
and the four-rolling surfaces are integrated in a cylindrical robust rack-frame having a piston around it.
The rack-frame is slidably fitted into a cylinder at the bottom of the conventional cylinder.
Four gear-wheels are pivotally mounted on the wrist-pin at the small end of the connecting rod. Each gear-wheel
has a roller of diameter equal to the pitch-circle diameter of the gear wheel and abuts on a rolling-surface.
Each one of the gear-wheels is meshed at one side with one of the piston-racks and at its opposite side to a control-rack.
To see the change of the compression ratio:
Open the first animation
Press . key to enter to the "step by step" mode
Press a few times . or / keys to move piston at TDC
Press (or keep pressed) the SpaceBar key.
The displacement of the rack-frame rotates the gear-wheels
that displace the piston and change the compression ratio.
Finally double click or press ESC to quit animation
The rotation of the crankshaft causes the reciprocation / rotational oscillation (because of their meshing with the control-
racks) of the gear-wheels. The gear-wheels force the piston-racks and the piston to reciprocate at double stroke than the
wrist pin stroke.
Feeding oil at one side of the piston of the rack-frame and releasing oil from the other side of the rack-frame piston, the
control-racks move along the cylinder axis and the compression ratio changes continuously.
The inclination (typically: 20 degrees) of the piston-rack teeth surface relative to the vertical to the cylinder axis plane,
generates a heavy thrust load on the gear-wheel. In order the piston-rack to apply to the gear-wheel a force
F parallel to the cylinder axis, the inclination of the surface of the teeth of the piston-rack generates an additional
vertical to the cylinder axis force F/3 (because tan(20deg)=0.36 which is about 1/3). The wrist pin takes the thrust forces
from all gear-wheels and cancels them: the two are equal in size and opposite in direction than the other two. This
cancellation frees the contact between the rollers and the rolling-surfaces from loads caused by the inclination of the
piston-rack teeth surface. It is the two-side arrangement of the piston-racks that makes the difference compared to the
state-of-the-art, where the one-sided piston-rack arrangement loads the contact between the roller and the rolling-surface
with a force having maximum equal to 1/3 of the maximum inertia force applied on the piston (TDC) or to 1/3 of the maximum
gas pressure force on the piston, whichever is higher.
Compared to the one-sided arrangement of the state-of-the-art, in the two-sided (symmetrical) arrangement, the maximum
force between the roller and the rolling-surface is more than three times lower and is caused by the inclination of the
connecting rod relative to the cylinder axis and not by the inclination of the piston-rack teeth surface. The connecting
rod inclination varies from zero to a maximum, being always way smaller than the constant inclination of the piston-rack
teeth surface. The inclination of the connecting rod gets significant values when the inertia force necessary to accelerate
the piston or the gas pressure force on the piston are far from their maximum values. The resulting load on the roller is
several times lower than the load on the rollers of the one-sided arrangements.
The inertia and combustion forces on the piston are equally shared between the four piston-racks, then they pass to all
four gear-wheels and then to the four control-racks, i.e. the loads are taken by more teeth / teeth surface than in the
The engine block is similar in size and weight to the conventional engine block of same stroke.
In the oppositely-reciprocating-rack-frame arrangement, below, a rack-frame is pivotally mounted, by the wrist pin,
on the small end of the connecting rod.
The piston comprises two projections/legs each having two "back to back" racks.
Two gear-shafts have, each, gearwheels of different pitch circle diameter. The big ones are meshed with the piston racks, the
small ones are meshed with the racks on the rack-frame.
The rotation of the crankshaft causes the reciprocation of the rack-frame that causes the rotational oscillation
of the gear-shafts that cause the reciprocation of the piston racks and of the piston.
With ratio of piston mass to rack-frame mass (including the wrist pin mass and the mass of the reciprocating part of the
connecting rod) equal to the ratio of the small gear wheels pitch circle to the big gear wheel pitch circle, the inertia
forces per cylinder are completely balanced. This way a three in-line without any balance shaft becomes as inertia
vibration free as a conventional six in-line engine, and a five in-line without any balance shaft becomes as inertia
vibration free as the V-12 and the Wankel rotary engine.
The symmetrical loading of the piston racks enables lightweight, yet robust, piston structure. The opposite happens for
the rack-frame: its racks are one-sided loaded which means need for heavy structure to avoid bending, the racks have to be
secured to each other by traverse beams, the wrist pin and the upper part of the connecting rod reciprocate together with
the rack-frame. Using proper gear wheels, the height of the engine is substantially smaller, the friction is also
decreased because the slider speed is by far smaller than the piston speed, the inertia forces are smaller and the
engine is balanced as regards the inertia forces per cylinder.
Last but not least, this arrangement shifts the combustion to the slow dead center providing additional time to the
fuel to get prepared and burned at good conditions, like the
OPRE engine. The connecting rod is heavily loaded only in tension
(Pulling Rod Engine).