HONDA vtec engine

[Mechta]

There are two different threads going on here. One is about engines with a wide RPM range, and the other is gearboxes.

The advantage of a high revving engine is that it gets its power from its speed and not its capacity, so it can be smaller. If it has variable valve timing, of which V-TEC is one manufacturer's term for it; it can also idle comfortably without the wild valve overlaps that a 'stage 3' racing camshaft would have to get power at high RPM. To make this high RPM usable on an aeroplane propeller, which should work on the biggest volume of air as possible and keep the blade tip speed subsonic, a gearbox and, ideally, a variable pitch propeller, are required.

Now onto gearboxes. A gearbox works fine when it has a smooth constant torque coming into it and its not being asked to accelerate or decelerate. On a gas turbine, the torque is a continuous load so there isn't a problem.


On a car, between the engine and gearbox is a flywheel and a clutch or torque converter. On a direct drive aeroplane engine, the propeller IS the flywheel. The problems start when the engine and its flywheel are separated by a gearbox.

When the piston travels up the cylinder compressing the fuel mixture, this takes energy, so the engine slows down. When the mixture ignites, the engine accelerates. This results in torque reversal, which is why at a slow idle the engine rocks backwards and forwards, especially if it is on anti-vibration mounts, as the flywheel effect causes the crankshaft to try and go at constant speed, and the engine tries to rock in the opposite direction to accomodate the acceleration and deceleration.

If a gearbox is now added to the drivetrain it will try and accelerate and decellerate the propeller. The propeller being a good flywheel, takes no notice, so the changes in speed try and change the direction of the bit in between, AKA the gears. On a car if it is at a slow idle, the likelihood is the clutch is depressed, and the gearbox is disconnected from the engine.

In the aeroplane, if there is no clutch, the torque reversal is hammering away at the gears. This is worse at low RPMs as there is a longer duration and hence greater amplitude of movement (the hammer hits from further away). Some microlight engines use a centrifugal clutch, so the gearbox is only driven when the engine is above the damaging torque reversal RPM. The alternative as per Thielert is to put a torque limiter in, which is a clutch with a preset load on it. If the torque reversal gets too much, the clutch slips.