A jet turbine has one gear
Seventeen of EV·ENGINE's machines shift gears. Three do not: the jet turbine, the steam locomotive and the radial prop plane each hold a single figure for road speed per 1000 rpm, so engine speed follows road speed directly across the whole rev range. The gearbox model still runs underneath, with nothing left for it to do.
One ratio for the whole rev range
Most machines in EV·ENGINE shift through several ratios, picked to keep the engine near its shift point through most of a run: the V10 is one of them. The jet turbine carries one ratio: 13.3 km/h per 1000 rpm, a single figure that covers idle at 1,500 rpm all the way to the 9,000 rpm redline. The steam locomotive and the radial prop plane each carry one ratio too, both at 42 km/h per 1000 rpm, though nothing else about the two machines matches: the locomotive idles at 430 rpm and reaches a 2,600 rpm redline, the radial idles at 500 and reaches 2,800.
A single ratio means there is exactly one rpm for any given road speed, for the whole length of a run. Nothing decides which gear the car is in, because there is no choice to make, and the one multiplier has to cover the entire spread alone. On the jet turbine that spread runs from an idle of 1,500 rpm to a redline of 9,000, all carried by the same 13.3 km/h per 1000 rpm. On the steam locomotive the spread is far narrower, 430 rpm to 2,600, carried by the same 42 km/h per 1000 rpm that also carries the radial prop plane from 500 to 2,800.
Each preset still carries a shift figure: 5,200 rpm for the jet turbine, 1,400 for the locomotive, 1,700 for the radial. On a machine with several ratios this is the point at which pressing on trades the current gear for the next one. On a machine with a single ratio there is no next gear to trade into, so the figure sits in the preset unused.
What a single gear removes
The gearbox that shifted four times in half a second could only happen to a machine with several ratios to fall through: lift off high in one gear, and the model cascaded down through every ratio left before the bug was caught. A machine with a single ratio has nowhere to cascade to. Whatever road speed the GPS reports, one multiplication turns it into rpm, and that multiplication is the entire gear model.
What is left to shape the response is the smoothing built into the signal itself. The jet turbine takes 220 ms to settle to a new road speed and its needle a further 200 ms to catch the resulting rpm; the radial prop plane, at 170 ms and 130 ms, catches up faster. Neither figure has anything to do with gearing: they are the only thing standing between a change in road speed and a change in the note, on a machine with no shift point of its own to add one.
Why an unevenness of 1 still works
Firing unevenness is what turns a piston engine's beat into growl: the crank throws are not evenly spaced, so torque arrives unevenly and the ear hears a half-order note beneath the main one. The jet turbine preset sets that figure to 1, next to nothing, against 12 firing-equivalent events per revolution. Combustion in a turbine does not arrive in discrete strokes the way it does in a piston engine, so the model gives it almost no unevenness to work with: 12 evenly spaced events per revolution and a near-flat unevenness figure together produce a tone closer to a pure spooling whine than a piston beat.
The steam locomotive sits at the opposite end: 0.5 events per revolution, one chuff every two turns of the driving wheel, against an unevenness figure of 45, the highest of the three. A single large piston stroke has nowhere to hide its unevenness behind other cylinders, so the model gives it plenty to work with. The radial prop plane sits between the two: 4.5 events per revolution and an unevenness of 15, enough to keep the nine-cylinder beat audible without it dominating the note.
The three machines also disagree on pipe length, which the exhaust comb filter uses to fix its first delay: 45 decimillimetres for the jet turbine against 220 for the locomotive, a spread wide enough to move the resonance from a short, high buzz to a long, low ring. Noise sits high on both the jet turbine and the locomotive, 175 and 200, though for different reasons: intake and exhaust roar on one, escaping steam on the other.
What to listen for
The jet turbine should sound continuous rather than beaten: as road speed climbs, the pitch should rise smoothly with no gear break, and the low end should stay present far longer than on either piston machine here, since the bass emphasis on this preset does not die away until 4,000 rpm rather than the 900 to 1,600 rpm range where the other two lose it.
The steam locomotive should do the opposite: at low speed each chuff should arrive as a distinct event, and only near its 2,600 rpm redline should the chuffs blur into a single texture. The radial prop plane sits between the two again, an audible nine-cylinder beat that never fully disappears even as its own single gear carries it up to 2,800 rpm.
Seventeen machines in EV·ENGINE spend part of every run deciding which gear to be in. Three do not, and what is left once that decision is gone is the plain relationship between a firing pattern, an unevenness figure and a pipe length, unfiltered by any shift point at all.
Hear it
Ten seconds each: an acceleration run through the gears, then the overrun. The same presets the numbers above come from.
- JET TURBINEturbofan · spool
- STEAM LOCOMOTIVEpiston chuff · steam
- RADIAL PROP PLANE9-cyl radial · warbird