Third and final part of our file on power, the importance of engine speed, but also its constraints, and a technical solution towards which more and more manufacturers seem to be moving: the turbo!
Engine speed
The third major element concerning power is the engine speed, the number of revolutions per minute that it makes. It is a measure of how often a motor can cycle its energy release. Power is simply the result of torque and engine speed. It therefore depends directly on these two forces. The higher the engine speed, the more power it will have, within a certain limit.
There are limits to the benefit of using RPM to increase horsepower, because as RPM increases, friction losses also increase.
When the engine speed of a Ducati Panigale Superleggera V4 can reach up to 17 rpm, we say to ourselves that the limits of physics can be pushed quite far. What about MotoGP? Dorna rightly considers investing money in technology that has no mass application to be a bad investment. Thus, the maximum revs are limited, not just by the electronics, but by the sizing of the components, which is regulated through the use of larger pistons and increasingly shorter strokes. Thus, Dorna set a limit at 000 x 81 mm, or a bore/stroke ratio of 48,5. This also requires specific distributions, with pneumatic or desmodromic return valves, because the conventional spring system is too constrained at such speeds. Without forgetting that the faster an engine runs, the more fuel it consumes…
And the turbo?
Reaching high revs is an obstacle to meeting anti-pollution standards. Indeed, at very high revs, in order for the exhaust gases to exit the cylinder while having time for a fresh air/fuel mixture to be admitted, this requires significant valve crossing - when the intake valves open before the exhaust valves close. At low revs, however, this crossover of the valves causes emissions problems, because it allows time for unburned fuel to exit directly into the exhaust.
Rather than increasing the engine speed, another technique to obtain similar power would be to use a turbo. In turbocharged engines, power increases by compressing the mixture. But because of the effects mentioned above, the increase in power also remains limited if the entire engine is not dimensioned for it, reliability decreases and the risk of breakage increases. Unless it is developed for use with a turbo: Yamaha seems to be very interested in this at the moment...
Consequently, manufacturers are working to test engines on instrumented test benches, with hundreds of possible filling and ignition maps, to find the best power/consumption/pollution compromise.
