Power Band Differences

[calab]

I am wondering what the difference is in the power band between an electric motor and lets say a regular 2 stroke gasoline powered 65cc engine rated at 2.75HP, so lets say the electric motor similarly outputs 2000 watts on a continous basis without overheating. I remember reading somewhere that the electric motor has max instantaneous power all the way through the power band, whereas the gasoline engine is limited in this regard. These motorized bicycle engines are 2 stroke, but they also sell them as 4 strokes too, in the 2.5HP range 50cc range, so the power band between those two differently stroked motors would be different as well even if the cc's matched.

Heres a 50cc 4 stroke

 

[dogman dan]

I don't have any graphs, but all those guys behind me on a race track showed how vastly better my torque and power was in the first 50 feet leaving the start line. I couldn't win, because my brakes were crap and they had more track experience than I did. But it was fun passing nearly all the gas powered bikes, even if I started from the back of the lineup.

There was a small group of guys with highly modified or otherwise larger gas motors I couldn't beat. Up to 10hp motors those guys had. But even a 1000w kit with a pingbattery was beating a stock china girl 50cc easily, and giving a 90cc china girl bike serious competition.

The next race, I brought 3000w, and was running in the top 5. Almost won that race, but misunderstood a black flag for the end of the race and came in. That race the guy with the 10hp snowmobile motor broke down, so I had a chance.

Race after that, the gas bike guys really brought it, lots of new 10hp snowmobile motors. But Paul D brought a powerful mid drive and kicked their asses. It was the better torque leaving the corners that killed them. AND, his incredible riding skill. Not easy to finish without crashing on that cart track.

 

[Ypedal]

I"ve built a chinagirl and i have a Mitsubishi 50cc on a Staton Inc gearbox setup in the shed. As Dogman pointed out, it's no contest off the line, gas motors with slip clutches need time to get going where electric motors just go instantly

Efficiency is a whole separate thing, gas motors make good power in a limited rpm range while electric motors are far more efficient over a much broader range.

batteries vs gasoline is the equalizer in this game... but i ride my ebikes while the expensive gasser never even came out of the shed this last season :wink:

 

[Punx0r]

For a brushed DC motor it makes peak torque at 0rpm reducing linearly to zero torque at no load speed, with (IIRC) peak power at halfway.

Brushless motors are different, the extremes are still the same, but depending on the motor and controller characteristics, you can pretty much make whatever torque you like at any speed. They can produce constant power over a very wide speed range.

 

[speedmd]

Very different power band on the electric.

 

[dogman dan]

I'm not sure how to read that graph, but it looks a lot to me like the best combined torque and efficiency is the middle rpm?

Like the benefits of riding 3/4 throttle vs full? Highest torque is at 0 rpm, but efficiency is worst at 0 rpm. Fully wound out, torque is low.

Indeed, looks like a very broad band where torque is adequate, and efficiency good. Must be why we never shift our bikes after we put a motor on them.

 

[speedmd]

It is more a opposite power slope to the ICE below 5000 rpm. Below 2000 rpm the current is most likely limited to get the flat line. Interesting and much discussion on multiple threads is the extremely broad power band that can be achieved with a good motor / controller design. Even at 1000 rpms with a light load/throttle you can achieve 90% efficiency. Full throttle at 13000 rpm you can also have 90% plus eff. with the right control method. Nothing on the ice side can come close to this type of power band.