Is it true that electric horses are bigger than ICE horses?
In this article I will try to answer the question why many manufacturers of electric motors for sailboats write: this 3 kW electric motor is equivalent in power to 10 hp. gasoline. Is this a publicity stunt or is it really something new?
If you don't want to read on, here's the quick answer: no, the horsepower of an electric motor is exactly equal to that of an internal combustion engine .
Further for those who want to understand.
There is a useful nuance that ICE manufacturers do not use in their engines or use it poorly. This nuance is the efficiency of the screw . The equality of horsepower on the screw does not mean the equal efficiency of the entire propulsion system.
The most effective slow-rotating screw of large diameter and small pitch. The difference in efficiency between a small fast spinning propeller and a large slow spinning propeller can be a factor of two or more (if we are talking about displacement speeds, not planing).
For a number of reasons, ICE manufacturers put a small screw in favor of versatility. For example, in the PLM, the propeller speed reaches 2000 per minute, and for the speeds at which the hull of a displacement sailboat can move, it is desirable to have sufficient thrust at revolutions up to 600-700 per minute. PLA at 600-700 rpm will have very weak thrust due to the small propeller area of the blades. As for stationary internal combustion engines, the manufacturer also tries not to put a large propeller in order not to slow down the boat too much when it is sailing. All this leads to losses in effectiveoperation of the entire propulsion system. It is this fact that manufacturers of electric engines for sailboats use, indicating that their obviously weaker engine pulls as well as a more powerful internal combustion engine. For electric motors, they try to install large efficient propellers, if we are talking about a stationary propeller, then either a folding propeller to reduce the braking force of the propeller, or a conventional, but an engine with an energy regeneration function when sailing.
To increase the driving force of the electric propulsion system, it is best to increase the diameter of the screw, slightly worse - increase the pitch, and very badly - increase the speed. Keep in mind that a slow rotating propeller with a lot of pulling force requires more torque from the engine (according to my data, a two-blade 240mm folding propeller with a 7" pitch at 700 rpm requires a torque of 27 Nm on the shaft), so you will either have to do enoughnew reduction from the motor to the propeller shaft, or put the motor with a large torque. Here we must understand that a fast motor with reduction will weigh less than a slow motor with high torque and without reduction.
The idea of converting the PLM ICE to electric is flawed from the very beginning, because the screws suitable for it are ineffective from the factory.
Some useful formulas :
1) Pm = M*n / 9.5492 [W]
where Pm is the mechanical power in W,
M is the engine torque in Newtonometers,
n is the number of engine revolutions per minute,
9.5492 - conversion factor of revolutions per minute - to radians per second, determined by the "Table of conversion of units of measurement of angular velocity".
For example, we get the torque at 1000 rpm and 5 kW:
M = Pm / n * 9.5492 = 5000 W / 1000 * 9.5492 = 48 Nm (rounded)
2) Pe = Pm * η [W]
where Pe is the electrical power in W,
Pm - mechanical power in W,
η is the overall efficiency of the motor and its controller.
Usually, brush (collector) electric motors (as in electric forklifts) have an efficiency in the region of 0.8. Brushless motors (they have an electronic collector) have an efficiency of about 0.9.
The efficiency of an electric motor drops mainly when they try to remove a torque from it that is greater than the one for which it is designed. Those. high power at a speed much lower than the idle speed of the electric motor.
3) Pe = U * I [W]
где where Pe is the electrical power in W,
U - battery voltage in V,
I - battery current in A.
Brushless motors have such a thing as phase current and phase voltage. Typically, these motors are three-phase, and the motor controller acts as an electronic collector, switching phases in order to create a driving force and turn the motor rotor. The phase current can be greater than or equal to the battery voltage, and the phase voltage can be less than or equal to the battery voltage. The controller performs the function of a converter (like a DC-DC or transformer) in order to give more current and less voltage when spinning the engine, which will allow you to getmore torque when accelerating. When the motor reaches idle speed, the phase current is equal to the battery current and the phase voltage is equal to the battery voltage.