Why do cars have a better power to weight performance than electric cars, even though electric cars are called more "efficient"?
- 5 years agoFavorite Answer
This is a very good question, because there is a lot of general confusion surrounding these issues. It becomes more clear when we consider the fuel and the engine separately. Paradoxically, power depends heavily on the fuel system while fuel efficiency is primarily quality of the engine.
Power is the speed at which energy can be transformed to in the case of a car mechanical energy. The energy comes from the fuel. Gasoline is a highly dense fuel. Gasoline can therefore release a great deal of heat energy very quickly. For even more power special fuels are used. Like holding a tiger by the tail, such rapid energy conversion also tends to be very inefficient. IE it wastes a lot of fuel. While a bench test of a ICE may show as high as 35% efficiency when put in a vehicle: stop and go traffic, the required transmission, lots of variation in RPM, the vehicle efficiency is closer to 15% from tank to wheels.
Traditionally, power to weight was a strong indicator for acceleration. The battery electric car has a disadvantage. The energy density of batteries is much lower than gasoline. The typical EV battery holds only about the equivalent of 1 or 2 gallons of gasoline. The chemical reaction in batteries is also lower than burning gasoline so the energy available to an EV is much less than an ICE. In addition batteries tend to be heavier than a fuel tank. The power to weight ratio would tend to be less for a battery electric car.
But the electric car is also a "different animal," and the rules that apply for an ICE vehicle don't apply as well to electric cars. Power to weight is one of these rules because electric cars have different tricks to more than make up for the difference and allow electric cars to beat ICE vehicles consistently at drag strips and that also makes electric drive one of the top choices for KERS power boost in formula one racing.
While the fuel for the ICE is superior in power than the battery for the EV the ICE wastes most of that advantage in inefficiency. The Electric motor is highly efficient with some in solar cars measured at 99% efficiency. While the ICE requires a transmission because it only has torque in a relatively narrow band of RPM (3000 to 5000,) the electric motor does not require a transmission because it has torque from 0 RPM. With superior useable torque and much better efficiency the electric drive is a much better system with the only problem of supplying power to it.
There are a number of solutions for powering an electric motor in an EV. We could switch from batteries to capacitors. These have a much better power density by weight (than batteries) but they tend to hold less than a tenth the energy. We could electrify the roadways. This is what is done with trams, trolleys and subways and there is a method to electrify the roadways wirelessly. All these would increase the power available while maintaining the superior efficiency of an EV.
(solar cars, hybrids and fuel cell electric cars are mostly aimed at increasing the total amount of the energy supply rather than the potential rate of energy transfer (power) over what is available from batteries. An increased energy supply on the vehicle translates to more range, but does not directly affect power or efficiency.) :
- OscarLv 75 years ago
Power and efficiency are two different things. Power gets you there faster or will haul/pull a heavier load. Efficiency gets you there using less fuel be it gas or electricity. But you aren't going to move a 80,000 load with an electric car. At least not all at once and by the time you break it up to what it can haul, the efficiency is out the window to.
They are only efficient in certain limited circumstances. If the distance isn't too great or the weight to high. Like to the store for milk. Or to work and back say 40 mile round trip or so. Past that things get iffy. At night lights are required, batteries don’t hold a charge well in the cold and your need to run a heater or air-conditioner cuts back on mileage.
Plus batteries are still large, bulky, heavy and take a while to charge up. Usually overnight. So you aren't going to pull into a filling station and be out in a few minutes.
- KanoLv 75 years ago
Batteries. Batteries are heavy and incapable of releasing large amounts of energy quickly without being damaged in the process.