How do hybrid cars use less fuel? Isn't efficiency wasted in the extra conversions of energy?

Question

I'm thinking of hybrids that have a gas motor and electric. If they aren't charged with electricity externally, all of the energy to run it comes from gasoline. Hence, how is it more fuel efficeint to transfer energy via an elctric motor to a battery and then back to the wheels via the motor again to use for drive (at slow speeds or whenever they do this)?

Answer 1

Electric motors and generators are extremely efficient, in the high 90s. The losses are small compared to the inefficiencies of gasoline engines and regular transmissions. In a hybrid, the gas engine is smaller because it always runs at its most efficient speed, with no need to change speed as the car speeds up and slows down. You don't need that inefficient transmission always changing gears, and the gas engine can just shut off completely if you are stopped in traffic. This makes them very efficient in stop and go city traffic.

Answer 2

Joel has it mostly right, but let me try to be simpler.

Most of the time the gas engine in the hybrid is running at a fixed speed and load turning a generator.  You can get much better gas mileage by specializing the design to do that really well.

A large engine operating over a wide range of speeds and loads, can't do nearly as well.

An additional factor is to be able to use the generator as brakes, while also charging the battery, "regenerative braking".

Answer 3

Simply, gasoline engines can't be efficient at all speed, so compromises (that hurt efficiency) have to be made in normal cars so that an engine can run at idle, provide low-RPM, high-torque starting power, and top-end performance. Hybrid (gas) engines are engineered to be efficient at a moderate RPM/torque combinations, and the electrical system (battery and motor) take care of the extremes--eliminating need for idle, and providing power for short duration, extreme accelerations.

Answer 4

I think u are right as I have a 98 Honda Civic that gets better mileage than most of the hybrids I have Sean.

Answer 5

Yes it is inefficient because of energy conversion and the complexity and weight of the petrol engine so hybrids do use more fuel than a full electric vehicle (even if the electric comes from a fossil fuel source)

Infernal combustion engines are very inefficient because they rarely operate at optimum load.

The Prius petrol engine runs at a number of optimised set rpm. the normally wasted power is saved to the battery, and also captured when slowing down.

An electric motor delivers much better torque (acceletation) performance so can assist the petrol when accelerating.

Answer 6

What extra conversions?

A hybrid has two engines, electric and gas.  When the gas is called for, the electric recharges from the engine cycles that are wasted in a normal car, like idling at a light or while braking.

Answer 7

The electric motor is best for starting from a dead stop and for accelerating, because it has highest torque at stall.  The gas motor is used more for 'cruising' at steady speed, and it runs through a CV (continuously variable) transmission so it always runs at its optimum speed.  So both motors are used at their greatest efficiency.  That's the biggest fuel saver. (It's kind of strange to speed up and slow down and hear the engine always running at the same speed!) If you stomp on the accelerator to pass a car, for instance, rather than just dump a lot of gas into the engine, the electric motor comes on to assist.

You don't need a starter because the electric motor gets the car going.  The valves of the gas motor are operated by solenoids, so when it's time to get the motor going the valves are all opened and the clutch is let out, allowing the engine to get up to speed, then the valves begin to work properly and gas is fed to the motor and it's running. So it's not a big drain on the battery to get the engine going like it is in a conventional car.

Also there are 'dynamic brakes'.  Instead of converting energy to heat, when you step on the brakes the momentum of the car is converted back into electricity that goes into the batteries.  The brake pads touch the disks only at the very last second, so the brakes last nearly forever.

And when you're going downhill the same thing happens, the momentum of the car going downhill generates electricity.  In fact there's a 'gear' in the transmission that's like downshifting to go down a hill where the engine creates more resistance to slow you down, and more electricity is generated.

I think the main thing is that yes, there is some efficiency lost in the generation of electricity, but it's not nearly as much as the inefficiency in the gas engine in acceleration, and the energy lost in downshifting and braking.

Answer 8

Don't Hybrid cars use electricity as well as fuel?  I don't really know much about them, but I would think that the production of electricity is as damaging as the production of fuel.  Electric prices keep going up because of limited resources to produce it and there are even power outages from people using too much electricity.  If I am correct in that the hybrids use electricity, I think we need to re-think our priorities.

Answer 9

Some use a combination engine utilizing both gas and electric.  Efficiency is not wasted in the conversion, instead the electrical power is re-charged.  Hybrids are more fuel efficient period.

Answer 10

depends how you define efficiency.

do you mean it in the thermodynamic sense, or simply in terms of the fuel consumption rate?