In this question I’m comparing non-plug-in hybrid fuel/electric vehicles with fuel-only vehicles. Also, I’m not disputing that hybrid cars are more fuel efficient, I’m just trying to understand how.

If I remember two things from high school physics, it’s:

  1. Creating energy from nothing is against the law.
  2. Converting energy from kinetic to stored energy and back again is always less that 100% efficient - you end up with less kinetic energy than you put in, the rest being “lost” to the environment in the form of heat or whatever.

Both a fuel-only car and a hybrid derive all their locomotive energy from the fuel (ok, and gravity, if you’re going downhill, but let’s ignore that). But the hybrid involves an extra conversion to/from stored energy on the way, i.e. charging and discharging the battery, which according to the second principle above should make it less efficient.

Furthermore, the battery entails additional mass, which, if it were dead weight, would reduce the power/weight ratio of the car, reducing efficiency.

So to achieve greater efficiency, the hybrid engine must somehow make use of energy that would ordinarily be lost in the engine of the fuel-only car. In order for the hybrid to be more fuel efficient than the fuel-only engine, it needs to do this in a way that overcomes the above inefficiencies and then some.

So my question is, what broad physical/mechanical techniques does a hybrid engine use to achieve this, and (in rough numbers) how much of the efficiency gains come from the different techniques?

I know there’s regenerative braking, but how efficient is this in practice? I.e. how much of a car’s kinetic energy that would otherwise be lost through braking is actually captured and re-used as locomotive energy? And is this basically it, or are there other techniques that significantly contribute to the efficiency of a hybrid?

  • $\begingroup$ When they calculate the fuel used - is it the total or just the gasoline ie they assume the electricity does not count... $\endgroup$
    – Solar Mike
    Commented Apr 13, 2018 at 7:56
  • $\begingroup$ @SolarMike In the type of hybrid engine I’m asking about, the battery is charged by the engine. There’s no external power source so (gravity aside) all the energy for driving the car ultimately comes from the gasoline/diesel fuel. So it’s relevant to directly compare the gasoline/diesel usage of the two types of engine. $\endgroup$ Commented Apr 13, 2018 at 8:33
  • $\begingroup$ But then you have not included gravity to get the true energy consumption ... an energy balance should include all the terms on either side... $\endgroup$
    – Solar Mike
    Commented Apr 13, 2018 at 9:42
  • $\begingroup$ @SolarMike If you’re driving on the flat, gravity doesn’t come into it. But even if you’re not, I don’t think there’s a big difference in the way gravity would impact on the fuel efficiency of the two different types of cars. The conversion between gravitational potential energy and kinetic energy for the two systems is essentially the same. Even the extra mass of the battery doesn’t make a difference here as it cancels out (objects fall at the same rate under gravity regardless of mass as Galileo demonstrated). $\endgroup$ Commented Apr 13, 2018 at 9:48
  • 1
    $\begingroup$ From what I understand (bear in mind I'm no mechanic) the hybrid car uses the electric engine to handle the variations in the required powee, allowing the internal combustion engine to constantly operate in its 'ideal' output range which is significantly more efficient that forcing sub-optimal performance out of the internal combustion engine. $\endgroup$
    – Ummdustry
    Commented Apr 13, 2018 at 11:11

2 Answers 2


The difference is the regenerative braking. It take as much work to decelerate a car as to accelerate. I don't know the actual efficiency but even 50% recovers a lot of the energy that would have been lost as brake heat and brake dust.

The electric assist control is also tuned to let the gasoline engine operate in a range where it is most efficient.

According to this link the DOE puts efficiency at 60%.

  • $\begingroup$ this second point is probably more important as it is also why diesel electric propulsion is the prime choice in larger machines(ships). IC engines are significantly more efficient in a narrow band of their range. $\endgroup$
    – L Selter
    Commented Apr 16, 2018 at 7:40
  • $\begingroup$ @LSelter In the case of a car I don't agree $\endgroup$
    – paparazzo
    Commented Apr 16, 2018 at 7:54
  • $\begingroup$ Tesla and the DOE quote regen efficiency at around 64% (turn-around power, not just power pulled from the generator). Skeptics claim real-world efficiency is half that; I tend personally to have some faith in the DOE $\endgroup$ Commented Apr 16, 2018 at 14:31
  • $\begingroup$ @CarlWitthoft Even half that is still a lot of recovered energy. $\endgroup$
    – paparazzo
    Commented Apr 16, 2018 at 16:50
  • $\begingroup$ No argument there! I love watching my energy/km meter go green during long downhills. $\endgroup$ Commented Apr 16, 2018 at 19:04

Paparazzo pretty much nailed it, but I'd like to add a few things.

Regenerative braking is certainly a large part of it, but there are other synergies when utilizing multiple engines in an automobile, too.

  • The ICE can be smaller and sized closer to what is needed for normal operation instead of peak power since the electric motor helps the ICE during the less common acceleration events
  • The electric motor moves the vehicle at lower speeds where the ICE is less efficient, specifically when accelerating from a standstill
  • The electric motor helps move the vehicle at speeds at which the ICE would have to operate further away from its most optimal point, helping to keep the ICE within its optimal range
  • The ICE can be turned off during idling. This is called stop start and is not limited to hybrids
  • The ICE can be designed to be more efficient within its narrower operating range since it does not have to be a master of all trades
  • The batteries and the electric drive allow for regenerative braking which helps recoup some of the energy lost during normal operation

In addition to this list from geotab, here are a few more insights to energy balance around an automobile:

  • in certain hybrid setups (unlike through-the-road setups) the motor can charge the batteries during times of idle, utilizing the best load/speed for the motor
  • being able to undersize the ICE is inherently beneficial to thermodynamic efficiency (similar to geotab point #1). ICE's operate best under high load. Ergo, if you can undersize the motor and force it to constantly be under high load, then kick the electric motor in when the peak load exceeds the capabilities of the ICE enter image description here

Other considerations: I don't have any data for this and at the moment do not have time to research it, but I think that there could easily be a human factor to consider as well. I've driven with many individuals, and I don't think that any one will argue that different people have different driving styles. Considering that some people accelerate slowly, while other accelerate hard and brake hard. Considering the synergies of the hybrid electric-ICE power train, it's possible that the hybrid power train can more robustly serve the two different drivers under equally efficient conditions. Consider that the aggressive driver would lose substantial energy in the braking system - and to get the acceleration they desire would require a larger ICE (think big V8s). they would spend more time NOT under peak load, which is what an ICE needs to be efficient. The hybrid drive train would deliver their desired acceleration, while maximizing thermal efficiency AND recouping the energy spent at the above quoted 60% efficiency.

Power curves of ICE and electric drive trains

Then imagine that slow driving individual gets in the same vehicle; they would likewise be penalized for their slow driving in a large ICE. Considering the hybrid drivetrain, this individual would no longer be penalized for their tame driving habits. In essence, the hybrid drive train could better service larger extremes in driving style, and is over all more robust in terms of energy efficiency.

Just a thought :)


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