In my first post of a planned series on what we can do to reduce our nation's utilization of primary energy, I mentioned what might be done by replacing some of the current light vehicle fleet with more efficient vehicles. And, as I've mentioned on multiple occasions, my personal vehicle is the hybrid Lexus CT200h. The CT200h has an EPA combined city and highway estimate of 42 m.p.g., comprised of a highway estimate of 40 m.p.g. and a city estimate of 43 m.p.g.
Does the hybrid configuration confer an advantage? A comparison with a high-mileage non-hybrid vehicle will be enlightening. For this, I'll select the Chevy Cruze Eco. This vehicle achieves an EPA combined city and highway estimate of 33 m.p.g., comprised of a highway estimate of 42 m.p.g. and a city estimate of 28 m.p.g.
The first thing to note is that the Cruze's highway estimate is very slightly higher than the CT200h. This is because, on the highway, the CT200h's propulsion is provided by its internal combustion engine, with the electric motor only providing power for hill climbing and passing. And the CT200h is a significantly heavier vehicle, primarily due to the additional equipment required for the hybrid drive train (hybrid battery, auxiliary 12 volt battery, electric motor and generator, and other associated equipment). These Cruze advantages are slightly offset by the fact that in most hybrids (though not the CT200h) utilize a smaller internal combustion engine, which is sufficient due to being able to rely on supplemental torque and power from the electric motor when needed. This smaller engine can operate at a better point in its engine map and will suffer lower pumping losses. The vehicles have very similar drag coefficients and drag areas and so drag forces are roughly equivalent.
The second thing to note is that the city mileage for the CT200h is significantly higher. This is because city driving often consists of starts and stops, acceleration, etc. The hybrid vehicle utilizes regenerative braking (more later) to store some of the kinetic energy added in acceleration from stop to speed rather than dissipating all of it as heat. It also utilizes the very efficient electric motor to start from a dead stop. And, of course, plug in hybrids (the CT200h is not such a vehicle) store significant energy that doesn't come from burning gasoline at all. It must be noted that ALL of the energy used in my CT200h comes from burning gasoline, regardless of whether the battery pack is charged by coasting down a hill, the regenerative braking system, etc.
So it's pretty clear that hybrids have the potential to achieve superior fuel economy compared to similar non-hybrid models. The extent to which fuel can be saved by a specific driver will depend on a couple of things - particularly driving habits and the division between city and highway driving.
What are the particular aspects of the hybrid drive train that enable such a vehicle to achieve better fuel economy than an equivalent non-hybrid vehicle? As mentioned above, one of the major items is the hybrid's ability to utilize some of the energy added to the vehicle by the internal combustion engine that the non-hybrid wastes.
First among these is regenerative braking. This is a system wherein part of the braking action effected by pressing the brake pedal is provided by having the wheels turn a generator against the torque of the generator action (think of the exercise bicycle at a gym - as you turn up the resistance, the energy you input through the pedals powers a generator that may power a display, etc.). The generator charges the battery, whose energy can then be used for acceleration, or even as the sole motive force at low speed.
Second is recapture of energy during coasting. As I let off the accelerator to slow down, go down a hill, etc., again, the turning of the wheels is used to run the generator and charge the battery. In the CT200h, both of these actions can be monitored in a display in the dash and in more detail on the monitor that also displays the GPS map, and other information I may select.
Next and again as mentioned above, the non-hybrid must have a sufficiently large engine to provide power for all required driving regimes (plus some to spare). This is much more than is required for constant speed driving. For any internal combustion engine, specific fuel consumption is lowest (i.e., the engine is more efficient) when operating near its maximum torque specification. Thus, the non-hybrid vehicle operates at a relatively inefficient portion of its engine map during a large portion of miles driven.
Further, the larger engine has significantly larger pumping losses as low pressure in the cylinders during the intake stroke draws in the air/fuel mixture, and while pumping the burned air/fuel mixture out of the cylinder and into the exhaust system.
The internal combustion engine in the hybrid can typically be sized significantly smaller than the engine in a non-hybrid vehicle (though such is not the case in the CT200h vs. Cruze Eco comparison - the CT200h utilizes a 1.8 liter engine as compared to the base model Cruze Eco's 1.4 liter engine). This is because, in those regimes where high power is required (acceleration from a stop, hill climbing, passing, acceleration onto freeway, etc.) the internal combustion engine can be supplemented by the electric motor.
There are other "nickel and dime" enhancers of fuel economy. For example, when I stop at a light (and assuming the hybrid battery pack is sufficiently charged) the internal combustion engine turns off. This also occurs gliding down a hill, gliding to stop, etc.) and, of course, the stopped engine doesn't burn any fuel.
In summary, the main fuel savers are recovery rather than dissipation of kinetic energy, lower engine size, and operation at more efficient points on the internal combustion engine's map.