How does a hybrid car work?

I understand some of the fundamentals: a small, efficient gasoline engine keeps the batteries charged for an essentially electric car. However, can anyone answer the following questions for the latest generation of hybrid cars:

Does the gasoline engine run all the time, or is it shut off when the battery gets fully charged?
Does the gasoline engine assist in moving the car, or only used to charge the battery with the electric motor providing all the force to make the car go?
If the gas engine assists, what mechanism allows a smooth transistion between the electric and gas motive inputs?
Is the energy for braking used to provide electricity to help recharge?
If so, how is that effected?

I can’t speak for all hybrids, only mine (Toyota Prius, owned for 3 days).

The engine runs when power is needed (over about 20 mph) or when the battery is low. Both the electric motor and the engine can power the wheels, but when the power demand is low, the battery is charging. The motor seems to come on when a surge in power is needed (climbing a hill). If no power is needed, the engine shuts off. Going down a steep hill, the wheels will charge the battery with the engine off. The Prius uses a CVT (continuously variable transmission) to connect the engine to the drivetrain. I assume the electric motor is direct drive.

The New Accord hybrid uses the gas engine all the time, with electric motor for supplementing it. The result is modest gains in power and economy.

I think it is this ability to recover some of the energy that was used to climb the hill in the first place that accounts for the higher fuel mileage. I would also think they could use battery charging as dynamic braking to slow the car down.

I don’t see how there would be any benefit if the batteries were only charged by the engine. In fact I think a system that only charged by running the engine should be less efficient than a prime mover-electric system that used a generator to run the drive motors directly since there is some inefficiency in the battery.

I’ve always understood that, while there are conversion inefficiencies, the gasoline engine can be sized for average power requirements rather than max and gains much efficiency by running at optimal RPM at all times.

Probably depends on the vehicle but the ones I’m familiar with cycle the IC engine on and off - for example, turning it off when you are at rest for a while.

Both IC and the electric motor are used to move the vehicle. I think that the electric motor is used a lot when accelerating from zero, since it can provide a lot of torque quickly at low speeds.

Beats me :slight_smile:

Yes, and I’ve heard that’s where the biggest gains in mileage are achieved, since all the energy is otherwise lost. It’s called “regenerative braking” and you basically use the brakes to drive a generator to recharge the batteries (instead of just turning all the kinetic energy into heat and noise).

Here’s a nifty article from How Stuff Works on hybrids:

I’ve had my Prius for about 13 months. The Internal Combustion Engine (IC) and the electric motor/generators (there are two) are connected to the transmission through a power split device. An excellant explaination of how all this works is located here:

The Prius engine runs on the Atkinson/Miller Cycle rather than the Otto Cycle used by most other IC engines in automotive use. This is supposed to allow higher efficiencies at lower power levels.

Regenerative braking does recover some power, but not very much at all in real terms.

The electric motor provides most of the initial “off the line” acceleration. Electric motors provide max torque at zero rpm. The Prius accelerates quite well before the IC even has a chance to start.

The site I linked to above has a lot of good explainations.

Color me dense. I just can’t see how a significant reduction in the fuel used can be realized without a significant recovery of energy that would otherwise be lost in braking. Also by recovering on the down hills some of the energy used to climb the hills in the first place.

Without such recovery the only source of energy is the gasoline burned. It takes a certain amount of energy to move an auto at 60 mph for one hour. Without the recovery of otherwise wasted energy the amount of fuel used for equally efficient verhicles is the same irrespective of the maximum output capacity of the prime mover.

With an engine driving a generator to supply power to electric motors, the engine can be operated at its most efficient speed without the need for the intermediate step of charging batteries. I think the batteries are there to store the recovered energy so it can be used later as desired.

Don’t forget that most hybrids shut the gas engine off when the car is still. That is a time when convential cars are getting 0MPG.

Other things that help the Prius get good milage are a low coefficient of drag, and higher pressure tires. (the 2004 has less efficent tires that the older models, but they last a LOT longer)

One could design a convential car that had the same COD and same relatively small gas engine as the Prius. And it would get good milage. But it would have awful 0-60 times.

The Atkinson cycle is more efficient, but has lower low end torque that an Otto cycle engine. Luckily that is where electric motors shine.

I must point out that the Prius also has things that minimize emissions (storing cooland in a thermos to get the catalytic converted hot sooner) in addition to good mileage.


Good point. We haven’t yet specified exactly how much better fuel mileage the hybrids get. 10% - 20% or what? Not idling and accelerating on electric at start would help some. However, the battery power used to accelerate from stop has to be replaced at some time by running the gas engine when it otherwise would be off.

These aren’t necessarily unique to the hybrid.

With a better transmission the acceleration of a conventional car could be improved with somewhat less complications than the hybrid has. Probably in the future less complex hybrid control systems will appear. For the time being I would certainly avoid buying a used hybrid. And eye popping 0 - 60 times aren’t the be all and end all of owning a car.

Another factor. I had forgotten that the engine doesn’t use the Otto cycle. I’ll have to look up the Atkinson which wasn’t in the picture when I went to school.

Again, this isn’t necessarily unique to hybrids.

Don’t get me wrong, I think hybrids are a great idea and think that something like them is the car of the future. I’m just trying to get at a correct answer to the OP and your good post has advanced that aim by quite a bit.

According to the Toyota website, and I’m sure they are using EPA numbers, the Prius is rated at 51 Hiway MPG. The next highest rated vehicle is the Echo at 41 Hiway MPG. The Echo is a smaller vehicle than the Prius, weighs less too. The interior room of the Prius is close to that of the Camry, which is rated at 33 Hiway MPG. Yes, these are EPA numbers and probably not real world, but wouldn’t they be useful for comparison.

And BTW, I have gotten 49-50 MPG over a tank of fuel. Mostly Hiway and speeds above 70 MPH. I could do better, but can’t stand to slow down. :slight_smile:

Well, I’ve always understood that IC engines are most efficient with their throttle’s wide open (and the gear ratio set to keep the revs reasonably low). There’s not just a “most efficient speed” - unless the throttle is wide open, the engine is sucking air through a constriction. The effective compression ratio is also smaller since the cylinders are filling with a rarified charge.
Because of this, a small engine with it’s throttle completely open producing 30 HP will use less gas than a medium engine throttled back to 30 HP, and both will use less gas than a huge engine throttled back to 30 HP.

The engine-motor-generator-battery combo allows the engine to be run in two states - open throttle and off. In effect, instead of using the throttle to reduce the engine power, you’re switching the engine on and off for different lengths of time and using the time-averaged power. The batteries act as an energy reservoir to keep the power going to the wheels constant.

The motor-generator combo also allows an effectively infinitely-variable gear ratio. Arguably you could run the engine with the throttle wide open all the time and control power by cranking the effective gear ratio up and down to force the revs up and down. There’s obviously a limit to the degree of power variability you can achieve with this approach though.

Here is my quick take on the Honda insight I have owned since 2000.

The gas engine runs all the time excluding at stops. Then it sits idle till you place the car in gear again.

The Honda motor only assists when your “putting your foot in it.” The gas engine can and does add an additional 14 horsepower in passing ans starting situations.

A funky high tech transmission of some kind, how excatly it work I do not know.

Yes, regenative breaking and downshifting are two effective ways that the honda recovers energy that would be otherwise wasted. I will leave the honda in gear and allow the gears to slow me down, Watching the charging meter I can see it recharges better than the gas engine at times. Plus, I can tell adistance in stopping ability when the speed gets to low for the gear I am in to continue recharging. Breaking does an even better job of adding a quick charge as well.

Many other factors take effect for the good gas milage. Aerodynamics of the car help, The biggest factor is how it is driven. Slower take off, and staying at a consistant speed is a great help. My favorite tactic is to follow about 10 car lengths behind a semi on the interstate. This gives me an easy visual outside of the car to keep a constant speed on average 5 miles per hour above the speed limit. I have consistantly gotten above factory listed MPG this way. My record so far for a trip over 100 miles is standing at 84 MPG.
This I contribute to constant speeds, and learning how to drive in a much more fuel efficient manner.

From what I have gathered watching Hybrids over these few years. At the Beginning, Honda’s hybrids out performed Toyota by a landslide on the interstate but did not fare so well in the city. Since 2000 Toyota has made bigger strides all around than Honda in updating the technology and better milage.

My 2000 insight would still win in a coast to coast MPG race against any factory toyota hybrid on the market. Yet, the insight is not longer produced, and it has serious lack of space compared to the toyota fleet. Plus, Toyota will soon be able to surpass this and will larger cars.

I feel Honda is going to win the Beta award for thier hybrid technology and Toyota the VHS.


I think this might well be true of a carbureted engine but I’m not sure it applies to computer controlled fuel injection. I can see any reason why a restriction in the air intake should affect the fuel used with the latter stetup.

It’s been a long time since I ran engine tests, but I seem to recall that there is an efficiency maximum somewhere other than at wide open throttle.

Having gotten out some very old references I think what you are talking about is volumetric efficiency.

This is the ratio of the weight of air taken in at some particular throttle setting to the weight of air taken when intake manifold pressure is at atmospheric. With the throttle less than wide open the manifold pressure is below atmospheric (unsupercharged engine) so the weight of air is less than at atmospheric and the efficiency less than 100%. With the throttle wide open the intake manifold pressure is nearly atmospheric and the volumetric efficiency is near 100%.

However I do believe that there is a maximum in the overall combined volumetric, thermal and mechanical efficiency at some point below wide open throttle.

And yes, a small engine running at 30 hp will have a higher mechanical efficiency than a big one at the same output. Mechanical efficiency is a measure of how much power it takes to run the engine itself. Since there is more engine to run with a bigger engine it’s mechanical efficiency at low power output is not as good as a small engine running near its maximum output.

Interesting aside: people have started modifying (NY Times - registration required) the Toyota Prius to plug in to a wall outlet and to use better batteries.

Stoichoimetry. The fuel injection has to add the right quantity of fuel to match the oxygen in the intake air. You can go a little lean or a little rich, but too far one way or the other and things start going wrong.

Depends on the load characteristics. A wide open throttle is most efficient provided that the revs don’t get too high. If your setup is giving an efficiency maximum somewhere other than a wide open throttle for a particular power output, you can always beat it with a higher gear ratio and a wide open throttle. Of course, a higher gear ratio may not actually be available! Also, if the power level is low you might not be able to run the engine slow enough to take advantage of this.

Aha! Here’s where we’re talking about different things. The effect I’m talking about is much larger than the mechanical efficiency, and it has to do with the thermodynamics of heat engines.

Heat engines are generally modelled as having a heat source at one temperature, T[sub]H[/sub], and a heat sink at a lower temperature T[sub]C[/sub]. The maximum possible efficiency from a “perfect” heat engine is given by (T[sub]H[/sub]-T[sub]C[/sub])/T[sub]H[/sub]. That’s as good as you can get, ever. In a real engine, you have additional losses on top of this. The pont being, the higher T[sub]H[/sub], the more efficient the engine.
Now, when an IC engine is running at open throttle, it draws a mass of air into each cylinder at one bar on the intake cycle. Your carb or fuel injectors add an appropriate quantity of fuel to that air on its way in. Then the mix gets compressed, say by a factor of 9 or so depending on the engine, which heats it up considerably. The spark lights it off and you get a big hike in temperature and pressure and I’m sure you know the rest! But that peak temperature and pressure governs your effective T[sub]H[/sub] in a gas engine.

If you have the intake throttled down so your engine is sucking though a small hole, you draw the same volume of air in but it is rarified - at a pressure lower than one bar. E.g. lets say your throttle is set so the cylinder fills with air at half a bar. You have have the mass of air in the cylinder. The carb or fuel injectors add only half as much fuel - no choice, there’s only half as much oxygen there to burn it with!

When the piston compresses the mix, it still compresses it by the same factor of 9, BUT the air was only at half a bar to start with so it doesn’t get as hot or reach the same pressure as a full charge. (This is equivalent to open throttle but a smaller compression ration than 9.) Then, when the spark lights it off you get a lower peak pressure and temperature, your effective T[sub]H[/sub] is less. The power stroke extracts less work from the hot gas, and a greater proportion of the energy goes out the exhaust pipe.

This is the main reason why a small engine running at open throttle and reasonable revs will be more efficient than a large one throttled back to the same power, and it’s a much larger effect than the mechanical efficiency.

"You have have the mass of air…"

Sigh. And I previewed. Twice. That should be half the mass of air.

The Toyota Prius dual gas-electric assist system is one way for a hybrid car to operate. Another is to use a larger battery pack, and have a small gas or diesal engine running a genrator continuously, charging the batteries. This system can rely on a single electric motor, or 4 smaller motors, each driving a wheel. I believe that a hybrid of this type was constructed by GM in the mid-70’s, and that it worked quite well. The main problem, unless you use some rather exotic batteries, you need a very heavy lead-acid battery pack.
But anyway, the Prius system seems to work pretty well. No doubt we will see more and better hybrid designs, as the price of gasoline climbs.