ELi5: Would the emissions from a hybrid vehicle and a conventional vehicle be the same if they get the same MPG?

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Sorry if this is an exceptionally stupid question but I hope I’m in good company. If a hybrid vehicle gets 35 mpg, and there’s a comparable non-hybrid vehicle that also gets 35 mpg, do they effectively end up putting out the same amount of emissions? Or does it not really matter what the mpg is and there’s more to it? Thanks so much!

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The tailpipe emissions would be the same only if driving conditions do not change.
the hybrid vehicle will produce less emissions out the tailpipe if it goes downhill, or if it switches over to electric for a while. during braking, regenerative braking recharges the battery and reduces emissions.
You need to look at the bigger picture. producing a hybrid car requires more materials, more manufacturing, more rare earths, more exotic materials such as lithium for the batteries. Most raw materials require lakes of diesel fuel to mine and refine. lithium is just awful to produce. Recycling of or disposing of a hybrid car is more energy intensive than a regular car. the cost of the hybrid is higher, which means that you have to work harder/longer to afford it, which increases your carbon footprint.

You cannot simply compare MPG on the sticker at the dealership. The MPG figure is only the tip of the iceburg.

In terms of carbon dioxide emissions they will be exactly the same. However if the hybrid car is equipped with a plug in system you can still use less fuel then a conventional ICE car. But other tailpipe emissions like nitrus oxide, ozone and sot particles can still be quite different. And hybrid cars tends to be heavier so you get more dust from the tyres and asphalt which can also be quite devastating to the local environment. As for brake dust this can go either way, heavier hybrid cars need more braking power and will therefore make more brake dust but on the other hand they often have regenerative braking and will use their actual brakes less.

If they have the same mpg, they are burning the same amount of fuel to cover the same distance, regardless of hybrid or full internal combustion. Hybrids are supposed to have a higher mpg because they can use the electric motor to move in specific situations, meaning the vehicle is now covering distance without burning fuel. This is supposed to allow the vehicle to travel farther while burning the same amount of fuel as a full combustion engine. But if the hybrid isn’t increasing the mpg of the vehicle over that of a comparable vehicle with a traditional engine, you aren’t reducing emissions in a meaningful manner.

The actual emissions will depend on the individual vehicles themselves and how they are configured for emissions controls. Different compression ratios, fuel-air mixtures, combustion temps, and even brand of fuel (different brands use different additives) will have effects on emissions.

The issue is the way you phrased the question, MPG literally means gallons of fuel consumed to travel the same distance, so in your question if both vehicles consume the same quantity of fuel to travel the same distance, they will producing the same emissions, as emissions are directly linked to the chemistry of gasoline combustion.

The point is that hybrids will have reduced emissions because they will use less fuel to travel the same distance, so a gasoline car will get 35 MPG and the equivalent hybrid will get 50, the 15 MPG difference is reduced emissions.

One point not made yet is that gasoline engines are not universally efficient at all speeds, meaning a car might get 15 miles per gallon when driving 0-15 miles per hour, get 30 miles per gallon from 15-40 miles per hour, and 22 miles per gallon when driving at 40-75 miles per hour. Electric motors *are* universally efficient, meaning they consume the same amount of energy to go from 0-15 as they 50-75 miles per hour (I’m referring to the engine, the actual performance of the car with aerodynamics is more complicated).

This is why we have two different kinds of hybrid car, *parallel hybrids* can swap back and forth from gasoline engine to electric motor, which means you might be running the gas engine at speeds where it’s pretty inefficient. *Series hybrids* only use their electric motors to drive the car and use the gasoline engine as a generator to produce electricity for the electric motor. This configuration is more efficient as you can regulate the gasoline engine to only operate at it’s peak performance levels to charge the battery and let the electric motor handle the rest.

Is it a plug-in hybrid? Then it’s certainly lower, as the plug-in power comes from a power plant, and those are all much more carbon efficient than an auto engine.

If not, then the hybrid is probably a little better. The mileage test procedure does include some slowing and stopping, (the “city” number) to reflect actual driving. Those slowing and stopping operations are much more efficient in a hybrid, as the car’s energu goes into the batteries rather than just into heat.

Under the exact conditions of the mpg test, and assuming the fuels used and combustion efficiency are the same (one car is not running a differently optimized combustion cycle than the other) then yes their emissions will be the same.